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.