Ionomer-free electrodes in PEM water electrolyzers: a critical review

Achieving large-scale production of clean hydrogen, which emits zero local-carbon emissions when powered by renewables, is a prerequisite to advance the hydrogen economy and to delay the escalating global temperatures. While proton-exchange-membrane water electrolyzers (PEMWE) are projected to play a vital role for the market, the technology still encounters challenges associated with cost and scale-up. One viable approach is to reduce the amount of platinum-group-metal (PGM) usage in the PEMWE. Recent studies have introduced novel electrodes designs that eliminate ionomer layers (polymeric layers that conduct protons), while maintaining high performance at low iridium loadings. These ionomer-free electrode designs not only feature high performance, but also enable facile fabrication processes and reuse of iridium after long-term operation, significantly contributing to cost reductions. This paper provides a comprehensive review on the ionomer-free electrodes for PEMWE, exploring its benefits, operation principles, and designs that have been studied in the literature to enhance catalytic activity and prolong durability.