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Rational designing of electrode materials having high surface area can accomplish the enhanced charge‐storing ability of the electrochemical energy storage devices. Therefore, the surface area of cobalt vanadium oxide (CVO) material is controlled by changing growth dynamics in successive ionic layer adsorption and reaction methods. Structural analysis confirms the formation of hydrous cobalt vanadium oxide nanoparticles (Co3V2O8⋅nH2O) thin film electrodes, and alteration in the surface area with change in growth dynamics is observed in Brunauer–Emmett–Teller analysis. The CVO1:1 thin film electrode prepared at optimal growth dynamics illustrates high specific capacitance (Cs) (capacity) of 793 F g−1 (396.7 C g−1) at 0.5 A g−1, respectively. Moreover, aqueous hybrid supercapacitor devices constructed using CVO1:1 as cathode exhibit high Cs of 133.5 F g−1 at 1.1 A g−1, specific energy (SE) of 47.7 Wh kg−1 with specific power (SP) of 0.90 kW kg−1. The solid‐state hybrid supercapacitor devices also offer high Cs of 102.9 F g−1 at 0.3 A g−1, SE of 36.6 Wh kg−1 at SP of 0.30 kW kg−1. In the SILAR approach, the dipping time plays a critical role in improving the surface area of the material and, consequently, electrochemical performance, as the current work amply indicates. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microflower to sheet evolution observed in Ni 3-x Co x (PO 4) 2 with Ni:Co ratio variation. • The optimum composition (∼1:1) of nickel and cobalt delivers highest capacitance. • HAAS device delivers 42.3 Wh kg−1 energy density at 1 kW kg−1 power density. • HASS device exhibits energy density of 36.2 Wh kg−1 at 160 W kg−1 power density. 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 1.38 Co 1.62 (PO 4) 2 ·8H 2 O) 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. [ABSTRACT FROM AUTHOR]

To achieve higher supercapacitive performance of active material, several materials with precise structures and properties have been prepared using different chemical synthesis methods. Recently, amorphous materials are gaining much attention as an electrode in supercapacitor application as it provides superior electrochemical properties due to disorder in structure. So, in this investigation, a facile, binder free successive ionic layer adsorption and reaction (SILAR) method is adopted for the preparation and deposition of amorphous, hydrous nickel phosphate thin films on stainless steel substrates. The amorphous nickel phosphate shows mesoporous, clusters of particles like morphology. In the electrochemical study, the amorphous, hydrous nickel phosphate electrode demonstrates a superior specific capacitance of 1700 F g−1 (specific capacity- 814 C g−1) at 0.5 mA cm−2 current density along with excellent capacitive retention (96.55%) and coulombic efficiency (98.62%) over 5000 cycles. Furthermore, fabricated hybrid supercapacitor device using the nickel phosphate as cathode and reduced graphene oxide as anode exhibits specific capacitance of 113.5 F g−1 at 3 mA cm−2 current density with a high 40.37 Wh kg−1 energy density at 1.689 kW kg−1 power density alongwith excellent cyclic stability (95.09% retention after 5000 cycles). The obtained results illustrate that the amorphous, hydrous nature of nickel phosphate is a beneficial and superior choice as a cathode material in high-performing hybrid asymmetric supercapacitor devices. • A facile, binder free SILAR method used for deposition of amorphous, hydrous nickel phosphate thin films on SS substrates. • The prepared electrode shows specific capacitance of (814 C g-1) 1700 F g-1 with 96.5 % cyclic stability over 5000 cycles. • AHASD device offers 113.5 F g-1 specific capacitance with 40.37 Wh kg-1 energy density at 1.689 kW kg-1 power density. [ABSTRACT FROM AUTHOR]