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Supercapacitors are candidates with the greatest potential for use in sustainable energy resources. Extensive research is being carried out to improve the performances of state-of-art supercapacitors to meet our increased energy demands because of huge technological innovations in various fields. The development of high-performing materials for supercapacitor components such as electrodes, electrolytes, current collectors, and separators is inevitable. To boost research in materials design and production toward supercapacitors, the up-to-date collection of recent advancements is necessary for the benefit of active researchers. This review summarizes the most recent developments of water-in-salt (WIS) and deep eutectic solvents (DES), which are considered significant electrolyte systems to advance the energy density of supercapacitors, with a focus on two-dimensional layered nanomaterials. It provides a comprehensive survey of 2D materials (graphene, MXenes, and transition-metal oxides/dichalcogenides/sulfides) employed in supercapacitors using WIS/DES electrolytes. The synthesis and characterization of various 2D materials along with their electrochemical performances in WIS and DES electrolyte systems are described. In addition, the challenges and opportunities for the next-generation supercapacitor devices are summarily discussed. [ABSTRACT FROM AUTHOR]

A novel ternary composite composed of ammonium nickel molybdate, nickel sulfide and reduced graphene oxide (ANM‐NiS‐rGO) has been synthesized by using a two‐step hydrothermal method for the first time. The morphological analysis reveals that the rGO nanosheets in the ternary composite are decorated with the electrochemically active NiS nanoparticles and ANM microflowers. Because the ternary composite exhibits a porous and 3D flower‐like structures, it possesses the large surface area of 135 m2 g−1 and high pore volume of 0.258 cm3 g−1. On the other hand, ANM‐NiS‐rGO was investigated as positive electrode for battery‐type hybrid supercapacitor that shows a specific capacity of 150 mAh g−1 at a current density of 1 A g−1. For comparison, the corresponding components such as ANM‐rGO and NiS‐rGO were also investigated. It was found that the ternary composite shows a higher specific capacity, better rate capability and cyclic stability compared with the binary composites as the former possesses higher surface area and electrical conductivity. To demonstrate the practical applications, a hybrid supercapacitor was fabricated by using ANM‐NiS‐rGO and rGO as the positive and negative electrodes, respectively. This device can be operated in the voltage range of 0–1.75 V and shows a high specific capacity of 28 mAh g−1 at a current density of 2 Ag−1. Furthermore, the device delivers a maximum energy density of 24.2 Wh kg−1 at a power density of 1.75 kW kg−1. These results suggest that the porous and hierarchically structured ANM‐NiS‐rGO has high potential in practical energy storage applications. [ABSTRACT FROM AUTHOR]

Graphical abstract The composites of carbon nanotubes and graphene oxide with carbonized polyaniline have been synthesized and investigated their electrochemical performance as supercapacitors. The free-standing and binder-free electrode materials showed an improved specific capacitance with better rate capability and cyclic stability. Highlights • Free-standing and binder-free electrode material of carbonized PANI over CNTs and GO. • The carbonized PANI formed coral-like nanostructures over the surface of CNTs. • The hierarchically structured composite exhibited excellent electrochemical performance. Abstract Composite materials with carbon nanotubes (CNTs) and graphene oxide (GO) are considered to be superior for energy storage applications due to their intriguing structural and electrical properties. The electrochemical performance of CNT/GO-based electrodes can be improved by introducing porous carbon materials into them. Herein, we have carbonized the composite consisted of polyaniline (PANI), GO and CNTs on stainless steel plates and investigated their electrochemical performances as a free-standing and binder-free electrode for supercapacitors. The carbonized PANI with a coral-like nanostructure was formed over CNT and GO surface. Furthermore, the introduction of GO efficiently prevents the excessive agglomeration of PANI in the resultant composite materials. The carbonized composite electrode exhibited the high specific capacitance of 465 mF cm−2 at a current density of 1 mA cm−2. Moreover, the carbon electrode obtained by annealing at a higher temperature of 550 °C showed the excellent rate capability and cyclic stability. For example, the 78% of initial capacitance retention was observed when the current density changed from 1 mA cm−2 to 8 mA cm−2, and the capacitance retention of 84% was achieved after 1000 charge/ discharge cycles. This 3D hierarchically structured carbon electrode can be considered as a potential candidate for next-generation energy storage devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • Composite hydrogel of perylenetetracarboxylic diimide and reduced graphene oxide is prepared. • Electrolyte of ammonium acetate with a record concentration of 32 molality (m) is used. • Battery-supercapacitor hybrid device well operates at 1.9 V from –20 to 50 °C. • It delivers an energy density of 12.9 W h kg−1 at a power density of 827 W kg−1. • The device retains 74 % of the initial capacity after 3000 cycles at 1 A g−1. Developing battery-supercapacitor hybrid devices (BSHs) is viewed as an efficient route to shorten the gap between supercapacitors and batteries. In this study, a composite hydrogel consisting of perylene tetracarboxylic diimide (PTCDI) and reduced graphene oxide (rGO) is tested as the anode for BSHs in the electrolyte of ammonium acetate (NH 4 Ac) with a record concentration of 32 molality (m). This water-in-salt electrolyte exhibits a wide electrochemical stability window of 2.13 V and high conductivity of 23.3 mS cm−1 even at −12 °C. Molecular dynamics calculations and spectroscopic measurements reveal that a favorable water-acetate interaction occurs in a high concentration NH 4 Ac electrolyte. On the other hand, the study of electrode kinetics in 32 m NH 4 Ac demonstrates a high capacitive contribution to charge storage in PTCDI-rGO although an electrode redox reaction involves reversible enolization of carbonyl groups in PTCDI. This result suggests fast NH 4 +-ion intercalation kinetics in charge–discharge processes. Furthermore, the electrode performance is improved by optimizing the loading amount of rGO in composites. The best-performing composite electrode delivers the maximum capacity of 165 mAh g−1 at 0.5 A g−1 and sustains a great capacity retention of 66% at 8 A g−1. Finally, an all-organic BSH device is tested in a broad temperature window from −20 to 50 °C and is well operated at 1.9 V regardless of operating temperatures. Due to the synergetic effect of splendid electrolyte properties and high anode capacities, BSH devices possess the maximum energy density of 12.9 Wh kg−1 at the power density of 827 W kg−1 and retain 74 % of the initial capacity after 3000 cycles at 1 A g−1. Our study paves a novel route towards designing inexpensive and environmentally friendly BSH devices with high performances. [ABSTRACT FROM AUTHOR]

CNT@NiSe/SS serves as a free-standing electrode for both supercapacitors and overall water splitting. The HSC exhibits high energy density of 32.1 Wh kg−1 at a power density of 823 W kg−1. CNT@NiSe/SS shows excellent HER and OER activities with the lowest overpotential of 174 mV at 10 mA cm−2 and 267 mV at 50 mA cm−2, respectively. The electrolyzer requires 1.71 V to achieve 10 mA cm−2 and shows a negligible increment in potential after 24 h of continuous operation. • CNT@NiSe/SS serves as the free-standing electrode for both supercapacitors and overall water splitting. • The HSC exhibits high energy density of 32.1 Wh kg−1 at the power density of 823 W kg−1. • CNT@NiSe/SS shows the high HER and OER activities with η 10 = 174 mV and η 50 = 267 mV. • The electrolyzer requires 1.71 V to achieve 10 mA cm−2 and shows the negligible increment in potential after 24 h. NiSe nanoparticles are electrodeposited over a forest of carbon nanotubes (CNTs) to form an intertwined and porous network. The assynthesized composite (denoted as CNT@NiSe/SS) is used as a free-standing and multifunctional electrode for both supercapacitors and overall water splitting applications. For a supercapacitor application, CNT@NiSe/SS exhibits higher specific capacity and improved rate capability compared with individual NiSe and CNTs. A hybrid supercapacitor device consisting of battery-like CNT@NiSe/SS and EDLC-like graphene delivers a maximum energy density of 32.1 Wh kg−1 at a power density of 823 W kg−1 and has excellent stability after a floating test of 50 h. On the other hand, CNT@NiSe/SS also serves as a bifunctional electrocatalyst with high activity for overall water splitting. The CNT@NiSe/SS electrode displays excellent hydrogen and oxygen evolution reaction performance with the lowest overpotential of 174 mV at 10 mA cm−2 and 267 mV at 50 mA cm−2, respectively. The symmetrical two-electrode system requires an operating potential of 1.71 V to achieve a current density of 10 mA cm−2. Furthermore, this electrolyzer shows a negligible increment in potential after 24 h of continuous water splitting. The outstanding performances of CNT@NiSe/SS can be attributed to the synergistic effect of NiSe and CNTs. [ABSTRACT FROM AUTHOR]

A high-performing electrolyte plays an imperative role in the pursuit of high energy density, green, low-cost, and safe supercapacitors. To this end, a novel deep eutectic solvent (DES) composed of N-methylacetamide and lithium perchlorate has been engineered by the addition of co-solvents including water and N,N -dimethylformamide (DMF). Interestingly, each co-solvent makes a specific contribution to DES performance as water for flame resistance and DMF for antifreeze. The optimized hybrid DES is tested as the electrolyte for symmetric supercapacitors with polyaniline/reduced graphene oxide composite electrodes (PANI-rGO). The hybrid DES with high ionic conductivity imparts a high operation voltage of 2.2 V to devices whereas the PANI-rGO electrode with the pseudocapacitive behavior provides the high capacitances. Due to the above synergistic effects, our device delivers the maximum energy density of 28.2 W h kg−1 at a power density of 1.1 kW kg−1 and shows moderate cyclic stability at room temperature. More importantly, the supercapacitors can well function in a low temperature range from −20 to 25 °C. The device operated at 25 °C achieves the maximum capacitance of 41.9 F g−1 due to improved ionic conductivity of the DES electrolyte. The current study opens a new route towards sustainable and affordable energy storage devices. [Display omitted] • DES electrolyte with N,N -dimethylformamide as co-solvent shows 2.2V window. • PANI-rGO exhibits pseudocapacitive behavior in the hybrid DES electrolyte. • Fabricated symmetric supercapacitor has potential to operate from −20 to 70 °C. • It delivers an energy density of 28.2 W h kg−1 at a power density of 1.1 kW kg−1 [ABSTRACT FROM AUTHOR]

Aqueous all-organic battery-supercapacitor hybrid (BSH) devices are emerging as promising alternative energy storage systems to conventional metal-ion batteries/supercapacitors in terms of their easy processability, high tunability, cost-effectiveness, and recyclability. In this study, we demonstrate an all-organic BSH device using a record concentration of ammonium acetate (30 m NH 4 Ac) as a cheap and green electrolyte. This novel water-in-salt system possesses an electrochemical stability window up to 2.1 V. In addition, organic polymers comprising of perylenetetracarboxy diimide core and the mixed aromatic amine groups are employed as negative and positive electrodes, respectively. The polymers show low water solubility, good crystallinity, and nano-morphology, which make them decent candidates for the all-organic aqueous BSH device. More importantly, electrochemical kinetic analysis of polymer electrodes reveals that the negative and positive electrodes display pseudocapacitive and battery-type behaviors, respectively. For the first time, an all-organic BSH device with 30 m NH 4 Ac is fabricated, which delivers a specific capacity of 42 mA h g−1 at 200 mA g−1 and capacity retention of ca. 63% after 5000 cycles. It further shows the maximum energy density of 16.5 Wh kg−1 and power density of 719 Wh kg−1, depicting great potential in energy storage applications. [Display omitted] • All-organic battery-supercapacitor hybrid device with 1.9 V has been demonstrated • High concentrated aqueous ammonium acetate of 30 m is used as a cheap electrolyte • The hybrid device exhibited the specific capacity of 42 mA h g−1 at 200 mA g−1 • After 2000 cycles of GCD, the hybrid device possessed 60% of capacity retention [ABSTRACT FROM AUTHOR]