Influence of supporting electrolyte on the electrocatalysis of CO2 reduction by cobalt protoporphyrin

Abstract

The electrochemical conversion of CO2 into high-value fuels is a promising technique to reduce CO2emission and relief the crisis of fossil fuel depletion. Here we report the influence of supporting electrolyte on the electrochemical reduction of carbon dioxide catalyzed by cobalt protoporphyrin. Phosphate anion strongly coordinates to the cobalt center leading to the positive movement of CoIII/CoII redox peak and the difficulty formation of carbon monoxide. Alkali cations, such as Li+, Na+ and K+, exhibit different catalytic activity and product selectivity. The efficiency of the hydrogen evolution reaction from direct proton (H+) reduction is increasing with the increase of the cation size. That is because the smaller cations preserve larger hydration number of water leading to geometrical hindrance, which hinders the diffusion of H+ to the electrode surface. On the contrary, the formation of CO and CH4 is decreasing with increasing of the cation size. This is resulted from the coordination of alkali cations to the oxygen atom of the key intermediate of CO2 electrochemical reduction, CO2-• anion, through ion-pairing. The smaller the cation size is, the stronger the ion-pairing is, helping the cleavage of one of the C-O bond. The deeper insight into the ionic effects and reaction mechanism of the electrochemical reduction of CO2 suggest new strategies for the utilization of CO2.