research articles on production

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• 24 May 2021

Can Fabric Waste Become Fashion’s Resource?

COVID-19 worsened the textile waste crisis. Now it's time for the fashion industry to address this spiraling problem, say Geoffrey Jones and Shelly Xu. Open for comment; 0 Comments.

• 20 Oct 2015
• Working Paper Summaries

Internalizing Global Value Chains: A Firm-Level Analysis

Manufacturing activities that used to be performed in close proximity are increasingly fragmented across firms and countries. This paper provides strong evidence that considerations driven by contractual frictions critically shape firms' ownership decisions along their value chains.

• 06 Feb 2012
• Research & Ideas

Kodak: A Parable of American Competitiveness

When American companies shift pieces of their operations overseas, they run the risk of moving the expertise, innovation, and new growth opportunities just out of their reach as well, explains HBS Professor Willy Shih, who served as president of Eastman Kodak's digital imaging business for several years. Key concepts include: Outsourcing ends up chipping away at America's "industrial commons"—the collective R&D, engineering, and manufacturing capabilities that are crucial to new product development. If the United States wants to keep from slipping any further in its ability to compete on the industrial stage, the government must increase its support of scientific research and collaborate with the business and academic world. Open for comment; 0 Comments.

• 28 Mar 2011

Why Manufacturing Matters

After decades of outsourcing, America's ability to innovate and create high-tech products essential for future prosperity is on the decline, argue professors Gary Pisano and Willy Shih. Is it too late to get it back? From HBS Alumni Bulletin. Closed for comment; 0 Comments.

• 01 Mar 2010

A Golden Opportunity for Ford and GM

With Toyota caught in a downshift, competitors should make aggressive moves to capitalize, says HBS professor Bill George. For starters, they need to improve their auto lineups for the long term. He explains how Ford and GM can best navigate the industry landscape ahead. Key concepts include: For U.S. automakers to accelerate production while Toyota remains wounded is not a long-term strategy for success. The companies should cut costs while simultaneously transforming their organizations and revamping product lineups. Ford and GM could secure market share gains by investing windfall profits into making products more competitive for the next decade. In this regard, Ford has the jump on GM. Chrysler is missing a golden opportunity to revamp, reposition, and reorganize. Closed for comment; 0 Comments.

• 10 Jan 2008
• Sharpening Your Skills

Sharpening Your Skills: Operations Management

Closed for comment; 0 Comments.

• 05 Sep 2006

HBS Cases: Porsche’s Risky Roll on an SUV

Why would a company want to locate in a high-cost, high-wage economy like Germany? Porsche's unusual answer has framed two case studies by HBS professor Jeffrey Fear and colleague Carin-Isabel Knoop. Closed for comment; 0 Comments.

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It’s Manufacturing’s Turn for Special Treatment

• Gary P. Pisano
• March 05, 2012

Understanding the Rise of Manufacturing in India

• Vijay Govindarajan
• Gunjan Bagla
• September 18, 2015

Is U.S. Manufacturing Making a Comeback?

• Harold L. Sirkin
• September 30, 2011

Learning Across Lines: The Secret to More Efficient Factories

• Michael A. Lapre
• Luk N. Van Wassenhove
• From the October 2002 Issue

Link Manufacturing Process and Product Life Cycles

• Robert H. Hayes
• Steven C. Wheelwright
• From the January 1979 Issue

Read a Plant--Fast

• R. Eugene Goodson
• From the May 2002 Issue

The CEO of General Electric on Sparking an American Manufacturing Renewal

• Jeffrey R. Immelt
• From the March 2012 Issue

Yesterday's Accounting Undermines Production

• Robert S. Kaplan
• From the July 1984 Issue

Entrepreneurs Take On Manufacturing

• February 22, 2016

How Kenvue De-Risked Its Supply Chain

• Michael Altman
• Atalay Atasu
• Evren Özkaya
• October 18, 2023

Why the U.S. Tech Sector Doesn't Need Domestic Manufacturing

• David B. Yoffie
• October 05, 2009

Competing Through Manufacturing

• From the January 1985 Issue

The Feudal World of Japanese Manufacturing

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• From the November–December 1990 Issue

How to Address the Supply-Chain Staffing Crisis

• Joe McKendrick
• September 18, 2023

Time to Reform Job Shop Manufacturing

• James E. Ashton
• Frank X. Cook, Jr.
• From the March–April 1989 Issue

How Deep Tech Can Drive Sustainability and Profitability in Manufacturing

• François Candelon
• Daniel Küpper
• John Paschkewitz
• Vinit Patel
• September 28, 2023

The Increase in U.S. Manufacturing Jobs Is Nothing to Cheer About

• Uday Karmarkar
• January 20, 2011

Netflix’s “American Factory” and the New Geography of Manufacturing

• Irene Yuan Sun
• October 10, 2019

Hidden Factory

• Jeffrey G. Miller
• Thomas E. Vollmann
• From the September 1985 Issue

How Off-the-Shelf Tech Can Make Factories More Profitable

• Abhinav Agrawal
• Rob Sterner
• September 14, 2023

• Roy D. Shapiro
• September 27, 1988

TRIAS: Decision on Cable Ladder Production

• Singfat Chu
• Leo Hermanto
• February 19, 2016

Mercury Athletic: Valuing the Opportunity

• Timothy A. Luehrman
• Joel L. Heilprin
• September 18, 2009

Boeing 787: Manufacturing a Dream

• Rory McDonald
• Suresh Kotha
• February 12, 2015

Production Planning at Viktor Lenac Shipyard

• David Currie
• Giorgio Sinkovic
• November 23, 2004

Lilium: Preparing for Takeoff

• Navid Mojir
• Vincent Dessain
• Mette Fuglsang Hjortshoej
• Emer Moloney
• February 15, 2022

Display Technologies, Inc. (Abridged)

• Jonathan West
• H. Kent Bowen
• July 29, 1998

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• Janice H. Hammond
• June 26, 1995

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• John S. Haywood-Farmer
• Eric Janssen
• January 21, 2011

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• Paul W. Marshall
• Derek Lewis
• February 04, 2005

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• R. Paul Olsen
• August 01, 1974

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• Phillip E. Pfeifer
• April 05, 1991

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• Pankaj Ghemawat
• Jose Luis Nueno Iniesta
• April 01, 2003

Skutis: Negotiating Production in China

• Stephen Grainger
• May 01, 2018

Maestro Pizza (E): Turning up the Heat

• Ramon Casadesus-Masanell
• Fares Khrais
• May 26, 2022

Three Jays Corporation

• August 26, 2014

Creating a Process-Oriented Enterprise at Pinnacle West

• February 05, 2010

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• W. Earl Sasser Jr.
• September 01, 1973

AB InBev: Brewing Up Forecasts during COVID-19

• C. Fritz Foley
• Emilie Billaud
• September 27, 2023

Overview of Service Design in the Performing Arts

• Beatriz Munoz-Seca Fernandez-Cuesta
• Marta Rodriguez
• January 17, 2011

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Intercropping on Mars: A promising system to optimise fresh food production in future martian colonies

Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

Affiliation Centre for Crop System Analysis, Wageningen University & Research, Wageningen, The Netherlands

Roles Conceptualization, Funding acquisition, Resources, Supervision, Writing – review & editing

Affiliation Wageningen Environmental Research, Wageningen University & Research, Wageningen, The Netherlands

Roles Data curation, Methodology, Supervision

• Rebeca Gonçalves, 
• G. W. Wieger Wamelink, 
• Peter van der Putten, 
• Jochem B. Evers

• Published: May 1, 2024
• https://doi.org/10.1371/journal.pone.0302149
• Peer Review
• Reader Comments

Future colonists on Mars will need to produce fresh food locally to acquire key nutrients lost in food dehydration, the primary technique for sending food to space. In this study we aimed to test the viability and prospect of applying an intercropping system as a method for soil-based food production in Martian colonies. This novel approach to Martian agriculture adds valuable insight into how we can optimise resource use and enhance colony self-sustainability, since Martian colonies will operate under very limited space, energy, and Earth supplies. A likely early Martian agricultural setting was simulated using small pots, a controlled greenhouse environment, and species compliant with space mission requirements. Pea ( Pisum sativum) , carrot ( Daucus carota ) and tomato ( Solanum lycopersicum ) were grown in three soil types (“MMS-1” Mars regolith simulant, potting soil and sand), planted either mixed (intercropping) or separate (monocropping). Rhizobia bacteria ( Rhizobium leguminosarum ) were added as the pea symbiont for Nitrogen-fixing. Plant performance was measured as above-ground biomass (g), yield (g), harvest index (%), and Nitrogen/Phosphorus/Potassium content in yield (g/kg). The overall intercropping system performance was calculated as total relative yield (RYT). Intercropping had clear effects on plant performance in Mars regolith, being beneficial for tomato but mostly detrimental for pea and carrot, ultimately giving an overall yield disadvantage compared to monocropping (RYT = 0.93). This effect likely resulted from the observed absence of Rhizobia nodulation in Mars regolith, negating Nitrogen-fixation and preventing intercropped plants from leveraging their complementarity. Adverse regolith conditions—high pH, elevated compactness and nutrient deficiencies—presumably restricted Rhizobia survival/nodulation. In sand, where more favourable soil conditions promoted effective nodulation, intercropping significantly outperformed monocropping (RYT = 1.32). Given this, we suggest that with simple regolith improvements, enhancing conditions for nodulation, intercropping shows promise as a method for optimising food production in Martian colonies. Specific regolith ameliorations are proposed for future research.

Citation: Gonçalves R, Wamelink GWW, van der Putten P, Evers JB (2024) Intercropping on Mars: A promising system to optimise fresh food production in future martian colonies. PLoS ONE 19(5): e0302149. https://doi.org/10.1371/journal.pone.0302149

Editor: Adalberto Benavides-Mendoza, Universidad Autónoma Agraria Antonio Narro, MEXICO

Received: April 28, 2023; Accepted: March 28, 2024; Published: May 1, 2024

Copyright: © 2024 Gonçalves et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript.

Funding: The author(s) received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Introduction

As we enter a new age of space exploration, having a permanent settlement on Mars is now a reality in the not-so-distant future. Conquering the red planet is an exciting endeavour, but also one that brings several scientific, economic, and strategic benefits to our own planet and species.

Whilst rovers have begun to pave our way on Mars, the next stage of exploration and research would need to involve sending humans to perform the work themselves, as humans are much more autonomous and efficient at accomplishing field tasks and carrying out research [ 1 ]. However, having humans on Mars means they need to have their basic necessities covered, and this includes food. To supply and resupply from Earth all the food needs of a long-term settlement is both impractical as well as economically inviable, which means that the crew will have to make use of local Martian resources to produce at least part of their food needs locally [ 2 ].

The production of fresh food would in fact be an indispensable factor for any Martian colony. A bioregenerative food system, one where all resources are provided and produced in situ with minimal or no resupply from Earth, carries many advantages over the pre-packaged dehydrated foods that astronauts usually eat. As well as having the need for less food to be brought aboard the spacecraft (a factor that also significantly reduces the financial cost of the mission), a chief advantage of fresh food over dehydrated food is the retainment of nutrients essential to human health, especially antioxidants such as vitamin C and β-carotene, both of which are partially reduced or completely destroyed in the process of dehydration [ 3 – 5 ]. Although supplementary pills could be considered, fresh food provides a much better bioavailability of nutrients in the body, mediated via a complex array of other compounds present [ 6 ], and it further decreases the need to rely on resupplies from Earth for essential dietary requirements. The act of growing fresh food itself can also have a great positive impact on the mental well-being of colonists. Research on long-duration space missions, as well as analogue isolation simulations on Earth, have identified psychological and psychosocial factors as two of the most critical problems astronauts will face in their off-world missions [ 7 , 8 ]. Practising gardening activities has been repeatedly shown to be a significant and effective reliever of stress, anxiety and depression [ 9 , 10 ], and astronauts themselves have in the past expressed deep satisfaction at tending to plants in space [ 11 ].

Prospects and challenges of Martian agriculture

Several methods for implementing bioregenerative food systems on Mars have been studied, most of which were focused on artificial growing media such as hydroponics and aquaponics. However, soil-based systems, which would involve making use of the available in situ Martian regolith, have recently come to new light as having fundamental features for the long-term sustainability of a colony on Mars. Such a system confers many practical and economic advantages over artificial growing media, such as offering better buffering capacity for operational errors and technical breaks [ 12 ], and harbouring a microbiome environment that could compensate for toxic build-up of trace gases in the growth chamber [ 11 ]. Moreover, inedible crop parts and food waste, as well as human waste, can be composted and added to the soil to provide nutrients for the next harvest. This adds a circularity factor to the system that would minimize, and eventually eliminate, any need for expensive resupplies of nutrients from Earth [ 13 ].

Gardening on Mars is viable. Although Mars has a very thin atmosphere (1% that of Earth), it contains the necessary elements for plant growth, such as carbon dioxide (CO 2 ) and nitrogen (N 2 ) [ 14 ], and the planet is abundant in (frozen) water present both in the polar caps and in several places under the Martian surface [ 15 ]. Because of its thin atmosphere, Mars has high radiation levels on the surface which can be harmful to both plants and humans [ 16 ], and the planet holds an average global temperature of -65°C [ 17 ]. To counteract these conditions, colonies on Mars will likely consist of closed habitats that can shield both plants and humans from harmful radiation, and that can maintain the optimum temperature and atmospheric conditions for life within the colony [ 18 ]. Gravity on Mars is about 38% that of Earth [ 17 ]. However, experiments done on plant growth under microgravity conditions aboard the International Space Station (ISS) showed that plants, including crop species such as lettuce, cabbage and mustard, could be successfully grown, harvested and safely consumed by astronauts [ 19 ], showing promise that the lower gravity will not be a major impediment for normal plant development on Mars.

There are some potential obstacles with the Mars regolith that need to be overcome in order to successfully grow plants using in situ resources. For example, samples from Mars missions have shown the presence of heavy metals and perchlorates in the Martian regolith across different landing sites [ 20 ], harmful to both plants and humans. However, phytoremediation and bioremediation offer potential solutions to this, where specific plants or microbes can be cultivated to remove such elements from the soil with up to 100% efficacy [ 21 – 23 ]. Moreover, there’s the suggestion that perchlorates would not be present in deeper layers of Martian regolith, since their formation on Mars is proposed to be largely due to the regolith interaction with cosmic radiation and other atmospheric processes [ 24 ]. Additional concerns include the regolith’s high pH (~8.5) and nutrient deficiency (especially of nitrogen), both detrimental to plant growth and health. To address these, the addition of compost to the soil could lower the pH [ 25 ] while also providing the lacking nutrients. An initial batch of fertilizer could be incorporated in first missions’ cargo supply, until a cyclic bioregenerative system could be established. Such system could use inedible parts from the crops grown in situ as compost for the next crop generation, or make use of human faeces as manure [ 26 ].

Optimising resource use in Martian colonies

On Earth, it is possible to test the viability of using Martian soil (and thus of having a Martian soil-based system) for crop growth using Mars regolith simulants available. Several crop growth experiments have been successfully carried out on Earth using such simulants, with over 20 different species of crops reported to be able to grow and produce yield [ 27 – 29 ]. However, the main focus of these pioneering experiments was to test whether plants could germinate, grow and produce yield in the Mars regolith. No attention has yet been given to developing the efficiency of the system. Since colonies on Mars will probably operate under a limited amount of energy, space and other resources, any crop growth method that increases the productivity and circularity of the system, in a way that optimizes the use of such space and resources, will be paramount to the success and viability of the mission.

Intercropping is an ancient technique used on Earth that involves growing two or more crop species simultaneously in the same field [ 30 ]. This differs from the “traditional” monocropping system, where only a single crop species is planted in the same field. Intercropping has long been known to produce several beneficial effects compared to monocropping, such as increasing plant biomass and yield of one or more of the component species, improving the nutritional value of yield, and optimizing the use of resources in the system [ 31 – 33 ]. Such effects have been observed both in the field as well as in greenhouse pot experiments [ 34 – 36 ]. The beneficial effects of intercropping are often measured as a more efficient use of resources compared to the performance of each separate species in a monocropping system.

One way intercrop productivity benefits can arise is when the interspecific competition (competition between different species) in the intercropping system is lower than the intraspecific competition (competition amongst the same species) [ 37 ]. Another way is by means of species complementarity , where both (or all) intercropped species can mutually benefit each other within the system. Complementarity can work either by 1) niche differentiation , where companion species that are adapted to different niches (i.e. a shallow root species together with a deep root species) can make a more complete use of all available resources in the system, resulting in less competition for resources; or by 2) resource facilitation , where one companion species provides or facilitates the up-take of a limiting resource to the other companion species [ 38 ]. A classic example of resource facilitation in intercropping is seen through legumes, which can provide nitrogen (N) into the intercropping system thanks to their symbiotic relationship with rhizobia bacteria. The bacteria infect the legume´s roots to form “root nodules”, wherein they turn atmospheric nitrogen (N 2 ) into the usable form of ammonia (NH 3 ) via a process called N fixation [ 39 ]. This process helps relieve the competitive pressure on the intercropping system, either via the addition of N in soil [ 40 ] or via direct N transfer from the legume to the non-legume [ 41 ], and often results in the optimization of resources such as lower N fertilizer requirements [ 31 ].

The benefits of intercropping towards optimizing yield and resource use make it a prime candidate method to be applied on a soil-based system on Mars. To date, no experiments have ever been done to investigate the effects of intercropping on crops grown in Mars regolith. In this study, we aim to test the viability and prospect of implementing an intercropping system as a potential food production method in future Mars colonies.

Study objective and research question

Our study objective is to investigate the effects of intercropping on crop performance grown in a Mars regolith simulant (from now on also referred to as just “Mars regolith”). This is an unprecedented and novel approach to tackling the challenge of optimizing resource use efficiency for soil-based food production in Martian colonies. We chose three companion species, namely pea ( Pisum sativum ), carrot ( Daucus carota ) and tomato ( Solanum lycopersicum ), for their complementary and nutritional properties, and for the fact that they have all been previously shown to be able to grow and produce yield in Mars regolith simulant [ 29 ]. We quantify plant performance as above-ground biomass (g), yield (g), harvest index (%), and nutritional content of yield as levels of nitrogen (N), phosphorus (P) and potassium (K) (g/kg). The overall intercropping system performance is also quantified using the Relative Yield Total (RYT) index, where a value of higher than 1 indicates an overall yield advantage between all species in the intercropping system—compared to their monocropped counterparts—and a value of less than 1 indicates an overall yield disadvantage. Nodulation by rhizobia bacteria in pea roots will also be a key variable observed in this experiment, to interpret the effects that an N-fixing species would have on the interspecific competition within the intercropping system.

Our research question was whether we would see any beneficial effects of intercropping on the performance of pea, carrot and tomato grown in Mars regolith simulant. We hypothesise that, for all soil types, all three species grown in the intercropping system will give higher values for each of the plant performance indicators, compared to their monocropped counterparts. In turn, we also hypothesise that the intercropping system will show an overall yield advantage when compared to monocropping, giving an RYT value of higher than 1, for all three soil types. If plants perform better when intercropped, by making use of the exact same resource input as when they are monocropped, this could prove the efficacy of intercropping as a method for increasing the efficiency of the system as a whole.

Materials and methods

Greenhouse conditions.

We performed a pot experiment in a greenhouse on Earth, in Wageningen (51.9692° N, 5.6654° E). During the experimental period, average temperature in the greenhouse was 20.4 ± 1.7°C, relative humidity was 60.9 ± 15%, and day/night cycle was 16/8h. Ambient air was used and no extra CO 2 was added. Lamps yielding 600 watt (HPS 230 volt) were switched on if sunlight intensity was below 150 watt/m 2 , and switched off if sunlight intensity was above 250 watt/m 2 .

Mars regolith simulant

The regolith analogue used in this experiment was the MMS-1 Mars Regolith Simulant, of unsorted grade (grain <3.17mm, density 1.25g/cm 3 ), provided by The Martian Garden Company (Austin, USA). This simulant was manufactured under commission by researchers at the National Aeronautics and Space Administration (NASA)’s Jet Propulsion Laboratory (JPL) [ 42 ]. MMS-1 was designed to be the closest simulate of the regolith found on the Martian surface, especially when in contact with water. For a comparison of physical and chemical properties of the MMS-1 simulant and actual Martian regolith see S1 Appendix .

Earth soils and soil preparation

Two “Earth” soils were included in the experimental design: (1) common potting soil, composed mainly of peat with fertilizer (PGmix 15%N-10%P2O5-20%K2O), and (2) river sand. The potting soil was sieved to remove lumpy matter and improve homogeneity. Sand was specially chosen for it being nutrient poor, similar to Mars regolith (see also [ 28 ], to test if they would produce comparable results.

For the Mars regolith and the sand treatments, a small amount of organic soil (derived from the sieved potting soil) was mixed at 10% of the total volume, to improve root aggregation and water retention [ 43 ]. Table 1 shows the pH and Nutrient analysis from samples of the three soils.

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Results shown for Mars regolith simulant and sand include a 10% volume of organic matter mixed with each.

https://doi.org/10.1371/journal.pone.0302149.t001

Species selection

The species selected for this experiment were pea ( Pisum sativum cv. ‘Prince Albert’, Tuin Plus), carrot ( Daucus carota cv. ‘Nantes 2’, Buzzy), and dwarf cherry tomato ( Solanum lycopersicum cv. ‘Tiny Tim’, Buzzy). These species represent 3 out of the 27 potential food candidates to be incorporated into a space habitat, as proposed in NASA’s Life Support Values and Assumptions Document [ 44 ], based on various selection criteria such as potential yield, harvest index, horticultural requirements and macronutrient content needed to supplement a crew’s diet.

These species have also been chosen for their particular complementary properties which can be mutually beneficial to each other. As a legume, pea can serve as N-fixers in the system in symbiosis with rhizobia bacteria. Carrot helps aerate the soil, which can improve water and nutrient uptake by companion plants [ 45 ]. Tomato can provide shade for the temperature sensitive carrot and support for the climbing pea [ 46 ], and both carrot and tomato release root exudates such as flavonoids that can promote root nodule formation in pea [ 47 ]. Furthermore, all three species have compostable crop waste that can be mixed with the soil to provide key nutrients to the subsequent crop, reducing the need for fertilizers and promoting circularity in the system [ 48 ].

Finally, these species were chosen for their nutrient composition. All three species, in particular carrot and tomato, are high in antioxidants such as vitamin C and β-carotene [ 49 – 51 ]. These are an essential part of the human diet, but most importantly are amongst the few key nutrients that are mostly or completely lost in dehydrated foods [ 4 ].

Rhizobia bacteria addition

To provide the peas with their symbionts for N-fixing, 0,75ml of Rhizobium leguminosarum (biovar viciae strain 248 OD600 ~ 0.784) was added to every pot, including pots not containing pea plants. Inoculation was done 19 days after sowing. The selection of R . leguminosarum was based on its well-documented symbiotic relationship with P . sativum [ 52 ]. This particular strain was chosen due to its prior successful nodulation with P . sativum in a previous in-house experiment, where the same Mars regolith simulant (MMS-1) was used, thus aiming to control the potential variable of the bacteria not adapting to Martian regolith conditions.

Experimental design

We used a randomised complete block design with the three experimental species grown in three types of soil (MMS-1 Mars regolith simulant, sand and potting soil), divided into two cropping systems (intercropping and monocropping). This gave 12 different treatments with five replicas for each treatment, resulting in 60 pots. The experimental design and treatments can be seen in Fig 1 .

A: Overview of experimental treatments. B: Randomised complete block design with five replicas, and intercropping treatment pots randomly rotated. C: Photo of the experiment (day 48).

https://doi.org/10.1371/journal.pone.0302149.g001

For the monocropping treatments, three plants of the same species were grown in each pot, whilst for the intercropping treatments, one plant of each species was grown in each pot. 5L Pots were used, 16cm deep and 20cm wide at the top. Pots were filled to a 13cm depth, with 4.55Kg Mars regolith, 5Kg sand and 1.3Kg potting soil. A mesh was placed on the bottom of each pot to keep the soil in place.

Seeds were sown on 1st December 2021 and the experiment ran for 105 days, with the harvest taking place on 16 th March 2022. Seeds were over-sown to ensure the required germination. Thinning of germinated seeds was carried out 10–14 days later.

Watering and nutrient solution

Water saturation point for each soil was measured before the start of the experiment (Mars regolith = 35.7%, potting soil = 119%, sand = 27%). Watering was done once a day, every day, by pouring water on the saucer beneath the pot. For this, a random sample of three pots from each soil type was weighed and the amount of water to be given was determined to be just below the saturation point for that soil.

100ml of 2EC Hoagland nutrient solution was given once a week for the first 68 days, then increased to twice a week for the remainder of days to compensate for the increased biomass from the growing plants. The solution was poured on the topsoil surface. Solution contents can be found in S2 Appendix .

Harvesting and measurements

At harvest, above-ground biomass and yield were weighed for all three species. Both variables were measured in grams per plant (for monocropping treatments these values were added and divided by the number of plants in the pot, so to get the average value per pot). Above-ground biomass included stem, leaves and branches. Yield fresh weight was done separately for ripe and immature tomato fruits, and for pea pods and pea seeds, where only the seeds were considered for yield analysis. Harvesting of pea yield was also done throughout the experiment, as the first pea seeds started to mature on day 65. The first tomato fruits only started to ripen two weeks before the harvest date, so they were left on the plants to be harvested all at once. For carrot yield, the whole carrot from the crown to the tap root was weighed. All pea roots were washed and visually assessed for the presence of rhizobia nodules, which were also dissected and checked for nodule activity using a light microscope (active nodules show an inner pink coloration [ 53 ])

All fresh weighed samples were put in ovens to dry at 70°C for 48 hours.

Yield of the three species was analysed for NPK content. Three replicas of each treatment were grinded and analysed using digestion H2SO4/H2O2/Se + NP on SFA and H2SO4/H2O2/Se + K on ICP-OES. For tomato, only ripe (red) tomato fruits were analysed.

Calculations

Where Y is the yield (g), and ABG is the above ground biomass (g).

The division of the intercropping yield over the monocropping yield of the same species gives the relative yield (RY) for that species ( Eq 2 ). A RY of > 0.33 (0.33 being the ratio of each species present in the intercropping treatment) indicates that the species over-yielded in the intercropping treatment compared to its monocropping treatment.

Where Y i is the yield from the intercropping treatment (per species) (g), and Y m is the yield from the monocropping treatment (averaged between the number of plants in the pot) (g).

Where RYt, RYp and RYc are the relative yields of tomato, pea and carrot (respectively). If RYT > 1, this indicates relative yield (and hence resource use) advantage from intercropping, and RYT < 1 indicates relative yield disadvantage from intercropping.

Data analysis

All data was checked for normality and homogeneity using a W test for normality and a Barlett’s test for homogeneity. When needed, LOG 10 transformations were done to meet the two assumptions, and tests were then run on the transformed data. There were only three parameters that did not fit homogeneity, tomato yield P content (P = 0.024), and harvest index for peas and for carrots (P = 0.011), but those were still included in the analysis.

A two-way ANOVA was performed on the following variables: above-ground biomass dry weight, yield dry weight, harvest index, N content in yield (NY), P content in yield (PY) and K content in yield (KY). For tomato yield, the data also included the dry weight of the immature fruits harvested, since they would become ripe and consumable if harvested later. The ANOVA was run separately for each species, with cropping type and soil type at the interaction level, and block effects accounted for. A post-hoc Fisher’s protected LSD test (P<0.05) was performed for significant effects of soil type and the interaction between soil type and cropping type.

An analysis of deviance was performed on the ratios between intercropping and monocropping for each species in every soil (calculated from LOG-transformed data), and the p-values were obtained for the null hypothesis that the ratio equals 1 (i.e. no difference between intercropping and monocropping treatments).

There was a missing value in one of the replicas sent for the NPK analysis of pea yield from the intercropping Mars regolith treatment due to insufficient pea material available. This means that for this treatment, only 2 replicas were analysed for NPK. There were no other missing or deleted values in the data set.

Mars regolith

Tomato showed increased performance in the intercropping treatment compared to the monocropping treatment, with significantly higher above-ground (AG) biomass (P = 0.037), yield (P = 0.043), and Potassium content in yield (KY) (P<0.001, Table 2 , Figs 2 and 3 ).

A: Intercropping treatment (left) and monocropped tomato (right). B: Intercropping treatment (left) and monocropped pea (right). C: Intercropping treatment (left) and monocropped carrot (right). Arrows point to the small carrot leaves from the intercropping treatment. Pictures were taken on the day of harvest (day 105).

https://doi.org/10.1371/journal.pone.0302149.g002

A: Above-ground biomass. B: Yield. C: Harvest index. D: N content in yield (NY). E: P content in yield (PY). F: K content in yield (KY). “Inter” = Intercropping treatment, “Mono” = Monocropping treatment. Letters indicate significant differences between soils within the same cropping treatment and within the same species, at the 0.05 level (Fisher’s LSD test). Asterisks (*) indicate significant differences between intercropping and monocropping treatments within the same soil and within the same species, at the 0.05 level (Analysis of Deviance). Error bars represent standard error (SE).

https://doi.org/10.1371/journal.pone.0302149.g003

https://doi.org/10.1371/journal.pone.0302149.t002

Contrary to tomato, carrot showed decreased performance in the intercropping treatment compared to the monocropping treatment, with significantly lower AG biomass (P = 0.023), yield (P = 0.022), NY (P<0.001) and PY (P = 0.011, Table 2 ).

Pea showed no significant difference in performance between the intercropping and monocropping treatments for any of the variables. However, like carrot, the observed trend showed lower values in the intercropping treatments compared to the monocropping treatments for all variables, with the p-value for N content close to the threshold (P = 0.060, Table 2 ).

There was no significant difference in harvest index between intercropping and monocropping for any of the three species.

Comparison between soils

Potting soil produced the same pattern of results as those seen in the Mars regolith simulant, when comparing the intercropping and monocropping treatments ( Table 2 and Fig 3 ). The only exception was KY, where in tomato it showed no significant difference, and in pea it was significantly lower in intercropping compared to monocropping (P = 0.029).

Sand also produced a similar pattern of results to Mars regolith, when comparing the intercropping and monocropping treatments ( Table 2 and Fig 3 ), with three exceptions where the values were inverted: Tomato showed no significant difference in KY, but showed an indication of lower K in intercropping; Pea showed significantly higher AG biomass (P = 0.002) and yield (P = 0.012) in intercropping compared to monocropping; and carrot showed no difference in NY, but with an indication of higher N in intercropping. A last exception was carrot PY, where there was no significant difference between cropping treatments, but the pattern followed being lower for intercropping compared to monocropping.

For all three species, AG biomass, yield, and harvest index mostly followed the pattern of being significantly the highest on potting soil, followed by sand, and lastly by Mars regolith (P<0.001). Where the difference was not significant (P>0.05), the values always followed the same pattern, except in the harvest indexes of tomato intercrop and monocrop, pea intercrop and carrot monocrop, where sand values were slightly higher than in potting soil ( Fig 3A–3C ). The only significant exception to this pattern were carrot AG biomass and yield, where both were lower in the potting soil intercropping treatment compared to the sand monocropping treatment (P<0.001, Figs 4 and 5 ). A noteworthy change was seen in pea AG biomass and yield, where both went from being significantly lower in sand compared to potting soil in monocropping, to having no significant difference between the two soils in intercropping (AG biomass: P = 0.030; yield: P = 0.015) ( Fig 3A and 3B ).

A: Sand. B: Mars regolith simulant. C: Potting soil.

https://doi.org/10.1371/journal.pone.0302149.g004

Labels in pictures indicating “Organic soil” refer to the potting soil treatment. A: Intercropped tomato. B: Monocropped tomato. C: Intercropped peas. D: Monocropped peas. E: Intercropped carrots. F: Intercropped carrots.

https://doi.org/10.1371/journal.pone.0302149.g005

For NPK content in yield, the pattern seen above was inverted, where Mars regolith was mostly found to give the highest yield NPK values, and potting soil the lowest values, for both cropping treatments. Mars regolith had significantly higher values than both sand and potting soil for tomato NY and KY (P<0.001), pea P (P = 0.007) and K (P<0.001), and carrot N (P<0.011). No significant difference was found amongst any other treatments. The only exception to this pattern was tomato PY, where it was significantly higher in Potting soil compared to Mars regolith monocrop, and higher in sand monocrop compared to Mars regolith monocrop (P = 0.041, Fig 3D–3F ).

Sand was the only soil to present an overall yield advantage in the intercropping system compared to monocropping (RYT = 1,32, Table 3 and Fig 6 ), where both tomato and pea showed yield advantage in intercropping over monocropping (tomato relative yield (RY) = 0.54; pea RY = 0,62), and carrot showed yield disadvantage (RY = 0,15). Potting soil showed no overall yield advantage (RYT = 1,00), where tomato over-yielded (RY = 0,66) but both pea and carrot under-yielded (pea RY = 0,28; carrot RY = 0,06). Mars regolith showed under-yielding in the intercropping system compared to monocropping (RYT = 0,93), where tomato over-yielded (RY = 0,64) but both pea and carrot under-yielded (pea RY = 0,18; carrot RY = 0,11).

Ratios above 1 indicate the variable had a higher value in intercropping relative to monocropping, ratios below 1 indicate the variable had a lower value in intercropping relative to monocropping. RYT (total relative yield) refers to overall yield advantage or disadvantage in the intercropping system. If RYT > 1, this indicates yield advantage from intercropping over monocropping, RYT < 1 indicates yield disadvantage from intercropping over monocropping, and RYT = 1 indicates no difference in yield between the two cropping systems. Variables indicated are above-ground biomass dry weight (g), yield dry weight (g), harvest index (%), and N, P and K content in yield (g/kg)) for each of the three species (tomato, pea and carrot). Asterisks (*) indicate significant differences between intercropping and monocropping values within the same variable, at the 0.05 level.

https://doi.org/10.1371/journal.pone.0302149.g006

https://doi.org/10.1371/journal.pone.0302149.t003

Presence of rhizobia bacteria

Visual analysis of roots of peas in both monocropping and intercropping treatments revealed presence of Rhizobia infected nodules in sand and potting soil, but not in Mars regolith (except for one replica from a monocropping treatment which had 3 nodules). Between 17 and 150 nodules were found across the other treatments. There was no significant difference in the number of nodules between potting soil and sand (Fisher’s post hoc test P<0.05), although there was on average more nodules found in sand than in potting soil ( Fig 7 ). Both sand and potting soil showed a significantly higher number of root nodules than Mars regolith (P<0.001). There was also on average more nodules in intercropping compared to monocropping for both soils, although there was considerable variation between replicas, and thus this difference was not significant (P = 0.100). Dissection of several nodules per replica showed a pink coloration inside, indicating that the nodules were active ( Fig 8 ).

Values are indicated for the monocropping and intercropping treatments from each of the three soils (Mars regolith, potting soil and sand). Data labels indicate the average value. Letters indicate significant differences between cropping treatment and between soils, at the 0.05 level (Fisher’s LSD test). Error bars represent standard error (SE).

https://doi.org/10.1371/journal.pone.0302149.g007

A: Pea roots from Potting soil monocropping treatment, with intact nodules. B: Pea roots from sand monocropping treatment, with nodules dissected, where a pink coloration can be seen, indicating the nodules were active in N-fixing. C: Pea roots from Mars regolith simulant (intercropping treatment), showing thicker and more tortuous roots. D: Pea roots from Mars regolith (monocropping treatment), from the only replica that showed the presence of nodules. These roots also appear thicker and more tortuous.

https://doi.org/10.1371/journal.pone.0302149.g008

Intercropping effects on RYT for Mars regolith

Overall, the intercropping system in Mars regolith showed yield disadvantage over monocropping, with an RYT (total relative yield) value of 0.93. Intercropping advantages or disadvantages can indicate the amount of interspecific competition or facilitation that is occurring within an intercropping system [ 54 ]. An RYT of less than 1 means that interspecific competition in intercropping was higher than intraspecific competition in monocropping. This is very likely due to the fact that pea did not form root nodules in symbiosis with the Rhizobia in Mars regolith, thus negating its role as N-fixers and the key advantage of having a legume in an intercropping system. Many studies have demonstrated that when intercropped, pea can compensate for increased nutrient competition, as well as for N deficiency in the soil, by releasing root exudates (i.e. flavonoids) that can promote root nodule formation [ 47 , 55 ]. This in turn increases fixation of atmospheric N and helps pea both escape the competitive pressure, as well as relieve the pressure for the other species by leaving more N available in the soil. Without the formation of nodules this was not possible, and instead of relieving the competitive pressure, pea became an added competitive load to the intercropping system in Mars regolith.

The contrast in performance between tomato and the other two species can be attributed to the fact that different crops often show different levels of competition strength when intercropped together. Tomato is a dominant species within intercropping systems, and has been shown to have yield advantages when intercropped with legumes [ 56 ]. In another study by Wu et al. [ 36 ], where tomatoes were intercropped with potato onion, intercropping promoted the growth of tomato, whilst it inhibited the growth of potato onion.

Tomato is also generally known to be “heavy feeders” with a high demand for nutrient supply [ 57 ], which could mean that they monopolise the resource uptake in the pot. This higher competitive strength of tomato could explain why it had increased yield performance in intercropping compared to monocropping, since it was put at an advantage compared to pea and carrot, whilst losing this advantage when grown with other tomato plants. It could also explain why in Mars regolith pea and carrot had lower performance in the intercropping treatment, as a higher resource uptake would confer the tomato more nutrients for growth, whilst leaving less resources available to the other two species. Carrot, which depends solely on the nutrients available in the soil, would naturally be at a disadvantage when grown with tomato.

Interestingly, in sand where we saw an adequate number of nodules in pea roots (“adequate” meaning an average of 30 or more nodules per plant [ 58 ]), there was an overall yield advantage in the intercropping system (RYT = 1.32), where although intercropped carrots underperformed, both pea and tomato showed significantly higher yield compared to monocropping. This higher RYT owes to the fact that pea performed particularly well when intercropped in sand, giving equal or higher values in all variables compared to monocropping.

The success of peas in sand is most likely due to the higher number of root nodules that were formed, particularly in the intercropping treatment, allowing for more atmospheric N-fixing. The higher number of root nodules in the sand treatment compared to potting soil can probably be explained by the lower N content available in sand, a factor that has been shown to promote nodulation [ 55 ]. Lower N availability has also been linked to an increase in RYT before: in a pea-barley intercrop study, RYT was highest for treatments with no fertilizer compared to treatments with applied fertilizer [ 59 ]. The high resource demand from the aggressive tomato in the sand intercropping treatment probably caused further decrease in N availability in the soil, which could explain the higher nodulation seen in this treatment ( Fig 7 ).

Potential factors affecting yield performance on Mars regolith

There are a few reasons why nodulation in pea may have been hindered in both Mars treatments. Many studies have shown nodulation and R . leguminosarum survival to greatly decrease when under a variety of biotic and abiotic stresses, including high salinity [ 60 ] and soil sterility [ 61 ], both of which are features of the MMS-1 Mars regolith used in this study. A potentially aggravating condition was that the regolith was very compact in comparison to both potting soil and sand. Although it was of “unsorted grade”, it was composed mostly of very fine particles that gave it a clay-like texture when wet, and it did not drain well. The roots of the peas in Mars regolith reflected this in that they were smaller, thicker and more tortuous than the roots of peas in sand (and also in potting soil) ( Fig 8 ), which is a common response to denser and coarser soils [ 62 ]. Since soil texture is known to affect salinity, where clay and moist soils tend to be more saline [ 63 ], this could have added a salt stress factor to the Mars regolith, negatively affecting rhizobia bacteria. Moreover, the Mars regolith was low in organic matter. Sterilised soils have been shown to severely hamper formation of nodules, when compared to unsterilised soils containing more varied microbial communities [ 61 ]. The chemical composition of the Martian regolith has also been suggested to affect nodulation. A study involving legume-Rhizobia symbiosis in Mars regolith simulants showed more nodulation in the MMS-2 regolith simulant compared to MMS-1, where MMS-2 contains added compounds such as Iron Oxide, Magnesium Oxide, Sulfates and Silicates [ 64 ]. Another similar study on legume-Rhizobia interactions in MMS-1 suggested that the high pH of the regolith may impair Fe uptake by the plants, Fe being a key factor for plants to sustain a healthy relationship with rhizobia, which would in turn negatively impact nodulation [ 65 ].

The compactness factor of the regolith could have also posed a problem for plants themselves, regardless of how it may or may not have affected Rhizobia nodulation. Compact soils allow for lower gas diffusion and water conductivity, which can lead to anaerobic conditions in the soil and significantly reduce N availability and nutrient uptake [ 62 ], with more compact soils being linked to lower yields [ 66 ]. The organic matter present in Earth soils greatly contributes to soil quality, both in reducing soil compactness, as well as being rich in nutrients and the micro-organisms that play a part in making nutrients available for plant uptake [ 67 ]. This could also explain why the absolute values for yield were the highest in potting soil, and why sand also showed higher yield values compared to Mars (although sand is also poor in organic matter, it is not as sterile as the Mars regolith, and may have harboured a wider microbiome that aided in plant performance). The sterile nature of Mars regolith means that, although it has the chemical elements necessary to meet the requirements of plant growth ( Table 1 ), these elements could be mostly absent in the bio-available forms that are necessary for their uptake [ 68 ], which are often made available by the activity of micro-organisms in the soil.

Nutrient analysis

Potassium content in yield (KY) for Mars regolith intercropped tomato was significantly higher than KY for both sand and potting soil tomato, from both cropping treatments. This could be explained by the fact that optimum K uptake occurs only above 6.5 pH, and is increased further with increased soil moisture [ 69 ], giving Mars regolith an advantageous condition for K uptake especially when compared to potting soil.

PY in Mars monocropped tomato was significantly lower than in Mars intercropped tomato. This could be explained by the fact that P uptake is severely compromised between pH 8 and pH 9, which is exactly where the Mars regolith falls within. However, it is unclear why this was also not the case for carrot and pea PY from Mars regolith, or for the Mars intercropped tomato treatment.

NY for both tomato and carrot were significantly higher in Mars regolith compared to potting soil, even though potting soil had vastly more nitrogen available ( Table 1 ). Although higher N availability in soil has been linked to higher yields [ 70 ], studies have also shown that this higher availability has either no correlation or a negative correlation [ 71 ] with N content in yield. This could explain why in potting soil we see the highest values for yield but also the lowest values for N content in yield, and vice-versa for Mars regolith.

Mars regolith limitations and proposed soil ameliorations

Physical and chemical properties of the Mars regolith, such as soil compactness and high pH, may have made it a hostile environment for survival and nodulation of rhizobia bacteria, as well as causing reduced nutrient availability and bioavailability in the soil for ideal plant development, impeding the plants from taking full advantage of their complementarity properties in the intercropping system. For instance, when nodulation occurred and peas were optimised for N-fixing, such as in the sand treatment, the intercropping system as it was designed in this experiment gave significant overall yield advantages, indicating the potential for such a method to be used to optimise crop growth on Mars.

Therefore, a chief solution for the improvement of the system would be to work on the sustainable amelioration of the Mars regolith, taking into account a realistic starting scenario on future Martian colonies. For example, initially selecting a higher grain grade of the regolith on Mars, at the onset of the Martian agricultural system, could potentially already aid in reducing soil compactness, improving drainage, salinity conditions, gas diffusion and nutrient availability.

Following the first crop harvest, using inedible parts to mix in as compost to the regolith could also greatly improve soil conditions. The increased organic matter would not only improve conditions for the survival and nodulation of Rhizobia bacteria, but also support the growth of all other species present by improving the regolith’s nutrient content, bioavailability and uptake. Adding compost to MMS-1 Mars regolith results in higher plant biomass, with the highest results at 30:70 simulant:compost mixture [ 43 ]. Other combinations or varying ratios of candidate species can also be tested in order to find the optimum arrangement for the most efficient species complementarity. For example, a study by El-Gaid et al. [ 72 ] on intercropping tomato and bean found that the highest RYT values were achieved in a combination of 1 tomato plant: 3 bean plants (RYT = 1.26).

Of course, the absence of nodulation and reduced plant biomass and yield on the Mars regolith can be attributed not only to its physical properties and lack of micro-organisms and nutrients, but also to its intrinsic chemical composition. Further research would need to be conducted to isolate and identify which and how each of the chemical factors of MMS-1 would influence both in the nodulation of rhizobia as well as in the performance of plants.

Finally, actual Mars regolith and the initial conditions of a Martian colony would pose some additional challenges which, due to the emerging nature of this field of study, were purposefully “bypassed” here in order to reduce variables and produce more reliable results with respect to the focal study system. For example, we manually added a minimal amount of nutrients required for plant growth, disregarding the inherent sterility of the in situ regolith and presuming that such nutrients could be supplied from Earth or acquired through an established bioregenerative system on Mars. Furthermore, our simulant did not contain any perchlorates, a compound known to be present on the surface of Mars, nor did we analyse the effects of the regolith’s heavy metals on plant growth. Although perchlorate and heavy metal presence can be remedied with the addition of plants or microbes to the soil [ 21 , 23 ], further research could include toxicity remediation and study the holistic effects that such measures would have on the microbial community and overall conditions for plant growth in the Martian regolith. As we build upon our collective knowledge for Martian agriculture, future studies can incorporate findings to produce a more overarching understanding of the complete scenario on Mars, including all its challenges and adopted solutions.

Conclusions

In this study we sought to simulate an agricultural scenario likely to be encountered by early Martian colonists, such as the use of small pots, a controlled greenhouse environment, and a nutrient poor Mars regolith simulant with only essential nutrients added. Under such circumstances, we found that an intercropping system can be successful in optimising resource use efficiency if soil conditions are favourable to plant growth and nodulation of N-fixing bacteria, as we saw in the sand treatment (RYT = 1.32). On Mars regolith simulant, intercropping had an overall yield disadvantage compared to monocropping (RYT = 0.93). We postulated that this was most likely due to the absence of rhizobia nodulation in the Mars regolith, which negated the role of pea as a Nitrogen-fixer, impeding crops in the intercropping system from taking full advantage of their complementary properties. We identified that some of the physical and chemical properties of the Mars regolith simulant, such as elevated compactness and high pH, may have created a hostile environment for the survival and nodulation of rhizobia bacteria, while also limiting nutrient availability and bioavailability in the soil necessary for normal crop development. As further research, we suggest considering our proposed soil ameliorations to the Mars regolith, such as utilizing a higher grain grade to reduce soil compactness, and simulate a cyclic system in order to use past harvest waste as compost to increase soil pH and nutrient availability. We acknowledge that certain challenges with the actual regolith and starting conditions on Mars, such as the presence of perchlorates and the initial absolute sterility of the regolith, have been purposefully bypassed in order to maintain the focal objective of the study. Although solutions to such challenges exist, future studies should aim to ultimately integrate all these factors into a single system. As we build towards a more complete and comprehensive knowledge of Martian agriculture, focusing on improving regolith conditions could be key to unlocking the potential of rhizobia-legume interactions and species complementarity, and advance intercropping as a leading method to optimise fresh food production in future Martian colonies.

Supporting information

S1 appendix. comparison of the mineralogical, physical and chemical properties between the mars regolith from the rocknest eolian deposit in the gale crater on mars and the mms-1 mars regolith simulant..

Values are given in weight percentage (wt%). The table contains compiled data taken from multiple sources and own measurements.

https://doi.org/10.1371/journal.pone.0302149.s001

S2 Appendix. pH, EC and nutrient contents of Hoagland nutrient solution.

https://doi.org/10.1371/journal.pone.0302149.s002

Acknowledgments

The authors wish to thank Zoë Berkers for help with measurements and harvest, and Paul Goedhart for his advice on the statistics.

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Hydrogen production, storage, utilisation and environmental impacts: a review

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• Published: 06 October 2021
• Volume 20 , pages 153–188, ( 2022 )

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• Ahmed I. Osman   ORCID: orcid.org/0000-0003-2788-7839 1 ,
• Neha Mehta 1 , 2 ,
• Ahmed M. Elgarahy 3 , 4 ,
• Mahmoud Hefny 5 , 6 ,
• Amer Al-Hinai 7 ,
• Ala’a H. Al-Muhtaseb 8 &
• David W. Rooney 1  

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A Correction to this article was published on 31 March 2022

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Dihydrogen (H 2 ), commonly named ‘hydrogen’, is increasingly recognised as a clean and reliable energy vector for decarbonisation and defossilisation by various sectors. The global hydrogen demand is projected to increase from 70 million tonnes in 2019 to 120 million tonnes by 2024. Hydrogen development should also meet the seventh goal of ‘affordable and clean energy’ of the United Nations. Here we review hydrogen production and life cycle analysis, hydrogen geological storage and hydrogen utilisation. Hydrogen is produced by water electrolysis, steam methane reforming, methane pyrolysis and coal gasification. We compare the environmental impact of hydrogen production routes by life cycle analysis. Hydrogen is used in power systems, transportation, hydrocarbon and ammonia production, and metallugical industries. Overall, combining electrolysis-generated hydrogen with hydrogen storage in underground porous media such as geological reservoirs and salt caverns is well suited for shifting excess off-peak energy to meet dispatchable on-peak demand.

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Introduction

The continual growth and rapid urbanisation of the world population and economy have resulted in an enormous increase in energy need, urging the switch from fossil-based fuels into alternative clean renewables (Dawood et al. 2020 ). Consequently, global decarbonisation in the transportation, industry and electricity generation sectors is crucially needed to mitigate anthropogenic climate change (Fawzy et al. 2020 ; Osman et al. 2021a ). In this context, there has been a growing interest from scholars and industries with versatile production routes. There is abundant availability of renewable sources used in hydrogen production; however, the variable and intermittent nature of these resources is the major challenge in the transition towards a hydrogen economy. Hence, this calls for technical accommodation, especially for balancing variable renewable supply, i.e. solar, wind and others, and varying energy demand. Furthermore, cost-effective production methods, policies, research and development and hydrogen infrastructure development are areas that need more investigation when transitioning towards the hydrogen economy.

More than 100 current and planned hydrogen production technologies are reported to date, with over 80% of those technologies are focused on the steam conversion of fossil fuels and 70% of them are based on natural gas steam reforming. However, in order to minimise carbon footprint emissions, a wider range of hydrogen extraction processes, such as methane pyrolysis and seawater electrolysis using alternative energy sources, must be addressed. All hydrogen production routes are highlighted in Fig.  1 .

Hydrogen production routes, including renewables, fossil fuels and nuclear, with hydrogen being produced in power plants, pharmaceutical applications, synthetic fuels or their upgrades in transportation, ammonia synthesis, metal production or chemical industry applications

Hydrogen is the most abundant element in the universe, and due to its reactivity, it only exists on earth in compounds such as water and organic materials. It is an odourless, flammable and colourless gas, which is leading to its safety concern, especially if a leak is not detected and gas collects in a confined area; it can ultimately ignite and causes explosions. Furthermore, metal hydrogen embrittlement is an issue as it could damage pipelines and containers due to its small molecular size; thus, it escapes through materials. The higher heating value (HHV) of hydrogen is 141.8 MJ/kg at 298 K, and the lower heating value is 120 MJ/kg at the same temperature. This is significantly higher than that of most fuels such as gasoline with a value of 44 MJ/kg at 298 K. However, liquid hydrogen has a lower energy density by volume than hydrocarbon fuels such as gasoline by a factor of four with a density of 8 MJ/l versus density of 32 MJ/l. While hydrogen gas has a high energy density by weight but a low energy density by volume compared to hydrocarbons, it requires a larger tank to store. For example, as opposed to liquified natural gas, liquified hydrogen contains 2.4 times the energy but takes 2.8 times the volume to store. At the same time, the low temperature for liquified hydrogen storage at ambient pressure and a temperature of −253 °C raises quite a few risks. When exposed, it can cause cold burns; furthermore, leakage can result in a combination of liquefied air and hydrogen, resulting in an explosive mixture or the formation of flammable or explosive conduits (Atilhan et al. 2021 ; El-Halwagi et al. 2020 ).

Like electricity, hydrogen is an energy carrier and not an energy source; using it to store renewable energies instead of being wasted when not in use is crucial since it is storable, utilisable and transportable (Parra et al. 2019 ; Abe et al. 2019 ).

Hydrogen cleanness and colour coding

Dawood et al. (Dawood et al. 2020 ) reported the four main stages in hydrogen economy: production, storage, safety and utilisation, where hydrogen purification and compression (subsystems) need to be considered along with the life cycle assessment (LCA) when selecting the production method for hydrogen. Hydrogen cleanness level is described in the literature with many colour coding: mainly green, blue and grey, which relies only on the production route, i.e. hydrogen origin, and fails to assess the deep cleanness of the produced hydrogen (Merzian and Bridges 2019 ), for instance: (1) Grey hydrogen is produced using fossil fuels such as natural gas, one tonne of hydrogen produced in this way is responsible for 10 tonnes of carbon dioxide (Dvoynikov et al. 2021 ), as shown in Fig.  2 ; (2) blue hydrogen is produced from fossil fuels like grey hydrogen but with combination of carbon capture and storage to mitigate emissions; (3) green hydrogen is typically produced from 100% renewable sources such as wind or solar energies with lower carbon footprint; (4) brown hydrogen is produced from gasification of coal-based fuel; and (5) turquoise hydrogen is produced from the thermal decomposition of natural gas, i.e. methane pyrolysis or cracking by spitting methane into hydrogen and carbon at a temperature range from 600 to 1200–1400 °C (Dvoynikov et al. 2021 ). This process produces black carbon (soot) as a by-product instead of carbon oxide emissions in the grey hydrogen, allowing for the sequestration of carbon emissions in the form of solid carbon. However, carbon stability in this black soot is critical for long-term carbon sequestration, along with the utilisation of renewable energy sources in the high-temperature process to achieve carbon neutrality. Interestingly, hydrogen could be produced with a negative carbon footprint via biogas pyrolysis.

Hydrogen colour coding for various manufacturing processes. Green hydrogen is produced using renewable energy sources such as solar or wind energy, followed by water electrolysis. Grey and brown hydrogen are produced by methane steam reforming and coal gasification, respectively, and when combined with carbon capture and storage, blue hydrogen is produced. Turquoise hydrogen is produced through the pyrolysis of methane, with solid carbon as a by-product

However, this colour coding is not precise as it assumes that green hydrogen always has low-carbon emission than blue or grey hydrogen, which is not applicable in all cases. Blue hydrogen, for example, is regarded as less safe than green hydrogen, even though it releases no carbon at the point of use or during the entire process, while green hydrogen may do. For instance, bioenergy feedstocks such as biomass emit greenhouse gas emissions such as CH 4 , SO x , NO x and CO 2 during their growth or thermochemical conversions. Furthermore, the carbon capture and storage technique used in the blue hydrogen reduces toxic emissions significantly. The manufacture of photovoltaic panels as renewable energy technology also has a significant carbon footprint and generates various types of waste, liquid and gaseous by-products that are hazardous to the environment. Starting from the extraction of quartz and other materials used to manufacture solar panels, this is coupled with the carbon and sulphur emission in the energy-intensive process when producing metallurgical silicon. Moreover, the solar panel has a 30-year lifespan, and then, it must be handled as a particular waste at its end of life.

A recent LCA study compared environmental impacts for steam methane reforming with water electrolysis using wind, solar photovoltaic, hydropower, solar thermal and biomass gasification as energy sources (Al-Qahtani et al. 2021 ). It was concluded that among all the technologies evaluated, solar photovoltaic electrolysis had the most damaging environmental implications because of the significant acidification potential in the photovoltaic panel production phase and the relatively poor efficiency of photovoltaic systems.

Thus, measuring the emitted greenhouse gas emissions accurately in the entire production process along with the life cycle of the equipment used is crucial. This is required to determine how green is the green hydrogen and how blue is the blue hydrogen. A recently proposed model for improved hydrogen colour coding consisted of a hydrogen cleanness index followed by the number of depth levels (Han et al. 2021 ). For instance, 80 green-4 means hydrogen is produced via renewable resources; however, it is not a zero-emission process, only 80% green, due to emissions related to the process. The number after the colour, which in this case is 4, indicates that greenhouse gas emissions (CO 2-e ) linked with the purification during the production route have been considered. This model still requires much more analysis to decide the start and end of the continuum thresholds for each colour, as well as the evaluation depth levels and related weight for each level.

Hydrogen production routes

According to the International Energy Agency (IEA), green hydrogen could help reduce our carbon footprint if major challenges such as infrastructure, logistics, cost-effective manufacturing methods and safety are overcome. Globally, hydrogen is responsible for about 843 metric tonnes of CO 2 emissions per annum, equal to the combined total emissions of the UK and Indonesia (IEA 2019 ). The global hydrogen demand is projected to increase from 70 million tonnes in 2019 to 120 million tonnes by 2024 (Global hydrogen market insights 2020 ; Atilhan et al. 2021 ; Safari and Dincer 2020 ). In 2025, the largest global green hydrogen plant will be built, with a capacity of 237,250 tonnes per annum, i.e. 650 tonnes/day hydrogen output through electrolysis and 4 gigawatts of renewable energy from wind, solar and storage.

A wide range of resources is available for hydrogen production, mainly fossil-based and renewable fuels (Dawood et al. 2020 ; Saithong et al. 2019 ; Osman et al. 2020 a). The former is the more mature and most common used industrially as it is a cost-effective method that deploys cracking or reforming fossil-based fuels. In 2016, hydrogen production globally was about 85 million tonnes used in petroleum, metal industry, fertiliser, food processing, semiconductor production, power plants and generations (Chen and Hsu 2019 ; El-Emam and Özcan 2019 ; Acar and Dincer 2019 ).

There are many ways to extract hydrogen from hydrogen-containing materials, either hydrocarbon or non-hydrocarbon, such as photonic, electric, chemical, bioenergy, heat and a combination of those methods together (Abe et al. 2019 ; El-Emam and Özcan, 2019 ; Osman et al. 2020 b). Table 1 shows different hydrogen production routes with different energy sources, technology readiness level (TRL) and their % energy efficiency.

Advances and challenges in water electrolysis

Water is typically purified and then sent to an electrolyser, which produces hydrogen and oxygen. The hydrogen is then dried, purified and compressed from a 10.3 to 413.7 bar pressure, and then stored in a tank. Although the electrolysis pathway offers a 100% renewable route for hydrogen production, it represents less than 5% of worldwide hydrogen production (Han et al. 2021 ). Despite this low percentage contribution, water electrolysis is gaining momentum for various reasons such as zero-carbon emissions, the absence of unwanted by-products such as sulphates, carbon oxides and nitrogen oxides, and high hydrogen purity. The cost of producing hydrogen through electrolysis would be reduced by approximately 70% over the next decade, allowing for the widespread adoption of a green hydrogen production approach.

By 2040, the worldwide market for hydrogen electrolysers is expected to have grown by 1000-fold. Aurora Energy Research predicted that about 213.5 gigawatts of projects will be completed over the next 19 years; this compares to an estimated 200 megawatt that is currently in service. They reported that 85 per cent of anticipated projects are in Europe, with Germany accounting for 23 per cent of expected global electrolyser capacity. The European Union has already set a goal of 40 gigawatts of electrolyser capability by 2030 (Research, 2021). If all this power is available, it will supply up to 32 million tons of hydrogen per year, which is already half of the currently demanded hydrogen. In a 1.5-degree climate change mitigation scenario, meeting 24% of energy demand with hydrogen will necessitate massive amounts of additional renewable electricity generation. To power electrolysers in this scenario, approximately 31,320 terawatt-hours of electricity would be required, i.e. more than is currently produced globally from all sources combined (BNEF 2020 ). Besides, an investment of more than $11 trillion in manufacturing, storage and transportation infrastructure would be required.

Proton exchange membrane (PEM) along with alkaline anion exchange membrane (AEM) and concentrated potassium hydroxide solution KOH are the most common techniques used in low-temperature water electrolysis. The key benefit of alkaline anion exchange membrane electrolysis over other methods is lower cost since no platinum group metals are used as catalysts herein. The main challenge, however, is the low rate of hydrogen production and the instability of the alkaline method owing to its susceptibility to pressure drop (Dvoynikov et al. 2021 ; Yu et al. 2019 ). A typical electrolysis system consists of two metal electrodes, an anode and a cathode, separated by a membrane and immersed in an electrolyte solution (Zhu et al. 2019 ). As an electric current flows through the solution, oxygen and hydrogen bubbles rise above the anode and cathode, respectively. Both electrodes are typically coated with a catalyst to reduce the amount of energy needed to liberate hydrogen from water.

However, large amounts of freshwater would be needed to generate hydrogen, and these supplies are already depleted worldwide; thus, the utilisation of seawater will be an option to overcome this issue. However, seawater utilisation in hydrogen production is associated with challenges such as the corrosion of chloride ions in seawater to the anode metal. Hung et al. reported a solution to this issue by designing the anode material as a porous nickel foam pan collector coated with an active and inexpensive nickel and iron catalyst, which showed strong conductivity and corrosion resistance. It is worth noting that, while using freshwater is more expensive than using seawater, the cost of water usually accounts for less than 2% of the total cost of hydrogen production via electrolysis (Milani et al. 2020 ). The affordability and accessibility of freshwater is one side of the coin, while inexpensive and sustainable green energy alternatives are the other, and the proximity of these two supplies, i.e. renewable energy and freshwater, does not always coincide. The main areas that need further investigation in water electrolysis are reducing the capital cost of electrolysis technology, finding water resources and increasing efficiency.

According to the recent literature summarised in Table 1 , membrane reactor technology is increasingly being recognised as an encouraging route to expand clean hydrogen production paths from hydrocarbons and hydrogen purification. At least 99.8% can be achieved without any gas purification using a proton exchange membrane analyser (Jorschick et al. 2021 ).

Recently, it was reported for Australia that the levelised cost of hydrogen (LCOH) for steam methane reforming could reach a cost of $(1.88–2.30)/kg H 2 and $(2.02–2.47)/kg H 2 for coal gasification production routes. In comparison, the LCOH via electrolysis technologies costs between $4.78 and $5.84/kg H 2 for alkaline electrolysis and $6.08–7.43/kgH 2 for proton exchange membrane technologies (Milani et al. 2020 ).

When using partial methane oxidation for hydrogen production via synthesis gas, the average cost is 1.33 euros/kg H 2 , while the cost of large-scale H 2 processing ranges between 1 and 1.5 euro/kg H 2 (Dvoynikov et al. 2021 ). It is important to note that the economic viability of using natural gas or related petroleum gas for hydrogen production should be seen in the light of transportation systems or the direct use of hydrogen on-site of the gas or oil plant.

In terms of blue hydrogen, carbon capture and utilisation lower greenhouse gas emissions but raise the overall production cost. Chemical looping reforming, for instance, has a comparatively short life cycle, global warming potential and low fossil fuel intake. Nevertheless, adding carbon capture and liquefaction process units raises the expense of the steam methane reforming by 18% and autothermal reforming processes by 2% (Atilhan et al. 2021 ). The process of liquefying hydrogen absorbs approximately 30% of the energy content of hydrogen. Additionally, keeping liquified hydrogen under one atmospheric pressure and at a low temperature of −253 °C is difficult. Furthermore, evaporation and leakage can occur even with robust insulation, losing typically 1 per cent of the stored volume per day (Atilhan et al. 2021 ).

Biomass gasification

Biomass gasification is seen as one of the most feasible, sustainable and potentially carbon-neutral alternatives to generate hydrogen (Saidi et al. 2020 ). Since biomass is a renewable feedstock that absorbs atmospheric carbon dioxide during growth, it has a much lower net CO 2 footprint than fossil-based fuels. However, the economic feasibility of hydrogen output from biomass must be closely related to the availability and affordability of raw materials in the local area. The biomass physicochemical properties, distribution and hydrogen rate are the main attributes of the supply materials. Since biomass feedstocks vary widely in structural composition and shape, all of these characteristics must be taken into account when combining the feedstock with the appropriate conversion technology (Srivastava et al. 2020 ).

Consequently, moisture, energy and ash contents are the core criteria for evaluating biomass utilisation in this route. The hydrogen yield from biomass is comparatively poor since the hydrogen content of biomass is roughly 5.9 wt% compared to 25 wt% for methane (natural gas), and the energy content is also low due to high oxygen content within the biomass of 40%. Thus, techno-economic studies backed by adequate life cycle assessment evaluation are crucial in this matter. Since biomass has a lower density, transportation and storage costs for either biomass feedstock or the produced hydrogen should be well justified in terms of economies of scale. In certain ways, these characteristics would make it impossible for biomass-based hydrogen production to compete with common natural gas such as steam methane reforming method unless new regulatory frameworks such as carbon tax favour competitively sustainable hydrogen production routes.

Biomass gasification, like coal, is the most practical process for biomass feedstocks because it produces the best yield at high temperatures, generally, 500–1400 °C, where the overall reaction is presented in Eq.  1 . Interestingly, the integration between biomass gasification and carbon capture and storage can potentially lead to an overall negative carbon footprint.

Advances and challenges in fossil-based hydrogen production route

The breakdown of the long-chain hydrocarbon via gasification, reforming or pyrolysis reaction routes is required for hydrogen production from fossil-based feedstocks. The primary product in the reforming reaction is the synthesis gas (a mixture of H 2 and CO), followed by H 2 separation via autothermal reforming, steam methane reforming, partial oxidation or membrane reforming. Another well-known method that is commonly used in hydrogen production is the gasification of fossil fuels, such as coal gasification (Milani et al. 2020 ).

Al-Qahtani et al. evaluated and compared the most common hydrogen generation routes on a monetary basis, such as steam methane reforming, coal or biomass gasification, methane pyrolysis with or without carbon capture and storage technology. Besides, the hydrogen production from the water via electrolysis derived from solar or nuclear energy were also assessed. They reported that, at the moment, steam methane reforming with carbon capture and storage appeared to be the most viable alternative (Al-Qahtani et al. 2021 ).

Steam methane reforming and methane pyrolysis

The primary feedstock for steam methane reforming is natural gas, predominantly methane mixed with other hydrocarbons and carbon dioxide (Osman 2020 ) Natural gas and steam reaction occur in a two-step reaction, as shown in Eq.  2 at high temperatures, followed by an interaction between the carbon monoxide and the produced hydrogen along with the unreacted natural gas. Following that, more steam is supplied to react with carbon monoxide in a water–gas shift reaction (WGSR), as shown in Eq.  3 , to recover further hydrogen and convert carbon monoxide into carbon dioxide. The entire process efficiency is around 76% (Al-Qahtani et al. 2021 ). The entire process releases a significant amount of carbon dioxide emissions, which may be decreased by installing carbon capture and storage technology, removing and separating the flue gases from the product stream. Following that, an amine solvent such as monoethanolamine absorbs about 90% of the carbon dioxide emission, and then, the processed flue gas stream is released into the environment. Afterwards, carbon dioxide is thermally desorbed and compressed to 110 bars for storage. The integration between steam methane reforming and carbon capture and storage (SMR + CCS) technologies has an energy efficiency of 68 per cent, owing mostly to the energy necessary to regenerate the monoethanolamine and the power required for compression. After the WGSR, hydrogen is further purified to 99.99 per cent in both situations, with or without carbon capture and storage, in a pressure swing adsorption unit, which is also utilised in the gasification technology such as coal or biomass gasification routes.

Regarding methane pyrolysis at high temperatures, thermally or catalytically, the processes degrade hydrocarbons into hydrogen and solid carbon, as shown in Eq.  4 . Because there is no oxygen in the process, no carbon oxides are generated, possibly removing the requirement for subsequent processing stages such as the WGSR and lowering the capital and operating expenditures compared to steam methane reforming (Al-Qahtani et al. 2021 ). The greater H 2 content in the product gas stream has the potential to reduce downstream clean-up operations significantly. The cost of methane pyrolysis is heavily influenced by the natural gas prices, processing method and solid carbon by-product.

Coal gasification

During the coal gasification process at high temperatures ranging from 800 to 1300 °C and 30–70 bar pressures, coal is partially oxidised in oxygen or air atmosphere into synthesis gas, as shown in Eq.  5 . The synthesis gas is typically composed of carbon monoxide and dioxide, hydrogen and unreacted methane, where the WGSR process (Eq.  3 ) enriches the syngas further to recover additional hydrogen. Thus, combining Eqs.  3 and 5 will lead to the overall reaction as in Eq.  6 . Coal gasification is less efficient than steam methane reforming with 55%, although it has a larger single-train capacity.

Bibliometric analysis

Key research studies were identified to summarise state of the art and discover knowledge gaps in the hydrogen production and LCA research arenas. The advanced search tool for publications from the Web of Science was used for this study, using the terms ‘Hydrogen production’ AND ‘ Life cycle assessment ’ as inputs. The results were manually scanned, and 24 most complete and relevant studies published from 2019 to 2021 were selected for review in the present study.

• Life cycle assessment

Life cycle assessment (LCA) is recognised as a comprehensive tool to evaluate environmental impacts associated with products and processes. There are many hydrogen production methods, such as steam methane reforming, electrochemical routes through water electrolysis using renewable power sources, thermochemical pathways involving renewable feedstock as the hydrogen carrier and biological processes (Valente et al. 2021 ; Owgi et al. 2021 ). However, environmental sustainability based on LCA remains one of the key requirements for selecting these processes for hydrogen production (Falcone et al. 2021 ). This is because policymakers need to adopt transformative solutions based on robust data and evidence-based research to identify processes that go beyond a one-fits-all approach.

To this end, we reviewed 24 LCA studies published from 2019 to 2021 on hydrogen production and life cycle assessment (Table 2 ). The four main stages defined by ISO 14040 and IS0 14,044 for conducting LCA are: (1) goal and scope definition, (2) life cycle inventory analysis, (3) environmental impacts assessment and (4) life cycle interpretation (Al-Muhtaseb et al. 2021 ).

Goal and scope of the life cycle assessment

The first stage of LCA consists of defining a goal and the scope of the study. This stage determines whether a study would be attributional or consequential, what functional unit will be considered to evaluate environmental impacts and the extent of the system boundary. This is an important initial step as the questions to be answered determine the results and associated policy implications.

Types of life cycle assessment: attributional and consequential

Life cycle assessment studies can be broadly classified into two categories: (1) Attributional LCA incorporates immediate physical flows such as raw materials, energy and emissions involved across the life cycle of a product (Jeswani et al. 2020 ), and (2) consequential LCA accounts for how physical flows can change as a consequence of an increase or decrease in demand for the product system under study (Earles and Halog 2011 ). It includes unit processes inside and outside the product's immediate system boundaries; therefore, consequential LCA studies are more suited for policy decisions. However, as LCA for hydrogen production remains at an embryonic stage, attributional studies are more commonly found. Nevertheless, both attributional and consequential approaches were considered for the purpose of this study.

Functional unit

In LCA, the functional unit is a measure of the purpose of the studied system, and it provides a reference by which the inputs and outputs can be related. This enables the comparison of two essentially different systems. The definition of the functional unit is intricately linked to the goal of an LCA study. It was observed that ~ 42% of the reviewed studies used ‘kg of hydrogen produced’ as the functional unit (Fig.  3 ). While some studies provided results considering hydrogen as an energy carrier and therefore recorded functional unit as ‘energy produced in MJ or kWh’. Very few studies reported ‘distance travelled in km’ as a functional unit when hydrogen was utilised as fuel for vehicles. The choice of different functional units for the same product, i.e. hydrogen, shows the challenges associated with comparing LCA models.

Types of functional units used in the life cycle assessment studies reviewed in the present work (N = 24)

System boundary

In LCA, the system boundary definition profoundly impacts the materials, processes and emissions considered for evaluation. As such, system boundary limits can also considerably influence the calculation of environmental impacts (Collotta et al. 2019 ). The two commonly studied kinds of system boundary for hydrogen production are ‘cradle-to-gate’ or ‘well-to-pump’ that includes processes only until production and ‘cradle-to-grave’ or ‘well-to-wheel’, which incorporates emissions during end use as well.

The generalised system boundary used for conducting the LCA of hydrogen production and consumption includes: (1) raw materials and primary energy sources such as natural gas, coal, biomass, nuclear energy and water; (2) the hydrogen production processes, for instance, water electrolysis and thermochemical processes. Some processes may also consider hydrogen purification as a subsystem to the production; (3) storage of hydrogen in underground caves or compressed tanks; (4) transportation of hydrogen in liquified or compressed gaseous form using trucks and tube trailers or pipelines; (5) emissions during end use such as by hydrogen trains or generation of power using hydrogen; and (6) finally, waste treatment processes from these systems such as emissions to land, air and water (Fig.  4 ).

Generalised system boundary used for conducting life cycle assessment of hydrogen production and consumption. This includes various raw materials such as solar, wind, biomass, coal, water and natural gas

During the review, we observed that studies employed an array of processes and limits in system boundary for conducting LCA of hydrogen production and consumption (Table 3 ). There were only a handful of studies that considered emissions during the use phase. However, given the increasing interest in using hydrogen as a clean energy carrier, it is important to consider the emissions during the use phase and conduct LCAs that present ‘well-to-wheel’ estimates.

Allocation approaches

The allocation approach refers to both ‘partitioning’ and system expansion/substitution method. The allocation approach has been identified to significantly control the values obtained for environmental impacts (Finnveden et al. 2009 ). Allocation approaches are required because the life cycle of a product can consist of many multifunctional processes. Therefore, it is imperative to allocate the environmental impacts between the different coproducts generated by the same process in a justified manner.

Life cycle inventory analysis

Life cycle inventory analysis includes data collation for all the inputs and outputs for processes within the system boundary. In general, the more the processes included in the system boundary, the more complex, challenging and cumbersome is the inventory analysis. This also explains the fact that many studies did not include all the processes ranging from raw material acquisition to end-of-life management (summarised in Table 3 ). The two different kinds of data to be collected for an LCA study are: (1) foreground data for foreground systems which includes primary data that can be easily modified or improved and (2) background data for background systems typically comes from Life Cycle Inventory databases (Silva et al. 2020 ). Background systems support the foreground systems. Table 2 details the databases/data sources incorporated in LCA studies on hydrogen production such as Ecoinvent, expert communications, Greenhouse gases, Regulated Emissions and Energy use in Transportation.

Environmental impacts assessment

Midpoint and endpoint indicators.

Global warming potential due to emissions of greenhouse gases and depletion of fossil fuels was the centre of the attention in the environmental indicators for hydrogen production, with 100% of the studies accounting for either of these two categories (Table 4 ). More than half (54%) of the reviewed studies computed environmental impacts in categories that go beyond global warming potential and net energy use/performance. These environmental impacts included but were not limited to acidification, eutrophication, abiotic depletion, marine, freshwater and terrestrial ecotoxicity, and human toxicity.

Global warming potential expressed as kg CO 2 equivalent relates to greenhouse gas emissions; abiotic depletion recorded in kg Sb equivalent is linked to depletion of minerals, peat and clay; acidification reported in kg SO 2 equivalent is due to the emission of acidifying substances; eutrophication measured as kg PO 4 3− equivalent is due to release of nutrients; particulate matter formation calculated as PM 2.5/PM 10 equivalent relates to the emission of PM 2.5 (particulate matter with ≤ 2.5 µm in diameter) and/or PM10 (particulate matter with ≤ 10 µm in diameter). Photochemical oxidation (commonly called as ‘summer smog’) occurs in stagnant air, in the presence of pollutants such as NO x , non-methane VOCs and others. Ozone layer depletion evaluates the global loss of ozone gas caused by trichlorofluoromethane (CFC-11) of the same mass. Land use calculated in m 2 is categorised as the transformation of urban, agricultural and natural land. Damage to terrestrial, freshwater and marine ecosystems is measured by ecotoxicity potential. Finally, human toxicity is caused due to the potential human health impacts of carcinogenic and non-carcinogenic pollutants.

The midpoint categories are aggregated to present results as endpoint categories such as human health, damage to ecosystem quality in the form of loss of species and resources depletion (Osman et al. 2021b ). It is argued that the environmental impacts should be presented as midpoint categories to prevent oversimplification or misinterpretation of environmental impacts (Kalbar et al. 2017 ). This is because endpoint indicators entail weighting of impacts. Evidently, only one study was identified that presented environmental impacts for both midpoint and endpoint indicators (Ozturk and Dincer 2019 ).

Uncertainty and sensitivity analysis

Uncertainty arises in LCA studies due to sparse and imprecise nature of the available data and model assumptions (Cherubini et al. 2018 ). It is, therefore, imperative to consider and compute these uncertainties quantitatively to reach transparent, robust and trustworthy decisions.

There has been a vast development on the methods to imbibe these uncertainties in LCA models such as parameter variation and scenario analysis, classical statistical theory (e.g. probability distributions and tests of hypothesis); Monte Carlo simulations, bootstrapping and other sampling approaches; nonparametric statistics, Bayesian analysis, fuzzy theory; and the use of qualitative uncertainty methods (Finnveden et al. 2009 ).

This review recorded that 67% of the studies used scenario analysis to account for parameter uncertainty (Fig.  5 ). Together with comparative studies mentioned in (Table 2 ) and scenario analysis in Fig.  5 , this value reaches 96%, i.e. all but one study performed comparative and/or scenario analysis (Cvetković et al. 2021 ). This can be attributed to the dearth of the data and the serious effort required to conduct an LCA of biohydrogen production via anaerobic digestion (Cvetković et al. 2021 ). Furthermore, it was noted that 8% of the studies employed Monte Carlo simulations to propagate parameter uncertainties in the model.

Details of the scenario, sensitivity and Monte Carlo simulations (to propagate uncertainty) conducted in the reviewed studies (N = 24). Scenario analysis was conducted in 67% of the reviewed studies

Sensitivity analysis is conducted to distinguish processes in the hydrogen production chain that contribute to the burdensome environmental footprints. Relatedly, if environmental impacts are to be minimised, these will be the processes where future research should focus on (Al-Muhtaseb et al. 2021 ). 42% of the studies reviewed here conducted sensitivity analysis.

Interpretation of results

This stage of the LCA includes making interpretations, drawing conclusions and distinguishing the processes that can be improved to increase the environmental feasibility of the system. This stage could also involve presenting and communicating results to stakeholders. Table 5 summarises key findings from the reviewed studies.

Key findings and recommendations for future life cycle assessment studies

Life cycle assessment is a complex tool that sits at the interface between science, engineering and policy. Despite this inherent complexity, it is recognised as a comprehensive tool to evaluate environmental impacts associated with products and processes. We reviewed LCA studies published from 2019 to 2021. This section draws recommendations for policymakers to create a sustainable hydrogen economy and LCA practitioners to conduct future studies.

During the review, no two LCA studies were identified to be similar. Differences in the geographical and temporal span, functional units and system boundaries considered, and environmental impact categories were reported. Therefore, it is recommended that the policymakers pay heed to the modelled processes and extent of the system boundary for making decisions for creating a sustainable hydrogen economy.

Most of the studies did not encompass processes, inputs and outputs for ‘cradle-to-grave’ LCA analysis. Thus, future studies should conduct ‘cradle-to-grave’ evaluation for robust decision-making.

About 54% of the reviewed studies computed environmental impacts in categories that go beyond global warming potential and depletion of fossil fuels. It is crucial to assess environmental impacts in more categories. Otherwise, there can be the issue of burden shifting, where hydrogen production processes are developed to mitigate climate change and energy security, however, leading to severe environmental and human health impacts such as acidification, eutrophication and human toxicity.

Finally, focusing on production pathways, only eight studies were identified that computed environmental impacts for biohydrogen, showing that there is a considerable knowledge gap in production processes utilising bio-based feedstocks.

Hydrogen underground storage

There are ambitious goals of the Paris agreement for climate change to be met than ever by 2050. However, the continuous increase in carbon dioxide (CO 2 ) emission generated by the use and storage of fossil fuels has created a clear demand for alternative sources of clean and renewable energy (Ochedi et al. 2021 ). Solar and wind energy, however, provide intermittent and volatile power sources (as shown in Fig.  6 ) that are requiring backup solutions and/or energy storage at scales comparable to their power generation capacity (i.e. longer-term TWh storage solutions). In particular, some industrial sectors are hard to be decarbonised. To help balance the energy supply and demand, a capability of various energy storage technologies, with a dynamic combination of daily, weekly and seasonal storage, can reduce CO 2 emissions per unit of energy provided.

[A] Diurnal time series shows the matching of load, wind and solar of a typical day during the winter season for Europe with 15th and 85th percentiles for each average day time series. [B] Annual time series of weekly averages illustrate the seasonal correlation (i.e. excess/shortage) of load, wind and solar. Electricity generation and demand normalised over the corresponding average value. [C] Schematic round-trip efficiency for a short-term (e.g. battery, brown line) and long-term (e.g. power to hydrogen, black line) storage technology. The figures were adapted from (2017) and (Gabrielli et al., 2020 )

To date, the technical feasibility and economic attractiveness for developing large-scale, lithium-ion-based and seasonal energy storage batteries can be challenging to be implemented and provide an energy supply during high demand times. Such shortfall can be eliminated by storing the excess renewable energy chemically—in the form of hydrogen—in the subsurface aquifers, salt caverns and/or exhausted hydrocarbon reservoirs in the so-called Underground Seasonal Hydrogen Storage (USHS). The usage of hydrogen as an energy carrier can be a promising solution for clean energy because of its non-toxicity, high specific energy and non-CO 2 emission after combustion. The challenge is to find hydrogen storage materials with high capacity. USHS, therefore, can be one of the most promising solutions for offsetting seasonal mismatch between energy generation and demand (Fig.  6 ), firstly for medium- and long-term storage while increasing contribution to low-carbon energy supply. Despite the vast opportunity provided by USHS, maturity still is considered low, with several uncertainties and challenges (Heinemann et al. 2021 ).

Hydrogen-based economy requires a large gas transport infrastructure. It has been suggested that existing natural gas pipe networks could be used to transport hydrogen (Melaina et al. 2013 ; Panfilov 2016 ). The gases would be transported as a mixture and separated afterwards. Some methods for separating mixtures of methane and hydrogen, particularly gas membrane separation, appear promising (Ockwig and Nenoff 2007 ).

Geologically, underground formations are suitable for storing hydrogen, which may then be used as a carrier of chemical energy produced in times of surplus energy production, stored for several months and ultimately retrieved for re-electrification when it is needed most (Bauer et al. 2013 ; Bauer et al. 2017 ). As an illustration of the possible storage potential, a system volumetric capacity (i.e. the Net Energy Density) of hydrogen-based flow battery stores approximately 2.7 kWh/L (NREL) of electrolyte, and hence, an exhausted million-barrel oil field would hold > 3 TWh of electricity. This is equivalent to 30 weeks’ output from a large offshore wind farm which is far more than is needed to eliminate the intermittency issues associated with such a facility. Hence, it was proved that only a few offshore gas fields are required to store enough energy as hydrogen to balance the entire seasonal demand for UK domestic heating (Mouli-Castillo et al. 2021 ).

Thermophysical properties of hydrogen

After hydrogen is produced at the surface from one of the technologies, it must be transported to a seasonal storage facility in a liquid or gas phase. Moreover, hydrogen can also be stored on the surfaces of solids (i.e. by adsorption) or within solids (i.e. by absorption) (El-Eskandarany 2020 ). During the loading cycle, where the power demand is at a peak, hydrogen can be easily re-converted for electrical generation.

Hydrogen can be considered as an ideal gas that may occur in various states over a wide temperature range and even at high pressures. Here, the thermophysical properties of hydrogen at the conditions relevant to the underground hydrogen storage were provided. One of its most important thermophysical characteristics is its low density, making it necessary for any practical application to compress the hydrogen or liquefy it. At intended storage depths, the density and dynamic viscosity of hydrogen are iteratively calculated using equation of state (EOS) and following (Span et al. 2020 ). Primarily, the hydrogen density (kg/m 3 ) mainly increases with increasing pressure while dynamic viscosities (μPa.s) significantly increase with increasing temperature, as shown in Fig.  7 . At low temperatures of − 262 °C, hydrogen is solid with a density of 70.6 kg/m 3 . At higher temperatures, hydrogen is a gas with a small density of 0.089 kg/m 3 at 0 °C and at a pressure of 1 bar. The extent of hydrogen's liquid state can be presented as a narrow zone between the triple and critical points, with a density of 70.8 kg/m 3 at − 253 °C.

(left) Density [kg/m 3 ], (right) dynamic viscosity [µPa.s] of hydrogen at representative P–T conditions which are typical for Underground Hydrogen Storage system. The calculations were carried out by the authors, using the fundamental properties of Hydrogen as an ideal gas. By the time pressure of > 35 MPa is reached, a deviation of 15% from the real values is expected

Three potential technologies for hydrogen storage, therefore, can be considered according to combinations of pressure and temperature relevant to the storage conditions (Table 6 ):

Cryo-compressed hydrogen storage (CcH 2 ) and liquid hydrogen (LH 2 ) storage: storage of hydrogen as a liquid requires cryogenic temperatures because the boiling point of hydrogen at one-atmosphere pressure is − 253 °C with a density of close to 71 kg/m 3 . These properties make storing hydrogen under standard atmospheric pressure and temperature extremely difficult due to the high cost and safety issues. Whereas other gases can be liquefied around the standard temperature of 20 ºC, this is unfortunately practically impossible for hydrogen. Therefore, hydrogen needs compression into cryogenic vessels that can be pressurised to 25–35 MPa. Accordingly, the size of liquid hydrogen requires larger tanks reaching about three times larger than the currently used gasoline tank (El-Eskandarany, 2020 ).

For pressure ranges between 5 and 30 MPa and temperature between 25 and 130 °C, hydrogen can safely be stored as a gas in underground geological formations. For USHS, hydrogen must be transported to a wellhead for underground storage. The hydrogen must then be compressed to be injected at sufficient pressure to enter the geological formation at the in situ pressure and temperature. Different potential geological storage sites for USHS are shown in Fig.  10 and will be discussed in more detail in the following sections.

Additionally, pressurised hydrogen gas takes a great deal of volume compared with, for example, gasoline with equal energy content—about 30 times bigger volume at 10 MPa gas pressure (El-Eskandarany 2020 ). USHS basically implies the reduction of the enormous volume of hydrogen gas due to the reservoir pressure gradient (Fig.  8 ). One kilogram of hydrogen in ambient temperature and at atmospheric pressure occupies a volume of 11 m 3 .

Normalised volume of hydrogen at the pressure–temperature (over the range of geothermal gradients) conditions plotted as a function of depth. Grey horizontal line at 800 m marks the minimum depth recommended for hydrogen injection, where it can be found as a supercritical phase at pressure and temperature conditions relevant for USHS (above 1.3 MPa)

Fluid dynamics of hydrogen in a brine-saturated porous medium

In the context of the USHS system, the cyclic injection of hydrogen into (and possible retrieval from) a brine-filled permeable formation is part of multi-phase flow problems that have been studied extensively (Hashemi et al. 2021 ; Liebscher et al. 2016 ). In this case, a two-phase hydrogen–brine system is immiscible—the fluids are separated by a capillary interface. Likewise, the CO 2 geological storage, an important first approximation to the behaviour of the hydrogen–brine system, is found via applying a group of dimensionless ratios and solubility (and hence its mobility) that analyse the dynamics of two-phase immiscible flow systems (Ringrose et al. 2021 ). Viscous/capillary ( N vc ) and gravity/viscous ( N gv ) ratios are, respectively, the characteristic time ratios for fluid to flow in the transverse direction due to capillary and gravity forces to that in the horizontal direction due to viscous forces using the assumption of (Zhou et al. 1997 ). The two fluids here are assumed to be vertically segregate due to the gravity and density difference. Both ratios can be formulated in Eqs.  7 and 8 as follows:

where u x is the total flow velocity in the horizontal (x) direction, ∆x and ∆z are the system dimensions, μ nw is the viscosity of the non-wetting phase (hydrogen), k av is the average permeability, ∆ρ is fluid density difference, g is the acceleration due to gravity and (dP c /dS w ) is the capillary pressure gradient as a function of wetting-phase saturation.

Around the injection/production wellbore, viscous-dominated conditions are expected to occur due to the high-pressure gradient (Ringrose et al. 2021 ). However, within the reservoir and away from the injection/production wellbore region, gravity-dominated conditions are expected to occur. Such ratios, therefore, can be used to expect the fluid dynamic behaviour of the hydrogen-brine flow system and determine which factors are likely to be most critical, particularly when assessing large-scale macroscopic fluid flow, where the capillary and gravity forces become important enough to be not neglected.

Another important factor for USHS is the solubility of hydrogen in the resident formation fluid (water/brine). Therefore, forecasting the phase equilibria (solubility of hydrogen in brine and water content in the hydrogen-rich phase) under the geological storage conditions (i.e. at different temperatures, pressure and molality) is necessary for the study of hydrogen mobility and reactivity, as well as the control, monitoring and optimisation of the storage. Based on new experimental datasets, Chabab et al. developed predictive models to estimate the water content in the hydrogen-rich phase and precisely capture the salting-out effect on hydrogen solubility (Fig.  9 ) (Chabab et al. 2020 ).

Solubility of hydrogen in pure water as well as the brine of different molalities (up to 5 M), as a function of pressure (up to 25 MPa), and at the temperature of 50ºC [a] and 100ºC [b]. The symbols represent experimental results from the literature (Chabab et al., 2020 ). The solid, dotted and dashed lines represent the hydrogen solubilities calculated by the e-PR-CPA, SW and geochemical models, respectively. The figure is modified from Chabab et al., ( 2020 )

Large-scale hydrogen geological storage

A promising solution to help balances the energy supply from renewable intermittent sources and demand is hydrogen as an energy carrier for clean energy and must be accompanied by energy storage systems. The benefits of using hydrogen are because of its non-toxicity, high specific energy and non-CO 2 emission after combustion. However, the challenge is to find hydrogen storage materials with high capacity. Large-scale underground storage of natural gas has been practised successfully for many decades, with a global total of 413 billion standard cubic metres (BSCM) of natural gas storage accommodated in depleted gas fields (80%), underground aquifers (12%), and engineered salt caverns (8%) (Perry 2005 ), as shown in Fig.  10 . Here, these types of underground hydrogen storage systems have been considered (Lord et al. 2014 ; Panfilov 2010 ).

Schematic diagram of different processes which are associated with hydrogen production using electrolysis, seasonal storage in geological formations and/or salt caverns, utilisation for ammonia production and re-electrification of hydrogen using fuel cells. The figure shows different potential storage mediums for the hydrogen in the underground geological formations: reservoir/aquifer and salt caverns. The dimensions are not to scale

Depleted hydrocarbon reservoirs

More often than not, depleted hydrocarbon reservoirs are appealing targets for USHS because of their storage capacity, proven seal, previous knowledge of reservoirs characterisation and existing infrastructure (i.e. natural gas pipeline network). Nevertheless, various physical, chemical and microbial processes are associated with USHS in hydrocarbon reservoirs (Heinemann et al. 2021 ) (summarised in Fig.  10 ).

While one can transfer know-how and technology from underground natural gas storage and underground carbon storage, some of the challenges USHS faces are peculiar. In both compressed gas and liquid forms, the low density of hydrogen makes the seasonal storage of hydrogen in porous media (and possible retrieval) problematic. With a mass–density ratio of less than 0.01 compared to water for most relevant subsurface storage conditions, H 2 is very light. Consequently, an H 2 plume would experience strong buoyancy forces (i.e. the stronger the buoyancy forces, the higher the potential for hydrogen leakage), and water upconing towards the extraction borehole may occur (Heinemann et al. 2021 ; Sainz-Garcia et al. 2017 ).

This limitation is felt most strongly during the hydrogen retrieval from the subsurface. The gas saturation around the production well required to keep a gas well flowing is of major concern since it will impact and reduce the production and ultimately will kill the well. The thinner the hydrogen plume will be, the lower gas saturation and the higher accumulation of resident formation brine in the downhole. Therefore, the dynamics of the USHS system require a wellbore model capable of describing/predicting the conditions (pressure and temperature) in the extraction borehole as the fluid(s) flow up (or the liquid accumulation at the bottom of) the borehole.

Water upconing is the change in the hydrogen–water contact profile due to drawdown pressures. This phenomenon can be seen as the name implies: a cone of water formed below the perforations. One way to avoid upconing during H 2 production is the use of a cushion gas (Kim et al. 2015 ; Oldenburg 2003 ), usually a cheaper and denser gas like nitrogen (N 2 ), which helps prevent water flooding of the gas plume when H 2 is being produced. This concept is well known in underground natural gas storage and has previously been proposed for USHS (Cao et al. 2020 ).

Additionally, it is important to note that USHS involves cyclic hydrogen injection (i.e. during power surplus) into and withdrawal (i.e. during energy demand) from the geological formations, where changes in the reservoir pressure may induce fatigue in the caprock and lowering the fracturing pressure at which hydrogen commences to leak through a seal rock. Therefore, assessing the sealing capacity to hydrogen (or hydrogen column height) will be crucial to keeping the risk of the potential upward leakage of hydrogen through the sealing caprock at a minimum. Seal rocks have fine pore and pore throat sizes that, in turn, generate hydraulically tight low-permeability caprocks with high capillary threshold pressures. High threshold pressures, together with wettability and interfacial tension (IFT) properties, determine the final column height that a seal can hold, thereby affecting the ultimate reservoir storage volumes. Compared to the underground natural gas storage, higher capillary entry pressures are expected to occur for hydrogen due to its higher interfacial tension (Hassanpouryouzband et al. 2021 ; Naylor et al. 2011 ). Therefore, hydrogen can be stored at a higher pressure in the reservoir than methane, with a reduced risk of geomechanical failure.

On the hydrogen injection into a storage reservoir, a very small fraction of hydrogen will dissolve into the formation fluids (Chabab et al. 2020 ), and water vapour may contaminate the hydrogen phase due to chemical disequilibrium. Hydrogen losses through diffusion need to be considered, as the diffusion ability of hydrogen is several times more than that of CO 2 and methane, to such an extent that hydrogen can travel between the structures of ice-like crystals (Hassanpouryouzband et al. 2020 ).

In order to show the influence of the large density difference (Fig.  11 ) between the injected gas (hydrogen) and the resident formation fluid (brine) on the hydrogen plume migration during the seasonal storage period, we numerically simulate the injection of 10-ton kg of hydrogen over 10 days and its storage for 35 days. We used the numerical simulator PorousFlow Module, open-source software for solving parallel tightly coupled nonlinear THM processes in porous media (Wilkins et al. 2021 ; Wilkins et al. 2020 ). It is based on the MOOSE framework (Gaston et al. 2009 ) and its internal architecture relies on state-of-the-art libraries for finite element analysis (Kirk et al. 2006 ) and nonlinear iterative algebraic solvers (Balay et al. 2019 ). The simulation results are shown in Fig.  11 . It is shown from the simulation standpoint that the leakage rate of hydrogen is going to be the biggest challenge due to the very high mobility of hydrogen, the small molecule size, the high dispersion rate and the large density difference between the hydrogen and brine. Therefore, a proper tightness assessment of the caprock above the reservoir is required to prove its effectiveness for any possible hydrogen leakage. In addition, we propose expressly storing H 2 /CH 4 gas mixtures to improve the density contrast with the water. The mixed gas can, upon demand, then be extracted and transported in the same natural gas pipelines.

Hydrogen–brine displacement in an idealised 2D horizontal cross section (i.e. geological storage formation). The injection wellbore is located at the left-hand side of the simulated domain. The subfigures are showing only the first 50 m horizontal distance from the injection well with 10 × horizontal exaggeration. The horizontal exaggeration is 10x. [A] the reservoir is fully saturated with brine (i.e. before the hydrogen injection start). The migration of the hydrogen phase after [B] 9 days, [C] 23 days, [D] 36 days and [E] 45 days

Subsurface microorganisms, including methanogens, sulphate reducers, homoacetogenic bacteria and iron(iii) reducers can make use of H 2 as an electron donor, which may lead to an unwanted accumulation of biomass in the vicinity of the injection borehole and/or loss of H 2 (Ganzer et al. 2013 ; Hagemann et al. 2015 a). The local rate of the biochemical reactions depends on the number of the particular microorganism (Hagemann et al. 2015 b). Hence, an important problem for the modelling of USHS is the description of microbial growth and decay functions. Microbial conversion of hydrogen can only occur if the hydrogen is in the aqueous phase. A mixture of hydrogen with another gas means it will have a lower partial pressure and hence lower solubility in water. It was stated that if the temperature of the formation is higher than 122ºC or the salinity is higher than 5 M NaCl, the hydrogenotrophic microbial activity becomes highly unlikely (Thaysen and Katriona 2020 ). Hence, if a storage reservoir is hot enough, one can combine hydrogen storage with CO 2 , since methanogenic microbial activity will be limited by the temperature constraint. Further, a high-pressure environment is toxic for some microorganisms.

Considering the deep depleted gas-condensate reservoirs, the risks are minimised here due to the presence of well-defined geological traps related to previously formed gas reservoirs. Unfortunately, the risk of migration from the target storage formation cannot be eliminated completely, particularly due to the re-pressurisation and change of the stresses and the long-term well integrity issues of the casing and cement.

Salt caverns

Another underground storage medium, which could be used under certain conditions and locations, is the usage of salts caverns as high-pressure gas storage facilities (Fig.  10 ) (Gabrielli et al. 2020 ; Hassanpouryouzband et al. 2021 ; Pudlo et al. 2013 ; Foh et al. 1979 ). Based on energy storage capacity (GWh) and discharge timescale, storing hydrogen in salt caverns can afford utility-scale, long-duration energy storage to meet the market need to shift excess off-peak energy to meet dispatchable on-peak demand. Salt caverns can hold substantial promise due to the self-sealing nature of the salt and the ability to customise the size and often shape of the caverns (Lord et al. 2014 ). However, the inaccessibility of the salt caverns in the area where hydrogen production is can be a limiting factor.

Salt caverns can be artificially constructed in the salt formation (or salt dome) by injecting water through an access wellbore, dissolving the salt and generating large volumes of brine in the so-called solution mining process. This process is associated with retrieving a large quantity of brine which requires disposal in an eco-environmental way. Finding suitable disposal repositories for brine disposal can be economically problematic due to higher costs for constructing longer pipelines which eventually may slow down or even hinder the permitting process. During the hydrogen withdrawing from the caverns under constant pressure, part of this saturated brine can be injected into the caverns to maintain the caverns' pressure and stability. Cushion gas, therefore, is not needed under these operating conditions (Foh et al. 1979 ; Taylor et al. 1986 ).

Compared to depleted oil and gas reservoirs, the key advantages for storing hydrogen in salt caverns are: (1) salt surrounding the caverns is highly impermeable and virtually leakproof where the only possibility for gas loss is escaped through leaky wells (Lord et al. 2014 ). (2) Salt does not react with hydrogen (Bünger et al. 2016 ). (3) Withdrawal of ‘discharge’ of hydrogen is highly flexible in rate, duration and volume with lower cushion gas requirements to avoid rock breakage. (4) Caverns are a mature, financeable storage technology that has been successfully used to store compressed gases for over 75 years with possible extensions for USHS.

The city of Kiel’s public utility, as an illustration, has been storing town gas with a hydrogen content of 60–65% in a gas cavern with a geometric volume of about 32,000 m 3 and a pressure of 8–16 MPa at a depth of 1330 m since 1971 (Kruck et al. 2013 ; Carpetis, 1988 ) estimated the hydrogen storage capacity for cavern volume of 500,000 m 3 and a casing shoe depth of 1000 m a pressure range of 180 to 60 bar is suitable of 4.0 Mio kg hydrogen (47 Mio m 3 (st)) and a cushion gas of 2.2 Mio kg (26 Mio m 3 (st)). For an economic prospect, the total installed costs, including wellbore drilling, compressors and gas treatment, were estimated to be about € 100 million (Michalski et al., 2017 ). Compared to energy storage in Li-ion batteries with a cost of 100 €/kWh, USHS in salt caverns offers a significant cost reduction potential in the total investment cost by a factor of 100.

Storage of hydrogen in the form of methane (natural gas) may be a preferable alternative for overcoming the storage problems associated with storing pure hydrogen in geological formations. When there is a surplus of renewable energy in the summer, hydrogen can be produced through water electrolysis. Furthermore, when this hydrogen and carbon dioxide combine in the methanation reaction, methane is produced, which can then be stored in a geological reservoir for winter use. This could be accomplished through a methane reforming reaction followed by using a fuel cell to generate electricity that can be fed into the power grid.

In short, hydrogen storage in a geological medium can offer a viable option for utility-scale, long-duration energy storage, allowing the hydrogen economy to grow to the size necessary to achieve net-zero emissions by 2050. While the operational experience of storing town gas in salt caverns provides considerable proof of its viability and operational best practice, full-scale deployment of USHS has yet to be evaluated for any associated risks and public acceptance of viewpoints, similar to the potential for induced seismicity.

• Hydrogen utilisation

Fuel and power systems

Globally, the heat generated from domestic as well as industrial activities contributes by 33 and 50% of the carbon dioxide emissions and universal energy consumption rate, respectively (Dodds et al. 2015 ). The majority of gaseous emitted by the conventional burning process of natural gas are implicated in numerous environmental contamination issues (i.e. greenhouse gaseous emissions). The primary source of carbon dioxide emissions was energy consumption, with a global emissions rate of 33.1 gigatonnes in 2018, mainly resulting from the burning of fossil fuels. Contrarily, applying hydrogen gas as an alternative fuel to natural gas has proved to be an efficient pathway to reduce greenhouse gaseous emissions. Once it is generated from renewable energy sources, as shown in Fig.  1 , it can directly participate in the decarbonisation process in the energy sector thanks to its reacting nature, whether combusted or utilised in the fuel cell. The hydrogen is currently produced by conventional (non-renewable sources) of 18%, 30% and 48% from coal, heavy oil/naphtha and natural gas, respectively, which was negatively responsible for releasing about million 560 tonnes of carbon dioxide per year (Lui et al. 2020 ).

Moreover, given the costly natural gas employed throughout the power-producing framework (i.e. requires a huge area to store), hydrogen appears to be a viable option as a fuel feeding to gas turbines (Bicer and Khalid 2020 ). The utilisation of hydrogen in the central heating system instead of natural gas offers numerous merits: comparable operational activity and an increased heat generation rate with minimal harmful emissions (Dodds et al. 2015 ). Several factors, such as the Wobbe index, should be considered before forwarding hydrogen to various appliances. Generally, Wobbe index values differ considering the chemical composition of the gas. The Wobbe index number of pure hydrogen is about 48 MJ/m 3 ; it falls within the permissible natural gas integrity extent for the vast majority of burners (Zachariah-Wolff et al. 2007 ). Supplying the operating system with a fuel beyond the Wobbe index band can negatively result in some operational problems (i.e. incomplete combustion and burner overheating). Clearly, attributing to the hydrogen's higher combustion velocity compared with the natural gas fuel, advanced burners with specialised technical specifications must be operated with hydrogen as a fuel feed stream.

Furthermore, the overabundant electricity generated from power facilities can be transformed into hydrogen, which can be either directed to the existing natural system (direct consumption) or chemically converted into chemicals used in different industrial aspects (Collet et al. 2017 ). Besides, hydrogen can be used individually in the aerospace industry or in combination with oxygen as propellants. The mentioned liquid mixture (oxygen and liquid) generates a large amount of energy and makes it more suitable for space applications. Because of releasing water during hydrogen combustion, in addition to its high efficacy compared with gasoline, these characters qualify it to be employed as an automotive fuel (Gurz et al. 2017 ).

Hydrogen employment in power systems

Hydrogen is enormously used to store and transport energy in a variety of power applications, typically illustrated in Fig.  1 and discussed as follows (Parra et al. 2019 ):

Storing of energy and auxiliary services

Given the hydrogen's high storing efficacy, hydrogen-based energy storage has gained traction for storing energy over a medium/long term and in auxiliary services in the last decades. It can meet energy storage requirements over a broad timescales to avoid any defect (shortage) that may occur between the product and the demand (required) of energy (Al Shaqsi et al. 2020 ). Recently, renewable energy production has grown rapidly; however, certain renewable energy supplies are sporadic and seasonally dependent. As a result, the produced renewable energy should be stored in a dependable form that is resistant to the fluctuation in those energy sources (Mehrjerdi et al. 2019 ). In particular, the most popular types of energy storage are: (1) power-to-power, (2) power-to-heat and (3) power-to-gas (Widera 2020 ). Hydrogen, in comparison, has a large energy storing capacity, a great storing time and flexibility. It has the ability to reduce energy volatility and absorb the surplus of energy production. Practically, it can deal with the economic and seasonal variations issues. Hydrogen can exceptionally balance between the resultant and required energies by storing the surplus energy when the production rate exceeds the required one as well as in times at which the electricity's price is minimal and reuse it in the opposite cases. Contrarily, hydrogen can be forwarded to generate electricity in the high energy demand.

Moreover, the storing capacity of hydrogen is higher than batteries, as it may range to weeks or months, unlike batteries that may extend (limited) for hours (Bocklisch 2016 ). Otherwise, hydrogen can be subjected to transform renewable resources to produce energy during different climatic conditions in different seasons. The storage capacity of hydrogen is estimated to reach up to megawatt-hours (1000 Kilowatts hours), even terawatts-hours, which is considered a high value by considering that of batteries (i.e. kilowatts hours). A slew of hydrogen power storage plants has been commenced worldwide, showing the technology's potency for the large scale. Examples of power plants established to produce and store hydrogen are Underground Sun Storage, Orsted and SoCalGas in Austria, Denmark and USA, respectively (Home | SoCalGas, https://www.socalgas.com ).

In the Underground Sun Storage, the energy derived from wind and solar renewable resources is stored beneath the earth's surface. Referring to the difficult storing of the produced energy from renewable resources, the rest released power in reprocessed into hydrogen via electrolysis process and conserved for the futuristic challenges. The findings of the plant outlines revealed that it has the efficiency to equilibrate the basic energy requirements in line with the various seasonal variations. Other projects were established to face the shortage between the system supply and demand. Orsted plant was designed to operate the electrolysers by subjecting the oversupply of energy generated from wind farms to them. Another project launched by SoCalGas on campus succeeded in directly converting the produced hydrogen from the solar electric system into methane inside a bioreactor.

Besides, hydrogen is hugely accounted as an assistant tool for providing the energy sector (grid) with the necessary services such as frequency maintenance and voltage strengthening via electrolysers and fuel cells (Bird et al. 2016 ). In the HAEOLUS facility (Haeolus. https://www.haeolus.eu/ ), the oversupply of wind generation is directly fed into an electrolyser to generate hydrogen, which is subsequently forwarded into fuel cells to be used later for various purposes (utilities, data transmittance, systems controlling and others) (Larscheid et al. 2018 ). Another form of energy storage can be achieved by regulating the grid frequency near its normal value (50–60 Hz) by injecting or consuming energy in a coordinated manner to maintain the gap between the product and the required power. Numerous regulation reserves have been installed in different European grid systems. Commonly, frequent containment and restoration reserves have been used to handle the frequencies through the distributed control systems. The first mentioned controlling scenario supplies a steady feed stream in case of occurring a sudden corruption in frequency in a very short period, whereas the latter can tolerate a longer corruption beyond the 30 s. The twice services can be attained via electrolysers and fuel cells by incrementing or decreasing their power setpoints related to frequency signals (Alshehri et al. 2019 ).

Besides, hydrogen-based equipment can contribute to voltage support by adjusting their power factor to meet the local voltage support requirements, which can be accomplished using inverter or rectifier monitoring systems (Alshehri et al. 2019 ). Some troubles such as blackout can occur in power plants, which was conventionally faced using a diesel Genset. The use of fuel cells may have the advantage to realise this scope given its no emissions and noiseless nature. These studies imply the profitability of hydrogen scaling up in the power sector.

Power-to-gas

Power-to-gas is a process in which electrical energy is used to generate a combustible gas. Since hydrogen is thought to be a combustible gas with a large power density, power-to-hydrogen technologies are increasing (Eveloy and Gebreegziabher 2018 ). Because of the combustibility nature of hydrogen, it has been inserted into gas applications. The hydrogen generated from the electrolyser can be converted into methane by the methanation process, which is either pumped to the natural gas grid operating system or stored to achieve the financial budget for the energy market (Gondal 2019 ). By the literature, numerous pilot projects have been commenced worldwide with the highest establishment rate of 85% in Europe, followed by the USA and Japan (Thema et al. 2019 ). Among different European countries, Germany constructed a power-to-gas plant with a maximum production capacity of (40–100 megawatts) to be directed for industrial purposes, and it will pump in the natural gas grid operating system from 2022 (Romeo et al. 2020 ).

Furthermore, several power-to-gas infrastructures have been installed in the regions rich in solar and wind renewable resources. A realistic study is displayed in the HAEOLUS project (north of Norway). Chiefly, its core idea was based on using 2.5 Megawatts proton exchange membrane electrolyser to transform the produced wind power generated from wind farms into hydrogen, which can be consumed in various aspects. HyCAUNAIS project displays the viability of running a resilient power to gas facility in conjunction with the methanation approach by equipping a nominal 1 megawatts electrolysis area to produce hydrogen, which was methanated and inserted into natural gas grid operating system or combined with biomethane generation area from landfill biogas (HYCAUNAIS – Storengy – Europe en BFC. https://www.europe-bfc.eu/beneficiaire/hycaunais-storengy/ ).

Lately, fuel cells have gained worldwide attention as efficient and environmentally friendly energy generators. Practically, they are integrated electrochemical devices widely used to convert the delivered chemical energy into its electrical counterpart via redox reactions (Yuan et al. 2021 ). Regarding their efficacy for energy generation, they can be served as energy carriers. Fuel cells are composed of two electrodes (i.e. anode and cathode) separated by electrolytes responsible for the migration of ions between electrodes (Ogawa et al. 2018 ). There are numerous types of fuel cells such as alkaline fuel cell, direct carbon fuel cell, direct methanol fuel cell, microbial fuel cells, molten carbonate fuel cells, phosphoric acid fuel cell, proton exchange membrane fuel cells and solid acid fuel cells.

Table 7 displays different types of fuel cells with their operational conditions and efficiency%. During system operation, hydrogen is passed to the anode while oxygen is passed to the cathode. At the anode, the hydrogen molecules are split into protons and electrons by a catalyst. The positive hydrogen particles can pass through the membrane to the cathode side, but the negative cannot. However, electrons change their path by being forced to the circuit and generating electric current. At the cathode, the hydrogen protons, electrons and oxygen combine to produce a water molecule which is the end product of this reaction. Among different types of fuels (i.e. hydrocarbons and chemical hydrides), applying hydrogen in fuel cells is eco-friendly because it does not expel any pollutants (Psoma and Sattler 2002 ). It works within low temperatures ranges comparing with the internal combustion engine. As mentioned before, the end product of the hydrogen-based fuel cell is water, whereas the end products of diesel/natural gas-based fuel cells are carbon dioxide and greenhouse gases (Xu et al. 2021 ). The main differences between fuel cells and traditional batteries are presented as follow: (1) operational mode of fuel cells is mostly like the traditional batteries, but the latter requires an electrical powering to run, (2) batteries can store hydrogen, unlike fuel cells that can provide a continuous electricity supply wherever hydrogen (fuel) and oxygen (oxidising agent) are available from outside sources. In addition to the mentioned differences, the batteries electrodes are steadily consumed during their extended usage, which entirely differs (not found) in the fuel cells (Spingler et al. 2017 ; Aydın et al. 2018 ).

Co-generation and tri-generation distribution systems

Interestingly, fuel cells can be employed to optimise the efficiency of different power systems and reduce the overall production cost of these processes in several aspects, including co-generation systems (i.e. heat + power/cold + power) or tri-generation systems (i.e. cold + heat + power). Co-generation is the sequential generation of two different forms of beneficial energy from a primary single source (fuel cells). In that case, the electricity generated from fuel cells is used to meet the electrical demand, and the released heat is directed towards the heating activities. As a result, total efficiency will be about 95%. Systematically, co-generation fuel cell systems consist of different components, including fuel processors, power suppliers, heat recovery unit, energy (thermal/electrochemical) storage unit, control devices, additional apparatus (i.e. pumps) and stack. Commercially, a large number of facilities have been launched to improve the performance of co-generation systems. Different co-generation projects were erected around the world. In Japan, the plant installed by the ENE-FARM project (300,000 units/2018) simultaneously supplied the home with electricity and heat necessary for daily activities by using proton exchange membrane fuel cells ranged from 0.3 to 1 kilowatt. Initially, liquefied petroleum gas feedstock streams are fed into a reformer, where they are converted into hydrogen, which is further combined with oxygen inside the fuel cells to produce water, electricity and heat used later for various residential purposes (Yue et al. 2021 ). Recently, the manufacturing of micro-co-generation fuel cells has grown in Europe. Besides, more than 1000 micro-combined heat and power fuel cells were launched in 10 European countries between 2012 and 2017. The primary European plant for a micro-co-generation fuel cell was the ENE. Field project (ene.field. http://enefield.eu/ ). An LCA study was successfully performed for the mentioned project, and simply it revealed that co-generation fuel cell was environmentally in nature compared with other gas boilers and heat pumps strategies considering its less greenhouse gaseous emissions. PACE was another project, firstly started in 216, whereas about 2800 of combined heat and power fuel cells are fabricated. Briefly, the overall development in the electrical efficiency through the two inspected projects were 60 and 95%, respectively (Home - PACE Pathway to a competitive European fuel cell micro-cogeneration market. https://pace-energy.eu/ ).

Tri-generation strategy is an improved strategy of co-generation in which a single primary source achieves the required cooling by thermally driven equipment. The working principle of heat pumps mainly stands on producing cooling from a thermal source. Typically, this can be achieved by using condenser and evaporator types of equipment. The gas released from absorbent/adsorbent is cooled down in the condenser and converted into a liquid by releasing its heat (refrigeration process). Then, the cooled down fluid continues to an evaporator, whereas it is evaporated by losing its contained heat. Significantly, the tri-generation fuel cells simultaneously reduce carbon emissions and enhance energy efficacy (Yue et al. 2021 ). Fong and Lee ( 2014 ) reported that employing a 593 kilowatts solid oxide fuel cell and absorption chillers, the carbon emissions were notably decreased by about 50% with an increase in the energy efficacy up to 75% (Fong and Lee 2014 ). A simulated 339 kilowatts solid oxide fuel cell combined with a combustor and a heat recuperation system proficiently recovered about 267 kilowatts of heat with an efficacy of 84%. Besides, they announced that 339 kilowatts solid oxide fuel cells provided with an absorption chiller generated about 303.6 kilowatts of cold with an efficacy of 89% (Yu et al. 2011 ).

Transportation sector

Compared with conventional battery-powered powertrains, vehicles based on hydrogen fuel (hydrogen-fuelled vehicles) represent a promising solution to surpass them. Globally, the sales rate of hydrogen-fuelled vehicles is anticipated to be 3% and enhanced up to 36% in 2030 and 2050, respectively (Path to Hydrogen Competitiveness: A Cost Perspective - Hydrogen Council. https://hydrogencouncil.com/en/ ). Currently, innumerable vehicles companies are developing their operating system to be hydrogen-based, attributing to its dependability and quality. Toyota has evolved Mirai fuel cell vehicles by using proton exchange membrane fuel cells with a volume power density and maximum power productivity of 3.1 km/L and 144 kilowatts, respectively. The hydrogen-fuelled vehicles can be driven by different forms of hydrogen (i.e. liquid and compressed). The compressed (high pressurised) hydrogen is the most appropriate form in the vehicles storage system of Clarity and NEXO; hydrogen-based fuel cell vehicles developed by Honda and Hyundai companies, respectively. At the same time, liquid hydrogen operates Hydrogen 7 vehicle improved by BMW company (Yue et al. 2021 ). Moreover, regional multi-unit trains powered by hydrogen have been entered into service in Europe and are projected to gain more economic benefits. Approximately 30% of presently employed diesel fleets may be phased out in the future (Study on the use of Fuel Cells and Hydrogen in the Railway Environment - Shift2Rail. https://shift2rail.org/publications/study-on-the-use-of-fuel-cells-and-hydrogen-in-the-railway-environment/ ).

Among different modes of transportation, the aviation division is regarded as the fastest transportation mode with anticipated annual growth in air traffic. The most common aircraft fuel is kerosene. Various aviation fuels often display a set of specifications, such as resistance to corrosion and severe temperature changes (Tzanetis et al. 2017 ). It is worth noting that petroleum accounts for the majority of the fuel used in the aviation sector. To improve energy preservation and reduce the negative environmental effects of fossil fuels, alternative, less harmful fuels such as liquid hydrogen are developed and thought to be eco-friendly. Table 8 presents some variations in the physicochemical properties between hydrogen and kerosene fuels. Refrigerated hydrogen fuel can be potentially better than kerosene as aviation fuel. It emits fewer greenhouse gaseous emissions and is easily produced from a variety of sources. Aside from that, the operating hydrogen-fuelled aircraft is characterised by minimal maintenance costs, long lifetime engines, high energy content and better combustion.

Furthermore, some constraints may arise during hydrogen utilisation as aviation fuel, such as depressed ignition energy, high flammability and the possibility of unburned traces forming that promotes metal embrittlement. Furthermore, the hydrogen admission with the onboard technology instead of inserting into the grid commercially allows its manufacturing companies to resell it (Nanda et al. 2017 ). The National Renewable Energy Laboratory manifested that the hydrogen cost in the mentioned case ranges from 3 to 10 USD/Kg, while the most traded hydrogen cost is about 13.99 USD/Kg. To sum up, liquid hydrogen presents admirable efficacy as an aviation fuel for reducing greenhouse gaseous emissions, resulting in a significant improvement in air quality. Furthermore, by using hydrogen-based aviation fuels, over-reliance on traditional fuels could be decreased. The total cost of aircraft powered by liquid hydrogen is predominately associated with the cost of production and storage technologies (Eichman et al. 2012 ).

Recently, the global navigation movement in terms of maritime shipping has become increasingly important in the movement of different types of goods worldwide, which is in line with tremendous industrial progress in various fields. Unfortunately, this, in turn, led to an increase in the consumption of conventional fuels (i.e. diesel and heavy fuels). Regrettably, the pollution created by ships significantly implicates about 2.5% of the universal greenhouse gaseous emissions. Furthermore, bunkering activities broadly contribute to the leakage of heavy fuels in the aquatic environment, consequently posing a threat to the ecosystem. It was announced that carbon dioxide emissions associated with shipping activities release about 3.3% of the global emissions (Vogler and Sattler 2016 ). Other gaseous emissions such as nitrogen oxide and sulphur oxide are also associated with shipping activities. Accordingly, the maritime industry seeks more environmentally alternative fuels than conventional ones to overcome these obstacles (Prussi et al. 2021 ). Numerous suitable substitutes in different states, gas (i.e. hydrogen, propane) and liquid (i.e. bio-oil, methanol and ethanol) are used to compensate for the usage of traditional fuels (Al-Enazi et al. 2021 ; Abou Rjeily et al. 2021 ). Among them, hydrogen can be employed in maritime activities in two routes: (1) internal combustion engines and (2) fuel cells (Banawan et al. 2010 ). Relatively, fuel cells meet the energy requirements needed by ships sailing for long distances travelling and supply the ancillary energy requirements of larger ships in contrast to the other battery-powered ones. Numerous studies have been conducted to assess the feasibility of using hydrogen in maritime activities. Deniz and Zincir ( 2016 ) stated that hydrogen had a durable, safe and bunker capability criterion, qualifying as a favourable fuel for shipping. Although they reported that liquefied natural gas has the preference to be used as an alternative fuel, they recommended more research studies on utilising hydrogen as an effective alternative fuel (Deniz and Zincir 2016 ).

Production of hydrocarbon fuels

Production of hydrocarbon fuels via fischer–trospch pathway.

Syngas (synthesis gas), a mixture of carbon monoxide and hydrogen, is a product of different thermochemical conversion processes (i.e. pyrolysis, gasification and others) and can be utilised by two scenarios: (1) direct fuel or (2) transformed into transportation fuels via Fischer–Trospch synthesis process and syngas fermentation (Wainaina et al. 2018 ). The two strategies are categorised as gas-to-liquid transformation strategies that can generate hydrocarbon fuels and alcohols based on syngas feedstock stream (Gruber et al. 2019 ). Normally, the Fischer–Trospch strategy (exothermic) operates at 200–350 °C and 1.5–4 MPa for reaction temperature and pressure, respectively (Okolie et al. 2019 ). Majorly, it comprises three main stages: (1) syngas production, (2) syngas treatment and (3) transforming into hydrocarbon fuels associated with their upgrading. Besides the production process of transportation fuel, other valuable products (i.e. paraffin, naphtha and others) can be produced. Significantly, the as-produced green fuels based on the Fischer–Trospch process have numerous advantages over petroleum-based fuels. They have excellent burning characters, elevated smoking points and free of heavy contaminants. The physicochemical properties of resultant fuels depend heavily on reaction conditions (i.e. reactor type, heating rate, residence time and others) (Sun et al. 2017 ). The given equations from (Eqs. 9 , 10 , 11 , 12 , 13 , 14 and 15 ) explicates the synthesis of different products (i.e. alkanes, alkenes, oxygenated products, methanol, ethanol and dimethyl ether via the Fischer–Trospch process by participating in hydrogen. The hydrogen/carbon monoxide ratio is a critical controlling parameter in the Fischer–Trospch synthesis process (Bermudez and Fidalgo 2016 ). Different types of catalysts (i.e. copper-based catalysts) can be used to optimise the yield of the Fischer–Trospch process.

Synthesis of alkanes:

Synthesis of alkenes:

Synthesis of alcohols:

Synthesis of carbonyl:

Synthesis of ethanol:

Synthesis of methanol:

A ratio of H 2 / CO of 2:1 is preferable for the synthesis of hydrocarbon fuels via water—gas shift reaction as given in Eq.  15 :

Dimethyl ether is admirable commonly realised as an efficient alternate for diesel fuel (Kim and Park 2016 ). Distinctly, numerous physicochemical features characterise liquefied petroleum gas, such as anti-corrosive, anti-carcinogenic, less nitrogen oxide and carbon monoxide emissions during its burning, less engine noise and high cetane number (Dincer and Bicer, 2020 ). In general, dimethyl ether can be produced by (1) direct route (combined single step of methanol synthesis and dehydration) or (2) indirect route (separated methanol synthesis and dehydration steps) as shown in Eqs. ( 16 , 17 and 18 ) (Gogate, 2018 ):

Direct route (single step):

Indirect route (two steps):

Dehydration of methanol:

Production of hydrocarbon fuels via Syngas fermentation pathway

Syngas fermentation (biorefining) pathway is regarded as the interconnection between the biochemical and thermochemical scenarios (Thi et al. 2020 ). It produces value-added products (i.e. alcohols) from syngas by flexibly employing several groups of microorganisms at different reaction temperatures of 37–40 °C and 55–90 °C for mesophilic (i.e. Clostridium autoethanogenum ) and thermophilic (i.e. Moorella thermoacetica ), respectively. During the process, the feedstock of syngas can be simply converted into alcohols (i.e. ethanol) via two subsequent stages via (1) producing acetyl coenzyme A and then (2) its transformation into ethanol. Other alcohols and chemicals (i.e. acetate, butanol and formate) can be synthesised by acetogenic bacteria (Park et al. 2017 ). Regarding several operational advantages characterised to syngas fermentation such as (1) no necessity for using costly pretreatment step, (2) process' versatility with different biomass composition, (3) independent on the hydrogen/carbon monoxide ratio in the feedstock upstream, (4) high selectivity of as-used microorganisms and (5) moderate (ambient) working parameters with no necessity for catalysts usage or its poising trouble, they support it over the Fischer–Trospch process. However, there are some operational challenges such as (1) weak solubility of the gas in the liquid state, (2) complicated bioreactor design, (3) existence of impurities and (4) low yield of production. Briefly, integration between different thermochemical, biochemical and hydrothermal routes can effectively compensate for the shortage of individual techniques and maximise productivity (Rigueto et al. 2020 ).

Refining of crude oil and petroleum products

Commercially, hydrogen is conceived as an upgrading (improving) agent for crude oil products and petroleum distillates in terms of hydrocracking and hydroprocessing and processes. The hydrocracking process is defined as treating heavier hydrocarbons with hydrogen to simultaneously split them into lighter derivatives and enhance the hydrogen/carbon ratio (El-Sawy et al. 2020 ). In hydroprocessing, various heteroatoms such as nitrogen, sulphur, oxygen and heavy metals are majorly captured from petroleum products via different treatment processes named: hydrodenitrogenation (Dasgupta and Atta 2020 ), hydrodesulphurisation (Han et al. 2018 ), hydrodeoxygenation (Yfanti and Lemonidou 2020 ) and hydrodemetallisation (Rana et al. 2020 ), respectively, as displayed in Eqs. ( 19 – 21 ).

Hydrodenitrogenation:

Hydrodesulphurisation:

Hydrodeoxygenation:

This can be achieved by reacting the upstream feedstock (heavy oils and petroleum products) with hydrogen through catalytic reaction, resulting in removing these contaminants and saturating the aromatics (C–C) bonds. The elimination process of these contaminates from feedstocks directly contributes to fuel upgrading because they deactivate the as-used catalysts due to their adsorption on the surfaces of the catalyst (blocking of active catalyst sites). Recently, the appeal for inserting hydrogen in hydroprocessing has been increasingly growing (Al Obaidi et al. 2018 ). From the environmental point of view, the key cause of this pattern is the combination of strict environmental legislation governing gaseous greenhouse emissions and other particulate contaminants, as well as product quality specifications. Generally, numerous upgrading techniques are directed to improve the physicochemical properties of heavy oils by decreasing their viscosity and carbon/hydrogen ratio at the same time (Misra et al. 2017 ).

Production of ammonia

Ammonia is deemed one of the essential chemicals largely employed in industrial fertiliser activities with huge global production rates. The biggest ammonia production plant has projected to achieve a daily capacity rate of 3300 metric tons (Brightling 2018 ). Broadly, ammonia can be introduced as fertiliser in the agriculture sector. Additionally, it is provided to various industries such as polymers processing, explosives, refrigerant, pharmaceuticals, gas sensors and fuel cells. The ammonia synthesis process is promoted by the catalytic reaction between hydrogen and nitrogen elements through the Haber process (Arora et al. 2018 ). It is performed in the as-designed reactor under operating conditions of 20–30 Mpa and 300–500 °C for pressure and temperature, respectively, using KOH-promoted finely divided iron catalysts with the required energy of 2.5 EJ (Tolod et al. 2019 ).

Moreover, the hydrogen addressed to the ammonia synthesis process is primarily derived from steam gas reforming, which is not regarded as environmentally friendly. Accordingly, there is an increasing interest in other green and sustainable ammonia synthesis pathways, such as electrochemical hydrogen manufacturing techniques and photocatalytic nitrogen fixation (artificial photocatalysis). The distinctiveness of the electrochemical ammonia synthesis pathways routes is controlled by the employed energy sources. Hydrogen can be generated from water employing an electrolysis process using renewable green sources (i.e. wind and solar energy) and, hence, reduce harmful greenhouse gaseous emissions (Bicer and Dincer 2017 ).

Metallurgical industries

Generally, hydrogen can produce oxy-hydrogen flames in industrial metallurgical activities and act as a reducing agent to obtain metals from their ores. During the oxy-hydrogen flames synthesis process (exothermic reaction), hydrogen is allowed to react with oxygen at very high temperatures (3000 °C) to produce oxy-hydrogen flames, used later for cutting and welding working on non-ferrous metals (Polverino et al. 2019 ). Otherwise, hydrogen is reliably characterised by its high ability to recover (reduce) metals from the aqueous solutions of their salts (hydrogen reduction). The metals may be powdered for later metallurgical usage or incorporated into a composite material. Chemically, hydrogen can interact with the elements of periodic tables in three ways: (1) ionic bond formation between the elements of Ia and IIa groups, (2) interstitial solid solution between the elements of VIa, VIIa and VIII groups and (3) metallic bond between elements of IIIa, Iva and Va groups. Additionally, the electrostatic shielding phenomenon can be generated by attributing the hydrogen's capability to capture free electrons and the self-trapping of metals. Likewise, the small particle size of hydrogen effectively facilitates the process of metal–hydrogen interaction (Agrawal et al. 2006 ).

To ensure the long-term production of clean and green hydrogen, it is crucial to conduct a critical assessment of various production routes and their environmental impacts, as well as seasonal storage and utilisation options. Hydrogen is produced from either fossil-based or renewable feedstocks; however, each route has advantages and disadvantages. The current hydrogen colour coding is imprecise, assuming that green hydrogen always has lower carbon emissions than blue or grey hydrogen, which is not always accurate.

Water electrolysis is gaining momentum; however, meeting 24% of energy demand with hydrogen in a 1.5-degree scenario of climate change mitigation will necessitate massive amounts of additional renewable electricity generation. In this scenario, approximately 31,320 terawatt-hours of electricity would be required to power electrolysers, which is more than is currently produced globally from all sources combined. Furthermore, > $11 trillion in manufacturing, storage and transportation infrastructure would be needed. The affordability and accessibility of freshwater is one side of the coin, and the proximity of these two supplies, i.e. renewable energy and freshwater, is the other. Water electrolysis research priorities include lowering the capital cost of electrolysis technology, locating water resources, find utilisation routes for the produced oxygen and increasing the efficiency of the process.

In terms of biomass gasification, the economic feasibility of producing hydrogen from biomass must be closely related to the availability and affordability of raw materials in the surrounding area. The main characteristics of the supply materials are the biomass physicochemical properties, distribution and hydrogen rate. Because biomass feedstocks differ greatly in structural composition and shape, all of these factors must be considered when combining the feedstock with the appropriate conversion technology. In conclusion, there are challenges associated with the most common hydrogen generation routes, such as steam methane reforming, water electrolysis, coal or biomass gasification, methane pyrolysis with or without carbon capture and storage technology.

To understand advances in evaluating environmental impacts due to hydrogen production, we performed an intensive critical review of 24 life cycle assessment studies published from 2019 to 2021, including methods and findings. The important methodological approaches and key findings observed were:

No two life cycle assessment studies were identified to be similar. There were differences in the geographical and temporal span, functional units and system boundaries considered, and environmental impact categories assessed. Therefore, it is recommended that the policymakers pay heed to the modelled processes and extent of the system boundary for making decisions for creating a sustainable hydrogen economy.

Many life cycle assessment studies did not encompass processes, inputs and outputs for ‘cradle-to-grave’ analysis. Thus, future research should pay more attention to ‘cradle-to-grave’ evaluation for robust decision-making.

In addition to the global warming potential and depletion of fossil fuels, environmental impacts in more categories for hydrogen production processes must be evaluated.

Furthermore, large-scale energy storage is key in securing the energy supply chain for the next energy transition using electrolysis-generated hydrogen. The Underground Seasonal Hydrogen Storage (USHS) holds great potential to overcome the natural temporal fluctuations inherent in renewable energy production at the scale required to achieve net-zero by 2050. The selection of geological porous media for USHS should be based on a comprehensive geological investigation that includes an assessment of their utility on both a basin and regional scale, fluid flow behaviour of hydrogen in brine-saturated subsurface reservoirs, an assessment of storage capacity, the safety of long-term storage, geochemical and biological reactions triggered by hydrogen injection, the geomechanical response of the subsurface to hydrogen storage and other measures. The discussed procedures can lead to informed decision-making in terms of forecasting best-operating strategies and ensuring safe and efficient hydrogen storage installation. Further research to integrate the theoretical studies with existing experimental USHS trials is required to minimise the uncertainty that might be associated with the feasibility of large-scale hydrogen storage. Finally, blending the need with the various utilisation routes such as fuel production, ammonia production, metallurgical industries and power systems is crucial in the hydrogen economy.

Change history

31 march 2022.

A Correction to this paper has been published: https://doi.org/10.1007/s10311-022-01432-x

Abbreviations

Carbon capture storage and utilisation

Levelised cost of hydrogen

Water–gas shift reaction

Underground Seasonal Hydrogen Storage

Proton exchange membrane fuel cells

Phosphoric acid fuel cells

Solid oxide fuel cells

Molten carbonate fuel cells

Direct methanol fuel cells

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Acknowledgements

The authors would like to thank OQ Oman for their generous financial support (project code: CR/DVC/SERC/19/01). The authors would also like to acknowledge the support of the Sustainable Energy Research Centre at Sultan Qaboos University. Ahmed Osman and David Rooney wish to acknowledge the support of The Bryden Centre project (Project ID VA5048). The Bryden Centre project is supported by the European Union’s INTERREG VA Programme, managed by the Special EU Programmes Body (SEUPB). Neha Mehta acknowledges funding from the Centre for Advanced Sustainable Energy (CASE). CASE is funded through Invest NI’s Competence Centre Programme and aims to transform the sustainable energy sector through business research.

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Osman, A.I., Mehta, N., Elgarahy, A.M. et al. Hydrogen production, storage, utilisation and environmental impacts: a review. Environ Chem Lett 20 , 153–188 (2022). https://doi.org/10.1007/s10311-021-01322-8

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SCIENCE & ENGINEERING INDICATORS

Publications output: u.s. trends and international comparisons.

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Publication Output by Region, Country, or Economy and by Scientific Field

This section of the report outlines trends over time in publication output across regions, countries, or economies and by fields of science. This section also provides insights into the research contributions of different regions, countries, or economies and how the focus of their scientific publications has changed over time. In addition, the section highlights variations in the distribution of publications across scientific fields for different regions, countries, or economies and examines trends over time in closed-access and open-access (OA) publications. This section also summarizes federal funding acknowledgments as a source of data to shed light on published research that received federal funding. (See sidebar Using Funding Acknowledgments to Track Federally Funded Research Over Time .)

Output by Region, Country, or Economy

Total worldwide S&E publication output reached 3.3 million articles in 2022, based on entries in the Scopus database. Indicators 2018 : Bibliometric Data Filters )." data-bs-content="Publication output includes only those indexed in the Scopus database. The publication output discussion uses fractional counting, which credits coauthored publications according to the collaborating institutions or regions, countries, or economies based on the proportion of their participating authors. Country assignments refer to the institutional address of authors, with partial credit given for each international coauthorship. As part of the data analysis, filters were employed on the raw Scopus S&E publication data to remove publications with questionable quality, which appear in what are sometimes called predatory journals (NSB Indicators 2018 : Bibliometric Data Filters )." data-endnote-uuid="a5778137-c92e-4694-991d-a1545bdcca18">​ Publication output includes only those indexed in the Scopus database. The publication output discussion uses fractional counting, which credits coauthored publications according to the collaborating institutions or regions, countries, or economies based on the proportion of their participating authors. Country assignments refer to the institutional address of authors, with partial credit given for each international coauthorship. As part of the data analysis, filters were employed on the raw Scopus S&E publication data to remove publications with questionable quality, which appear in what are sometimes called predatory journals (NSB Indicators 2018 : Bibliometric Data Filters ). Approximately 86% of publications in 2022 came from regions, countries, or economies with high-income and upper-middle-income economies ( Figure PBS-1 ). The number of publications from all income-level groups grew between 2003 and 2022 ( Table SPBS-2 ). Also, the number of publications from upper-middle-income economies grew more quickly than the number from high-income economies during the more recent period between 2010 and 2022.

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S&E publications, by income group: 2003–22

Article counts refer to publications from a selection of conference proceedings and peer-reviewed journals in S&E fields from Scopus. Articles are classified by their year of publication and are assigned to a region, country, or economy on the basis of the institutional address(es) of the author(s) listed in the article. Articles are credited on a fractional count basis (i.e., for articles produced by authors from different regions, countries, or economies, each region, country, or economy receives fractional credit on the basis of the proportion of its participating authors). Data are not directly comparable with data from Science and Engineering Indicators 2022 ; see the Technical Appendix for information on data filters. Low-income regions, countries, or economies are not included in this figure because of their low publication output. Data by region, country, or economy and income group are available in Table SPBS-2 . Regions, countries, or economies are allocated to income groups based on World Bank data, using their current designation. For example, all of China’s publications from 2003 to 2022 are counted as part of the upper-middle-income category because that is China's current designation.

National Center for Science and Engineering Statistics; Science-Metrix; Elsevier, Scopus abstract and citation database, accessed April 2023; World Bank Country and Lending Groups, accessed May 2023.

Science and Engineering Indicators

In 2022, six regions, countries, or economies each produced more than 100,000 articles: China, the United States, India, Germany, the United Kingdom, and Japan. Together, these leading regions, countries, or economies accounted for over 50% of the worldwide total in 2022 ( Figure PBS-2 ; Table PBS-1 ). Figure PBS-2 and Table PBS-1 , or whole counting, as in Table SPBS-17 . There is a slight difference between the United States and China when looking at the whole-counting total production numbers. Using whole counting for 2022, the United States had 605,633 articles, whereas China had 976,141. A whole-counting measure allocates one full count to each region, country, or economy with an author contributing to the article; in fractional counting, each region, country, or economy receives a proportion of the count based on the number of authors from that region, country, or economy. For example, if an article had four authors—two from the United States, one from China, and one from Brazil—the fractional scores would be half for the United States, a quarter for China, and a quarter for Brazil. The difference between whole and fractional counting indicates that the United States has more authors working with Chinese authors than China has working with U.S. authors." data-bs-content="The proportion of output attributable to the large producers is consistent whether using fractional counting, as in Figure PBS-2 and Table PBS-1 , or whole counting, as in Table SPBS-17 . There is a slight difference between the United States and China when looking at the whole-counting total production numbers. Using whole counting for 2022, the United States had 605,633 articles, whereas China had 976,141. A whole-counting measure allocates one full count to each region, country, or economy with an author contributing to the article; in fractional counting, each region, country, or economy receives a proportion of the count based on the number of authors from that region, country, or economy. For example, if an article had four authors—two from the United States, one from China, and one from Brazil—the fractional scores would be half for the United States, a quarter for China, and a quarter for Brazil. The difference between whole and fractional counting indicates that the United States has more authors working with Chinese authors than China has working with U.S. authors." data-endnote-uuid="a3ba485a-d862-4069-8166-933aa601ea66">​ The proportion of output attributable to the large producers is consistent whether using fractional counting, as in Figure PBS-2 and Table PBS-1 , or whole counting, as in Table SPBS-17 . There is a slight difference between the United States and China when looking at the whole-counting total production numbers. Using whole counting for 2022, the United States had 605,633 articles, whereas China had 976,141. A whole-counting measure allocates one full count to each region, country, or economy with an author contributing to the article; in fractional counting, each region, country, or economy receives a proportion of the count based on the number of authors from that region, country, or economy. For example, if an article had four authors—two from the United States, one from China, and one from Brazil—the fractional scores would be half for the United States, a quarter for China, and a quarter for Brazil. The difference between whole and fractional counting indicates that the United States has more authors working with Chinese authors than China has working with U.S. authors. In absolute numbers, the growth in worldwide annual publication output (from 2.0 million in 2010 to 3.3 million in 2022) was driven in particular by two countries: China (42% of additional publications during that period) and India (11%) together accounted for more than half of that increase in publications ( Figure PBS-3 ). Russia, South Korea, Iran, and Brazil made notable contributions to the growth in the number of publications from the rest of the world from 2010 to 2022 ( Figure PBS-3 ; Table SPBS-2 ). Generally, the set of the top 15 producers of S&E articles was the same each year between 2010 and 2022, with the exception of Iran replacing Taiwan in the top 15 beginning in 2014 ( Table PBS-1 ; Table SPBS-2 ).

S&E publications for 10 leading regions, countries, or economies: 2022

Article counts refer to publications from a selection of conference proceedings and peer-reviewed journals in S&E fields from Scopus. Articles are classified by their year of publication and are assigned to a region, country, or economy on the basis of the institutional address(es) of the author(s) listed in the article. Articles are credited on a fractional count basis (i.e., for articles produced by authors from different regions, countries, or economies, each region, country, or economy receives fractional credit on the basis of the proportion of its participating authors). Data by all countries, regions, and economies are available in Table SPBS-2 .

National Center for Science and Engineering Statistics; Science-Metrix; Elsevier, Scopus abstract and citation database, accessed April 2023.

S&E publications in all fields for 15 largest producing regions, countries, or economies: 2012 and 2022

na = not applicable.

The regions, countries, or economies are ranked based on the 2022 total. Article counts refer to publications from conference proceedings and peer-reviewed journal articles in S&E and indexed in Scopus (see Technical Appendix for more details). Articles are classified by their year of publication and are assigned to a region, country, or economy on the basis of the institutional address(es) of the author(s) listed in the article. Articles are credited on a fractional count basis (i.e., for articles from multiple regions, countries, or economies, each region, country, or economy receives fractional credit on the basis of the proportion of its participating authors). Detail may not add to total because of regions, countries, or economies that are not shown. Proportions are based on the world total excluding unclassified addresses (data not presented). Details and other regions, countries, or economies are available in Table SPBS-2 .

S&E publications, by selected region, country, or economy and rest of world: 2003–22

Article counts refer to publications from a selection of conference proceedings and peer-reviewed journals in S&E fields from Scopus. Articles are classified by their year of publication and are assigned to a region, country, or economy on the basis of the institutional address(es) of the author(s) listed in the article. Articles are credited on a fractional count basis (i.e., for articles produced by authors from different countries, each country receives fractional credit on the basis of the proportion of its participating authors). Data for all regions, countries, and economies are available in Table SPBS-2 .

The U.S. trend of moderate but increasing publication output varied by state. The National Science Board’s (NSB’s) State Data Tool ( https://ncses.nsf.gov/indicators/states/ ) provides state-level data based on each state’s doctorate population and research and development (R&D) funding. Indicators include academic S&E article output per 1,000 science, engineering, and health doctorate holders in academia (NSB 2021a) and academic S&E article output per $1 million in academic S&E R&D funding (NSB 2021b).

Output by Scientific Field

The distribution of publications by field of science across region, country, or economy may indicate research priorities and capabilities. Health sciences was the field of science in which most articles were published in 2022, representing almost a quarter of all publications ( Table SPBS-2 and Table SPBS-10 ). Other fields with large numbers of publications included engineering (17% of publications in 2022), biological and biomedical sciences (13%), and social sciences (5%) ( Table SPBS-2 , Table SPBS-5 , Table SPBS-8 , and Table SPBS-16 ). In the United States, the European Union (EU-27), and Japan, health sciences publication output in 2022 far exceeded that of any other field. Table SPBS-17 through Table SPBS-31 )." data-bs-content="The use of whole counting or fractional counting to tally the publication output of nations can change the calculated publication count based on the degree to which a region, country, or economy is involved in international collaborations. Under whole counting, a nation receives credit for any publication with an author from that nation. Under fractional counting, the nation’s credit for a publication is prorated based on the share of the publication’s coauthors who are located in that nation ( Table SPBS-17 through Table SPBS-31 )." data-endnote-uuid="05c8023f-5250-4386-96cf-f321c7f293c4">​ The use of whole counting or fractional counting to tally the publication output of nations can change the calculated publication count based on the degree to which a region, country, or economy is involved in international collaborations. Under whole counting, a nation receives credit for any publication with an author from that nation. Under fractional counting, the nation’s credit for a publication is prorated based on the share of the publication’s coauthors who are located in that nation ( Table SPBS-17 through Table SPBS-31 ). Meanwhile, of the other top producers, publications from China were most highly concentrated in engineering (25%), and publications from India were published predominantly in computer and information sciences (21%) ( Figure PBS-4 ).

Distribution of national S&E research portfolios across scientific fields, by selected region, country, or economy: 2022

EU-27 = European Union.

Articles refer to publications from a selection of conference proceedings and peer-reviewed journals in S&E fields from Scopus. Articles are classified by their year of publication and are assigned to a region, country, or economy on the basis of the institutional address(es) of the author(s) listed in the article. Articles are credited on a fractional count basis (i.e., for articles from multiple countries, each country receives fractional credit on the basis of the proportion of its participating authors). See Table SPBS-1 for countries included in the EU; beginning in 2020, the United Kingdom was no longer a member of the EU. See Table SPBS-3 through Table SPBS-16 for data on all regions, countries, and economies by each S&E field.

Fields within life sciences were dominant in the United States in 2022, with more than half of all U.S. publications in health sciences (37%) or biological and biomedical sciences (14%) ( Figure PBS-5 ). There were fewer U.S. publications in engineering (11%), computer and information sciences (7%), and physics (5%). In comparison with the United States, China had a stronger focus on publications in engineering and in the physical sciences and information sciences. In 2022, 25% of China’s publications were in engineering, 11% were in computer and information sciences, and 9% were in physics ( Figure PBS-6 ). Compared with the United States, China had a lower percentage of its publications in health sciences (14%) and biological and biomedical sciences (12%). In 2022, China also had a much lower percentage of its publications in social sciences (1%) when compared with the United States (8%).

U.S. S&E publication portfolio, by field of science: 2022

Articles refer to publications from a selection of conference proceedings and peer-reviewed journals in S&E fields from Scopus. Articles are classified by their year of publication and are assigned to a region, country, or economy on the basis of the institutional address(es) of the author(s) listed in the article. Articles are credited on a fractional count basis (i.e., for articles from multiple countries, each country receives fractional credit on the basis of the proportion of its participating authors). See Table SPBS-3 through Table SPBS-16 for data on all regions, countries, and economies by each S&E field.

S&E publication portfolio from China, by field of science: 2022

Articles refer to publications from a selection of conference proceedings and peer-reviewed journals in S&E fields from Scopus. Articles are classified by their year of publication and are assigned to a region, country, or economy on the basis of the institutional address(es) of the author(s) listed in the article. Articles are credited on a fractional count basis (i.e., for articles from multiple countries, each country receives fractional credit on the basis of the proportion of its participating authors). See Table SPBS-3 through Table SPBS-16 for data on all regions, countries, and economies and by each S&E field.

All the leading regions, countries, or economies saw an increase in their output of health sciences publications between 2010 and 2022. This increase is to be expected, given the context of increasing publication rates in general over that period, with overall number of publications increasing by 71% ( Table SPBS-2 ), while publications in health sciences increased by 66% ( Table SPBS-10 ). Russia had the highest relative growth rate among the 20 leading regions, countries, or economies in health sciences, increasing its publication output by almost 450% between 2010 and 2022 ( Table SPBS-10 ). China and Iran each increased their output of health sciences publications by more than 250% over this period, while India’s health sciences publication output increased by more than 180%. The United States increased its output of health sciences publications by 32% over this period, while Germany, France, the United Kingdom, and Japan had the smallest increases, each with less than 20%.

Leading regions, countries, or economies also saw increases in engineering publications. The fastest growing between 2010 and 2022 were India (up 378%) and Russia (up 230%) ( Table SPBS-8 ). China increased its output of engineering publications by 176% from 2010 to 2022, while France, the United States, and Japan all saw declines in newly published engineering articles per year over this period (3%, 13%, and 26%, respectively).

In the United States, publication output varied from that of other regions, countries, or economies with respect to scientific fields. Of the fields not already mentioned, the fastest growing from 2010 to 2022 were psychology (up 39% from 2010 to 2022) and the social sciences (up 38%) ( Figure PBS-7 ). Meanwhile, fields with the largest decreases in U.S. publications included physics (down 31% from 2010 to 2022) and materials science (down 16%).

Index of U.S. publications, by field: 2010–22

Using funding acknowledgments to track federally funded research over time.

Federally funded research is an important component of the research ecosystem and is often envisioned as a means of supporting science performed for public benefit that may not otherwise be motivated by commercial interest (Bornmann 2013; Stephan 2012; Yin et al. 2022). Federal research funding supports applied and basic research (see Indicators 2022 report “ Research and Development: U.S. Trends and International Comparisons ”) and has long been linked to successful expansions in scientific production—through the increased productivity and impact of individual researchers and laboratories (Ebadi and Schiffauerova 2016) and the national scale (Leydesdorff and Wagner 2009). This sidebar explores funding acknowledgments, as recorded in Scopus, as an emerging source to help illustrate the extent to which published research is supported by federal agencies and the trends in federally funded research. Specifically, the share of published research acknowledging support by federal funding was highest in chemistry and smaller in other fields, such as the social sciences ( Table PBS-A ). These differences may be driven by factors such as the resource costs to conduct research and by field differences, such as the overall frequency of publication, team size, and cultural differences among the disciplines. The time period analyzed in this sidebar is 2018–22, unless otherwise indicated.

Funding acknowledgments can shed light on the ability and priorities of federal funding to support discovery as measured by peer-reviewed journal articles and conference proceedings. However, some benefits and limitations of this emerging data source are important to highlight so as to accurately interpret these trends. Each peer-reviewed journal article and conference proceeding in the Scopus database includes a field for funding acknowledgments that are extracted by algorithmic (software) means. In some cases where the acknowledgments field is incomplete, funding information from agencies is also used to identify funded publications in Scopus. Using this field, it is possible to observe the conversion of federal funds to published research outputs, but a direct linkage between funding inputs and published discoveries remains challenging. First, extraction of this information into a structured field is a relatively new effort and is most complete for the most recent 4 years. Figure SPBS-1 shows how funding acknowledgment sections have grown in coverage since 2003 and that funding information was indexed for 68% of all publications in 2022. * Many factors may have contributed to this growth in addition to improved extraction, including increasing pressure and requirements from funders to include funding acknowledgments, standardization of acknowledgment language, and incentives to demonstrate high publication output—because future funding is tied to past conversion of funds into publications—while receiving funding. † Last, this inquiry helps explore research that acknowledges any federal funding but does not only account for publications that source all their funding from a single source. In practice, a publication may be generated using funding from multiple sources within the federal government, or from additional sources in state government, local government, or the private sector.

U.S. S&E publications with and without acknowledgments of U.S. federal funding: 2003–22

Articles are classified by their year of publication and are assigned to a region, country, or economy on the basis of the institutional address(es) of the author(s) listed in the article. Whole counting is used. An article is considered to be federally funded if the funding information tied with the publication record in Scopus links it with one of the U.S. federal agencies. Not all Scopus publications have funding information available, and coverage has evolved with time. For more information, see Figure SPBS-1 . For a breakdown of federally funded papers by funding agency, see Table SPBS-90 .

National Center for Science and Engineering Statistics; Science-Metrix; Elsevier, Scopus abstract and citation database, accessed April 2023.

Figure PBS-A tracks the growth of federally funded publications relative to the total research production in the United States. Other than a small downturn from 2021 to 2022, every year has seen an increase from the previous year in the number of publications that acknowledge funding support from federal agencies. The most comprehensive data from the past 4 years show variation among subject areas in the percentage of publications that acknowledge federal support. Table PBS-A shows number and share of publications appearing between 2018 and 2022 that acknowledged funding from federal sources and those acknowledging funding from other sources. During this time, more than 50% of publications in the following subject fields acknowledged federal funding support: chemistry (55% of publications), biological and biomedical sciences (53%), astronomy and astrophysics (53%), and physics (52%). Only two subject areas have less than 30% of publications with federal funding acknowledged: agricultural sciences (28%), and social sciences (15%). Otherwise, all other fields had between 30% and 50% of their publications acknowledging federal funding.

U.S. S&E publications, by U.S. federal funding status and field: 2018–22

In conclusion, federal funding plays an important role in the current research environment in the United States. Of the 606,144 articles published in journals and conference proceedings in 2022, 35% acknowledged support from federal agencies ( Figure PBS-A ). Ultimately, acknowledgment of federal funding can help show trends in the conversion of grants into published research over time and show variation at the subject or field level.

* Missing data in funding fields in a Scopus entry may mean that the research did not receive funding, the authors did not cite any funding despite receiving it, or the algorithm was unable to extract the acknowledgment. Of the articles from 2003 that had an entry for funding acknowledgment (27% had text in the funding field in Scopus), around 76% acknowledged a federal funding source. Comparatively, of the publications in 2022 with indexed funding information (68%), 52% acknowledged a federal source. The growth of coverage of funding not being tied to federal funding acknowledgments provides evidence that the data source has become more dependable over time. Data for the percentage of publications with indexed funding sources by year and field can be found in Figure SPBS-1 .

† Table SPBS-90 displays the number of articles and conference proceedings acknowledging federal funding at the agency level and sub-agency level. These counts represent the number of supported articles as acknowledged and attributed in Scopus from 2003 to 2022.

Output and Open Access

There is growing support for the availability of S&E publications through OA sources among government and private funders, institutions, and scientists themselves. Some of these funders have imposed requirements on their grantees to publish their research results in OA journals. In the United States, the Office of Science and Technology Policy announced that all federal agencies should update their public access policies as soon as possible to ensure that results of their funded research are publicly available, with full implementation of these policies by the end of 2025 (Brainard and Kaiser 2022). Meanwhile, restricted access to scientific literature may impede researchers’ ability to stay informed (Larivière and Sugimoto 2018; Piwowar et al. 2018). As alternatives to traditional closed-access journals (where readers must subscribe to gain access or pay per article), articles may be made OA through several avenues, with different levels of availability and durability.

There are four commonly defined types of OA: Gold, Hybrid, Bronze, and Green. Gold OA denotes articles published in journals that are entirely OA as a matter of journal policy. Hybrid OA denotes articles for which the authors have elected to pay a fee for publication as OA rather than as closed access. Bronze OA denotes articles that appear as OA after an embargo period of closed access or articles that appear available as OA despite lacking license information to guarantee OA in the long term. Green OA denotes articles that are self-archived by authors in OA repositories, which are often maintained, curated, and administered by universities or other institutions. The Hybrid and Bronze categories have been combined as Other Journal-Based OA in this report because of their similar structure as journal-hosted types of OA that allow only conditional—and potentially revocable—OA.

The number of articles published annually in closed-access journals increased by 112% between 2003 and 2022 ( Figure PBS-8 ). Over the same period, annual publishing of Green OA articles increased by 228%, while Other Journal-Based OA articles (Hybrid and Bronze OA) increased by 198%. Gold OA articles (which are published in OA journals with no restrictions) had the largest percentage growth, from 19,089 articles in 2003 to 991,805 articles in 2022, an increase of over 5,000%. Hence, although the majority (77%) of S&E articles in 2003 whose access status is known were published in closed-access journals, fewer than half (49%) were in closed-access journals in 2022.

S&E publications, by publication access type: 2003–22

OA = open access.

Articles refer to publications from a selection of conference proceedings and peer-reviewed journals in S&E fields from Scopus. Articles are classified by their year of publication. OA types are mutually exclusive. For articles published under multiple OA types, the article will be counted as part of only the first type it matches in this list: Gold OA, Other Journal-Based OA, or Green OA. Summing all OA and closed-access article counts results in a smaller number of articles than for all S&E because the access status of some articles (e.g., those without digital object identifiers) cannot be reliably ascertained. Green articles are published in toll-access journals but archived in an OA archive, or "repository." These repositories may be discipline specific (like arXiv) or institutional repositories operated by universities or other institutions. Green articles may be published versions or preprints and can have any license or no license. Bronze (Other Journal-Based OA) articles are free to read on the publisher's website, without a license that grants any other rights. There may be a delay between publication and availability to read, and often articles can be removed unilaterally by the publisher. Hybrid (Other Journal-Based OA) articles are free to read at the time of publication, with an open license. These are usually published in exchange for an article processing charge. Gold articles have all the same characteristics as Hybrid articles but are published in all-OA journals, which are in turn called "Gold journals" or just "OA journals."

To conclude this section, the findings of the output analysis reveal the growth in scientific publications over time, with upper-middle-income economies exhibiting particularly large percentage increases. Meanwhile, the distribution of publications across scientific fields shows that life sciences dominated in the United States, Europe, and Japan, whereas publications in engineering and computer sciences dominated in China and India. In OA, the dramatic growth of Gold OA publications and the steady growth of publications in other OA categories show an increased shift toward open science. However, OA can impede the dissemination of some scientific research. Publishing research as OA often requires authors to pay article processing fees, which may be prohibitive for scientists in less-developed nations or whose funders do not subsidize those fees. https://www.elsevier.com/about/policies/pricing . Wiley APCs are at https://authorservices.wiley.com/author-resources/Journal-Authors/open-access/article-publication-charges.html . Springer Nature APCs are at https://www.springernature.com/gp/open-research/journals-books/journals ." data-bs-content="Many publishers make their article processing charges (APCs) known publicly. For example, a list of Elsevier APCs can be found at https://www.elsevier.com/about/policies/pricing . Wiley APCs are at https://authorservices.wiley.com/author-resources/Journal-Authors/open-access/article-publication-charges.html . Springer Nature APCs are at https://www.springernature.com/gp/open-research/journals-books/journals ." data-endnote-uuid="9a612a36-466d-4d60-af1a-57e93bd50e76">​ Many publishers make their article processing charges (APCs) known publicly. For example, a list of Elsevier APCs can be found at https://www.elsevier.com/about/policies/pricing . Wiley APCs are at https://authorservices.wiley.com/author-resources/Journal-Authors/open-access/article-publication-charges.html . Springer Nature APCs are at https://www.springernature.com/gp/open-research/journals-books/journals . The fees can be seen as shifting the costs of accessing research from readers and libraries to authors (Larivière and Sugimoto 2018).

Related Content

Sustainable Energy & Fuels

A light-driven photosynthetic microbial fuel cell for carbon-negative bioelectricity production †.

* Corresponding authors

a School of Chemical Engineering, Pusan National University, Busan, Republic of Korea E-mail: [email protected] Fax: +82.51.510.3943 Tel: +82.51.510.2393

b School of Civil & Environmental Engineering, Yonsei University, Seoul, Republic of Korea

c Institute for Environmental Energy, Pusan National University, Busan, Republic of Korea

Microbial fuel cells (MFCs) can convert chemical energy into electrical energy directly through the decomposition of organic matter by electroactive bacteria (EAB). In this process, many research groups have investigated MFCs under dark conditions, but few studies have examined those operated under light conditions. This study compared the photosynthetic MFC under light conditions (P-MFC) and MFC under dark conditions (D-MFC) for bioelectricity production and power density. The electroactive photosynthetic microbial community was enriched in the anode chamber of P-MFC. The acetate consumption and COD removal rate of the P-MFC were two times faster than that of D-MFC. The volume of effluent biogas ( e.g. , CO 2 and CH 4 ) from the decomposition of organic matter in the P-MFC was significantly lower than that from the D-MFC. Under light conditions, the electroactive photosynthetic microbial community assimilates the CO 2 produced by organic decomposition. Field emission scanning electron microscopy of P-MFC revealed aggregated electroactive cells with a fibrous appendage on the carbon surface. P-MFC also revealed a higher maximum power density (836 mW m −2 ) than D-MFC (592 mW m −2 ). This study provides a new concept for photosynthetic microbial fuel cells for bioelectricity production without CO 2 emissions.

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A light-driven photosynthetic microbial fuel cell for carbon-negative bioelectricity production

W. G. Park, M. Kim, S. Li, E. Kim, E. J. Park, J. Yoo, N. Maile, J. Jae, H. Kim and J. R. Kim, Sustainable Energy Fuels , 2024, Advance Article , DOI: 10.1039/D3SE01487H

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May 2, 2024 by Betsy Bird Leave a Comment

Research and Wishes: A Q&A with Nedda Lewers About Daughters of the Lamp

May 2, 2024 by Betsy Bird   Leave a Comment

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Let me ask you something. We’re all adults here. In spite of the fact that this site is entirely dedicated to children’s literature, I don’t fool myself into believing that any child is currently reading any of my posts (except for homework assignments which, as we all know, is 100% just fine). And since we are all adults, we know that when it comes to your lifelong career, figuring out what you’d like to do is more of a winding road than a concrete encrusted path.

Today, I’m talking to someone who started out as an educator and now has a middle grade fantasy debut that her publisher (a boutique outfit you might have heard of called Penguin Random House) is promoting out the wazoo. That’s sort of the dream, isn’t it? But how do you get there? For that, it’s time for some questions and some answers.

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But first! I need to be a bit less oblique about the subject of today’s Q&A. Nedda Lewers is the author of the new Daughters of the Lamp series, released just this past February. In terms of the first installment, here’s how her publisher describes it:

“Sahara Rashad lives by logic. Loves science. And always has a plan. Except her dad just whisked her away to her uncle’s wedding in Egypt, upending every single plan she had for the summer. In Cairo, Sahara’s days are filled with family—and mystery. First, Sahara’s cousins claim the pretentious bride-to-be is actually a witch. Then her late mother’s necklace starts glowing—and disappears. Sahara’s attempts to recover the necklace lead her to the greatest mystery yet. Deep in an underground chamber lies Ali Baba’s magical treasure. Hidden from a line of sorcerers who threatened to use its powers for evil, the treasure was given to Sahara’s ancestor Morgana for safekeeping and passed down from mother to daughter for generations. Now only Sahara stands in the sorcerers’ way. Can the girl who’s never believed in magic trust the unknown and claim her legacy as the treasure’s keeper?”

Let’s get to those questions, shall we?

Betsy Bird: Nedda! I appreciate you talking with me today and congrats on your middle grade debut! DAUGHTERS OF THE LAMP has been garnering starred reviews and praise left, right and central. Tell us a little bit about where the idea for this book came from.

Nedda Lewers: Hi Betsy! I am a huge fan of your blog. Thank you for having me on it today. It’s been so surreal and an honor to experience the warm reception Daughters of the Lamp has received before and after its debut.

Where did the idea for the story come from? That is a question I have thought a lot about but is still difficult to answer. Even though the idea for the book had been taking root and growing inside me for a long time, I believe that all art is a collaboration between the artist and the creative forces of the universe. When I began to focus on writing seven years ago, I quickly decided I wanted to write a children’s fantasy (those were my favorite stories when I was a kid) and that the main character would be Egyptian American. There were two reasons for the latter. One, it is a significant layer of the lens through which I see the world, and two, growing up, I always wished that the protagonists in the magical adventures I was reading looked and sounded more like me and the people around me. It was also important for me to incorporate the Arab folklore I grew up hearing as a child since they’d played a pivotal role in my story constructs. So, though I brought these inspirations/motivations to the page when I set out to write Daughters of the Lamp , they were transformed by the mysterious alchemy that works through us during the creative process.

  BB: You’ve mentioned in other interviews that, “I enjoyed writing, but I knew nothing about crafting a novel. At forty-one, I was a beginner again, taking writing workshops online, reading books on craft, studying works by other middle grade authors, and making new friends in the writing world virtually and in person.” I think a lot of people have the idea that they could maybe write for kids at some point, but they’d have no idea where to start. Where did you turn to for advice when you were first beginning? And from your perspective, how difficult was it to find your footing?

Nedda: I hear that from people all the time, too. And I tell them what I tell students during school visits. It took me seven years to create and ultimately get Daughters of the Lamp published. That process involved a gagillion small steps. Focusing on the small step(s) you can take every day rather than the end goal, which can feel daunting, worked for me. In the beginning, those steps were heavily comprised of revisiting numerous Arab folktales from my childhood as well as discovering new ones. When I began drafting, I found it very helpful to connect regularly with other writers in my community and virtually who held me accountable to getting words down on the page and helped me hone my craft. In many ways, the writing process is a solitary endeavor, so having people to talk with and bounce ideas off of was critical for my development as a writer and my mental health.

It took time to find my footing and my writing community, but putting my work and myself out there for workshops and mentorship programs made a huge difference. I particularly enjoy Jessica Brody’s Writing Mastery Academy website, where you can take prerecorded classes at your own pace. Her Save the Cat Novel Writing and Creating Dynamic Characters with Mary Cole courses are excellent.

BB: You integrate the story of Ali Baba continually throughout the book with the character of Morgana. Ali Baba is such an interesting story, and unfortunately a bunch of American kids don’t really encounter it all that often. Why did you specifically pick that particular Scheherazade tale? 

Nedda: Once I had decided I wanted to incorporate Arab folklore into my story, I went straight to the source of many of the tales I’d heard as a child— my parents. I asked them what their favorite stories were when they were growing up in Egypt. Both mentioned “Ali Baba and the Forty Thieves.” Though I had heard the tale numerous times and watched television adaptations of it when I traveled to Egypt as a child, it wasn’t until 2017 that I actually read a version of it. It was then that the character of Morgana, Ali Baba’s loyal servant, leaped off the page. I could not get over all the clever and courageous ways she saved Ali Baba from the thieves seeking revenge against him for finding their precious hidden chamber of treasures. I knew I had to incorporate her into my novel. But this time, I wanted her to be in charge of her narrative, to be the hero of her story. What if a magical legacy started with her and got passed down from generation to generation of the women in her family until one thousand years later, it reached twelve-year-old Egyptian-American Sahara Rashad? That was a story I really wanted to tell. I love reading books that show how our ancestors’ journeys, triumphs, and struggles impact our own lives in the present. The Joy Luck Club and Holes are two of my favorites.

BB: What kind of research did you do to make Morgana’s story as accurate as possible? Can you tell us some of the sources you used when looking into tenth-century Baghdad?

Nedda: Great question. I love research and have found that the discoveries I make during the process add a unique dimension to my story and often inspire some of my favorite scenes. From the get-go, I set out to make Morgana’s world as accurate as possible for the time and place she lived. Thanks to my local library’s research database, I was able to access university papers and articles on the medieval Islamic period. The books, Daily Life in the Medieval Islamic World by James E. Lindsay and Medieval Islamic Civilization: An Encyclopedia edited by Josef W. Meri, were extremely helpful during this phase for getting a sense of the social, cultural, and political elements of the Middle East in the 10 th century. I also consulted maps based on the time to plan Morgana’s journey when she leaves Baghdad, as well as online photos of museum artifacts to inform the physical aspects of her world.

BB: In your own life growing up you had family, much like your main character Sahara, that lived in Egypt and that you visited. But while Sahara shares that fact with you, no one would say that she IS you. When crafting a character that has similarities to your own life, how do you find ways to distinguish that person from yourself? What do you do to hone in on their character? How do you make them unique?

Nedda: I would say part of my growth as a writer has been learning to draw on my personal experiences and emotions, as these are critical in writing relatable characters, while being conscious of not creating a character that mirrors me. This process requires both intentionality and craft. One way I hope I achieved it was by thinking about who Sahara was before readers meet her on the page. I wrote passages of past experiences she had that were different than my own and had a lot to do with the wound of experiencing loss at a young age. Those scenes didn’t end up in the book, but they impact Sahara’s motivations in the present and inform her logic-minded and plan-obsessed personality, which serves to protect her from the volatility of life. While Sahara is extremely skeptical of magic when we meet her, I am the opposite. Especially when I was a child—if there was a star in the sky to wish on, I found it.

BB: Was there anything in the book that you originally included and then had to eventually remove?

Nedda: Not so much removed as replaced. Sahara’s first chapter initially took place during a relay race at school instead of the end-of-year fair. But as I tweaked it to reveal to readers more about Sahara—how she operates in life, what she is grappling with— and to set the stage for what’s to come, the elements of the fair, particularly her response to the rigged games and the palm reader whom she nicknames “Woo-Woo Rapunzel,” served the story better.

BB: And how much did the first draft of your books resemble the final product? And what did your editor do specifically to guide the process?

Nedda: Daughters of the Lamp is not only my debut but also the first novel I have ever written. As you can imagine, there was a lot of learning and revising along the way to get what I envisioned in my head and what I had written on paper to match. Integrating feedback from critique partners and beta readers, my Author Mentor Match mentor, my agent, and my editor was invaluable. While most of the big picture elements had been solidified by the time the manuscript got to my editor, Polo Orozco, he was pivotal in helping me ensure that the themes I wanted to explore were pulled all the way through the novel and embedded in such a way that they didn’t feel didactic but organic to the plot. He had such an understanding of the heart of the story that if something I’d included veered from that, he helped me course correct. My experience working with Polo was different on Book 2 of the series because he was involved from the beginning stages of outlining. That was so much fun!

BB: Finally, I know you’ve a sequel on the horizon in (checks watch) June! What else are you working on these days? What else is in your future?

Nedda: I know—I can’t believe the sequel, Children of the Wind , comes out so soon. I’m thrilled that kids who enjoy the first book won’t have to wait long for the second! Currently, I am working on a new middle grade story that involves Egyptology and the challenging issue of repatriation. I love writing middle grade and the opportunity it affords to explore with readers the complicated and messy areas in life, the ones that much to Sahara’s chagrin, don’t fit neatly into a box or a plan.

Big thanks to Nedda for so kindly answering all my questions today. And no need to wait. You can grab yourself a copy of Daughters of the Lamp immediately at your local bookstore or library, so run on out and get it if you can!

Filed under: Interviews

About Betsy Bird

Betsy Bird is currently the Collection Development Manager of the Evanston Public Library system and a former Materials Specialist for New York Public Library. She has served on Newbery, written for Horn Book, and has done other lovely little things that she'd love to tell you about but that she's sure you'd find more interesting to hear of in person. Her opinions are her own and do not reflect those of EPL, SLJ, or any of the other acronyms you might be able to name. Follow her on Twitter: @fuseeight.

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