Insights into the Composition and Function of a Bismuth-Based Catalyst for Reduction of CO₂ to CO

The electrochemical conversion of CO₂ to CO is a process important for generation of carbon-based fuels using energy produced from intermittent, renewable energy sources. Electrodeposited thin films of bismuth-based materials are promising platforms for this transformation when used for CO₂ electrolysis in the presence of imidazolium-based ionic liquids including [BMIM]OTf and [EMIM]OTF. In this study, the composition and function of a bismuth-based carbon monoxide evolving catalyst (Bi-CMEC) has been probed. In particular, the composition of both the electrodeposited bismuth material and the catholyte has been scrutinized in an effort to understand the factors that drive efficient catalyst operation. A combination of XPS depth profiling, XANES, and EXAFS experiments were employed to identify the bulk composition of the bismuth catalyst, which is shown to be composed of both metallic and oxidized phases. The identity of the catholyte solvent has been shown to influence the nature and efficacy of CO₂ reduction by the Bi-based catalyst system, as mass transport of both CO₂- and imidazolium-based ionic liquid to the electrode surface dramatically impacts the kinetics and selectivity for CO production at the cathode/electrolyte interface.