A Study of the Effects of Ions in Aqueous Electrolyte on CO2 electrochemical Reduction to Efficiently Produce Formate

Abstract

It has early been known that the electrolyte cations and anions in aqueous can influence the electrochemical reduction of carbon dioxide (CO2) over the electrodes. This can be attributed to cation hydration and hydrolysis, and the specific adsorption of the anion, which occur near and on the cathode, respectively. In the experiment, we first found that the order of cations as to the effects on faradaic efficiencies (FEs) achieved when using SnO2/carbon paper electrode in the electrochemical reduction of CO2 to formate was found to be FENH4+ < FELi+ < FENa+ < FEK+ < FERb+ <FECs+ , whereas that of anions was FEHCO3- < FECl- < FEBr- < FEI-. From to Cs+, the pKa (Ka: dissociation constant) for cation hydrolysis decreases, and the pH near the cathode gets low, resulting in an increase in the local concentration of dissolved CO2. The consequences are an increase in cathode activity and FEs for HCOO- production. For anions, specific adsorption of the anion forms SnO2-X layer (X = NH4 +, Cl-, Br- and I-) on the surface of SnO2 electrode. Electron-transfer to CO2 may occur via the SnO2-X–C bond. It can be concluded that the stronger the adsorption of the anion to the electrode, the easilier transfer of electrons, the higher FE for formate of electrochemical reduction of CO2. Keywords: SnO2, electrolyte, cations, anions, electrochemical reduction of carbon dioxide