Study On The Synthesis Of Electrocatalysts For Oxygen Reduction Reaction By Using Template To Control The Self-assembly And Pyrolysis Of Metalloporphyrin

Polymer electrolyte membrane fuel cells(PEMFCs)are highly efficient and clean energy conversion devices with the advantages of high energy density,low operating temperature,fast start-up and sustainable power generation.Due to the slow ORR kinetics of cathodic oxygen reduction of PEMFCs,a large number of Pt group precious metal-based(PGM)electrocatalysts are needed to improve the reaction rate.The high price and low reserves of PGM have limited the development of PEMFCs.In order to solve this problem,there are mainly two strategies,one is to reduce the consumption of PGM,and the other is to use non-PGM materials to completely get rid of its dependence.The main research direction of ORR electrocatalysts is to develop non-noble metal electrocatalysts(NPMEs)with high ORR activity and durability.This thesis carries out the following research work:(1)The solubility-induced self-assembly of FeCPP construction unit on the carbon template(BP2000)was developed in this work.Owing to the deprotonation and protonation of the carboxyl group,the solubility of FeCPP could be adjusted.Then,the FeCPP precipitated out of the solution and self-assembled on the template BP2000.After the optimization of the pyrolysis temperature and the loading of FeCPP,a series of electrocatalysts were prepared after pyrolysis under argon atmosphere.The ORR performance is evaluated with the half-wave potential(E1/2)of 0.88 V(vs.RHE)and 0.79 V(vs.RHE)in alkaline and acidic solution,respectively.The ORR activity is dramatically enhanced in comparation with the pyrolyzed and mechanically mixed FeCPP and BP2000,and the E1/2 of the self-assembled electrocatalysts were increased by 80 mV and 100 mV in alkaline and acidic solution,respectively.This is probably ascribed to the well dispersion of FeCPP on the template,thus leading to more exposed Fe-based active centers.According to the SCN-poisoning,the ORR active sites in acidic solution are mainly Fe-N-C species.(2)Two kinds of porphyrins of FeNMePyP and CoTPPS4 with opposite charges were used to ionically self-assemble to form the nanowire networks(NWNWs)structure,and then the self-assembled NWNWs were dispersed on template(EC600).By adjusting the loading of ionically self-assembled NWNWs assembled on carbon,the interaction between double porphyrins assemblies and carbon was regulated,making the ionically self-assembled NWNWs evenly dispersed on the template.The formation of metal-based nanoparticles was avoided,which are not conducive to the ORR activity and stability.As evidenced by high-angle annular dark field scanning transmission electron microscopy and 57Fe Mossbauer spectroscopy,the synthesized electrocatalyst contains only atomically dispersed Fe and Co sites.The electrostatic interaction between iron porphyrin and cobalt porphyrin,as well as the strong interaction between self-assembled porphyrin nanostructures and the template should be the probable reasons for the formation of atomic-scale Fe/Co-N-C electrocatalyst.In alkaline electrolyte,the electrocatalyst shows comparable ORR activity,better stability and anti-methanol toxicity as the commercial Pt/C.The half-wave potentials of the synthesized catalyst and commercial Pt/C are 0.87 V(vs.RHE)and 0.88 V(vs.RHE),respectively.After the 2500 cycles of accelerated durability test(ADT),the half-wave potentials of the them degrade 17 mV and 74 mV,respectively.According to the RRDE experiment,the ORR mechanism of the synthesized electrocatalyst is an indirect 4e-route.(3)The self-assembled hemin@ZIF-8 hybrid precursor was obtained via the hydrogen bond and coordination interaction between hemin,2-methylimidazole,and Zn(?).The hemin molecules directed the formation of ZIF-8,meanwhile,the hemin molecules were in-situ encapsulated into the nanocages of ZIF-8.The obtained hybrid precursor was heat-treated to form the atomically dispersed Fe-N-C electrocatalyst with high Fe content.The resultant NPME exhibits an excellent ORR activity with a E1/2 of 0.83 V(vs.RHE)in acidic electrolyte,which is only 30 mV less than that of the commercial Pt/C and higher than most of the reported NPMEs.According to SCN-poisoning,atomic-resolution transmission electron microscope transmission electron microscopy,X-ray absorption fine spectroscopy,and 57Fe Mossbauer spectroscopy,the excellent ORR activity with a direct 4e-ORR mechanism in acidic solution likely originates from densely distributed Fe-N-C active sites,corresponding to a total Fe weight ratio of 4.5%that is the highest among all of atomically dispersed Fe-based NPMEs.The E1/2 of our NPME decreases 20 mV after 3000 cycles of ADT in acidic solution.The existence of individual Fe sites in the nanopores should strengthen the carbonized ZIF-8 framework with 84%retention of microporous area.This robust structure should be beneficial to the excellent durability of our NPME.