Unveiling the Origin of pH-Dependent Catalytic Performance of Bi₂O₃ Nanostructure for Electrochemical CO₂ Reduction

Over the past decade, the electrochemical conversion of CO₂ into valuable chemicals and fuels has garnered increasing interest as a promising pathway toward a carbon-neutral circular economy. This study investigates Bi₂O₃ gas diffusion electrodes (Bi-GDEs) for the conversion of CO₂ to formic acid/formate (HCOOH/HCOO^-), which demonstrate excellent selectivity at high current densities. The catalyst is synthesized through a one-pot microwave-assisted process that is rapid, energy-efficient, and scalable, utilizing the green solvent ethylene glycol. The resulting Bi₂O₃ nanostructure achieves near-unit selectivity for CO₂ conversion, with a faradaic efficiency exceeding 95% for HCOOH/HCOO^- formation across a wide pH range. The catalytic activity is strongly pH-dependent, with an increase in the pH reducing the overpotential at a given current density. To elucidate the origin of this pH-dependent activity, operando Raman spectroscopy, post-mortem scanning electron microscopy (SEM), and electrical double layer characterization were performed. Operando Raman results reveal that Bi₂O₃ undergoes reduction more readily in highly acidic or basic electrolytes, whereas its reduction is inhibited near neutral pH. However, at highly negative potentials relevant to CO₂RR, cationic Bi species fully convert to metallic Bi. Despite structural variations at different electrolyte pH values, metallic Bi remains the active phase, explaining the high selectivity of Bi-GDEs across a broad pH range. Post-mortem SEM images highlight the influence of electrolyte pH on morphological evolution under CO₂RR conditions. At the highest pH of 14, a hierarchical dendritic structure emerges, showing an increase of 100% in double layer capacitance, which evidences the significant enhancement in the electrochemical active surface area and consequently the CO₂RR activity.