Anion-exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts,membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much ofthe recent progress is due to improvements in materials chemistry, MEA designs, andoptimized operation conditions. Research on anion-exchange polymers (AEPs) has focusedon the cationic head/backbone/side-chain structures and key properties such as ionicconductivity and alkaline stability. Several approaches, such as cross-linking, microphase, andorganic/inorganic composites, have been proposed to improve the anion-exchangeperformance and the chemical and mechanical stability of AEMs. Numerous AEMs nowexceed values of 0.1 S/cm (at 60−80 °C), although the stability specifically at temperaturesexceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still alimiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have thehighest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to beunderstood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolutionreaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH) 2 on Pt show competitiveactivities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbonsupports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needsto be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardizedevaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzercells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm 2, respectively.