Microenvironment Engineering in Electrochemical CO2 Reduction: From Multiscale Reaction Fields to Coupled Operating Windows.

The electrochemical CO2 reduction reaction (CO2RR) provides a promising route for converting CO2 into fuels and chemical feedstocks, but practical implementation remains constrained by the simultaneous requirements of high selectivity, energy efficiency, carbon utilization, and stability at industrially relevant current densities. The key state variables governing the CO2RR microenvironment, including local pH, reactant availability, ion distribution, solvation structure, interfacial electric field, and wetting state, are defined and linked to the reaction and degradation pathways that control performance. Microenvironment regulation is organized into three functionally distinct but bidirectionally coupled levels: chemical regulation shapes local physicochemical state, physical regulation governs their spatiotemporal distribution and accessibility, and system-level operation establishes the macroscopic boundary conditions and fluxes required to sustain favorable interfacial states. Practical CO2RR is therefore framed as maintaining activity, selectivity, carbon utilization, energy efficiency, and durability within a coupled operating window. In situ characterization, multiscale simulation, and data-driven approaches are further discussed as tools for identifying and controlling these coupled effects. This framework connects local reaction fields with system-specific, experimentally testable operating windows for practical CO2RR.