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The design of microstructures and the optimum selection of electrode materials have substantial effects on the electrochemical performances of supercapacitors. A core–shell structured CuCo2S4@Ni(OH)2 electrode material was designed, with CuCo2S4 nanotubes as the core wrapped by interlaced Ni(OH)2 nano-sheets as the shell. The hydrothermal and electro-deposition processes were adopted to synthesize CuCo2S4@Ni(OH)2 materials. The CuCo2S4 nanotubes can both provide specific capacitance and act as a “superhighway” for electrons due to their highly conductive skeleton structure. The Ni(OH)2 nano-sheets will boost the electrochemically active sites and enhance the specific surface area. Meanwhile, the mutually restricted core–shell CuCo2S4@Ni(OH)2 electrode could regulate the volume deformation to improve its stability. The CuCo2S4@Ni(OH)2 electrode had a maximum specific capacitance of 2668.4 F g−1 at a current density of 1 A g−1 and a superior cycling stability of 90.3% after 10 000 cycles. Moreover, a CuCo2S4@Ni(OH)2//active carbon asymmetric supercapacitor with a maximum energy density of 44 W h kg−1 was assembled, suggesting that CuCo2S4@Ni(OH)2 is a successful binder-free electrode material for high performance supercapacitors.

Reduced graphene oxide (RGO) with a porous nanostructure and large surface area suffers from poor volumetric capacitance when it is employed in supercapacitors. This paper reports a facile, scalable and efficient method to produce self-assembled reduced graphene oxide with high packing density (1.55 g cm−3) via phosphoric acid treatment (PAT) at moderate temperatures (200–400 °C). The PAT process could not only controllably preserve redox-active C–OH and CO groups which can provide large and stable pseudocapacitance, but also contribute to the formation of a well self-assembled compact structure. Due to the pseudocapacitive behavior of the electrochemically active oxygen functional groups as well as the compact layered structure, the reduced graphene oxide (RGO-MP40) exhibits an ultrahigh volumetric capacitance (482 F cm−3 and 384 F cm−3 at a mass loading of 2.5 mg cm−2 and 19.7 mg cm−2, respectively). This novel strategy holds great promise for designing graphene-based materials for compact energy-storage devices.

A high-shear mixer was used in the process of chemical precipitation to prepare graphene oxide (GO) and Ni(OH)2 composites with different weight fractions of GO from 0 wt% to 2.6 wt%. The GO/Ni(OH)2 composite with 1.5 wt% GO prepared by high-shear mixing (1.5GO/Ni(OH)2–H) displayed a high specific capacitance (1836 F g−1 at 2 mV s−1), which was beyond that (1581 F g−1 at 2 mV s−1) of GO/Ni(OH)2 obtained by impeller agitator stirring (1.5GO/Ni(OH)2–M). 1.5GO/Ni(OH)2–H also showed excellent cycling stability, retaining 91.7% of its initial specific capacitance after 2000 cycles. Furthermore, the asymmetric device (the 1.5GO/Ni(OH)2–H as the positive electrode and active carbon as the negative electrode) exhibited a high energy density of 41.2 W h kg−1 at a power density of 1500 W kg−1 with a voltage of 1.5 V. In addition, the effects of high-shear mixing and impeller agitator stirring on the microstructures and electrochemical properties of the composites were discussed, which indicated that high-shear mixing could exfoliate graphene oxide and restrain the grain growth of Ni(OH)2, thus leading to excellent electrochemical performance.

The variation of oxygen functional groups (OFGs) is extremely complicated during the thermal treatment of graphene oxide (GO). Selectively improving the content of electrochemically active OFGs through multi-step thermal treatment is a promising strategy to enhance the charge storage capability of graphene-based supercapacitor electrodes. In this work, a two-step thermal treatment method was used to produce reduced graphene oxide (RGO) with selectively increasing the content of electrochemically active quinone-type carbonyl groups (C˭O). The first step is thermal reduction at 500 °C in Ar atmosphere, during which GO is exfoliated into separated sheets with porous structure and larger surface area; the second step is air oxidation at 400 °C, which facilitates the selective formation of quinone-type C˭O groups and enhances the mesoporous fraction. Due to the pseudocapacitive behavior of quinone-type C˭O groups and low ion diffusion resistance, the electrode prepared by RGO exhibits a high specific capacitance of 303 F g−1 at 0.2 A g−1 with excellent rate capability (202 F g−1 at 10 A g−1) and notable cycling performance (~ 116% capacitance retention after 1500 cycling at 1 A g−1). The present work proposed a facile and environmentally friendly two-step thermal treatment strategy in effectively controlling the surface property and microstructure of graphene materials, which has significant implications for mass producing graphene materials for supercapacitor electrodes.

In this study, a new, effective strategy is reported for the fabrication of composites using manganese oxide (MnO2) grown in situ on three-dimensional (3D) graphene by the reverse microemulsion (water-in-oil) method. A uniform coating of nanoscale MnO2 layers can be observed on the internal surface of 3D graphene, which could benefit rapid ionic and electronic transport. The electrochemical performance of the MnO2/3D graphene composites is optimized by the control of the composite structure and mass loading of MnO2. The MnO2/3D graphene composite thus prepared exhibits a significantly high specific capacitance of 659.7 F g−1 at 0.3 A g−1 and an excellent retention life of 106% after 1000 cycles. The facile synthesis and excellent electrochemical performance of the MnO2/3D graphene composites indicate that the developed method demonstrates potential applications for the fabrication of novel electrode materials for use in energy storage devices.

Manganese oxide (MnO2)/three-dimensional (3D) reduced graphene oxide (RGO) composites were prepared by a reverse microemulsion (water/oil) method. MnO2 nanoparticles (3–20 nm in diameter) with different morphologies were produced and dispersed homogeneously on the macropore surfaces of the 3D RGO. Scanning electron microscopy and transmission electron microscopy were applied to characterize the microstructure of the composites. The MnO2/3D RGO composites, which were annealed at 150 °C, displayed a significantly high specific capacitance of 709.8 F g−1 at 0.2 A g−1. After 1000 cycles, the capacitance retention was measured to be 97.6%, which indicates an excellent long-term stability of the MnO2/3D RGO composites.

Herein, graphene oxide (GO) with a 3D structure was prepared on the surface of Ni foam (NF) via electrophoretic deposition, and then reacted in situ to form 3D reduced graphene oxide (RGO) via thermal reduction. Ni–Co–S nanosheet arrays were produced on various substrates (RGO/NF, GO/NF and NF) via a facile one-step electrochemical deposition. Scanning electron microscopy (SEM) demonstrated that RGO nanosheets were vertically wrapped on NF by thermal reduction of GO. Furthermore, interconnected and hierarchical porous Ni–Co–S nanosheets were uniformly coated on RGO. The electrochemical performance of the ternary material on different substrates was investigated. The physical structures, combined with the advantages of both ternary Ni–Co–S and RGO, exhibited excellent electrochemical performance. When incorporated as electrode materials for supercapacitors, the synthesized samples possess high specific capacitance and a long cycle life. With the synergistic effect of RGO and ternary Ni–Co–S, a high performance is achieved. The specific capacitance of Ni–Co–S/RGO/NF (2643 F g−1) demonstrated an enhancement compared with Ni–Co–S/GO/NF (2083 F g−1) and Ni–Co–S/NF (1329 F g−1) at a current density of 10 A g−1. Additionally, the retention of specific capacitance of Ni–Co–S/RGO/NF after 2500 cycles displayed a superior cyclic stability of 84.22% at a current density of 50 A g−1.