Interfacial Microenvironmental Engineering for Acidic CO ₂ Electroreduction in Proton Exchange Membrane Electrolyzers

ABSTRACT Electrochemical CO ₂ reduction reaction (CO ₂ RR) in proton exchange membrane electrolyzers offers a pathway to close the carbon cycle and produce sustainable fuels and chemicals at industrial‐scale current densities. Acidic operation enables compact reactor architectures and superior single‐pass carbon utilization, yet faces persistent challenges including parasitic hydrogen evolution, catalyst corrosion and deactivation, along with constrained local CO ₂ transport. This review highlights recent progress in interfacial microenvironmental engineering that reconcile acidic operation with selective and durable CO ₂ conversion. We organize strategies across four critical interfaces: (1) the gas diffusion layer‐catalyst layer interface, where engineered hydrophobicity and pore structure promote CO ₂ delivery while maintaining a stable three‐phase boundary; (2) the catalyst‐electrolyte electric double layer interface, where control over interfacial fields, pH gradients, and adsorbate binding energetics suppresses the hydrogen evolution reaction while promoting CO ₂ RR selectivity; (3) at the catalyst layer‐membrane interface, where optimized ionomer distribution and tailored proton conductivity balance local proton availability, mitigating catalyst degradation; and (4) the integrated membrane‐electrode assembly, where harmonized ion transport, CO ₂ flux, and electron conduction stabilizes the microenvironment for long‐term durability. By consolidating mechanistic insights and practical design principles, this review provides a roadmap for rational interfacial engineering to realize efficient, durable, and scalable acidic CO ₂ electroreduction in PEM electrolyzers.