View source for High-temperature electrolysis

{{short description|Technique for producing hydrogen from water}}
[[Image:High-temperature electrolysis.png|right|350px|thumb|High-temperature electrolysis schema.]]
[[File:Decarbonization of economy via H2.png|thumb|410x410px|Decarbonization of Economy via hydrogen produced from HTE]]
'''High-temperature electrolysis''' (also '''HTE''' or '''steam electrolysis,''' or '''HTSE''') is a technology for producing [[hydrogen]] from water at high temperatures or other products, such as iron or carbon [[nanomaterials]], as higher energy lowers needed electricity to split molecules and opens up new, potentially better electrolytes like molten salts or hydroxides.<ref>{{cite journal|author1=Hauch, A. |author2=Ebbesen, S. D. |author3=Jensen, S. H. |author4=Mogensen, M. |title=Highly Efficient high temperature electrolysis|journal=J. Mater. Chem.|year=2008|volume=18|issue=20 |pages=2331–2340|doi=10.1039/b718822f}}</ref><ref>{{Cite journal |last1=Licht |first1=Stuart |last2=Wu |first2=Hongjun |date=2011-12-22 |title=STEP Iron, a Chemistry of Iron Formation without CO 2 Emission: Molten Carbonate Solubility and Electrochemistry of Iron Ore Impurities |url=https://pubs.acs.org/doi/10.1021/jp2078715 |journal=The Journal of Physical Chemistry C |language=en |volume=115 |issue=50 |pages=25138–25147 |doi=10.1021/jp2078715 |issn=1932-7447}}</ref><ref>{{Cite journal |last1=Licht |first1=Stuart |last2=Cui |first2=Baochen |last3=Wang |first3=Baohui |date=2013-09-01 |title=STEP carbon capture – The barium advantage |url=https://www.sciencedirect.com/science/article/pii/S2212982013000097 |journal=Journal of CO2 Utilization |volume=2 |pages=58–63 |doi=10.1016/j.jcou.2013.03.006 |bibcode=2013JCOU....2...58L |issn=2212-9820}}</ref><ref>{{Cite journal |last1=Ren |first1=Jiawen |last2=Yu |first2=Ao |last3=Peng |first3=Ping |last4=Lefler |first4=Matthew |last5=Li |first5=Fang-Fang |last6=Licht |first6=Stuart |date=2019-11-19 |title=Recent Advances in Solar Thermal Electrochemical Process (STEP) for Carbon Neutral Products and High Value Nanocarbons |url=https://pubs.acs.org/doi/10.1021/acs.accounts.9b00405 |journal=Accounts of Chemical Research |language=en |volume=52 |issue=11 |pages=3177–3187 |doi=10.1021/acs.accounts.9b00405 |pmid=31697061 |issn=0001-4842}}</ref> Unlike electrolysis at room temperature, HTE operates at elevated temperature ranges depending on the thermal capacity of the material.<ref>{{Citation |last=Valderrama |first=César |title=High-Temperature Electrolysis |date=2016 |encyclopedia=Encyclopedia of Membranes |pages=937–939 |editor-last=Drioli |editor-first=Enrico |url=https://doi.org/10.1007/978-3-662-44324-8_2122 |access-date=2024-04-14 |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-662-44324-8_2122 |isbn=978-3-662-44324-8 |editor2-last=Giorno |editor2-first=Lidietta}}</ref> Because of the detrimental effects of burning fossil fuels<ref>{{Cite web |title=Fact Sheet {{!}} Climate, Environmental, and Health Impacts of Fossil Fuels (2021) {{!}} White Papers |url=https://www.eesi.org/papers/view/fact-sheet-climate-environmental-and-health-impacts-of-fossil-fuels-2021 |access-date=2024-04-14 |website=www.eesi.org}}</ref> on humans and the environment, HTE has become a necessary alternative and efficient method by which hydrogen can be prepared on a large scale and used as fuel. The vision of HTE is to move towards decarbonization<ref>{{Cite journal |last1=Shiva Kumar |first1=S. |last2=Lim |first2=Hankwon |date=November 2022 |title=An overview of water electrolysis technologies for green hydrogen production |url=https://doi.org/10.1016/j.egyr.2022.10.127 |journal=Energy Reports |volume=8 |pages=13793–13813 |doi=10.1016/j.egyr.2022.10.127 |bibcode=2022EnRep...813793S |issn=2352-4847}}</ref><ref name=":0">{{Cite journal |last1=Zainal |first1=Bidattul Syirat |last2=Ker |first2=Pin Jern |last3=Mohamed |first3=Hassan |last4=Ong |first4=Hwai Chyuan |last5=Fattah |first5=I.M.R. |last6=Rahman |first6=S.M. Ashrafur |last7=Nghiem |first7=Long D. |last8=Mahlia |first8=T M Indra |date=January 2024 |title=Recent advancement and assessment of green hydrogen production technologies |url=https://doi.org/10.1016/j.rser.2023.113941 |journal=Renewable and Sustainable Energy Reviews |volume=189 |pages=113941 |doi=10.1016/j.rser.2023.113941 |bibcode=2024RSERv.18913941Z |issn=1364-0321}}</ref> in all economic sectors. The material requirements for this process are: the heat source, the electrodes, the electrolyte, the electrolyzer membrane, and the source of electricity.    <gallery widths="300" heights="300">
</gallery>


==Principle==
The process utilizes energy (in the form of heat) from sources to convert water into steam, which is then passed into an electrolytic system (made up of two electrodes connected to the source of current, an electrolyte, and a membrane). At high temperatures (over 650 °C in most topologies), the materials used to construct the cells become conductive. Therefore, electrochemical reactions begin to occur, and the cell begins to function once it has reached the proper temperature and electricity is supplied<ref name=":2">{{Citation |last1=Crema |first1=Luigi |title=5 High-temperature electrolysis: efficient and versatile solution for multiple applications |date=2021-09-07 |pages=219–268 |url=https://www.degruyter.com/document/doi/10.1515/9783110596274-013/html |access-date=2024-04-14 |publisher=De Gruyter |language=en |doi=10.1515/9783110596274-013 |isbn=978-3-11-059627-4 |last2=Testi |first2=Matteo |last3=Trini |first3=Martina}}</ref> while it is being fed with steam. The steam will eventually split into hydrogen (cathode) and oxygen (anode)<ref>{{Citation |last=Mougin |first=J. |title=Hydrogen production by high-temperature steam electrolysis |date=2015 |work=Compendium of Hydrogen Energy |pages=225–253 |url=https://doi.org/10.1016/B978-1-78242-361-4.00008-X |access-date=2024-04-14 |publisher=Elsevier |doi=10.1016/b978-1-78242-361-4.00008-x|isbn=978-1-78242-361-4 }}</ref> according to the equations below:
:<chem>Overall: 2H2O -> 2H2 + O2</chem>
:<chem>Cathode: 2H2O ->2H + 2OH^{-}</chem>
:<chem>Anode: 2OH^{-} ->  H2O + (1/2)O2</chem>

==Efficiency==
[[File:Theoretical thermal water splitting efficiencies.webp|thumb|right|300px|Theoretical thermal water splitting efficiencies.<ref>https://inldigitallibrary.inl.gov/sites/sti/sti/4480292.pdf {{Bare URL PDF|date=August 2024}}</ref> <br> 60% efficient at 1000°[[Celsius|C]] <br> 
[[Steam reforming]] of [[Hydrocarbon|hydrocarbons]] to hydrogen is 70-85% efficient<ref>{{cite journal | doi=10.1155/2013/690627 | title=Hydrogen Production Technologies: Current State and Future Developments | date=2013 | last1=Kalamaras | first1=Christos M. | last2=Efstathiou | first2=Angelos M. | journal=Conference Papers in Energy | volume=2013 | pages=1–9 | doi-access=free }}</ref>
]] 
High temperature electrolysis is more efficient economically than traditional room-temperature [[electrolysis of water|electrolysis]] because some of the energy is supplied as heat, which is cheaper than electricity, and also because the electrolysis reaction is more efficient at higher temperatures.  In fact, at 2500&nbsp;°C, electrical input is unnecessary because water breaks down to hydrogen and oxygen through [[thermolysis]].  Such temperatures are impractical; proposed HTE systems operate between 100&nbsp;°C and 850&nbsp;°C.<ref name="Badwal 2012">{{cite journal|last=Badwal|first=SPS|author2=Giddey S |author3=Munnings C |journal=WIREs Energy and Environment|year=2012|doi=10.1002/wene.50|url=http://wires.wiley.com/WileyCDA/WiresArticle/wisId-WENE50.html|title=Hydrogen production via solid electrolytic routes|volume=2|issue=5|pages=473–487|s2cid=135539661 }}</ref><ref>[http://www.hi2h2.com/ Hi2h2 - High temperature electrolysis using SOEC]</ref><ref>[https://cordis.europa.eu/project/id/212903/reporting Final Report Summary – WELTEMP (Water Electrolysis at Elevated Temperatures)]</ref>

If one assumes that the electricity used comes from a [[heat engine]], it takes 141.86<!-- 6 August 2021: Is five digits of precision appropriate here? --> megajoules (MJ) of heat energy to produce one kg of hydrogen{{clarify|reason=at what assumed reaction conditions, and why is this less than the numbers to follow?|date=August 2021}}, for the HTE process itself and for the electricity required. At 100&nbsp;°C, 350 MJ of thermal energy are required (41% efficient). At 850&nbsp;°C, 225 MJ are required (64% efficient). Above 850&nbsp;°C, one begins to exceed the capacity of standard chromium steels to resist corrosion,<ref>{{cite web 
  |url=https://www.azom.com/article.aspx?ArticleID=1175 
  |access-date=6 August 2021
  |title=Stainless Steel - High Temperature Resistance 
  |date=8 January 2002 
  |website=azom.com 
  |publisher=AZO Materials 
  |quote=Most austenitic steels, with chromium contents of at least 18%, can be used at temperatures up to 870°C and [specialized grades] even higher. 
}}</ref> and it's already no easy matter to design and implement an industrial scale chemical process to operate at such a high temperature point.<!-- Courtesy of Captain OR Obvious -->

==Materials==
Solid oxide electrolysis cells (SOECs) are electrochemical devices that function at high temperatures and are used for high-temperature electrolysis. These cells' ingredients ensure that the device will function well both physically and electrochemically at high temperatures.<ref name=":2" /> Therefore, the selection of materials for the electrodes and electrolyte in a [[solid oxide electrolyser cell]] is essential. One option being investigated for the process<ref>Kazuya Yamada, Shinichi Makino, Kiyoshi Ono, Kentaro Matsunaga, Masato Yoshino, Takashi Ogawa, Shigeo Kasai, Seiji Fujiwara, and Hiroyuki Yamauchi "High Temperature Electrolysis for Hydrogen Production Using Solid Oxide Electrolyte Tubular Cells Assembly Unit", presented at AICHE Annual Meeting, San Francisco, California, November 2006. 
[http://aiche.confex.com/aiche/2006/techprogram/S3253.HTM abstract]</ref> used [[yttria-stabilized zirconia]] (YSZ) electrolytes, [[Nickel]] (Ni)-[[cermet]] steam/Hydrogen electrodes, and d Oxide of [[Lanthanum oxide]] (La<sub>2</sub>O<sub>3</sub>), [[Strontium]] and [[Cobalt]] oxygen electrodes.

==Economic potential==

Even with HTE, electrolysis is a fairly inefficient way to store energy.  Significant conversion losses of energy occur both in the electrolysis process, and in the conversion of the resulting hydrogen back into power.

At current hydrocarbon prices, HTE can not compete with [[pyrolysis]] of hydrocarbons as an economical source of hydrogen, which produces carbon dioxide as a by-product.

HTE is of interest as a more efficient route to the production [[Green hydrogen|"green" hydrogen]], to be used as a [[carbon neutral]] fuel and general energy storage. It may become economical if cheap non-fossil fuel sources of heat (concentrating solar, nuclear, geothermal, waste heat) can be used in conjunction with non-fossil fuel sources of electricity (such as solar, wind, ocean, nuclear).

Possible supplies of cheap high-temperature heat for HTE are all nonchemical, including [[nuclear reactor]]s, [[solar power|concentrating solar thermal collectors]], and [[Geothermal power|geothermal]] sources. HTE has been demonstrated in a laboratory at 108&nbsp;kilojoules (electric) per gram 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=2008-09-19
 |publisher=[[Science Daily]]
}}</ref> but not at a commercial scale.<ref>{{cite web
 | url= http://www.hydrogen.energy.gov/pdfs/nuclear_energy_h2_plan.pdf
 | title= Nuclear hydrogen R&D plan
 | date= March 2004
 | publisher= [[United States Department of Energy|U.S. Dept. of Energy]]
 | access-date= 2008-05-09
 | archive-url= https://web.archive.org/web/20130924021755/http://www.hydrogen.energy.gov/pdfs/nuclear_energy_h2_plan.pdf
 | archive-date= 2013-09-24
 | url-status= dead
 }}</ref>

'''<big>Advantages and Challenges</big>'''

Obviously, the most notable advantage of HTE is that it provides an opportunity for which green hydrogen is prepared on a large scale, because it has the potential for zero emissions.<ref name=":0" /> The process provides an improved reaction kinetics for the splitting of water molecule.<ref>{{Citation |last1=Elder |first1=Rachael |title=High Temperature Electrolysis |date=2015 |work=Carbon Dioxide Utilisation |pages=183–209 |url=https://doi.org/10.1016/B978-0-444-62746-9.00011-6 |access-date=2024-04-14 |publisher=Elsevier |doi=10.1016/b978-0-444-62746-9.00011-6 |last2=Cumming |first2=Denis |last3=Mogensen |first3=Mogens Bjerg|isbn=978-0-444-62746-9 }}</ref> Part of the electricity requirement is replaced with heat, which makes it a bit cheaper because electricity is more expensive than heat. 

However, HTE technology suffered limitations due to:

# Above 100 °C, the electrolysis of liquid water requires pressurization, and is therefore limited by the working pressures that can be reasonably attained.<ref>{{Cite journal |last1=Lohmann-Richters |first1=F. P. |last2=Renz |first2=S. |last3=Lehnert |first3=W. |last4=Müller |first4=M. |last5=Carmo |first5=M. |date=2021-11-01 |title=Review—Challenges and Opportunities for Increased Current Density in Alkaline Electrolysis by Increasing the Operating Temperature |journal=Journal of the Electrochemical Society |volume=168 |issue=11 |pages=114501 |doi=10.1149/1945-7111/ac34cc |issn=0013-4651|doi-access=free |bibcode=2021JElS..168k4501L }}</ref>
# creating materials that are both chemically and physically stable in conditions of intense oxidation and reduction, as well as high working temperatures.<ref name=":1">{{Citation |last1=Acar |first1=Canan |title=3.1 Hydrogen Production |date=2018-01-01 |work=Comprehensive Energy Systems |pages=1–40 |editor-last=Dincer |editor-first=Ibrahim |url=https://www.sciencedirect.com/science/article/pii/B9780128095973003047 |access-date=2024-04-14 |place=Oxford |publisher=Elsevier |doi=10.1016/b978-0-12-809597-3.00304-7 |isbn=978-0-12-814925-6 |last2=Dincer |first2=Ibrahim}}</ref>
# chemical and physical stability at low electrical conductivities, high working temperatures, and/or ionic concentrations.<ref name=":1" />

===Alternatives===
There are hundreds of [[thermochemical cycle]]s known to use heat to extract hydrogen from water. For instance, the thermochemical [[sulfur-iodine cycle]]. Since the electricity generation step has a fairly low efficiency and is eliminated, thermochemical production might reach higher efficiencies than HTE. However, large-scale thermochemical production will require significant advances in materials that can withstand high-temperature, high-pressure, highly corrosive environments.

==United States Department of Energy==
{{See also|Next Generation Nuclear Plant}}
The [[United States Department of Energy|DOE]] [[Office of Nuclear Energy]] has demonstration projects to test 3 nuclear facilities with high-temperature electrolysis in the United States at:<ref>{{cite web | url=https://www.energy.gov/ne/articles/3-nuclear-power-plants-gearing-clean-hydrogen-production | title=3 Nuclear Power Plants Gearing up for Clean Hydrogen Production }}</ref>

* [[Nine Mile Point Nuclear Generating Station]] in [[Oswego, New York|Oswego, NY]]
* [[Davis–Besse Nuclear Power Station]] in [[Oak Harbor, Ohio]]
* [[Prairie Island Nuclear Power Plant]] in [[Red Wing, Minnesota]]

==Mars ISRU==
{{see also|In-situ resource utilization}}
High temperature electrolysis with [[solid oxide electrolyser cell]]s was used to produce 5.37 grams of oxygen per hour on [[Mars]] from atmospheric carbon dioxide for the [[Mars Oxygen ISRU Experiment]] in the [[NASA]] [[Mars 2020]] [[Perseverance (rover)|Perseverance]] rover, using zirconia electrolysis devices.<ref name="Wall 2014">{{cite news |last=Wall |first=Mike |url=http://www.space.com/26705-nasa-2020-rover-mars-colony-tech.html |title=Oxygen-Generating Mars Rover to Bring Colonization Closer |work=Space.com |date=August 1, 2014 |access-date=2014-11-05 }}</ref><ref name=GFSC>[https://web.archive.org/web/20141106213923/http://ssed.gsfc.nasa.gov/IPM/Final/PDF/ThursPM.pdf The Mars Oxygen ISRU Experiment (MOXIE)] PDF. Presentation: MARS 2020 Mission and Instruments". November 6, 2014.</ref><ref>{{Cite web|last=Potter|first=Sean|date=2021-04-21|title=NASA's Perseverance Mars Rover Extracts First Oxygen from Red Planet|url=http://www.nasa.gov/press-release/nasa-s-perseverance-mars-rover-extracts-first-oxygen-from-red-planet|access-date=2021-04-22|website=NASA}}</ref>

==See also==
* [[Office of Nuclear Energy]]
* [[High-pressure electrolysis]]

==References==
*[https://www1.eere.energy.gov/solar/pdfs/doctor.pdf U.S. DOE high-temperature electrolysis]

==Footnotes==
{{reflist}}

{{electrolysis}}

{{DEFAULTSORT:High-Temperature Electrolysis}}
[[Category:Electrolysis]]
[[Category:Hydrogen production]]