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.