A Tandem (Bi2O3 → Bimet) Catalyst for Highly Efficient ec-CO2 Conversion into Formate: Operando Raman Spectroscopic Evidence for a Reaction Pathway Change

A novel bismuth oxide nanofoam, produced by means of the dynamic hydrogen bubble template (DHBT) electrodeposition approach followed by thermal annealing at 300 °C for 12 h, demonstrates excellent electrocatalytic selectivity toward formate production with faradaic efficiencies (FEs) never falling below 90% within an extended potential window of ∼1100 mV (max. FEformate = 100% at −0.6 V vs RHE). These promising electrocatalytic characteristics result from the coupling of two distinct reaction pathways of formate formation in the aqueous CO2-sat. 0.5 M KHCO3 electrolyte, which are active on (i) the partly reduced Bi2O3 foam at low overpotentials (sub-carbonate pathway) and (ii) on the corresponding metallic Bimet foam catalyst at medium and high overpotentials (Bi–O pathway). For the first time, operando Raman spectroscopy provides experimental evidence for the embedment of CO2 into the oxidic Bi2O3 matrix (sub-carbonate formation) at low overpotentials prior to and during the CO2 reduction reaction (CO2RR). The gradual transition of the formed carbonate/oxide composite catalyst into its fully metallic state is monitored by operando Raman spectroscopy as a function of electrolysis time and applied potential. The observed structural and compositional alterations correlate with changes in the faradaic efficiency and partial current density of formate production (PCDformate), which reaches a maximum value of PCDformate = −84.1 mA cm–2 at −1.5 V vs RHE. The so-called identical location scanning electron microscopy technique was applied to monitor morphological changes that take place on the nanometer length scale upon sub-carbonate formation and partial electro-reduction of the oxidic precursor during the CO2RR. However, the macroporous structure of the foam catalyst remains unaffected by the (oxide/carbonate → metal) transition and the catalytic CO2RR.