Operando monitoring of gas bubble evolution in water electrolysis by single high-frequency impedance

Abstract

Gas bubble management is highly demanded in water electrolysis and the lack of real-time monitoring of gas bubbles has slowed down the progress. Here, we demonstrate operando single frequency impedance measurement as an electrochemical means to detecting gas bubble evolution during water splitting reactions. At optimum high frequencies, where the contribution of faradaic charge transfer and mass transport as well as the phase component of the impedance are minimized, the dynamic variation of the resistance response can be correlated to the effect of gas bubbles. The amplitude of the resistance fluctuations indicates the impact of gas bubble evolution on the available active surface in a non-periodic pattern, where a bigger amplitude points to a larger number of gas bubbles and their sluggish growth and detachment over electrodes. Accordingly, the dynamic resistance variation varies with surface wettability and electrode configuration from flat two-dimensional to porous three-dimensional electrodes. Coupling this technique with operando optical microscopy unravels the correlation of the dynamic variation amplitude with gas bubble characteristics, i.e., size and release rate. The approach is applied to a bifunctional hetero-hierarchical Ni(OH)₂@N-NiC catalyst to confirm the operando monitoring of ultrafast hydrogen and oxygen bubble evolution due to its superaerophobicity and anisotropic morphology. This facile operando approach is applied for monitoring gas bubble evolution in non-transparent full water electrolyser cells, and is useful for developing gas-repelling electrodes, as well as a range of gas evolving applications beyond.