View source for Anion exchange membrane electrolysis

{{Short description|Splitting of water using a semipermeable membrane}}

{{Infobox
 |title       = AEM electrolysis
 |image = [[File:AEM water electrolysis working principle with HER and OER.png|frameless]]
 |caption = AEM water electrolysis working principle with HER and OER.
}}

'''Anion exchange membrane''' '''(AEM) electrolysis''' is the [[electrolysis of water]] that utilises a semipermeable membrane that conducts [[hydroxide]] ions (OH<sup>−</sup>) called an [[anion exchange membrane]]. Like a [[proton-exchange membrane]] (PEM), the membrane separates the products, provides electrical insulation between electrodes, and conducts ions. Unlike PEM, AEM conducts hydroxide ions. The major advantage of AEM water electrolysis is that a high-cost [[noble metal]] [[catalyst]] is not required, low-cost transition metal catalyst can be used instead.<ref name="john2014">{{cite journal |last1=Varcoe |first1=John R. |last2=Atanassov |first2=Plamen |last3=Dekel |first3=Dario R. |last4=Herring |first4=Andrew M. |last5=Hickner |first5=Michael A. |last6=Kohl |first6=Paul. A. |last7=Kucernak |first7=Anthony R. |last8=Mustain |first8=William E. |last9=Nijmeijer |first9=Kitty |last10=Scott |first10=Keith |last11=Xu |first11=Tongwen |last12=Zhuang |first12=Lin |title=Anion-exchange membranes in electrochemical energy systems |journal=Energy Environ. Sci. |date=2014 |volume=7 |issue=10 |pages=3135–3191 |doi=10.1039/C4EE01303D|doi-access=free |hdl=10044/1/24509 |hdl-access=free }}</ref><ref name="dario2018">{{cite journal |last1=Dekel |first1=Dario R. |title=Review of cell performance in anion exchange membrane fuel cells |journal=Journal of Power Sources |date=January 2018 |volume=375 |pages=158–169 |doi=10.1016/j.jpowsour.2017.07.117|doi-access=free |bibcode=2018JPS...375..158D }}</ref> AEM electrolysis is similar to [[alkaline water electrolysis]], which uses a non-ion-selective separator instead of an anion-exchange membrane.

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==Advantages and Challenges==
===Advantages===
Of all water electrolysis methods, AEM electrolysis can combine the advantages of [[alkaline water electrolysis]] (AWE) and PEM electrolysis.<ref name="Juchan2021">{{cite journal |last1=Yang |first1=Juchan |last2=Jang |first2=Myeong Je |last3=Zeng |first3=Xiaojun |last4=Park |first4=Yoo Sei |last5=Lee |first5=Jooyoung |last6=Choi |first6=Sung Mook |last7=Yin |first7=Yadong |title=Non-precious electrocatalysts for oxygen evolution reaction in anion exchange membrane water electrolysis: A mini review |journal=Electrochemistry Communications |date=October 2021 |volume=131 |pages=107118 |doi=10.1016/j.elecom.2021.107118|doi-access=free }}</ref><ref name="paige2021">{{cite journal |last1=Shirvanian |first1=Paige |last2=Loh |first2=Adeline |last3=Sluijter |first3=Soraya |last4=Li |first4=Xiaohong |title=Novel components in anion exchange membrane water electrolyzers (AEMWE's): Status, challenges and future needs. A mini review |journal=Electrochemistry Communications |date=November 2021 |volume=132 |pages=107140 |doi=10.1016/j.elecom.2021.107140|hdl=10871/127974 |s2cid=240613300 |hdl-access=free }}</ref>
[[Polymer electrolyte membrane electrolysis]] uses expensive [[platinum-group metals]] (PGMs) such as [[platinum]], [[iridium]], and [[ruthenium]] as a catalyst. Iridium, for instance, is more scarce than platinum; a 100 MW PEM electrolyser is expected to require 150&nbsp;kg of Iridium, which will cost an estimated 7 million [[United States dollar|USD]].<ref name="Henkensmeier2021">{{cite journal |last1=Henkensmeier |first1=Dirk |last2=Najibah |first2=Malikah |last3=Harms |first3=Corinna |last4=Žitka |first4=Jan |last5=Hnát |first5=Jaromír |last6=Bouzek |first6=Karel |title=Overview: State-of-the Art Commercial Membranes for Anion Exchange Membrane Water Electrolysis |journal=Journal of Electrochemical Energy Conversion and Storage |date=1 May 2021 |volume=18 |issue=2 |doi=10.1115/1.4047963|doi-access=free }}</ref> Like [[alkaline water electrolysis]], electrodes in AEM electrolysis operate in an alkaline environment, which allows non-noble, low-cost catalysts based on Ni, Fe, Co, Mn, Cu, etc to be used.<ref name="john2014" /><ref name="Juchan2021" /><ref name="paige2021" /><ref name="Qiucheng2022">{{cite journal |last1=Xu |first1=Qiucheng |last2=Zhang |first2=Liyue |last3=Zhang |first3=Jiahao |last4=Wang |first4=Jingyu |last5=Hu |first5=Yanjie |last6=Jiang |first6=Hao |last7=Li |first7=Chunzhong |title=Anion Exchange Membrane Water Electrolyzer: Electrode Design, Lab-Scaled Testing System and Performance Evaluation |journal=EnergyChem |date=September 2022 |volume=4 |issue=5 |pages=100087 |doi=10.1016/j.enchem.2022.100087|s2cid=251354531 |url=https://backend.orbit.dtu.dk/ws/files/291084151/1_s2.0_S2589778022000197_main.pdf }}</ref><ref name="Grace2020">{{cite journal |last1=Lindquist |first1=Grace A. |last2=Xu |first2=Qiucheng |last3=Oener |first3=Sebastian Z. |last4=Boettcher |first4=Shannon W. |title=Membrane Electrolyzers for Impure-Water Splitting |journal=Joule |date=December 2020 |volume=4 |issue=12 |pages=2549–2561 |doi=10.1016/j.joule.2020.09.020|doi-access=free }}</ref>

AEM electrolyser can run on pure water or slightly alkaline solutions (0.1-1M KOH/NaOH) unlike highly concentrated alkaline solutions (5M KOH/NaOH) in AWE.<ref name="Juchan2021" /><ref name="shiva2022">{{cite journal |last1=Shiva Kumar |first1=S. |last2=Lim |first2=Hankwon |title=An overview of water electrolysis technologies for green hydrogen production |journal=Energy Reports |date=November 2022 |volume=8 |pages=13793–13813 |doi=10.1016/j.egyr.2022.10.127|doi-access=free }}</ref> This reduces the risk of leakage.<ref name="Juchan2021" /> Using an alkaline solution, usually KOH/NaOH increases membrane conductivity and adds a hydroxide ion conductive pathway, which increases the utilisation of catalyst. The current density of an AEM electrolyser without a PGM catalyst operating at 1 A/cm<sup>2</sup> was reported to require 1.8 volts and 1.57 volts in pure water-fed and 1 M KOH-fed, respectively.<ref name="dongguo2021">{{cite journal |last1=Li |first1=Dongguo |last2=Motz |first2=Andrew R. |last3=Bae |first3=Chulsung |last4=Fujimoto |first4=Cy |last5=Yang |first5=Gaoqiang |last6=Zhang |first6=Feng-Yuan |last7=Ayers |first7=Katherine E. |last8=Kim |first8=Yu Seung |title=Durability of anion exchange membrane water electrolyzers |journal=Energy & Environmental Science |date=2021 |volume=14 |issue=6 |pages=3393–3419 |doi=10.1039/d0ee04086j|doi-access=free }}</ref> Electrolyte can be fed on both anode and cathode side or anode side only.<ref name="Dongyu2019">{{cite journal |last1=Xu |first1=Dongyu |last2=Stevens |first2=Michaela Burke |last3=Cosby |first3=Monty R. |last4=Oener |first4=Sebastian Z. |last5=Smith |first5=Adam M. |last6=Enman |first6=Lisa J. |last7=Ayers |first7=Katherine E. |last8=Capuano |first8=Christopher B. |last9=Renner |first9=Julie N. |last10=Danilovic |first10=Nemanja |last11=Li |first11=Yaogang |last12=Wang |first12=Hongzhi |last13=Zhang |first13=Qinghong |last14=Boettcher |first14=Shannon W. |title=Earth-Abundant Oxygen Electrocatalysts for Alkaline Anion-Exchange-Membrane Water Electrolysis: Effects of Catalyst Conductivity and Comparison with Performance in Three-Electrode Cells |journal=ACS Catalysis |date=4 January 2019 |volume=9 |issue=1 |pages=7–15 |doi=10.1021/acscatal.8b04001|s2cid=105391723 }}</ref>

In the zero-gap design of AWE, the electrodes are separated only by a diaphragm which separates the gases. The diaphragm only allows water and hydroxide ions to pass through, but does not completely eliminate gas cross-over. Oxygen gas can enter the hydrogen half-cell and react on the cathode side to form water, which reduces the efficiency of the cell. Gas cross-over from the H<sub>2</sub> to the O<sub>2</sub> evolution side can pose a safety hazard because it can create an explosive gas mixture with >4%mol H<sub>2</sub>.<ref name="shiva2022" /><ref name="Hamish2020">{{cite journal |last1=Miller |first1=Hamish Andrew |last2=Bouzek |first2=Karel |last3=Hnat |first3=Jaromir |last4=Loos |first4=Stefan |last5=Bernäcker |first5=Christian Immanuel |last6=Weißgärber |first6=Thomas |last7=Röntzsch |first7=Lars |last8=Meier-Haack |first8=Jochen |title=Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions |journal=Sustainable Energy & Fuels |date=2020 |volume=4 |issue=5 |pages=2114–2133 |doi=10.1039/c9se01240k|doi-access=free }}</ref> The AEM electrolyser was reported to maintain H<sub>2</sub> crossover to less than 0.4% for the 5000 h of operation.<ref name="marta2023">{{cite journal |last1=Moreno-González |first1=Marta |last2=Mardle |first2=Peter |last3=Zhu |first3=Shan |last4=Gholamkhass |first4=Bobak |last5=Jones |first5=Scot |last6=Chen |first6=Nathan |last7=Britton |first7=Benjamin |last8=Holdcroft |first8=Steven |title=One year operation of an anion exchange membrane water electrolyzer utilizing Aemion+® membrane: Minimal degradation, low H2 crossover and high efficiency |journal=Journal of Power Sources Advances |date=January 2023 |volume=19 |pages=100109 |doi=10.1016/j.powera.2023.100109|doi-access=free }}</ref>

AEM based on an aromatic polymer backbone is promising due to its significant cost reduction. Compare to [[Nafion]] membrane use in PEM, the production of Nafion required highly toxic chemicals, which increased the cost (>1000$/m<sup>2</sup>)<ref name="Mohamed2022">{{cite journal |last1=Mamlouk |first1=Mohamed |title=Alkaline Anion Exchange Membrane (AEM) Water Electrolysers—Current/Future Perspectives in Electrolysers for Hydrogen |journal=Comprehensive Renewable Energy |date=2022 |pages=473–504 |doi=10.1016/B978-0-12-819727-1.00103-5|isbn=9780128197349 |s2cid=239231605 }}</ref><ref name="Piyachai2021">{{cite journal |last1=Khomein |first1=Piyachai |last2=Ketelaars |first2=Wesley |last3=Lap |first3=Tijs |last4=Liu |first4=Gao |title=Sulfonated aromatic polymer as a future proton exchange membrane: A review of sulfonation and crosslinking methods |journal=Renewable and Sustainable Energy Reviews |date=March 2021 |volume=137 |pages=110471 |doi=10.1016/j.rser.2020.110471|s2cid=228845451 |doi-access=free }}</ref> and [[fluorocarbon]] gas is produced at the production stage of
tetrafluoroethylene, which poses a strong environmental impact.<ref name="carlo2022">{{cite journal |last1=Santoro |first1=Carlo |last2=Lavacchi |first2=Alessandro |last3=Mustarelli |first3=Piercarlo |last4=Di Noto |first4=Vito |last5=Elbaz |first5=Lior |last6=Dekel |first6=Dario R. |last7=Jaouen |first7=Frédéric |title=What is Next in Anion-Exchange Membrane Water Electrolyzers? Bottlenecks, Benefits, and Future |journal=ChemSusChem |date=22 April 2022 |volume=15 |issue=8 |pages=e202200027 |doi=10.1002/cssc.202200027|pmid=35263034 |pmc=9310600 |bibcode=2022ChSCh..15E0027S |hdl=10281/380104 |hdl-access=free }}</ref> Fluorinated raw materials are inessential for AEM, allowing for a wider selection of low-cost polymer chemistry.<ref name="dario2018" />

===Challenges===

[[File:Degraded AEM after elongation testing.jpg|thumb|Degraded AEM Fumatech FAA3-PK-75 after mechanical strength testing to evaluate the mechanical properties of the membrane.<ref name="malikah2021">{{cite journal |last1=Najibah |first1=Malikah |last2=Tsoy |first2=Ekaterina |last3=Khalid |first3=Hamza |last4=Chen |first4=Yongfang |last5=Li |first5=Qingfeng |last6=Bae |first6=Chulsung |last7=Hnát |first7=Jaromír |last8=Plevová |first8=Michaela |last9=Bouzek |first9=Karel |last10=Jang |first10=Jong Hyun |last11=Park |first11=Hyun S. |last12=Henkensmeier |first12=Dirk |title=PBI nanofiber mat-reinforced anion exchange membranes with covalently linked interfaces for use in water electrolysers |journal=Journal of Membrane Science |date=December 2021 |volume=640 |pages=119832 |doi=10.1016/j.memsci.2021.119832|s2cid=239639812 |doi-access=free }}</ref>]]

AEM electrolysis is still in the early research and development stage, while alkaline water electrolysis is in the mature stage and PEM electrolysis is in the commercial stage. There is less academic literature on pure-water fed AEM electrolysers compared to the usage of KOH solution.<ref name="Hamish2020" />
The major technical challenge facing a consumer level AEM electrolyser is the low durability of the membrane, which refers to the short device lifetime or longevity. The lifetimes of PEM electrolyser stacks range from 20,000 h to 80,000 h.<ref name="dongguo2021" /><ref name="shiva2022" /> Literature surveys have found that AEM electrolyser durability is demonstrated to be >2000 h, >12,000 h, and >700 h for pure water-fed (Pt group catalyst on anode and cathode), concentrated KOH-fed, and 1wt% K<sub>2</sub>CO<sub>3</sub>-fed respectively.<ref name="dongguo2021" />

To overcome the obstacles for a large scale usage of AEM, increasing ionic conductivity and durability is essential. Many AEM breakdown at temperatures higher than 60&nbsp;°C, AEM that can tolerate the presence of O<sub>2</sub>, high pH, and temperatures exceeding 60&nbsp;°C are needed.<ref name="Naiying2022" /><ref name="kishore2023">{{cite journal |last1=Chand |first1=Kishore |last2=Paladino |first2=Ombretta |title=Recent developments of membranes and electrocatalysts for the hydrogen production by anion exchange membrane water electrolysers: A review |journal=Arabian Journal of Chemistry |date=February 2023 |volume=16 |issue=2 |pages=104451 |doi=10.1016/j.arabjc.2022.104451|hdl=11567/1101845 |s2cid=254002110 |hdl-access=free }}</ref>

==Science==
===Reactions===
[[Oxygen evolution reactions]] (OER) need four electrons to produce one molecule of O<sub>2</sub>, consume multiple OH<sup>−</sup> anions, and form multiple adsorbed intermediates on the surface of the catalyst. These multiple steps of reaction create a high energy barrier and thus a high [[overpotential]], which causes the OER to be sluggish. The performance of the AEM electrolyser largely depends on OER. The overpotential of OER can be reduced with an efficient catalyst that breaks the reaction's intermediate bond.
[[Hydrogen evolution reaction]] (HER) kinetics in alkaline solutions are slower than in acidic solutions because of additional proton dissociation and the formation of hydrogen intermediate (H*) that is not present in acidic conditions.<ref name="Naiying2022">{{cite journal |last1=Du |first1=Naiying |last2=Roy |first2=Claudie |last3=Peach |first3=Retha |last4=Turnbull |first4=Matthew |last5=Thiele |first5=Simon |last6=Bock |first6=Christina |title=Anion-Exchange Membrane Water Electrolyzers |journal=Chemical Reviews |date=13 July 2022 |volume=122 |issue=13 |pages=11830–11895 |doi=10.1021/acs.chemrev.1c00854|pmid=35442645 |pmc=9284563 }}</ref><ref name="Juchan2021" />

====Anode reaction====
{| style="border:1px solid #ccc; width:60%;"
|-
| <math>4\mathrm{OH}^- \rightarrow 3\mathrm{OH}^{-} + \mathrm{e}^- + \mathrm{OH}^*</math>
| style="text-align:right; vertical-align:top;"| <math>\left ( 1 \right )</math>
|}

{| style="border:1px solid #ccc; width:60%;"
|-
| <math>\mathrm{OH}^* + 3\mathrm{OH}^* + \mathrm{e}^- \rightarrow 2\mathrm{OH}^- + \mathrm{e}^- + \mathrm{O}^* + \mathrm{H}_2 \mathrm{O}</math>
| style="text-align:right; vertical-align:top;"| <math>\left ( 2 \right )</math>
|}

{| style="border:1px solid #ccc; width:60%;"
|-
| <math>\mathrm{O}^* + \mathrm{H}_2 \mathrm{O} + 2\mathrm{OH}^- + 2\mathrm{e}^- \rightarrow \mathrm{OH}^- + 3\mathrm{e}^- + \mathrm{OOH}^* + \mathrm{H}_2 \mathrm{O}</math>
| style="text-align:right; vertical-align:top;"| <math>\left ( 3 \right )</math>
|}

{| style="border:1px solid #ccc; width:60%;"
|-
| <math>\mathrm{OOH}^* + \mathrm{H}_2 \mathrm{O} + \mathrm{OH}^- + 3\mathrm{e}^- \rightarrow 4\mathrm{e}^- + 2\mathrm{H}_2 \mathrm{O} + \mathrm{O}_2</math>
| style="text-align:right; vertical-align:top;"| <math>\left ( 4 \right )</math>
|}

Where the * indicate species adsorbed to the surface of the catalyst.

====Cathode reaction====
The reaction starts with water adsorption and dissociation in Volmer step and either hydrogen desorption in the Tafel step or Heyrovsky step.
{| style="border:1px solid #ccc; width:60%;"
|-
| Volmer step: <math>2\mathrm{H}_2 \mathrm{O} + 2\mathrm{e}^- \rightarrow 2\mathrm{H}^* + 2\mathrm{OH}^{-}</math>
| style="text-align:right; vertical-align:top;"| <math>\left ( 5 \right )</math>
|}

{| style="border:1px solid #ccc; width:60%;"
|-
| Tafel step: <math>2\mathrm{H}^* \rightarrow \mathrm{H}_2</math>
| style="text-align:right; vertical-align:top;"| <math>\left ( 6 \right )</math>
|}

{| style="border:1px solid #ccc; width:60%;"
|-
| 
Heyrovsky step: <math>\mathrm{H}_2 \mathrm{O} + \mathrm{H}^* + \mathrm{e}^- \rightarrow \mathrm{H}_2 + \mathrm{OH}^-</math>
| style="text-align:right; vertical-align:top;"| <math>\left ( 7 \right )</math>
|}

===Anion exchange membrane===
{{Further|Anion exchange membrane}}
Hydroxide ion intrinsically has lower mobility than H<sup>+</sup>, increasing ion exchange capacity can compensate for this lower mobility but also increase swelling and reduce membrane mechanical stability. Cross-linking membranes can compensate for membrane mechanical instability. The [[quaternary ammonium]] (QA) headgroup is commonly employed to attach polymer matrices in AEM. The head group allows anions but not cations to be transported. QA AEMs have low chemical stability because they are susceptible to OH<sup>−</sup> attack. Promising head group candidates include imidazolium-based head group and nitrogen-free head groups such as phosphonium, sulphonium, and ligand-metal complex. Most QAs and imidazolium groups degrade in alkaline environments by [[Hofmann degradation]], [[SN2 reaction|S<sub>N</sub>2 reaction]], or ring-opening reaction, especially at high temperatures and pH.<ref name="Naiying2022" /><ref name="kimberly2018">{{cite journal |last1=Hagesteijn |first1=Kimberly F. L. |last2=Jiang |first2=Shanxue |last3=Ladewig |first3=Bradley P. |title=A review of the synthesis and characterization of anion exchange membranes |journal=Journal of Materials Science |date=August 2018 |volume=53 |issue=16 |pages=11131–11150 |doi=10.1007/s10853-018-2409-y|bibcode=2018JMatS..5311131H |hdl=10044/1/60973 |s2cid=139833074 |hdl-access=free }}</ref>

Polymeric AEM backbones are cationic-free base polymers. Poly(arylene ether)-based backbones, polyolefin-based backbones, polyphenylene-based backbones, and backbones containing cationic moieties are some examples.<ref name="Naiying2022" />

Some of the best-performing AEMs are HTMA-DAPP, QPC-TMA, m-PBI, and PFTP.<ref name="carlo2022" />

===Membrane electrode assembly===
A [[membrane electrode assembly]] (MEA) is made of an anode and cathode catalyst layer with a membrane layer in between. The catalyst layer can be deposited on the membrane or the substrate. Catalyst-coated substrate (CCS) and catalyst-coated membrane (CCM) are two approaches to preparing MEA. A substrate must conduct electricity, support the catalyst mechanically, and remove gaseous products.

Nickel is typically used as a substrate for AEM, while titanium is for PEM; both nickel and titanium can be used on AEM. Carbon materials are not suitable for the anode side because of their degradation by HO<sup>−</sup> ions, which are nucleophiles.<ref name="Hamish2020" /> On the cathode, nickel, titanium, and carbon can be readily used. The catalyst layer is typically made by mixing catalyst powder and [[ionomer]] to produce an ink or slurry that is applied by spraying or painting. 
<ref name="Naiying2022" /><ref name="Hamish2020" /> Other methods include electrodeposition, magnetron sputtering, chemical electroless plating, and screen printing onto the substrate.<ref name="Hamish2020" /><ref name="ester2021">{{cite journal |last1=López-Fernández |first1=Ester |last2=Sacedón |first2=Celia Gómez |last3=Gil-Rostra |first3=Jorge |last4=Yubero |first4=Francisco |last5=González-Elipe |first5=Agustín R. |last6=de Lucas-Consuegra |first6=Antonio |title=Recent Advances in Alkaline Exchange Membrane Water Electrolysis and Electrode Manufacturing |journal=Molecules |date=20 October 2021 |volume=26 |issue=21 |pages=6326 |doi=10.3390/molecules26216326|pmid=34770735 |pmc=8587517 |hdl=10261/256196 |hdl-access=free |doi-access=free }}</ref>

Ionomers act as a binder for the catalyst, substrate support, and membrane, which also provide OH<sup>−</sup> conducting ions and increase electrocatalytic activities.<ref name="paige2021" /><ref name="Naiying2022" /><ref name="Hamish2020" />

==See also==
{{portal|Energy|Renewable energy}}
{{colbegin}}
*[[Electrochemistry]]
*[[Electrochemical engineering]]
*[[Electrolysis]]
*[[Hydrogen production]]
*[[Photocatalytic water splitting]]
*[[Timeline of hydrogen technologies]]
*[[Electrolysis of water]]
*[[PEM fuel cell]]
*[[proton-exchange membrane]]
*[[Hydrogen economy]]
*[[High-pressure electrolysis]]
{{colend}}

==References==
{{Reflist}}

{{DEFAULTSORT:Polymer electrolyte membrane electrolysis}}
[[Category:Electrolysis]]
[[Category:Hydrogen economy]]
[[Category:Hydrogen production]]
[[Category:Electrolytic cells]]