Microsheets like nickel cobalt phosphate thin films as cathode for hybrid asymmetric solid-state supercapacitor: Influence of nickel and cobalt ratio variation

A rational combination of metal cations is necessary to obtain a synergistic effect leading to improved electrochemical energy storage performance of bimetal compounds. So, the effect of concentration variation of reactants (nickel and cobalt ratio) in nickel cobalt phosphate material and their influence on physicochemical properties and electrochemical capacitive performances are investigated. The morphological evolution from microflowers (nickel phosphate) to microsheets (cobalt phosphate) is observed with increasing cobalt content in nickel cobalt phosphate thin films. The optimum nickel and cobalt composition (∼1:1) in nickel cobalt phosphate (Ni_1.38Co_1.62(PO₄)₂·8H₂O) thin film electrode demonstrate maximum specific capacitance (capacity) of 1116 F g⁻¹ (446 C g⁻¹) at 0.5 A g⁻¹ current density. Also, a fabricated hybrid asymmetric aqueous device consists of nickel cobalt phosphate thin film as cathode and rGO as an anode in 1 M KOH electrolyte delivers a higher specific capacitance of 120 F g⁻¹ with a high energy density of 42.3 Wh kg⁻¹ at a power density of 1 kW kg⁻¹. Moreover, a hybrid asymmetric solid-state device fabricated using PVA-KOH gel electrolyte exhibits a maximum specific capacitance of 102 F g⁻¹ with an energy density of 36.2 Wh kg⁻¹ at 160 W kg⁻¹ power density and 83.7 % initial capacitance retention after 4000 GCD cycles. Such excellent electrochemical capacitive performance of hybrid asymmetric devices is attributed to the synergy between nickel and cobalt species in nickel cobalt phosphate thin film electrodes, suggesting its potential application as a cathode in hybrid energy storage devices.