Non-precious hydrogen evolution reaction catalysts: Stepping forward to practical polymer electrolyte membrane-based zero-gap water electrolyzers

Water electrolysis (WE) technologies powered by renewable energy sources are promising methods for sustainable hydrogen production as they do not contribute toward resource depletion and pollutant emissions. However, to benefit from these technologies, their high capital costs must be addressed. The development of non-precious catalysts for hydrogen evolution reaction, which is a hydrogen-generating electrochemical reaction in WE, addresses this issue. However, most non-precious hydrogen evolution catalysts have only been evaluated at the half-cell level, and their practical applicability to WE has not been confirmed. Herein, we review non-precious hydrogen evolution reaction catalysts implemented in anion and proton exchange membrane WE, which are efficient WE methods owing to their low ohmic losses and ease of product gas separation. We also discuss the importance of implementing non-precious catalysts in water electrolyzers and standardizing an evaluation method.