Operando electrochemical NMR spectroscopy reveals a water-assisted formate formation mechanism

The affinity of oxygen (O)-bound species is a key factor in CO2 reduction (CO2R) reactions (including C1 and C2+ products), although existing experimental methods cannot quantitatively track the O atoms active within CO2R reactions in real time. Among the diversified products from CO2R reactions, the formate (HCOO−) possesses the highest profit per mole of electrons. Here, we report an operando electrochemical nuclear magnetic resonance (NMR) method, which allows to quantitatively describe the complex species containing O atoms during the electrochemical CO2R reactions. Based on Cu and bimetallic Cu-based materials (Bi2CuO4 and In2Cu2O5) systems, we found that by introducing Bi and In metal adsorption sites, the O atoms of adsorbed H2O can directly involve in the formation of HCOO− through a water-assisted mechanism (∗COOH− regeneration), thereby improving the selectivity of liquid HCOO− product mostly from 34.2% to 98%. This strategy gives valuable insights into the design of HCOO−-favored catalysts.