Fabrication of Metallic Nickel-Cobalt Phosphide Hollow Microspheres for High-Rate Supercapacitors

Electrode materials for high-rate supercapacitors are greatly desired. Among actively studied electrode materials, phosphide compounds with metallic features are recognized as such promising supercapacitor materials. In this work, considering the benefits of multimetal redox centers and superior electrical conductivity of ternary nickel-cobalt metal phosphides for improving the supercapacitor performance, urchin-like nickel-cobalt phosphide hollow spheres were prepared with a mild hydrothermal method followed by phosphorization. The formation process was explored using time-dependent experiments. The prepared material exhibited an excellent capacity of 761 C g^-1 at a current density of 1 A g^-1, with 693 C g^-1 remaining even at 20 A g^-1, which exhibited a high-rate capability with approximately 91.1% retention of its initial capacity. Moreover, the asymmetric supercapacitor assembled with the prepared NiCoP hollow sphere and activated carbon exhibited a maximum energy density of 35.6 W h kg^-1 and maximum power density of 8387 W kg^-1. Such a high performance of the as-prepared material originated from a combination of superior electrical conductivity, a synergistic effect in Ni and Co, and an effective urchin-like hollow structure. Hence, the as-prepared ternary nickel-cobalt metal phosphide could be applied as an effective supercapacitor electrode material with a high power density and long-term stability for practical applications.