An all-solid-state hybrid supercapacitor based on hierarchical CoS2/CuCo2S4 offers excellent rate capacitance, low charge-transfer resistance, high-rate energy density, and extended service stability.

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 batterytype diffuse-porous Co 9 S 8 −NiCo 2 S 4 /defective reduced graphene oxide (Co 9 S 8 −NiCo 2 S 4 /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 Co 9 S 8 −NiCo 2 S 4 /D-rGO demonstrates thorough kinetic reversibility, enhanced rate efficiency, bias-potentialindependent 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 Co 9 S 8 −NiCo 2 S 4 /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 Co 9 S 8 −NiCo 2 S 4 /D-rGO∥FeS/ND-rGO device is ascribed to the electromicrostructural compatibility of Co 9 S 8 −NiCo 2 S 4 /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 Co 9 S 8 −NiCo 2 S 4 /D-rGO∥FeS/ND-rGO hybrid pseudocapacitor device. KEYWORDS: Co 9 S 8 −NiCo 2 S 4 , defective rGO, nitrogen-doped defective rGO, all-solid-state hybrid pseudocapacitor, electromicrostructural compatibility, high-rate energy density

In order to improve the electro-microstructural physiognomics of electrode materials for applications in better efficiency supercapacitors, herein graphitic carbon nitride (GCN)heterostructurized CoS−NiCo 2 S 4 is designed using a controlled material growth synthesis procedure. The developed CoS− NiCo 2 S 4 /GCN possesses ample hydrophilicity, possible charge transfer between GCN and CoS−NiCo 2 S 4 , uniform phase distribution, and distinctive microstructural characteristics. The preliminary electrochemical studies in the three-electrode setup show GCN-induced lower charge transfer resistance and very unique Warburg profile corresponding to extremely low diffusion resistance in CoS−NiCo 2 S 4 /GCN as compared to pristine CoS− NiCo 2 S 4 . Furthermore, GCN is found to significantly induce surface-controlled (capacitive-type) charge storage and frequency-independent specific capacitance up to 10 Hz in CoS−NiCo 2 S 4 . Furthermore, the CoS−NiCo 2 S 4 ||N-rGO and CoS−NiCo 2 S 4 /GCN||N-rGO all-solid-state hybrid supercapacitor (ASSHSC) devices were fabricated using N-rGO as the negative electrode material, and the inducing effect of GCN on the supercapacitive charge storage performance of the devices is thoroughly studied. Results demonstrate that the mass specific capacitance and areal capacitance of CoS−NiCo 2 S 4 /GCN||N-rGO are ∼2 and ∼4 times more than those of the CoS−NiCo 2 S 4 ||N-rGO ASSHSC device, respectively. Furthermore, the CoS−NiCo 2 S 4 /GCN||N-rGO offers more energy density, rate energy density, and additional charge− discharge durability (over ∼10,000 cycles) than the CoS−NiCo 2 S 4 ||N-rGO ASSHSC device. The multifold performance improvement of CoS−NiCo 2 S 4 with GCN heterostructurization is ascribed to GCN-induced supplemented porosity and pore widening, ionic nonstoichiometry (Ni 2±δ , Co 2±δ , and Co 3±δ ), wettability, integrated enhancement in the conductivity, and electroactive-ion accessibility in the CoS−NiCo 2 S 4 /GCN heterocomposite. The present study offers vital physicoelectrochemical insights toward the future development of low cost and high-performance electrode materials, and their implementation in high-rate and operationally stable all-solid-state hybrid supercapacitor devices, for application in the next-generation front-line technologies.

To revolutionize the charge storage efficiency of electrode materials for their utilizations in high Ragone efficient electrochemical energy storage devices, herein, a slow-precipitation-induced material growth approach has been innovated to design a hetero oxide–sulfide [MnO2/NiS–MnS (MnO2/Ni–Mn–S)] material with smaller crystallite size, ultrathin assembled-sheet-like microstructure, and perceptible phase physiognomies (α-MnO2, MnS, and α-NiS). The electroredox assessment of MnO2/Ni–Mn–S illustrates high pseudocapacitive energy storage efficiency, significant redox reversibility, lowly constrained bulk accessibility of the OH– ions at higher rate electrochemical reaction conditions, dominance of semi-infinite diffusion-controlled electrochemical processes, and extremely low charge-transfer resistance (∼1.45 Ω), total series resistance (∼0.51 Ω) and diffusion (Warburg) resistance. A fabricated 1.8 V MnO2/Ni–Mn–S||nitrogen-doped reduced graphene oxide (N-rGO) all-solid-state hybrid supercapacitor (ASSHSC) device with N-rGO as the negative electrode material delivers high area and mass specific capacitance/capacity, ∼100% Columbic efficiency at high-rate operating conditions, and very low charge-transfer and Warburg resistance. The MnO2/Ni–Mn–S||N-rGO ASSHSC device also delivers excellent Ragone efficiency (E D = 31.5 W h kg–1 at P D = 937.5 W kg–1 and E D = 15.5 W h kg–1 at P D = 2767.5 W kg–1) and ∼97.6% retention of charge storage after 11,000 uninterrupted charge–discharge cycles. The significantly improved supercapacitive charge storage efficacy of MnO2/Ni–Mn–S is ascribed to the cohesive redox activity of Ni2+, Ni3+, Mn2+, and Mn3+ and nonstoichiometric Ni2±δ, Ni3±δ, Mn2±δ, and Mn3±δ ions, rich ion-disseminating bulk, S2– vacancy-induced electronic conductivity, and suitable electro-microstructural physiognomies for the electrochemical processes.

The development of ultra-efficient electrode materials is the contemporary necessity for the advancement in the supercapacitor device technologies. In this context, herein, sluggish nucleation and growth kinetics strategy has been...