Energy harvesting from NiCo₂S₄/Co_<i>x</i>S_<i>y</i> nanoflakes: a two-fold strategy by morphology control and using redox-additive electrolytes

Designing materials with an appropriate (nano) architecture could prove to be an effective strategy to improve several electrochemical properties to yield high-energy storage devices for their practical applications. Here, a two-fold approach of choosing an appropriate synthesis method (pulsed electrodeposition) and a redox-additive electrolyte has been adopted for this purpose. A customized pulsed electrodeposition technique, used for obtaining macroporous NiCo₂S₄/Co_<i>x</i>S_<i>y</i> nanoflakes, has been designed, which shows improved energy storage (as pseudocapacitors) application. Specific capacitance, when used in electrode form, has been further enhanced by choosing an optimized electrolyte consisting of 1 M KOH and 0.05 M K₃[FeCN₆], where the latter has been used as a redox-additive electrolyte. The multifunctional electrode shows a high capacitance of ∼7000 mF cm^-2. The addition of the redox-additive material to the electrolyte increases the diffusion coefficient (4-fold), which in turn increases the energy storage properties of the material. The improved electrochemical properties have been utilized in designing a prototype solid-state symmetric supercapacitor device, which shows a capacitance value of 380 F g^-1 with a high power density and energy density. Additionally, the device exhibits an exceptionally high capacitance retention of 100% after 2000 continuous switchings at high current density. The device performance under real-life on-field conditions has also been demonstrated. This work significantly shows that surface morphology modification by adopting a custom pulsed electrodeposition technique and redox-additive electrolyte optimization is an effective strategy to enhance the electrochemical energy storage performance.