Mechanistic Insights on CO2 Reduction Reactions at Platinum/[BMIM][BF4] Interfaces from In Operando Spectroscopy

CO2 electrocatalysis in room-temperature ionic liquids is interesting for production of chemicals with a higher energy content and for potential Power-to-X applications. Controlling CO2 reduction reactions (CO2RR) is, however, also highly demanding as existing overpotentials need to be reduced and high selectivity toward the desired products needs to be achieved. For that reason, mechanistic insights into CO2RR are highly desirable. Here, we have applied cyclic voltammetry, in operando IR absorption, and sum-frequency generation (SFG) spectroscopy to address CO2RR at Pt electrode surfaces that were brought in contact with [BMIM] [BF4] and water as coreactant. Cycling the Pt electrode potential from 0.1 to -1.4 V vs SHE, leads to the formation of an imidazolium carboxylic acid species ([BMIM]-COOH) for which we provide spectroscopic evidence. Once formed, this species can be further catalyzed to formic acid during an anodic potential sweep which starts to form at an electrode potential of 1.6 V. During cathodic cycling, SFG spectroscopy demonstrates that CO is formed in addition to [BMIM]-COOH on the Pt catalyst surface, but onset potentials as cathodic as 1.05 V vs SHE are needed. This is different from [EMIM] [BF4] electrolytes and points to additional steric effects. The CO adsorbate layer poisons the Pt catalyst surface and leads to a deactivation over many potential cycles. From our spectroscopic analysis, we provide detailed information on two products of CO2RR, that is, CO and formic acid, as well as on their formation potentials and mechanisms.