Enhancing Carbon Efficiency in Electrochemical CO2 Reduction at Silver Gas Diffusion Electrodes – The Effect of Acidic Electrolytes Explained via TFFA Modeling

Abstract

Recently, there has been a growing focus on enhancing carbon efficiency in the field of electrochemical CO2 reduction, quantifying the proportion of CO2 converted electrochemically relative to the total amount consumed. The competition between homogeneous reactions forming carbonates and the electrochemical conversion of CO2 is causing lowered carbon efficiencies in typical buffer or alkaline electrolytes. Consequently, strategies aimed at shifting the equilibrium of these reactions have to be employed. In this work, the application of acidic electrolytes is presented as a promising pathway to overcome this limitation. Experimental investigations at silver gas diffusion electrodes (GDEs) in acidic 0.5 M K2SO4 electrolyte show an improvement of carbon efficiency by up to 20% compared to 1 M KHCO3 electrolyte, while the Faradaic efficiency at high current densities is unaffected. Nevertheless, degradation of silver GDEs occurs in acidic conditions, emphasizing the need for development of more stable catalysts. Furthermore, a spatially resolved mathematical model provides insights into the local reaction environment within the GDE. This highlights the importance of local alkalinity to control the rate of electrochemical CO2 reduction. While the formation of hydroxide ions by the electrochemical reactions creates an alkaline environment favoring CO2 reduction, the acidic bulk electrolyte hinders the formation of carbonates.