| The proton exchange membrane fuel cells?PEMFCs?is a kind of new energy technologies with environmental friendly,high energy density and conversion efficiency,inicio rapido,and extensive fuel source,and is considered as an ideal power source in the future.In order to improve sluggish oxygen reduction reaction kinetics process occuring at the cathode,a large number of precious metal platinum catalysts are needed to improve the reaction rate.However,Due to the high price and few reserves of the Platinum,the high cost from a large number of platinum catalysts using as cathode catalysts impedes the large-scale commercialisation of PEMFCs.In recent years,developing a carbon based oxygen reduction catalyst with low cost,high activity and stability for replacing the traditional Pt based catalyst has become a hot topic in the field of fuel cell electrocatalysis.In this thesis,we designed and prepared a series of high-performance Fe-Nx/C oxygen reduction catalysts by employing co-doping,organic iron sources introduced by the self-assembly,chemical etching-immersion method,etc.The research results in the study are mainly included as follows:?1?We have designed and prepared a series of Fe in-situ doping ZIF-8 polyhedrons with different iron content and particle size by doping Iron acetylacetonate during self-assembly of ZIF-8 with 2-methylimidazole and zinc nitrate.Subsequently,Fe and N codoped carbon polyhedron catalysts were prepared by high temperature pyrolysis.This kind of catalyst has the characteristics of ZIF-8 original polyhedron morphology,high dispersion iron,high content pyridine nitrogen and graphite nitrogen,hierarchical porous structure,and high specific surface area,etc.We found that the introduction of organic iron can not only adjust the particle size of ZIF-8,but also catalyst's specific surface area,porous structure,total nitrogen content and the composition ratio of various nitrogen species.When mole ratio of Fe/Zn is 0.84,the catalyst exhibited the optimal oxygen reduction activity:in 0.1 M KOH,the catalyst had a current density at 0.70 V 1.26 times higher than that of a commercial Pt/C catalyst;in 0.1 M HClO4,the half-wave potential of our catalyst reach 0.77 V and is comparable to that of commercial Pt/C.At the same time,the catalyst has 4 electronic dominance ORR reaction in both alkaline and acid medium,and has a better stability than commercial Pt/C catalyst.?2?By using a two dimensional structural g-C3N4 as a template and nitrogen source,iron-tannins as the carbon source and iron source,a highly active iron carbide anchoring graphene-like constructing porous carbon nano-networks oxygen reduction catalyst were prepared by high temperature pyrolysis.The optimum catalyst exhibit a superior oxygen reduction?ORR?activity and durability under the 0.1 M KOH with a half-wave potential?0.86 V vs.RHE?40 m V more positive than that of a commercial Pt/C catalyst.And the oxygen reduction process almost completely followed the 4 electron mechanism.By contrast experiment,we found that the introduction of g-C3N4 as template agent and nitrogen source in the precursors could promote the formation of three-dimensional porous structure and Fe-Nx activity sites;The introduction of tannic acid could inhibit the formation of larger particles of iron species,and improve specific surface area of catalysts and promote the formation of carbon defects,meanwhile could tune the nitrogen content and the ratio of active nitrogen species in the catalyst.?3?The hollow ZIF-8 polyhedron is first synthesized by a tannic acid chemical etching process,followed by impregnating ferric acetylacetonate and physicslly mixing with g-C3N4,well defined Fe–N co-doped hollow carbon nanopolyhedra catalyst with atomiclly dispersive Fe–Nx active sites have been prepared by high-temperature pyrolysis.Electrochemical tests showed that the resulting optimal catalyst exhibited a half-wave potential 30 mV more positive than that of commercial Pt/C catalyst in alkaline medium and a half-wave potential of0.78 V comparable to that of commercial Pt/C catalysts in acidic medium;the as-prepared hollow nanopolyhedra exhibits superior stability both in acidic and alkaline mediums,and high catalytic efficiency?nearly 100%selectivity for the four-electron ORR process?.EXAFS indicates that iron and nitrogen are bonded into Fe-N in the optimal sample C-FeHZ8@g-C3N4-950.Except that high ORR performance both under acidic and alkaline mediums in the RDE test,in the H2-O2 PEMFC test,the membrane electrode assembly?MEA?with the optimal Fe-N-C hollow nanopolyhedra catalysts as cathode catalyst contained a current density of 750 m A·cm-2@0.6 V and 400 mA·cm-2@0.7 V for the PEMFC,and the maximum output power is 628 mW·cm-2.We found that the introduction of g-C3N4 could not only adjust nitrogen content and ratio of active nitrogen species but also suppress formation of iron species particles;finally,it promotes formation of atomiclly Fe-Nx activity sites.?4?We developed Fe/N co-doped porous carbon polyhedron with atomiclly Fe-Nx active sites with ferrocene as the organic iron source and ZIF-8 as the precursor,follow by pyrolysis.The optimum catalyst not only exhibits excellent oxygen reduction catalytic performance but also high stability in alkaline and acid medium:in an alkaline medium,half wave potential is50 mV higher than commercial Pt/C catalyst;in an acidic medium,a half-wave potential of0.78 V is comparable to that of commercial Pt/C catalysts.In addition,oxygen reduction reaction was carried out by four electron mechanism in both medium.Furthermore,in the H2-O2 PEMFC test,the membrane electrode assembly?MEA?with the optimal catalyst C-FeZIF-1.44-950 as the cathode catalyst contained a current density of 1100 mA·cm-2@0.6V and 637 mA·cm-2@0.7 V for the PEMFC,and the maximum output power yielded 775mW·cm-2.We found that the introduction of ferrocene could not only promote the formation of atomiclly Fe-Nx activity and pyridine nitrogen and graphite nitrogen in the catalyst but also tune the pore structure of the carbon polyhedron. |
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