Anode Catalysts in Anion‐Exchange‐Membrane Electrolysis without Supporting Electrolyte: Conductivity, Dynamics, and Ionomer Degradation

Abstract

Anion‐exchange‐membrane water electrolyzers (AEMWEs) in principle operate without soluble electrolyte using earth‐abundant catalysts and cell materials and thus lower the cost of green H2. Current systems lack competitive performance and the durability needed for commercialization. One critical issue is a poor understanding of catalyst‐specific degradation processes in the electrolyzer. While non‐platinum‐group‐metal (non‐PGM) oxygen‐evolution catalysts show excellent performance and durability in strongly alkaline electrolyte, this has not transferred directly to pure‐water AEMWEs. Here, AEMWEs with five non‐PGM anode catalysts are built and the catalysts’ structural stability and interactions with the alkaline ionomer are characterized during electrolyzer operation and post‐mortem. The results show catalyst electrical conductivity is one key to obtaining high‐performing systems and that many non‐PGM catalysts restructure during operation. Dynamic Fe sites correlate with enhanced degradation rates, as does the addition of soluble Fe impurities. In contrast, electronically conductive Co3O4 nanoparticles (without Fe in the crystal structure) yield AEMWEs from simple, standard preparation methods, with performance and stability comparable to IrO2. These results reveal the fundamental dynamic catalytic processes resulting in AEMWE device failure under relevant conditions, demonstrate a viable non‐PGM catalyst for AEMWE operation, and illustrate underlying design rules for engineering anode catalyst/ionomer layers with higher performance and durability.

Impact of Catalyst Reconstruction on the Durability of Anion Exchange Membrane Water Electrolysis

“…Since R CL is undertaken by the ionomer and the external electrolyte (KOH in this case), , and the KOH concentration has not changed during the test, we believe the increase of R CL after the durability test should be ascribed to the conductivity loss of the ionomer. This view has been further proved by characterizing the functional group (quaternary-N group) degradation in the ionomer. , Figure S5a and b shows the N 1s spectrum in the cathode and anode catalytic layers, respectively. The peak at 402.7 eV was assigned to positively charged quaternary-N, and the peaks at 400 and 399 eV were assigned to uncharged tertiary-N and secondary-N in the polyelectrolyte, respectively .…”

“…This view has been further proved by characterizing the functional group (quaternary-N group) degradation in the ionomer. 29,57 Figure S5a and b shows the N 1s spectrum in the cathode and anode catalytic layers, respectively. The peak at 402.7 eV was assigned to positively charged quaternary-N, and the peaks at 400 and 399 eV were assigned to uncharged tertiary-N and secondary-N in the polyelectrolyte, respectively.…”

“…28 These studies make us reasonably speculate that the OER catalysts in AEMWE would undergo similar structural transformations. Unlike the solid/liquid interface in a KOH solution, the catalyst/polymer electrolyte interface in AEMWE is a solid/solid interface, 10,14,29 which is significantly affected by the catalyst structure. Because the electrochemical reactions take place at the interface between the ionconducting ionomer and the electron-conducting catalyst, the structural changes of the catalysts are very likely to destroy the contact interface between the catalyst and the ionomer, resulting in the loss of active reaction sites.…”

Membrane Strategies for Water Electrolysis

“…70 Highly conductive, chemically stable, and mechanically robust AEMs would be highly desirable, but AEMWE performance and durability would not solely depend on the AEM properties. The cell performance is strongly governed by the interface between AEI binders and electrocatalysts 71 and their interface with the membrane; therefore, pairing AEMs with compatible, performance-boosting ionomers brings substantial impact. 72 Improvement of the membrane− electrode interface is a critical requirement.…”

A NiCo2O4 electrocatalyst with a thin graphitic coating for the anion exchange membrane water electrolysis of wastewater

“…However, the non-precious metals used in AEMWEs have low catalytic activity and efficiency as a result of significant activation ( η act ) and ohmic ( η ohm ) losses. 8–11 The activation loss is intrinsic to non-precious metal electrocatalysts, whereas the ohmic loss is caused by the system configuration. Thus, for AEMWEs to become next-generation water electrolysis systems, highly active and advanced electrocatalysts are required.…”