Planar and rigid conventional electronics are intrinsically incompatible with curvilinear and deformable devices. The recent development of organic and inorganic flexible and stretchable electronics enables the production of various applications, such as soft robots, flexible displays, wearable electronics, electronic skins, bendable phones, and implantable medical devices. To power these devices, persistent efforts have thus been expended to develop a flexible energy storage system that can be ideally deformed while maintaining its electrochemical performance. In this review, the enabling technologies of the electrochemical and mechanical performances of flexible devices are summarized. The investigations demonstrate the improvement of electrochemical performance via the adoption of new materials and alternative reactions. Moreover, the strategies used to develop novel materials and distinct design configurations are introduced in the following sections.
The spread of wearable and flexible electronics devices has been accelerating in recent years for a wide range of applications. Development of an appropriate flexible power source to operate these flexible devices is a key challenge. Supercapacitors are attractive for powering portable lightweight consumer devices due to their long cycle stability, fast charge-discharge cycle, outstanding power density, wide operating temperatures and safety. Much effort has been devoted to ensure high mechanical and electrochemical stability upon bending, folding or stretching and to develop flexible electrodes, substrates and overall geometrically-flexible structures. Supercapacitors have attracted considerable attention and shown many applications on various scales. In this review, we focus on flexible structural design under six categories: paper-like, textile-like, wire-like, origami, biomimetics based design and micro-supercapacitors. Finally, we present our perspective of flexible supercapacitors and emphasize current technical difficulties to stimulate further research.
Ordered mesoporous carbons (OMCs) possess great advantages, such as large surface area, uniform pore distribution, high porosity, and physical and chemical stability. However, the monotonic and long porous channels in OMCs hinder their further application, especially in energy storage. Here, we synthesized mesoporous carbon hollow spheres (MCHSs) with a "Dualtemplating method" using dandelion-like silica spheres (DSSs) as the template.Through the dual-templating method, the MCHSs directly replicated the mesoporous edge of DSSs as a thick mesoporous shell but substituted the clogged central core with hollow-core. The combined structure with mesoporous carbon spheres and hollow-core has various advantages over conventional OMCs.The radial and hierarchical pores in the sphere provide a large surface area (1319 m 2 g À1 ), short diffusion path, and open-pore that facilitates ion transfer to any direction. Simultaneously, the hollow sphere carved in the center of the MCHSs allows space for the improvement of ion mobility and electrolyte retention. Also, the dense structure of the MCHSs allows more compact packing and high tap density when MCHSs were applied as an electrode. The MCHSs exhibit high specific capacitance and present a well-developed EDLC shape at all scan rates (10 to 1000 mV s À1 ), the results show a superior electrochemical performance compared with other recent mesoporous carbon allotropes.
Coffee is one of the largest agricultural products; however, the majority of the produced coffee is discarded as waste sludge by beverage manufacturers. Herein, we report the use of graphitic porous carbon materials that have been derived from waste coffee sludge for developing an energy storage electrode based on a hydrothermal recycling procedure. Waste coffee sludge is used as a carbonaceous precursor for energy storage due to its greater abundance, lower cost, and easier availability as compared to other carbon resources. The intrinsic fibrous structure of coffee sludge is based on cellulose and demonstrates enhanced ionic and electronic conductivities. The material is primarily composed of cellulose-based materials along with several heteroatoms; therefore, the waste sludge can be easily converted to functionalized carbon. The production of unique graphitic porous carbon by hydrothermal carbonization of coffee sludge is particularly attractive since it addresses waste handling issues, offers a cheaper recycling method, and reduces the requirement for landfills. Our investigations revealed that the graphitic porous carbon electrodes derived from coffee sludge provide a specific capacitance of 140 F g−1, with 97% retention of the charge storage capacity after 1500 cycles at current density of 0.3 A g−1.
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