Operando Raman Spectroscopy Reveals Backpressure-Controlled Dynamics in Diluted CO₂ Electrolysis

Understanding the competition between the electrochemical CO₂ reduction reaction (eCO₂RR) and the hydrogen evolution reaction (HER) is crucial for efficient CO₂ conversion, particularly under limited reactant availability using diluted CO₂. We investigate the interplay between the operating conditions at various differential backpressures concerning CO₂ availability, water behavior at the electrified catalyst layer, and how these factors modulate the eCO₂RR-to-formate selectivity and HER activity. We show that H₂ formation can be suppressed while maintaining high formate selectivity even using highly diluted CO₂ (2.25%) by increasing the differential backpressure from the gas side of the gas-diffusion cathode in the flow-through electrolyzer, which enhances CO₂ transport to the catalyst layer through reshaping the triple-phase boundary. Operando Raman spectroscopy reveals the accumulation and perturbation of water on the catalyst layer at negative potentials, accompanied by weakening of the hydrogen-bond network. Moreover, even higher differential backpressure minimizes carbonate formation, preserves eCO₂RR kinetics, and promotes the displacement of surface water, which in turn minimizes HER.