Preparation And Supercapacitive Performance Investigation Of Mn/NiCo-Based Electrode Materials

The limited fossil fuel reserves and severe climate change have stimulated a rapid development of efficient energy storage devices.Among the various energy storage systems,supercapacitors have been studied extensively owning to their promising characteristics,such as fast recharge capability,great power density and excellent lifespan.Supercapacitors are categorized into two types,including electric double-layer capacitors and pseudocapacitors.Generally,the pseudocapacitors show a higher capacitance and energy density than that of the electric double-layer capacitors.Mn-based electrode material is a promising candidate for supercapacitor applications owing to its large theoretical capacity and low cost,the construction of Mn-based nanomaterials and their composites with a hierarchical structure is of far-reaching significance for improving the performance of electrode materials.Besides,transition metal sulfides hold immense promise for use in energy storage device due to their higher electronic conductivity and theoretical capacity,compared with their corresponding metal oxides.However,in practice,the cycle stability of sulfides electrodes is always poor.In these cases,we have synthesized a series of Mn/NiCo-based oxides and sulfides with a hierarchical structure through a combination technique,including electrospinning,calcination and hydrothermal method.In addition,we have also explored the applications of these nanomaterials in supercapacitor device.The main research contents are listed as follows:（1）Hierarchically double-walled MnO₂ hollow nanofibers（MHNFs）are fabricated through a facile surface-modification-assisted strategy.The synthesized MHNFs are characterized by diverse techniques,and their capacitive performances have also been investigated by the electrochemical tests.According to the electrochemical measurements,the as-synthesized MHNFs achieve a specific capacitance value of 222.4 F g^-1,as well as superior cycling capability（94%after 5000 cycles）.The MHNFs and activated carborn are employed to assemble an asymmetric supercapacitor in a 1M Na₂SO₄ solution.The device shows the highest energy density of 29.3 W h kg^-1 at 90 W kg^-1,and the highest power density of 7200 W kg^-1 at 7.2 W h kg^-1,within a voltage window of 0-1.8 V.These results make the as-fabricated MHNFs a suitable electrode material for high efficient electrochemical supercapacitor.（2）Mn₂O₃@MnO₂ composite nanofibers,with an optimum composition by adjusting the KMnO₄ precursor concentration,have been successfully synthesized through a facile electrospinning technique along with a hydrothermal route.The hierarchically core-shell structure,as well as the synergistic effect between the two components,enables the as-prepared Mn₂O₃@MnO₂ nanocomposite electrode to achieve a desirable specific capacitance(225.0 F g^-1 at 0.2 A g^-1)and cycle stability（86.5%retention after 5000 cycles）.Besides,asymmetric supercapacitor assembled with the as-prepared Mn₂O₃@MnO₂composite and activated carbon as the positive and negative electrode,respectively,exhibits a satisfactory energy storage performance.More importantly,different electrode preparation techniques,including different substrate as a current collector,as well as the pressing process being employed in the electrochemical measurements,leads to a different electrochemical performance for the as-prepared electrode material.（3）MnS nanofibers have been synthesized by an electrospinning technique combined with a sulfidation process at a high temperature,in which KMnO₄ and S powder are used as Mn and S sources,respectively.Because of its unique one-dimensional nanofiber structure,MnS electrode exibites a high specific capacitance(642.0 F g^-1 at 1 A g^-1)and cycle stability.In order to explore the influence of temperature on the crystal phases and morphology of the products,Mn-based oxides are calcinated at different temperatures（300-800°C）.In addition,the relationship between the internal structure and the properties for the electrode materials are analyzed by comparing the electrochemical properties of the products obtained at different calcination temperature in an alkaline electrolyte（6 M KOH）.（4）Hollow C/NiCo₂S₄ nanosphere composites have been designed and successfully synthesized through a facile strategy by combining the hydrothermal and in situ template removal method,with SiO₂ nanosphere as a template and glucose as a carbon source.As a result,the as-prepared hollow C/NiCo₂S₄ nanosphere electrode exhibites a high specific capacitance of 1545 F g^-1 at 2 A g^-1,and an exceptional cyclic performance retaining90.1%retention after 6000 cycles at 10 A g^-1.Moreover,as the hollow C/NiCo₂S₄nanosphere is employed as a positive electrode for a asymmetric supercapacitor,a satisfactory energy density and good cycle stability are achieved.The superior electrochemical performance for the C/NiCo₂S₄ electrode to the state-of-the-art Ni-Co sulfide electrodes may profit from the undermentioned features:the interconnected NiCo₂S₄ nanosheets generated on the carbon sphere surface can not only act as a reservoir for charge storage,but also offer a high specific surface area to create more active sites for better electrolyte infiltration,easier accessibility transfer of the electrolyte ions and shorter ions diffusion path;the inner void space of nanospheres as an expansion buffer can effectively mitigate the volume expansion,thus resulting in an improved cycling stability;and the designed highly conductive carbon layer facilitates the transportation and diffusion of electrolyte ion,thereby the charge rate of the electrode is enhanced.In addition,the presence of carbon can also hinder the aggregation of NiCo₂S₄ nanosheets during the successive charge-discharge processes,which can enhance the structural stability of the electrode.