Hierarchical nickel-cobalt selenide nanoparticles/nanosheets as advanced electroactive battery materials for hybrid supercapacitors

Transition metal selenides are very ideal as electroactive battery material for hybrid supercapacitors owing to their high electrochemical activity and metallic electrical conductivity, but their environmental friendly fabrication and complex structure construction is still a big challenge. Here, a simple low-temperature selenization method has been developed to green synthesize a series of Ni-Co selenide samples with a hierarchical nanoparticle/nanosheet structure. The hierarchical structure has been constructed by the recrystallization of sample during the selenization process, which can be greatly influenced by the Co composition. The specific hierarchical structure provides more electroactive sites for charge storage, and the optimized synergy between the Ni and Co compositions further greatly tunes the charge storage performance, as a result, the Ni0.67Co0.33Se2 shows the best performance with a specific capacity of 447 C g-1 at 1 A g-1, which is much higher than the Ni-Co selenides with the other Ni to Co ratios and the Ni-Co oxides. The Ni0.67Co0.33Se2 retains 97% of the specific capacity after 2000 cycles, demonstrating its excellent cycling stability. A hybrid supercapacitor has been assembled using Ni0.67Co0.33Se2 as the positive electrode, which shows both high specific energy and specific power performances.