Investigate The Energy Storage Performance Of Binary Transition Metal Oxide Based Supercapacitors

Because of its high power density,fast charge/discharge and long cycling life, supercapacitors have become new energy storage devices,which can be used in many fields,such as electric vehicles.Binary transition metal oxides as electrode materials for supercapacitors have also received widespread attention in recent years.They possess a variety of electrochemically active/inactive ions,various oxidation states,but also have better electrical conductivity and more active sites.However,the synthesis of binary transition metal oxides with large energy density,high loading,high rate performance and long cycling life is still challenging.These problems severely limit the development of binary transition metal oxides and are not conducive to their further wide application.Therefore,new methods need to be explored to improve its performance.In this thesis,carbon nanotubes are combined with different binary transition metal oxides,and the binary transition metal oxides are systematically regulated by doping and vulcanization,respectively.The optimal growth parameters are obtained by combining various characterization methods,and the mechanism of performance enhancement is analyzed.Finally,the binary transition metal oxide electrode materials with high specific capacitance and high-performance supercapacitors are obtained.The main research contents and results are as follows:（1）Synthesis of manganese doped nickel molybdate（MNMO）nanostructures as positive electrode were by hydrothermal method.Firstly,carbon nanotubes（CNTs）were grown on the carbon cloth（CC）by chemical vapor deposition（CVD）,and then different proportions of MNMO nanostructures were synthesized by hydrothermal method.The manganese/nickel molar ratio（i.e.Mn:Ni=0:1,0.1:1,0.2:1,1:1,5:1）was systematically regulated to obtain the optimum doping ratio of 0.2:1 under best electrochemical performance.The optimized samples were used as electrode materials.The specific capacitance at 1 A g^-1 can reach 1262.6 F g^-1,which is much larger than the undoped nickel molybdate electrode(1 A g^-1,618.55 F g^-1)and has long cycling life（after 14,000 cycles,the specific capacitance still remains 74.4%）.The improvement in energy storage performance is attributed to the fact that manganese doping can not only provide large surface area and good conductivity（favorable for fast electron transfer,electrolyte ion diffusion and shortening ion diffusion path）,but also can utilize abundant redox reactions of Ni²⁺and Mn²⁺to promote their synergy and improve the electrochemical performance of materials.The asymmetric supercapacitor is assembled with MNMO-0.2:1 as positive electrode,carbon cloth/carbon nanotubes@active carbon（CC/CNTs@AC）as negative electrode.At a power density of 864.5 W kg^-1,the energy density of the CC/CNTs@MNMO//CC/CNTs@AC device is 64.95 Wh kg^-1 and has long cycling life,after 5000 cycles(at 20 A g^-1),the specific capacitance maintains 77%.（2）Synthesis of sulfided iron molybdate（Fe-Mo-S）nanostructures as negative electrode were by hydrothermal method.The CC/CNTs substrate was first synthesized by CVD,and then the Fe-Mo-S nanostructures were prepared by hydrothermal method on the CC/CNTs substrate.By studying the electrochemical performance of the materials at different vulcanization amounts(MoO₄^2-:S^2-molar ratio of 1:0,1:0.33,1:0.67,1:1.33,1:2.67 in the precursor solution)to acquire the optimum vulcanization amount.The Fe-Mo-S-3 electrode was obtained at the optimal amount of vulcanization(MoO₄^2-:S^2-=1:0.67),and its active material can be uniformly coated on the surface of CNTs.Its specific capacitance can reach 632.34 F g^-1 at 2 A g^-1;and the specific capacitance still retains 414.98 F g^-11 at 50 A g^-1 with better rate performance.The main contribution of vulcanization is that the ion diffusion ability of electrode materials can be enhanced and the ion diffusion resistance can be reduced under the appropriate amount of vulcanization.The uniform coated nanosheets formed can also enhance the electron transport and improve the charge collection ability.It can also promote the redox reaction and enhance the redox peak,thus enhancing the specific capacitance.With a power density of 1699.83 W Kg^-1,the energy density of CC/CNTs@Fe-Mo-S//CC/CNTs@MNMO device can reach 62.29 Wh Kg^-1,and the cycling life is long,after 5000 cycles(20 A g^-1),the specific capacitance maintains38.28%.（3）Synthesis of nickel-doped iron molybdate（NFMO）nanostructures as negative electrode were by hydrothermal method.The CC/CNTs substrate was first prepared by CVD;Secondly,using hydrothermal method to prepare the NFMO nanostructures on the CC/CNTs substrate.Through regulating the molar ratio of nickel and iron elements（Ni:Fe=0:1,0.1:1,0.25:1,1:1,4:1,1:0）,synthesized a series of samples and their properties and performance was characterized separately to obtain the optimal doping ratio sample（NFMO-0.25:1）.The optimized sample was used as electrode material with a specific capacitance of 795.97 F g^-1 at 1 A g^-1.Suitable nickel element doping can reduce the charge adsorption energy storage behavior,enhance the diffusion behavior,and also contribute to ion transport,while reducing the charge transfer resistance and increasing the surface area.The synergistic effect between elements also enriches the redox reaction,which can also improve the electrochemical performance.The asymmetric device（CC/CNTs@NFMO//CC/CNTs@MNMO）is assembled with NFMO-0.25:1 as the negative electrode and CC/CNTs@MNMO as the positive electrode.It has a high specific capacitance of 205.40 F g^-1 at 1 A g^-1.At a power density of 849.91 W kg^-1,the energy density of CC/CNTs@NFMO//CC/CNTs@MNMO devices is as high as 82.44 Wh kg^-1;has long cycling life,after 1000 cycles(10 A g^-1),the specific capacitance of the device remains87.06%.In summary,the electrochemical properties of the binary transition metal oxide molybdate materials were improved by doping and vulcanization,and the specific capacitance of MNMO positive electrode,Fe-Mo-S negative electrode and NFMO negative electrode can reach 1262.6,632.34 and 795.97 F g^-1,respectively.Moreover,the supercapacitor with energy density up to 82.44 Wh kg^-1 was obtained based on the optimal positive and negative electrode matching assembly.The results of this thesis will provide some reference value for the research and development of binary transition metal oxide-based supercapacitors.