Light tuning CO/H₂ composition on Ag: Unraveling CO₂ mass transfer and electron-phonon coupling in plasmon-enhanced electrocatalysis

Plasmon-enhanced electrocatalysis (PEEC) is an emerging approach to mitigate CO₂ emissions. The mechanisms behind CO₂ adsorption and reduction at the catalyst-electrolyte interface in PEEC still need to be further explored. Herein, we employ a well-defined Ag nanostructure to elucidate these pivotal issues. By shining light with wavelengths of 625, 525, 405 nm on Ag, an adjustable CO/H₂ ratio from 35 to 1 can be obtained. The reaction pathway changing under plasmonic excitation does not originate from the lowered CO₂ mass transfer in the vicinity of Ag, as the electrochemical quartz crystal microbalance results unravels that a slightly elevated temperature in bulk electrolyte caused by light irradiation cannot weaken the CO₂ adsorption at the Ag catalyst-electrolyte interface. Theoretical calculations reveal that optical excitation towards shorter wavelengths leads to a progressive lowered energy barrier for H₂ formation together with an enhanced energy barrier for *COOH formation. Although thermodynamically suppressed, CO₂ reduction can still be improved kinetically by optimizing the excitation wavelength and intensity, being accompanied with the enhanced photocurrent. Transient absorption spectroscopy results further correlates the higher photocurrent with a prolonged electron-phonon coupling time, verifying that the improvement of CO₂ reduction kinetics in PEEC can be realized by hot electron harnessing.