The Design Of Electrodes With Fractal Structure And Their Application For Supercapacitor And Oxygen Evolution Reaction

In order to maintain the economic growth of modern society,it is urgent to find new clean energy and improve the utilization efficiency of primary energy.All kinds of clean energy obtained in nature(eg.Solar,Tidal,Geothermal and Wind power are intermittent)can only be converted into energy with stable output after storage and conversion,so as to match the energy demand of human beings.The research on theory,technology and equipment of novel and efficient electrochemical energy storage and conversion devices has become the focus in current energy development.Among them,Supercapacitor(electrochemical capacitor)and electrocatalytic water splitting are acted as an important technology for electrochemical energy storage and a crucial way of energy conversion,respectively.They are two green and clean energy storage and conversion methods,which considered as significant ways to effectively alleviate energy demand and ecological environment problems.The design,performance and production cost of electrode materials are critical to the effective implementation of energy storage and conversion processes.Therefore,the high performance electrodes were constructed by controlling the fractal growth of substrate and optimizing the structure of electrode materials,which is the key to improve the performance for supercapacitors and electrocatalytic water splitting,and is also one of the important parts in the effective implementation of energy storage and conversion process.Based on the fundamental research in our laboratory and the development trend in the field of energy storage and conversion,we starts from the design of a unique three-dimensional structure self-supporting electrode,and systematically explores the regulation rules of the structure,synthesis conditions and preparation methods of the electrode on the performance of Supercapacitor and Oxygen evolution reaction(OER).This paper mainly research on two aspects:the first part,taking the metal fractal material as the base,Fe₂O₃ and Ni Fe₂O₄ were selected as the anode materials of supercapacitor.The two-dimensional layered structure of materials was constructed mainly by electrodeposition and oxidation method.The morphology,structure and composition of the material were controlled by the preparation temperature,time and oxidation method,and the capacitance performance of different electrodes was investigated.The second part,the dendrite and nickel foam acted as fibrous and flat substrates,respectively,constructing Ni-Fe dendrite,Zn-Fe dendrite and V-Ni₂P as OER catalyst by method of metal doping.We optimize the composite material via reaction concentration,temperature and other conditions,and explore the catalytic activity and stability of different electrode.The dissertation brings about conclusions below:(1)The iron dendrite substrate(FDM)was prepared by controlled electrodeposition method,and then the ferric oxide nanosheets were assembled on the surface of the matrix by the green and mild water vapor oxidation process to prepare the fibrous electrode.The influences of reaction temperature and time on the electrochemical properties of Fe₂O₃@FDM were discussed.The obtained Fe₂O₃@FDM anode manifests high specific capacitances(2166 m F cm^-2 at 1 m A cm^-2).The FDM provide large surface area for Fe₂O₃ nanosheets assembling,afford channels for ion diffusion,and play a supporting role for the material and enhances the stability of the electrode.Further,a ASC is assembled by Ni(OH)₂@Ni dendrite matrix(Ni(OH)₂@NDM)cathode and Fe₂O₃@Fe dendrite matrix(Fe₂O₃@FDM)anode.Single fiber has the ability to start up a clock and also drive a calculator.This part of the work is based on the metal fractal structure to provide a new idea for the development of fiber-shaped Supercapacitor anodes.(2)The Ni-Fe dendritic array matrix was fabricated by the facile electrodeposition process for the first time.Then the metal on the surface of dendrite was converted into Ni Fe₂O₄via in situ air calcination oxidation method.The as-prepared Ni Fe₂O₄@NFM(Ni Fe₂O₄@Ni Fe dendrite)electrode appears the low overpotential of 234 m V(vs.RHE)at 10 m A cm^-2.Furthermore,the Ni Fe₂O₄@NFM-3(prepared at 400?)revealed high area capacitance about 1560 m F cm^-2 at current density of 1m A cm^-2,achieving the construction of bifunctional electrode for oxygen evolution reaction(OER)and energy storage.The unique bi-functional characteristics of the fiber electrode make it possible to be used in energy storage and conversion devices,which lay the research foundation for the application of metal fractal structure in different fields.(3)The fractal structure composite electrode was prepared by controllable electrodeposition.It was also discussed that the transformation law of structure and morphology of metal dendrites during electrodeposition.Nickel and Zinc elements were introduced to form Ni-Fe and Zn-Fe composite dendrites.The element doping is beneficial to improve the electrical conductivity and adjust crystal structure of electrode,and the unique structure of electrode is conducive to the rapid transfer of charge,so the composite dendrites exhibit outstanding OER activity.Ni-Fe dendrite electrode requires overpotential of 248 m V(vs.RHE)(10 m A cm^-2),and the Tafel slope was about 55 m V dec^-1.According to the previous two works,we further improve the regulation mechanism of fractal structure,providing a research foundation for the design of three-dimensional fibrous catalyst electrode.(4)V-Ni₂P nanosheets were assembled on the surface of nickel foam by simple hydrothermal method and phosphorization to obtain the V-Ni₂P/NF electrode.In the process,Ni Foam acts as the nickel source and substrate.V-Ni₂P presented a unique nano-flake structure with a larger contact area and abundant active sites,attributing to the introduction of vanadium in nanosheets,which is beneficial to the adjustment of the electronic structure for the material,the improvement of its catalytic activity and the acceleration of catalytic reaction kinetics.The V-Ni₂P/NF-2 electrode displayed excellent catalytic activity with a low overpotential of 228 m V at 80 m A cm^-2,and the electrode also reveal a small Tafel slope and excellent electrochemical stability.The electrode can also be used as the catalyst for hydrogen evolution reaction under alkaline conditions,indicating that the electrode can be used as a bifunctional electrode to realize the water splitting under alkaline conditions.The main research work of this paper supplies the research foundation for the design and construction of electrodes in the field of energy storage and conversion,and provides a new idea for the development and utilization of materials with fractal structure.