Engineering graphene for high-performance supercapacitors: Enabling role of colloidal chemistry

The high electrical conductivity and high specific surface area of graphene are traditionally regarded as the most intriguing features for its promise as the electrode material for supercapacitors. In this perspective, we highlight that from the engineering point of view, the unique colloidal chemistry of chemically functionalized graphene is the key property that has made graphene stand out as a promising nanoscale building block for constructing unique nanoporous electrodes for capacitive energy storage, We present several examples to demonstrate bow the non-covalent colloidal forces between graphene sheets can be harnessed to engineer the nanostructure of graphene-based bulk electrodes for supercapacitors based on both the electrical double layer storage and the redox reaction or pseudo-capacitance mechanisms. The colloidal engineering strategy can be extended to enable other nanomaterials to achieve high energy storage performance.

Microwave-hydrothermal preparation of a graphene/hierarchy structure MnO_2 composite for a supercapacitor

Graphene/hierarchy structure manganese dioxide （GN/MnO2） composites were synthesized using a simple microwave-hydrothermal method. The properties of the prepared composites were analyzed using field emission scanning electron microscopy （FE-SEM）, X-ray diffraction （XRD）, transmission electron microscopy （TEM）, and X-ray photoelectron spectroscopy （XPS） measurements. The electrochemical performances of the composites were analyzed using cyclic voltammetry, electrochemical impedance spectrometry （EIS）, and chronopotentiometry. The results showed that GN/MnO2 （10 wt% graphene） displayed a specific capacitance of 244 F/g at a current density of 100 mA/g. An excellent cyclic stability was obtained with a capacity retention of approximately 94.3% after 500 cycles in a 1 mol/L Li2SO4 solution. The improved electrochemical performance is attributed to the hierarchy structure of the manganese dioxide, which can enlarge the interface between the active materials and the electrolyte. The prepa- ration route provides a new approach for hierarchy structure graphene composites; this work could be readily extended to the preparation of other graphene-based composites with different structures for use in energy storage devices.

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）.

Mesoporous tubular graphene electrode for high performance supercapacitor

We report the fabrication of mesoporous tubular graphene（MTG） by a chemical vapor deposition method using Mg O@Zn O core-shell structure as the template. The unique bi-directional ions transfer in unstack graphene layers and high mesopore ratio of MTGs allows capacitance reach 15 μF/cm^2 at 0.5 A/g, and11 μF/cm^2 at 10 A/g, which is closer to theoretical value（21μF/cm^2） than SWCNTs and DWCNTs at either low or high rate. Meanwhile, MTGs exhibited good structural stability, high surface area（701 m^2/g）, high conductivity（30 S/cm） and low oxygen ratio（0.7 atom%）, allowing excellent SC performance. The 4 V EDLC using MTGs and EMIMBF_4 electrolyte exhibited high energy density in wide range of high power density and excellent cycling stability, showing strong potential in EDLC and other electrochemical energy storage systems, in addition, showing significant factor of ion transfer distance for high performance SCs especially operating at high voltage using ionic liquid electrolyte.

Urchin-like FeOOH hollow microspheres decorated with MnO_2 for enhanced supercapacitor performance

Ultrathin MnO_2 decorated hierarchical urchinlike FeOOH hollow micro-nanospheres have been designed and synthesized through a facile hydrothermal route.The microspheres are made of FeOOH nanofibers with a diameter of 10 nm.Due to the synergetic effect between the unique FeOOH hollow micro/nanostructures and ultrathin MnO_2 layer,the as-fabricated FeOOH@MnO_2 hybrid electrode exhibits a high specific capacitance of 1192 F g^（-1）at a current density of 1 A g^（-1）.It also reveals high rate capabilities and superior stability.Moreover,the asymmetric supercapacitor（ASC）assembled from the FeOOH@MnO_2 and the active carbon（AC）delivers a high energy density of 40.2 W h kg^（-1）at a power density of 0.78 kW kg^（-1）,and the energy density could remain 10.4 W h kg^（-1）under a condition of high power density of 11.7 kW kg^（-1）.

Flexible asymmetric supercapacitor based on MnO_2 honeycomb structure

A flexible asymmetric supercapacitor with high energy density was constructed 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. By controlling the electroplating time, the MnO2 nanosheets can be self-assembled to honeycomb structure and showed excellent electrochemical performance in 1 mol/L Na2SO4 electrolyte with SC950-EP30 performing the best. It exhibited a high specific capacitance（1110.85 F/g at a current density of 1 A/g based on the mass of MnO2） and superior rate capability（77.44% capacity retention from 1 A/g to 10 A/g）.Thus, the optimal asymmetric device assembled with this material as positive electrode can deliver a maximum energy density of 43.84 Wh/kg and a maximum power density of 6.62 kW/kg.