Electrochemical Energy Stoarge Of Transition Metal Molybdate With Modification

The electrode materials is one of the key factors affecting the energy storage performance of supercapacitors.Transition metal molybdate have the advantages of simple preparation,high theoretical capacity and environmental friendliness.However,the electrochemical performance of transition metal molybdate is still far from its ideal value.Therefore,how to further modify the transition metal molybdate,improve its electrochemical performance and make it better used in supercapacitors has great research significance and value.The cobalt monoxide（CoO）is a kind of material with electrochemical activity,high theoretical capacity,good electrical conductivity.It can form nanohybrid materials with transition metal molybdate.By adjusting the synthesis conditions and controlling the morphology and three-dimensional structure of the materials,specific surface area of the materials can be improved,reactive sites can be added,electron transfer path can be optimized,electron and ion transfer can be accelerated,so as to improve the electrochemical energy storage performance of the materials.High temperature hydrogenation and reduction method can produce oxygen vacancy in transition metal molybdate.The oxygen vacancy changes its electron structure.The conduction band of the system moves down,and the defect state appears in the gap between the valence band and the conduction band.The band gap is reduced to reduce the distance from the electron to the conduction band,improve the conductivity of the material,and thereby improve the electrochemical energy storage performance of the material.In this paper,the transition metal molybdate,including nickel molybdate（NiMoO₄）,zinc molybdate（ZnMoO₄）,cobalt molybdate（Co MoO₄）,were modified from different aspects via synthesizing building nanohybrids and introducing oxygen vacancy,so as to improve their electrochemical properties.The specific research works of this paper are as follows:（1）Electroactive CoO nanostars modified NiMoO₄ nanoneedles（NiMoO₄/CoO）and its electrochemical propertiesThe NiMoO₄/CoO nanohybrid was synthesized by two step hydrothermal method on conductive nickel foam matrix.The specific surface area(55 m²g^-1)of the NiMoO₄/CoO nanohybrid is higher than that of the NiMoO₄（25 m² g-1）.The modification of CoO nanostars improves the specific surface area of the NiMoO₄/CoO nanohybrid and contributes to the electrochemical efficiency of the NiMoO₄/CoO nanohybrid.According to the model,modification of CoO nanostars increases the density of reactive sites of the nanohybrid,which is conducive to enhancing electrochemical reactivity.In 6 M KOH electrolyte solution,the specific capacitance of the NiMoO₄/CoO nanocomposite was 2332 F g^-1 when the current density was 2 m A cm^-2,a significant improvement over that of the NiMoO₄(1331 F g^-1).At current density of 20 m A cm^-2,the specific capacitance retention of NiMoO₄ and NiMoO₄/CoO nanocomposites is 75.2%and 98.1%,respectively after 5000 cycles.In addition,NiMoO₄/CoO nanohybrid was used as the positive electrode,exfoliated graphite carbon paper（EGCP）as the negative electrode,and polyvinyl ole-potassium hydroxide（PVA-KOH）as the gel electrolyte to construct the asymmetric all-solid supercapacitor NiMoO₄/CoO//EGCP.NiMoO₄/CoO//EGCP asymmetric supercapacitors have an output voltage of 1.5 V.When the power density is 750 W kg^-1,the energy density is 71.4 Wh kg^-1.When the current density is 5 A g^-1,the specific capacitance retention remains at 83.5%after 5000 cycles.（2）Electroactive CoO nanorods modified ZnMoO₄ nanosheets（ZnMoO₄/CoO）and its electrochemical propertiesThe ZnMoO₄/CoO nanohybrid was synthesized by two step hydrothermal method on conductive nickel foam matrix.Density functional theory calculations show that the ZnMoO₄/CoO nanocomposite has a smaller band gap（0.007 e V）than ZnMoO₄（3.019e V）.The ZnMoO₄/CoO nanohybrid shows more electronic state density distribution near the Fermi energy level.CoO nanoribbons and ZnMoO₄ nanosheets are intertwined,which could reduce interface transfer resistance,improve the interfacial properties of the nanohybrid and accelerate electron transfer and ion transfer rates,thus improving its electrochemical performance.In 6 M KOH electrolyte solution,the specific capacitance of the ZnMoO₄/CoO nanocomposite was 1941 F g^-1 when the current density was 2 m A cm^-2,a significant improvement over that of the ZnMoO₄(716F g^-1).At current density of 20 m A cm^-2,the specific capacitance retention of ZnMoO₄and ZnMoO₄/CoO nanocomposites is 62.5%and 100.5%,respectively after 5000cycles.In addition,ZnMoO₄/CoO nanohybrid was used as the positive electrode,EGCP as the negative electrode,and PVA-KOH as the gel electrolyte to construct the asymmetric all-solid supercapacitor ZnMoO₄/CoO//EGCP.ZnMoO₄/CoO//EGCP asymmetric supercapacitors have an output voltage of 1.6 V.When the power density is 800 W kg^-1,the energy density is 58.6 Wh kg^-1.When the current density is 5 A g^-1,the specific capacitance retention remains at 81.8%after 5000 cycles.（3）Oxygen vacancy modified ZnMoO₄ porous nanosheet（ZnMoO₄-OV）and its electrochemical propertiesZnMoO₄-OV porous sheet containing oxygen vacancy was prepared by hydrothermal and hydrogenation reduction on conductive nickel foam matrix.Compared with ZnMoO₄ nanosheet,ZnMoO₄-OV porous nanosheet has higher specific surface area and more diverse pore diameter distribution.Density functional theory calculation results show that the band gap of ZnMoO₄-OV（1.92 e V）is smaller than that of ZnMoO₄（3.019 e V）.The electron state density distribution shows that ZnMoO₄-OV has more electron state distribution than ZnMoO₄ near the Fermi energy level.Oxygen vacancy can increase the adsorption capacity of the electrode materials toOH^-in electrolyte solution,and participate in the electrochemical reaction as the active site,thus enhancing the redox reactivity of ZnMoO₄.In 6 M KOH electrolyte solution,when the current density is 2 m A cm^-2,the specific capacitance of ZnMoO₄ and ZnMoO₄-OV is 797 F g^-1 and 1674 g^-1,respectively.When the current density is 20 m A cm^-2,the specific capacitance retention rate of ZnMoO₄ and ZnMoO₄-OV is 68.3%and 97.7%after 5000 cycles,respectively.In addition,ZnMoO₄-OV was used as the positive electrode,AC as the negative electrode,and PVA-KOH as the gel electrolyte to construct the asymmetric all-solid supercapacitor ZnMoO₄-OV//AC.ZnMoO₄-OV//AC asymmetric supercapacitors have an output voltage of 1.6 V.When the power density is 800 W kg^-1,the energy density is 60.1 Wh kg^-1.When the current density is 5 A g^-1,the specific capacitance retention remains at 87.4%after 5000 cycles.（4）Oxygen vacancy modified Co MoO₄ porous nanosheet（Co MoO₄-OV）and its electrochemical propertiesCo MoO₄-OV porous sheet containing oxygen vacancy was prepared by hydrothermal and hydrogenation reduction on conductive nickel foam matrix.Compared with Co MoO₄ nanosheet,Co MoO₄-OV porous nanosheet has higher specific surface area and more diverse pore diameter distribution.Density functional theory calculation results show that the band gap of Co MoO₄-OV（0.041 e V）is smaller than that of Co MoO₄（1.683 e V）.The electron state density distribution shows that Co MoO₄-OV has more electron state distribution than Co MoO₄ near the Fermi energy level.Introducing oxygen vacancy can increase carrier density,accelerate electron transfer and improve conductivity,thus enhancing redoxreactivity of Co MoO₄-OV.In 6 M KOH electrolyte solution,when the current density is 2 m A cm^-2,the specific capacitance of Co MoO₄ and Co MoO₄-OV is 917 F g^-1 and 1989 g^-1,respectively.When the current density is 20 m A cm^-2,the specific capacitance retention rate of Co MoO₄ and Co MoO₄-OV is 92.5%and 98.8%after 5000 cycles,respectively.In addition,Co MoO₄-OV was used as the positive electrode,AC as the negative electrode,and PVA-KOH as the gel electrolyte to construct the asymmetric all-solid supercapacitor Co MoO₄-OV//AC.Co MoO₄-OV//AC asymmetric supercapacitors have an output voltage of 1.6 V.When the power density is 800 W kg^-1,the energy density is 62.3 Wh kg^-1.When the current density is 5 A g^-1,the specific capacitance retention remains at 91%after 5000 cycles.