Pyrolysis Of Self-assembled Iron Porphyrins As Oxygen Reduction Electrocatalysts

Proton exchange membrane fuel cell(PEMFC)has attracted much attention due to its advantages of high energy conversion rate,low pollution,high reliability and low noise.As a key material in the fuel cells,Pt/C electrocatalysts are commonly used to improve the dynamic of oxygen reduction reaction(ORR)in fuel cells.While,the high price and limited reserve of Pt have greatly hindered the commercialization of fuel cells.It is of great importance to design and synthesis of new non-noble metal electrocatalysts(NNMEs)for the replacement of Ptbased electrocatalysts.In this thesis,series of NNMEs were fabricated by pyrolysis of selfassembled iron porphyrins.The main research conclusions are as follows:(1)Using an inexpensive iron porphyrin(Hemin,abbreviated as Hm)as the precursor,the SA-Hm(950)electrocatalyst was obtained by self-assembly of Hm through first dissolving and then precipitating with subsequent pyrolysis at 950?.In both alkaline and acidic electrolyte,the SA-Hm(950)electrocatalyst displays better ORR performance than that of electrocatalyst by direct pyrolysis of hemin.In the alkaline electrolyte,at an electrocatalyst loading of 0.8mg cm^-2 on the working electrode,the onset potential(E_onset)and half-wave potential(E_1/2)of SAHm(950)electrocatalyst are 0.893 and 0.807V(vs.RHE),which are 11 and 24mV higher than E_onset(0.882V vs.RHE)and E_1/2(0.783V vs.RHE)of the electrocatalyst by direct pyrolysis of hemin,respectively.The main reason for the enhanced activity is that self-assembly technology improves the dispersion of hemin,reduces the size of iron-based particles and effectively inhibits the aggregation of particles,exposing more available active sites.The E_onset and E_1/2 of SA-Hm(950)are 15 and 25 mV lower than those of commercial Pt/C(E_onset=0.908V and E_1/2=0.832V vs.RHE)at the Pt loading of 5 ?gpt cm^-2 on the working electrode,respectively.In the durability test,after 1500 scanning cycles,the current density of commercial Pt/C at 0.8V(vs.RHE)decreases by 52%,while the current density of SA-Hm(950)electrocatalyst at 0.8V(vs.RHE)only decreases by 8.8%.(2)The oxygen reduction electrocatalyst was prepared by self-assembly of hemin on the surface of the commercial carbon black with subsequent pyrolysis at 950?,in which the dispersion of iron based species is effectively improved.In the alkaline electrolyte,at the electrocatalyst loading of 0.2 mg cm^-2 on the working electrode,the SA-Hm/C(950)electrocatalyst displays nearly the same E_1/2(0.875V vs.RHE)with that of commercial Pt/C(E_1/2=0.873V vs.RHE)at the Pt loading of 40 ?gpt cm^-2 on the working electrode.Under the catalysis of SA-30Hm/EC600(950)electrocatalyst,the intermediate HO2-yield is less than 0.7%and the average electron transfer number is 3.99,indicating that the ORR process mainly takes place with a directly four electron transfer.The high density of iron based active species with well dispersion is the main reason for the outstanding performance of SA-30Hm/EC600(950)electrocatalyst.In the durability test,after 2000 scanning cycles,the current density of commercial Pt/C at 0.8V(vs.RHE)reduces by 54%,while the current density of SA30Hm/EC600(950)at 0.8V(vs.RHE)diminishs by 26%.Assemblying SA-30Hm/EC600(950)as the cathode electrocatalyst in the alkaline membrane fuel cell,the power density obtained in the single cell test reaches 259.2 mW cm^-2 with the cathodic electrocatalyst loading of 2.4 mg cm^-2 and the alkaline polymer content of 35 wt%.(3)At room temperature,iron porphyrins with opposite charges(Fe?NMePyP and Fe?TPPS4)were ionic self-assembled into nanoflowers.After pyrolysis at 775 ?,the morphology of nanoflower-like structure was well retained.Molecular dynamics(MD)reveal that charge mismatched dimers are formed with oppositely charged iron porphyrins.The adjacent mismatched dimers interact with each other through ?-?,steric hindrance,van der Waals force and static electricity interactions to obtain twisted molecular chains.Such molecular chains further entangle to form a stable spheroid.It is found that the spheroids hold excellent thermal stability by the MD simulation at high temperature,well explaining the phenomenon that the morphology of nano-flowers are maintained after pyrolysis at high temperature.In 0.1 M KOH solution,at the electrocatalyst loading of 1.2 mg cm^-2 on the working electrode,the E_onset and E_1/2 of nano-flower like Fe?NMePyP/Fe?TPPS4-RT-775 electrocatalyst are 0.926 and 0.842V(vs.RHE),which are 8 and 16 mV lower than the E_onset(0.934V vs.RHE)and E_1/2(0.858V vs.RHE)of commercial Pt/C at the Pt loading of 20?gpt cm^-2 on the working electrode,respectively.In 0.1 M HClO4 solution,the E_onset and E_1/2 of Fe?NMePyP/Fe?TPPS4-RT-775 are 0.875 and 0.792V(vs.RHE),which are 68 and 77mV lower than E_onset(0.943V vs.RHE)and E_1/2(0.869V vs.RHE)of commercial Pt/C at the Pt loading of 10 ?gpt cm^-2 on the working electrode,respectively.In both 0.1M KOH and 0.1M HClO₄ solution,the obtained electrocatalysts exhibit better durability than those of commercial Pt/C.This approach provides an effective synthetic route for design and synthesis of NNMEs with controllable structures.