Zi-Yu Zhang

Design and synthesis of NiCo2O4–reduced graphene oxide composites for high performance supercapacitors

In the present work, we used charge-bearing nanosheets as building blocks to construct a binary composite composed of NiCo2O4 and reduced graphene oxide (RGO). Co–Ni hydroxides intercalated by p-aminobenzoate (PABA) ion and graphite oxide (GO) were exfoliated into positively charged hydroxide nanosheets and negatively charged graphene oxide nanosheets in water, respectively, and then these oppositely charged nanosheets were assembled to form heterostructured nanohybrids through electrostatic interactions. The subsequent thermal treatment led to the transformation of the hydroxide nanosheets into spinel NiCo2O4 and also to the reduction of graphene oxide. The as-obtained NiCo2O4–RGO composite exhibits an initial specific capacitance of 835 F g−1 at a specific current of 1 A g−1 and 615 F g−1 at 20 A g−1. More interestingly, the specific capacitance of the composite increases with cycling numbers, reaches 1050 F g−1 at 450 cycles and remains at 908 F g−1 (higher than the initial value) after 4000 cycles. The high specific capacitance, remarkable rate capability and excellent cycling ability of the composites mean that they show promise for application in supercapacitors. Comparison with the capacitive behavior of pure NiCo2O4 and NiCo2O4 mechanically mixed with RGO displays the importance of the self-assembly of the nanosheets in making a wide range of graphene-based composite materials for applications in electrochemical energy storage.

Three-dimensional graphene hydrogel supported ultrafine RuO2 nanoparticles for supercapacitor electrodes

In the present work, a three-dimensional (3D) porous framework of RuO2/reduced graphene oxide hydrogels (RuO2/RGOH) was prepared by a facile one-step hydrothermal method. In this hybrid hydrogel, RuO2 nanoparticles were homogeneously dispersed on the exfoliated RGO sheets. The as-prepared RuO2/RGOH electrode shows excellent supercapacitive performances with high specific capacitance (345 F g−1 for 15% RuO2 loading), good rate capability and a long electrochemical cycling life (without decaying after 2000 cycles). Furthermore, RuO2 in the hybrid can contribute a capacitance as high as 1365 F g−1, which is comparable to its theoretical value. These excellent results originate from the factors that the 3D porous network structure provides a more accessible surface area and facilitates an electron and proton injecting/expelling process in the electrochemical reaction. This work provides a facile method for preparing graphene-based composite materials with remarkable capacitive performances.

Preparation of a two-dimensional flexible MnO2/graphene thin film and its application in a supercapacitor

A novel two-dimensional (2D) free standing and flexible MnO2/graphene film (MGF) supercapacitor electrode is successfully fabricated by a spin-coating and hydrothermal process. The MnO2 nano-sheets are successfully aligned vertically only on one side of the graphene thin film. Raw amphiphilic graphene oxide film is helpful in effectively promoting the dispersion of well-defined MnO2 nanosheets, which can form a porous network and cover the film surface. The graphene film acts as a substrate where MnO2 nano-sheets grow in situ, and meanwhile it is used as a base current collector with a large accessible surface area and without binders for electrochemical testing. The MGF exhibits excellent electrochemical performance in a three electrode configuration, including a high specific capacitance of up to 280 F g−1 and outstanding cycle stability (no obvious decay after 10 000 cycles). In addition, the symmetric MGF supercapacitor shows a specific capacitance of up to 77 F g−1 under a cell voltage of 1.0 V. After 10 000 cycles, the capacity retention rate is 91% at a current density of 1 A g−1. At the same time, the symmetric supercapacitor also has a high energy density of 10.7 W h kg−1 at a power density of 500 W kg−1.

Design and synthesis of an organic (naphthoquinone) and inorganic (RuO2) hybrid graphene hydrogel composite for asymmetric supercapacitors

The naphthoquinone and RuO2 hybrid graphene hydrogel (NQ–RuO2/SGH) composite, which can be a suitable positive electrode material of an asymmetric supercapacitor, is prepared via a two-step process. The RuO2/graphene hydrogel composite (RuO2/SGH) is first synthesized through a simple hydrothermal method by using oxidized graphite (GO) and RuCl3 as raw materials, and then, 1,4-naphthoquinone (NQ) molecules are adsorbed on the exposed graphene hydrogel surface of the RuO2/SGH composite via a π–π stacking interaction. The electrochemical tests show that the NQ–RuO2/SGH composite delivers a high specific capacitance (450.8 F g−1) even at a low RuO2 loading mass (14.6%), meanwhile, an asymmetric supercapacitor (ASC) is constructed by using NQ–RuO2/SGH as the positive electrode and a nitrogen-doped porous carbon material (MNC) as the negative electrode. The electrochemical measurements prove that the energy density of the ASC can reach up to 16.3 W h kg−1 in a 1 M H2SO4 electrolyte.