Study On Synthesis Of Transition Metal Molybdates By Molten Salt Method And Its Electrochemical Performance

The excessive use of traditional energy is a global crisis today.In order to alleviate this phenomenon,sustainable and clean energy has come to be noticed.At the same time,there is a pressing need for compatible energy storage technologies to improve energy efficiency.Among them,supercapacitors have been paid more and more attraction because of their advantages such as high power density,fast charging and discharging speed,and long cycle life.As an important component of supercapacitors,electrode material has a significant influence on the electrochemical performance of supercapacitors.Transition metal molybdates have been used as electrode material in supercapacitors due to their excellent electrochemical properties.Among them,NiMoO₄ is regarded as an ideal electrode material due to its advantages of excellent electrochemical performance,low cost and environmental friendliness.However,it still has the disadvantage of low magnification performance.In addition,now most of NiMoO₄ is synthesized by hydrothermal method which is not suitable for large-scale production.Molten salt synthesis method is a material synthesis method with high product purity,simple operation,low equipment requirements,and it is suitable for large-scale application.Therefore,in this topic,NiMoO₄ material with excellent performance was fabricated by molten salt synthesis method and modified to further improve its electrochemical performance.The specific research are as follows:NiMoO₄ material was prepared by molten salt synthesis method,and KNO₃-Na NO₃ was used as a molten salt reaction system with molar ratio of 1:1.The effects of molar ratio of raw material to molten salt,reaction temperature and reaction time on the electrochemical performance of NiMoO₄ material were studied.Research has shown that the NiMoO₄ nanorods prepared exhibit the most superior electrochemical performance under the conditions of a ratio of 1:30 between raw materials and molten salt,a reaction temperature of 360℃and a reaction time of 6 h.In a three electrode system,using 2 mol L^-1 KOH as the electrolyte,the specific capacitance of NiMoO₄ nanorods can reach 1124.5 F g^-1 at a current density of 1 A g^-1.The assembled NiMoO₄//AC ASC can achieve a specific capacitance of 114.7 F g^-1 at a current density of 0.25 A g^-1,and it can exhibit a maximum energy density of 35.8 Wh kg^-1.Meanwhile,its specific capacitance can be maintained at 85.4%after 4000 cycles of charge and discharge.In order to further improve its electrochemical performance,Co was doped into NiMoO₄to prepare Ni_（1-x）Co_xMo O₄（x=0.2,0.4,0.6,0.8）materials.Among them,the Ni_0.6Co_0.4Mo O₄（NC-2）material prepared at x=0.4 exhibited the best electrochemical performance.In a three electrode system,using 2 mol L^-1 KOH as the electrolyte,the specific capacitance of NC-2 can reach 1481.3 F g^-1 at a current density of 1 A g^-1.The specific capacitance of the assembled NC-2//AC ASC can achieve 186.8 F g^-1 at a current density of0.25 A g^-1.NC-2//AC ASC can exhibit a maximum energy density of 58.4 Wh kg^-1.And its specific capacitance can be maintained at 100.0%after 4000 cycles of charge and discharge.The results indicate that NC-2(Ni_0.4Co_0.6Mo O₄)is a promising electrode material for supercapacitors.Similarly,in order to improve the electrochemical performance of NiMoO₄,NiMoO₄/Zn Mo O₄ composite materials were prepared by molten salt synthesis method.The ratio of Ni/Zn in the raw materials was adjusted to prepare composite materials with different properties.The prepared NZ-2 material exhibited the best electrochemical performance when the molar ratio of Ni to Zn in the raw material is 8:2.In a three electrode system,using 2 mol L^-1 KOH as the electrolyte,the specific capacitance of NZ-2 can reach 1479.8 F g^-1 at a current density of 1 A g^-1.The specific capacitance of the assembled NZ-2//AC ASC can achieve 175.8F g^-1 at a current density of 0.25 A g^-1,and the maximum energy density can achieve 54.9 Wh kg^-1,Moreover,its specific capacitance can be maintained at 87.1%after 4000 cycles of charge and discharge.The results show that NiMoO₄/Zn Mo O₄ composites have great application prospects in the field of supercapacitors.The electrochemical properties of NiMoO₄,NC-2 and NZ-2 were compared.At a current density of 1 A g^-1,the specific capacitance of NiMoO₄ nanorods was 1124.5 F g^-1,while the specific capacitance of NC-2 and NZ-2 increased to 1481.3 F g^-1 and 1479.8 F g^-1,respectively.Among the assembled asymmetric capacitors NiMoO₄//AC,NC-2//AC and NZ-2//AC,the maximum energy densities of NC-2//AC and NZ-2//AC were up to 58.4 Wh kg^-1 and 54.9 Wh kg^-1,both higher than NiMoO₄//AC(35.8 Wh kg^-1).Therefore,the modification of the material significantly optimized the performance of the material and increased its practicality.