Ongoing technological advances in diverse fields including portable electronics, transportation, and green energy are often hindered by the insufficient capability of energy-storage devices By taking advantage of two different electrode materials, asymmetric supercapacitors can extend their operating voltage window beyond the thermodynamic decomposition voltage of electrolytes while enabling a solution to the energy storage limitations of symmetric supercapacitors This review provides comprehensive knowledge to this field We first look at the essential energy-storage mechanisms and performance evaluation criteria for asymmetric supercapacitors to understand the wide-ranging research conducted in this area Then we move to the recent progress made for the design and fabrication of electrode materials and the overall structure of asymmetric supercapacitors in different categories We also highlight several key scientific challenges and present our perspectives on enhancing the electrochemical performance of future asymmetric supercapacitors

Design and Mechanisms of Asymmetric Supercapacitors.

In recent years, tremendous research effort has been aimed at increasing the energy density of supercapacitors without sacrificing high power capability so that they reach the levels achieved in batteries and at lowering fabrication costs For this purpose, two important problems have to be solved: first, it is critical to develop ways to design high performance electrode materials for supercapacitors; second, it is necessary to achieve controllably assembled supercapacitor types (such as symmetric capacitors including double-layer and pseudo-capacitors, asymmetric capacitors, and Li-ion capacitors) The explosive growth of research in this field makes this review timely Recent progress in the research and development of high performance electrode materials and high-energy supercapacitors is summarized Several key issues for improving the energy densities of supercapacitors and some mutual relationships among various effecting parameters are reviewed, and challenges and perspectives in this exciting field are also discussed This provides fundamental insight into supercapacitors and offers an important guideline for future design of advanced next-generation supercapacitors for industrial and consumer applications

Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities

Fiber-based structures are highly desirable for wearable electronics that are expected to be light-weight, long-lasting, flexible, and conformable Many fibrous structures have been manufactured by well-established lost-effective textile processing technologies, normally at ambient conditions The advancement of nanotechnology has made it feasible to build electronic devices directly on the surface or inside of single fibers, which have typical thickness of several to tens microns However, imparting electronic functions to porous, highly deformable and three-dimensional fiber assemblies and maintaining them during wear represent great challenges from both views of fundamental understanding and practical implementation This article attempts to critically review the current state-of-arts with respect to materials, fabrication techniques, and structural design of devices as well as applications of the fiber-based wearable electronic products In addition, this review elaborates the performance requirements of the fiber-based wearable electronic products, especially regarding the correlation among materials, fiber/textile structures and electronic as well as mechanical functionalities of fiber-based electronic devices Finally, discussions will be presented regarding to limitations of current materials, fabrication techniques, devices concerning manufacturability and performance as well as scientific understanding that must be improved prior to their wide adoption

Fiber‐Based Wearable Electronics: A Review of Materials, Fabrication, Devices, and Applications

Heteroatom doping can endow graphene with various new or improved electromagnetic, physicochemical, optical, and structural properties. This greatly extends the arsenal of graphene materials and their potential for a spectrum of applications. Considering the latest developments, we comprehensively and critically discuss the syntheses, properties and emerging applications of the growing family of heteroatom-doped graphene materials. The advantages, disadvantages, and preferential doping features of current synthesis approaches are compared, aiming to provide clues for developing new and controllable synthetic routes. We emphasize the distinct properties resulting from various dopants, different doping levels and configurations, and synergistic effects from co-dopants, hoping to assist a better understanding of doped graphene materials. The mechanisms underlying their advantageous uses for energy storage, energy conversion, sensing, and gas storage are highlighted, aiming to stimulate more competent applications.

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Heteroatom-doped graphene materials: syntheses, properties and applications

Ionic liquids offer a unique suite of properties that make them important candidates for a number of energy related applications. Cation–anion combinations that exhibit low volatility coupled with high electrochemical and thermal stability, as well as ionic conductivity, create the possibility of designing ideal electrolytes for batteries, super-capacitors, actuators, dye sensitised solar cells and thermo-electrochemical cells. In the field of water splitting to produce hydrogen they have been used to synthesize some of the best performing water oxidation catalysts and some members of the protic ionic liquid family co-catalyse an unusual, very high energy efficiency water oxidation process. As fuel cell electrolytes, the high proton conductivity of some of the protic ionic liquid family offers the potential of fuel cells operating in the optimum temperature region above 100 °C. Beyond electrochemical applications, the low vapour pressure of these liquids, along with their ability to offer tuneable functionality, also makes them ideal as CO2 absorbents for post-combustion CO2 capture. Similarly, the tuneable phase properties of the many members of this large family of salts are also allowing the creation of phase-change thermal energy storage materials having melting points tuned to the application. This perspective article provides an overview of these developing energy related applications of ionic liquids and offers some thoughts on the emerging challenges and opportunities.

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Energy applications of ionic liquids

A novel type of flexible fiber/wearable supercapacitor that is composed of two fiber electrodes - a helical spacer wire and an electrolyte - is demonstrated. In the carbon-based fiber supercapacitor (FSC), which has high capacitance performance, commercial pen ink is directly utilized as the electrochemical material. FSCs have potential benefits in the pursuit of low-cost, large-scale, and efficient flexible/wearable energy storage systems.

Fiber Supercapacitors Utilizing Pen Ink for Flexible/Wearable Energy Storage

Nitrogen-doped graphene was demonstrated as an efficient and alternative metal-free electrocatalyst for dye-sensitized solar cells. Electrochemical measurements showed that the nitrogen-doping process can remarkably improve the catalytic activity of graphene toward triiodide reduction, lower the charge transfer resistance, and thus enhance the corresponding photovoltaic performance. Furthermore, the nitrogen doping levels ranging from 3.5 at% to 18 at%, as well as the nitrogen states (including pyrrolic, pyridinic and quaternary configurations) in graphene, were controlled to interpret the roles of graphene structure in catalytic activity and device performance. The results suggested that the nitrogen states, rather than the total N content, have a significant effect on the catalytic activity. Both pyridinic and quaternary nitrogen states can provide active sites for promoting triiodide reduction reaction, probably due to the shift in redox potential and the lowered adsorption energy.

Nitrogen-doped graphene for dye-sensitized solar cells and the role of nitrogen states in triiodide reduction

With the emergence of flexible/stretchable electronic devices, flexible supercapacitors (SCs) have attracted widespread interest in developing lightweight, thin, elastic and efficient portable/wearable energy storage devices that show promising applications in hybrid electric vehicles, uninterruptible power supplies and “smart textiles”. Along with a brief description on the general information about flexible SCs, this feature article focuses on recent advances in flexible SCs including the planar-structured flexible SCs and the new-type fiber SCs as well as fiber integrated/hybrid energy systems containing fiber SCs. The construction of efficient active materials on flexible substrates (e.g. plastics, papers, fabrics/textiles, fibrous substrates, etc.) and feasible strategies to achieve high-performance flexible SCs are highlighted. Perspectives on the research trends of flexible SCs toward practical applications are proposed.

Flexible planar/fiber-architectured supercapacitors for wearable energy storage

N,N-Dimethylformamide (DMF) has been shown to be an efficient precursor solvent for the one-step deposition of perovskite thin films in photovoltaic applications. Here, the specific advantage DMF introduces during the perovskite crystallization process is elucidated through comparison with dimethylacetamide (DMAc), one of its homologues. The unique presence of a DMF-induced intermediate phase was verified for the first time and its positive functions to inhibit uncontrolled perovskite precipitation and facilitate homogeneous nucleation were demonstrated. When combined with a double blocking layer structure to prevent shunting, our planar heterojunction (PHJ) perovskite solar cells achieved a high power conversion efficiency of up to 13.8%. Our results uncover the origin of the widespread adoption of DMF in perovskite thin film deposition, and represent a helpful step towards judicious perovskite morphological control.

Understanding the solvent-assisted crystallization mechanism inherent in efficient organic–inorganic halide perovskite solar cells

Memristors are devices that can store and process information based on their switchable internal resistance. Although these devices offer better performance than the conventional technology, the use of materials such as complex metal oxides usually requires high-temperature annealing processing or vacuum processing such as sputtering, which complicates the fabrication of the devices and hinders their development for practical use. Here we show a high-performance memristor based on organometal trihalides and electrochemical active metals, which achieved an on–off current ratio of 1.9 × 109. The devices can be solution-processed at low temperature and in air, which may be further developed into printable electronics. We explored the influence of different metal electrodes and device structures on memristor performance and the results indicated the great potential of methyl ammonium lead halide perovskite for information storage and computing. Our work provides new application prospects for these materials and may also contribute to the better understanding of other perovskite-based optoelectronic devices.

Ming Peng

Papers

Fiber Supercapacitors Utilizing Pen Ink for Flexible/Wearable Energy Storage

Nitrogen-doped graphene for dye-sensitized solar cells and the role of nitrogen states in triiodide reduction

Flexible planar/fiber-architectured supercapacitors for wearable energy storage

Understanding the solvent-assisted crystallization mechanism inherent in efficient organic–inorganic halide perovskite solar cells

High-performance perovskite memristor based on methyl ammonium lead halides