Zhongai Hu

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.

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.

Graphene hydrogels non-covalently functionalized with alizarin: an ideal electrode material for symmetric supercapacitors

In the present work, the anthraquinone derivative alizarin (AZ) with a multi-electron redox center as the functionalizing molecule has been immobilized onto three-dimensional (3D) self-assembled graphene hydrogels (SGHs) through a non-covalent functionalization strategy. The excellent electrical conductivity and interconnected macroporous framework of SGHs facilitate unconstrained electrolyte ion diffusion and electron transportation. Moreover, the surface confined redox reactions and fast kinetic feature of AZ molecules result in an outstanding electrochemical capacitive performance. In the three-electrode system, the AZ-functionalized SGHs (AZ–SGHs) electrodes exhibit a larger specific capacitance (as high as 350 F g−1 at 1 A g−1, two times higher than that of bare SGHs) and ultrahigh rate capability (61% capacitance retention at 200 A g−1) in aqueous electrolyte solutions. More importantly, when the resultant AZ–SGHs electrodes are integrated into a symmetric supercapacitor (SSC), the electrode material shows a good self-synergy and potential self-matching behavior due to two pairs of redox peaks with mirror symmetry. As a result, the AZ–SGHs SSC exhibits an excellent energy storage performance. In a voltage range from 0 to 1.4 V, a maximum energy density of 18.2 W h kg−1 is achieved at a power density of 700 W kg−1.

Non-covalently functionalizing a graphene framework by anthraquinone for high-rate electrochemical energy storage

Anthraquinone (AQ) molecules with electrochemically reversible redox couples (anthraquinone/anthracenol) have been selected to functionalize a graphene framework (GF) through non-covalent modification. The π–π stacking interactions between components induce a favorable molecular orientation so that the aromatic ring of AQ is parallel to the sp2 network of GF. In this case, the fast Faradaic reactions between anthraquinone and anthracenol generate additional pseudocapacitance for enhancing the supercapacitive performance of GF. In the three-electrode configuration, AQ-functionalized GF (AQ/GF) shows a high capacitance value (396 F g−1 at 1 A g−1, two times higher than bare GF), ultrahigh rate capability (64% capacitance retention at 100 A g−1) and long cycle life (97% retention after 2000 cycles). For further practical application, a novel asymmetric supercapacitor with high energy and power densities has been assembled by using AQ/GF as negative electrode and GF as positive electrode in H2SO4 aqueous electrolyte. Maximum energy (13.2 Wh kg−1) and power (9175.3 W kg−1) densities have been obtained for the GF//AQ/GF device.

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.

Green and all-carbon asymmetric supercapacitor based on polyaniline nanotubes and anthraquinone functionalized porous nitrogen-doped carbon nanotubes with high energy storage performance

Aqueous electrolyte-based asymmetric supercapacitors (ASCs) are one of the hot topics in the field of energy storage due to their high ionic conductivity, their environmental friendliness and lower cost. However, most research work on ASCs has involved non-renewable metal oxides (or hydroxides). Herein, an all-carbon and high-energy asymmetric supercapacitor (ASC) is constructed using polyaniline nanotubes (PNTs) as the positive electrode and anthraquinone-functionalized porous nitrogen-doped carbon nanotubes (AQ@PNCNTs) as the negative electrode. The PNTs are prepared by a facile chemical self-assembly method, and further carbonization/activation of the PNT precursor results in the formation of the porous nitrogen-doped carbon nanotubes (PNCNTs). Under solvothermal conditions, PNCNTs serve as a conductive substrate to adsorb anthraquinone (AQ) molecules, which can contribute additional electrochemical capacitance to the overall capacitance of the electrode. The as-assembled AQ@PNCNTs//PNTs ASC exhibits excellent supercapacitive performances in 1 M H2SO4 aqueous electrolyte. In particular, the device can deliver an energy density as high as 32.7 W h kg−1 at a power density of 700 W kg−1. Even at the power density of 14.0 kW kg−1, the energy density still remains at 20.2 W h kg−1. This strategy provides a feasible way to construct green supercapacitors with high power density and energy density.

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.

Nitrogen-doped heterostructure carbon functionalized by electroactive organic molecules for asymmetric supercapacitors with high energy density

Chemical oxidation is employed to lengthwise unzip and transverse cut multi-walled carbon nanotubes (MWCNTs) to form heterostructure carbon nanotubes (HCNTs) that are residual tubes with randomly distributed graphene layers on the tube wall. Then, we coat polyaniline nanoparticles on HCNTs through in situ polymerization, in which the HCNTs are served as core and polyaniline is regarded as shell. The resultant core–shell structure is converted to a nitrogen-doped heterostructure carbon (NHC) through pyrolysis by following alkali activation. Subsequently, the NHC is used as conductive substrate to adsorb tetrachlorobenzoquinone (TCBQ) and anthraquinone (AQ) molecules via π–π stacking interaction to get the functionalized nitrogen-doped heterostructure carbon (TCBQ–NHC and AQ–NHC), respectively. As a result, multielectron reactions in positive and negative potential ranges are implanted in two electrodes, respectively. Electrochemical measurements show that the TCBQ–NHC and AQ–NHC electrodes achieve specific capacitances of 365 and 331 F g−1 at 1 A g−1 in potential windows of 0–1.0 and −0.4 to 0.6 V, respectively. Furthermore, the as-constructed AQ–NHC//TCBQ–NHC asymmetric supercapacitor (ASC) can deliver high energy density (20.3 W h kg−1) at the power density of 0.7 kW kg−1 with long cycle life (the capacitance remains 98% of the initial value after 5000 cycles).

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.