Molecular level insights on the pulsed electrochemical CO₂ reduction

Electrochemical CO₂ reduction reaction (CO₂RR) occurring at the electrode/electrolyte interface is sensitive to both the potential and concentration polarization. Compared to static electrolysis at a fixed potential, pulsed electrolysis with alternating anodic and cathodic potentials is an intriguing approach that not only reconstructs the surface structure, but also regulates the local pH and mass transport from the electrolyte side in the immediate vicinity of the cathode. Herein, via a combined online mass spectrometry investigation with sub-second temporal resolution and 1-dimensional diffusion profile simulations, we reveal that heightened surface CO₂ concentration promotes CO₂RR over H₂ evolution for both polycrystalline Ag and Cu electrodes after anodic pulses. Moreover, mild oxidative pulses generate a roughened surface topology with under-coordinated Ag or Cu sites, delivering the best CO₂-to-CO and CO₂-to-C₂₊ performance, respectively. Surface-enhanced infrared absorption spectroscopy elucidates the potential dependence of *CO and *OCHO species on Ag as well as the gradually improved *CO consumption rate over under-coordinated Cu after oxidative pulses, directly correlating apparent CO₂RR selectivity with dynamic interfacial chemistry at the molecular level.