Restricting proton transfer via carbon nanohorn-supported monomolecular cobalt phthalocyanine enhances CO2-to-CO electrocatalysis in acidic media

Electrocatalytic reduction of CO 2 to CO in acidic media is a compelling approach toward closing carbon cycle, as it can increase CO 2 utilization while circumvent carbonate precipitation. Nonetheless, a cardinal challenge remains the inherent competition from hydrogen evolution reaction (HER), driven by high proton availability at catalyst surface, which severely suppresses CO 2 reduction selectivity and activity. Herein, we engineer a catalyst comprising monomolecularly dispersed cobalt phthalocyanine on carbon nanohorns (CNHs) with abundant topological defects, which orchestrates efficient CO 2 electroreduction in acidic media via restricting proton transfer. This design constructs an electron-deficient Co center and surrounded with a proton-deficient microenvironment, achieving CO electrogeneration with 95.9% Faradaic efficiency (FE) and 259.1 mA cm –2 partial current density at pH = 0.5 while maintaining > 85% FE across 38 h durability at pH 1. Mechanistic investigations reveal that the CNHs support simultaneously restricts both key proton-supply pathways: the defect-rich structure suppresses surface hydrogen spillover along the carbon framework, while the dahlia-like architecture of CNHs aggregate hinder axial hydronium diffusion from the bulk electrolyte. The resulting reduction in proton availability around Co centers, combined with limited hydronium access from the bulk, stabilizes crucial *COOH and *CO intermediates and accelerates CO 2 reduction kinetics. By showcasing how carbon-support engineering can modulate proton-transfer pathways, this work offers a viable and generalizable strategy toward high-performance CO 2 electrolysis in strongly acidic media, advancing the design of robust molecular catalysts for practical application.