CO2 Electroreduction on Silver and Silver-Based Cathodes in Aqueous and Nonaqueous Solutions: A Review

The scientific literature on the electrochemical conversion of CO2 on silver and silver-based cathodes in aqueous solutions and nonaqueous media has been analyzed. It has been shown that the most problematic issues regarding cathodic reduction and, accordingly, utilization of this greenhouse gas into valuable products (CO, HCOOH, CH4, CH3OH, (COOH)2) are the process speed and its selectivity. In this regard, we present the peculiarities of the electrocatalytic activity of the silver surface in comparison with other metals and the prospects of silver-based cathodes in technological and economic perspectives. According to the publications of the last decade, special attention was paid to (1) silver cathodes with a highly developed surface, such as foam and nanoporous electrodes, (2) electrodes with AgNPs deposited on the substrate, and (3) binary silver-containing cathodes. These types of cathodes can increase the rate of electrochemical conversion of CO2 to CO by an order of magnitude or more, providing Faradaic efficiency of more than 90%. This is especially important given that carbon(II) oxide is currently the most cost-effective product. One of the most promising and actively developing areas is the electrochemical conversion of CO2 at a silver cathode in a nonaqueous medium, in particular in ionic liquids and organic aprotic solvents. In comparison with aqueous solutions, they exhibit an order of magnitude higher solubility of CO2, no cathodic hydrogen evolution, and significantly higher electrochemical stability. These advantages make it possible to carry out electrochemical conversion at high cathodic current values and ensure high selectivity for target products. It has been shown that the combination of a nonaqueous medium with silver-containing cathodes increases both the productivity and selectivity of CO2 reduction to CO and oxalate.