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

Cobalt phthalocyanine (CoPc) is an active electrocatalyst for the sequential electrochemical reductions of CO2-to-CO and CO-to-methanol (CH3OH), and it has been shown to be active for the conversion of CO2-to-CH3OH through a cascade catalysis reaction. However, in gas-fed flow electrolyzers equipped with gas diffusion electrodes (GDEs), the reduction of CO2 by CoPc selectively produces CO with minimal CH3OH formation. Herein, we show that the limited performance of the CO2-CO-CH3OH cascade reactions by CoPc is primarily due to the competitive binding between the CO2 and CO species. Through microkinetic analyses, we determine that the effective equilibrium constant for CO2 binding is three times higher than that for CO binding. The stronger CO2 binding suppresses the CO-to-CH3OH reaction even at moderate local CO2 concentrations. Because the GDE configuration enhances the CO2 mass transport, gas-fed flow electrolyzers exacerbate this suppression of CH3OH formation from the CO2RR. In contrast, CH3OH formation is observed when the local concentration of the CO2 is low, compared to the local CO concentration. To promote methanol formation via CO2 reduction, we propose applying modifications to the coordination environments of CoPc to strengthen the binding of CO and regulate the transport of CO2.