Hierarchical Interfacial Engineering of Gas Diffusion Electrodes for Acidic CO2 Electroreduction in Membrane Electrode Assemblies

Acidic CO2 electroreduction in membrane electrode assemblies suppresses parasitic carbonate formation and improves carbon utilization. Yet, practical implementations remain hindered by proton accumulation and alkali cation buildup that give rise to failure modes associated with hydrogen evolution, salt precipitation, and catalyst flooding. Here, we report a multiscale interfacial design for gas diffusion electrodes that addresses these challenges across multiple length scales. Our design employs a multilayer architecture in which a microscale hydrophobic porous silica overlayer sits below a nanoscale quaternary ammonium polyelectrolyte coating. The former regulates electrolyte intrusion via capillary pressure while the latter electrostatically restricts cation accumulation. Incorporation of a hydrophilic macromolecular component into the polyelectrolyte further induces complexation-driven charge densification while simultaneously establishing percolating water channels that enhance interfacial hydration and reduce cell voltage. Our design provides CO Faradaic efficiencies of 99% sustained over 162 h, with single-pass CO2 conversion efficiency of 77% at 200 mA cm-2, on a model Ag catalyst. Furthermore, we show that the design readily generalizes across catalyst platforms and translates to scale, delivering stable operation in a 25 cm2 electrolyzer with CO production rates of ∼62 mL min-1. Lastly, techno-economic analysis projects a pathway to cost-competitive CO production, highlighting hierarchical interfacial engineering as an effective strategy for acidic CO2 electrolysis at industrially relevant scales.