Role of selenium on the pseudocapacitance of nickel-cobalt selenide

Metal selenides are necessary electrode materials in electrochemical energy storage , demonstrating comparable or even superior electrochemical performance to metal oxides . However, the role of Se of metal selenide on the pseudocapacitance is still opaque. Therefore, NiO/NiCo 2 O 4 /CoO and NiCoSe 2 spheres are contrastively synthesized and evaluated in an electrochemical system. The specific capacitance of the NiCoSe 2 electrode is 685 F·g −1 (411C·g −1 , 114.2 mAh·g −1 ) at 1 A·g −1 , which is 5.27 times as much as that of the NiO/NiCo 2 O 4 /CoO electrode (130 F·g −1 , 78 C·g −1 , 21.7 mAh·g −1 ). During charging-discharging for metal selenides electrodes, the new species of CoOOH and NiOOH appear, and the selenium in the NiCoSe 2 sphere disappears, indicating that the hydroxides but not the selenide participate in the electrochemical pseudocapacitance. Selenium does not participate in pseudo-capacitance reactions and promotes the transformation of the active phase. In addition, excellent electrical conductivity of selenium increase the electrochemical performance. The asymmetric device with NiCoSe 2 as a positive electrode shows a superior energy density of 24 Wh·kg −1 at 800 W·kg −1 and outstanding stability (146% retention after 5000 cycles). Notably, two asymmetric devices in series can be charged to 3.21 V by a solar-panels, which can successfully drive different loads, indicating the outstanding performance of the devices.