A Reduced Imidazolium Cation Layer Serves as the Active Site for Electrochemical Carbon Dioxide Reduction

•A suppression of HER after the first cycle of CV was found in the presence of EMIM-BF4 on a copper electrode.•A reduced EMIM+ cation layer can prohibit HER and may initiate CO2 reduction.•DFT calculation showed that the reduction from aqueous EMIM+ cation to adsorbed EMIM radical on Cu(111) occurs at −0.7 V.•Interaction of organic molecule with electrode material is vital for CO2 activation.