Synthesis Of Molybdenum-based Metal Oxides And Their Applications In High Energy Density Supercapacitors

Supercapacitor,also known as electrochemical capacitor,is a new type of energy storage device between traditional capacitors and batteries.It has high power density,short charge and discharge time,long cycle life and wide operating temperature range,maintenance-free and environmentally friendly,which can be applied to smart grids,electric buses,consumer electronics,smart wearable devices,and other fields.However,its lower energy density limits its wider use as an independent power source compared to rechargeable batteries.In order to improve the energy density of supercapacitors,research on transition metal oxides based on pseudocapacitive storage mechanism is a hotspot of supercapacitor electrode materials in recent years.Among them,molybdenum oxides,such as molybdenum oxide and nickel molybdate,are excellent candidates of electrode materials for supercapacitors due to the high theoretical specific capacity,high electrochemical activity,abundant reserves,low cost,and environmental friendliness.In particular,the orthogonal phase molybdenum oxide??-MoO₃?with a two-dimensional layered structure,as a typical intercalation-pseudocapacitive material,has reversible Faradaic reactions not only occurs on the surface but also in the interlayer structure of the bulk without the control of ion diffusion.The rapid internal embedding/extraction of the crystal structure,which the charge storage behavior between the traditional pseudocapacitive behavior and battery behavior,is expected to provide a higher energy density for the device.In this work,molybdenum-based metal oxides were used as research objects to study the methods to improve the conductivity,specific capacitance,and cycling stability,so as to obtain molybdenum-based metal oxide electrode materials with good electrochemical properties,and supercapacitors with high energy density and good mechanical properties.The prime contents and conclusions of the research are as follows:?1?The flexible paper-like MoO₃ nanobelts freestanding film was prepared and its application as electrode material in solid-state symmetric supercapacitor was explored.It was to be found that the flexible membrane exhibited obviously good electrochemical performances,mainly because of the three-dimensional network structure formed only by the overlapping and stacking of nanobelts without binders and additives,which provides large electroactive contact surfaces to make full use of the active materials and additionally relieves the volume expansion in the repeated charge-discharge process.Thereby,the electrochemical performances of the flexible electrode can be improved,so that the assembled device has a higher energy density.?2?The silver quantum dots?Ag QDs?modified MoO₃ nanobelts and manganese oxide?MnO₂?nanowire freestanding films were successfully fabricated,and their applications in solid-state asymmetric supercapacitors were explored.The results showed that these two kinds of flexible membranes and the solid-state asymmetric supercapacitor possessed excellent electrochemical performance mainly owing to the following reasons:?i?a good ohmic contact was formed on the surface between Ag QDs and semiconductors,which produces a high conductive region to reduce the charge transfer resistance and increase the rate of ion diffusion and charge transfer;?ii?in order to overcome the effective barrier caused by the built-in electric field,the composite electrode can undertake a higher potential to increase the areal capacitance of the asymmetric supercapacitor;?iii?the unique chemical and thermal properties of Ag QDs,which is benefit to prevent the agglomeration of nanostructured materials in the three-dimensional network structure,and release the energy changes caused by the volume expansion in the repeated electrolyte ion intercalation/de-intercalation process,thus improving the cycling stability;?iv?the paper-like freestanding film electrodes without binder and conductive additives are beneficial to reduce the internal resistant and fully make use of the active materials to obtain higher areal capacitance.?3?The nickel molybdate?NiMoO₄?modified MoO₃ core-shell nanobelts were presented and successfully prepared,and its application as positive electrode material in supercapacitors was explored.It can be seen that the active electrode material had high specific capacitance and the assembled device has a higher energy density,which can be ascribed to:?i?by coupling two high electrochemical active materials,a high capacitance can be obtained;?ii?MoO₃ nanobelts provide numerous of sites for the direct growth of ultrathin porous NiMoO₄ layer,leading to a large increase of the surface area and abundant open space to enhance the utilization of electrode materials;?iii?the ultrathin mesoporous NiMoO₄ shell supplies a high surface area and abundant open space to act as“ion-buffering reservoirs”,and shortens the ionic diffusion distance,promoting the electrolyte ions transport into the interior MoO₃,relieving the volume expansion in the repeated charge-discharge process,thus increasing the cycling stability.?4?The Ag QDs modified mesoporous NiMoO₄ microspheres were presented and successfully prepared,and its application as anode materials in solid-state asymmetric supercapacitors was discussed.The results showed that the hybrid electrode and the asymmetric device had good electrochemical performances,which can be explained by the following reasons:?i?the abundant mesoporous of the NiMoO₄ microspheres exhibit large specific surface area for ion adsorption,and rich active sites from the inside and outside surfaces of microspheres for Faradic redox reaction,which is beneficial for achieving high specific capacitance of the electrode;?ii?the introduction of Ag QDs on the surfaces of the mesoporous NiMoO₄ microspheres greatly improves the electrical conductivity and chemical stability of the electrode.It benefits to reduce the interfacial resistance between the electrode and electrolyte,and promote the rapid diffusion of electrolyte ions into the interior of the electrode,hence resulting in great rate capability;?iii?the synergistic effect between mesoporous NiMoO₄ microspheres and Ag QDs enhances the structural stability of the composite and the electronic interaction on the local interfaces,which results in the better cycling stability of the material.In this dissertation,molybdenum-based metal oxide electrode materials for supercapacitors with different high specific capacitances were synthesized,and the mechanism of promoting the electrochemical performances of the devices was explored.The silver quantum dots modification,which enhancement mechanism was analyzed in detail,is a simple,effective and universal strategy to improve the electrical conductivity of transition metal oxide electrode materials,which further explores the selection and preparation of advanced electrode materials for high energy density supercapacitors.