The impact of variability

Alkaline water electrolysis is a mature technology for making hydrogen but integration with renewable energy sources, which have inherently variable output, poses challenges: renewably powered electrolysers must operate efficiently regardless of fluctuating power and also be tolerant to repeated startup and shutdown. One challenge of shutdowns is that so-called reverse currents can be induced; in this situation, the roles of the anode and cathode are effectively inverted. It is known that this can lead to performance degradation, but a greater understanding of the mechanisms at play is still needed. Now, C. Buddie Mullins and colleagues in the USA and Denmark use in situ techniques to comprehensively explore the effects of variable operation and simulated shutdowns on the stability of transition metal-based electrocatalysts in alkaline water electrolysers.

The researchers find that during simulated shutdowns the electrode catalysts are severely degraded to an extent not observed during steady-state operation. For example, under reverse current conditions, in situ surface-enhanced Raman spectroscopy suggests strong reduction of the oxygen electrode’s surface, which is only partially reversible. Moreover, the team also observe changes in the thickness and composition of catalyst films, alongside dissolution of the catalyst support under reverse currents and dissolution of the catalysts during reoxidation. The work highlights the need for new materials that are more resilient under real-world, dynamic testing conditions and for the development of effective accelerated stress tests that take variable operation into account.