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.