Tailoring 3D flower-like NiO-Bi₂O₃ composite nanoarchitectures for high-rate supercapacitor and efficient alkaline hydrogen evolution

The development of high-performance electrocatalysts through cost-effective, sustainable, and scalable methods is essential for practical energy storage and conversion applications, as alternatives to expensive noble metal-based catalysts. In this study, NiO–Bi₂O₃ mixed metal oxides were synthesized via a co-precipitation method using nickel and bismuth precursors. The formation mechanism was systematically investigated by varying the Ni:Bi molar ratios (3:1, 1:1, 1:3, and 1:5). The resulting NiO–Bi₂O₃ materials were characterized using XRD, FE-SEM, HR-TEM, BET, XPS, and EDS techniques. Variations in the precursor ratio significantly affected the morphology and crystallinity of the products. Notably, the NiO–Bi₂O₃ sample with a 1:3 Ni to Bi ratio formed a flower-like hierarchical nanostructure with abundant oxygen vacancies, which are lattice defects that contribute to its electronic conductivity. As revealed by XPS analysis, the flower-like three-dimensional NiO–Bi₂O₃ electrode achieved a highest specific capacitance of 1218.5 F·g⁻¹ at 1 A·g⁻¹ in 6 M KOH, outperforming other ratios. An asymmetric supercapacitor was assembled using the optimized NiO–Bi₂O₃ as the anode and NiCo-LDH as the cathode. The device delivered an energy density of 114.62 Wh·kg⁻¹ at a power density of 750 W·kg⁻¹ and maintained 98.3 % of its initial capacitance after 5000 charge–discharge cycles. The same electrode also demonstrated excellent HER activity, requiring overpotentials of 73 and 138 mV at 10 and 50 mA·cm⁻², respectively, with a Tafel slope of 35 mV·dec⁻¹ in 1 M KOH. These results highlight the potential of NiO–Bi₂O₃ composite nanostructures for advanced supercapacitor and HER applications.