Operando Benchtop NMR Quantifies Carbonation, Water Crossover, and Liquid Products for High-Current Electrochemical CO₂ Reduction

Operando characterization is crucial for understanding the selectivity and stability of the electrochemical CO₂ reduction reaction (eCO₂RR). Existing operando techniques normally use single-compartment cells operating at low currents. However, high current densities on the order of 100 mA cm^–2 are required for practical applications. Under a high current, reaction pathways and electrolyte dynamics can change, and stability issues such as salt precipitation and water crossover become more pronounced. Here, we developed an inline operando NMR method that is compatible with high-current reaction conditions. Demonstrating this on a copper-catalyzed eCO₂RR at 100 mA cm^–2, the operando NMR revealed a fast decrease of Faradaic efficiency for formate and ethanol within half an hour of reaction, accompanied by a pH decrease from 14 to 8 and a continuous accumulation of bicarbonate in the electrolyte. Water crossover was simultaneously observed and quantified via a deuteration technique and became more severe at high currents. This study revealed a highly dynamic electrolyte environment of copper-catalyzed eCO₂RR. Using a gas diffusion flow cell and a benchtop NMR system, this operando approach is accessible by non-NMR experts and readily applicable to a wide range of catalysts, electrolyte compositions, and reactor designs for eCO₂RR.