Bingshu Guo

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.

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).

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.