Electrochemical carbon dioxide and bicarbonate reduction on copper in weakly alkaline media

The electrochemical reduction of CO2 on copper is an intensively studied reaction. However, there has not been much attention for CO2 reduction on copper in alkaline electrolytes, because this creates a carbonate buffer in which CO2 is converted in HCO3 (-) and the pH of the electrolyte decreases. Here, we show that electrolytes with phosphate buffers, which start off in the alkaline region and, after saturation with CO2, end up in the neutral region, behave differently compared to CO2 reduction in phosphate buffers which starts off in the neutral region. In initially alkaline buffers, a reduction peak is observed, which is not seen in neutral buffer solutions. In contrast with earlier literature reports, we show that this peak is not due to the formation of a CO adlayer on the electrode surface but due to the production of formate via direct bicarbonate reduction. The intensity of the reduction peak is influenced by electrode morphology and the identity of the cations and anions in solution. It is found that a copper nanoparticle-covered electrode gives a rise in intensity in comparison with mechanically polished and electropolished electrodes. The peak is observed in the SO4 (2-)-, ClO4 (-)-, and Cl-- containing electrolytes, but the formate-forming peak is not seen with Br- and I-.