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Graphene material has a large theoretical specific surface area, excellent theoretical conductivity and mechanical flexibility, and is a promising electrode material for supercapacitors. However, two-dimensional graphene sheets are easy to stack, which affects its electrical properties. Specifically, a sponge-like composite hydrogel for high-performance supercapacitors was prepared by onestep hydrothermal method from activated carbon and graphene oxide. Benefiting from the introduced nitrogen-containing groups and the greatly increased specific surface area, the GAC-2 nitrogen-doped activated carbon/graphene hydrogel electrode showed high specific capacitance of 505.6 F g-1. In addition, the composite hydrogel presented an excellent 3D network structure with abundant internal structural pores, in which graphene and activated carbon were cross-linked by strong covalent bonds. This unique structure greatly improves the mechanical flexibility of the composite electrode (the capacitance retention rate is approximately 87.7% after 500 bending tests). And the self-assembled flexible supercapacitor shows an energy density of 26.9 Wh kg-1 at 242 W kg-1 and outstanding capacitance retention rate of about 92.1% after 5000 charge-discharge cycles, confirming its potential application in supercapacitors. [ABSTRACT FROM AUTHOR]

Waste fiberboards were used to produce N, O co-doped carbon dots (CDs) by acid etchingmethod. The graphene hydrogels decorated with N, O co-doped CDs (GCD) were synthesized via hydrothermalmethod and optimized through differentmass ratios. The optimal composite electrode exhibited a high specific capacitance of 335.1 F g-1 at 1 A g-1, and a good mechanical flexibility with about 90.6% capacitance retention after 500 bending/unbending cycles. Furthermore, the assembled flexible supercapacitor owned an ideal cycling stability of 83.4% after 10000 charge/discharge cycles at 5 A g-1. Such results demonstrated that N, O co-doped CDs played an essential and unique role in composite graphene hydrogel for improving the electrochemical performance of the flexible supercapacitor. [ABSTRACT FROM AUTHOR]

In this paper, litchi-like microspheres structures of NiCo2Se4 @carbon microspheres (NCSC) hybrids were successfully synthesized via a simple two-step hydrothermal method. The effects of the content of NiCo2Se44 on the structure of NCSC composites are investigated. The morphology and nanostructure of the as-obtained NCSC hybrids were examined by XRD, XPS, FESEM and TEM. The results indicate that the carbon microspheres (CM) were uniformly covered with NiCo2Se44 and with the increase of NiCo2Se44 content, the superficies of NCSC also exhibits homogeneous rough nanoparticles and regular spheres connect with each other. Meanwhile, the electrochemical tests indicate a suitable mass ratio of NiCo2Se44 and carbon microspheres (2:3) showed the highest specific capacitance value (1394 F g-1 at the current density of 0.5 A g-1, 766 F g-1 even at the current density of 50 A g-1), excellent rate property and outstanding cycling stability (80.093% of the initial value after 10000 cycles at the current density of 5 A g-1). Benefiting from the above integrated advantages, asymmetric supercapacitors (ASC) fabricated with NCSC composites as the positive electrode and porous activated carbon as negative electrode operating at the voltage window of 0-2 V deliver a high energy density of 101 Wh kg-1 at a power density of 0.749 kW kg-1, with the specific capacitance retention of 81.095% after 10000 cycles at the current density of 10 A g-1, superior to most of reported supercapacitors. [ABSTRACT FROM AUTHOR]

A simple and template-free method for fabrication of a modified porous graphene hydrogel electrode was developed. The method involved adding natural organic phytate in the pure graphene hydrogels. Benefiting from the introduced phosphate groups and positively charged metal ions, the flexible magnesium phytate/graphene (Mg-P/G) hydrogel electrode showed large specific capacitance of 281 F/g with a small charge transfer resistance of 0.3 Ω. Furthermore, the flexible supercapacitor showed prominent cycling stability (about 100% retention in capacitance over 10000 cycles) and excellent mechanical flexibility (about 97.5% of its original capacitance after 500 bending cycles). [ABSTRACT FROM AUTHOR]