Optimized core–shell polypyrrole-coated NiCo₂O₄ nanowires as binder-free electrode for high-energy and durable aqueous asymmetric supercapacitor

Cathode design is of essence for fabricating aqueous asymmetric supercapacitor (ASC) device accompanying with high energy density and long cycle life. Nickel cobaltite, owing to the high theoretical capacity and good redox reversibility, is one of the most promising cathode materials for ASC device. However, the poor conductivity and cycling stability lead to a low practical capacity. A novel core–shell structure polypyrrole-coated NiCo₂O₄ nanowires are rationally designed and synthesized via hydrothermal synthesis of the NiCo₂O₄ nanowires and subsequently electrodeposition of the polypyrrole (PPy) outer shell. By adjusting the PPy load mass, the optimized electrode shows a high specific capacitance (2302 F g⁻¹ at 2 A g⁻¹), excellent rate properties and enhanced cycling performance (93.9% retention of capacity after 5000 cycles). Furthermore, an ASC device based on the PPy-coated NiCo₂O₄ nanowires as cathode and activated carbon as anode demonstrates a high energy density of 45.6 Wh kg⁻¹ at a power density of 362 W kg⁻¹.