Electrolytes have been identified as some of the most influential components in the performance of electrochemical supercapacitors (ESs), which include: electrical double-layer capacitors, pseudocapacitors and hybrid supercapacitors. This paper reviews recent progress in the research and development of ES electrolytes. The electrolytes are classified into several categories, including: aqueous, organic, ionic liquids, solid-state or quasi-solid-state, as well as redox-active electrolytes. Effects of electrolyte properties on ES performance are discussed in detail. The principles and methods of designing and optimizing electrolytes for ES performance and application are highlighted through a comprehensive analysis of the literature. Interaction among the electrolytes, electro-active materials and inactive components (current collectors, binders, and separators) is discussed. The challenges in producing high-performing electrolytes are analyzed. Several possible research directions to overcome these challenges are proposed for future efforts, with the main aim of improving ESs' energy density without sacrificing existing advantages (e.g., a high power density and a long cycle-life) (507 references).

A review of electrolyte materials and compositions for electrochemical supercapacitors

https://cyberleninka.org/article/n/768408.pdf

Mechanical properties of graphene and graphene-based nanocomposites

Flexible solid-state supercapacitors (FSSCs) are frontrunners in energy storage device technology and have attracted extensive attention owing to recent significant breakthroughs in modern wearable electronics In this study, we review the state-of-the-art advancements in FSSCs to provide new insights on mechanisms, emerging electrode materials, flexible gel electrolytes and novel cell designs The review begins with a brief introduction on the fundamental understanding of charge storage mechanisms based on the structural properties of electrode materials The next sections briefly summarise the latest progress in flexible electrodes (ie, freestanding and substrate-supported, including textile, paper, metal foil/wire and polymer-based substrates) and flexible gel electrolytes (ie, aqueous, organic, ionic liquids and redox-active gels) Subsequently, a comprehensive summary of FSSC cell designs introduces some emerging electrode materials, including MXenes, metal nitrides, metal–organic frameworks (MOFs), polyoxometalates (POMs) and black phosphorus Some potential practical applications, such as the development of piezoelectric, photo-, shape-memory, self-healing, electrochromic and integrated sensor-supercapacitors are also discussed The final section highlights current challenges and future perspectives on research in this thriving field

Towards flexible solid-state supercapacitors for smart and wearable electronics

Volumetric performance, a much more reliable and precise parameter for evaluating the charge-storage capacity of supercapacitors compared with gravimetric performance, has aroused more and more interest in recent years owing to the rapid development of miniaturized, portable and wearable electronic devices as well as electric vehicles. Various carbon materials are widely used as electrode materials in supercapacitors. However, their intrinsically low specific capacitance and relatively low bulk density lead to a relatively low volumetric performance, significantly limiting their future application. This critical review points out the crucial importance of volumetric performance and reviews recent achievements of high volumetric performances obtained through the rational design and development of novel carbon-based materials. Particular emphasis is focused on discussing the factors influencing the volumetric performance of carbon materials from a structural design point of view. We then make an in-depth summary of various promising approaches used to make significant research breakthroughs in recent years. Current challenges, future research directions and opportunities in this fascinating field of supercapacitors with high gravimetric and volumetric performances are also discussed.

Carbon materials for high volumetric performance supercapacitors: design, progress, challenges and opportunities

High-energy yarn supercapacitors are desirable for safe and wearable electronics. Here, Kou et al. use a coaxial wet-spinning assembly method to fabricate core-sheath fibres of polymer-wrapped carbon nanomaterials and demonstrate high-performance supercapacitor applications.

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Coaxial wet-spun yarn supercapacitors for high-energy density and safe wearable electronics

We report a facile strategy to synthesize silica nanoparticles-coated graphene oxide (GO–SiO2) nanohybrids in a water–alcohol mixture at room temperature. AFM observations revealed that silica nanoparticles with ca. 50 nm in size were densely and evenly covered on graphene oxide sheets. Due to the space layer of silica nanoparticles, micro-scale GO–SiO2 hybrid plates could be individually dispersed in water and polar organic solvents, promising good solution-based processibility. The growth process of GO-supported silica is traced by TGA and XRD measurements, showing that 24 hours is enough to achieve a fine cover effect for the disappearance of (002) diffraction peak of GO. Based on the high dense overlaying of silica nanoparticles, up to micro-scale silica sheets with thickness of ca. 8 nm were readily fabricated by burning GO–SiO2 at 650 °C in air. Likewise, a centimeter-scale semitransparent film of silica nanosheets was prepared by calcining a GO–SiO2 film. Interestingly, the GO–SiO2 nanohybrids exhibit excellent hydrophilic nature and can be directly applied as a general kind of building blocks to construct large-area superhydrophilic surfaces on arbitrary substrates (e.g., lotus leaf, ceramic tile and polypropylene) through the simple drop-coating method. Such a coating methodology paves the way for making large-area superhydrophilic surface without extra process treatments and damaging the intrinsic structure of substrates.

Making silica nanoparticle-covered graphene oxide nanohybrids as general building blocks for large-area superhydrophilic coatings

Macroscopic assembled, self-standing graphene and graphene oxide (GO) films have been demonstrated as promising materials in many emerging fields, such as Li ion battery electrodes, supercapacitors, heat spreaders, gas separation, and water desalination. Such films were mainly available on centimeter-scale via the time- and energy-consuming vacuum-filtration method, which seriously impedes their progress and large-scale applications. Due to the incompatibility between large-scale and ordered assembly structures, it remains a big challenge to access large-area assembled graphene thick films. Here, we report for the first time a fast wet-spinning assembly strategy to produce continuous GO and graphene thick films. A 20 m long, 5 cm wide, well-defined GO film was readily achieved at a speed of 1 m min–1. The continuous, strong GO films were easily woven into bamboo-mat-like fabrics and scrolled into highly flexible continuous fibers. The reduced graphene films with high thermal and moderate electrical conduc...

Wet-Spun Continuous Graphene Films

Fiber-based micro-supercapacitors (F-mSCs) are new members of the energy storage family, which facilitate SCs with flexibility and expand their application to fields such as tiny, flexible and wearable devices. One of the biggest challenges for F-SCs is to enhance the energy density (E) and keep the flexibility at the same time. Here, for the first time we assembled a type of fiber-based asymmetric micro-supercapacitors (F-asym-mSCs) with two different graphene fiber-based electrodes. The excellent electrochemical performances (59.2 mF cm−2 and 32.6 mF cm−2) of both electrodes offered a chance to achieve high performance two-ply F-asym-mSCs. The potential window of F-asym-mSCs was expanded to 1.6 V, and both the area energy density (EA: 11.9 μW h cm−2) and the volume energy density (EV: 11.9 mW h cm−3) are the highest E ever reported in F-SCs. The F-asym-mSCs exhibit good cycling stability with a 92.7% initial capacitance retention after 8000 cycles and can be integrated into a fiber-like device to realize the flexibility of fibers.

Graphene fiber-based asymmetric micro-supercapacitors

We have explored a new method to produce flexible and all-solid-state graphene fiber supercapacitors (GFSs) from wet-spun graphene fibers. The GFSs exhibited high capacitance (3.3 mF cm−2) and good stability (almost no changes occur after 5000 charge cycles and bending cycles). Moreover, we decorated GFSs with polyaniline nanoparticles and the resulting pseudocapacitors exhibited a capacitance of 66.6 mF cm−2.

Liang Kou

Papers

Coaxial wet-spun yarn supercapacitors for high-energy density and safe wearable electronics

Making silica nanoparticle-covered graphene oxide nanohybrids as general building blocks for large-area superhydrophilic coatings

Wet-Spun Continuous Graphene Films

Graphene fiber-based asymmetric micro-supercapacitors

Flexible high performance wet-spun graphene fiber supercapacitors