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 _x B) and nickel boride/graphene (Ni _x B/G) are chosen as positive materials of supercapacitors. The Ni _x B/G displays higher specific capacitance (1822 F g ^-1 ) than that of Ni _x B (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 _x B/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 _x B, Ni(BO ₂ ) ₂ , and graphene that fully enhances the diffusion of OH ^- and the electron transport. During the cycles, the prepared ultrafine Ni _x B 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(BO ₂ ) ₂ amorphous shell will make it easy to accept extra electrons, enhancing the adsorption of OH ^- at the shell surface. Moreover, Ni(BO ₂ ) ₂ makes strong adhesion between Ni _x B (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 _x B/G endow the electrode good capacitive performance.