Specifically Adsorbed Anions Enhance CO₂ Electrochemical Reduction to CO over a Gallium Catalyst in Organic Electrolytes

Abstract

The effects of anions (ClO4–, Cl–, Br–, and I–) on CO selectivity in organic electrolytes have been studied. A gallium electrode with a low CO faradic efficiency was chosen to reveal the anion effects on CO2 conversion. We found that the CO2 reduction in a tetrabutylammonium chloride (Bu4NCl) solution exhibited more than 2 times higher CO faradic efficiency than that in the commonly used tetrabutylammonium perchlorate (Bu4NClO4) solution. Also, Tafel plot measurement results showed that the presence of Cl– promoted an electron-transfer process, thus enhancing the reaction rate of CO2 reduction. X-ray photoelectron spectroscopy (XPS) results further revealed that the solvated Cl– form the Ga–Cl bonds on the Ga surface. Electrons could flow from the electrode to the adsorbed Cl– and then flow to the CO2, resulting in the restriction of CO2 near the Ga electrode. In the reaction, CO2 was converted to a CO2•– radical by receiving an electron, which flowed from Cl– to CO2 via the Cl–―C bond. The obtained CO2•– further combined with CO2 to form the adduct of (CO2)2•–. After the second electron reaction, (CO2)2•– was converted to CO.