Microenvironment Engineering for High-Current-Density Electrochemical CO2 Reduction

Electrochemical CO2 reduction reaction (ECO2RR) is a promising technology for converting rapidly rising atmospheric CO2—driven by fossil fuel consumption and industrial processes—into a circular carbon economy. In particular, ECO2RR is expected to enable renewable-based long-duration energy storage (LDES) systems through the highly efficient conversion of CO2 into high-value multi-carbon (C2+) compounds. However, scaling ECO2RR to the industrial level remains challenging because, under high-current operation, the CO2 consumption rate exceeds its supply rate, causing a sharp decline in local CO2 concentration. The resulting increase in local pH promotes both carbonate formation and electrode flooding within the gas diffusion electrode (GDE), creating a wetting-induced mass transfer bottleneck. To address this challenge, this review categorizes and analyzes recent strategies for CO2 microenvironment engineering that overcome mass transfer limitations at high-current densities, focusing on two complementary approaches: (1) enhancing gas-phase CO2 supply while suppressing flooding through nano/microscale hydrophobic polymers and structural gradient designs, and (2) enhancing active CO supply in the liquid phase through electrolyte composition optimization. We further show that these strategies are not mutually independent but create complementary structural and chemical synergies, and we propose future directions for simultaneously improving high-current operability and C2+ product selectivity.