Electrochemical CO<sub>2</sub> Reduction to Methanol by Cobalt Phthalocyanine: Quantifying CO<sub>2</sub> and CO Binding Strengths and Their Influence on Methanol Production
Libo Yao, Kevin E. Rivera Cruz, Paul M. Zimmerman, Nirala Singh, Charles C. L. McCrory
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 .