English

• Due to outstanding storage capacity, ultra-high power density, super long cycle life time, moderate energy density and high safety, supercapacitors have enthusiastically become a new type of energy storage device and energy storage technology after lithium ion battery. In the recent decade, scientists and engineers around the world have extensively and thoroughly studied physics, chemistry, materials, chemical engineering, electronics, transportation engineering, energy science and technology, aeronautical and astronautics science and technology, which promoted the rapid application of supercapacitors in electric automobile, engineering machinery, industrial energy saving, wind power and many other fields. This monograph focuses on three reviews and fifteen original research papers, highlighting main research progress in micro-supercapacitors (MSCs), graphene-based supercapacitors, flexible supercapacitors, hybrid supercapacitors, covering the new energy storage principle, novel electrode materials, neoteric electrolyte, and late-model device technology.

  The boom development of wearable and portable electronics has intensively stimulated the demand of microscale energy storage devices with multiple compatible features of lightweight, tailored size, outstanding flexibility, and high energy density. MSCs, as a newly-developed class of microscale electrochemical energy storage devices, have gained considerable attentions. Wu et al. (https://doi.org/10.1016/j.cclet.2017.08.007) develolped a simplified mask-assisted fabrication of all-solid-state MSCs with high areal capacitance based on graphene and MnO₂ nanosheets. Lai et al. (https://doi.org/10.1016/j.cclet.2018.01.007) reported a paper-based flexible all-solid-state asymmetric MSCs fabricated by pencil drawing methodology. Yang et al. (https://doi.org/10.1016/j.cclet.2018.01.024) explored the laser processed MSCs based on carbon nanotubes and MnO₂ nanosheets composite and the fabricated devices showed excellent electrochemical performance and aesthetic property. Similarly, Yuan et al. (https://doi.org/10.1016/j.cclet.2018.01.012) used the similar strategy of lasertreated polymer derivatives to construct all-solid-state pseudocapacitive MSCs based on MnO₂ from the reduction of KMnO₄. With the characteristics of high capacities, environmentally friendly and low cost, metal oxides are widely applied as active materials of MSCs. Shen et al. (https://doi.org/10.1016/j.cclet.2017.12.007) summarized the recent progress of metal oxides based on-chip MSCs with various approaches for the synthesis of metal oxides nanostructures and developments on the fabrication of MSCs. Wang et al. (https://doi.org/10.1016/j.cclet.2017.12.019) reviewed several kind of novel and unconventional multifunctional integrated supercapacitors and outlined the enormous progress on multifunctional integrated supercapacitors.

  Compared with activated carbon, graphene is the ideal supercapacitor electrode owing to its high specific surface area, excellent electron conductivity and thermal conductivity, high mechanical strength, high energy density and high power density of the electrode material requirements of supercapacitors. Qian et al. (https://doi.org/10.1016/j.cclet.2018.01.027) reported a method for fabricatinga mesoporoustubular grapheneelectrode thatexhibited high energy density in wide range of high power density and excellent cycling stability in an ionic liquid electrolyte EMIMBF₄ electrolyte. To achieve superior pseudocapacitive lithium storage, Wang et al. (https://doi.org/10.1016/j.cclet.2017.09.063) designed and prepared a "soft" graphene oxide-organopolysulfide nanocomposites. Moreover, three-dimensional graphene was used by Wang and Yang et al. (https://doi.org/10.1016/j.cclet.2018.01.017) as a support to prepare vertical crosslinking MoS₂/three-dimensional graphene composited with superior and stable electrochemical capacitive performance. Wu et al. (https://doi.org/10.1016/j.cclet.2018.01.051) demonstrated a method to fabricate a composite of reduced graphene oxide with hollow Co₉S₈ derived from metal organic framework for highly stable supercapacitors.

  The emerging flexible supercapacitors, with higher energy density than conventional physical capacitors, higher charging/discharging rate capability, and longer life-cycles than primary/secondary batteries, have become one of the most intense research focuses in the electrical energy storage field. Huang et al. (https://doi.org/10.1016/j.cclet.2017.12.028) constructed a flexible asymmetric supercapacitor with high energy density by using a flexible substrate of carbonized silk-fabrics decorated with carbon nanotube, electroplating MnO₂ nanosheets and dip-coating activated carbon powders as the positive and the negative electrodes, respectively. What's more, Liu et al. (https://doi.org/10.1016/j.cclet.2018.01.013) outlined recent progress towards the development of flexible supercapacitors based on macroscopic carbon nanotubes-based electrodes, including one dimensional (1D) fibers, 2D films, and 3D foams, with a focus on electrode preparation and configuration design as well as their integration with other multifunctional devices.

  Hybrid capacitors, also known as asymmetric electrochemical capacitors, can better satisfy the application requirements for energy storage devices of high energy density and high power density, having become the inevitable choice for the development of excellent specific energy supercapacitors. Shi et al. (https://doi. org/10.1016/j.cclet.2018.01.031) reported a high-performance lithium-ion capacitor constructed from a mesoporous carbon as positive electrode and a hard carbon as negative electrode. Zhang et al. (https://doi.org/10.1016/j.cclet.2018.01.029) opened up a novel cathode that boron and nitrogen dual-doped carbon for high performance hybrid ion capacitors. Liu et al. (https://doi.org/10.1016/j.cclet.2018.01.011) designed a free-standing battery-type electrode of bismuth oxide nanoflake@carbon film for aqueous sodium ion hybrid supercapacitors.

  In addition, Wu et al. (https://doi.org/10.1016/j.cclet.2017.11.024) developed powdery carbon aerogel with an ideal hierarchical pore structure showing impressive capacitive performances when utilized as electrodes for organic electrolyte supercapacitor. Jiang et al. (https://doi.org/10.1016/j.cclet.2017.11.035) systematically studied and compared the temperature stability of activated carbon supercapacitors assembled with liquid aqueous electrolytes by using in situ electrodeposited. Lu et al. (https://doi.org/10.1016/j. cclet.2017.10.030) achieved a template directed synthesis of holey carbon nanosheet/nanotube material by tuning the structure of hard template kaolinite, which showed promising electrochemical energy storage capacity.

  Although we only introduced several typical applications of supercapacitors here, energy density is a key factor restricting the development of other applications of supercapacitors under the premise of guaranteeing long-term cycling stability, high-rate performance, and power density. Thereby, designing supercapacitors configuration with high energy density to achieve the excellent electrochemical capacitance performance is the major issue and challenge. We hope that this special issue brings readers a timely overview on the latest breakthroughs in the related area.

  Finally, we would like to express our sincere thanks to all the authors, referees and Editorial Board. We always welcome any comment, suggestion, and feedback.

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