Redox Additive-Improved Electrochemically and Structurally Robust Binder-Free Nickel Pyrophosphate Nanorods as Superior Cathode for Hybrid Supercapacitors

For several decades, one of the great challenges for constructing a high-energy supercapacitor has been designing electrode materials with high performance. Herein, we report for the first time to our knowledge a novel hybrid supercapacitor composed of battery-type nickel pyrophosphate one-dimensional (1D) nanorods and capacitive-type N-doped reduced graphene oxide as the cathode and anode, respectively, in an aqueous redox-added electrolyte. More importantly, ex situ microscopic images of the nickel pyrophosphate 1D nanorods revealed that the presence of the battery-type redox additive enhanced the charge storage capacity and cycling life as a result of the microstructure stability. The nickel pyrophosphate 1D nanorods exhibited their maximum specific capacitance (8120 mF cm<sup>–2</sup> at 5 mV s<sup>–1</sup>) and energy density (0.22 mWh cm<sup>–2</sup> at a power density of 1.375 mW cm<sup>–2</sup>) in 1 M KOH + 75 mg K<sub>3</sub>[Fe­(CN)<sub>6</sub>] electrolyte. On the other side, the N-doped reduced graphene oxide delivered an excellent electrochemical performance, demonstrating that it was an appropriate anode. A hybrid supercapacitor showed a high specific capacitance (224 F g<sup>–1</sup> at a current density of 1 A g<sup>–1</sup>) and high energy density (70 Wh kg<sup>–1</sup> at a power density of 750 W kg<sup>–1</sup>), as well as a long cycle life (a Coulombic efficiency of 96% over 5000 cycles), which was a higher performance than most of those in recent reports. Our results suggested that the materials and redox additive in this novel design hold great promise for potential applications in a next-generation hybrid supercapacitor.