Electrostructural Compatibility of Battery-Type Diffuse-Porous Co9S8–NiCo2S4/Defective Reduced Graphene Oxide and Flaky FeS/Nitrogen-Doped Defective Reduced Graphene Oxide for Ultra-High-Performance All-Solid-State Hybrid Pseudocapacitors

In order to engineer pseudocapacitor devices with high rates of energy and power delivery, and long cycle life, herein facile controlled material growth strategies are adopted to synthesize battery-type diffuse-porous Co9S8–NiCo2S4/defective reduced graphene oxide (Co9S8–NiCo2S4/D-rGO) and flaky FeS/nitrogen-doped defective reduced graphene oxide (FeS/ND-rGO) as positive and negative electrode materials, respectively. The physicochemical studies demonstrate microstructural distinctiveness in the context of permitting the bulk diffusibility of electrolyte ions, uniform heterostructurization, and added number of reactive equivalents in the electrode materials. During electrochemical studies, the Co9S8–NiCo2S4/D-rGO demonstrates thorough kinetic reversibility, enhanced rate efficiency, bias-potential-independent series resistance, charge-transfer resistance and relaxation time, and Warburg profile corresponding to minimum diffusion resistance. Similarly, FeS/ND-rGO offers good kinetic reversibility and a wide negative potential window. Further, the fabricated Co9S8–NiCo2S4/D-rGO∥FeS/ND-rGO all-solid-state hybrid pseudocapacitor device majorly shows diffusion-controlled charge storage physiognomies and lowly impeded charge transfer, operates at a wide potential window of 1.9 V, and delivers high rate specific capacitance and capacity, promising rate specific energy density at high power density, and 96.9% capacitance/capacity retention after 11 000 successive charge–discharge cycles. The enhanced pseudocapacitive charge storage efficiency of the Co9S8–NiCo2S4/D-rGO∥FeS/ND-rGO device is ascribed to the electromicrostructural compatibility of Co9S8–NiCo2S4/D-rGO and FeS/ND-rGO; nonstoichiometry induced multiple redox-active Co, Ni, and Fe ions; ion-buffering-pool-like behavior of materials’ bulk; and integrated charge transfer efficiency of D-rGO and ND-rGO. Additional electrochemical studies also reveal that the use of the solid electrolyte (PVA-KOH) offers a sufficient advantage over the liquid electrolyte (aqueous KOH) in the Co9S8–NiCo2S4/D-rGO∥FeS/ND-rGO hybrid pseudocapacitor device.