Hierarchically Uniform 2D Porous Sheet-like NiO/NiCo₂O₄: A High-Performance Positrode Material for All-Solid-State Hybrid Pseudocapacitors with Superior Ragone and Cyclic Efficiencies

As electrochemical energy storage devices with superior Ragone and cyclic efficiencies are vital for numerous modern-day technologies, herein, a kinetically controlled crystal growth strategy has been innovated to design a very uniform 2D porous sheet-like NiO/NiCo₂O₄ positrode material of very high surface area (235 m² g^–1) for the fabrication of a high-performance hybrid pseudocapacitor of all-solid-state architecture. The electrochemical investigation of NiO/NiCo₂O₄ in a three-electrode setup demonstrates the physiognomies such as rich redox reversibility, >95% diffusion-controlled charge storage, high rate charge storage efficiency, negligible iR drop, insignificant charge transfer, and series resistance of 0.45 and 1.21 Ω, respectively, and typical Warburg response indicative of facilitated diffusion of the electrolyte ions during the charge storage process, which illustrates the material’s archetypal suitability as an excellent positrode material for application in high-performance pseudocapacitors. The fabricated NiO/NiCo₂O₄||N-rGO all-solid-state hybrid pseudocapacitor (ASSHPC) device with PVA-KOH and N-rGO as the solid-gel separator electrolyte and negatrode material, respectively, demonstrates the physiognomies such as hybrid (semi-infinite diffusion and surface controlled) charge storage, imperceptible iR drop even under high applied current density conditions, superior rate specific capacity/capacitance, extremely low charge transfer, and series resistance of 0.3 and 1.0 Ω, respectively, very low relaxation time constant of ∼0.69 s, excellent Ragone efficiency (energy densities of 38 and 25 Wh kg^–1 at power densities of 2346 and 10,976 W kg^–1, respectively), and 98.4% retention in area specific capacity over 12,500 charge–discharge cycles. It is found that the multiple oxidation states of the Ni and Co ions in NiO/NiCo₂O₄ and the archetypical bulk porosity in its microstructure offer physicoelectrochemical compatibility with N-rGO, which facilitates lowly resisted electrolyte ion diffusion, increased redox-active sites, and shortened diffusion path length of the electroactive ions during the charge storage process in the high-performance ASSHPC device. The optimized approach in the study will boost the advancement of positrode material systems to develop extremely Ragone and cyclic efficient pseudocapacitor devices for integrations in prospective electronic architectures.