Mixed Phase Nickel‐Cobaltite for High Energy Density Asymmetric Supercapacitor using Cathodic Overpotential Synthesis Route

Abstract For flexible solid‐state electronics, it is essential to accelerate the transition to renewable materials and environmentally friendly manufacturing procedures. Symmetric supercapacitors (SCs) have a limited voltage window and capacity, hampering their use in practical applications such as portable electronics, hybrid vehicles, and transportation. To improve the energy density of supercapacitors, the strategy of using an advanced pseudo‐capacitive electrode in an asymmetric device configuration is feasible and effective. Here, we report on the design and synthesis of hierarchical honeycomb‐like morphology with controlled Ni/Co molar ratios of NiCo₂O₄ (NCO) coatings on flexible stainless‐steel substrates utilizing high cathodic overpotential deposition. The NCO electrode retains 90.5% of the three‐electrode capacitance (911 F g⁻¹, or 594 C g⁻¹) even after increasing the current density to 10 A g⁻¹ (∼10 times). Furthermore, we have fabricated a 1.5 V asymmetric supercapacitor (ASC, NCO//AC); such a device delivers maximum specific energy and specific power of 76.8 Wh kg⁻¹ and 16858 W kg⁻¹ in 2 M KOH electrolyte and excellent cycling stability (∼88% retention after 5000 cycles).