Ionomers modulate the microenvironment in electrocatalytic CO₂ reduction

The global shift toward a low-carbon society has accelerated the development of electrocatalytic CO₂ reduction reaction (CO₂RR) technology, which shows great potential in simultaneously addressing environmental pollution and energy crises. In the CO₂RR system, microenvironment modulation can effectively enhance catalytic activity, product selectivity, operational stability, and energy efficiency. The introduction of ionomers into catalyst layers enables precise control of the microenvironment at the catalyst surface through their unique structural properties, significantly improving CO₂RR performance. In this review, we first provide a concise overview of the key components and main influencing factors of the reaction microenvironment and the structures together with the functional mechanisms of commonly used ionomers. We then systematically discuss how various ionomers modulate the microenvironment, including their effects on CO₂ mass transport, stabilization and diffusion of intermediates, ion species and concentrations at the surface (affecting the pH, K⁺ distribution and interfacial electric field), surface morphology and hydrophobicity of catalysts, and structures of interfacial water. Finally, we present a comprehensive summary that identifies current practical challenges of ionomer applications from multiple perspectives while proposing feasible solutions and outlining future research directions for this field.