Role of H₂O for CO₂ Reduction Reactions at Platinum/Electrolyte Interfaces in Imidazolium Room-Temperature Ionic Liquids

CO2 reduction reactions (CO2RR) are interesting for power-to-x applications and have been studied on Pt electrodes in 1-ethyl-3-methylimidazolium dicyanamide [EMIM][DCA] as well as in 1-ethyl-3-methylimidazolium [EMIM][BF4], 1-butyl-3-methylimidazolium [BMIM][BF4] and 1-ocytl-3-methylimidazolium tetrafluoroborate [OMIM][BF4] electrolytes. Cyclic voltammetry indicates a strong increase in activity for CO2RR with water concentration, which was investigated on a molecular level by using IR absorption spectroscopy (IRAS) and sum-frequency generation (SFG) to address both bulk and surface-adsorbed species. IRAS demonstrates that the formation of an imidazolium carboxylic acid intermediate occurs at electrode potentials as high as -0.4 V, which depend on the choice of the RTIL and the water concentration. In addition, SFG spectroscopy provides evidence for the formation of CO on Pt atop sites and was used to determine the onset potential for the formation CO. In [BMIM][BF4] and [OMIM][BF4] electrolytes, the formation of CO is negligible even at very negative potentials of -1.5 V, but for [EMIM][DCA] and [EMIM][BF4] the formation of CO is observed and the onset potential shifts significantly with the H2O concentration. Taking the Stark tuning rate (40 cm(-1)/V) of the CO band into account, we conclude that the CO coverage is fairly low, yet the presence of CO leads to deactivation of the Pt surface and to a decrease in reduction currents.