Development of amorphous Fe‐doped nickel‐cobalt phosphate (FexNiCo(PO4)2) nanostructure for enhanced performance of solid‐state asymmetric supercapacitors

Tremendous efforts have been made to create significant energy storage devices using nanoscale design and hybrid techniques. Toward this end, herein, we have fabricated, a binder-free, amorphous iron-doped nickel-cobalt phosphate (FexNiCo(PO4)2, ie, F-NCP) thin film on stainless steel substrate using a facile successive ionic layer adsorption and reaction (SILAR) method. Furthermore, the influence of Fe doping concentration on physico-chemical properties is investigated. The various F-NCP-series electrodes contain nanoparticle-like morphology that is beneficial for charge transfer and efficient diffusion of electrolytes in supercapacitors. Such nanoparticle-like morphology and the synergy among iron, cobalt, and nickel elements in the F-NCP-3 electrode deliver a maximum specific capacity of 987 C g−1 at a current density of 2.1 A g−1 with excellent cyclic retention of 95.3% after 5000 galvanostatic charge-discharge cycles. Especially, when an asymmetric solid-state supercapacitor (ASSS) is fabricated in polyvinyl alcohol-KOH gel electrolyte with reduced graphene oxide (rGO) as a negative electrode, the designed F-NCP-3//rGO ASSS device shows the wide (1.6 V) potential window, and a maximum specific capacitance of 116 F g−1 at 1.5 A g−1. In addition, the ASSS device gives a higher energy density of 41.26 Wh kg−1 at 1.22 kW kg−1 power density and exhibits superior cyclic stability (93% after 5000 cycles). The suggested asymmetric configuration makes a promising alternative of the cathode material to construct energy storage devices for various portable electronic systems.