Structural Dynamics and Evolution of Bismuth Electrodes during Electrochemical Reduction of CO2 in Imidazolium-Based Ionic Liquid Solutions

Real-time changes in the composition and structure of bismuth electrodes used for catalytic conversion of CO2 into CO were examined via X-ray absorption spectroscopy (including XANES and EXAFS), electrochemical quartz crystal microbalance (EQCM), and in situ X-ray reflectivity (XR). Measurements were performed with bismuth electrodes immersed in acetonitrile (MeCN) solutions containing a 1-butyl-3-methylimidazolium ([BMIM](+)) ionic liquid promoter or electrochemically inactive tetrabutylammonium supporting electrolytes (TBAPF(6) and TBAOTf). Altogether, these measurements show that bismuth electrodes are originally a mixture of bismuth oxides (including Bi2O3) and metallic bismuth (Bi-0) and that the reduction of oxidized bismuth species to Bi-0 is fully achieved under potentials at which CO2 activation takes place. Furthermore, EQCM measurements conducted during cyclic voltammetry revealed that a bismuth-coated quartz crystal exhibits significant shifts in resistance (Delta R) prior to the onset of CO2 reduction near -1.75 V vs Ag/AgCl and pronounced hysteresis in frequency (Delta f) and Delta R, which suggests significant changes in roughness or viscosity at the Bi/[BMIM](+) solution interface. In situ XR performed on rhombohedral Bi (001) oriented films indicates that extensive restructuring of the bismuth film cathodes takes place upon polarization to potentials more negative than -1.6 V vs Ag/AgCl, which is characterized by a decrease of the Bi (001) Bragg peak intensity of >= 50% in [BMIM] OTf solutions in the presence and absence of CO2. Over 90% of the reflectivity is recovered during the anodic half-scan, suggesting that the structural changes are mostly reversible. In contrast, such a phenomenon is not observed for thin Bi (001) oriented films in solutions of tetrabutylammonium salts that do not promote CO2 reduction. Overall, these results highlight that Bi electrodes undergo significant potential-dependent chemical and structural transformations in the presence of [BMIM](+) based electrolytes, including the reduction of bismuth oxide to bismuth metal and changes in roughness and near-surface viscosity.