Steering the Selectivityof Electrochemical CO₂ Reduction on the Cu Catalyst viathe Interplay between theElectrode Morphology and Electrolyte Anion Identity

Electrochemical carbon dioxide (CO₂) reduction (ECR) holds promise as a viable pathway for the generation of fuels and chemicals. Several strategies have been explored to enhance the product selectivity of ECR on copper (Cu) catalysts. A systematic approach to optimize the local reaction microenvironment, however, remains elusive. Engineering the electrode structure and reaction microenvironment is a facile but effective strategy for steering the product selectivity of ECR reactions and can enable the rational design of highly selective Cu electrodes. Herein, we demonstrate that the synergy between an optimized Cu gas diffusion electrode (GDE) morphology and electrolyte anion identity can steer ECR product selectivity toward ethylene (C₂₊) or methane via the local CO₂ availability, pH, and electrode morphology regulation. We show that using a relatively thin 100 nm Cu catalyst layer (CL) sputtered on an optimized macropore-sized hydrophobic poly(tetrafluoroethylene) substrate promotes methane selectivity at high reaction rates. We achieved a methane partial current density of 126 mA cm^–2 and a Faradaic efficiency (FE) of 42%. In contrast, a relatively thick 500 nm Cu CL favors ethylene production, reaching a high FE of 52% at 250 mA cm^–2 (with a total C₂₊ value of 77%) in a near-neutral KHCO₃ electrolyte. Utilizing KI electrolyte significantly enhances methane selectivity, achieving ca. 56% at a partial current density of 168 mA cm^–2 while effectively suppressing the hydrogen evolution reaction (HER) on the thin CL. Furthermore, on the relatively thick CL, a higher C₂₊ FE of 84% was achieved at 250 mA cm^–2, demonstrating the impact of electrolyte anion identity and CL thickness on product selectivity in ECR. In addition, we find that a further increase in the Cu CL thickness does not result in a superior C₂₊ performance in KI compared to the KHCO₃ electrolyte. Our result highlights the critical role of the interplay between Cu electrode morphology and the electrolyte anion identity, which can facilitate efficient CO₂ mass transport, enable selective Cu sites, and tune local pH – thereby steering ECR product selectivity.