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.
Bibliographical noteFunding Information:
The electron density profiles obtained from fitting the low-angle reflectivity signal of the Bi film plotted in Figure 2a (solid black trace) describe a graphene-coated SiC substrate topped by a bismuth-based bilayer, while the profiles extracted from fitting the reflectivity in the CTR regime (red trace) describe a Bi (001) film (∼80% coverage) on top of graphene-coated SiC with a smaller amount of intermixed amorphous bismuth material. Given the discrepancies between the electron density profiles obtained through fitting of the low-angle reflectivity (solid black trace) and higher-angle CTR (red trace) signals of the pristine film, it is evident that the topmost bismuth layer detected in the low-angle regime consists of an amorphous bismuth material (most likely Bi2O3) with no contribution to the reflectivity signal in the higher-angle CTR regime. This hypothesis is supported by the fact that both the low-angle and