Interfacial Engineering of Nickel Boride/Metaborate and Its Effect on High Energy Density Asymmetric Supercapacitors

Solid materials with special atomic and electronic structures are deemed desirable platforms for establishing clear relationships between surface/interface structure characteristics and electrochemical activity. In this work, nickel boride (Ni_xB) and nickel boride/graphene (Ni_xB/G) are chosen as positive materials of supercapacitors. The Ni_xB/G displays higher specific capacitance (1822 F g^-1) than that of Ni_xB (1334 F g^-1) at 1 A g^-1, and it still maintains 1179 F g^-1 at 20 A g^-1, suggesting the high rate performance. The asymmetric supercapacitor device (Ni_xB/G//activated carbon) also delivered a very high energy density of 50.4 Wh kg^-1, and the excellent electrochemical performance is ascribed to the synergistic effect of Ni_xB, Ni(BO₂)₂, and graphene that fully enhances the diffusion of OH^- and the electron transport. During the cycles, the prepared ultrafine Ni_xB nanoparticles will be gradually in situ converted into β-Ni(OH)₂ which has a smaller particle size than that prepared by other methods. This will enhance the utilization of Ni(OH)₂ and decrease the ion diffusion distance. The electron deficient state of B in Ni(BO₂)₂ amorphous shell will make it easy to accept extra electrons, enhancing the adsorption of OH^- at the shell surface. Moreover, Ni(BO₂)₂ makes strong adhesion between Ni_xB (or β-Ni(OH)₂) and graphene and protects the core structure in a stable state, extending the cycle life. The above properties of Ni_xB/G endow the electrode good capacitive performance.