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Although nanostructured NiCo 2 O 4 is the most appealing and studied electrode material by far as it is cheap, nontoxic, and efficient for supercapacitor application, it still lacks suitable device application comprising a wide voltage window and high operating current density, high specific energy and power, and absolute stability. Here, we report a successful application of our NiCo 2 O 4 nanoparticles (NPs) in a highly efficient electrochemical supercapacitor device, which is brought to reality because of the NPs' fast ion intercalation/extraction and fast reversible Faradaic surface reactions. The as-synthesized spinel NiCo 2 O 4 NPs are 65 nm in average diameter, have 59 m 2 /g of surface area, and 0.44 cm 3 /g of pore volume. Charge storage capability and reliability of the material as an active electrode have been demonstrated in terms of a conventional three-electrode method and in a coin-cell device. Specific capacity values of 150.56 to 41.66 mAh•g −1 (1084 to 300 Fg −1 in terms of capacitance) have been realized in the applied potential window (0.0−0.5 and 0.0−1.5 V) and in current densities of 2 to 10 Ag −1 , which are >98% stable in any of these given conditions up to 20 000 measured charge−discharge cycles. The NiCo 2 O 4 NP electrode exhibits a maximum energy density of 10.42 Wh kg −1 and a power density of 3750 Wkg −1 at 10 Ag −1 and makes itself an efficient electrode material for superior supercapacitor devices. These high performances are corroborated to the ultrafast intercalation reaction of electrolyte ions with nickel cobaltate at the electrode surface.

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Addition of redox additives in electrolytes to enhance the electrochemical activity at electrode–electrolyte interfaces is one of the prime approaches these days to develop high-energy-density supercapacitors. Here, we report an investigation on a quasi-solid-state supercapacitor, fabricated with a nonaqueous, gel polymer electrolyte (GPE) based on a mixture of a plastic crystal succinonitrile and an ionic liquid 1-butyl-1-methylpyrrolidinium bis­(trifluoromethyl sulfonyl) imide, added with a redox additive hydroquinone (HQ), immobilized in a polymer poly­(vinylidine fluoride-co-hexafluoropropylene). The HQ-incorporated GPE is observed to be a freestanding, easily processible, and reusable film, showing excellent flexibility and thermal stability up to ∼100 °C. The high ionic conductivity (∼4.2 mS cm–1) and wide electrochemical stability window (∼5.0 V) through linear sweep voltammetry measurements make the optimum composition of GPE a potential electrolyte for high-energy-density supercapacitors. The symmetric supercapacitor coin cells have been fabricated with peanut shell-derived porous carbon electrodes separated by GPE films. The electrochemical activity due to the presence of HQ at carbon electrode–GPE interfaces introduces additional pseudocapacitance over the double-layer capacitance, leading to enhanced overall specific capacitance (289 F g–1), and hence corresponds to the high specific energy (∼40 Wh kg–1) and maximum power (∼20 kW kg–1). The capacitor cell shows prolonged cyclic profile up to ∼10 000 charge–discharge cycles with ca. 85–93% Coulombic efficiency.

Redox‐active liquid or polymer‐based aqueous electrolytes are widely reported in carbon‐based supercapacitors, enhancing their performance substantially due to the involvement of fast redox reaction(s) at the electrode–electrolyte interface. Here, a nonaqueous, ionic liquid (IL)‐based redox‐active gel polymer electrolyte (GPE) as a potential supercapacitor electrolyte is demonstrated. This electrolyte, comprising IL 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide added with redox agent NaI, immobilized in poly(vinylidenefluoride‐co‐hexafluoropropylene), offers excellent thermal, mechanical, and electrochemical stability with high ionic conductivity of ≈3.81 × 10−3 S cm−1 at room temperature. The redox‐additive NaI not only increases the ionic conductivity of electrolyte but also introduces redox behavior at the interfaces of porous activated carbon supercapacitor resulting in high specific capacitance (≈334 F g−1) and high specific energy and power (≈26.1 and ≈18 kw kg−1, respectively). The capacitor with redox‐active nonaqueous GPE offers stable performance up to 10 000 charge–discharge cycles with ≈5% initial fading only. This study opens a novel approach to prepare highly stable redox‐active GPEs for high‐performance supercapacitors.

Sodium ion batteries (NIBs) have gained remarkable attention as a potential alternative of lithium ion batteries for upcoming large-scale applications ranging from small electronic equipment to electric vehicles. In recent...

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