Stimulating the Pre-Catalyst Redox Reaction and the Proton–Electron Transfer Process of Cobalt Phthalocyanine for CO₂ Electroreduction

Abstract

The mechanism for electrochemical carbon dioxide reduction reaction (CO2RR) to carbon monoxide on cobalt phthalocyanine (CoPc) in aqueous electrolytes remains debatable, impeding the design of high-performance catalysts. By using a quasi-empirical protocol with density functional theory calculations, we identify the mechanisms of two important steps for CO2RR on CoPc: the reduction of CoIIPc to form catalytically active [CoIIPc]2– for CO2 adsorption and the proton–electron transfer to the key intermediate [CoPc-COO]2– to form [CoPc-COOH]2–. According to the charge states and pKa analysis, the formation of the adsorbed carboxyl (*COOH) takes place via the concerted proton–electron transfer process at low potentials, and the sequential proton–electron transfer process becomes thermodynamically favored at more reductive potentials, which successfully elucidates the potential-dependent reaction kinetics of CO2RR on CoPc catalysts. Electron-withdrawing substituents of CoPc would enhance the reduction of CoPc but hinder the protonation of *CO2, which accounts for previous conflicting results. Our findings not only deepen the understanding of CoPc-catalyzed CO2RR but also provide a guideline for molecular engineering of CoPc-based catalysts.