Bismuth-based cathodes are known for their exceptional performance as electrocatalytic platforms for the reduction of CO2 into CO, a valuable commodity chemical and synthetic liquid fuel precursor. Bismuth electrodes can deliver CO with high selectivity, faradaic efficiencies and competing kinetics when the reduction of CO2 is carried out in acetonitrile solutions containing imidazolium-based ionic liquids (ILs). Still, the fundamental processes leading to the activation of CO2 at the Bi–IL interface remain elusive. Seeking to understand how this electrode/electrolyte interface behaves under catalytic and non-catalytic conditions, we conducted in situ X-ray reflectivity and crystal truncation rod (CTR) measurements as a function of potential applied, in acetonitrile solutions containing millimolar concentrations of 1-butyl-3-methylimidazolium triflate ([BMIM]OTf) or tetrabutylammonium hexafluorophosphate (TBAPF6), a redox inactive electrolyte. For these studies, bismuth thin film electrodes (~ 6 nm thick) were sputtered onto flat graphene-coated SiC (001) substrates. Ex situ atomic force microscopy and in situ X-ray reflectivity showed that these films have micrometer-sized terraces of very low-roughness (rms £ 5 Å). Additionally, CTR analyses revealed that about 80% of bismuth within the films was crystalline with the (001) plane parallel to the underlying substrate surface, and intermixed with a smaller fraction (~20%) of amorphous bismuth oxides. The Bi films underwent a reversible re-structuring in the presence of the IL as the applied potential was cycled between the open circuit potential (OCP) and -1.9 V (vs. Ag/AgCl). These structural changes were observed with or without CO2 present in solution. Specifically, the variation of the Bi (001) film Bragg peak intensity (at momentum transfer Q = 1.56 Å-1) implies that significant structural or conformational changes to the Bi films occurred just below the onset potential of CO2 activation. In contrast, bismuth film electrodes did not exhibit such re-structuring in TBAPF6 solutions. These results will be discussed in the context of a potential dependent re-structuring of the Bi–IL interface and its possible role on the catalytic reduction of CO2 into CO.

Bismuth-Ionic liquid interface for electrocatalytic CO2 conversion under the lens of x-ray reflectivity

253rd National Meeting (2017)

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