Preventing the Deactivation of Gold Cathodes During Electrocatalytic CO2 Reduction While Avoiding Gold Dissolution

Abstract

The electrochemical reduction of CO2 on gold cathodes was investigated, and the major products were found to be CO, H2 and formate, which is consistent with existing literature. The Faradaic efficiency for CO production decreased from around 60 to 10% over the course of 4 h when the electrolysis was performed at – 5 mA cm–2 in 0.2 M KHCO3 saturated with CO2. This deactivation was accompanied by an increase in the selectivity of the cathode towards H2 and formate production, which is normally attributed to the deposition of metals from trace impurities in the electrolyte or surface-bound species formed during the reaction. In this case, the deactivation was found to be due to the deposition of Cu, Zn and possibly Fe from the electrolyte, with the presence of Fe strongly enhancing H2 production, the Cu deposition increasing the formate production rate and Zn enhancing both H2 and formate production. While the accumulation of these poisons can be prevented with periodic anodic treatments (using methods previously described in the literature), these treatments lead to significant gold dissolution, with up to 450 ppb of gold found in the electrolyte after 4 h of electrolysis, and thus is unsuitable for use in long-term CO2 reduction systems. This dissolution is expected to alter the surface structure and thus selectivity of the cathode. Therefore, alternative electrochemical cleaning protocols (periodic cyclic voltammetry, open-circuit and low anodic current treatments) were investigated as methods to remove these poisons without significant gold corrosion occurring. The best approach to prevent the deactivation of gold cathodes during CO2 reduction is to cycle the potential between − 0.5 and 0.5 V vs Ag|AgCl every 15 min during long-term electrolysis. It is also shown that simply interrupting the CO2 reduction process every 15 min with 4 min at open circuit can also partially prevent the deactivation of the CO2 reduction reaction as will short anodic current pulses.