Operando NMR quantifies liquid product, water crossover and carbonates for electrochemical CO2 reduction

Operando analysis is crucial for understanding the selectivity and stability of the electrochemical CO2 reduction reaction (eCO2RR). Existing operando techniques normally adapt single-compartment cells operating at low currents. However, high current densities on the order of 100 mA cm-2 are required for practical applications, and under these conditions, selectivity and reaction pathways can differ. Here, we developed an inline operando NMR method compatible with high-current reaction conditions. Demonstrating on a copper-catalyzed eCO2RR at a current of 100 mA cm-2, our NMR study revealed a fast decrease of Faradaic efficiency for formate and ethanol within the first few hours of reaction, accompanied by a pH decrease from 14 to 8 within the first hour and a continuous concentration increase of bicarbonate. At 200 mA cm-2, the bicarbonate concentration reached the saturation point of 3.34 M within five hours. Water crossover was simultaneously observed and quantified via a deuteration technique and showed a strong current dependency. Our NMR observations revealed a highly dynamic environment of copper-catalyzed eCO2RR at high currents and will further aid the design and optimization of this reaction. Using on a common flow cell and a benchtop NMR system, the new operando approach is accessible by non-NMR experts and readily applicable to a wide range of catalysts, electrolyte compositions and reactor designs for eCO2RR.