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A flexible self-supporting and binder-free supercapacitor electrode is fabricated by embedding a polypyrrole (PPy) nanostructured network into only one side of a graphene thin film via a spin coating method and a subsequent hydrothermal process. Based on the amphiphilic nature of the graphene oxide (GO) thin film, the well-defined PPy nanoparticles are well dispersed and form a reticular structure on the thin film surface. After reduction treatment at a temperature of 180 °C, the reduced graphene oxide (rGO) thin film not only provides a large accessible surface area for the dispersion of PPy nanoparticles but also acts as a binder-free conductive current collector. The polypyrrole/graphene film (PGF) electrode material exhibits an excellent electrochemical performance in the three-electrode configuration, including a high specific capacitance of up to 455 F g−1 at 1 A g−1 and outstanding cycling stability (97% capacitance retention after 1000 cycles). In addition, the symmetric PGF supercapacitor shows a specific capacitance of up to 49.3 F g−1 at a cell voltage of 1.4 V and exhibits a maximum energy density of 13.4 W h kg−1 at a power density of 700 W kg−1, with 96% capacitance retention after 1000 cycles. This work shows impressive potential in fabricating high-performance flexible electrodes for supercapacitors. [ABSTRACT FROM AUTHOR]

Redox-active covalent organic frameworks (COFs) are an emerging class of energy storage materials with notably abundant active sites, well-defined 1D channels and high surface areas. However, during the process of crystalline framework formation, error checking and proofreading will inevitably lead to internal structural defects. The morphological control of COFs is still a difficult task. Herein, solvent effects and pre-polymerization steps are used to control the preferential growth direction of the COF nucleus (extend along the conjugate plane in the a and b axis/vertically stacked along the c axis). As a result, the well-crystalline COFs with flake-flower (COFs-F) and rod-flower morphologies (COFs-R) are successfully prepared by controlling kinetic parameters. By comparing the electrochemical test results in different electrolytes, it is confirmed that the integration of carbonyl with anthraquinone blocks during the irreversible enolization process can achieve maximum utilization of the COFs skeleton. Benefiting from the high density redox active sites and more regular one-dimensional pores, COFs-R exhibits a specific capacitance of 486.3 F g−1. Since the tight stacking along the c axis can relieve the collapse to a certain extent, after 10 000 cycles at 0.5 A g−1, the capacitance of COFs-R is still maintained at 93.2%. The kinetic analysis shows that the capacitance contribution rate of COFs-R achieves 95.3% at 100 mV s−1. Density functional theory (DFT) shows that COFs-R has a narrow band gap and a low energy barrier for electron transport. Due to the unique advantages of the morphology and high density of active sites, the COFs proposed in this work can provide a new candidate for the next generation of intelligent energy storage materials. [ABSTRACT FROM AUTHOR]

2,6-Diaminoanthraquinone (DQ) molecules were covalently modified onto the surface of graphene (GO) via a nucleophilic displacement reaction between the epoxy groups on the surface of GO and the –NH2 groups of DQ molecules in the presence of ammonia to form a composite material (labeled as DQ–RGO). The rapid reversible faradaic reactions of DQ molecules are realized by means of the good conductivity of graphene. Therefore, the DQ–RGO composite material can combine the Faraday pseudocapacitance of DQ with the double-layer capacitance of graphene, thus displaying outstanding electrochemical performance in acidic electrolyte solution, including a high specific capacitance (332 F g−1 at 1 A g−1), excellent rate capability (the capacitance retention rate is 72.9% at 50 A g−1) and good cycling stability (maintaining 81.8% of the initial capacitance after 5000 cycles). Meanwhile, a thionine-functionalized graphene hydrogel (Th–GH) was also prepared via a non-covalent strategy. Finally, the DQ–RGO composite acted as the negative electrode and Th–GH acted as the positive electrode to construct an asymmetric supercapacitor (ASC). The ASC exhibited an energy density of 14.2 W h kg−1 along with a power density of 0.763 kW kg−1 and long cycling durability (maintaining 80% of the initial capacitance after 8000 cycles at 5 A g−1). More importantly, two such devices in series successfully illuminated 16 red light-emitting diodes (LEDs), demonstrating its outstanding energy storage performance. This work can provide ideas for the construction of green, all-carbon and excellent electrochemical performance devices. [ABSTRACT FROM AUTHOR]

Monolayer HSr2Nb3O10 nanosheets with a thickness of about 1.95 nm were synthesized via a top-down approach. The structure and morphology were well characterized by XRD, SEM and TEM analyses. The sample served as an efficient photocatalyst for hydrogen evolution with the in situ growth of highly dispersed Pt clusters (about 2 nm). The photocatalytic hydrogen evolution rate of the Pt/HSr2Nb3O10 nanosheets was 530.2 μmol gcatalyst−1 h−1 under the optimal conditions of 1 wt% of Pt (PtCl4 as the precursor), triethanolamine (TEOA) as the sacrificial reagent and the aqueous solution pH = 10.7, which was about 2.2 times higher than that of the layered HSr2Nb3O10. [ABSTRACT FROM AUTHOR]

In this study, uniform nitrogen-doped hollow carbon spheres (NHCSs) have been synthesized using sphere-like SiO2 as a template and polydopamine as a carbon source. It is found that NHCSs have an ultrahigh specific surface area of 870.1 m2 g−1 and high porosity. 9,10-Phenanthrenequinone (PQ) molecules are selected to decorate NHCSs to obtain electrode materials (NHCSs/PQ) for supercapacitors, which show a clear capacitance increase in comparison with pure NHCSs when tested in 1 M H2SO4 aqueous solution. In a three-electrode system, the NHCSs/PQ (mass ratio 4 : 2) electrodes exhibit larger specific capacitance (as high as 252 F g−1 at 1 A g−1) and an ultrahigh rate capability (80.8% capacitance retention at 30 A g−1). Furthermore, the asymmetric supercapacitor is assembled using NHCSs/PQ as a positive electrode material and nitrogen-doped porous carbon (NPCs) that has adsorbed anthraquinone as a negative electrode material, respectively. The device exhibits a high energy density of 8.34 W h kg−1 along with a power density of 600 W kg−1. [ABSTRACT FROM AUTHOR]

A novel tetrahydroxy-anthraquinone zeolitic framework (TZM) with a Viburnum blossom-like structure is synthesized via a simple solvothermal method by using tetrahydroxy-anthraquinone to substitute imidazolate without carbonization at high temperature. Considering the excellent electrochemical performance of the TZM, including a high specific capacitance of 2030 F g−1 at a current density of 1 A g−1, and good cycling stability with a capacitance retention of 94% of the initial capacitance after 1000 cycling charge/discharge measurements, an aqueous TZM//AC asymmetric supercapacitor (ASC) in 1 M KOH electrolyte is successfully fabricated by using the resultant TZM as the positive electrode and activated carbon nanosheets (ACS) as the negative electrode, respectively. The as-assembled aqueous ASC delivers a high energy density of 47.7 W h kg−1 at a power density of 750 W kg−1 with a high potential window of 0–1.5 V. Moreover, for the purpose of exploring the practical application of the active materials, an all-solid-state ASC TZM//ACS device with PVA-KOH gel electrolyte and separator is assembled and connected, which can light up a red light-emitting diode (LED). Its good performance demonstrates that the TZM//ACS ASC is a promising energy-storage system. [ABSTRACT FROM AUTHOR]

As a new type of green electrode material for supercapacitors, redox-active organic molecules have shown great potential due to their fast and reversible redox reactivity, excellent cycle stability and sustainability. However, their electrically insulating nature limits any practical application when they are used alone. Herein, dissected carbon nanotubes (DCNTs) were prepared through a single-step chemical oxidation method to build a unique conductive substrate with 1D nanotubes and 2D graphene nanoribbons. Also, thanks to the abundant oxygen-containing functional groups on the sidewalls of DCNTs, redox-active organic p-phenylenediamine (PPD) molecules can be strongly and massively anchored by a covalent strategy. In this design, DCNTs can serve as a conductive network to ensure efficient electron transport while PPD molecules guarantee a high specific capacitance via the fast and reversible redox reaction. Under the synergy of the two parts, the PPD covalently functionalized DCNT (PPD-C-DCNT) electrode material displays a high capacitance value of 388 F g−1 at 1 A g−1 (ten times higher than that of pristine MWCNTs), a wide electrochemical stable potential window of −0.8 to 0.8 V vs. the SCE reference electrode in 1 mol L−1 H2SO4 and an outstanding electrochemical stability of 85.7% after 10 000 cycles at 5 A g−1. Finally, a novel symmetric supercapacitor (PPD-C-DCNT SSC) was assembled to evaluate the actual energy storage properties of the electrode material. As a result, the PPD-C-DCNT SSC exhibits a maximum energy density of 19.1 W h kg−1 at a power density of 800 W kg−1 in a cell voltage ranging from 0 to 1.6 V. [ABSTRACT FROM AUTHOR]

In the present work, a four-step strategy has been used to prepare rhombic arrays of porous CoP2 nanowires on carbon cloth (denoted as CoP2/CC) for an efficient hydrogen evolution reaction (HER). The four-steps involve the hydrothermal synthesis of the CoxZn1−xOHF precursor, the formation of Co2O3@ZnO through a thermal treatment, alkaline etching of ZnO and phosphorization. Specifically, the ratio of Co to Zn in the precursor affects significantly the morphology and HER performance of CoP2/CC. CoP2/CC-4 (which was obtained at the optimal Co–Zn ratio), as a self-supported and binder-free electrode, exhibits a superior catalytic performance toward HER in both acid and alkaline media. The overpotential of HER is as low as 56 mV in 0.5 M H2SO4 and 72 mV in 1 M KOH at a current density of 10 mA cm−2. Correspondingly, the Tafel slope of the electrode reaction is not larger than 67 mV dec−1 and 88 mV dec−1 in acid and alkaline solution, respectively. Furthermore, the catalyst maintained a high stability for HER. This work displays a new idea to fabricate efficient electrocatalysts for HER or other electrochemical reactions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

C3N4/CdS–CdWO4 core–shell heterostructure photocatalysts were prepared via a facile solvent volatilization method. High-resolution transmission electron microscopy (HRTEM) analysis demonstrated that an interfacial structure was present between the C3N4 shell and the CdS–CdWO4 core, and the specific function of the modification of C3N4 was investigated by room-temperature photoluminescence spectroscopy and transient photocurrent measurement. The heterostructure photocatalysts displayed enhanced photocatalytic activity in the degradation of RhB dye than that of the single components and binary composites. Among the as-prepared samples, the best photocatalytic performance was achieved by 10 wt% C3N4/CdS–CdWO4, which was about 5.7 and 14.6 times that of CdS–CdWO4 and C3N4, respectively, under UV-visible light irradiation. Owing to the formation of a C3N4/CdS–CdWO4 ternary heterostructure, its efficient pollutant-removing ability and stability can be attributed to the improved separation of photogenerated carriers and transfer efficiency for the intensive built-in electric field intensity. Radical trapping experiments proved that h+ and ṖO2− play crucial roles in the photocatalytic process. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

CO 2 , the main greenhouse gas, has received considerable attention due to environmental issues. From a scientific perspective, CO 2 as a cheap and abundant carbon source, could be applied in synthesizing more valuable chemicals such as urea, formic acid, and cyclic carbonates. However, the high bond energy of C[double bond, length as m-dash]O (750 kJ mol -1 ) and the non-polarity property make CO 2 molecules difficult to activate. In this paper, we have carefully designed a low-cost, stable and, most importantly, highly efficient Co-based heterocatalyst Co@N x C functionalized by nitrogen-doped carbon to activate CO 2 molecules and convert it into cyclic carbonates. The CO 2 conversion process could be triggered at very mild conditions (60 °C and 1 bar CO 2 ). We carefully adjusted the nitrogen content in the carbon support to enhance the catalytic performance of Co centers via the interface effect. Consequently, the optimal catalyst displayed extraordinary activity toward the cycloaddition of CO 2 with styrene oxide as high conversion (92%) and selectivity (>99%) were achieved in 4 h without byproducts., Competing Interests: The authors declare that they have no competing interests., (This journal is © The Royal Society of Chemistry.)

Monolayer HSr 2 Nb 3 O 10 nanosheets with a thickness of about 1.95 nm were synthesized via a top-down approach. The structure and morphology were well characterized by XRD, SEM and TEM analyses. The sample served as an efficient photocatalyst for hydrogen evolution with the in situ growth of highly dispersed Pt clusters (about 2 nm). The photocatalytic hydrogen evolution rate of the Pt/HSr 2 Nb 3 O 10 nanosheets was 530.2 μmol g catalyst -1 h -1 under the optimal conditions of 1 wt% of Pt (PtCl 4 as the precursor), triethanolamine (TEOA) as the sacrificial reagent and the aqueous solution pH = 10.7, which was about 2.2 times higher than that of the layered HSr 2 Nb 3 O 10 .