| Fuel cell is a kind of novel green energy with broad prospect,which has advantages of high energy conversion efficiency,less pollution and easily controlled design scale.Proton exchange membrane fuel cell(PEMFC)is a mature technology that has been studied extensively,and is applicable to many fields.But the use of noble metal platinum as catalyst greatly increases costs and limits the systematically deploying of this technology on a large scale.At present,high efficiency,long life and low cost non-noble metal electrocatalysts for the oxygen reduction reaction are very important and active research field.Recently,pyrolytic carbon loaded or unloaded transition metal organic/inorganic complexes,conductive polymer loaded transition metal catalyst,nitrogen atom-coordinated transition metal catalyst and carbon materials catalyst such as heteroatom-doped carbon materials(carbon nanotube or needles and graphene)have received much concern due to excellent catalytic performance and high stability.With strong design ability,dopamine can polymerize on almost all of the material surface by oxidation under alkaline conditions,which can react secondary,coordinate or electroless deposit with metal ions,and produce carbon based materials with high electrical conductivity via high temperature pyrolysis.Hence,in this thesis,environmentally friendly and non-toxic dopamine was selected as precursor,and by molecular design,heteroatom-doped carbon based multi-element composite materials were constructed.A variety of preparation methods were adopted to fabricate carbon based multi-element composite catalysts and non-noble metal carbon catalysts with different structures,morphologies and sizes to efficiently and stably catalyze oxygen reduction reaction.Meanwhile,the effects of some factors,such as the effect of chemical composition,microscopic morphology and size,and the chemical state of surface atoms of the catalyst on its electro-catalytic property were studied,as well the bonding sites were determined and the catalytic mechanism was proposed.The following research activities are investigated in this thesis:1.The reaction kinetics of polydopamine and its electrochemical behaviour were studied first,and the composition and structure of the polymer film were analyzed from the viewpoint of electrochemistry.And then the polydopamine film was deposited on the surface of multi-wall carbon nanotubes,and through the secondary reactions,multi-heteroatom-doped carbon nanotubes catalyst was obtained after high temperature pyrolysis.The film thickness and the kind and number of heteroatoms were adjusted by controlled the reaction conditions to change surface chemical composition and combinations to improve its catalytic activity.The results show that the electrocatalytic acticity of carbon-nitrogen-metal is much better than that of carbon nanotube.And the oxygen reduction potential for N,S,F doped carbon nanotubes is close to commercialized 20%Pt/C,and is much better than that of carbon nanotubes.The oxygen reduction reaction contains four electronic selectivity,and within 30000 s,its electrochemical stability is better that Pt/C catalyst,indicating that this doped carbon nanotubes catalyst has good oxygen reduction catalyst performance.2.Candle soot(CS)collected through the combustionof candle is constituent of onion like carbon nanoparticles,hereinto a large number of sp2 hybrid graphite carbon are present which result in high electrical conductivity.A series of doped transition metal/CS catalysts were fabricated by ultrasonic equal volume impregnation method,and the results of electrochemical oxygen reduction performance tests reveal that the order of catalytic properties for different metals was Fe>Co>Ni.Besides,the effects of postreatment method on the specific surface area,pore structure,particle size,conductivity,and functional groups on CS surface were also studied.When the CS was treated by carbon dioxide,its specific surface area can reach as high as 949.9 m2 g-1,and its conductivity is also satisfying.2 mol%Fe/CS catalysts with different carriers were prepared by different treatment methods,and their electrochemical performances of oxygen reduction were compared.The effect of Fe loading quantity on the oxygen reduction performance of C03 treated CS was studied and the results show that the catalytic performance increased with the Fe content increasing.When the Fe loading quantity is as much as 10 mol percentage,the Fe/CS catalyst owns the best catalytic activity.The results obtain from BET,XPS,XRD,TEM and SEM also demonstrate that the catalytic activity of catalyst is determined simultaneously by active component distribution state,carrier intrinsic properties,metal oxide particles size and the number of active site.Based on the above research,dopamine polymerizations,deposits and coordinates with on CS carrier with high specific area via a one step reaction.After high temperature pyrolysis,N and metal co-doped C-N-M structure was obtained,and the deposition conditions,N doping content,metal type,and metal particle size were judged to determine their effect on oxygen reduction reaction performance of composite catalysts.3.Graphene oxide(GO)were prepared according to Hummers method with some modifications,and the physical and chemical properties of the as-prepared GO were characterized by many experimental methods.Results show that the GO dispersed in solution in a single atomic layer structure,and its surface exists a large number of oxygen-containing groups.The dopamine polymerized and deposited on GO surface to achieve intercalating modification.Then transition element Mn was doped,and after high temperature pyrolysis,C-N-Mn was obtained.By controlling the polymerization conditions and chelating metal concentration,the atomic ratio,microtopography of catalyst and its chemical properties can be optimized,and then improve its conductivity and electrocatalysis oxygen reduction performance.The electrochemical results show that this catalyst has a high oxygen reduction catalytic activity and four electronic selectivity. |
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