In situ spectroscopy and diffraction to look inside the next generation of gas diffusion and zero-gap electrolyzers

Electrolyzers allow for the sustainable conversion of chemical waste (e.g. nitrous oxides, NO_x, or carbon dioxide, CO₂) into valuable chemicals or building blocks (e.g. ammonia or hydrocarbons). There is a constant search for new and improved materials (electrocatalysts) that can facilitate these complex chemical reactions with optimized activity, selectivity, and stability. In order for electrolyzers to become economically feasible, it is of utmost importance that they perform at high current density >100 mA/cm² (activity), since this scales with chemical reaction rate. However, if high current density is only achieved for a short period of time (stability), the electrolyzer has to be regenerated, which is a costly endeavor. For this purpose, chemical engineers have focused on gas diffusion electrodes (GDE) or membrane electrode assemblies (MEA) in recent years, but these cell configurations are prone to rapid deactivation and salting. In situ spectroscopy and diffraction techniques can shed light on the parameters that influence catalyst (de)activation, but application of the technique of choice depends heavily on the reaction conditions and hence is not straightforwardly applied to electrolyzers that operate at high current density. This review addresses the recent developments within the community for in situ characterization of GDE and MEA electrolyzers, and opportunities for future studies are highlighted, which are aimed to stimulate discussion and advancement of the field.