Electrochemical CO₂ Reduction to Methanol by Cobalt Phthalocyanine: Quantifying CO₂ and CO Binding Strengths and Their Influence on Methanol Production

Abstract

Cobalt phthalocyanine (CoPc) is an active electrocatalyst for the sequential electrochemical reductions of CO 2 -to-CO and CO-to-methanol (CH 3 OH), and it has been shown to be active for the conversion of CO 2 -to-CH 3 OH through a cascade catalysis reaction. However, in gas-fed flow electrolyzers equipped with gas diffusion electrodes (GDEs), the reduction of CO 2 by CoPc selectively produces CO with minimal CH 3 OH formation. Herein, we show that the limited performance of the CO 2 –CO–CH 3 OH cascade reactions by CoPc is primarily due to the competitive binding between the CO 2 and CO species. Through microkinetic analyses, we determine that the effective equilibrium constant for CO 2 binding is three times higher than that for CO binding. The stronger CO 2 binding suppresses the CO-to-CH 3 OH reaction even at moderate local CO 2 concentrations. Because the GDE configuration enhances the CO 2 mass transport, gas-fed flow electrolyzers exacerbate this suppression of CH 3 OH formation from the CO 2 RR. In contrast, CH 3 OH formation is observed when the local concentration of the CO 2 is low, compared to the local CO concentration. To promote methanol formation via CO 2 reduction, we propose applying modifications to the coordination environments of CoPc to strengthen the binding of CO and regulate the transport of CO 2 .