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

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.