Microenvironment Engineering with Supercritical CO2 for Selective CO2-to-CH4 Electrocatalysis.

Supercritical carbon dioxide (ScCO2), characterized by its high diffusivity, tunable density, and unique solvent properties, presents a novel opportunity to manipulate the local environment in electrocatalytic reactions. This study demonstrates the efficacy of ScCO2 as a reaction medium to dramatically enhance the selectivity of the electrocatalytic CO2 reduction reaction (ECO2RR) toward methane (CH4) on a Pd-decorated porous CuZn (CuZn@Pd) catalyst. Under optimized ScCO2 conditions (50 °C, 16 MPa, -1.2 V vs Pt), the Faradaic efficiency (FE) for CH4 reaches a remarkable 65%, a substantial increase from the 33% FE observed under ambient aqueous conditions. Concurrently, the competing hydrogen evolution reaction (HER) is effectively suppressed, with the H2 FE plummeting to only 3%. Comprehensive characterization and electrochemical analysis reveal that the ScCO2 medium enhances CO2 mass transport, increases its local concentration at the catalyst surface, and modifies the interfacial environment. Density functional theory (DFT) calculations provide atomic-level insight, showing that ScCO2 facilitates the key *CO to *COH step at Pd sites, reducing the reaction energy barrier and steering the pathway selectively toward CH4. This work establishes supercritical fluid medium engineering as a powerful strategy for controlling product selectivity in electrocatalysis.