Spectro-Electrochemical Insights into Electrocatalytic CO₂ Reduction in Acidic Media through Model Catalyst Design

Electrocatalytic CO₂ reduction (eCO₂R) under acidic conditions is the game changer of resourceful CO₂ utilization owing to the alleviated carbon loss but faces severe competition from the hydrogen evolution reaction (HER) that greatly curtails the electric current efficiency. Leveraging the eCO₂R side of the teeterboard calls for a fundamental understanding of the triphasic electrode process involving a complex arrangement of electric double layers (EDLs). Herein, a series of model catalysts with tailored cavernous parameters are fabricated to geometrically and spectroscopically decipher the competing HER and eCO₂R processes that engage different proton sources. A comprehensive set of in situ/operando spectro-electrochemical tools, including differential electrochemical mass spectrometry, vibrational spectra (Raman and Infrared), and rotating disk electrode, are combined to interrogate the geometrically modulated HER and eCO₂R kinetics with exceptional temporal and spectral resolutions. We uncover that an overcrowded EDL zone comprising capacitively stored K⁺ cations in closely packed nanocages disfavors eCO₂R at high current densities via limited reaction volume, restrained CO₂ supply, and escalated water dissociation. We further show that, by meticulously crafting the mesoporous cavity structure and suitably expanding the hierarchical EDL zone, great acidic eCO₂R performance of near-unity CO Faradaic efficiency can be achieved across a wide current range over a prolonged operation. This work offers profound insights into the spatial regulation of mass transport, local chemical environment, and EDL arrangement via tailored catalyst geometry toward high-efficiency eCO₂R.