Liner Nanostructure of Metal Boride as a Cathode Material\nfor High-Performance and Flexible Solid-State Asymmetric Hybrid Battery\nSupercapacitors

The\ndevelopment of a high-performance solid-state asymmetric hybrid\nflexible supercapacitor device with high capacitance, rate capability,\nhigh energy density, and flexible stability remains challenging. Herein,\ncobalt metal-free crystalline NiCuB alloy particles with nanowire-type\nmorphology are developed by a new synthesis process for this sustainable\ndevelopment. The optimized composition Ni<sub>0.6</sub>Cu<sub>0.4</sub>B is found to have the maximum synergy effect among the metals and\nthe metalloid. A three-electrode charge storage study shows a specific\ncapacitance of 1621 F g<sup>–1</sup> at 5.6 A g<sup>–1</sup> for this material along with high rate capability and energy efficiency.\nA highly flexible all-solid-state asymmetric hybrid supercapacitor\n(HSC) device is fabricated by using this Ni<sub>0.6</sub>Cu<sub>0.4</sub>B as the positive electrode, α-Fe<sub>2</sub>O<sub>3</sub>/g-C<sub>3</sub>N<sub>4</sub> as the negative electrode, and KOH-PVA gel as\nthe solid electrolyte. The hybrid flexible device offered a maximum\ncapacitance of 325 F g<sup>–1</sup> at 6.2 A g<sup>–1</sup> applied current density. The charge storage characteristics remain\nconsistent when the engineered HSC device is bent at 60, 90, 135,\nand 170°. The device can maintain 93% Coulombic efficiency at\n15,000 measured charge–discharge cycles after bending at angle\n≤150° over 1000 bending cycles. The HSC device delivers\na high energy density of 57.8 Wh kg<sup>–1</sup> and a power\ndensity of 16,380 W kg<sup>–1</sup>. A diffusion-controlled\ncharge-storage mechanism is predominant, along with a high diffusion\ncoefficient of electrolytes and a suitable pore size distribution.