Anion exchange membrane electrolysis

Of all water electrolysis methods, AEM electrolysis can combine the advantages of alkaline water electrolysis (AWE) and PEM electrolysis.{{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 }}{{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 }}

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 kg of Iridium, which will cost an estimated 7 million USD.{{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 }} 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.{{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 }}{{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 }}

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.{{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 }} This reduces the risk of leakage. 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/cm² was reported to require 1.8 volts and 1.57 volts in pure water-fed and 1 M KOH-fed, respectively.{{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 }} Electrolyte can be fed on both anode and cathode side or anode side only.{{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 }}

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 H₂ to the O₂ evolution side can pose a safety hazard because it can create an explosive gas mixture with >4%mol H₂.{{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 }} The AEM electrolyser was reported to maintain H₂ crossover to less than 0.4% for the 5000 h of operation.{{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 }}

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$/m²){{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 }}{{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 }} and fluorocarbon gas is produced at the production stage of

tetrafluoroethylene, which poses a strong environmental impact.{{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 }} Fluorinated raw materials are inessential for AEM, allowing for a wider selection of low-cost polymer chemistry.

File:Degraded AEM after elongation testing.jpg

The major technical challenge facing a consumer level AEM electrolyser is the short device lifetime or longevity. The lifetimes of PEM electrolyser stacks range from 20,000 h to 80,000 h. 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% K₂CO₃-fed respectively.

In particular, many AEM breakdown at temperatures higher than 60 °C, AEM that can tolerate the presence of O₂, high pH, and temperatures exceeding 60 °C are needed.{{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 }}

The Oxygen Evolution Reaction (OER) involves complex processes and a high energy barrier and thus a high overpotential. The performance of the AEM electrolyser largely depends on OER. The overpotential of OER can be lowered with a suitable catalyst. Researches shows that Ni-Fe based catalysts are one of the most promising catalysts for OER in AEM electrolyser. {{cite journal |last1=Kumar |first1=S. Shiva |last2=Lim |first2=Hankwon |title=An overview of water electrolysis technologies for green hydrogen production |journal=Energy Reports |date=25 October 2022 |volume=8 |pages=13793-13813 |doi=10.1016/j.egyr.2022.10.127 }}

Hydrogen evolution reaction (HER) kinetics in alkaline solutions is usually faster.{{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 }}

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

The reaction starts with water adsorption and dissociation in Volmer step and either hydrogen desorption in the Tafel step or Heyrovsky step.

{{Further|Anion exchange membrane}}

Hydroxide ion intrinsically has lower mobility than H⁺, 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⁻ 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, S_N2 reaction, or ring-opening reaction, especially at high temperatures and pH.{{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 }}

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.

Some of the best-performing AEMs are HTMA-DAPP, QPC-TMA, m-PBI, and PFTP.

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⁻ ions, which are nucleophiles. 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.

Other methods include electrodeposition, magnetron sputtering, chemical electroless plating, and screen printing onto the substrate.{{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 }}

Ionomers act as a binder for the catalyst, substrate support, and membrane, which also provide OH⁻ conducting ions and increase electrocatalytic activities.

{{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

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{{Reflist}}

{{DEFAULTSORT:Polymer electrolyte membrane electrolysis}}

Category:Electrolysis

Category:Hydrogen economy

Category:Hydrogen production

Category:Electrolytic cells