Self-template synthesis of yolk-shelled NiCo2O4 spheres for enhanced hybrid supercapacitors

Abstract A self-template method is developed for hierarchically yolk-shelled NiCo 2 O 4 spheres (YS-NiCo 2 O 4 ) through a controlled hydrolysis process and followed by a thermal annealing treatment. The yolk-shelled NiCo 2 O 4 spheres possess out-shell consisting of hundreds of ultrathin sheets with 3–5 nm in thickness and solid yolk composing of a large number of nanoparticles. The YS-NiCo 2 O 4 generates a large specific surface area of 169.6 m 2  g −1 . Benefit from the large specific surface area and rich oxygen vacancy, the as-fabricated YS-NiCo 2 O 4 as electrode materials for supercapacitor exhibits high specific capacitance of 835.7 F g −1 at 0.5 A g −1 , an enhanced rate capability and excellent electrochemical stability with 93% retention after 10,000 cycles even at 10 A g −1 . Moreover, a hybrid supercapacitor combined with YS-NiCo 2 O 4 and graphene shows a high energy density of 34.7 Wh kg −1 at the power density of 395.0 W kg −1 at 0.5 A g −1 , even at 20 A g −1 , the hybrid supercapacitor still delivers the energy density of about 12.1 Wh kg −1 and the power density of 11697 W kg −1 . The desirable performance of yolk-shelled NiCo 2 O 4 suggests it to be a promising material as supercapacitor electrodes.