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.