Vibrational sum-frequency generation study of the CO2 electrochemical reduction at Pt/EMIM-BF4 solid/liquid interfaces

Abstract

Electrochemical reduction of CO2 on polycrystalline Pt catalysts in a 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM]BF4 room-temperature ionic liquid (RTIL) is not impaired by well-known effects such as high ohmic losses or vigorous hydrogen evolution reactions. Vibrational broadband sum-frequency generation (SFG) combined with cyclic voltammetry has been applied to identify the formation of a [CO2-EMIM] complex at the interface which gives rise to a SFG active vibrational mode at 2355cm−1. The [CO2-EMIM] complex serves as a stable precursor to CO2 reduction and is linked to a remarkable small potential barrier for CO2 reduction and subsequent CO production. Vibrational SFG spectra reveal CO linearly bonded to Pt atop sites as the predominant surface-adsorbed reduction product. Extended electrochemical reduction of CO2 results in an intense CO band and an electrochemical Stark tuning rate of 33cm−1/V at potentials >−0.2V vs. SHE which is indicative of a close-packed CO adlayer. Anomalous Stark tuning for potentials <−0.2V is observed and attributed to hydrogen co-adsorption from residual H2O into the CO adlayer. CO modification of Pt leads to considerable reduction of the observed reduction current density that is caused by a poisoning effect of the CO adlayer that persists at the interface for +0.5 to −0.8V.