CO2 Conversion at High Current Densities: Stabilization of Bi(III) Containing Electrocatalysts under CO2 Gas Flow Conditions

Abstract

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)-to-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 subcarbonate phases comprising the active composite catalyst layer (CL) during the CO2RR.

Comparison of Electrochemically Deposited Bi and Sn Catalysts onto Gas Diffusion Electrodes for the Electrochemical CO2 Reduction Reaction to Formate

“…The most common catalysts for the eCO 2 RR reaction to formate/formic acid are based on tin (Sn) and bismuth (Bi). Both of these elements exhibit high selectivity towards formate/formic acid and high overpotentials towards the hydrogen evolution reaction [4,[13][14][15][16][17][18][19]. Additionally, these catalysts are of interest because of their non-toxic properties and low cost [20].…”

“…Through a suitable choice of substrate, process, and electrolyte parameters, the nucleation rate and morphology of the precipitates can be influenced. As shown in the literature, defects, grain sizes, and orientation of the catalyst material that can be influenced in this way have a decisive influence on the selectivity and activity of the catalyst in the electrochemical CO 2 reduction [14][15][16][17][18][31][32][33][34][35][36][37]. Compared with the other techniques available, electrodeposition is the most straightforward method to fabricate large electrodes for real industrial electrolysers.…”