Hongying Wu

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.

Growth of 3D SnO2 nanosheets on carbon cloth as a binder-free electrode for supercapacitors

Three-dimensional (3D) lamellar SnO2 is grown on a carbon cloth (CC) substrate (denoted as 3D lamellar SnO2/CC) through hydrothermal reactions and subsequent thermal treatments. The resulting 3D lamellar SnO2/CC can be directly used as an electrode in supercapacitors without the necessity for addition of either binder or conductive species, and achieves a specific capacitance as high as 247 F g−1 at a current density of 1 A g−1 within a potential window ranging from −0.6 to 0.3 V because of the unique porous structure accessible to electrolyte ions. In order to match the capacitive behaviors of 3D lamellar SnO2/CC in the two-electrode systems, reduced graphene oxide/carbon cloth (rGO/CC) is prepared by starting from GO. The rGO/CC and 3D lamellar SnO2/CC are respectively used as positive and negative electrodes to assemble an asymmetric supercapacitor. The device exhibits not only an excellent cycle stability (76.9% after 10 000 cycles at 3 A g−1), but also high energy density of 22.8 W h kg−1 at a power density of 850 W kg−1 under a cell voltage of 1.7 V. Moreover, the as-fabricated supercapacitor has green and environmentally friendly features because an aqueous neutral electrolyte is employed in it.

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.

Electrodeposition of honeycomb-shaped NiCo2O4 on carbon cloth as binder-free electrode for asymmetric electrochemical capacitor with high energy density

Combining high-capacitive metal oxides and excellent conductive carbon substrates is a very significant strategy to achieve high-performance electrodes for electrochemical capacitors (ECs). Herein, the bimetallic (Ni, Co) hydroxide is uniformly grown on the electro-etched carbon cloth (CC) by a facile co-electrodeposition method, and then the honeycomb-shaped NiCo2O4/CC (HSNC) composite is formed by transforming the hydroxide precursor into its bimetallic oxides through the subsequent thermal treatment. The special structure of the HSNC as binder-free electrode is responsible for its excellent electrochemical performance with carbon-like power feature. The experimental results show that HSNC electrode exhibits a high specific capacitance with remarkable cycle stability (94.3% after 10 000 cycles at 10 A g−1) in the three-electrode configuration. To evaluate further the capacitive performance of the as-prepared binder-free electrode in a full cell set-up, an asymmetric electrochemical capacitor (AEC) is assembled by using the HSNC as the positive electrode and reduced graphene oxide/carbon cloth (rGO/CC) as the negative electrode in KOH electrolyte. The as-assembled device presents an energy density as high as 32.4 W h kg−1 along with power density of 0.75 kW kg−1, comparing with nickel-metal hyoride battery (Ni-MH) batteries (30.0 W h kg−1 at 0.35 kW kg−1). Even at the power density of 37.7 kW kg−1 (50-time increase, a full charge–discharge within 3.5 s), energy density still holds at 17.8 W h kg−1, indicating an outstanding rate capability. Furthermore, the as-fabricated device exhibits a long cycle lifetime (76.5% after 10 000 cycles at 3 A g−1) with a cell voltage of 1.5 V.

Organic multi-electron redox couple-induced functionalization for enabling ultrahigh rate and cycling performances of supercapacitors

In the present work, the danthron molecule (1,8-dihydroxyanthraquinone, DT) with multi-electron redox centers as a novel organic electrochemically active material for supercapacitors has been decorated on reduced graphene oxide nanosheets (RGNs) via a facile one-step reflux method. The resultant danthron functionalized RGNs (DT–RGNs) composite electrode material not only provided a fast and reversible 4e−/4H+ redox reaction because of two types of redox-active organic functional groups (carbonyl and hydroxyl) in DT, but also preserved the unique electrode architecture with the required conductivity of the graphene nanosheets. In the three-electrode system, the optimized electrode (DT–RGNs 3 : 5) exhibited an excellent capacitance of 491 F g−1 at 1 A g−1 which is three times higher than that of bare RGNs. Most importantly, the DT–RGNs electrode showed an ultrahigh rate capability of 80.8% capacitance retention at 100 A g−1 and a superior electrochemical stability of 98.8% after 10 000 cycles at 10 A g−1, outstripping a great amount of reported organic and inorganic electrodes. Meanwhile, the effect of intramolecular and/or intermolecular hydrogen bonds between carbonyl and hydroxyl on the electrochemical properties of the DT–RGNs electrode was investigated. Finally, the novel symmetric supercapacitor (DT–RGNs SSC) was assembled to evaluate the actual energy storage properties of electrode materials.

Dissected carbon nanotubes functionalized by 1-hydroxyanthraquinone for high-performance asymmetric supercapacitors

In the present paper, 1-hydroxyanthraquinone (HAQ) has been adsorbed onto dissected carbon nanotubes (rDCNTs) with reduced graphene oxide layers through noncovalent interaction. As a result, we realized the functionalization of rDCNTs, which means multi-electron electrochemical active groups have been transplanted to the carbon-based materials to further improve the pseudocapacitance. The surface area of dissected carbon nanotubes is increased by several times compared to MWCNTs by an oxidative unzipping process while the conductive backbones of MWCNTs are preserved. The special structure and electrical conductivity of the composites guarantee an outstanding super-capacitive performance for the as-prepared material. In the three-electrode configuration, the HAQ-functionalized rDCNTs (HAQ-rDCNTs) electrode exhibits a higher specific capacitance value (as high as 324 F g−1 at 1 A g−1, two times higher than bare DCNTs) and an ultrahigh rate capability (77.7% capacitance retention at 50 A g−1) in aqueous electrolyte solutions. For further practical application, a novel asymmetric supercapacitor (ASC) has been assembled by using DCNTs as the positive electrode and HAQ-rDCNTs as the negative electrode in a H2SO4 electrolyte. As the result, the device shows an excellent energy storage performance. At a voltage of 1.4 V, the as-fabricated ASC exhibits a high energy density of 12.3 W h kg−1 at a power density of 700 W kg−1.