Abstract:
Electrochemical CO₂ reduction (CO₂RR) to value-added chemicals offers a promising pathway for mitigating carbon emissions while simultaneously producing sustainable fuels and chemicals. Among the various CO₂RR products, formate is particularly attractive due to its potential applications as an energy storage vector, feedstock for chemical synthesis, and fuel for fuel cells. In this work, bismuth-based gas diffusion electrodes (GDEs) were synthesized through electrodeposition using two different electrolyte systems: a deep eutectic solvent (DES) composed of choline chloride and ethylene glycol (1:2) and a conventional aqueous electrolyte. The influence of the electrolyte environment on catalyst formation, morphology, electrochemical behavior, and catalytic performance toward CO₂RR was systematically investigated.Electrodeposition in the DES electrolyte resulted in the formation of nanowire-like structures with a high electrochemically active surface area, whereas deposition in the aqueous electrolyte produced spherical nanostructures. Electrochemical measurements revealed that catalysts electrodeposited in DES under varying deposition current densities exhibited distinct catalytic behaviors, which were systematically evaluated. It was found that catalysts prepared under different deposition current densities and solvent systems displayed varying selectivity toward formate during electrochemical CO₂ reduction. Among them, the catalyst prepared in DES at a deposition current density of 4 mA cm⁻² exhibited the highest formate selectivity, achieving a Faradaic efficiency of 97.7% at a current density of 30 mA cm⁻². In contrast, the catalyst prepared in the aqueous electrolyte at 18 mA cm⁻² maintained high selectivity at elevated current densities, achieving formate Faradaic efficiencies of 88.9% and 77.0% at 150 and 200 mA cm⁻², respectively. Post-reaction characterization revealed that both catalysts underwent electrochemical restructuring during activation, transforming into nanoflower-like morphologies that likely represent the catalytically active phase for CO₂ reduction. Additional characterization using SEM, PXRD, XPS, electrochemical impedance spectroscopy (EIS), and ICP–MS provided insights into the structural evolution, surface chemistry, charge-transfer properties, catalyst loading, and dissolution behavior under electrochemical conditions. Finally, the scalability of the electrodeposition process was demonstrated by fabricating large-area electrodes up to 10 × 15 cm², where the DES-derived catalyst maintained a formate Faradaic efficiency of approximately 95.5% at 100 mA cm⁻². These findings demonstrate that DES-assisted electrodeposition offers a sustainable and scalable strategy for preparing highly selective bismuth catalysts for electrochemical CO₂ conversion to formate.
