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 science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Metal oxide nanostructures are promising electrode materials for lithium-ion batteries and supercapacitors because of their high specific capacity/capacitance, typically 2-3 times higher than that of the carbon/graphite-based materials. However, their cycling stability and rate performance still can not meet the requirements of practical applications. It is therefore urgent to improve their overall device performance, which depends on not only the development of advanced electrode materials but also in a large part "how to design superior electrode architectures". In the article, we will review recent advances in strategies for advanced metal oxide-based hybrid nanostructure design, with the focus on the binder-free film/array electrodes. These binder-free electrodes, with the integration of unique merits of each component, can provide larger electrochemically active surface area, faster electron transport and superior ion diffusion, thus leading to substantially improved cycling and rate performance. Several recently emerged concepts of using ordered nanostructure arrays, synergetic core-shell structures, nanostructured current collectors, and flexible paper/textile electrodes will be highlighted, pointing out advantages and challenges where appropriate. Some future electrode design trends and directions are also discussed.

Recent Advances in Metal Oxide-based Electrode Architecture Design for Electrochemical Energy Storage

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin-mountable, and wearable strain sensors are needed for several potential applications including personalized health-monitoring, human motion detection, human-machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin-mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.

Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review

Electrochemical supercapacitors can deliver high levels of electrical power and offer long operating lifetimes, but their energy storage density is too low for many important applications. Pseudocapacitive transition-metal oxides such as MnO(2) could be used to make electrodes in such supercapacitors, because they are predicted to have a high capacitance for storing electrical charge while also being inexpensive and not harmful to the environment. However, the poor conductivity of MnO(2) (10(-5)-10(-6) S cm(-1)) limits the charge/discharge rate for high-power applications. Here, we show that hybrid structures made of nanoporous gold and nanocrystalline MnO(2) have enhanced conductivity, resulting in a specific capacitance of the constituent MnO(2) (~1,145 F g(-1)) that is close to the theoretical value. The nanoporous gold allows electron transport through the MnO(2), and facilitates fast ion diffusion between the MnO(2) and the electrolytes while also acting as a double-layer capacitor. The high specific capacitances and charge/discharge rates offered by such hybrid structures make them promising candidates as electrodes in supercapacitors, combining high-energy storage densities with high levels of power delivery.

Nanoporous metal/oxide hybrid electrodes for electrochemical supercapacitors

In recent years, much effort have been dedicated to achieve thin, lightweight and even flexible energy-storage devices for wearable electronics. Here we demonstrate a novel kind of ultrathin all-solid-state supercapacitor configuration with an extremely simple process using two slightly separated polyaniline-based electrodes well solidified in the H(2)SO(4)-polyvinyl alcohol gel electrolyte. The thickness of the entire device is much comparable to that of a piece of commercial standard A4 print paper. Under its highly flexible (twisting) state, the integrate device shows a high specific capacitance of 350 F/g for the electrode materials, well cycle stability after 1000 cycles and a leakage current of as small as 17.2 μA. Furthermore, due to its polymer-based component structure, it has a specific capacitance of as high as 31.4 F/g for the entire device, which is more than 6 times that of current high-level commercial supercapacitor products. These highly flexible and all-solid-state paperlike polymer supercapacitors may bring new design opportunities of device configuration for energy-storage devices in the future wearable electronic area.

Highly Flexible and All-Solid-State Paperlike Polymer Supercapacitors

Thermoelectric materials are of great interest for applications as heat pumps and power generators, which can realize conversion between thermal and electrical energy without moving mechanical components or hazardous working fluids. Especially since the combustion of fossil fuel has caused very alarming environmental problems, the conversion of waste heat to electric power by means of thermoelectric devices has become more urgent. The performance of thermoelectric materials is quantified by a figure of merit, given by ZT1⁄4 SsT/k, where S, s, T, and k are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. To have a high ZT value at room temperature, high S, high s, and low k are required. Various attempts have been made to enhance the efficiency of thermoelectric materials. However, there is a strong correlation of these three parameters according to theWiedemann–Franz law, which makes it a very challenging task. Therefore, very slow improvement was achieved in the past, until some promising approaches were reported recently, which involve quantum-well structures, crystals with complex electronic structures, thin and multilayer films and so-called phonon-glass/electroncrystal compound materials. Synthesizing composites was considered an effective strategy to achieve improved material performances by combining the advantages of each component. Synergistic enhancement effects have been found in some research fields, such as dielectric permittivity and thermal conductivity. However, for thermoelectric materials, it was believed impossible to enhance thermoelectric properties through composites, because early theoretical numerical simulations indicated that the Seebeck coefficient and the figure of merit of the composites could not be higher than the maximum of one of its components. Recently, when we tried to use carbon nanotubes (CNTs) to improve the transport properties of polyaniline (PANI, which is a typical kind of conductive polymer) through composites, we found that the thermoelectric performance of the composites could be remarkably enhanced compared with both of their bulk parent samples, which is not consistent with the early theoretical conclusions about composites. Morphology characterization showed that the PANI component uniformly coated the surface of each individual CNTand of each CNT bundle. It is a composite with a low-dimensional network structure. Here, we explore the reason for these observations and surmise that this method may be extended to be a facile and general strategy to synthesize nanocomposites with enhanced thermoelectric properties. The CNT/PANI nanocomposites used in the study were fabricated by a simple two-step method, that is, the formation of a thick CNT network and the polymerization of the PANI component. First, a freestanding CNTnetwork made of randomly entangled individual CNTs and CNT bundles was obtained by filtering a uniform CNT suspension through a microporous membrane with the aid of vacuum. Second, the in situ chemical polymerization approach was used to synthesize a PANI layer uniformly coated on the prepared CNT network. Details of the process are given in the Experimental section. Macroscopically, the pristine CNT sheet surface looks very smooth and shiny. From scanning electron microscopy (SEM) observations, it was apparent that individual CNTs and their bundles randomly intertwined together to form a good CNT network (Fig. 1A). The original CNTdiameters were in the range of about 15–30 nm. After PANI polymerization (0.2 M aniline), the diameters increased to about 60–200 nm. Figure 1B indicates that PANI formed a wholly uniform coating layer on the surface of the CNTs. It is notable that the framework of the network was retained after the PANI coating process in the liquid phase. This is indeed a well-formed randomly distributed nanostructural system. Macroscopically, its surface became a little rough, however, it still remained flexible (inset of Fig. 1B). It can be rolled up, bent, or twisted easily, and even folded without cracking. This mechanical nature is superior to that of conventional fragile thermoelectric films. The transmission electron microscopy (TEM) images in Figure 1C and 1D further demonstrate the nanostructure of individual CNTs and CNT bundles coated with PANI, where some PANI coating was destroyed through intensive ultrasonication during the specimen preparation. Clear interfaces (indicated by white arrows) can be seen around the outer walls of the CNTs. The thickness of the PANI coating layer is about 50–90 nm. These observations provide strong evidence that the PANI chains grow on the outer walls of the CNTs. The formation mechanism can be understood briefly as follows. In themixed solution, aniline hydrochlorides are oxidized by ammonium peroxidisulfate (APS) to form insoluble oligomers and polymers of polyaniline. The p-bonded surface of the CNTs interacts strongly with the conjugated structure of polyaniline, which facilitates the deposition of polymer chains at the surface of the CNTs, forming a tubular coating layer. Here the individual CNTs and the CNT network serve as the core and the template, respectively.

A Promising Approach to Enhanced Thermoelectric Properties Using Carbon Nanotube Networks

The rapid development of miniaturized electronic devices has led to a growing need for rechargeable micropower sources with high performance. Among different sources, electrochemical microcapacitors or microsupercapacitors provide higher power density than their counterparts and are gaining increased interest from the research and engineering communities. To date, little work has appeared on the integration of microsupercapacitors onto a chip or flexible substrates. This review provides an overview of research on microsupercapacitors, with particular emphasis on state-of-the-art graphene-based electrodes and solid-state devices on both flexible and rigid substrates. The advantages, disadvantages, and performance of graphene-based microsupercapacitors are summarized and new trends in materials, fabrication and packaging are identified.

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A Review of Graphene‐Based Electrochemical Microsupercapacitors

Flexible carbon nanotube/polyaniline paper-like films and their enhanced electrochemical properties

In this work, we show that embedding super-aligned carbon nanotube sheets into a polymer matrix (polydimethylsiloxane) can remarkably reduce the coefficient of thermal expansion of the polymer matrix by two orders of magnitude. Based on this unique phenomenon, we fabricated a new kind of bending actuator through a two-step method. The actuator is easily operable and can generate an exceptionally large bending actuation with controllable motion at very low driving DC voltages (<700 V/m). Furthermore, the actuator can be operated without electrolytes in the air, which is superior to conventional carbon nanotube actuators. Proposed electrothermal mechanism was discussed and confirmed by our experimental results. The exceptional bending actuation performance together with easy fabrication, low-voltage, and controllable motion demonstrates the potential ability of using this kind of actuator in various applicable areas, such as artificial muscles, microrobotics, microsensors, microtransducers, micromanipulatio...

Chuizhou Meng

Papers

Highly Flexible and All-Solid-State Paperlike Polymer Supercapacitors

A Promising Approach to Enhanced Thermoelectric Properties Using Carbon Nanotube Networks

A Review of Graphene‐Based Electrochemical Microsupercapacitors

Flexible carbon nanotube/polyaniline paper-like films and their enhanced electrochemical properties

High-performance, low-voltage, and easy-operable bending actuator based on aligned carbon nanotube/polymer composites.