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

The broadband and tunable high-performance microwave absorption properties of an ultralight and highly compressible graphene foam (GF) are investigated. Simply via physical compression, the microwave absorption performance can be tuned. The qualified bandwidth coverage of 93.8% (60.5 GHz/64.5 GHz) is achieved for the GF under 90% compressive strain (1.0 mm thickness). This mainly because of the 3D conductive network.

Broadband and Tunable High‐Performance Microwave Absorption of an Ultralight and Highly Compressible Graphene Foam

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

ConspectusIn nanotechnology, the creation of new nanoparticles consistently feeds back into efforts to design and fabricate new macroscopic materials with specific properties. As a two-dimensional (2D) building block of new materials, graphene has received widespread attention due to its exceptional mechanical, electrical, and thermal properties. But harnessing these attributes into new materials requires developing methods to assemble single-atom-thick carbon flakes into macroscopically ordered structures. Because the melt processing of carbon materials is impossible, fluid assembly is the only viable approach for meeting this challenge. But in the meantime, researchers need to solve two fundamental problems: creating orientational ordering in fluids and the subsequent phase-transformation from ordered fluids into ordered solid materials. To address these problems, this Account highlights our graphene chemistry methods that take advantage of liquid crystals to produce graphene fibers.We have successfully...

Graphene in macroscopic order: liquid crystals and wet-spun fibers.

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

Graphene–bismuth oxide nanotube fiber as electrode material for constituting flexible supercapacitors using a PVA/H3PO4 gel electrolyte is reported with a high specific capacitance (Ca) of 69.3 mF cm−2 (for a single electrode) and 17.3 mF cm−2 (for the whole device) at 0.1 mA cm−2, respectively. Our approach opens the door to metal oxide–graphene hybrid fibers and high-performance flexible electronics.

Chao Gao

Papers

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

Graphene in macroscopic order: liquid crystals and wet-spun fibers.

Wet-Spun Continuous Graphene Films

Graphene fiber-based asymmetric micro-supercapacitors

Bismuth oxide nanotubes–graphene fiber-based flexible supercapacitors