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 (NixB) and nickel boride/graphene (NixB/G) are chosen as positive materials of supercapacitors. The NixB/G displays higher specific capacitance (1822 F g-1) than that of NixB (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 (NixB/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 NixB, Ni(BO2)2, and graphene that fully enhances the diffusion of OH- and the electron transport. During the cycles, the prepared ultrafine NixB 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 NixB (or β-Ni(OH)2) and graphene and protects the core structure in a stable state, extending the cycle life. The above properties of NixB/G endow the electrode good capacitive performance.