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The preparation of reduced graphene oxide (rGO) by an oxidation‐reduction chemical reaction is very extensive. However, during the oxidation process, the obtained graphene oxide (GO) is usually not uniform in size and contains some incompletely oxidized graphite particles. To obtain uniformly small‐sized GO and remove impurities, we put forward here a simple suction filtration process, that is, when GO passes through the filter, it is preferentially selected. In this way, we provide a facile but effective method for the screening of GO and then obtain wrinkled flower‐like rGO (WF rGO) with excellent electrochemical performance. We also perform a design of experiments to optimize the reduction procedure. It is found that such factors as the reaction time, temperature, reductant and KOH content all affect the microstructure and electrochemical properties of rGO. With the optimized reduction procedure used, the specific capacitance of rGO can reach 231 F/g at the current density of 0.5 A/g. Furthermore, through the suction filtration along with the optimization of reduction, the WF rGO shows a much higher specific capacitance of 272 F/g. On this basis, the energy density of a symmetric rGO//rGO capacitor device arrives at 10.9 Wh/kg at a power density of 0.5 kW/kg and 8.2 Wh/kg at 12.6 kW/kg, and the capacitance keeps 91.5 % after 10,000 cycles. Moreover, the WF rGO//WF rGO device possesses a significantly higher energy density of 12.1 Wh/kg at a power density of 0.5 kW/kg and 9.7 Wh/kg at 24.2 kW/kg, and the capacity perfectly remains 94 % of its initial value after 10,000 cycles. [ABSTRACT FROM AUTHOR]

With the increasing demand for high‐performance energy storage devices, single materials of ordinary structures for electrode materials have become increasingly difficult to meet people's needs. Therefore, composites of inimitable structures have drawn considerable attention. In this work, nitrogen‐doped porous graphene coated with Fe3O4 nanoparticles (NPGF) is prepared by an efficient and green pyrolysis method. Structural and compositional characterizations confirm that the NPGF nanohybrids possess uniformly distributed pore structure and quite pure composition which is free of any impurities. In addition, electrochemical characterization verifies the excellent electrochemical performance, such as high‐specific capacitance (713 F g−1 at 1 A g−1), prominent rate capability (capacitance retention of 77.3% and 67.9% when the current density is increased respectively from 1 to 10 and 20 A g−1), and outstanding cycling stability (capacitance retention of 94.3% after 3000 cycles). Such promising results suggest that the NPGF nanohybrids have great application prospects in future high‐performance supercapacitors. [ABSTRACT FROM AUTHOR]

Supercapacitors are a kind of novel energy storage devices with long cycle stability and high power density. Electrode materials selection is one of the key factors that affect the properties of supercapacitors. Biomass-derived electrode materials - being low cost, renewable and environmentally friendly - are therefore attracting researchers’ attention. In this work, we adopted a simple process of carbonization and activation with rice plant soot, a common biomass material, as carbon source, and finally obtained the nanoscale porous carbon electrode (NPCE) materials. Then, the electrochemical properties of the as-prepared NPCE materials were tested in 6 M KOH, and the results indicated that the specific capacitance could reach 216 Fg−1. Therefore, this low-cost, highly efficient technique is a significant milepost towards environmentally sustainable and commercially feasible fabrication of carbon electrode materials from biomass sources. [ABSTRACT FROM AUTHOR]

3 D capacitance: Hierarchical porous carbon‐based electrode materials with a composite structure are prepared from a biomass waste by a facile carbonization and activation process without using any additional templates. Benefiting from the composite structure, the ions experience a variety of environments, which contribute significantly to the excellent electrochemical properties of supercapacitors. [ABSTRACT FROM AUTHOR]

• Binder-free electrode of 3D rGO/α-Ni(OH) 2 @nickel foam with a layer-by-layer structure is fabricated. • An extremely high specific capacitance of 2118 F/g is obtained at 1 A/g. • Specific capacitance remains perfect with huge increase in the mass loading. • Energy density of supercapacitors reaches as high as 109.9 Wh/kg. The conventional fabrication of composite electrodes often requires extraordinarily complex electrode preparation steps. We provide here a separate dip-coating method for ultrasmall α-Ni(OH) 2 nanoparticles grown between reduced graphene oxide (rGO) sheets on nickel foam, forming a direct binder-free electrode of 3D rGO/α-Ni(OH) 2 @nickel foam (GNH@NF- n , layer number n = 5, 8, 10, 15, 20) with a layer-by-layer structure. At a charge and discharge current density of 1 A/g, the specific capacitance of GNH@NF-5 reaches 2118 F/g (capacity: 323.5 mAh/g). When the current density is changed from 1 to 10 A/g, the specific capacitance retains 74.8% of the maximum value (capacity retains 88.4%). Changes in structure and electrochemical performance are analyzed as the number of layers increases, i.e., as the mass loading increases. When the mass loading increases by a factor of 5.2 from 1.4 to 7.3 mg/cm2, the GNH@NF- n electrode maintains a very high specific capacitance, with a value as high as 89% of the capacitance (or capacity) for the lowest mass loading. Dipping separately in nickel-based and rGO solutions (separate dip-coating method) is more effective in achieving the layer-by-layer structure and high electrochemical performance than dipping in a mixed solution (mixed dip-coating method). Furthermore, asymmetric GNH@NF-5//rGO supercapacitors are assembled and exhibit a high specific energy of 109.9 Wh/kg at a specific power of 1.3 kW/kg and a high cyclic stability with capacitance retention of 86.1% after 10,000 charge and discharge cycles. Image, graphical abstract High-performance binder-free electrode of 3D rGO/α-Ni(OH) 2 @nickel foam with a layer-by-layer structure is fabricated by a facile dip-coating method. Coating is achieved by dipping into separate nickel-based and rGO solutions. The electrode with this unique structure exhibits very interesting electrochemical behaviors. An extremely high specific capacitance of 2118 F/g is obtained at 1 A/g; in addition, specific capacitance remains perfect with huge increase in the mass loading. Hence, one can imagine an increase in the areal capacitance of this kind of material in supercapacitor applications, rendering it a more competitive electrode material for practical applications. Furthermore, asymmetric GNH@NF-5//rGO supercapacitors are assembled and exhibit a very high specific energy of 109.9 Wh/kg at a specific power of 1.3 kW/kg. [ABSTRACT FROM AUTHOR]

Advanced electrode materials are of vital importance to the application of high-performance supercapacitors. However, most of the electrode materials are limited by their low specific capacitance and/or poor cycling stability. In this work, nitrogen-doped graphene-encapsulated Fe 3 O 4 nanoparticles (Fe 3 O 4 @NG) were synthesized through a simple one-step green and scalable dry pyrolysis method, in which the uniform growth of Fe 3 O 4 nanoparticles with a diameter of about 30–60 nm, the reduction of graphene oxide (GO), and the introduction of nitrogen atoms on graphene could be achieved simultaneously. The structure, composition, and electrochemical performance of the Fe 3 O 4 @NG samples were systematically characterized. Compared to pristine Fe 3 O 4 , Fe 3 O 4 @NG showed superior electrochemical performances, including an ultra-high specific capacitance of up to 740 F g−1 at the current density of 1 A g−1, a greatly improved rate capability of 56.8% with the increase in current density from 1 to 20 A g−1, and an excellent cycling stability with the retention ratio of 90.9% after 3000 cycles. Furthermore, after being placed in the external environment for one year, the specific capacitance retention of Fe 3 O 4 @NG could be as high as 98%, proving again that the as-prepared Fe 3 O 4 @NG exhibited perfect structural stability and excellent stable electrochemical properties. All of the results demonstrate an extraordinary performance of Fe 3 O 4 @NG, thus being potential for future practical applications. Image 1 • Fe 3 O 4 @N-doped graphene nanocomposites were synthesized through a simple one-step green and scalable dry pyrolysis method. • An ultra-high specific capacitance of up to 740 F g−1 is obtained. • A greatly improved rate capability and an excellent cycling stability are simultaneously obtained. • After being placed in the external environment for one year, the specific capacitance retention could be as high as 98%. [ABSTRACT FROM AUTHOR]

Different microstructured MnO 2 was synthesized as a pseudocapacitive material by using a facile hydrothermal method. MnSO 4 ·H 2 O and (NH 4 ) 2 S 2 O 8 were reacted at 120 °C for 12 h to form MnO 2 , for which the morphology and phase composition of the MnO 2 formed could be well controlled. The obtained MnO 2 samples were comparatively characterized in terms of their crystallographic structure, microstructure and electrochemical performance. The results revealed that the MnO 2 was controllably formed in different microstructures (nanorod, hollow urchin and smooth ball) with different crystalline phase compositions. The average specific capacitance of MnO 2 nanorod, hollow urchin and smooth ball at a scan rate of 5 mV/s was 317, 204 and 276 F/g, respectively, indicating that MnO 2 nanorod displayed the best electrochemical capacity. All the synthesized MnO 2 , however, showed a good reversibility and cycling stability, roughly 70% of the initial capacitance retained after 2000 charge/discharge cycles. This work sheds light on a facile approach for synthesis of high-performance metal oxide electrode materials with controllable microstructure. [ABSTRACT FROM AUTHOR]