Hydrogen production

{{Short description|Industrial production of molecular hydrogen}} [[Hydrogen]] gas is produced by several industrial methods.<ref>{{cite journal |last1=Fan |first1=Lixin |last2=Tu |first2=Zhengkai |last3=Chan |first3=Siew Hwa |title=Recent development of hydrogen and fuel cell technologies: A review |journal=[[Energy Reports]] |date=2021 |volume=7 |pages=8421–8446 |doi=10.1016/j.egyr.2021.08.003 |doi-access=free|bibcode=2021EnRep...7.8421F }}</ref> Nearly all of the world's current supply of hydrogen is created from fossil fuels.<ref>{{cite news |last1=Reed |first1=Stanley |last2=Ewing |first2=Jack |title=Hydrogen Is One Answer to Climate Change. Getting It Is the Hard Part |url=https://www.nytimes.com/2021/07/13/business/hydrogen-climate-change.html |work=The New York Times |date=13 July 2021 }}</ref><ref name="rosenow-2022">{{cite journal |last1=Rosenow |first1=Jan |date=27 September 2022 |title=Is heating homes with hydrogen all but a pipe dream? An evidence review |journal=Joule |volume=6 |issue=10 |pages=2225–2228 |doi=10.1016/j.joule.2022.08.015 |doi-access=free |bibcode=2022Joule...6.2225R }}</ref> Most hydrogen is ''gray hydrogen'' made through [[steam methane reforming]]. In this process, hydrogen is produced from a chemical reaction between steam and [[methane]], the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide.<ref name=":7">{{Cite web |last1=Bonheure |first1=Mike |last2=Vandewalle |first2=Laurien A. |last3=Marin |first3=Guy B. |last4=Van Geem |first4=Kevin M. |date=March 2021 |title=Dream or Reality? Electrification of the Chemical Process Industries |url=https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |url-status=live |archive-url=https://web.archive.org/web/20210717132733/https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |archive-date=17 July 2021 |access-date=6 July 2021 |website=CEP Magazine |publisher=[[American Institute of Chemical Engineers]]}}</ref> When [[carbon capture and storage]] is used to remove a large fraction of these emissions, the product is known as ''[[blue hydrogen]]''.<ref name=":25">{{cite journal |last1=Griffiths |first1=Steve |last2=Sovacool |first2=Benjamin K. |last3=Kim |first3=Jinsoo |last4=Bazilian |first4=Morgan |last5=Uratani |first5=Joao M. |title=Industrial decarbonization via hydrogen: A critical and systematic review of developments, socio-technical systems and policy options |journal=Energy Research & Social Science |date=October 2021 |volume=80 |article-number=102208 |doi=10.1016/j.erss.2021.102208 |bibcode=2021ERSS...8002208G |url=https://pure.au.dk/portal/da/publications/industrial-decarbonization-via-hydrogen(a0793820-c310-4750-8ee6-138e11a277fe).html }}</ref>

''[[Green hydrogen]]'' is usually understood to be produced from [[Renewable energy|renewable]] electricity via [[electrolysis]] of water.<ref name=":3">{{cite journal |last1=Squadrito |first1=Gaetano |last2=Maggio |first2=Gaetano |last3=Nicita |first3=Agatino |title=The green hydrogen revolution |journal=Renewable Energy |date=November 2023 |volume=216 |article-number=119041 |doi=10.1016/j.renene.2023.119041 |doi-access=free |bibcode=2023REne..21619041S }}</ref><ref name=":2" /> Less frequently, definitions of ''green hydrogen'' include hydrogen produced from other low-emission sources such as [[Biomass (energy)|biomass]].<ref name=":32">{{cite journal |last1=Squadrito |first1=Gaetano |last2=Maggio |first2=Gaetano |last3=Nicita |first3=Agatino |title=The green hydrogen revolution |journal=Renewable Energy |date=November 2023 |volume=216 |article-number=119041 |doi=10.1016/j.renene.2023.119041 |doi-access=free|bibcode=2023REne..21619041S }}</ref> Producing green hydrogen is currently more expensive than producing gray hydrogen, and the efficiency of energy conversion is inherently low.<ref>{{cite journal |last1=Johnson |first1=Nathan |last2=Liebreich |first2=Michael |last3=Kammen |first3=Daniel M. |last4=Ekins |first4=Paul |last5=McKenna |first5=Russell |last6=Staffell |first6=Iain |title=Realistic roles for hydrogen in the future energy transition |journal=Nature Reviews Clean Technology |date=2025-04-22 |volume=1 |pages=351–371 |doi=10.1038/s44359-025-00050-4 |url=https://www.nature.com/articles/s44359-025-00050-4}}</ref><ref name=":18">{{Cite web |last1=Evans |first1=Simon |last2=Gabbatiss |first2=Josh |date=30 November 2020 |title=In-depth Q&A: Does the world need hydrogen to solve climate change? |url=https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |url-status=live |archive-url=https://web.archive.org/web/20201201155033/https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |archive-date=1 December 2020 |access-date=1 December 2020 |website=[[Carbon Brief]]}}</ref> Other methods of '''hydrogen production''' include [[biomass]] [[gasification]], [[methane pyrolysis]], extraction of underground [[natural hydrogen]],<ref name="natural-hydrogen1">{{cite web |title=Natural Hydrogen: A Potential Clean Energy Source Beneath Our Feet |url=https://e360.yale.edu/features/natural-geologic-hydrogen-climate-change |access-date=2024-03-23 |website=Yale E360 |language=en-US}}</ref><ref name="Hassanpouryouzband foraging or farming">{{cite journal |last1=Hassanpouryouzband |first1=Aliakbar |last2=Wilkinson |first2=Mark |last3=Haszeldine |first3=R. Stuart |title=Hydrogen energy futures – foraging or farming? |journal=Chemical Society Reviews |date=2024 |volume=53 |issue=5 |pages=2258–2263 |doi=10.1039/D3CS00723E |doi-access=free |pmid=38323342 |bibcode=2024CSRev..53.2258H |hdl=20.500.11820/b23e204c-744e-44f6-8cf5-b6761775260d |hdl-access=free}}</ref> and ''in situ'' hydrogen synthesis.<ref name="insitu1">{{cite journal |last1=Hanfi |first1=Mohamed Abdalsalam |last2=Alade |first2=Olalekan Saheed |last3=Tanimu |first3=Abdulkadir |last4=Mahmoud |first4=Mohamed |last5=Alarifi |first5=Sulaiman A. |title=Catalytic and Noncatalytic in Situ Hydrogen Production from Heavy Oil: A Review of Experimental Studies |journal=ACS Omega |year=2024 |volume=9 |issue=51 |pages=50118–50133 |doi=10.1021/acsomega.4c07383 |doi-access=free|pmid=39741833 |pmc=11683620 }}</ref><ref name="insitu2">{{cite web |title=In-Situ Hydrogen Production Process (Press release) |website=The Journal of Petroleum Technology |date=July 22, 2024 |url=https://jpt.spe.org/skoltech-pioneers-in-situ-hydrogen-production-process|access-date=2025-09-19}}</ref>

As of 2023, less than 1% of dedicated hydrogen production is low-carbon, i.e. blue hydrogen, green hydrogen, and hydrogen produced from biomass.<ref name=":23">{{Cite web |date=10 July 2023 |title=Hydrogen |url=https://www.iea.org/energy-system/low-emission-fuels/hydrogen |access-date=2023-09-21 |website=IEA |at="Energy" section |language=en-GB}}</ref>

In 2020, roughly 87 million tons of hydrogen was produced<ref name="Leigh2021">{{cite web |last1=Collins |first1=Leigh |date=2021-05-18 |title=A net-zero world 'would require 306 million tonnes of green hydrogen per year by 2050': IEA {{!}} Recharge |url=https://www.rechargenews.com/energy-transition/a-net-zero-world-would-require-306-million-tonnes-of-green-hydrogen-per-year-by-2050-iea/2-1-1011920 |url-status=live |archive-url=https://web.archive.org/web/20210521172338/https://www.rechargenews.com/energy-transition/a-net-zero-world-would-require-306-million-tonnes-of-green-hydrogen-per-year-by-2050-iea/2-1-1011920 |archive-date=2021-05-21 |website=Recharge {{!}} Latest renewable energy news |language=en}}</ref> worldwide for various uses, such as [[oil refining]], in the production of [[ammonia]] through the [[Haber process]], and in the production of [[methanol]] through reduction of [[carbon monoxide]]. The global hydrogen generation market was fairly valued at US$155 billion in 2022, and expected to grow at a compound annual growth rate of 9.3% from 2023 to 2030.<ref>{{Cite web|url=https://www.grandviewresearch.com/industry-analysis/hydrogen-generation-market|title=Hydrogen Generation Market Size And Share Report, 2030|website=www.grandviewresearch.com}}</ref>

== Overview ==

Molecular hydrogen was discovered in the [[Kola Superdeep Borehole]]. It is unclear how much molecular hydrogen is available in natural reservoirs, but at least one company<ref>{{Cite web |title=Natural Hydrogen Energy LLC |url=http://www.nh2e.com/ |url-status=live |archive-url=https://web.archive.org/web/20201025191409/http://nh2e.com/ |archive-date=2020-10-25 |access-date=2020-09-29}}</ref> specializes in drilling wells to extract hydrogen. Most hydrogen in the [[lithosphere]] is bonded to oxygen in water.

Manufacturing elemental hydrogen requires the consumption of a hydrogen carrier such as a fossil fuel or water. The former carrier consumes the fossil resource and in the steam methane reforming (SMR) process produces greenhouse gas carbon dioxide. However, in the newer [[methane pyrolysis]] process no carbon dioxide is produced. These processes typically require no further energy input beyond the fossil fuel. [[File:SMR+WGS-1.png|thumb|Illustrating inputs and outputs of steam reforming of natural gas, a process to produce hydrogen. {{as of|2020}}, the carbon sequestrastion step is not in commercial use.]] [[Chemical decomposition|Decomposing]] water, the latter carrier, requires electrical or heat input, generated from some [[primary energy]] source (fossil fuel, [[nuclear power]] or a [[renewable energy]]). Hydrogen produced by electrolysis of water using renewable energy sources such as wind and [[solar power]], referred to as [[green hydrogen]].<ref>{{cite web |title=Definition of Green Hydrogen |url=http://www.cleanenergypartnership.de/fileadmin/pdf/Definition_of_Green_Hydrogen.pdf |access-date=2014-09-06 |publisher=[[Clean Energy Partnership]]}}{{dead link|date=September 2017|bot=InternetArchiveBot|fix-attempted=yes}}</ref> When derived from natural gas by zero greenhouse emission methane pyrolysis, it is referred to as turquoise hydrogen.<ref name="Schneider et al 20202">{{cite journal |last1=Schneider |first1=Stefan |last2=Bajohr |first2=Siegfried |last3=Graf |first3=Frank |last4=Kolb |first4=Thomas |date=October 2020 |title=State of the Art of Hydrogen Production via Pyrolysis of Natural Gas |journal=ChemBioEng Reviews |volume=7 |issue=5 |pages=150–158 |doi=10.1002/cben.202000014 |doi-access=free}}</ref>

When fossil fuel derived with [[greenhouse gas emissions]], is generally referred to as [[grey hydrogen]]. If most of the carbon dioxide emission is captured, it is referred to as blue hydrogen.<ref>{{Cite web |last=Sampson |first=Joanna |date=11 February 2019 |title=Blue hydrogen for a green future |url=https://www.gasworld.com/blue-hydrogen-for-a-green-future/2016596.article |url-status=live |archive-url=https://web.archive.org/web/20190509143817/https://www.gasworld.com/blue-hydrogen-for-a-green-future/2016596.article |archive-date=2019-05-09 |access-date=2019-06-03 |website=gasworld |language=en}}</ref> Hydrogen produced from coal may be referred to as brown or black hydrogen.<ref>{{Cite web |title=Brown coal the hydrogen economy stepping stone {{!}} ECT |url=http://www.ectltd.com.au/brown-coal-the-hydrogen-economy-stepping-stone/ |url-status=live |archive-url=https://web.archive.org/web/20190408101511/http://www.ectltd.com.au/brown-coal-the-hydrogen-economy-stepping-stone/ |archive-date=2019-04-08 |access-date=2019-06-03 |language=en-US}}</ref>

=== {{anchor|white|grey|black|brown|yellow|red|pink|violet|blue|turquoise|green}}Classification based on production method === Hydrogen is often referred to by various colors to indicate its origin (perhaps because gray symbolizes "dirty hydrogen"<ref name=":1" />).<ref>{{Cite web |title=Hydrogen Color Explained |url=https://www.sensonic.com/en/blog/hydrogen-color-explained--3242/ |access-date=2023-11-22 |website=Sensonic |language=en}}</ref><ref name="national-grid-group-undated">{{cite web |author=national grid |title=The hydrogen colour spectrum |url=https://www.nationalgrid.com/stories/energy-explained/hydrogen-colour-spectrum |access-date=2022-09-29 |work=National Grid Group |location=London, United Kingdom}}</ref><ref name="Hassanpouryouzband foraging or farming"/> {| class="wikitable" |+Colors that refer to method of production<ref>{{Cite web |title=What potential for natural hydrogen? |url=https://www.energy-observer.org/resources/natural-hydrogen |access-date=2023-07-03 |website=Energy Observer |language=en}}</ref> ! colspan=2|Color ! width=350|Production source ! width=370|Process / method / chemistry !Greenhouse gas footprint<ref>{{Cite web |title=How Green Are the National Hydrogen Strategies? |url=https://www.researchgate.net/publication/358515834 |access-date=10 October 2024}}</ref> !Notes ! References |- | width=130|[[Green hydrogen|Green]] || width=20 style="background:#90EE90;"| | Renewable electricity: wind, solar, hydro, tidal, geothermal. May also include electricity from low-emission sources such as [[Biomass (energy)|biomass]]. |[[Electrolysis]] of water 2 H2O → 2 H2 + O2 |Minimal | | <ref name=":2">{{Cite web |last=Deign |first=Jason |date=2020-06-29 |title=So, What Exactly Is Green Hydrogen? |url=https://www.greentechmedia.com/articles/read/green-hydrogen-explained |url-status=deviated |archive-url=https://web.archive.org/web/20220323195427/https://www.greentechmedia.com/articles/read/green-hydrogen-explained |archive-date=2022-03-23 |access-date=2022-02-11 |website=Greentechmedia}}</ref> |- | Turquoise || {{coltit|turquoise}} | Fossil hydrocarbons: natural gas, a.k.a. [[Methane]] | [[Methane pyrolysis]] (thermal splitting) CH4 → C + 2 H2 |Minimal |Solid carbon byproduct | {{r|bmwi-2020|pages=28}} {{r|van-de-graaf-etal-2020|pages=2}} |- | Blue || {{coltit|1278C0}} | Fossil hydrocarbons: natural gas | Gas reforming with [[carbon capture and storage]] 1st stage: CH4 + H2O → CO + 3 H2

2nd stage: CO + H2O → CO2 + H2 |Low |CCS networks required | {{r|bmwi-2020|pages=28}} |- | Gray || {{coltit|C0C0C0}} | Fossil hydrocarbons: natural gas |[[Steam reforming]] of natural gas 1st stage: CH4 + H2O → CO + 3 H2

2nd stage: CO + H2O → CO2 + H2 |High |CO2 produced | {{r|bmwi-2020|pages=28}} {{r|sansom-etal-2020|pages=10}} {{r|van-de-graaf-etal-2020|pages=2}} |- |Black || {{coltit|000000}} |Fossil hydrocarbons: [[Coal]] ([[anthracite]]) |Coal [[carbonisation]] or [[gasification]] 1st stage: 3 C (i.e., coal) + O2 + H2O → H2 + 3 CO

2nd stage: CO + H2O → CO2 + H2

C24H12 + 12 O2 → 24 CO + 6 H2 |Very high |CO2 produced |<ref name=":0">{{Cite web |title=The colors of hydrogen: Expanding ways of decarbonization |url=https://spectra.mhi.com/the-colors-of-hydrogen-expanding-ways-of-decarbonization |access-date=7 October 2024}}</ref> |- | Brown || {{coltit|664800}} | Fossil hydrocarbons: brown coal ([[lignite]]) | Coal carbonisation or gasification as black hydrogen |Very high |CO2 produced | {{r|bruce-etal-2018|pages=91}} |- |Red || {{coltit|red}} |Nuclear power |Nuclear heat: thermolysis [[Thermochemical cycle|Thermochemical water splitting]] H2O(''l'') ⇌ H2(''g'') + 1/2 O2(''g'')   |Minimal | |<ref name=":0" /> |- |Mauve|| {{coltit|E0B0FF}} |Decay heat from radioactive isotopes |Radiogenic heat: thermolysis [[Thermochemical cycle|Thermochemical water splitting]] H2O(''l'') ⇌ H2(''g'') + 1/2 O2(''g'')   |Minimal | |<ref name=":0" /> |- |Pink|| {{coltit|pink}} |Nuclear power |Nuclear electricity plus water: electrolysis 2 H2O → 2 H2 + O2 |Minimal | |<ref name=":0" /> |- | Purple || {{coltit|BA55D3}} | Nuclear power | Nuclear heat plus water: [[Electrolysis]] and thermolysis 2 H2O → 2 H2 + O2 |Minimal |Also contributing steam to natural gas reforming | {{r|van-de-graaf-etal-2020|pages=2}}<ref name=":1">{{Cite news |title=Can a viable industry emerge from the hydrogen shakeout? |newspaper=The Economist |url=https://www.economist.com/business/2023/07/03/can-a-viable-industry-emerge-from-the-hydrogen-shakeout |access-date=2023-09-26 }}</ref> |- | Yellow || {{coltit|yellow}} | Solar photovoltaics | Electrolysis 2 H2O → 2 H2 + O2 |Minimal | | <ref name="national-grid-group-undated" /> |- | [[Gold hydrogen|Gold]] || {{coltit|gold}} | Hydrogen | Microbial activity in depleted oil wells, drilling |Low |CCS networks required | <ref name="department-of-earth-sciences-2022">{{cite web |author=Department of Earth Sciences |date=12 September 2022 |title=Gold hydrogen |url=https://www.earth.ox.ac.uk/2021/09/gold-hydrogen-oxford-earth-sciences-in-the-times/ |access-date=2022-09-29 |work=Department of Earth Sciences, Oxford University |location=Oxford, United Kingdom}}</ref><ref name="Hassanpouryouzband foraging or farming"/> |- |White|| {{coltit|white}} |Hydrogen occurring naturally in underground deposits |Drilling, mining |Minimal | |<ref name=":0" /> |}

== Current production methods == {{see also|Timeline of hydrogen technologies}}

=== Steam reforming – gray or blue ===

Hydrogen is industrially produced from [[steam reforming]] (SMR), which uses natural gas.<ref>{{cite web |date=December 2008 |title=Actual Worldwide Hydrogen Production from ... |url=http://www.fair-pr.de/background/worldwide-hydrogen-production-analysis.php |archive-url=https://web.archive.org/web/20150202194422/http://www.fair-pr.de/background/worldwide-hydrogen-production-analysis.php |archive-date=2015-02-02 |access-date=2008-05-09 |publisher=Arno A Evers}}</ref> The energy content of the produced hydrogen is around 74% of the energy content of the original fuel,<ref>{{cite book |doi=10.1016/B978-0-12-409548-9.10117-4 |chapter=Production of Hydrogen |title=Encyclopedia of Sustainable Technologies |date=2017 |last1=Velazquez Abad |first1=A. |last2=Dodds |first2=P.E. |pages=293–304 |isbn=978-0-12-804792-7 }}</ref> as some energy is lost as excess heat during production. In general, steam reforming emits carbon dioxide, a greenhouse gas, and is known as gray hydrogen. If the carbon dioxide is captured and stored, the hydrogen produced is known as blue hydrogen.

Steam methane reforming (SMR) produces hydrogen from natural gas, mostly methane (CH4), and water. It is the cheapest source of industrial hydrogen, being the source of nearly 50% of the world's hydrogen.<ref>{{cite journal |last1=Dincer |first1=Ibrahim |last2=Acar |first2=Canan |title=Review and evaluation of hydrogen production methods for better sustainability |journal=International Journal of Hydrogen Energy |date=September 2015 |volume=40 |issue=34 |pages=11094–11111 |doi=10.1016/j.ijhydene.2014.12.035 |bibcode=2015IJHE...4011094D }}</ref> The process consists of heating the gas to {{Convert|700–1100|C|F|abbr=on|sigfig=2}} in the presence of steam over a [[nickel]] [[catalyst]]. The resulting [[endothermic reaction]] forms carbon monoxide and molecular hydrogen (H2).<ref name="rotech">{{cite book|last1=Press|first1=Roman J.|title=Introduction to hydrogen Technology|last2=Santhanam|first2=K. S. V.|last3=Miri|first3=Massoud J.|last4=Bailey|first4=Alla V.|last5=Takacs|first5=Gerald A.|publisher=John Wiley & Sons|year=2008|isbn=978-0-471-77985-8|page=249}}</ref>

In the [[water-gas shift reaction]], the carbon monoxide reacts with steam to obtain further quantities of H2. The WGSR also requires a catalyst, typically over [[iron oxide]] or other [[oxide]]s. The byproduct is CO2.<ref name="rotech"/> Depending on the quality of the [[feedstock]] (natural gas, [[naphtha]], etc.), one ton of hydrogen produced will also produce 9 to 12 tons of CO2, a greenhouse gas that may be [[Carbon capture and storage|captured]].<ref>{{cite web |title= Hydrogen Production via Steam Reforming with CO2 Capture | first = Guido | last = Collodi | date = 2010-03-11 | website = CISAP4 4th International Conference on Safety and Environment in the Process Industry | url = http://www.aidic.it/CISAP4/webpapers/7Collodi.pdf | access-date = 2015-11-28}}</ref>

For this process, high temperature steam (H2O) reacts with methane (CH4) in an endothermic reaction to yield [[syngas]].<ref>{{cite web | title = HFCIT Hydrogen Production: Natural Gas Reforming | publisher = U.S. Department of Energy | date = 2008-12-15 | url = http://www1.eere.energy.gov/hydrogenandfuelcells/production/natural_gas.html}}</ref>

:CH4 + H2O → CO + 3 H2

In a second stage, additional hydrogen is generated through the lower-temperature, [[Exothermic reaction|exothermic]], water-gas shift reaction, performed at about {{Convert|360|C|F|abbr=on}}:

:CO + H2O → CO2 + H2

Essentially, the [[oxygen]] (O) atom is stripped from the additional water (steam) to oxidize CO to CO2. This oxidation also provides energy to maintain the reaction. Additional heat required to drive the process is generally supplied by burning some portion of the methane.

===Electrified Steam Methane Reforming=== In May 2019, Science published the results of a Danish study in which the tin catalyst is heated electrically. This reduces natural gas consumption (and CO2-emission) by a third, while the improved heating increases the overall efficiency. SMR needs 4,2 kWh/Nm3 H2, eSMR 3,6 kWh/Nm3 H2 (2,6 kWh natural gas and 1.0 kWh electricity).<ref>{{cite journal |last1=From |first1=Thomas N. |last2=Partoon |first2=Behzad |last3=Rautenbach |first3=Marené |last4=Østberg |first4=Martin |last5=Bentien |first5=Anders |last6=Aasberg-Petersen |first6=Kim |last7=Mortensen |first7=Peter M. |title=Electrified steam methane reforming of biogas for sustainable syngas manufacturing and next-generation of plant design: A pilot plant study |journal=Chemical Engineering Journal |date=January 2024 |volume=479 |article-number=147205 |doi=10.1016/j.cej.2023.147205 |bibcode=2024ChEnJ.47947205F |url=https://pure.au.dk/portal/en/publications/289d9169-5a24-4da0-b347-7088a36efe81 }}</ref>

===From water=== {{Main|Water splitting}} Methods to produce hydrogen without the use of fossil fuels involve the process of [[water splitting]], or splitting the water molecule (H2O) into its components oxygen and hydrogen. When the source of energy for water splitting is renewable or low-carbon, the hydrogen produced is sometimes referred to as [[green hydrogen]]. The conversion can be accomplished in several ways, but all methods are currently considered more expensive than fossil-fuel based production methods.

=== Electrolysis of water – green, pink or yellow ===

{{Main|Electrolysis of water}} {{see also|High-temperature electrolysis|High-pressure electrolysis}}Hydrogen can be made via [[high pressure electrolysis]], low pressure electrolysis of water, or a range of other emerging electrochemical processes such as high temperature electrolysis or carbon assisted electrolysis.<ref>{{cite journal |last1=Badwal |first1=Sukhvinder P. S. |last2=Giddey |first2=Sarbjit S. |last3=Munnings |first3=Christopher |last4=Bhatt |first4=Anand I. |last5=Hollenkamp |first5=Anthony F. |date=24 September 2014 |title=Emerging electrochemical energy conversion and storage technologies |journal=Frontiers in Chemistry |volume=2 |page=79 |bibcode=2014FrCh....2...79B |doi=10.3389/fchem.2014.00079 |pmc=4174133 |pmid=25309898 |doi-access=free}}</ref> However, current best processes for water electrolysis have an effective electrical efficiency of 70-80%,<ref>{{cite web |author=Werner Zittel |author2=Reinhold Wurster |date=1996-07-08 |title=Chapter 3: Production of Hydrogen. Part 4: Production from electricity by means of electrolysis |url=http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html#3.4 |url-status=live |archive-url=https://web.archive.org/web/20070207080325/http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html#3.4 |archive-date=2007-02-07 |access-date=2010-10-01 |work=HyWeb: Knowledge – Hydrogen in the Energy Sector |publisher=Ludwig-Bölkow-Systemtechnik GmbH}}</ref><ref>{{cite web |author=Bjørnar Kruse |author2=Sondre Grinna |author3=Cato Buch |date=2002-02-13 |title=Hydrogen – Status and Possibilities |url=http://www.bellona.org/reports/hydrogen |archive-url=https://web.archive.org/web/20110702210000/http://www.bellona.org/reports/hydrogen |archive-date=2011-07-02 |publisher=The Bellona Foundation |format=PDF |quote=Efficiency factors for PEM electrolysers up to 94% are predicted, but this is only theoretical at this time.}}</ref><ref>{{cite web |title=high-rate and high efficiency 3D water electrolysis |url=http://www.grid-shift.com/white_papers/docs/3D_Water_Electrolysis_Abstract%202.htm |archive-url=https://web.archive.org/web/20120322204531/http://www.grid-shift.com/white_papers/docs/3D_Water_Electrolysis_Abstract%202.htm |archive-date=2012-03-22 |access-date=2011-12-13 |publisher=Grid-shift.com}}</ref> so that producing 1 kg of hydrogen (which has a [[specific energy]] of 143 MJ/kg or about 40 kWh/kg) requires 50–55 kWh of electricity.

In parts of the world, steam methane reforming is between $1–3/kg on average excluding hydrogen gas pressurization cost. This makes production of hydrogen via electrolysis cost competitive in many regions already, as outlined by Nel Hydrogen<ref>{{cite web |title=Wide Spread Adaption of Competitive Hydrogen Solution |url=http://nelhydrogen.com/assets/uploads/2018/03/2018-03-02-FC-EXPO-Nel_FINAL.pdf |url-status=live |archive-url=https://web.archive.org/web/20180422202814/http://nelhydrogen.com/assets/uploads/2018/03/2018-03-02-FC-EXPO-Nel_FINAL.pdf |archive-date=2018-04-22 |access-date=22 April 2018 |website=nelhydrogen.com |publisher=Nel ASA}}</ref> and others, including an article by the IEA<ref>{{cite web |last1=Philibert |first1=Cédric |title=Commentary: Producing industrial hydrogen from renewable energy |url=https://www.iea.org/newsroom/news/2017/april/producing-industrial-hydrogen-from-renewable-energy.html |url-status=live |archive-url=https://web.archive.org/web/20180422202826/https://www.iea.org/newsroom/news/2017/april/producing-industrial-hydrogen-from-renewable-energy.html |archive-date=22 April 2018 |access-date=22 April 2018 |website=iea.org |publisher=International Energy Agency}}</ref> examining the conditions which could lead to a competitive advantage for electrolysis.

A small part (2% in 2019<ref>{{Harvnb|IEA H2|2019|p=37}}</ref>) is produced by electrolysis using electricity and water, consuming approximately 50 to 55 kilowatt-hours of electricity per kilogram of hydrogen produced.<ref>{{cite web |title=How Much Electricity/Water Is Needed to Produce 1 kg of H2 by Electrolysis? |url=https://cleanenergypartnership.de/en/faq/hydrogen-production-and-storage/?scroll=true |url-status=live |archive-url=https://web.archive.org/web/20200617074422/https://cleanenergypartnership.de/en/faq/hydrogen-production-and-storage/?scroll=true |archive-date=17 June 2020 |access-date=17 June 2020}}</ref>

[[File:Hydrogen production via Electrolysis.png|alt=Hydrogen production via Electrolysis graphic|thumb|upright=1.35|Illustrating inputs and outputs of electrolysis of water, for production of hydrogen and no greenhouse gas]] Water electrolysis is using electricity to split water into hydrogen and oxygen. As of 2020, less than 0.1% of hydrogen production comes from water electrolysis.<ref>{{Cite web|last=Petrova|first=Magdalena|date=2020-12-04|title=Green hydrogen is gaining traction, but still has massive hurdles to overcome|url=https://www.cnbc.com/2020/12/04/green-hydrogen-is-gaining-traction-but-it-must-overcome-big-hurdles.html|access-date=2021-06-20|website=[[CNBC]]|language=}}</ref> [[Electrolysis of water]] is 70–80% efficient (a 20–30% conversion loss)<ref>{{cite web|title=ITM – Hydrogen Refuelling Infrastructure – February 2017|url=http://www.level-network.com/wp-content/uploads/2017/02/ITM-Power.pdf|website=level-network.com|access-date=17 April 2018|ref=pg12}}</ref><ref>{{cite web|title=Cost reduction and performance increase of PEM electrolysers|url=http://www.fch.europa.eu/sites/default/files/Nov22_Session3_Panel%205_Slot%202_NOVEL-MEGASTACK_Thomassen%20%28ID%202891376%29.pdf|website=fch.europa.eu|publisher=Fuel Cells and Hydrogen Joint Undertaking|access-date=17 April 2018|ref=pg9}}</ref> while [[steam reforming]] of natural gas has a [[thermal efficiency]] between 70 and 85%.<ref name="Steam reforming">{{cite journal|title=Hydrogen Production Technologies: Current State and Future Developments|journal=Conference Papers in Energy|year=2013|doi=10.1155/2013/690627|doi-access=free|last1=Kalamaras|first1=Christos M.|last2=Efstathiou|first2=Angelos M.|volume=2013|pages=1–9}}</ref> The electrical efficiency of electrolysis is expected to reach 82–86%<ref>{{cite web|title=Cost reduction and performance increase of PEM electrolysers|url=http://www.fch.europa.eu/sites/default/files/Nov22_Session3_Panel%205_Slot%202_NOVEL-MEGASTACK_Thomassen%20%28ID%202891376%29.pdf|website=fch.europa.eu|publisher=Fuel Cell and Hydrogen Joint Undertaking|access-date=17 April 2018|ref=pg21}}</ref> before 2030, while also maintaining durability as progress in this area continues apace.<ref>{{cite web|title=Report and Financial Statements 30 April 2016|url=http://www.itm-power.com/wp-content/uploads/2016/07/ITM-Annual-Report-2016.pdf|website=itm-power.com|access-date=17 April 2018|ref=pg27}}</ref>

Water electrolysis can operate at {{Convert|50-80|C|F|sigfig=2}}, while steam methane reforming requires temperatures at {{Convert|700-1100|C|F|sigfig=2}}.<ref>{{cite web|title=Hydrogen Production: Natural Gas Reforming|url=https://www.energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming|website=energy.gov|publisher=US Department of Energy|access-date=17 April 2018}}</ref> The difference between the two methods is the primary energy used; either electricity (for electrolysis) or natural gas (for steam methane reforming). Due to their use of water, a readily available resource, electrolysis and similar water-splitting methods have attracted the interest of the scientific community. With the objective of reducing the cost of hydrogen production, renewable sources of energy have been targeted to allow electrolysis.<ref name="Hordeski"/>

There are three main types of [[electrolytic cell]]s, [[Solid oxide electrolyzer cell|solid oxide electrolyser cells]] (SOECs), [[Polymer electrolyte membrane electrolysis|polymer electrolyte membrane cells]] (PEM) and [[alkaline electrolysis cell]]s (AECs).<ref>{{cite journal |last1=Badwal |first1=Sukhvinder P.S. |last2=Giddey |first2=Sarbjit |last3=Munnings |first3=Christopher |title=Hydrogen production via solid electrolytic routes |journal=WIREs Energy and Environment |date=September 2013 |volume=2 |issue=5 |pages=473–487 |doi=10.1002/wene.50 |bibcode=2013WIREE...2..473B }}</ref> Traditionally, alkaline electrolysers are cheaper in terms of investment (they generally use nickel catalysts), but less-efficient; PEM electrolysers, conversely, are more expensive (they generally use expensive [[platinum group]] metal catalysts) but are more efficient and can operate at higher [[current densities]], and can therefore be possibly cheaper if the hydrogen production is large enough.<ref>{{cite journal |last1=Sebbahi |first1=Seddiq |last2=Nabil |first2=Nouhaila |last3=Alaoui-Belghiti |first3=Amine |last4=Laasri |first4=Said |last5=Rachidi |first5=Samir |last6=Hajjaji |first6=Abdelowahed |title=Assessment of the three most developed water electrolysis technologies: Alkaline Water Electrolysis, Proton Exchange Membrane and Solid-Oxide Electrolysis |journal=Materials Today: Proceedings |date=2022 |volume=66 |pages=140–145 |doi=10.1016/j.matpr.2022.04.264 |bibcode=2022MTPro..66..140S }}</ref>

SOECs operate at high temperatures, typically around {{Convert|800|C|F|sigfig=2}}. At these high temperatures, a significant amount of the energy required can be provided as thermal energy (heat), and as such is termed [[high-temperature electrolysis]]. The heat energy can be provided from a number of different sources, including waste industrial heat, [[nuclear power stations]] or concentrated [[Solar thermal collector|solar thermal plants]]. This has the potential to reduce the overall cost of the hydrogen produced by reducing the amount of electrical energy required for electrolysis.<ref name="Ogden 1999">{{cite journal |last=Ogden |first=J.M. |title=Prospects for building a hydrogen energy infrastructure |journal=[[Annual Review of Energy and the Environment]] |year=1999 |volume=24 |pages=227–279 |doi=10.1146/annurev.energy.24.1.227| doi-access=}}</ref><ref>{{cite journal |last1=Hauch |first1=Anne |last2=Ebbesen |first2=Sune Dalgaard |last3=Jensen |first3=Søren Højgaard |last4=Mogensen |first4=Mogens |title=Highly efficient high temperature electrolysis |journal=Journal of Materials Chemistry |date=2008 |volume=18 |issue=20 |pages=2331 |doi=10.1039/b718822f }}</ref><ref>In the laboratory, water electrolysis can be done with a simple apparatus like a [[Hofmann voltameter]]:{{cite web |url= http://www.practicalphysics.org/go/Experiment_677.html |title=Electrolysis of water and the concept of charge |archive-url= https://web.archive.org/web/20100613112114/http://practicalphysics.org/go/Experiment_677.html |archive-date=2010-06-13 }}</ref><ref>{{cite press release |title=Nuclear power plants can produce hydrogen to fuel the 'hydrogen economy' |publisher=[[American Chemical Society]] |date=March 25, 2012 |url=http://portal.acs.org/portal/PublicWebSite/pressroom/newsreleases/CNBP_029640 |access-date=March 9, 2013 |archive-date=December 10, 2019 |archive-url=https://web.archive.org/web/20191210090606/http://portal.acs.org/portal/PublicWebSite/pressroom/newsreleases/CNBP_029640 }}</ref>

PEM electrolysis cells typically operate below {{Convert|100|C|F}}.<ref name="Ogden 1999"/> These cells have the advantage of being comparatively simple and can be designed to accept widely varying [[voltage]] inputs, which makes them ideal for use with renewable sources of energy such as [[Solar cell|photovoltaic solar panels]].<ref>{{cite journal |last1=Clarke |first1=R.E. |last2=Giddey |first2=S. |last3=Ciacchi |first3=F.T. |last4=Badwal |first4=S.P.S. |last5=Paul |first5=B. |last6=Andrews |first6=J. |title=Direct coupling of an electrolyser to a solar PV system for generating hydrogen |journal=International Journal of Hydrogen Energy |date=March 2009 |volume=34 |issue=6 |pages=2531–2542 |doi=10.1016/j.ijhydene.2009.01.053 }}</ref> AECs optimally operate at high concentrations of electrolyte (KOH or [[potassium carbonate]]) and at high temperatures, often near {{Convert|200|C|F}}.

====Efficiency and economics==== Efficiency of modern hydrogen generators is measured by ''energy consumed per standard volume of hydrogen'' (MJ/m3), assuming [[standard temperature and pressure]] of the H2. The lower the energy used by a generator, the higher would be its efficiency; a 100%-efficient electrolyser would consume {{convert|39.4|kWh/kg|MJ/kg|0}} of hydrogen,<ref name=EU140207>{{cite web |url= http://www.fch-ju.eu/sites/default/files/study%20electrolyser_0-Logos_0_0.pdf |title= Development of water electrolysis in the European Union |author= Luca Bertuccioli |work= Client Fuel Cells and Hydrogen Joint Undertaking |date= 7 February 2014 |display-authors= etal |access-date= 2 May 2018 |archive-date= 31 March 2015 |archive-url= https://web.archive.org/web/20150331145105/http://www.fch-ju.eu/sites/default/files/study%20electrolyser_0-Logos_0_0.pdf }}</ref> {{convert|{{#expr:141840*0.08988round0}}|J/L|MJ/m3|2}}.  Practical electrolysis typically uses a rotating electrolyser, where centrifugal force helps separate gas bubbles from water.<ref>{{cite journal |last1=Lao |first1=L. |last2=Ramshaw |first2=C. |last3=Yeung |first3=H. |title=Process intensification: water electrolysis in a centrifugal acceleration field |journal=Journal of Applied Electrochemistry |date=June 2011 |volume=41 |issue=6 |pages=645–656 |doi=10.1007/s10800-011-0275-2 |hdl=1826/6464 }}</ref> Such an electrolyser at 15 bar pressure may consume {{convert|50|kWh/kg|MJ/kg|0}}, and a further {{convert|15|kWh|MJ|0}} if the hydrogen is compressed for use in hydrogen cars.<ref>Stensvold, Tore (26 January 2016). [http://www.tu.no/artikler/coca-cola-oppskrift-kan-gjore-hydrogen-til-nytt-norsk-industrieventyr/276348 «Coca-Cola-oppskrift» kan gjøre hydrogen til nytt norsk industrieventyr]. ''[[Teknisk Ukeblad]]'', .</ref>

Conventional alkaline electrolysis has an efficiency of about 70%,<ref>{{cite book|last1=Stolten|first1=Detlef|title=Hydrogen Science and Engineering: Materials, Processes, Systems and Technology|date=Jan 4, 2016|publisher=John Wiley & Sons|isbn=978-3-527-67429-9 |page=898|url=https://books.google.com/books?id=we5bCwAAQBAJ|access-date=22 April 2018}}</ref> however advanced alkaline water electrolysers with efficiency of up to 82% are available.<ref>{{cite web |last1=thyssenkrupp |title=Hydrogen from water electrolysis – solutions for sustainability |url=https://www.thyssenkrupp-uhde-chlorine-engineers.com/en/products/water-electrolysis/ |website=thyssenkrupp-uhde-chlorine-engineers.com |access-date=28 July 2018 |archive-date=19 July 2018 |archive-url=https://web.archive.org/web/20180719085819/https://www.thyssenkrupp-uhde-chlorine-engineers.com/en/products/water-electrolysis/ }}</ref><ref>{{cite web |title=Report of CORDIS EU research results – Hydrogen from RES: pressurised alkaline electrolyser with high efficiency and wide operating range – June 2024 |url=https://cordis.europa.eu/project/id/278732/reporting |access-date=19 November 2025 |website= |ref=pg12}}</ref> Industrial PEM efficiency is expected to increase to approximately 86% before 2030.<ref>{{cite web |title=Cost reduction and performance increase of PEM electrolysers |url=http://www.fch.europa.eu/sites/default/files/Nov22_Session3_Panel%205_Slot%202_NOVEL-MEGASTACK_Thomassen%20%28ID%202891376%29.pdf |access-date=17 April 2018 |website=fch.europa.eu |publisher=Fuel Cells and Hydrogen Joint Undertaking |ref=pg9}}</ref>

In 2022, a Nature publication described a capillary-fed electrolysis cell, which reached 98% energy efficiency due to various design optimizations minimizing [[overpotential]]s.<ref>{{Cite journal |last1=Hodges |first1=Aaron |last2=Hoang |first2=Anh Linh |last3=Tsekouras |first3=George |last4=Wagner |first4=Klaudia |last5=Lee |first5=Chong-Yong |last6=Swiegers |first6=Gerhard F. |last7=Wallace |first7=Gordon G. |date=2022-03-15 |title=A high-performance capillary-fed electrolysis cell promises more cost-competitive renewable hydrogen |journal=Nature Communications |language=en |volume=13 |issue=1 |pages=1304 |doi=10.1038/s41467-022-28953-x |pmid=35292657 |issn=2041-1723|pmc=8924184 |bibcode=2022NatCo..13.1304H }}</ref>

[[File:H2 production cost ($-gge untaxed) at varying natural gas prices.jpg|thumb|upright=1.8|H2 production cost ($-gge untaxed) at varying natural gas prices]] As of 2020, the cost of hydrogen by electrolysis is around $3–8/kg.<ref name=Fickling>{{cite news |last1=Fickling |first1=David |title=Hydrogen Is a Trillion Dollar Bet on the Future |url=https://www.bloomberg.com/graphics/2020-opinion-hydrogen-green-energy-revolution-challenges-risks-advantages/ |website=Bloomberg.com |archive-url= https://web.archive.org/web/20201202090223/https://www.bloomberg.com/graphics/2020-opinion-hydrogen-green-energy-revolution-challenges-risks-advantages/ |archive-date=2 December 2020 |language=en |date=2 December 2020 |quote=green hydrogen .. current pricing of around $3 to $8 a kilogram .. gray hydrogen, which costs as little as $1 |url-status=live}}</ref> Considering the industrial production of hydrogen, and using current best processes for water electrolysis (PEM or alkaline electrolysis) which have an effective electrical efficiency of 70–82%,<ref>{{cite web|title=Chapter 3: Production of Hydrogen. Part 4: Production from electricity by means of electrolysis|work=HyWeb: Knowledge – Hydrogen in the Energy Sector |url=http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html#3.4 |author=Werner Zittel|author2=Reinhold Wurster|publisher=Ludwig-Bölkow-Systemtechnik GmbH|date=1996-07-08}}</ref><ref>{{cite web |url=http://www.bellona.org/reports/hydrogen |title=Hydrogen—Status and Possibilities |author=Bjørnar Kruse |author2=Sondre Grinna |author3=Cato Buch |date=2002-02-13 |format=PDF |publisher=The Bellona Foundation |archive-url= https://web.archive.org/web/20110702210000/http://www.bellona.org/reports/hydrogen |archive-date=2011-07-02 |quote=Efficiency factors for PEM electrolysers up to 94% are predicted, but this is only theoretical at this time. }}</ref><ref>{{cite web |url=http://www.grid-shift.com/white_papers/docs/3D_Water_Electrolysis_Abstract%202.htm |title=high-rate and high efficiency 3D water electrolysis |publisher=Grid-shift.com |access-date=2011-12-13 |archive-url= https://web.archive.org/web/20120322204531/http://www.grid-shift.com/white_papers/docs/3D_Water_Electrolysis_Abstract%202.htm |archive-date= 2012-03-22 }}</ref> producing 1 kg of hydrogen (which has a [[specific energy]] of 143 MJ/kg or about 40 kWh/kg) requires 50–55 kWh of electricity. At an electricity cost of $0.06/kWh, as set out in the Department of Energy hydrogen production targets for 2015,<ref>{{cite web|title=DOE Technical Targets for Hydrogen Production from Electrolysis|url=https://www.energy.gov/eere/fuelcells/doe-technical-targets-hydrogen-production-electrolysis |website=energy.gov|publisher=US Department of Energy|access-date=22 April 2018}}</ref> the hydrogen cost is $3/kg.

The US DOE target price for hydrogen in 2020 was $2.30/kg, requiring an electricity cost of $0.037/kWh, which is achievable given recent PPA tenders for wind and solar in many regions.<ref>{{cite web|last1=Deign|first1=Jason|title=Xcel Attracts 'Unprecedented' Low Prices for Solar and Wind Paired With Storage |url= https://www.greentechmedia.com/articles/read/record-low-solar-plus-storage-price-in-xcel-solicitation#gs.Jfm4ZWk |website=greentechmedia.com|publisher=Wood MacKenzie|access-date=22 April 2018}}</ref> In 2021, the US DOE established the Hydrogen Energy Earthshot (Hydrogen Shot) with a target of $1 (USD) for 1 kg of hydrogen in 1 decade, i.e., $1/kg by 2031 (known as "1 1 1").<ref>{{Cite news |title=What Is The DOE Hydrogen Energy Earthshots Initiative? |url=https://finance.yahoo.com/news/doe-hydrogen-energy-earthshots-initiative-143546436.html |archive-url=https://web.archive.org/web/20240624190951/https://finance.yahoo.com/news/doe-hydrogen-energy-earthshots-initiative-143546436.html |archive-date=2024-06-24 |access-date=2025-08-09 |work=Yahoo Finance |language=en-US}}</ref> This low price was selected to be competitive with the price of hydrogen from natural gas in the United States which is approximately $1.50/kg. In comparison, the cost baseline for hydrogen from electrolysis in 2020 was approximately $5/kg, requiring an 80% cost reduction to meet the Hydrogen Shot goal.<ref>{{cite journal |last1=Lund |first1=Per Christer |title=Norwegian Hydrogen Highway-HyNor : Strategy and Realization to Utilize Hydrogen as a Carrier for Clean Energy in One of the Worlds Leading Energy Nations |journal=Journal of the Society of Mechanical Engineers |date=2010 |volume=113 |issue=1104 |pages=858–861 |doi=10.1299/jsmemag.113.1104_858 |doi-access=free }}</ref><ref>{{cite report |last1=Satyapal |first1=Sunita |title=U.S. Department of Energy Hydrogen and Fuel Cell Technology Perspectives |date=2019 |doi=10.2172/1511435 |osti=1511435 }}</ref>

The report by IRENA.ORG is an extensive factual report of present-day industrial hydrogen production consuming about 53 to 70 kWh per kg could go down to about 45 kWh/kg {{chem|H|2}}.<ref>{{Cite web|url=https://irena.org/-/media/Files/IRENA/Agency/Publication/2020/Dec/IRENA_Green_hydrogen_cost_2020.pdf|title=accessed June 22, 2021}}{{Dead link|date=June 2025 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> The thermodynamic energy required for hydrogen by electrolysis translates to 33 kWh/kg, which is higher than steam reforming with carbon capture and higher than methane pyrolysis. One of the advantages of electrolysis over hydrogen from steam methane reforming (SMR) is that the hydrogen can be produced on-site, meaning that the costly process of delivery via truck or pipeline is avoided.

===Chemically assisted electrolysis === In addition to reduce the voltage required for electrolysis via the increasing of the temperature of the electrolysis cell it is also possible to electrochemically consume the oxygen produced in an electrolyser by introducing a fuel (such as carbon/coal,<ref>{{cite journal |last1=Giddey |first1=S. |last2=Kulkarni |first2=A. |last3=Badwal |first3=S.P.S. |title=Low emission hydrogen generation through carbon assisted electrolysis |journal=International Journal of Hydrogen Energy |date=January 2015 |volume=40 |issue=1 |pages=70–74 |doi=10.1016/j.ijhydene.2014.11.033 |bibcode=2015IJHE...40...70G }}</ref> [[methanol]],<ref name="Uhm et al 2012">{{cite journal |last1=Uhm |first1=Sunghyun |last2=Jeon |first2=Hongrae |last3=Kim |first3=Tae Jin |last4=Lee |first4=Jaeyoung |title=Clean hydrogen production from methanol–water solutions via power-saved electrolytic reforming process |journal=Journal of Power Sources |date=January 2012 |volume=198 |pages=218–222 |doi=10.1016/j.jpowsour.2011.09.083 |bibcode=2012JPS...198..218U }}</ref><ref name="Ju et al 2017">{{cite journal |last1=Ju |first1=HyungKuk |last2=Giddey |first2=Sarbjit |last3=Badwal |first3=Sukhvinder P.S. |title=The role of nanosized {{chem|Sn|O|2}} in Pt-based electrocatalysts for hydrogen production in methanol assisted water electrolysis |journal=Electrochimica Acta |date=March 2017 |volume=229 |pages=39–47 |doi=10.1016/j.electacta.2017.01.106 }}</ref> [[ethanol]],<ref name="Ju et al 2016">{{cite journal |last1=Ju |first1=HyungKuk |last2=Giddey |first2=Sarbjit |last3=Badwal |first3=Sukhvinder P.S. |last4=Mulder |first4=Roger J. |title=Electro-catalytic conversion of ethanol in solid electrolyte cells for distributed hydrogen generation |journal=Electrochimica Acta |date=September 2016 |volume=212 |pages=744–757 |doi=10.1016/j.electacta.2016.07.062 |bibcode=2016AcElc.212..744J }}</ref> [[formic acid]],<ref name="Lamy et al 2012">{{cite journal |last1=Lamy |first1=Claude |last2=Devadas |first2=Abirami |last3=Simoes |first3=Mario |last4=Coutanceau |first4=Christophe |title=Clean hydrogen generation through the electrocatalytic oxidation of formic acid in a Proton Exchange Membrane Electrolysis Cell (PEMEC) |journal=Electrochimica Acta |date=January 2012 |volume=60 |pages=112–120 |doi=10.1016/j.electacta.2011.11.006 |bibcode=2012AcElc..60..112L }}</ref> [[glycerol]],<ref name="Lamy et al 2012"/> etc.) into the oxygen side of the reactor. This reduces the required electrical energy and has the potential to reduce the cost of hydrogen to less than 40~60% with the remaining energy provided in this manner.<ref name="Sukhvinder P. S 2014">{{cite journal |last1=Badwal |first1=Sukhvinder P. S. |last2=Giddey |first2=Sarbjit S. |last3=Munnings |first3=Christopher |last4=Bhatt |first4=Anand I. |last5=Hollenkamp |first5=Anthony F. |title=Emerging electrochemical energy conversion and storage technologies |journal=Frontiers in Chemistry |date=24 September 2014 |volume=2 |page=79 |doi=10.3389/fchem.2014.00079 |pmid=25309898 |pmc=4174133 |bibcode=2014FrCh....2...79B |doi-access=free }}</ref>

Carbon/hydrocarbon assisted water electrolysis (CAWE) has the potential to offer a less energy intensive, cleaner method of using chemical energy in various sources of carbon, such as low-rank and high sulfur coals, biomass, alcohols and methane (Natural Gas), where pure CO2 produced can be easily sequestered without the need for separation.<ref>{{cite journal |last1=Ju |first1=HyungKuk |last2=Badwal |first2=Sukhvinder |last3=Giddey |first3=Sarbjit |title=A comprehensive review of carbon and hydrocarbon assisted water electrolysis for hydrogen production |journal=Applied Energy |date=December 2018 |volume=231 |pages=502–533 |doi=10.1016/j.apenergy.2018.09.125 |bibcode=2018ApEn..231..502J }}</ref>

==== Hydrogen from biomass – green ==== Biomass is converted into [[syngas]] by gasification and syngas is further converted into hydrogen by [[water-gas shift reaction]] (WGSR).<ref name="Carbon Neutral Fuels and Chemicals">{{cite journal |last1=Sasidhar |first1=Nallapaneni |title=Carbon Neutral Fuels and Chemicals from Standalone Biomass Refineries |journal=Indian Journal of Environment Engineering |date=30 November 2023 |volume=3 |issue=2 |pages=1–8 |doi=10.54105/ijee.B1845.113223 |doi-access=free }}</ref>

==== Hydrogen as a byproduct of other chemical processes ==== {{Main|Mekog}}

The industrial production of [[chlorine]] and [[caustic soda]] by electrolysis generates a sizable amount of Hydrogen as a byproduct. In the port of Antwerp a 1MW demonstration fuel cell power plant is powered by such byproduct. This unit has been operational since late 2011.<ref>{{cite web|url=http://www.nedstack.com/images/stories/news/documents/20120202_Press%20release%20Solvay%20PEM%20Power%20Plant%20start%20up.pdf |title=Solvay proudly presents Nedstack's Megawatt fuel cell |date=February 6, 2012|archive-url=https://web.archive.org/web/20141208084700/http://www.nedstack.com/images/stories/news/documents/20120202_Press%20release%20Solvay%20PEM%20Power%20Plant%20start%20up.pdf|archive-date=2014-12-08}}</ref> The excess hydrogen is often managed with a [[hydrogen pinch]] analysis.

Gas generated from [[Coke (fuel)|coke ovens]] in steel production is similar to [[Syngas]] with 60% hydrogen by volume.<ref>{{cite web |title=Different Gases from Steel Production Processes |url=https://www.clarke-energy.com/steel-production-gas/ |url-status=live |archive-url=https://web.archive.org/web/20160327004904/http://www.clarke-energy.com/gas-type/steel-production-gas/ |archive-date=27 March 2016 |access-date=5 July 2020}}</ref> The hydrogen can be extracted from the coke oven gas economically.<ref>{{cite web |title=Production of Liquefied Hydrogen Sourced by COG |url=https://www.nipponsteel.com/en/tech/report/nsc/pdf/n9226.pdf |url-status=live |archive-url=https://web.archive.org/web/20210208150429/https://www.nipponsteel.com/en/tech/report/nsc/pdf/n9226.pdf |archive-date=8 February 2021 |access-date=8 July 2020}}</ref>

When an existing [[blast furnace]] is modified to use biomass as its fuel, production of both green steel and green hydrogen/urea are feasible.<ref>{{Cite journal|date=November 2025|title=Retrofitting Blast Furnaces for Producing Green Steel and Green Urea|url=https://www.ijee.latticescipub.com/wp-content/uploads/papers/v5i2/B187105021125.pdf |journal=Indian Journal of Environment Engineering|language=en|volume=5|pages=19–25|doi=10.54105/ijee.B1871.05021125|issn= 2582-9289|last1=Nallapaneni|first1=Sasidhar|issue=2 |access-date=21 November 2025}}</ref>

===Other fossil fuel methods===

====Partial oxidation==== Hydrogen production from natural gas, heavier hydrocarbons, and alternative carbonaceous feedstocks is achieved by partial oxidation.<ref>{{cite journal | last1 = Khan | first1 = M. M. | last2 = Amjad | first2 = A. B. | title = Hydrogen production from municipal waste and low grade lignite blend | journal = Results in Engineering | volume = 24 | article-number = 103495 | date = December 1, 2024 | doi =10.1016/j.rineng.2024.103495 | doi-access = free }}</ref> A fuel-air or fuel-oxygen mixture is partially [[Combustion|combusted]], resulting in a hydrogen- and carbon monoxide-rich syngas. More hydrogen and carbon dioxide are then obtained from carbon monoxide (and water) via the water-gas shift reaction.<ref name="rotech"/> Carbon dioxide can be co-fed to lower the hydrogen to carbon monoxide ratio.

The partial oxidation reaction occurs when a [[stoichiometry|substoichiometric]] fuel-air mixture or fuel-oxygen is partially combusted in a reformer or partial oxidation reactor. A distinction is made between ''thermal'' partial oxidation (TPOX) and ''catalytic'' partial oxidation (CPOX). The chemical reaction takes the general form:

:2 C''n''H''m'' + ''n''O2 → 2''n'' CO + ''m''H2

Idealized examples for [[heating oil]] and coal, assuming compositions C12H24 and C24H12 respectively, are as follows: :C12H24 + 6 O2 → 12 CO + 12 H2 :C24H12 + 12 O2 → 24 CO + 6 H2

==== Plasma pyrolysis ==== The [[Kværner process]] or Kvaerner [[carbon black]] and hydrogen process (CB&H)<ref name="BHR">{{Cite web|url=http://www.interstatetraveler.us/Reference-Bibliography/Bellona-HydrogenReport.html|title=Hydrogen technologies|website=www.interstatetraveler.us}}</ref> is a [[Plasma (physics)|plasma]] [[pyrolysis]] method, developed in the 1980s by a [[Norway|Norwegian]] company of the same name, for the production of hydrogen and carbon black from liquid hydrocarbons (CnHm). Of the available energy of the feed, approximately 48% is contained in the hydrogen, 40% is contained in [[activated carbon]] and 10% in [[superheated steam]].<ref name=hfpe/> CO2 is not produced in the process.

A variation of this process was presented in 2009 using [[plasma arc waste disposal]] technology for the production of hydrogen, heat and carbon from methane and natural gas in a [[Plasma gasification|plasma converter]].<ref>{{Cite web |url=http://fuelcellsworks.com/news/2009/10/12/hydrogen-breakthrough-for-norwegian-company/ |title=Kværner-process with plasma arc waste disposal technology |access-date=2009-10-13 |archive-url=https://web.archive.org/web/20140313194912/http://fuelcellsworks.com/news/2009/10/12/hydrogen-breakthrough-for-norwegian-company/ |archive-date=2014-03-13 }}</ref>

==== Coal ==== For the production of hydrogen from coal, [[coal gasification]] is used. The process of coal gasification uses steam and oxygen to break molecular bonds in coal and form a gaseous mixture of hydrogen and carbon monoxide.<ref name="Hordeski">Hordeski, M. F. Alternative fuels: the future of hydrogen. 171–199 (The Fairmont Press, inc., 2007).</ref> Carbon dioxide and pollutants may be more easily removed from gas obtained from coal gasification versus coal combustion.<ref>{{cite web |title=Emissions Advantages of Gasification |url=https://netl.doe.gov/research/coal/energy-systems/gasification/gasifipedia/low-emissions |website=National Energy Technology Laboratory |publisher=U.S. Department of Energy}}</ref><ref>{{cite web |title=Emissions from burning coal |url=https://www.eia.gov/energyexplained/coal/coal-and-the-environment.php#:~:text=Several%20principal%20emissions%20result%20from,respiratory%20illnesses%20and%20lung%20disease |website=U.S. EIA |publisher=U.S. Energy Information Administration}}</ref> Another method for conversion is low-temperature and high-temperature [[coal carbonization]].<ref>{{cite journal |last1=Lee |first1=Woon-Jae |last2=Lee |first2=Yong-Kuk |title=Internal Gas Pressure Characteristics Generated during Coal Carbonization in a Coke Oven |journal=Energy & Fuels |date=May 2001 |volume=15 |issue=3 |pages=618–623 |doi=10.1021/ef990178a |bibcode=2001EnFue..15..618L }}</ref>

[[Coke oven]] gas made from pyrolysis (oxygen free heating) of coal has about 60% hydrogen, the rest being methane, carbon monoxide, carbon dioxide, ammonia, molecular nitrogen, and [[hydrogen sulfide]] (H2S). Hydrogen can be separated from other impurities by the [[pressure swing adsorption]] process. [[Steel industry in Japan|Japanese steel companies]] have carried out production of hydrogen by this method.

==== Petroleum coke ==== [[Petroleum coke]] can also be converted to hydrogen-rich [[syngas]] via coal gasification. The produced syngas consists mainly of hydrogen, carbon monoxide and H2S from the sulfur in the coke feed. Gasification is an option for producing hydrogen from almost any carbon source.<ref>{{cite journal |last1=Gemayel |first1=Jimmy El |last2=Macchi |first2=Arturo |last3=Hughes |first3=Robin |last4=Anthony |first4=Edward John |title=Simulation of the integration of a bitumen upgrading facility and an IGCC process with carbon capture |journal=Fuel |date=January 2014 |volume=117 |pages=1288–1297 |doi=10.1016/j.fuel.2013.06.045 |bibcode=2014Fuel..117.1288G }}</ref>

===Radiolysis=== Nuclear radiation can break water bonds through [[radiolysis]].<ref>{{Cite web|url=https://wipp.energy.gov/library/cra/2009_cra/references/Others/Spinks_Woods_1990_Radiation_Sources_Interaction_of_Radiation_with_Matter.pdf|title=''An Introduction to Radiation Chemistry Chapter 7''}}</ref><ref>{{Cite web|url=http://www.gammaexplorer.com/wp-content/uploads/2014/03/Nuclear-Hydrogen-Production-Handbook-2011.pdf|title=''Nuclear Hydrogen Production Handbook Chapter 8''}}{{Dead link|date=June 2025 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> In the [[Mponeng]] [[gold mine]], [[South Africa]], researchers found bacteria in a naturally occurring high radiation zone. The bacterial community which was dominated by a new [[phylotype]] of ''[[Desulfotomaculum]]'', was feeding on primarily [[Radiolysis|radiolytically]] produced hydrogen.<ref>{{cite journal |last1=Lin |first1=Li-Hung |last2=Wang |first2=Pei-Ling |last3=Rumble |first3=Douglas |last4=Lippmann-Pipke |first4=Johanna |last5=Boice |first5=Erik |last6=Pratt |first6=Lisa M. |last7=Lollar |first7=Barbara Sherwood |last8=Brodie |first8=Eoin L. |last9=Hazen |first9=Terry C. |last10=Andersen |first10=Gary L. |last11=DeSantis |first11=Todd Z. |last12=Moser |first12=Duane P. |last13=Kershaw |first13=Dave |last14=Onstott |first14=T. C. |title=Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome |journal=Science |date=20 October 2006 |volume=314 |pages=479–482 |doi=10.1126/science.1127376 |pmid=17053150 |issue=5798|bibcode = 2006Sci...314..479L |osti=920334 |url=https://digital.library.unt.edu/ark:/67531/metadc897899/ }}</ref>

===Thermolysis=== Water spontaneously dissociates at around 2500 °C, but this [[thermolysis]] occurs at temperatures too high for usual process piping and equipment resulting in a rather low commercialization potential.<ref>{{Cite web|title=Dream or Reality? Electrification of the Chemical Process Industries|url=https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852|access-date=2021-08-22|website=www.aiche-cep.com|language=en}}</ref>

==== Pyrolysis on biomass ==== [[Pyrolysis]] can be divided into different types based on the pyrolysis temperature, namely low-temperature slow pyrolysis, medium-temperature rapid pyrolysis, and high-temperature flash pyrolysis.<ref>{{cite journal |last1=Guoxin |first1=Hu |last2=Hao |first2=Huang |title=Hydrogen rich fuel gas production by gasification of wet biomass using a CO2 sorbent |journal=Biomass and Bioenergy |date=May 2009 |volume=33 |issue=5 |pages=899–906 |doi=10.1016/j.biombioe.2009.02.006 |bibcode=2009BmBe...33..899G }}</ref> The source energy is mainly solar energy, with help of [[Photosynthesis|photosynthetic microorganisms]] to decompose water or biomass to produce hydrogen. However, this process has relatively low hydrogen yields and high operating cost. It is not a feasible method for industry.{{Citation needed|date=August 2025}}

==== Nuclear-assisted thermolysis ==== The [[High-temperature-gas-cooled-reactor|high-temperature gas-cooled reactor (HTGR)]] is one of the most promising CO2-free nuclear technique to produce hydrogen by splitting water in a large scale. In this method, [[Sulfur–iodine cycle|iodine-sulfur (IS)]] thermo-chemical cycle for splitting water and high-temperature steam electrolysis (HTSE) were selected as the main processes for nuclear hydrogen production. The S-I cycle follows three chemical reactions:<ref>{{cite journal |last1=Ping |first1=Zhang |last2=Laijun |first2=Wang |last3=Songzhe |first3=Chen |last4=Jingming |first4=Xu |title=Progress of nuclear hydrogen production through the iodine–sulfur process in China |journal=Renewable and Sustainable Energy Reviews |date=January 2018 |volume=81 |pages=1802–1812 |doi=10.1016/j.rser.2017.05.275 |bibcode=2018RSERv..81.1802P }}</ref>

Bunsen reaction: I2+SO2+2H2O→H2SO4+2HI

HI decomposition: 2HI→H2+I2

[[Sulfuric acid]] decomposition: H2SO4→SO2+1/2O2+H2O

The hydrogen production rate of HTGR with IS cycle is approximately 0.68 kg/s, and the capital cost to build a unit of power plant is $100 million.

===Thermochemical cycle=== {{Main|thermochemical cycle}}

[[Thermochemical cycle]]s combine solely [[heat source]]s (''thermo'') with ''chemical'' reactions to split water into its hydrogen and [[oxygen]] components.<ref>[https://inlportal.inl.gov/portal/server.pt/gateway/PTARGS_0_2_816_259_0_43/http%3B/exps3.inl.gov%3B7087/publishedcontent/publish/communities/inl_gov/about_inl/home_page_fact_sheets/sheets/producing_hydrogen__the_thermochemical_cycles_4.pdf Producing hydrogen: The Thermochemical cycles]</ref> The term ''cycle'' is used because aside from water, hydrogen and oxygen, the chemical compounds used in these processes are continuously recycled. If electricity is partially used as an input, the resulting thermochemical cycle is defined as a [[wikt:hybrid|hybrid]] one.

The [[sulfur-iodine cycle]] (S-I cycle) is a thermochemical cycle processes which generates hydrogen from water with an efficiency of approximately 50%. The sulfur and iodine used in the process are recovered and reused, and not consumed by the process. The cycle can be performed with any source of very high temperatures, approximately 950 °C, such as by [[Concentrating solar power]] systems (CSP) and is regarded as being well suited to the production of hydrogen by [[Very high temperature reactor|high-temperature nuclear reactors]],<ref>[http://www.iea.org/techno/essentials5.pdf IEA Energy Technology Essentials – Hydrogen Production & Distribution] {{Webarchive|url=https://web.archive.org/web/20111103165110/http://iea.org/techno/essentials5.pdf |date=2011-11-03 }}, April 2007</ref> and as such, is being studied in the [[High-temperature engineering test reactor]] in Japan.<ref>{{cite web |url=http://httr.jaea.go.jp/eng/index.html |title=HTTR High Temperature engineering Test Reactor |publisher=Httr.jaea.go.jp |access-date=2014-01-23 |archive-date=2014-02-03 |archive-url=https://web.archive.org/web/20140203045100/http://httr.jaea.go.jp/eng/index.html }}</ref><ref>{{cite web|url=https://smr.inl.gov/Document.ashx?path=DOCS%2FGCR-Int%2FNHDDELDER.pdf |archive-url=https://web.archive.org/web/20161221005731/https://smr.inl.gov/Document.ashx?path=DOCS%2FGCR-Int%2FNHDDELDER.pdf |archive-date=2016-12-21 |title=Progress in Nuclear Energy Nuclear heat for hydrogen production: Coupling a very high/high temperature reactor to a hydrogen production plant|year= 2009}}</ref><ref>{{Cite web|url=http://www.iaea.org/NuclearPower/Downloadable/aris/2013/16.GTHTR300C.pdf|title=Status report 101 – Gas Turbine High Temperature Reactor (GTHTR300C)}}</ref><ref>{{Cite web |url=http://www.kns.org/jknsfile/v39/JK0390009.pdf |title=JAEA'S VHTR FOR HYDROGEN AND ELECTRICITY COGENERATION: GTHTR300C |access-date=2013-12-04 |archive-url=https://web.archive.org/web/20170810211559/https://www.kns.org/jknsfile/v39/JK0390009.pdf |archive-date=2017-08-10 }}</ref> There are other hybrid cycles that use both high temperatures and some electricity, such as the [[Copper–chlorine cycle]], it is classified as a hybrid [[thermochemical cycle]] because it uses an [[electrochemical]] reaction in one of the reaction steps, it operates at 530 °C and has an efficiency of 43 percent.<ref>Chukwu, C., Naterer, G. F., Rosen, M. A., "Process Simulation of Nuclear-Produced Hydrogen with a Cu-Cl Cycle", 29th Conference of the Canadian Nuclear Society, Toronto, Ontario, Canada, June 1–4, 2008. {{cite web |url=http://hydrogen.uoit.ca/assets/Default/documents/Public/CNS08-Chukwu.pdf |title=Process Simulation of Nuclear-Based Thermochemical Hydrogen Production with a Copper-Chlorine Cycle |access-date=2013-12-04 |archive-url=https://web.archive.org/web/20120220142521/http://hydrogen.uoit.ca/assets/Default/documents/Public/CNS08-Chukwu.pdf |archive-date=2012-02-20 }}</ref>

===Ferrosilicon method===

Ferrosilicon is used by the military to quickly produce hydrogen for [[balloon (aircraft)|balloons]]. The chemical reaction uses [[sodium hydroxide]], [[ferrosilicon]], and water. The generator is small enough to fit a truck and requires only a small amount of electric power, the materials are stable and not combustible, and they do not generate hydrogen until mixed.<ref>[https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091069_1993091069.pdf Report No 40: The ferrosilicon process for the generation of hydrogen]</ref> The method has been in use since [[World War I]]. A heavy steel [[pressure vessel]] is filled with sodium hydroxide and ferrosilicon, closed, and a controlled amount of water is added; the dissolving of the hydroxide heats the mixture to about 93 °C and starts the reaction; [[sodium silicate]], hydrogen and steam are produced.<ref>{{cite book |last1=Hargittai |first1=István |title=Candid Science: Conversations with Famous Chemists |date=2000 |publisher=Imperial College Press |isbn=978-1-86094-228-0 |page=261 }}</ref> The process is called silicol process and the overall reaction of the process is: {{NoteTag|note=The iron is intentionally omitted}}

: 2NaOH + [[FeSi|Si]] + H2O → Na2SiO3 + 2H2

The production method is economical to produce fuel cell quality hydrogen since low quality fesrrosilicon can be used to obtain high quality sodium silicate which commands good pricing.<ref>{{Cite journal |last1=Brack |first1=Paul |last2=Dann |first2=Sandie E. |last3=Wijayantha |first3=K. G. Upul |last4=Adcock |first4=Paul |last5=Foster |first5=Simon |date=November 2015 |title=An old solution to a new problem? Hydrogen generation by the reaction of ferrosilicon with aqueous sodium hydroxide solutions |journal=Energy Science & Engineering |language=en |volume=3 |issue=6 |pages=535–540 |doi=10.1002/ese3.94|s2cid=54929253 |doi-access=free |bibcode=2015EneSE...3..535B }}</ref>

===Photobiological water splitting=== [[File:Algae hydrogen production.jpg|thumb|upright|An [[algae bioreactor]] for hydrogen production.]] {{Main|Biological hydrogen production (Algae)}}

Biological hydrogen can be produced in an [[algae]] [[bioreactor]].<ref>{{cite journal |last1=Hemschemeier |first1=Anja |last2=Melis |first2=Anastasios |last3=Happe |first3=Thomas |title=Analytical approaches to photobiological hydrogen production in unicellular green algae |journal=Photosynthesis Research |date=December 2009 |volume=102 |issue=2–3 |pages=523–540 |doi=10.1007/s11120-009-9415-5 |pmid=19291418 |pmc=2777220 |bibcode=2009PhoRe.102..523H }}</ref> In the late 1990s it was discovered that if the algae are deprived of [[sulfur]] it will switch from the production of [[oxygen]], i.e. normal [[photosynthesis]], to the production of hydrogen. It seems that the production is now economically feasible by surpassing the 7–10 percent [[Energy efficiency (physics)|energy efficiency]] (the conversion of sunlight into hydrogen) barrier.<ref>{{Cite web|url=http://www.hydrogen.energy.gov/pdfs/progress08/ii_f_2_melis.pdf|title=DOE 2008 Report 25 %|access-date=2009-03-06|archive-date=2017-06-17|archive-url=https://web.archive.org/web/20170617112822/https://www.hydrogen.energy.gov/pdfs/progress08/ii_f_2_melis.pdf}}</ref> with a hydrogen production rate of 10–12 ml per liter culture per hour.<ref>{{cite conference|url=http://www.apecadvbioh2.org/Download/Renewable%20Energy%20Technology%20and%20Prospect%20on%20Biohydrogen%20Study%20in%20Thailand_Peesamai%20Jenvanitpanjakul.pdf |archive-url=http://webarchive.nationalarchives.gov.uk/20130704120720/http://www.apecadvbioh2.org/Download/Renewable%20Energy%20Technology%20and%20Prospect%20on%20Biohydrogen%20Study%20in%20Thailand_Peesamai%20Jenvanitpanjakul.pdf |archive-date=July 4, 2013 |title=Renewable Energy Technology And Prospect On Biohydrogen Study In Thailand |first=Peesamai |last=Jenvanitpanjakul |date=February 3–4, 2010 |conference=Steering Committee Meeting and Workshop of APEC Research Network for Advanced Biohydrogen Technology |publisher=[[Feng Chia University]] |location=[[Taichung]] }}</ref>

===Photocatalytic water splitting=== {{Main|Photocatalytic water splitting}}

The conversion of solar energy to hydrogen by means of water splitting process is one of the most interesting ways to achieve clean and [[renewable energy]] systems. However, if this process is assisted by photocatalysts suspended directly in water instead of using photovoltaic and an electrolytic system the reaction is in just one step, it can be made more efficient.<ref>{{cite journal |last1=Navarro Yerga |first1=Rufino M. |last2=Álvarez Galván |first2=M. Consuelo |last3=del Valle |first3=F. |last4=Villoria de la Mano |first4=José A. |last5=Fierro |first5=José L. G. |title=Water Splitting on Semiconductor Catalysts under Visible-Light Irradiation |journal=ChemSusChem |date=22 June 2009 |volume=2 |issue=6 |pages=471–485 |doi=10.1002/cssc.200900018 |pmid=19536754 |bibcode=2009ChSCh...2..471N }}</ref><ref>{{cite book |doi=10.1016/S0065-2377(09)00404-9 |chapter=Photocatalytic Water Splitting Under Visible Light: Concept and Catalysts Development |chapter-url=https://books.google.com/books?id=rs7VRicvWxoC&pg=PA111 |title=Photocatalytic Technologies |series=Advances in Chemical Engineering |year=2009 |last1=Navarro |first1=R.M. |last2=Del Valle |first2=F. |last3=Villoria De La Mano |first3=J.A. |last4=Álvarez-Galván |first4=M.C. |last5=Fierro |first5=J.L.G. |isbn=978-0-12-374763-1 |volume=36 |pages=111–43}}</ref><ref>{{cite journal |last1=Ropero-Vega |first1=J.L. |last2=Pedraza-Avella |first2=J.A. |last3=Niño-Gómez |first3=M.E. |title=Hydrogen production by photoelectrolysis of aqueous solutions of phenol using mixed oxide semiconductor films of Bi–Nb–M–O (M=Al, Fe, Ga, In) as photoanodes |journal=Catalysis Today |date=September 2015 |volume=252 |pages=150–156 |doi=10.1016/j.cattod.2014.11.007 }}</ref> Current systems, however have low performance for commercial implementation.<ref>{{cite journal |last1=Low |first1=Jingxiang |last2=Yu |first2=Jiaguo |last3=Jaroniec |first3=Mietek |last4=Wageh |first4=Swelm |last5=Al-Ghamdi |first5=Ahmed A. |title=Heterojunction Photocatalysts |journal=Advanced Materials |date=May 2017 |volume=29 |issue=20 |article-number=1601694 |doi=10.1002/adma.201601694 |pmid=28220969 |bibcode=2017AdM....2901694L }}</ref><ref>{{cite journal |last1=Djurišić |first1=Aleksandra B. |last2=He |first2=Yanling |last3=Ng |first3=Alan M. C. |title=Visible-light photocatalysts: Prospects and challenges |journal=APL Materials |date=March 2020 |volume=8 |issue=3 |page=030903 |doi=10.1063/1.5140497 |doi-access=free |bibcode=2020APLM....8c0903D }}</ref>

=== Biohydrogen routes ===

[[Biomass]] and waste streams can in principle be converted into [[biohydrogen]] with biomass [[gasification]], steam reforming, or biological conversion like biocatalysed electrolysis<ref name="Sukhvinder P. S 2014"/> or fermentative hydrogen production.<ref name=Ullmann/>

Among hydrogen production methods biological routes are potentially less energy intensive. In addition, a wide variety of waste and low-value materials such as agricultural biomass as renewable sources can be utilized to produce hydrogen via biochemical or thermochemical pathways.<ref name="Carbon Neutral Fuels and Chemicals"/> Nevertheless, at present hydrogen is produced mainly from fossil fuels, in particular, natural gas which are non-renewable sources. Hydrogen is not only the cleanest fuel but also widely used in a number of industries, especially fertilizer, petrochemical and food ones.<ref name="Hydrogen Step"/>

Biochemical routes to hydrogen are classified as dark and photo fermentation processes. In [[dark fermentation]], carbohydrates are converted to hydrogen by fermentative microorganisms including strict anaerobe and facultative anaerobic bacteria. A theoretical maximum of 4 mol H2/mol glucose can be produced.{{citation needed|date=September 2023}} Sugars are convertible to volatile fatty acids (VFAs) and alcohols as by-products during this process. Photo fermentative bacteria are able to generate hydrogen from VFAs. Hence, metabolites formed in dark fermentation can be used as feedstock in photo fermentation to enhance the overall yield of hydrogen.<ref name="Hydrogen Step">{{cite journal |last1=Asadi |first1=Nooshin |last2=Karimi Alavijeh |first2=Masih |last3=Zilouei |first3=Hamid |title=Development of a mathematical methodology to investigate biohydrogen production from regional and national agricultural crop residues: A case study of Iran |journal=International Journal of Hydrogen Energy |date=January 2017 |volume=42 |issue=4 |pages=1989–2007 |doi=10.1016/j.ijhydene.2016.10.021 |bibcode=2017IJHE...42.1989A }}</ref>

An [[enzyme]]-catalyzed process convert the common sugar [[xylose]] into hydrogen with nearly 100% of the theoretical [[Yield (chemistry)|yield]]. The process employs 13 enzymes, including a novel [[polyphosphate]] [[xylulokinase]] (XK).<ref>{{cite journal |last1=Martín del Campo |first1=Julia S. |last2=Rollin |first2=Joseph |last3=Myung |first3=Suwan |last4=Chun |first4=You |last5=Chandrayan |first5=Sanjeev |last6=Patiño |first6=Rodrigo |last7=Adams |first7=Michael WW |last8=Zhang |first8=Y.-H. Percival |title=High-Yield Production of Dihydrogen from Xylose by Using a Synthetic Enzyme Cascade in a Cell-Free System |journal=Angewandte Chemie International Edition |date=22 April 2013 |volume=52 |issue=17 |pages=4587–4590 |doi=10.1002/anie.201300766 |pmid=23512726 }}</ref>

==== Fermentative hydrogen production ==== {{Main|fermentative hydrogen production|dark fermentation}}

[[Fermentative hydrogen production]] converts organic substrates to hydrogen. A diverse group of [[bacteria]] promote this transformation. [[Photofermentation]] differs from [[dark fermentation]] because it only proceeds in the presence of [[light]]. For example, photo-fermentation with [[Rhodobacter sphaeroides]] SH2C can be employed to convert some fatty acids into hydrogen.<ref>{{cite journal |last1=Tao |first1=Y |last2=Chen |first2=Y |last3=Wu |first3=Y |last4=He |first4=Y |last5=Zhou |first5=Z |title=High hydrogen yield from a two-step process of dark- and photo-fermentation of sucrose |journal=International Journal of Hydrogen Energy |date=February 2007 |volume=32 |issue=2 |pages=200–206 |doi=10.1016/j.ijhydene.2006.06.034 |bibcode=2007IJHE...32..200T }}</ref>

Fermentative hydrogen production can be done using direct biophotolysis by green algae, indirect biophotolysis by cyanobacteria, photo-fermentation by anaerobic photosynthetic bacteria and dark fermentation by anaerobic fermentative bacteria. For example, studies on hydrogen production using ''H. salinarium'', an anaerobic photosynthetic bacteria, coupled to a hydrogenase donor like ''E. coli'', are reported in literature.<ref>{{cite journal |first1=Brijesh |last1=Rajanandam |first2=Siva |last2=Kiran |year=2011 |title=Optimization of hydrogen production by ''Halobacterium salinarium'' coupled with ''E coli'' using milk plasma as fermentative substrate |journal=Journal of Biochemical Technology |volume=3 |issue=2 |pages=242–4 |url=http://www.jbiochemtech.com/index.php/jbt/article/viewArticle/JBT321 |access-date=2013-03-09 |archive-date=2013-07-31 |archive-url=https://web.archive.org/web/20130731135527/http://www.jbiochemtech.com/index.php/jbt/article/viewArticle/JBT321 }}</ref> ''Enterobacter aerogenes'' is another hydrogen producer.<ref name="Organosolv">{{cite journal |last1=Asadi |first1=Nooshin |last2=Zilouei |first2=Hamid |title=Optimization of organosolv pretreatment of rice straw for enhanced biohydrogen production using Enterobacter aerogenes |journal=Bioresource Technology |date=March 2017 |volume=227 |pages=335–344 |doi=10.1016/j.biortech.2016.12.073 |pmid=28042989 |bibcode=2017BiTec.227..335A }}</ref>

==== Enzymatic hydrogen generation ==== Diverse enzymatic pathways have been designed to generate hydrogen from sugars.<ref>{{cite journal |last1=Zhang |first1=Y-H Percival |last2=Sun |first2=Jibin |last3=Zhong |first3=Jian-Jiang |title=Biofuel production by in vitro synthetic enzymatic pathway biotransformation |journal=Current Opinion in Biotechnology |date=October 2010 |volume=21 |issue=5 |pages=663–669 |doi=10.1016/j.copbio.2010.05.005 |pmid=20566280 |bibcode=2010COBt...21..663Z }}</ref>

==== Biocatalysed electrolysis ====

[[File:Microbial electrolysis cell.png|thumb|A microbial electrolysis cell]]

Besides dark fermentation, [[electrohydrogenesis]] (electrolysis using microbes) is another possibility. Using [[microbial fuel cell]]s, wastewater or plants can be used to generate power. Biocatalysed electrolysis should not be confused with [[biological hydrogen production]], as the latter only uses algae and with the latter, the algae itself generates the hydrogen instantly, where with biocatalysed electrolysis, this happens after running through the microbial fuel cell and a variety of aquatic plants<ref>{{cite journal |last1=Strik |first1=David P. B. T. B. |last2=Hamelers (Bert) |first2=H. V. M. |last3=Snel |first3=Jan F. H. |last4=Buisman |first4=Cees J. N. |title=Green electricity production with living plants and bacteria in a fuel cell |journal=International Journal of Energy Research |date=July 2008 |volume=32 |issue=9 |pages=870–876 |doi=10.1002/er.1397 |bibcode=2008IJER...32..870S}}</ref><ref>{{cite press release |title=Living plants produce energy |website=[[Wageningen University and Research Centre]] |url=http://www.glastuinbouw.wur.nl/UK/expertise/energy/innovations/plantenergy/ |archive-url=https://web.archive.org/web/20100517040527/http://www.glastuinbouw.wur.nl/UK/expertise/energy/innovations/plantenergy/ |archive-date=2010-05-17}}</ref> can be used. These include [[Glyceria maxima|reed sweetgrass]], cordgrass, rice, tomatoes, lupines and algae.<ref>{{cite book |first1=Ruud |last1=Timmers |year=2012 |title= Electricity generation by living plants in a plant microbial fuel cell |publisher=Wageningen University |type=PhD Thesis |url=http://library.wur.nl/WebQuery/clc/1992064 |isbn=978-94-6191-282-4}}{{Page needed|date=March 2013}}</ref> [[File:Nanogalvanic powder.jpg|thumb|Nano-galvanic aluminium-based powder developed by the [[United States Army Research Laboratory|U.S. Army Research Laboratory]]]]

=== Nanogalvanic aluminium alloy powder === {{Main|Aluminium-based nanogalvanic alloys}}

[[Aluminium alloy]] powder reacts with water to produce hydrogen gas upon contact with water. It reportedly generates hydrogen at 100 percent of the theoretical yield.<ref>{{Cite web|url=https://www.arl.army.mil/business/intellectual-property/alnanogalvanicpowder/|title=Aluminum Based Nanogalvanic Alloys for Hydrogen Generation|website=U.S. Army Combat Capabilities Development Command Army Research Laboratory|access-date=January 6, 2020}}</ref><ref>{{Cite news|url=https://www.army.mil/article/191212/army_discovery_may_offer_new_energy_source|title=Army discovery may offer new energy source|last=McNally|first=David|date=July 25, 2017|work=U.S. Army|access-date=January 6, 2020}}</ref> The process is not economical.

==Natural hydrogen== [[File:Mid-continental Rift System.webp|thumb|[[Midcontinent Rift System|Mid-continental Rift System]]]] {{main|Natural hydrogen}}

Hydrogen is also present naturally underground. This [[natural hydrogen]], also called white hydrogen or gold hydrogen, can be extracted from wells in a similar manner as fossil fuels such as oil and natural gas.<ref>{{Cite journal |last1=Gaucher |first1= Éric C. |author-link=Éric Claude Gaucher |date=February 2020 |title=New Perspectives in the Industrial Exploration for Native Hydrogen |journal=[[Elements (journal)|Elements]] |volume=16 |issue=1 |pages=8–9 |doi=10.2138/gselements.16.1.8|doi-access=free |bibcode= 2020Eleme..16....8G }}</ref><ref>{{cite journal |last1=Hand |first1=Eric |title=Hidden hydrogen |url=https://www.science.org/content/article/hidden-hydrogen-earth-may-hold-vast-stores-renewable-carbon-free-fuel |website=science.org |publisher=Science |pmid=36795818 |access-date=9 December 2023}}</ref> <ref name="Hassanpouryouzband foraging or farming"/>

White hydrogen could be found or produced in the [[Midcontinent Rift System|Mid-continental Rift System]] at scale for a renewable [[hydrogen economy]]. Water could be pumped down to hot iron-rich rock to extract the hydrogen.<ref>{{cite web | url=https://www.usgs.gov/news/featured-story/potential-geologic-hydrogen-next-generation-energy | title=The Potential for Geologic Hydrogen for Next-Generation Energy | U.S. Geological Survey | date=25 January 2018 }}</ref>

== Experimental production methods ==

=== Methane pyrolysis – turquoise === [[File:Methane_Pyrolysis-1.png|thumb|Illustrating inputs and outputs of methane pyrolysis, a process to produce Hydrogen]] [[Pyrolysis]] of methane (natural gas) with a one-step process<ref>{{cite web |last1=Fernandez |first1=Sonia |title=Researchers develop potentially low-cost, low-emissions technology that can convert methane without forming CO2 |url=https://phys.org/news/2017-11-potentially-low-cost-low-emissions-technology-methane.html |url-status=live |archive-url=https://web.archive.org/web/20201019193709/https://phys.org/news/2017-11-potentially-low-cost-low-emissions-technology-methane.html |archive-date=19 October 2020 |access-date=19 October 2020 |website=Phys-Org |publisher=American Institute of Physics}}</ref> bubbling methane through a molten metal catalyst is a "no greenhouse gas" approach to produce hydrogen that was demonstrated in laboratory conditions in 2017 and now being tested at larger scales.<ref name="BASF">{{cite web |last1=BASF |title=BASF researchers working on fundamentally new, low-carbon production processes, Methane Pyrolysis |url=https://www.basf.com/us/en/who-we-are/sustainability/we-produce-safely-and-efficiently/energy-and-climate-protection/carbon-management/interview-methane-pyrolysis.html |url-status=live |archive-url=https://web.archive.org/web/20201019120013/https://www.basf.com/us/en/who-we-are/sustainability/we-produce-safely-and-efficiently/energy-and-climate-protection/carbon-management/interview-methane-pyrolysis.html |archive-date=19 October 2020 |access-date=19 October 2020 |website=United States Sustainability |publisher=BASF}}</ref><ref name="Schneider et al 2020">{{cite journal |last1=Schneider |first1=Stefan |last2=Bajohr |first2=Siegfried |last3=Graf |first3=Frank |last4=Kolb |first4=Thomas |title=State of the Art of Hydrogen Production via Pyrolysis of Natural Gas |journal=ChemBioEng Reviews |date=October 2020 |volume=7 |issue=5 |pages=150–158 |doi=10.1002/cben.202000014 |doi-access=free }}</ref> The process is conducted at high temperatures (1065 °C).<ref name="pmid29146810">{{cite journal |last1=Upham |first1=D. Chester |last2=Agarwal |first2=Vishal |last3=Khechfe |first3=Alexander |last4=Snodgrass |first4=Zachary R. |last5=Gordon |first5=Michael J. |last6=Metiu |first6=Horia |last7=McFarland |first7=Eric W. |date=17 November 2017 |title=Catalytic molten metals for the direct conversion of methane to hydrogen and separable carbon |journal=Science |volume=358 |issue=6365 |pages=917–921 |bibcode=2017Sci...358..917U |doi=10.1126/science.aao5023 |pmid=29146810 |doi-access=free}}</ref><ref>{{cite journal |last1=Palmer |first1=Clarke |last2=Upham |first2=D. Chester |last3=Smart |first3=Simon |last4=Gordon |first4=Michael J. |last5=Metiu |first5=Horia |last6=McFarland |first6=Eric W. |date=January 2020 |title=Dry reforming of methane catalysed by molten metal alloys |journal=Nature Catalysis |volume=3 |issue=1 |pages=83–89 |doi=10.1038/s41929-019-0416-2 }}</ref><ref>{{cite web |last1=Cartwright |first1=Jon |title=The reaction that would give us clean fossil fuels forever |url=https://www.newscientist.com/article/mg23230940-200-crack-methane-for-fossil-fuels-without-tears |url-status=live |archive-url=https://web.archive.org/web/20201026044037/https://www.newscientist.com/article/mg23230940-200-crack-methane-for-fossil-fuels-without-tears/ |archive-date=26 October 2020 |access-date=30 October 2020 |website=NewScientist |publisher=New Scientist Ltd.}}</ref><ref>{{cite web |last1=Karlsruhe Institute of Technology |title=Hydrogen from methane without CO2 emissions |url=https://phys.org/news/2013-04-hydrogen-methane-co2-emissions.html |url-status=live |archive-url=https://web.archive.org/web/20201021215453/https://phys.org/news/2013-04-hydrogen-methane-co2-emissions.html |archive-date=21 October 2020 |access-date=30 October 2020 |website=Phys.Org}}</ref> Producing 1 kg of hydrogen requires about 18 kWh of electricity for process heat.<ref>[https://efre2022.hcei.tsc.ru/files/proceedings/C1-O-030501.pdf Proceedings] hcei.tsc.ru</ref> The pyrolysis of methane can be expressed by the following reaction equation.<ref>{{cite journal |last1=Lumbers |first1=Brock |last2=Agar |first2=David W. |last3=Gebel |first3=Joachim |last4=Platte |first4=Frank |title=Mathematical modelling and simulation of the thermo-catalytic decomposition of methane for economically improved hydrogen production |journal=International Journal of Hydrogen Energy |date=January 2022 |volume=47 |issue=7 |pages=4265–4283 |doi=10.1016/j.ijhydene.2021.11.057 |bibcode=2022IJHE...47.4265L |url=https://figshare.com/articles/journal_contribution/18394181 }}</ref>

: {{chem|CH|4}}(g) → C(s) + 2 {{chem|H|2}}(g) ΔH° = 74.8 kJ/mol

The industrial quality solid carbon may be sold as manufacturing feedstock, included in asphalt pavement, or landfilled.

Methane pyrolysis technologies are in the early development stages at several companies as of 2023. They have obstacles to overcome before commercialization.<ref>{{cite journal |last1=Patlolla |first1=Shashank Reddy |last2=Katsu |first2=Kyle |last3=Sharafian |first3=Amir |last4=Wei |first4=Kevin |last5=Herrera |first5=Omar E. |last6=Mérida |first6=Walter |title=A review of methane pyrolysis technologies for hydrogen production |journal=Renewable and Sustainable Energy Reviews |date=July 2023 |volume=181 |article-number=113323 |doi=10.1016/j.rser.2023.113323 |bibcode=2023RSERv.18113323P }}</ref>

=== Biological production === {{Main|Biohydrogen}}

[[Fermentative hydrogen production]] is the [[fermentative]] conversion of organic substrate to [[biohydrogen]] manifested by a diverse group of [[bacteria]] using multi [[enzyme]] systems involving three steps similar to [[Anaerobic digestion|anaerobic conversion]]. [[Dark fermentation]] reactions do not require light energy, so they are capable of constantly producing hydrogen from organic compounds throughout the day and night. [[Photofermentation]] differs from [[dark fermentation]] because it only proceeds in the presence of [[light]]. [[Electrohydrogenesis]] is used in [[microbial fuel cell]]s to produce hydrogen from organic matter.<ref>{{cite web |title=Hydrogen production from organic solid matter |url=http://www.biohydrogen.nl/hyvolution/25446/5/0/20 |url-status=live |archive-url=https://web.archive.org/web/20110720185743/http://www.biohydrogen.nl/hyvolution/25446/5/0/20 |archive-date=2011-07-20 |access-date=2010-07-05 |publisher=Biohydrogen.nl}}</ref>

Biological hydrogen can be produced in an [[algae]] [[bioreactor]]. In the late 1990s it was discovered that if the algae is deprived of [[sulfur]] it will switch from the production of [[oxygen]], i.e. normal [[photosynthesis]], to the production of hydrogen.<ref>{{cite journal |last1=Hemschemeier |first1=Anja |last2=Melis |first2=Anastasios |last3=Happe |first3=Thomas |date=December 2009 |title=Analytical approaches to photobiological hydrogen production in unicellular green algae |journal=Photosynthesis Research |volume=102 |issue=2–3 |pages=523–540 |bibcode=2009PhoRe.102..523H |doi=10.1007/s11120-009-9415-5 |pmc=2777220 |pmid=19291418}}</ref> Biological hydrogen can also be produced using feedstocks other than algae, the most common feedstock being waste streams. The process involves bacteria feeding on hydrocarbons and excreting hydrogen and CO2.<ref>{{cite web |date=September 20, 2007 |title=NanoLogix generates energy on-site with bioreactor-produced hydrogen |url=http://electroiq.com/blog/2007/09/nanologix-generates-energy-on-site-with-bioreactor-produced-hydrogen/ |archive-url=https://web.archive.org/web/20180515183752/http://electroiq.com/blog/2007/09/nanologix-generates-energy-on-site-with-bioreactor-produced-hydrogen/ |archive-date=2018-05-15 |access-date=14 May 2018 |website=Solid State Technology}}</ref>

=== Biocatalysed electrolysis === Besides regular electrolysis, electrolysis using microbes is another possibility. With biocatalysed electrolysis, hydrogen is generated after running through the microbial fuel cell and a variety of [http://www.glastuinbouw.wur.nl/UK/expertise/energy/innovations/plantenergy/ aquatic plants] {{Webarchive|url=https://web.archive.org/web/20100517040527/http://www.glastuinbouw.wur.nl/UK/expertise/energy/innovations/plantenergy/ |date=2010-05-17 }} can be used. These include [[Glyceria maxima|reed sweetgrass]], cordgrass, rice, tomatoes, lupines, and algae<ref>{{cite web |title=Power from plants using microbial fuel cell |url=https://translate.google.com/translate?js=n&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.resource-online.nl%2Fachtergrond.php%3Fid%3D147&sl=nl&tl=en&history_state0= |url-status=live |archive-url=https://web.archive.org/web/20210208150428/https://translate.google.com/translate?js=n&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.resource-online.nl%2Fachtergrond.php%3Fid%3D147&sl=nl&tl=en&history_state0= |archive-date=2021-02-08 |access-date=2010-07-05 |language=nl}}</ref>

=== High-pressure electrolysis === [[High pressure electrolysis]] is the electrolysis of water by decomposition of [[water]] (H2O) into [[oxygen]] (O2) and hydrogen gas (H2) by means of an electric current being passed through the water. The difference with a standard [[Electrolysis of water|electrolyser]] is the [[compressed hydrogen]] output around 120–200 [[Bar (unit)|bar]] (1740–2900 [[Pounds per square inch|psi]], 12–20 [[MPa]]).<ref>{{cite conference |last1=Janssen |first1=H. |last2=Emonts |first2=B. |last3=Groehn |first3=H. G. |last4=Mai |first4=H. |last5=Reichel |first5=R. |last6=Stolten |first6=D. |title=High-pressure electrolysis, the key technology for efficient H2 production |pages=172–177 |osti=20274275 |conference=HYPOTHESIS IV |date=2001 |publisher=Kluwer Academic |isbn=978-3-9807963-0-9 |oclc=496234379 }}</ref> By pressurising the hydrogen in the electrolyser, through a process known as chemical compression, the need for an external [[hydrogen compressor]] is eliminated,<ref name="Carmo 2013">{{cite journal |last=Carmo |first=M |author2=Fritz D |author3=Mergel J |author4=Stolten D |year=2013 |title=A comprehensive review on PEM water electrolysis |journal=Journal of Hydrogen Energy |volume=38 |issue=12 |pages=4901–4934 |doi=10.1016/j.ijhydene.2013.01.151|bibcode=2013IJHE...38.4901C }}</ref> the average energy consumption for internal compression is around 3%.<ref>{{cite web |title=2003-PHOEBUS-Pag.9 |url=http://www.fz-juelich.de/ief/ief-3/datapool/page/214/solar%20energy%2075%20469-478.pdf |archive-url=https://web.archive.org/web/20090327074743/http://www.fz-juelich.de/ief/ief-3/datapool/page/214/solar%20energy%2075%20469-478.pdf |archive-date=2009-03-27 |access-date=2010-07-05}}</ref> European largest (1 400 000 kg/a, High-pressure Electrolysis of water, alkaline technology) hydrogen production plant is operating at Kokkola, Finland.<ref>{{Cite web |date=December 2015 |title=Finland exporting TEN-T fuel stations |url=http://www.uusiteknologia.fi/2015/12/01/suomi-viemaan-vedyn-tankkausasemia/ |url-status=live |archive-url=https://web.archive.org/web/20160828184130/http://www.uusiteknologia.fi/2015/12/01/suomi-viemaan-vedyn-tankkausasemia/ |archive-date=2016-08-28 |access-date=2016-08-22}}</ref>

=== High-temperature electrolysis === Hydrogen can be generated from energy supplied in the form of heat and electricity through high-temperature electrolysis (HTE). Since some of the energy in HTE is supplied in the form of heat, less of the energy must be converted twice from heat to electricity, and then to hydrogen. Therefore, potentially less energy is required to produce hydrogen. Nuclear heat could be used to split hydrogen from water. High temperature (950–1000 °C) gas cooled nuclear reactors have the potential to split hydrogen from water by thermochemical means using nuclear heat. High-temperature electrolysis has been demonstrated in a laboratory, at 108 [[Megajoule|MJ]] (thermal) per kilogram of hydrogen produced,<ref name="INL">{{cite press release |url=https://www.sciencedaily.com/releases/2008/09/080918170624.htm |title=Steam heat: researchers gear up for full-scale hydrogen plant |date=2008-09-18 |access-date=2008-09-19 |publisher=[[Science Daily]] |archive-date=2008-09-21 |archive-url=https://web.archive.org/web/20080921145517/http://www.sciencedaily.com/releases/2008/09/080918170624.htm |url-status=live}}</ref> but not at a commercial scale. In addition, this is lower-quality "commercial" grade Hydrogen, unsuitable for use in fuel cells.<ref>{{cite web |date=March 2004 |title=Nuclear Hydrogen R&D Plan |url=http://www.hydrogen.energy.gov/pdfs/nuclear_energy_h2_plan.pdf |archive-url=https://web.archive.org/web/20080518073117/http://www.hydrogen.energy.gov/pdfs/nuclear_energy_h2_plan.pdf |archive-date=2008-05-18 |access-date=2008-05-09 |publisher=[[U.S. Dept. of Energy]]}}</ref>

=== Photoelectrochemical water splitting === {{main|Photoelectrolysis of water}}

Using electricity produced by photovoltaic systems offers the cleanest way to produce hydrogen. Water is broken into hydrogen and oxygen by electrolysis – a [[photoelectrochemical cell]] (PEC) process which is also named [[artificial photosynthesis]].<ref name="WaterSplitting">{{cite journal |last1=Valenti |first1=Giovanni |last2=Boni |first2=Alessandro |last3=Melchionna |first3=Michele |last4=Cargnello |first4=Matteo |last5=Nasi |first5=Lucia |last6=Bertoni |first6=Giovanni |last7=Gorte |first7=Raymond J. |last8=Marcaccio |first8=Massimo |last9=Rapino |first9=Stefania |last10=Bonchio |first10=Marcella |last11=Fornasiero |first11=Paolo |last12=Prato |first12=Maurizio |last13=Paolucci |first13=Francesco |date=December 2016 |title=Co-axial heterostructures integrating palladium/titanium dioxide with carbon nanotubes for efficient electrocatalytic hydrogen evolution |journal=Nature Communications |volume=7 |issue=1 |article-number=13549 |bibcode=2016NatCo...713549V |doi=10.1038/ncomms13549 |pmc=5159813 |pmid=27941752}}</ref> William Ayers at Energy Conversion Devices demonstrated and patented the first multijunction high efficiency photoelectrochemical system for direct splitting of water in 1983.<ref>William Ayers, US Patent 4,466,869 Photolytic Production of Hydrogen</ref> This group demonstrated direct water splitting now referred to as an "artificial leaf" or "wireless solar water splitting" with a low cost thin film amorphous silicon multijunction sheet immersed directly in water.<ref name="Water Splitting on Semiconductor Ca">{{cite journal |last1=Navarro Yerga |first1=Rufino M. |last2=Álvarez Galván |first2=M. Consuelo |last3=del Valle |first3=F. |last4=Villoria de la Mano |first4=José A. |last5=Fierro |first5=José L. G. |date=22 June 2009 |title=Water Splitting on Semiconductor Catalysts under Visible-Light Irradiation |journal=ChemSusChem |volume=2 |issue=6 |pages=471–485 |bibcode=2009ChSCh...2..471N |doi=10.1002/cssc.200900018 |pmid=19536754}}</ref><ref name="Advances in Chemical Engineering">{{cite book |last1=Navarro |first1=R.M. |title=Advances in Chemical Engineering - Photocatalytic Technologies |last2=Del Valle |first2=F. |last3=Villoria de la Mano |first3=J.A. |last4=Álvarez-Galván |first4=M.C. |last5=Fierro |first5=J.L.G. |year=2009 |isbn=978-0-12-374763-1 |volume=36 |pages=111–143 |chapter=Photocatalytic Water Splitting Under Visible Light |doi=10.1016/S0065-2377(09)00404-9 |ref=CONACYT Mexico}}</ref>

Hydrogen evolved on the front amorphous silicon surface decorated with various catalysts while oxygen evolved off the back metal substrate. A Nafion membrane above the multijunction cell provided a path for ion transport. Their patent also lists a variety of other semiconductor multijunction materials for the direct water splitting in addition to amorphous silicon and silicon germanium alloys. Research continues towards developing high-efficiency [[multi-junction cell]] technology at universities and the photovoltaic industry. If this process is assisted by photocatalysts suspended directly in water instead of using photovoltaic and an electrolytic system, the reaction is in just one step, which can improve efficiency.<ref name="Water Splitting on Semiconductor Ca" /><ref name="Advances in Chemical Engineering" />

=== Photoelectrocatalytic production === A method studied by Thomas Nann and his team at the University of East Anglia consists of a gold electrode covered in layers of indium phosphide (InP) nanoparticles. They introduced an iron-sulfur complex into the layered arrangement, which when submerged in water and irradiated with light under a small electric current, produced hydrogen with an efficiency of 60%.<ref>{{cite journal |last1=Nann |first1=Thomas |last2=Ibrahim |first2=Saad K. |last3=Woi |first3=Pei-Meng |last4=Xu |first4=Shu |last5=Ziegler |first5=Jan |last6=Pickett |first6=Christopher J. |date=22 February 2010 |title=Water Splitting by Visible Light: A Nanophotocathode for Hydrogen Production |journal=Angewandte Chemie International Edition |volume=49 |issue=9 |pages=1574–1577 |doi=10.1002/anie.200906262 |pmid=20140925 |bibcode=2010ACIE...49.1574N |doi-access=free}}</ref>

In 2015, it was reported that [[Panasonic Corp.]] has developed a [[photocatalyst]] based on [[niobium nitride]] that can absorb 57% of sunlight to support the [[decomposition]] of water to produce hydrogen gas.<ref name="asahi">{{cite news |last=Yamamura |first=Tetsushi |date=August 2, 2015 |title=Panasonic moves closer to home energy self-sufficiency with fuel cells |url=http://ajw.asahi.com/article/sci_tech/technology/AJ201508020014 |archive-url=https://web.archive.org/web/20150807010324/http://ajw.asahi.com/article/sci_tech/technology/AJ201508020014 |archive-date=August 7, 2015 |access-date=2015-08-02 |work=[[Asahi Shimbun]]}}</ref> The company plans to achieve commercial application "as early as possible", not before 2020.

=== Concentrating solar thermal === Very high temperatures are required to dissociate water into hydrogen and oxygen. A catalyst is required to make the process operate at feasible temperatures. Heating the water can be achieved through the use of water [[concentrating solar power]]. [[Hydrosol-2]] is a 100-kilowatt pilot plant at the [[Plataforma Solar de Almería]] in [[Spain]] which uses sunlight to obtain the required 800 to 1,200 °C to heat water. Hydrosol II has been in operation since 2008. The design of this 100-kilowatt pilot plant is based on a modular concept. As a result, it may be possible that this technology could be readily scaled up to the megawatt range by multiplying the available reactor units and by connecting the plant to [[heliostat]] fields (fields of sun-tracking mirrors) of a suitable size.<ref>{{cite web |date=2008-11-25 |title=DLR Portal – DLR scientists achieve solar hydrogen production in a 100-kilowatt pilot plant |url=http://www.dlr.de/en/desktopdefault.aspx/tabid-1/86_read-14380/ |url-status=live |archive-url=https://web.archive.org/web/20130622103525/http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10002/ |archive-date=2013-06-22 |access-date=2009-09-19 |publisher=Dlr.de}}</ref>

=== Thermochemical production === There are more than 352<ref>{{cite web |title=353 Thermochemical cycles |url=http://www.hydrogen.energy.gov/pdfs/review06/pd_10_weimer.pdf |url-status=live |archive-url=https://web.archive.org/web/20090205122514/http://www.hydrogen.energy.gov/pdfs/review06/pd_10_weimer.pdf |archive-date=2009-02-05 |access-date=2010-07-05}}</ref> thermochemical cycles which can be used for [[water splitting]],<ref>[http://shgr.unlv.edu/stchNew/source/login.asp UNLV Thermochemical cycle automated scoring database (public)]{{dead link|date=September 2017|bot=InternetArchiveBot|fix-attempted=yes}}</ref> around a dozen of these cycles such as the [[iron oxide cycle]], [[cerium(IV) oxide-cerium(III) oxide cycle]], [[zinc zinc-oxide cycle]], [[sulfur-iodine cycle]], [[copper-chlorine cycle]] and [[hybrid sulfur cycle]], [[aluminium aluminium-oxide cycle]], are under research and in testing phase to produce hydrogen and oxygen from water and heat without using electricity.<ref>{{cite web |title=Development of Solar-powered Thermochemical Production of Hydrogen from Water |url=http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf |url-status=live |archive-url=https://web.archive.org/web/20070417134156/http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf |archive-date=2007-04-17 |access-date=2010-07-05}}</ref> These processes can be more efficient than high-temperature electrolysis, typical in the range from 35% – 49% [[Lower heating value|LHV]] efficiency. Thermochemical production of hydrogen using chemical energy from coal or natural gas is generally not considered, because the direct chemical path is more efficient.

None of the thermochemical hydrogen production processes have been demonstrated at production levels, although several have been demonstrated in laboratories.

=== Kværner process === The [[Kværner process]] or Kvaerner [[carbon black]] and hydrogen process (CB&H)<ref name="BHR2">{{cite web |title=Bellona-HydrogenReport |url=http://www.interstatetraveler.us/Reference-Bibliography/Bellona-HydrogenReport.html |url-status=live |archive-url=https://web.archive.org/web/20160603020122/http://www.interstatetraveler.us/Reference-Bibliography/Bellona-HydrogenReport.html |archive-date=2016-06-03 |access-date=2010-07-05 |publisher=Interstatetraveler.us}}</ref> is a method, developed in the 1980s by a [[Norway|Norwegian]] [[Kværner|company of the same name]], for the production of hydrogen from [[hydrocarbons]] (CnHm), such as [[methane]], natural gas and [[biogas]]. Of the available energy of the feed, approximately 48% is contained in the hydrogen, 40% is contained in [[activated carbon]] and 10% in superheated steam.<ref name=hfpe>{{cite web|url=https://www.hfpeurope.org/infotools/energyinfos__e/hydrogen/main03.html|title=HFP Europe|date=September 2017}}{{dead link|date=November 2024}}</ref>

=== {{anchor|hydrogen well}}Extraction of naturally-occurring hydrogen=== {{main|Natural hydrogen}} {{As of|2019}}, hydrogen is mainly used as an industrial feedstock, primarily for the production of [[ammonia]] and [[methanol]], and in petroleum refining. Although initially hydrogen gas was thought not to occur naturally in convenient reservoirs, it is now demonstrated that this is not the case; a hydrogen system is currently being exploited near Bourakebougou, [[Koulikoro Region]] in Mali, producing electricity for the surrounding villages.<ref>{{cite journal |last1=Prinzhofer |first1=Alain |last2=Tahara Cissé |first2=Cheick Sidy |last3=Diallo |first3=Aliou Boubacar |date=October 2018 |title=Discovery of a large accumulation of natural hydrogen in Bourakebougou (Mali) |journal=International Journal of Hydrogen Energy |volume=43 |issue=42 |pages=19315–19326 |doi=10.1016/j.ijhydene.2018.08.193 |bibcode=2018IJHE...4319315P }}</ref> More discoveries of naturally occurring hydrogen in continental, on-shore geological environments have been made in recent years<ref>{{cite journal |last1=Larin |first1=Nikolay |last2=Zgonnik |first2=Viacheslav |last3=Rodina |first3=Svetlana |last4=Deville |first4=Eric |last5=Prinzhofer |first5=Alain |last6=Larin |first6=Vladimir N. |date=September 2015 |title=Natural Molecular Hydrogen Seepage Associated with Surficial, Rounded Depressions on the European Craton in Russia |journal=Natural Resources Research |volume=24 |issue=3 |pages=369–383 |bibcode=2015NRR....24..369L |doi=10.1007/s11053-014-9257-5 }}</ref> and open the way to the novel field of [[natural hydrogen|natural or native hydrogen]] (also known as white hydrogen<ref name=bbc2025>{{cite web | last=Farand | first=Chloe | title=The precious 'white gold' fuel buried in the Earth | website=BBC Home | date=24 July 2025 | url=https://www.bbc.com/future/article/20250723-the-worlds-race-to-drill-for-natural-white-hydrogen | access-date=1 June 2026}}</ref> and various other names) supporting [[energy transition]] efforts.<ref>{{cite journal |last1=Gaucher |first1=Eric C. |author1-link=Éric Claude Gaucher |date=1 February 2020 |title=New Perspectives in the Industrial Exploration for Native Hydrogen |journal=Elements |volume=16 |issue=1 |pages=8–9 |bibcode=2020Eleme..16....8G |doi=10.2138/gselements.16.1.8 |doi-access=free}}</ref><ref>{{cite journal |last1=Truche |first1=Laurent |last2=Bazarkina |first2=Elena F. |date=2019 |title=Natural hydrogen the fuel of the 21 st century |journal=E3S Web of Conferences |volume=98 |page=03006 |bibcode=2019E3SWC..9803006T |doi=10.1051/e3sconf/20199803006 |doi-access=free}}</ref> [[File:Mid-continental_Rift_System.webp|thumb|Mid-continental Rift System]] White hydrogen could be found or produced in the [[Midcontinent Rift System|Mid-continental Rift System]] in North America at scale for a renewable hydrogen economy. Water could be pumped down to hot iron-rich rock to produce hydrogen and the hydrogen could be extracted.<ref>{{Cite web|url=https://www.usgs.gov/news/featured-story/potential-geologic-hydrogen-next-generation-energy|title=The Potential for Geologic Hydrogen for Next-Generation Energy | U.S. Geological Survey|website=www.usgs.gov|date=25 January 2018 }}</ref>

==Environmental impact== Most hydrogen is produced from [[fossil fuels]], resulting in [[carbon dioxide emissions]].<ref>{{Cite web |title=Executive summary – Global Hydrogen Review 2023 – Analysis |url=https://www.iea.org/reports/global-hydrogen-review-2023/executive-summary |access-date=2024-05-13 |website=IEA |language=en-GB}}</ref> Hydrogen produced by this technology has been described as '''grey hydrogen''' when emissions are released to the atmosphere, and '''blue hydrogen''' when emissions are captured through [[carbon capture and storage]] (CCS).<ref>{{cite web |first=Uwe |last=Hessler| date=December 6, 2020|publisher=Deutsche Welle |title=First element in periodic table: Why all the fuss about hydrogen? |url=https://www.dw.com/en/first-element-in-periodic-table-why-all-the-fuss-about-hydrogen/a-53783698#:~:text=Hydrogen%20is%20the%20simplest%20atom,density%20of%20any%20crystalline%20solid |website=dw.com}}</ref><ref>[https://www.airproducts.com/company/news-center/2023/11/1106-air-products-blue-hydrogen-plant-europe-and-exxonmobil-long-term-agreement "Air Products to Build Europe's Largest Blue Hydrogen Plant and Strengthens Long-term Agreement"], [[Air Products]] press release, November 6, 2023. Retrieved 2023-11-14.</ref> Blue hydrogen has been estimated to have a [[greenhouse gas footprint]] that is 20% greater than burning gas or coal for heat and 60% greater when compared to burning diesel for heat, assuming US up- and mid-stream [[methane leakage]] rates and production via [[Steam reforming|steam methane reformers]] (SMR) retrofitted with carbon dioxide capture.<ref>{{Cite Q|Q108067259}}</ref> The emissions intensity of blue hydrogen is highly sensitive to upstream methane leakage, with leakage rates needing to be below 0.3–3.2% (depending on assumptions) for blue hydrogen to offer climate benefits relative to directly burning natural gas.<ref>{{cite journal |last1=Johnson |first1=Nathan |last2=Liebreich |first2=Michael |last3=Kammen |first3=Daniel M. |last4=Ekins |first4=Paul |last5=McKenna |first5=Russell |last6=Staffell |first6=Iain |title=Realistic roles for hydrogen in the future energy transition |journal=Nature Reviews Clean Technology |date=2025-04-22 |volume=1 |pages=351–371 |doi=10.1038/s44359-025-00050-4 |url=https://www.nature.com/articles/s44359-025-00050-4}}</ref>

The use of [[Autothermal reforming|autothermal reformers]] (ATR) with integrated capture of carbon dioxide allows higher capture rates at satisfactory energy efficiencies and life cycle assessments have shown lower greenhouse gas emissions for such plants compared to SMRs with carbon dioxide capture.<ref>{{cite journal |last1=Antonini |first1=Cristina |last2=Treyer |first2=Karin |last3=Streb |first3=Anne |last4=van der Spek |first4=Mijndert |last5=Bauer |first5=Christian |last6=Mazzotti |first6=Marco |title=Hydrogen production from natural gas and biomethane with carbon capture and storage – A techno-environmental analysis |journal=Sustainable Energy & Fuels |date=2020 |volume=4 |issue=6 |pages=2967–2986 |doi=10.1039/D0SE00222D |hdl=20.500.11850/422246 |hdl-access=free }}</ref> Application of ATR technology with integrated capture of carbon dioxide in Europe has been assessed to have a lower greenhouse gas footprint than burning natural gas, e.g. for the H21 project with a reported reduction of 68% due to a reduced carbon dioxide intensity of natural gas combined with a more suitable reactor type for capture of carbon dioxide.<ref>[https://zeroemissionsplatform.eu/wp-content/uploads/ZEP-paper-Facts-on-low-carbon-hydrogen-%E2%80%93-A-European-perspective-October-2021.pdf "Facts on low-carbon hydrogen – A European perspective"], ZEP Oct 2021. Confirmed 2023-12-12.</ref>

Hydrogen produced from [[renewable energy]] sources is often referred to as '''green hydrogen'''. Two ways of producing hydrogen from renewable energy sources are claimed to be practical. One is to use [[power to gas]], in which electric power is used to produce hydrogen from [[electrolysis of water]], and the other is to use [[landfill gas]] to produce hydrogen in a steam reformer. Hydrogen fuel, when produced by renewable sources of energy like wind or solar power, is a [[renewable fuel]].<ref>{{cite journal |url=http://www.nrel.gov/docs/fy04osti/36178.pdf#page=4 |publisher=[[National Renewable Energy Laboratory]] |journal=Research Review |title=New Horizons for Hydrogen |date=April 2004 |issue=2 |pages=2–9}}</ref><ref>Dvorak, Phred, [https://www.wsj.com/articles/green-hydrogen-gets-a-boost-in-the-u-s-with-4-billion-plant-11670458815 "WSJ News Exclusive: Green Hydrogen Gets a Boost in the U.S. With $4 Billion Plant]: The planned factory, a joint venture by [[Air Products]] and [[AES Corporation|AES]] ...", ''Wall Street Journal'', December 8, 2022. Retrieved 2023-11-14. {{subscription required}}</ref> Hydrogen produced from [[nuclear power|nuclear energy]] via electrolysis is sometimes viewed as a subset of [[green hydrogen]], but can also be referred to as '''pink hydrogen'''. The [[Oskarshamn Nuclear Power Plant]] made an agreement in January 2022 to supply commercial pink hydrogen in the order of kilograms per day.<ref>{{cite web |last1=Collins |first1=Leigh |title=World first for nuclear-powered pink hydrogen as commercial deal signed in Sweden {{!}} Recharge |url=https://www.rechargenews.com/energy-transition/world-first-for-nuclear-powered-pink-hydrogen-as-commercial-deal-signed-in-sweden/2-1-1155202 |website=Recharge {{!}} Latest renewable energy news |language=en |date=25 January 2022}}</ref>

{{as of|2020}}, estimated costs of production are $1–1.80/kg for grey hydrogen and blue hydrogen,<ref name=collin2020>{{cite web |last1=Collins |first1=Leigh |title=A wake-up call on green hydrogen: the amount of wind and solar needed is immense {{!}} Recharge |url=https://www.rechargenews.com/transition/a-wake-up-call-on-green-hydrogen-the-amount-of-wind-and-solar-needed-is-immense/2-1-776481 |website=Recharge {{!}} Latest renewable energy news |archive-url= https://web.archive.org/web/20210604060241/https://www.rechargenews.com/transition/a-wake-up-call-on-green-hydrogen-the-amount-of-wind-and-solar-needed-is-immense/2-1-776481 |archive-date=4 June 2021 |language=en |date=19 March 2020 |url-status=live}}</ref> and $2.50–6.80 for green hydrogen.<ref name=collin2020/>

94 million tonnes of grey hydrogen are produced globally using fossil fuels as of 2022, primarily natural gas, and are therefore a significant source of greenhouse gas emissions.<ref>{{Cite web |title=How does the energy crisis affect the transition to net zero? |url=https://www.eib.org/en/stories/energy-crisis-net-zero-transition |access-date=2022-12-23 |website=European Investment Bank |language=en}}</ref><ref>{{Cite web |title=Hydrogen – Fuels & Technologies |url=https://www.iea.org/fuels-and-technologies/hydrogen |access-date=2022-12-23 |website=[[International Energy Agency|IEA]] |language=en-GB}}</ref><ref>{{Cite journal |last=Castelvecchi |first=Davide |date=2022-11-16 |title=How the hydrogen revolution can help save the planet — and how it can't |journal=Nature |language=en |volume=611 |issue=7936 |pages=440–443 |doi=10.1038/d41586-022-03699-0 |pmid=36385542 |bibcode=2022Natur.611..440C }}</ref><ref>{{Cite web |title=Hydrogen |url=https://energy.ec.europa.eu/topics/energy-systems-integration/hydrogen_en |access-date=2022-12-23 |website=energy.ec.europa.eu |language=en}}</ref>

==Hydrogen uses== {{see also|Hydrogen economy}} Hydrogen is used for the conversion of heavy petroleum fractions into lighter ones via [[hydrocracking]]. It is also used in other processes including the [[aromatization]] process, [[hydrodesulfurization]] and the production of [[ammonia]] via the [[Haber process]], the primary industrial method for the production of synthetic nitrogen fertilizer for growing 47 percent of food worldwide.<ref>{{cite web |last1=Ritchie |first1=Hannah |author1-link=Hannah Ritchie |title=How many people does synthetic fertilizer feed? |url=https://ourworldindata.org/how-many-people-does-synthetic-fertilizer-feed |website=Our World in Data |date=7 November 2017 |publisher=Global Change Data Lab |access-date=16 September 2021}}</ref>

Hydrogen may be used in [[fuel cells]] for local electricity generation or potentially as a transportation fuel.

Hydrogen is produced as a [[by-product]] of industrial [[chlorine production]] by electrolysis. Although requiring expensive technologies, hydrogen can be cooled, compressed and purified for use in other processes on site or sold to a customer via pipeline, cylinders or trucks. The discovery and development of less expensive methods of production of bulk hydrogen is relevant to the establishment of a [[hydrogen economy]].<ref name=Ullmann>{{cite book |doi=10.1002/14356007.a13_297.pub2 |chapter=Hydrogen, 1. Properties and Occurrence |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2011 |last1=Häussinger |first1=Peter |last2=Lohmüller |first2=Reiner |last3=Watson |first3=Allan M. |isbn=978-3-527-30673-2}}</ref>

== See also == {{Commons}} {{div col|colwidth=20em}} * [[Ammonia production]] * [[Artificial photosynthesis]] * [[Biohydrogen]] * [[Hydrogen analyzer]] * [[Hydrogen compressor]] * {{section link|Hydrogen economy|Color codes}}{{Broken anchor|date=2024-03-23|bot=User:Cewbot/log/20201008/configuration|reason= The anchor (Color codes) [[Special:Diff/1215131921|has been deleted]].}} * [[Hydrogen embrittlement]] * [[Hydrogen leak testing]] * [[Hydrogen pipeline transport]] * [[Hydrogen purifier]] * [[Hydrogen safety]] * [[Hydrogen sensor]] * [[Hydrogen storage]] * [[Hydrogen station]] * [[Hydrogen tank]] * [[Hydrogen tanker]] * [[Hydrogen technologies]] * [[Hydrogen valve]] * [[Industrial gas]] * [[Liquid hydrogen]] * [[Next Generation Nuclear Plant]] (partly for hydrogen production) * [[Northern Gas Networks#Hy4heat|Hy4Heat]] * [[Lane hydrogen producer]] * [[Linde–Frank–Caro process]] * [[Underground hydrogen storage]]

== Notes == {{reflist|group=note}}

==References== {{reflist||refs= <ref name="bmwi-2020">{{cite book | author = BMWi | title = The national hydrogen strategy | date = June 2020 | publisher = Federal Ministry for Economic Affairs and Energy (BMWi) | location = Berlin, Germany | url = https://www.bmbf.de/files/bmwi_Nationale%20Wasserstoffstrategie_Eng_s01.pdf | access-date = 2020-11-27 | archive-date = 2020-12-13 | archive-url = https://web.archive.org/web/20201213170320/https://www.bmbf.de/files/bmwi_Nationale%20Wasserstoffstrategie_Eng_s01.pdf | url-status = live }}</ref> <ref name="van-de-graaf-etal-2020">{{cite journal |last1=Van de Graaf |first1=Thijs |last2=Overland |first2=Indra |last3=Scholten |first3=Daniel |last4=Westphal |first4=Kirsten |title=The new oil? The geopolitics and international governance of hydrogen |journal=Energy Research & Social Science |date=December 2020 |volume=70 |article-number=101667 |doi=10.1016/j.erss.2020.101667 |pmid=32835007 |pmc=7326412 |bibcode=2020ERSS...7001667V }}</ref> <ref name="sansom-etal-2020">{{cite book | last1 = Sansom | first1 = Robert | last2 = Baxter | first2 = Jenifer | last3 = Brown | first3 = Andy | last4 = Hawksworth | first4 = Stuart | last5 = McCluskey | first5 = Ian | title = Transitioning to hydrogen: assessing the engineering risks and uncertainties | date = 2020 | publisher = The Institution of Engineering and Technology (IET) | location = London, United Kingdom | url = https://www.theiet.org/media/4095/transitioning-to-hydrogen.pdf | access-date = 2020-03-22 | archive-date = 2020-05-08 | archive-url = https://web.archive.org/web/20200508054016/https://www.theiet.org/media/4095/transitioning-to-hydrogen.pdf | url-status = live }}</ref> <ref name="bruce-etal-2018">{{cite book | last1 = Bruce | first1 = S | last2 = Temminghoff | first2 = M | last3 = Hayward | first3 = J | last4 = Schmidt | first4 = E | last5 = Munnings | first5 = C | last6 = Palfreyman | first6 = D | last7 = Hartley | first7 = P | title = National hydrogen roadmap: pathways to an economically sustainable hydrogen industry in Australia | date = 2018 | publisher = CSIRO | location = Australia | url = http://ipswichgetup.grandkidzfuture.com/ewExternalFiles/Hydrogen%20CSIRO%202018.pdf | access-date = 2020-11-28 | archive-date = 2020-12-08 | archive-url = https://web.archive.org/web/20201208212256/http://ipswichgetup.grandkidzfuture.com/ewExternalFiles/Hydrogen%20CSIRO%202018.pdf | url-status = live }}</ref> }}

==Sources== *{{cite book |ref = {{harvid|IEA H2|2019}} | publisher=[[International Energy Agency]] | title=The Future of Hydrogen | year=2019 | url-access=registration | url=https://www.iea.org/reports/the-future-of-hydrogen }}

== Further reading == * {{cite book |editor1-last=Calise |editor1-first=Francesco |editor2-last=D'accadia |editor2-first=Massimo Dentice |editor3-last=Santarelli |editor3-first=Massimo |editor4-last=Lanzini |editor4-first=Andrea |editor5-last=Ferrero |editor5-first=Domenico |title=Solar Hydrogen Production |date=2019 |doi=10.1016/C2017-0-02289-9 |isbn=978-0-12-814853-2 }}

[[Category:Hydrogen production| ]]