Editing Electrolysis (section)

==Research trends==
===Electrolysis of carbon dioxide===
{{main|Electrochemical reduction of carbon dioxide }}
The electrochemical reduction or electrocatalytic conversion of [[carbon dioxide|CO<sub>2</sub>]] can produce value-added chemicals such as [[methane]], [[ethylene]], [[ethanol]], etc.<ref>{{cite journal|doi=10.1039/C4TA03893B|title=Sustainable production of formic acid by electrolytic reduction of gaseous carbon dioxide|journal=J. Mater. Chem. A|volume=3|issue=6|pages=3029|year=2015|last1=Lee|first1=Seunghwa|last2=Ju|first2=Hyungkuk|last3=Machunda|first3=Revocatus|last4=Uhm|first4=Sunghyun|last5=Lee|first5=Jae Kwang|last6=Lee|first6=Hye Jin|last7=Lee|first7=Jaeyoung|s2cid=98110035 }}</ref><ref>{{cite journal|doi=10.1021/jz1012627|title=Prospects of CO<sub>2</sub> Utilization via Direct Heterogeneous Electrochemical Reduction|journal=The Journal of Physical Chemistry Letters|volume=1|issue=24|pages=3451|year=2010|last1=Whipple|first1=Devin T.|last2=Kenis|first2=Paul J.A.|s2cid=101946630 }}</ref><ref>{{cite journal|doi=10.1016/j.cap.2011.01.003|title=Electrocatalytic reduction of CO<sub>2</sub> gas at Sn based gas diffusion electrode|journal=Current Applied Physics|volume=11|issue=4|pages=986|year=2011|last1=Machunda|first1=Revocatus L.|last2=Ju|first2=Hyungkuk|last3=Lee|first3=Jaeyoung|bibcode=2011CAP....11..986M}}</ref> The electrolysis of carbon dioxide gives formate or carbon monoxide, but sometimes more elaborate organic compounds such as [[ethylene]].<ref>{{cite book|last=Hori|first=Y|chapter=Electrochemical CO<sub>2</sub> Reduction on Metal Electrodes|year=2008|pages=141–153|title=Modern Aspects of Electrochemistry|volume=42|editor=C.G. Vayeanas, R. White and M.E. Gamboa-Aldeco|publisher=Springer|location=New York|edition=42|doi=10.1007/978-0-387-49489-0_3|isbn=978-0-387-49488-3}}.</ref>  This technology is under research as a carbon-neutral route to organic compounds.<ref>{{cite journal|doi=10.1021/cr300463y|pmid=23767781|title=Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO<sub>2</sub> Fixation|journal=Chemical Reviews|volume=113|issue=8|pages=6621–6658|year=2013|last1=Appel|first1=Aaron M.|last2=Bercaw|first2=John E.|last3=Bocarsly|first3=Andrew B.|last4=Dobbek|first4=Holger|last5=Dubois|first5=Daniel L.|last6=Dupuis|first6=Michel|last7=Ferry|first7=James G.|last8=Fujita|first8=Etsuko|last9=Hille|first9=Russ|last10=Kenis|first10=Paul J.A.|last11=Kerfeld|first11=Cheryl A.|last12=Morris|first12=Robert H.|last13=Peden|first13=Charles H.F.|last14=Portis|first14=Archie R.|last15=Ragsdale|first15=Stephen W.|last16=Rauchfuss|first16=Thomas B.|last17=Reek|first17=Joost N.H.|last18=Seefeldt|first18=Lance C.|last19=Thauer|first19=Rudolf K.|last20=Waldrop|first20=Grover L.|pmc=3895110}}</ref><ref>{{cite journal|doi=10.1039/C3CS60323G |pmid=24186433 |title=A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels |journal=Chem. Soc. Rev. |volume=43 |issue=2 |pages=631–675 |year=2014 |last1=Qiao |first1=Jinli |last2=Liu |first2=Yuyu |last3=Hong |first3=Feng |last4=Zhang |first4=Jiujun }}</ref>

===Electrolysis of acidified water===
{{main|Electrolysis of water}}
Electrolysis of water produces [[hydrogen]] and oxygen in a ratio of 2 to 1 respectively.
:2 H<sub>2</sub>O{{abbr|(l)|liquid}} → 2 H<sub>2</sub>{{abbr|(g)|gaseous}} + O<sub>2</sub>{{abbr|(g)|gaseous}} {{pad|2em}} ''E''° = +1.229&nbsp;V

The [[Energy conversion efficiency|energy efficiency]] of water electrolysis varies widely. The efficiency of an electrolyser is a measure of the enthalpy contained in the hydrogen (to undergo combustion with oxygen or some other later reaction), compared with the input electrical energy. Heat/enthalpy values for hydrogen are well published in science and engineering texts, as 144&nbsp;MJ/kg (40 kWh/kg).  Note that fuel cells (not electrolysers) cannot use this full amount of heat/enthalpy, which has led to some confusion when calculating efficiency values for both types of technology.  In the reaction, some energy is lost as heat. Some reports quote efficiencies between 50% and 70% for alkaline electrolysers (50 kWh/kg);<ref name="polly2023">{{cite web |last1=Martin |first1=Polly |title=Green hydrogen {{!}} Which type of electrolyser should you use? Alkaline, PEM, solid oxide or the latest tech? |url=https://www.hydrogeninsight.com/electrolysers/green-hydrogen-which-type-of-electrolyser-should-you-use-alkaline-pem-solid-oxide-or-the-latest-tech-/2-1-1480577 |website=rechargenews.com |language=en |date=5 July 2023}}</ref> however, higher practical efficiencies are available with the use of [[polymer electrolyte membrane electrolysis]] and catalytic technology, such as 95% efficiency.<ref name="carmo2013a">{{cite journal|last=Carmo|first=M |author2=Fritz D |author3=Mergel J |author4=Stolten D |title=A comprehensive review on PEM water electrolysis|journal=Journal of Hydrogen Energy|volume=38|issue=12|pages=4901|year=2013 |doi=10.1016/j.ijhydene.2013.01.151|bibcode=2013IJHE...38.4901C }}</ref><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
 |author1=Zittel, Werner
 |author2=Wurster, Reinhold
 |publisher=Ludwig-Bölkow-Systemtechnik GmbH
 |date=8 July 1996
 |url-status=dead
 |archive-url=https://web.archive.org/web/20070207080325/http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html#3.4
 |archive-date=7 February 2007
 |df=dmy
}}</ref>

The [[National Renewable Energy Laboratory]] estimated in 2006 that 1&nbsp;kg of hydrogen (roughly equivalent to 3&nbsp;kg, or 4 liters, of petroleum in energy terms) could be produced by wind powered electrolysis for between US$5.55 in the near term and US$2.27 in the longer term.<ref name="NRELElectrolysis">{{cite web|url=http://www.nrel.gov/docs/fy06osti/39534.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.nrel.gov/docs/fy06osti/39534.pdf |archive-date=2022-10-09 |url-status=live |title=Wind Energy and Production of Hydrogen and Electricity – Opportunities for Renewable Hydrogen – Preprint |access-date=20 October 2008 |author1=Levene, J.  |author2=Kroposki, B. |author3=Sverdrup, G. |date=March 2006 |work=National Renewable Energy Laboratory}}</ref>

About 4% of hydrogen gas produced worldwide is generated by electrolysis, and normally used onsite. Hydrogen is used for the creation of ammonia for fertilizer via the [[Haber process]], and converting heavy petroleum sources to lighter fractions via [[hydrocracking]]. Onsite electrolysis has been utilized to capture hydrogen for hydrogen fuel-cells in [[hydrogen vehicles]].

===Carbon/hydrocarbon assisted water electrolysis===
{{main|Hydrogen production}}
Recently, to reduce the energy input, the utilization of carbon ([[coal]]), [[Alcohol (chemistry)|alcohol]]s (hydrocarbon solution), and organic solution ([[glycerol]], formic acid, [[ethylene glycol]], etc.) with co-electrolysis of water has been proposed as a viable option.<ref>{{cite journal|doi=10.1016/j.apenergy.2018.09.125|title=A comprehensive review of carbon and hydrocarbon assisted water electrolysis for hydrogen production|journal=Applied Energy|volume=231|pages=502–533|year=2018|last1=Ju|first1=Hyungkuk| last2=Badwal|first2=Sukhvinder|last3=Giddey|first3=Sarbjit|bibcode=2018ApEn..231..502J |s2cid=117669840 }}</ref><ref>{{cite journal|doi=10.1016/j.electacta.2016.07.062|title=Electro-catalytic conversion of ethanol in solid electrolyte cells for distributed hydrogen generation|journal=Electrochimica Acta|volume=212|pages=744–757|year=2016|last1=Ju|first1=Hyungkuk|last2=Giddey|first2=Sarbjit|last3=Badwal|first3=Sukhvinder P.S.|last4=Mulder|first4=Roger J.}}</ref> The carbon/hydrocarbon assisted water electrolysis (so-called CAWE) process for hydrogen generation would perform this operation in a single [[electrochemical]] reactor. This system energy balance can be required only around 40% electric input with 60% coming from the chemical energy of carbon or hydrocarbon.<ref>{{cite journal|doi=10.1016/j.ijhydene.2014.11.033|title=Low emission hydrogen generation through carbon assisted electrolysis|journal=International Journal of Hydrogen Energy|volume=40|pages=70–74|year=2015|last1=Giddey|first1=S.|last2=Kulkarni|first2=A.|last3=Badwal|first3=S.P.S.|issue=1 |bibcode=2015IJHE...40...70G }}</ref> This process utilizes solid coal/carbon particles or powder as fuels dispersed in acid/alkaline electrolyte in the form of slurry and the carbon contained source co-assist in the electrolysis process as following theoretical overall reactions:<ref>{{cite journal|doi=10.1016/j.ijhydene.2018.03.195|title=Role of iron species as mediator in a PEM based carbon-water co-electrolysis for cost-effective hydrogen production|journal=International Journal of Hydrogen Energy|volume=43|issue=19|pages=9144–9152|year=2018|last1=Ju|first1=Hyungkuk|last2=Giddey|first2=Sarbjit|last3=Badwal|first3=Sukhvinder P.S.|bibcode=2018IJHE...43.9144J }}
</ref>

:Carbon/Coal slurry (C + 2H<sub>2</sub>O) → CO<sub>2</sub> + 2H<sub>2</sub> {{pad|2em}} ''E''′ = 0.21&nbsp;V (reversible voltage) / ''E''′ = 0.46&nbsp;V (thermo-neutral voltage)

or

:Carbon/Coal slurry (C + H<sub>2</sub>O) → CO + H<sub>2</sub> {{pad|2em}} ''E''′ = 0.52&nbsp;V (reversible voltage) / ''E''′ = 0.91&nbsp;V (thermo-neutral voltage)

Thus, this CAWE approach is that the actual cell overpotential can be significantly reduced to below 1.0&nbsp;V as compared to 1.5&nbsp;V for conventional water electrolysis.

===Electrocrystallization===
A specialized application of electrolysis involves the growth of conductive crystals on one of the electrodes from oxidized or reduced species that are generated in situ.  The technique has been used to obtain single crystals of low-dimensional electrical conductors, such as [[charge-transfer salt]]s and [[linear chain compound]]s.<ref>{{cite journal|doi=10.1021/ja00399a065|title=Superconductivity in an organic solid. Synthesis, structure, and conductivity of bis(tetramethyltetraselenafulvalenium) perchlorate, (TMTSF)<sub>2</sub>ClO<sub>4</sub>|year=1981|journal=Journal of the American Chemical Society|volume=103|issue=9|pages=2440|last1=Bechgaard|first1=K.|last2=Carneiro|first2=K.|last3=Rasmussen|first3=F. B.|last4=Olsen|first4=M.|last5=Rindorf|first5=G.|last6=Jacobsen|first6=C.S.|last7=Pedersen|first7=H.J.|last8=Scott|first8=J.C.|bibcode=1981JAChS.103.2440B }}</ref><ref>{{cite book | last1 = Williams | first1 = Jack M. | title = Inorganic Syntheses | year = 2007 | volume = 26 | pages = 386–394 | doi = 10.1002/9780470132579.ch70 | isbn = 978-0-470-13257-9 | chapter = Highly Conducting and Superconducting Synthetic Metals }}</ref>

=== Electrolysis of Iron Ore ===
The current method of producing steel from [[iron ore]] is very carbon intensive, in part to the direct release of CO<sub>2</sub> in the blast furnace. A study of steel making in Germany found that producing 1 ton of steel emitted 2.1 tons of [[Global warming potential|CO<sub>2</sub>e]] with 22% of that being direct emissions from the blast furnace.<ref>{{Cite journal |last1=Backes |first1=Jana Gerta |last2=Suer |first2=Julian |last3=Pauliks |first3=Nils |last4=Neugebauer |first4=Sabrina |last5=Traverso |first5=Marzia |date=19 March 2021 |title=Life Cycle Assessment of an Integrated Steel Mill Using Primary Manufacturing Data: Actual Environmental Profile |journal=Sustainability |language=en |volume=13 |issue=6 |pages=3443 |doi=10.3390/su13063443 |issn=2071-1050|doi-access=free |bibcode=2021Sust...13.3443B }}</ref> As of 2022, steel production contributes 7–9% of global emissions.<ref>{{Cite journal |last1=Lopes |first1=Daniela V. |last2=Quina |first2=Margarida J. |last3=Frade |first3=Jorge R. |last4=Kovalevsky |first4=Andrei V. |date=2022 |title=Prospects and challenges of the electrochemical reduction of iron oxides in alkaline media for steel production |journal=Frontiers in Materials |volume=9 |doi=10.3389/fmats.2022.1010156 |bibcode=2022FrMat...910156L |issn=2296-8016|doi-access=free }}</ref> Electrolysis of iron can eliminate direct emissions and further reduce emissions if the electricity is created from green energy.

The small-scale electrolysis of iron has been successfully reported by dissolving it in molten [[oxide]] salts and using a platinum anode.<ref>{{Cite journal |last1=Wiencke |first1=Jan |last2=Lavelaine |first2=Hervé |last3=Panteix |first3=Pierre-Jean |last4=Petitjean |first4=Carine |last5=Rapin |first5=Christophe |date=2018-01-01 |title=Electrolysis of iron in a molten oxide electrolyte |journal=Journal of Applied Electrochemistry |language=en |volume=48 |issue=1 |pages=115–126 |doi=10.1007/s10800-017-1143-5 |s2cid=102871146 |issn=1572-8838|doi-access=free }}</ref> Oxygen anions form oxygen gas and electrons at the anode. Iron cations consume electrons and form iron metal at the cathode. This method was performed a temperature of 1550&nbsp;°C which presents a significant challenge to maintaining the reaction. Particularly, anode [[corrosion]] is a concern at these temperatures.

Additionally, the low temperature reduction of iron oxide by dissolving it in alkaline water has been reported.<ref>{{Cite journal |last1=Yuan |first1=Boyan |last2=Haarberg |first2=Geir Martin |date=2009-03-20 |title=Electrodeposition of Iron in Aqueous Alkaline Solution: An Alternative to Carbothermic Reduction |url=https://iopscience.iop.org/article/10.1149/1.3114006/meta |journal=ECS Transactions |language=en |volume=16 |issue=36 |pages=31 |doi=10.1149/1.3114006 |bibcode=2009ECSTr..16J..31Y |s2cid=96771590 |issn=1938-5862}}</ref> The temperature is much lower than traditional iron production at 114&nbsp;°C. The low temperatures also tend to correlate with higher current efficiencies, with an efficiency of 95% being reported. While these methods are promising, they struggle to be cost competitive because of the large economies of scale keeping the price of blast furnace iron low.

=== Electrolysis of seawater ===
A 2020 study investigated direct electrolysis of seawater, alkaline electrolysis, [[Proton-exchange membrane|proton-exchange membrane electrolysis]], and [[Solid oxide electrolyser cell|solid oxide electrolysis]].<ref>{{Cite journal |last1=d’Amore-Domenech |first1=Rafael |last2=Santiago |first2=Óscar |last3=Leo |first3=Teresa J. |date=2020 |title=Multicriteria analysis of seawater electrolysis technologies for green hydrogen production at sea |url=https://linkinghub.elsevier.com/retrieve/pii/S1364032120304573 |journal=Renewable and Sustainable Energy Reviews |language=en |volume=133 |pages=110166 |doi=10.1016/j.rser.2020.110166|bibcode=2020RSERv.13310166D |s2cid=224843343 }}</ref> Direct electrolysis of seawater follows known processes, forming an electrolysis cell in which the seawater acts as the electrolyte to allow for the reaction at the [[anode]], {{chem2 | 2 Cl-(aq) -> Cl2(g) + 2e-}} and the reaction at the [[cathode]], {{chem2 | 2 H2O(l) + 2 e- -> H2(g) + 2OH-(aq) }}. The inclusion of [[magnesium]] and [[calcium]] [[ion]]s in the seawater makes the production of alkali [[hydroxide]]s possible that could form scales in the  electrolyser cell, cutting down on lifespan and increasing the need for maintenance. The alkaline electrolysers operate with the following reactions at the anode, {{chem2 | 2 OH-(aq) -> 1/2 O2(g) + H2O(l) + 2 e- }}and cathode, {{chem2 | 2 H2O(l) + 2 e- -> H2(g) + 2 OH-(aq) }}, and use high base solutions as electrolytes, operating at {{Convert|60-90|C|F|abbr=on}} and need additional separators to ensure the gas phase hydrogen and oxygen remain separate. The electrolyte can easily get contaminated, but the alkaline  electrolyser can operate under pressure to improve energy consumption. The electrodes can be made of inexpensive materials and there's no requirement for an expensive catalyst in the design.   

Many alternatives to the simple electrolyzer above are there.  There are micro-electrolyzer designs that are able to eliminate the separator requirement by designing the internal flow to separate the gases autonomously.  See for example US12116679B2 where the operating pressure is increased to the point of Chlorine liquefaction so that sea water electrolyzer can proceed in a locally alkaline electrolytic fluid.   Removing separators allows operating at very high temperatures.  The structural design allows for operations at upto 700 bar thereby eliminating the need for Hydrogen compressors.   

Proton-exchange membrane electrolysers operate with the reactions at the anode, {{chem2 | H2O(l) -> 1/2 O2(g) + 2 H+(aq) + 2 e- }} and cathode, {{chem2 | 2 H+(aq) + 2 e- -> H2(g) }}, at temperatures of {{Convert|60-80|C|F|abbr=on}}, using a solid polymer electrolyte and requiring higher costs of processing to allow the solid electrolyte to touch uniformly to the electrodes. Similar to the alkaline electrolyser, the proton exchange membrane  electrolyser can operate at higher pressures, reducing the energy costs required to compress the hydrogen gas afterward, but the proton exchange membrane  electrolyser also benefits from rapid response times to changes in power requirements or demands and not needing maintenance, at the cost of having a faster inherent degradation rate and being the most vulnerable to impurities in the water. 

Solid oxide electrolysers run the reactions {{chem2 | O2-(g) -> 1/2 O2(g) + 2 e- }} at the anode and {{chem2 | H2O(g) + 2 e- -> H2(g) + O2-(g) }} at the cathode. The solid oxide electrolysers require high temperatures ({{Convert|700-1000|C|F|abbr=on}}) to operate, generating superheated steam. They suffer from degradation when turned off, making it a more inflexible hydrogen generation technology. In a selected series of [[Multiple-criteria decision analysis|multiple-criteria decision-analysis]] comparisons in which the highest priority was placed on economic operation costs followed equally by environmental and social criteria, it was found that the proton exchange membrane electrolyser offered the most suitable combination of values (e.g., investment cost, maintenance, and operation cost, resistance to impurities, specific energy for hydrogen production at sea, risk of environmental impact, etc.), followed by the alkaline electrolyser, with the alkaline electrolyser being the most economically feasible, but more hazardous in terms of safety and environmental concerns due to the need for basic electrolyte solutions as opposed to the solid polymers used in proton-exchange membranes. Due to the methods conducted in multiple-criteria decision analysis, non-objective weights are applied to the various factors, and so multiple methods of decision analysis were performed simultaneously to examine the electrolysers in a way that minimizes the effects of bias on the performance conclusions.