Capabilities and limitations of rotating disk electrodes versus membrane electrode assemblies in the investigation of electrocatalysts

Cost-competitive fuel cells and water electrolysers require highly efficient electrocatalysts for the respective reactions of hydrogen oxidation and evolution, and oxygen evolution and reduction. Electrocatalyst activity and durability are commonly assessed using rotating disk electrodes (RDEs) or membrane electrode assemblies (MEAs). RDEs provide a quick and widely accessible testing tool, whereas MEA testing is more complex but closely resembles the actual application. Although both experimental set-ups allow investigation of the same reactions, there are scientific questions that cannot be answered by the RDE technique. In this Perspective, we scrutinize protocols widely used to determine the activity and durability of electrocatalysts, and highlight discrepancies in the results obtained using RDEs and MEAs. We discuss where the use of RDEs is appropriate and, conversely, where it leads to erroneous interpretations. Ultimately, we show that many of the current challenges for hydrogen and oxygen electrocatalysts require MEA testing and advocate for its greater adoption in the early stages of electrocatalyst development.