Cathodic Corrosion at the Bismuth–Ionic Liquid Electrolyte Interface under Conditions for CO₂ Reduction

Bismuth electrodes undergo distinctive electrochemically induced structural changes in nonaqueous imidazolium ([Im]+)-based ionic liquid solutions under cathodic polarization. In situ X-ray reflectivity (XR) studies have been undertaken to probe well-ordered Bi (001) films which originally contain a native Bi2O3 layer. This oxide layer gets reduced to Bi0 during the first cyclic voltammetry (CV) scan in acetonitrile solutions containing 1-butyl-3-methylimidazolium ([BMIM]+) electrolytes. Approximately 60% of the Bi (001) Bragg peak reflectivity is lost during a potential sweep between −1.5 and −1.9 V vs Ag/AgCl due to a ∼ 4–10% thinning and a ∼40% decrease in lateral size of Bi (001) domains, which are mostly reversed during the anodic scan. Repeated potential cycling enhances the thinning and roughening of the films, suggesting that partial dissolution of Bi ensues during negative polarization. The mechanism of this behavior is understood through molecular dynamics simulations using ReaxFF and density functional theory (DFT) calculations. Both approaches indicate that [Im]+ cations bind to the metal surface more strongly than tetrabutylammonium (TBA+) as the potential and the charge on the Bi surface become more negative. ReaxFF simulations predict a higher degree of disorder for a negatively charged Bi (001) slab in the presence of the [Im]+ cations and substantial migration of Bi atoms from the surface. DFT simulations show the formation of Bi···[Im]+ complexes that lead to the dissolution of Bi atoms from step edges on the Bi (001) surface at potentials between −1.65 and −1.95 V. Bi desorption from a flat terrace requires a potential of approximately −2.25 V. Together, these results suggest the formation of a Bi···[Im]+ complex through partial cathodic corrosion of the Bi film under conditions (potential and electrolyte composition) that favor the catalytic reduction of CO2.