Formation of mixed metal sulfides of NixCu1−xCo2S4 for high-performance supercapacitors
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Supercapacitors cannot fulfill their potential as energy storage devices without substantially improving their comparatively low energy density. This requires improving their capacitance. Unfortunately, predicting the capacitance of the carbon-based materials that typically make up supercapacitor electrodes is difficult. For example, remarkably we lack a theoretical understanding of the capacitance of even the most basic example of a carbon electrode: highly oriented pyrolytic graphite. (HOPG) This material has a capacitance that is an order of magnitude lower than both standard metals and theoretical expectations. Here, we use new quantum mechanical calculations in combination with a critical analysis of the literature to demonstrate that the standard explanations of this unusually low capacitance are inadequate. We then demonstrate that a layer of hydrocarbon impurities which has recently been shown to form on graphite is the most plausible explanation. We develop a model of this effect which accounts for the penetration of solvent into the hydrocarbon layer as the voltage increases. This model explains the characteristic V shape of the capacitance as a function of voltage. Finally, we present evidence that this layer also forms and limits the capacitance in real supercapacitor materials such as activated carbon. Methods of modifying or removing this layer could therefore potentially lead to significant improvements in the capacitance of typical supercapacitors.
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Abstract Herein, NiCo 2 O 4 /C core‐shell nanoneedles grown on Ni foam are prepared by a simple route and applied in supercapacitors. Electrochemical results show that the area specific capacitance of NiCo 2 O 4 /C reaches 2057 mF/cm 2 at 1 mA/cm 2 , which is about 4 times than that of the pristine NiCo 2 O 4 . The electrode exhibits excellent rate capability (92 % of capacitance retention at 10 mA/cm 2 ) and remarkable cycling stability (81 % of capacitance retention after 10000 cycles). The as‐fabricated all‐solid‐state asymmetric supercapacitor (NiCo 2 O 4 /C//AC(active carbon)) achieves an energy density of 2 mW h cm −3 at a power density of 37 mW cm −3 and remains 1.6 mW h cm −3 at a power density of 120 mW cm −3 . Moreover, the ASC device can retain 87 % capacitance after 4000 cycles. The result reveals that it is a feasible method to achieve excellent performance in energy storage devices by the dipping method.
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This paper revisits the supercapacitor capacitance characterization method 1A of the IEC standard 62391-1. For comparison, an alternative method using the total charge stored in the supercapacitor is proposed. These two methods are applied to three supercapacitor samples with different rated capacitances from different manufacturers at different terminal voltages. Results show that the capacitance determined using the IEC method decreases when the discharge current increases. Besides, the capacitance measured using the IEC method is lower than that estimated using the total charge method. These observations are explained by analyzing a five-branch RC ladder circuit model capturing the porous electrode structure and the charge redistribution process of supercapacitors.
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Graphene is a promising material for energy storage, especially for high performance supercapacitors. For real time high power applications, it is critical to have high specific capacitance with fast charging time at high current density. Using a modified Hummer's method and tip sonication for graphene synthesis, here we show graphene-based supercapacitors with high stability and significantly-improved electrical double layer capacitance and energy density with fast charging and discharging time at a high current density, due to enhanced ionic electrolyte accessibility in deeper regions. The discharge capacitance and energy density values, 195 Fg-1 and 83.4 Whkg-1, are achieved at a current density of 2.5 Ag-1. The time required to discharge 64.18 Whkg-1 at 5 A/g is around 25 sec. At 7.5 Ag-1 current density, the cell can deliver a specific capacitance of about 137 Fg-1 and maintain 98 % of its initial value after 10,000 cycles, suggesting that the stable performance of supercapacitors at high current rates is suitable for fast charging-discharging applications. We attribute this superior performance to the highly porous nature of graphene prepared with minimum restacking due to crimple nature wrinkles and the improved current collecting method.
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Here we proposed novel NiCo2O4 nanowires on carbon black layer that used as a superior electrode for supercapacitors, with an excellent pseudocapacitive performance. The NiCo2O4 nanowires-based electrode provide high values of specific capacitance of 3.31 F/cm2 corresponding to the current densities of 5 mA/cm2. The symmetrical supercapacitor based on NiCo2O4 nanowires-based electrode have a high areal specific capacitance of 42.6 mF/cm2 corresponding to the current densities of 0.5 mA/cm2. It exhibited good cycling life that the specific capacitance maintained with no decrease during 50000 cycles. Our work can be applied in mass applications of high-capacitance energy-storage supercapacitors.
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