Preparation And Properties Of Cobalt-based Self-supported Oxygen Diffusion Electrodes

With the development of economy and society,fossil energy is being consumed continuously,environmental pollution,shortage of resources and other problems followed.It is a great challenge for human society to develop efficient and clean new energy and energy conversion devices.Oxygen electrode reaction includes oxygen reduction reaction and oxygen evolution reaction,which are important parts of a series of energy conversion devices.However,oxygen electrode reactions have a relatively slow reaction kinetics,so it needs efficient electrocatalyst.However,commercial oxygen electrode catalysts are extremely dependent on precious metals（Pt,Ir,Ru,etc.）with high cost and scarce reserves,which limits their large-scale application and commercialization.Therefore,the development of non-noble metal oxygen electrode catalysts with high activity and low cost has important theoretical significance and practical application value.Although after the tireless efforts of many researchers,non-noble metal catalysts in the catalytic activity had gained obvious ascension,but most studies had focused on the preparation of powder catalyst,which need to join in the practical application of adhesive to assemble catalytic layer,this inevitably buried active center,limits its further application.The in-situ growth of catalytic materials on conductive substrates and the construction of self-supported electrodes can not only make the substrate and catalytic materials more closely combined,but also improve the utilization efficiency of catalyst.Therefore,the design and development of high activity and low cost self-supported oxygen diffusion electrode has important theoretical significance and application value.In this dissertation,a three-dimensional self-supported oxygen diffusion electrode was prepared by electrochemical deposition on a conductive substrate based on cobalt.The micromorphology and chemical structure of the self-supported electrode were synthesized by means of control.Furthermore,the performance of Co-based oxygen diffusion electrode was studied by means of multi-level evaluation,and the performance enhancement of Co-based oxygen diffusion electrode was realized.The specific research results are as follows:Firstly,a catalytic layer composed of Co S/Co O nanosheets was grown in situ on a conductive substrate by electrodeposition.Different kinds of self-supported electrodes were prepared by adjusting electrodeposition conditions,post-treatment methods and substrate regulation,and their oxygen reduction performance was evaluated.With the increase of the number of deposition cycles,the load of the catalyst increases,and the oxygen reduction reaction（ORR）activity of the self-supported electrode increases gradually.In addition,different protective atmosphere and activation temperature were used to heat the electrodeposited self-supporting electrode.The results show that under the protection of argon,with the increase of activation temperature,the nanosheets gradually disappear,and Co gradually aggregates to form granular structure.The high temperature calcination under the protection of argon-hydrogen mixture（10%hydrogen+90%argon）can effectively avoid the agglomeration of Co nanoparticles and obtain better ORR activity.In order to further improve the ORR performance of the synthesized self-supported electrode,the metal was coated by one-step electrodeposition of polypyrrole followed by one-step pyrolysis to form a catalytic electrode with multiple components acting synergically.During the pyrolysis process,polypyrrole can not only inhibit metal agglomeration,but also form abundant Co-N-C active centers.In addition,the high electrical conductivity and high specific surface area of polypyrrole also promote the overall electron transfer and active site dispersion of the electrode.The ORR activity of the self-supported electrode was significantly improved under the coupling of several factors.Secondly,the bimetallic hydroxide（Co Fe-LDH）nanosheet structure was grown in situ on the carbon nanotube film by electrodeposition method.The nanosheet grew vertically on the carbon nanotube film to form a self-supporting electrode.Cofe-LDH self-supported electrodes with different morphologies and catalytic activities were prepared by regulating the metal molar ratio of precursor,electrodeposition potential and electrodeposition time.Appropriate proportion of bimetal can prevent the agglomeration of LDH from electrochemical deposition,thus improving its catalytic activity.When Co content is 0.75,the maximum double layer capacitance of the electrode is 25.3 m F cm^–2,and the overpotential is only 285 m V at 10 m A cm^–2 in 1 M KOH solution.When the deposition potential is–1.1 V vs.Ag/Ag Cl,it has the best OER performance,not only has a smaller overpotential at high current density,but also has a Tafel slope of only 47.6 m V Dec^–1,further demonstrating its faster reaction kinetics,which is superior to commercial iridium dioxide catalyst.And Co0.75Fe0.25-LDH-1.1 V has good stability and durability.The high catalytic activity was attributed to the synergistic effect of Co and Fe bimetals and the structure of three-dimensional nano-plate with rich electrochemical active area.