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