Preparation And Properties Of Self-standing M-N/CFs Oxygen Reduction Catalysts Derived From Metal Chelating Fibers

Oxygen reduction reaction（ORR）is one of the key reactions in the new generation of energy conversion devices（such as metal-air cells,fuel cells,etc.）.The reaction has the problems of slow kinetic process and complex reaction process,which affect the conversion efficiency of the whole energy device.At present,the catalysts used for oxygen reduction reaction are mainly platinum-based catalysts,but they have the problems of low reserves,high prices,poor durability and easy poisoning.Therefore,efficient,stable and cheap non-noble metal catalysts are needed to promote the commercialization of these energy conversion devices.Transition metal-nitrogen/carbon materials（M-C/N）are considered as the most promising oxygen reduction catalysts because of their excellent catalytic activity,excellent stability and anti-poisoning.The ORR performance of M-N/C catalysts is mainly determined by the intrinsic activity and the density of exposed catalytic sites.The density of active sites can be improved through morphological and structural engineering.During the pyrolysis of metal chelating fibers,the external organic chelate ring cladding layer can prevent the agglomeration of metal species,realize the uniform distribution of active substances,and then expose more M-Nx active sites,thus improving the ORR electrocatalytic performance of the material.In this thesis,metal chelating fibrous membrane precursor was prepared by electrospinning and chemically modified adsorption,and then a self-standing M-N/CFs oxygen reduction catalyst was synthesized by pyrolysis.The effects of the microstructure,chemical composition and metal types of the M-N/CFs on the electrocatalytic activity of ORR was investigated.The M-N/CFs catalysts were used as an air cathode catalysts in zinc-air batteries.The main research contents and conclusions are as follows:（1）Polyacrylonitrile/cellulose acetate（PAN/CA）composite fibrous membranes were prepared by electrospinning,and amidoximated polyacrylonitrile/regenerated cellulose（AOPAN/RC）chelating fibrous membranes were obtained through chemical modification.The chelating fibrous membranes were pyrolyzed at high temperature after adsorbing iron ions to obtain a self-standing iron-nitrogen/carbon（Fe-N/CFs）catalyst.The influence of iron content and pyrolysis temperature on the structure and performance of the catalyst was explored:when the concentration of the iron solution was controlled to be 0.05M and the pyrolysis temperature was 900℃,the initial potential,half-wave potential and limit diffusion current density of the prepared Fe-N/CFs catalyst in alkaline medium are 1.01V,0.866 V and 5.37 mA cm^-2,respectively.It also shows good catalytic performance and stability in acidic and neutral media.When applied in an assembled Zn-air battery,Fe-N/CFs delivers a peak power density of 134 mW cm^-2.The excellent ORR catalytic activity of Fe-N/CFs and the excellent performance in zinc-air batteries can be attributed to the large pore structure of the carbon fiber interlaced network that facilitates mass transfer,the microporous/mesoporous surface contains abundant active sites,and the synergy between the active sites of N doping on carbon fibers,Fe-N_x and graphite carbon coated Fe₃O₄nanoparticles.（2）Different M-N/CFs catalysts were prepared with other transition metals（Co,Ni,Mn）,and the effects of transition metals on the structure and performance of M-N/CFs catalysts were discussed.It is found that different metals exist in different forms in M-N/CFs catalysts,Co and Ni mainly exist in the form of simple substances,while Mnexists in the form of MnO.The oxygen reduction activity of M-N/CFs catalysts containing transition metal is better than N/CFs,and Co-N/CFs has the best catalytic performance,which is comparable to commercial Pt/C.The peak power density of zinc-air battery assembled with Co-N/CFs is 104mWcm^-2,and it exhibits excellent rate performance and stability.