MXene-based materials for electrochemical energy storage

Rechargeable batteries and supercapacitors are widely investigated as the most important electrochemical energy storage devices nowadays due to the booming energy demand for electric vehicles and hand-held electronics. The large surface-area-to-volume ratio and internal surface areas endow two-dimensional（2D） materials with high mobility and high energy density; therefore, 2D materials are very promising candidates for Li ion batteries and supercapacitors with comprehensive investigations. In 2011, a new kind of 2D transition metal carbides, nitrides and carbonitrides, MXene, were successfully obtained from MAX phases. Since then about 20 different kinds of MXene have been prepared. Other precursors besides MAX phases and even other methods such as chemical vapor deposition（CVD） were also applied to prepare MXene, opening new doors for the preparation of new MXene. Their 2D nature and good electronic properties ensure the inherent advantages as electrode materials for electrochemical energy storage. In this review, we summarize the recent progress in the development of MXene with emphasis on the applications to electrochemical energy storage. Also, future perspective and challenges of MXene-based materials are briefly discussed regrading electrochemical energy storage.

Recent advances of graphene-based materials for high-performance and new-concept supercapacitors

Supercapacitors, with ultrahigh power density, superior rate capability, long-term cyclability, and exceptional safety, are regarded as one highly competitive candidate of electrochemical energy storage devices,filling the gap between batteries and conventional capacitors. Despite of tremendous effort, elaborated screening of high-performance electrode materials, e.g., graphene, is still intensively required. In this review, we describe the most recent progress in the research and development of graphene-based materials for high-performance and new-concept supercapacitors for the targeted applications in next-generation and smart electronics. First, the design and fabrication of high-performance supercapacitors, including electrical double layer capacitors, pseudocapacitors and hybrid supercapacitors, were summarized in term of the charge storage mechanism. Second, new-concept supercapacitors with multiple functionalities of high-voltage, fiber-shape, microscale and shape-diversity in order to fulfill the requirements of future electronics are reviewed. Accordingly, special emphasis is given to the structure-dependent-performance effects of pores, hybridization, dimensionalities of graphene-based materials on performance of supercapacitors, and tremendous potential of graphene-based planar micro-supercapacitors for the direct seamlessly integration with versatile micro-electronics. Finally, perspectives and challenges of graphene-based supercapacitors are briefly discussed.

Graphene fiber based supercapacitors:Strategies and perspective toward high performances

Modern wearable electronics are thirsty for flexible, lightweight energy storage and supply devices. Flexible fiber-shaped supercapacitors, possess good flexibility, high power density, fast charging capability and long cycle life, becoming a promising option for wearable devices. The past decade has witnessed the emergence of graphene fiber based supercapacitors（GFSCs） as one of the most active vicinity in fiber-supercapactiors, for their excellent properties including high surface area, chemical stability, excellent electrical conductivity, lightweight and mechanical properties. In this perspective, we introduced the basic energy storage mechanisms of GFSCs, followed by the analysis in improving their overall performances, recent advances, and a conclusive discussion on the challenges and opportunities.

Flexible Strip Supercapacitors for Future Energy Storage

Flexible strip supercapacitors are developed and their electrochemical properties are characterized. Activated carbon is used as the electrode material and it is found to have a good porous structure which provides a large surface area for energy storage.Furthermore, this activated carbon performs well. The manufacturing processes for the supercapacitors are described in detail and the preparation process has good reproducibility. The strip supercapacitors are combined in series and parallel to measure their electrical properties. The performances of these two samples in series or in parallel both follow the theoretical models. The electrochemical potential window of a series circuit of these two strip supercapacitors is 4.8 V. The energy and power of the series or parallel circuits are equal to the sums of the two strip supercapacitors.

Natural nanomaterial as hard template for scalable synthesizing holey carbon naonsheet/nanotube with in-plane and out-of-plane pores for electrochemical energy storage

Tuning porous structure of carbon nanomaterials has been found to be important for their performance enhancement in electrochemical energy storage applications. In this work we employed a natural nanomaterial kaolinite, which is abundant and cheap, as hard template to synthesis porous carbon nanomaterial. By tuning the structure of hard template kaolinite, we have achieved a template directed formation of holey carbon nanosheet/nanotube materials. This carbon nanomaterials with hierarchical in-plane and out-of-plane pores have shown electrochemical energy storage capacity of 286 F/g（equal to 314 F/cm^3） at 0.1 A/g and 85 F/g（equal to 93 F/cm^3） at 100 A/g, which is comparable to variety of reported carbon based electrochemical energy storage electrode materials.

Hollow Co_9S_8 from metal organic framework supported on rGO as electrode material for highly stable supercapacitors

Metal sulfides as a feasible candidate with high specific capacitance for supercapacitors suffer from sluggish ion/electron transport kinetics and rapid capacitance fading. Herein, we demonstrate a method to fabricate a composite of reduced graphene oxide（rGO） with hollow Co9S8 derived from metal organic framework（MOF）. Due to the combined highly conductive rGO substrates and hollow shell, the prepared r GO/Co9S8 composite exhibits a high specific capacitance of 575.9 F/g at 2 A/g and 92.0% capacitance retention after 9000 cycles. Its excellent electrochemical performance provides great promise for application, and this versatile method can be extended to prepare other similar nanocomposite.

Facile synthesis of nitrogen-doped graphene aerogels functionalized with chitosan for supercapacitors with excellent electrochemical performance

Three-dimensional porous nitrogen-doped graphene aerogels（NGAs） were synthesized by using graphene oxide（GO） and chitosan via a self-assembly process by a rapid method.The morphology and structure of the as-prepared aerogels were characterized.The results showed that NGAs possesed the hierarchical pores with the wide size distribution ranging from mesopores to macropores.The NGAs carbonized at different temperature all showed excellent electrochemical performance in 6 mol/L KOH electrolyte and the electrochemical performance of the NGA-900 was the best.When working as a supercapacitor electrode,NGA-900 exhibited a high specific capacitance（244.4 F/g at a current density of 0.2 A/g）,superior rate capability（51.0% capacity retention） and excellent cycling life（96.2% capacitance retained after 5000 cycles）.

Vertical crosslinking MoS_2/three-dimensional graphene composite towards high performance supercapacitor

Molybdenum disulfide（MoS2） has been stimulated in extensive researches due to its layered structure and the potential as an electrochemical energy material. However, the effects on electrochemical performance of concentration of MoS2 are rarely mentioned. In this paper, the effects of different concentrated layered MoS2 on the morphology and electrochemical properties of the composite of MoS2 and three-dimensional graphene（MoS2/3DG） were discussed. The results show that layered MoS2 was successfully compounded to 3DG and formed a vertical crosslinking structure. It can be observed that MoS2 nanosheets are vertically loaded on the inner and outer surface of graphee when the concentration of MoS2 is 0.20 mg/L. The specific capacitance of composite（MoS2（0.20 mg/L）/3 DG）reaches 2182.33 mF/cm^2 at the current density of 1 mA/cm^2, and the specific capacitance remains 116.83% after 5000 cycles. When the current density increased 100 times（from 1 mA/cm^2 to 100 mA/cm^2）, the specific capacitance retains 78.9%. Meanwhile, the hybrid energy storage devises can deliver an energy density of 130.34 Wh/m^2. The superior electrochemical properties are attributed to the synergistic effect of MoS2 and 3DG. Therefore, the material has a potential application on supercapacitor electrode material.