Sea-urchin-like nickel–cobalt phosphide/phosphate composites as advanced battery materials for hybrid supercapacitors

Abstract

In this study, sea-urchin-like bimetal nickel–cobalt phosphide/phosphate (Ni–Co–P/PO_x) samples were first synthesized by a low-temperature phosphating method. The phosphide phase shows a crystal nanoparticle morphology distributed in the matrix of the amorphous phosphate phase. By theoretical calculations, it has been found that NiCoP shows a metallic conductivity property because the density of states crosses the Fermi level, and it exhibits better electrochemical activity than Ni₂P and CoP owing to the presence of more electrons close to the Fermi level; moreover, the effects of phosphorization and bimetallic properties on the charge storage performance of Ni–Co–P/PO_x have been investigated, and the results indicate improved electrochemical activity, rate capability and cycling stability. Ni–Co–P/PO_x with the Ni to Co molar ratio of 1 : 1 shows the best performance, and it exhibits electrochemical activity greater than and rate performance comparable to that of the oxide. Ni–Co–P/PO_x with the Ni to Co molar ratio of 1 : 1 shows a specific capacity of 647 C g⁻¹ at 1 A g⁻¹ and specific capacity retentions of 67% and 94% after the specific current increases by 50 times and after 1000 cycles, respectively; this indicates that high electrochemical activity, rate performance and cycling stability performances have been simultaneously achieved. In addition, a Ni–Co–P/PO_x//reduced graphene oxide hybrid supercapacitor was assembled, which showed good electrocapacitive performance.