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

Highlights: • A self-template method is developed for hierarchically yolk-shelled NiCo₂O₄ spheres. • The yolk-shelled NiCo₂O₄ spheres possess sheet-liked out-shell and porous solid yolk. • YS-NiCo₂O₄ shows a remarkable capacitance retention of ∼93% after 10,000 cycles. • A hybrid supercapacitor shows a high energy density of 12.1 Wh kg⁻¹ and the power density of 11697 W kg⁻¹. 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 m² 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.