Nanostructured β-Bi₂O₃ Fractals on Carbon Fibers for Highly Selective CO₂ Electroreduction to Formate

3D Bi₂O₃ fractal nanostructures (f-Bi₂O₃) are directly self-assembled on carbon fiber papers (CFP) using a scalable hot-aerosol synthesis strategy. This approach provides high versatility in modulating the physiochemical properties of the Bi₂O₃ catalyst by a tailorable control of its crystalline size, loading, electron density as well as providing exposed stacking of the nanomaterials on the porous CFP substrate. As a result, when tested for electrochemical CO₂ reduction reactions (CO₂RR), these f-Bi₂O₃ electrodes demonstrate superior conversion of CO₂ to formate (HCOO⁻) with low onset overpotential and a high mass-specific formate partial current density of −52.2 mA mg⁻¹, which is ≈3 times higher than that of the drop-casted control Bi₂O₃ catalyst (−15.5 mA mg⁻¹), and a high Faradaic efficiency (FE_HCOO ⁻) of 87% at an applied potential of −1.2 V versus reversible hydrogen electrode. The findings reveal that the high exposure of roughened β-phase Bi₂O₃/Bi edges and the improved electron density of these fractal structures are key contributors in attainment of high CO₂RR activity.