Herein, we demonstrate the superior performance of novel bismuth subcarbonate ((BiO)2CO3) film 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 versus the reversible hydrogen electrode (RHE). Even higher values of PCDformate = –441.2 mA cm–2 (FEformate = 62%) were observed at 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, however, the reductive Bi(III)®Bi(0) transition already occurs at 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. Advanced X-ray diffraction computed tomography (XRD-CT) technique was applied to gain deeper insights into the spatial distribution of the metallic and carbonate phases comprising the active composite catalyst layer (CL) during the CO2RR.