Selective electroreduction of CO2 to formate on Bi and oxide-derived Bi films

Bi and oxide-derived Bi films were prepared through potentiostatic electrodeposition on titanium substrates, and subsequent electrochemical and thermal oxidation. The morphology of the films was studied through scanning electron microscopy (SEM), and their structure was analyzed via x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD). The electrocatalytic activity and stability of these films with regard to CO2 electroreduction in aqueous solution was then assessed. As-deposited Bi films exhibited a high activity (j(HCOOH) = 1.6 mA cm(geo)(-2) at -0.82 V vs RHE, which corresponds to an overpotential of 800 mV) and stability for CO2 conversion to formate. The activity of electrochemically oxide-derived Bi films, however, was significantly higher (j(HCOOH) = 8.3 mA cm(geo)(-2)) at only a slightly larger overpotential (900 mV). The formate faradic efficiency vs potential curve of both types of films were almost identical, although the Bi electrochemically active surface area was increased by a factor of four. This is believed to reflect the fact that both the hydrogen evolution and the CO2 electrochemical reduction reactions are occurring on the same site, or on two different sites affected in identical ways by the treatment.