Size-Dependent Cation Effect on Intermediate Binding and Water Network as Revealed by Raman Spectroscopy in Acidic Electroreduction of Carbon Dioxide

Recent advances in the acidic electroreduction of carbon dioxide (CO 2 ) demonstrate the critical role of alkali metal cations (AM + ) in driving CO 2 activation and subsequent elementary steps. However, the effect of cation radius on CO 2 reduction in acid remains inadequately understood. Herein, we investigate the size effect of AM + (Cs +, K +, Na +, Li + ) on CO 2 reduction using a polycrystalline copper cathode in a flow cell in acid. In 1 M Cs +, the production of multi-carbon (C 2+ ) products peaks with a Faradaic efficiency (FE C2+ ) of 63.2% at –500 mA cm −2 . On the contrary, Li + favors the production of methane with a FE CH4 of 33.1% even at –70 mA cm −2 . Kinetic analysis reveals the divergent roles of AM + in regulating the CO 2 activation step and the C−C coupling step, while in situ Raman spectroscopy demonstrates tunable surface coverage and configuration of *CO as well as the interface hydrophobicity in different AM + . By correlating electrochemical performance with spectroscopic findings, we elucidate how the cation size regulates the binding of key intermediates as well as the local water network, which in turn affect the activity of copper for CO 2 reduction in acid.