Operando Raman Spectroscopy Reveals Backpressure‐Controlled Dynamics in Diluted CO ₂ Electrolysis

ABSTRACT Understanding the competition between the electrochemical CO 2 reduction reaction (eCO 2 RR) and the hydrogen evolution reaction (HER) is crucial for efficient CO 2 conversion, particularly under limited reactant availability using diluted CO 2 . We investigate the interplay between the operating conditions at various differential backpressures concerning CO 2 availability, water behavior at the electrified catalyst layer, and how these factors modulate the eCO 2 RR‐to‐formate selectivity and HER activity. We show that H 2 formation can be suppressed while maintaining high formate selectivity even using highly diluted CO 2 (2.25%) by increasing the differential backpressure from the gas side of the gas‐diffusion cathode in the flow‐through electrolyzer, which enhances CO 2 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 eCO 2 RR kinetics, and promotes the displacement of surface water, which in turn minimizes HER.