Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

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

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

Wearable biosensors are garnering substantial interest due to their potential to provide continuous, real-time physiological information via dynamic, noninvasive measurements of biochemical markers in biofluids, such as sweat, tears, saliva and interstitial fluid. Recent developments have focused on electrochemical and optical biosensors, together with advances in the noninvasive monitoring of biomarkers including metabolites, bacteria and hormones. A combination of multiplexed biosensing, microfluidic sampling and transport systems have been integrated, miniaturized and combined with flexible materials for improved wearability and ease of operation. Although wearable biosensors hold promise, a better understanding of the correlations between analyte concentrations in the blood and noninvasive biofluids is needed to improve reliability. An expanded set of on-body bioaffinity assays and more sensing strategies are needed to make more biomarkers accessible to monitoring. Large-cohort validation studies of wearable biosensor performance will be needed to underpin clinical acceptance. Accurate and reliable real-time sensing of physiological information using wearable biosensor technologies would have a broad impact on our daily lives.

Wearable biosensors for healthcare monitoring.

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

Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors

We demonstrated the fabrication of functionalized graphene nanosheets via low temperature (300 °C) treatment of graphite oxide with a slow heating rate using Mg(OH)2 nanosheets as template. Because of its dented sheet with high surface area, a certain amount of oxygen-containing groups, and low pore volume, the as-obtained graphene delivers both ultrahigh specific gravimetric and volumetric capacitances of 456 F g(-1) and 470 F cm(-3), almost 3.7 times and 3.3 times higher than hydrazine reduced graphene, respectively. Especially, the obtained volumetric capacitance is the highest value so far reported for carbon materials in aqueous electrolytes. More importantly, the assembled supercapacitor exhibits an ultrahigh volumetric energy density of 27.2 Wh L(-1), which is among the highest values for carbon materials in aqueous electrolytes, as well as excellent cycling stability with 134% of its initial capacitance after 10,000 cycles. Therefore, the present work holds a great promise for future design and large-scale production of high performance graphene electrodes for portable energy storage devices.

Qian Wang

Papers

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

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

Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors

Template-assisted low temperature synthesis of functionalized graphene for ultrahigh volumetric performance supercapacitors.

Template-Directed Synthesis of Pillared-Porous Carbon Nanosheet Architectures: High-Performance Electrode Materials for Supercapacitors