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Low crystalline CoSx was fabricated by anion-exchange of 2-dimensional (2D) cobalt carbonate hydroxide (CoCH) using aqueous sodium sulfide solution at room temperature. It was proved that single crystalline CoCH nanoplates would transform into porous 2D CoSx with low crystallinity after the anion-exchange. When they were used as electrode materials for supercapacitor, the 2D CoSx material had a much higher specific capacitance than its precursor due to their different compositions and electroconductivities. CoSx had a high specific capacitance of 863 F g−1 at 1 A g−1 and a good stability during long time charge-discharge processes, about 64.7% of initial capacitance retention after 10000 cycles. Asymmetric hybrid devices using 2D CoSx as positive electrode and activated carbon as negative electrode were assembled, and the capacitor devices were able to achieve a high energy density of 33.56 Wh kg−1 at the power density of 400 W kg−1. The high performances of the porous CoSx make it a promising electrode material for energy storage.

Amorphous and crystalline copper cobalt sulfides are successfully synthesized via a hydrothermal method at different temperatures and researched as electrode materials for supercapacitor. The reaction mechanisms during the sulfidation and hydrothermal processes are investigated and clarified. It is demonstrated that the hydrothermal temperature is a crucial factor for the crystallinity, morphology and electrochemical performance of CuCo2S4. Among all the samples, the CuCo2S4 synthesized at 150 °C shows the highest specific capacitance of 515 F g-1 at 1 A g−1 as well as good cycling stability with ∼93.3% capacitance retention after 10000 cycles at 5 A g−1. An asymmetric device which is assembled by using optimized CuCo2S4 electrode as positive electrode and activated carbon as negative electrode material is able to deliver an ultrahigh energy density of 50.56 Wh kg−1 at a power density of 4.6 kW kg−1, and remains 20.93 Wh kg−1 at a high power density of 22.5 kW kg−1, with ∼99% capacity retention after 10000 cycles. Based on the results above, the CuCo2S4 materials prepared by our method possess a considerable potential as electrode materials for supercapacitor applications.

For rapid charge/discharge application, capacitive energy storage technology is an attractive approach. The low energy density, one of disadvantages, can be ameliorated by promising pseudocapacitive materials that harvest energy through redox reactions. In this work, a composite of bimetal oxide and carbon nanotubes (CNTs), NiCoO2@CNT, with a novel mesoporous grape−like structure is prepared by a mild method, in which metal oxide nanoparticles are bonded to CNTs like vines tightly. Due to the synergistic effect and the unique structure as well as the facilitation of CNTs, impressive properties are performed for the NiCoO2@CNT composite. With an excellent stability and rate capability, it achieves 1587 F g−1 at 1 A g−1. As revealed in the charge storage mechanism, the surface−controlled process plays a dominant role, which can be ascribed to the large proportion of redox reactions on the surface of small NiCoO2 nanoparticles and the presence of CNTs. NiCoO2@CNT and activated carbon are then applied to fabricate an asymmetric supercapacitor. It exhibits a high energy density of 41.8 Wh kg−1 at 412 W kg−1 and an outstanding cycling property with 92% maintained after 5000 cycles, indicating a great potential for the actual application.

Hybrid materials with three-dimensional hierarchical architectures have drawn considerable attention in the field of energy storage and conversion owing to the improved electrolyte-accessible surface area and shortened ion transfer paths. In this work, a novel hierarchical Co-Fe LDH@NiO composite with shell-core rod arrays grown on the surface of Ni foam is presented for the use of electrode material without any organic binders and conductive agents. It is found that the expected good conductivity, large electrolyte-accessible surface area and sufficient active sites cannot be well achieved at the same time. Owing to the facile electrolyte diffusion paths and large exposed surface area, Co-Fe LDH@NiO with 46.06% Co-Fe LDH has a specific capacity of 361 C g −1 at 1 A g −1 . However, at high current densities, its performance decreases quite dramatically due to the poor electrical conductivity of NiO support and the high interface contact resistance between NiO microrods and Co-Fe LDH nanosheets. The assembled hybrid supercapacitor based on Co-Fe LDH@NiO - Ni and activated carbon electrodes can deliver an energy density of 22 Wh kg −1 at the power density of 800 W kg −1 , showing a promising potential in energy storage and conversion.

Pure Ni-Mn layered double hydroxide (LDH), Co-Mn LDH with a flower-like morphology and sandwich-like Ni-Mn LDH/reduced graphene oxide (rGO), Co-Mn LDH/rGO hybrids are fabricated via a simple co-precipitation method. In the hybrids, Ni-Mn and Co-Mn hydroxide nanoflakes are tightly anchored on the both surfaces of rGO, leading to the composites with high specific surface areas. Electrochemical measurements prove that rGO can improve the capacitance and cyclic stability of the hybrid materials and that Ni-Mn LDH delivers a much higher specific capacitance but a worse cycling performance than Co-Mn LDH. A high specific capacitance of 1635 F g −1 at 1 A g −1 and a high rate retention of 71% at 10 A g −1 are achieved for Ni-Mn LDH/rGO. A hybrid capacitor with Ni-Mn LDH/rGO as positive electrode and activated carbon as negative electrode is assembled. It possesses a specific capacitance of 84.26 F g −1 at 1 A g −1 and an energy density of 33.8 Wh kg −1 within a potential window of 1.7 V.

A carbon nanotubes (CNTs)-nickel-cobalt oxyhydroxide nanoflake core-shell structure is designed and fabricated by a facile one-step chemical bath deposition method at 80 °C. After its annealing at 300 °C, a CNTs/Ni-Co oxide core-shell heterostructure is thus achieved. Phase characterization proves that Ni-Co oxyhydroxide and Ni-Co oxide as shells are (Ni,Co)OOH and NiCo2O4, respectively. Structural analysis confirms that the as-synthesized oxyhydroxide and oxide shells are conformally coated on the external surface of CNTs. Electrochemical data demonstrate that (Ni,Co)OOH/CNTs composite exhibits a high specific capacitance of 853 F g−1 at 5 mV s−1 and an excellent high rate capability, with 75% retention after a 10-fold increase in current density. Meanwhile, the hybrid of NiCo2O4/CNTs has an enhanced performance of 940 F g−1 at 5 mV s−1. Its rate capability is much better than the oxyhydroxide precursor because of its high conductivity. Both of these heterostructures have an excellent cycling performance of 100% capacitance retention after 10,000 cycles and Coulombic efficiency. An asymmetric capacitor is also fabricated with (Ni,Co)OOH/CNTs as positive electrode and activated carbon as negative electrode. It possesses a high specific capacitance of 164 F g−1 and a high energy density of 73.8 Wh Kg−1 within 1.8 V.

Pure NiAl-layered double hydroxides (LDHs) and graphene oxide (GO)/NiAl-LDHs composites are both fabricated by refluxing solution with the assistance of microwave irradiation within 2 hours. The structural characterization and morphological observation of the composites suggest that small NiAl-LDHs crystals are in situ grown on the surface of GO nanosheets, preventing the compact stacking of GO as an intercalated spacer. The electrochemical performances of pure LDHs and GO/NiAl-LDHs composites with different content of GO are investigated and compared. The GO/NiAl-LDHs composite containing 23 wt% GO exhibits a higher specific capacitance of 1630 F g−1 at 1 A g−1 in 6 M KOH solution and more stable rate capability than others. When GO is substituted by microwave reduced graphene oxide (rGO) or the GO in the GO/NiAl-LDHs composite is thermally reduced at a rational temperature, rGO/NiAl-LDHs composites are obtained with enhanced electrochemical behavior in rate capability and cycle life. Critical comparisons indicate that rGO/NiAl-LDHs composite prepared by thermal reducing GO/NiAl-LDHs has better cycling performance and rate capability than the counterpart directly prepared with rGO.

Three-dimensional network of cobalt and aluminum layered double hydroxide (LDH) nanosheets was synthesized on nickel foam by a simple hydrothermal method. The CoAl-LDH nonosheets were subsequently coated by ion sputtering with thin layers of Pt films to facilitate fast electron transport between current collector and the CoAl-LDH active materials. The optimal thickness of the Pt film acquiring the best performance was identified by applying various sputtering time in controlled experiments. The supercapacitor built by the CoAl-LDH nanosheets coated with Pt film sputtered for 5 min has a high specific capacitance (734.4 F g −1 at 3 A g −1 ), excellent rate capability as well as cycling stability. Moreover, it showed a long life of 77% retention after 6000 cycles and its general morphology was preserved after the test. The synergetic affect of conductive layer of Pt films and CoAl-LDH on the improvement of electrochemical properties was discussed and this would provide a useful clue in designing novel and effective electrode materials for supercapacitors.

A microwave-assisted reflux method was used to rapidly synthesize CoAl-layered double hydroxide (CoAl-LDH)/graphene oxide composite. In the synthetic procedure, graphite oxide (GO) was exfoliated to graphene oxide nanosheets in company with the in situ growth of CoAl-LDH. The as-prepared products showed lamellar structures. The aggregation of LDH was effectively prevented by the presence of graphene oxide nanosheets. Electrochemical performances were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests. All the composites had higher rate capability than pure CoAl-LDH, maintaining about 80% of discharge capacitance when the current density increased from 1 A g−1 to 20 A g−1. A maximum specific capacitance of 772 F g−1 was obtained at 1 A g−1 in 6 M KOH solution for the composite containing 12.9% GO. Moreover, the composite exhibited excellent long cycle life with about 73% specific capacitance retained at 6 A g−1 after 10,000 cycles.