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

A self-template method is developed for hierarchically yolk-shelled NiCo2O4 spheres (YS-NiCo2O4) through a controlled hydrolysis process and followed by a thermal annealing treatment. The yolk-shelled NiCo2O4 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-NiCo2O4 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-NiCo2O4 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-NiCo2O4 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 NiCo2O4 suggests it to be a promising material as supercapacitor electrodes. (C) 2017 Elsevier B.V. All rights reserved.