CO 2 Conversion at High Current Densities: Stabilization of Bi(III)-Containing Electrocatalysts under CO 2 Gas Flow Conditions

Herein, we demonstrate the superior performance of bismuth subcarbonate ((BiO)2CO3) layer catalysts for formate production using a fluidic CO2-fed electrolyzer device. The subcarbonate catalyst readily forms in situ from a CO2-absorbing Bi2O3 precursor material during the CO2 reduction reaction (CO2RR). In 1 mol dm–3 KOH electrolyte solution, a maximum Faradaic efficiency of FEformate = 97.4% (corresponding partial current density of formate formation: PCDformate = −111.6 mA cm–2) was achieved at a comparably low applied electrolysis potential of −0.8 V vs the reversible hydrogen electrode (RHE). Even higher values of PCDformate = −441.2 mA cm–2 (FEformate = 62%) were observed at a more cathodic potential, −2.5 V vs RHE. As the alkalinity of the liquid electrolyte is further increased (e.g., by using 5 mol dm–3 KOH solution), the performance of formate production is boosted beyond PCDformate values of −1 A cm–2. Combined X-ray diffraction and Raman spectroscopic investigations demonstrate an extraordinarily high stability of Bi(III) cations in the catalytically active subcarbonate catalyst phase down to cathode potentials of −1.5 V vs RHE. This stabilization effect can clearly be attributed to the high abundance of gaseous CO2 under the operating conditions of the gas-fed electrolyzer. In the absence of any CO2 supply, the reductive Bi(III) → Bi(0) transition already occurs under much milder conditions of −0.3 V vs RHE, as evidenced by in situ Raman spectroscopy in CO2-free 1 mol dm–3 KOH electrolyte solution. An advanced X-ray diffraction computed tomography technique was applied to gain deeper insights into the spatial distribution of the metallic and subcarbonate phases comprising the active composite catalyst layer during the CO2RR.