Elucidating the rate-limiting step of CO₂ electroreduction on metal phthalocyanines.

Abstract

Immobilized molecular catalysts, especially metal phthalocyanines, have garnered substantial interest for the electrochemical CO₂ reduction reaction (CO₂RR) due to their well-defined active sites and promising performance. Yet, the reaction mechanism, particularly the rate-limiting step, remains debated. Here, using electrochemical analysis and kinetic isotope effect measurements, we identify the rate-limiting step of CO₂RR to CO on immobilized metal phthalocyanines, with Au as a reference. Notably, cobalt phthalocyanine (CoPc) exhibits dispersion-dependent kinetics: protonation of adsorbed *CO₂ is rate-limiting on molecularly dispersed CoPc supported on carbon nanotubes (CoPc/CNTs), whereas CO₂ adsorption becomes rate-limiting on aggregated CoPc due to a weakened interfacial electric field at the Co active sites. This mechanistic distinction further elucidates the role of electrolyte anions: HCO₃^-, largely a spectator on Au, promotes CO₂RR on CoPc/CNTs by serving as a proton donor in the rate-limiting protonation step. These findings provide mechanistic insights into CO₂RR on metal phthalocyanines and guide the rational design of molecular electrocatalysts.