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Abstract NiFe 2 O 4 (NFO) with different morphologies were successfully synthesized via a facile hydrothermal method of different precipitants. Results showed that the NFO obtained using urea as precipitant (NFO-U) formed a nanosheet shape, while the powders prepared using CH 3 COONa (NFO-C) exhibited a nanoparticle structure. Both of the two NFO showed a good crystallinity. Conclusion was reached through the results of BET and electrochemical tests that layer structure would improve the properties through a larger specific surface area. Encouragingly, the specific capacitance of NFO-U could arrive 240.9 F/g at the current density of 1 A/g. It was worth noting that the cycle performance of NFO-U was excellent, of which the specific capacitance improved to 128% after 2000 cycles. In addition, an asymmetric supercapacitor (ASC) system was constructed utilizing NFO-U as positive electrode while activated carbon (AC) as negative electrode. The two-electrode system showed a good performance with energy density of 10.15 Wh/kg at a power density of 140 W/kg. Therefore, the construction of sheet-like structures greatly improved the electrochemical performance of NiFe 2 O 4 materials. [ABSTRACT FROM AUTHOR]

• The partial S-doping NiFe 2 O 4 was obtained through a facile hydrothermal method. • The d-spacing of NiFe 2 O 4 was expended caused by partial S-doping. • The impact of partial S-doping to NiFe 2 O 4 is exposed by VASP analysis. • The optimal electrode exhibited the highest specific capacitance of 284 F/g. Nickel ferrite exhibits many advantages as electrode materials in the application of supercapacitor, such as high energy capacity and excellent cycle stability. Whereas, compared with identical-metal sulfides, the specific capacitance of NiFe 2 O 4 is still insufficient. Hence, to combine the superiorities of oxides and sulfides, partial sulfur doping NiFe 2 O 4 is obtained through hydrothermal synthesis method. Analyzed by Vienna Ab-initio Simulation Package (VASP) software and high-resolution transmission electron microscopy, the lattice parameters of NiFe 2 O 4 are confirmed to be expanded by introducing sulfur atoms, which is beneficial to electrochemical performances. At a current density of 1 A/g, the specific capacitance of partial sulfur doping NiFe 2 O 4 is calculated to be 284 F/g, higher than that of the original NiFe 2 O 4 of 182.2 F/g. Moreover, a maximum energy density of 21.14 Wh/kg is achieved at a power density of 375 W/kg. This work shows that partial sulfur doping could improve the electrochemical performance of NiFe 2 O 4. To sum up, this work provides an effective way to enhance the poor electrochemical performance of oxides, greatly encouraging the application of oxides in energy storage field. [Display omitted] [ABSTRACT FROM AUTHOR]

Morphology of transition metal oxide largely affects its electrochemical energy storage properties. This also applies to its composites. In this paper, NiFe 2 O 4 /Graphene nanosheets composites (NFO/GNSs) are successfully prepared by a facile hydrothermal method. NiFe 2 O 4 (NFO) in the sample exhibits a unique nanosheet morphology, vertically arranging on the graphene nanosheets (GNSs). Microscopically, NFO nanosheets interpenetrate on the surface of graphene to form a continuous network structure. The sample exhibits excellent specific capacitance and rate performance. It is worth mentioning that the specific capacitance increases to 140% after 5000 cycles, showing superior cycle stability. This may be due to the conductive network formed by NFO nanosheets and GNSs. Excellent electrochemical performance indicates excellent application prospects of materials in supercapacitor electrodes. Image 1 • Unique NiFe 2 O 4 nanosheets structure with graphene nanosheets was successfully prepared by a hydrothermal method. • NiFe 2 O 4 nanosheets/graphene performed a better specific capacitance, arriving at 464.15 F/g at current density of 1 A/g. • The network structure formed between NiFe 2 O 4 nanosheets and graphene nanosheets provides excellent cycle performance. [ABSTRACT FROM AUTHOR]

Abstract Bimetallic carbides with high activity and stability are promising potential materials for energy-storage application. However, the researches about Fe 2 MoC as electrode material on supercapacitors are comparatively weak, and the processing methods of Fe 2 MoC were also relatively few. Herein, a simple hydrothermal method, combining with carbothermic treatment at 900 °C, is explored to fabricate molybdenum iron carbon (Fe 2 MoC) successfully. Chitosan is not only a carbon source, but also a chelating agent to form bimetallic carbide rather than two separated monometallic carbide during the high-temperature treatment. Fe 2 MoC nanoparticles possessing large specific surface, high activity, stability and small resistance were the promising candidate for electrode material. Systematic electrochemical characterizations have verified the Fe 2 MoC (chitosan as carbon source) possesses a specific capacitance (97.7 F/g at a current density of 0.5 A/g), high rate capability (97.0% capacitance retention from 0.5 to 10 A/g) and cycling stability (83.9% capacitance retention after 1000 cycles) in 1 M KOH. In addition, it offers the energy density of 6.74 Wh/kg at a power density of 21 kW/kg. In view of the low-cost and excellent performance, Fe 2 MoC will hold great promise in energy-storage field for supercapacitors. [ABSTRACT FROM AUTHOR]