Xianzhong Sun

One-Step Electrophoretic Deposition of Reduced Graphene Oxide and Ni(OH)2 Composite Films for Controlled Syntheses Supercapacitor Electrodes

A facile, rapid, scalable, and environmentally friendly electrophoretic deposition (EPD) approach has been developed for the fabrication of reduced graphene oxide (RGO) and Ni(OH)(2) syntheses based on EPD of graphene oxide (GO) and Ni(NO(3))(2) colloidal suspension. Nickel ion decoration made GO positively charged and further made cathodic EPD feasible. Direct assembly by one-step EPD facilitated transformation from GO to RGO and resulted in multilayer or flower-like RGO/Ni(OH)(2) hybrid films on different substrates. X-ray diffraction analysis suggested that the crystal structures of Ni(OH)(2) depended on the colloidal suspension and the substrate. Further transmission electron microscopy characterization indicated that Ni(OH)(2) nanoclusters composed of 5-10 nm nanoparticles in grain size were homogeneously dispersed and anchored on the RGO. The resulting 100% binder-free RGO/Ni(OH)(2) electrodes exhibited excellent pseudocapacitive behavior with high specific capacitance of 1404 F g(-1) at 2 A g(-1), high rate capability, and good electrochemical cyclic stability. These results paved the way for EPD to produce RGO-based nanocomposite films for high-performance energy storage devices.

Recent advances in porous graphene materials for supercapacitor applications

Driven by the environmental problems and energy crisis, the development of clean and renewable energy materials as well as their devices is urgently demanded. Supercapacitors, also called electrochemical capacitors, which store energy using either ion adsorption or fast surface redox reactions, are supposed to be a promising candidate for alternative electrical energy storage devices due to their high power density, exceptional cycle life, and low maintenance cost. The performance of supercapacitors is highly dependent on the properties of electrode materials. Graphene based materials exhibit great potential application for supercapacitors because of their unique structure and excellent intrinsic physical properties. Introducing porous structures to graphene is an effective strategy to obtain high surface area and high specific capacitance. In this review, we provide a brief summary of the recent developments about the synthesis and application of porous graphene materials for supercapacitors.

High-performance supercapacitors based on a graphene–activated carbon composite prepared by chemical activation

Graphene has been widely applied as a promising supercapacitor material based on the electric double-layer mechanism. In order to solve the dispersed problem of graphene, noncovalent functionalized graphene is prepared. However, not all of these functionalized graphene materials can be employed in supercapacitors due to non-electrochemically activated molecules absorbed on graphene. Here we find a route of chemical activation with KOH to transfer noncovalent functionalized graphene to a graphene–activated carbon composite with a high specific surface area. Stable graphene colloids absorbed by oligomers of p-phenylene diamine was produced during the reduction of graphite oxide. KOH can homogeneously contact the solid graphene nanosheets after drying the colloid. Chemical activation by annealing the graphene based hybrid with KOH leads to a greatly increased specific surface area of 798 m2 g−1. The resulting graphene–activated carbon composite has a good capacitance of 122 F g−1 and energy density of 6.1 Wh kg−1 in aqueous electrolyte. The supercapacitor exhibits maximum energy densities of 52.2 and 99.2 Wh kg−1 in an ionic liquid electrolyte at room temperature and 80 °C, respectively.

Chemically Crosslinked Hydrogel Film Leads to Integrated Flexible Supercapacitors with Superior Performance

A high-strength poly(vinyl alcohol) chemical hydrogel (PCH) film is prepared by coupling covalent crosslinking with a film-casting process. Conducting polyaniline (PANI) is then embedded in the PCH film by in situ growth to form a composite film with a PANI-hydrogel-PANI configuration, which leads to a new conceptual flexible supercapacitor with all-in-one configuration that exhibits superior electrochemical performance and mechanical flexibility.

Scalable Self-Propagating High-Temperature Synthesis of Graphene for Supercapacitors with Superior Power Density and Cyclic Stability

An ultrafast self-propagating high-temperature synthesis technique offers scalable routes for the fabrication of mesoporous graphene directly from CO2 . Due to the excellent electrical conductivity and high ion-accessible surface area, supercapacitor electrodes based on the obtained graphene exhibit superior energy and power performance. The capacitance retention is higher than 90% after one million charge/discharge cycles.

Flexible solid-state supercapacitors based on a conducting polymer hydrogel with enhanced electrochemical performance

In this paper, a conducting polyaniline hydrogel instead of traditional solid electrode materials is used as an electrode material to prepare high performance flexible solid-state supercapacitors. Conducting polymer hydrogels combine the properties of hydrogel with electrical conductivity, thus offering intrinsic porous conducting frameworks and promoting the transport of charges, ions, and molecules. According to our results, the capacitance of the polyaniline hydrogel electrode is quite remarkable (430 F g−1) in this prototype, flexible solid-state supercapacitor with a two-electrode configuration. Furthermore, this supercapacitor shows excellent rate capability, cyclic stability and bendable performance. Moreover, this supercapacitor can drive a glow armlet to work very well, which demonstrates that the device has great potential to work as a power source in real-life applications.

Shape-controlled synthesis of nanocarbons through direct conversion of carbon dioxide

Morphology control of carbon-based nanomaterials (nanocarbons) is critical to practical applications because their physical and chemical properties are highly shape-dependent. The discovery of novel shaped nanocarbons stimulates new development in carbon science and technology. Based on direct reaction of CO2 with Mg metal, we achieved controlled synthesis of several different types of nanocarbons including mesoporous graphene, carbon nanotubes, and hollow carbon nanoboxes. The last one, to our knowledge, has not been previously reported to this date. The method described here allows effective control of the shape and dimensions of nanocarbons through manipulation of reaction temperature. The formation mechanism of nanocarbons is proposed. As a proof of concept, the synthesized nanocarbons are used for electrodes in symmetrical supercapacitors, which exhibit high capacitance and good cycling stability. The reported protocols are instructive to production of nanocarbons with controlled shape and dimensions which are much desirable for many practical applications.

Self-generating graphene and porous nanocarbon composites for capacitive energy storage

Herein we develop a new method in the synthesis of graphene nanosheets and porous nanocarbon composites. The method is based on the thermolysis of polymers and the reassembly of decomposed products, which are derived from all the solid materials including polyvinylidene fluoride, potassium hydroxide and graphite oxide. The resultant yields are composed of single-to-few graphene layers and micro-to-mesoporous structural nanocarbons with specific surface areas and inner pore volumes of 896–2724 m2 g−1 and 0.48–2.05 cm3 g−1, respectively. Symmetrical supercapacitors based on as-prepared electrode materials with organic and ionic liquid electrolytes show a specific capacitance of 165 and 185 F g−1, respectively, as well as high volumetric capacitance, good rate-capability, and excellent cycling stability. It is also noted that the variation of porous architectures of the carbon framework (i.e., high pore volume but similar surface area) results in different electrochemistry, suggesting the significance of porosity optimization for supercapacitor electrodes.

Large-Scale Production of Nanographene Sheets with a Controlled Mesoporous Architecture as High-Performance Electrochemical Electrode Materials

Graphene is considered as a rising-star material because of its unique properties and it is a promising material for applications in many fields. In recent years, experiments on graphene fabricated by using versatile methods have shed light on the crucial problem of aggregation and restacking, which is induced by strong π-π stacking and van der Waals forces, but preparation methods for real-world applications are still a great challenge. Here we report a facile, rapid, and environmentally friendly process, the burn-quench method, that allows large-scale and controlled synthesis of ordered mesoporous nanographene with 1-5 layers, which has a high surface area and electric conductivity. Electrodes composed of nanographene with a mesoporous architecture used both in electrochemical capacitors and lithium-ion batteries have a high specific capacitance, rate capability, energy density, and cyclic stability. Our results represent an important step toward large-scale graphene synthesis based on this new burn-quench method for applications in high-performance electrochemical energy storage devices.

A two-step method for preparing Li4Ti5O12–graphene as an anode material for lithium-ion hybrid capacitors

Lithium-ion hybrid capacitors (LICs) are expected to fill the gap between lithium-ion batteries and electrochemical supercapacitors. In this paper, we synthesize Li4Ti5O12–graphene (LTO–G) by a two-step method and use it as the anode material in AC/LTO–G Li-ion hybrid capacitors. The LTO–G composite prepared by the two-step method shows the best electrochemical properties in various ways, and delivers a high specific capacity of 194 mA h g−1 at 0.1C, and 90 mA h g−1 at 28.6C. The AC/LTO–G capacitors can perform well between 1 and 2.5 V, and they deliver a reversible capacity of 80 mA h g−1 at 0.1C. The energy density based on the total active material for these capacitors is 15 W h kg−1 at 4000 W kg−1, and 30 W h kg−1 at 1000 W kg−1. After 10 000 cycles, these capacitors still deliver an energy density higher than 22 W h kg−1 at 1000 W kg−1. The highest energy density based the total mass of the device is 6.6 W h kg−1 and there is still much room for improvement, indicating that the LTO–G composite is a promising candidate anode material for Li-ion hybrid capacitors.