Design, Preparation And Performance Of Iron Porphyrin-based Electrocatalysts For Oxygen Reduction

Polymer electrolyte membrane fuel cells(PEMFCs)are highly efficient energy conversion devices with the advantages of high energy density,low operating temperature,fast start-up and zero pollution.Oxygen reduction reaction(ORR)largely governs the overall performance of PEMFCs.Commercial Pt/C has long been employed as the state-of-the-art electrocatalyst for ORR.The scarcity and high price of Pt,however,have restrained the broad application of PEMFCs.Thus,it is crucial to substitute commercial Pt/C with non-precious metal electrocatalysts.The ORR electrocatalysts based on metalloporphyrins are promising to replace Pt.In this dissertation,three kinds of metalloporphyrin-based non-precious metal electrocatalysts were synthesized.The main conclusions are as follows:(1)A series of hybrid electrocatalysts toward ORR were synthesized by ionic selfassembly of positively charged Fe?meso-tetra(N-methyl-4-pyridyl)porphyrin(FeNMePyP)with negatively charged H₃PMo₁₂O₄0(PMo₁₂)in ethanol solution under ambient conditions in the presence of suspended multi-wall carbon nanotubes(MWCNTs).Self-assembled FeNMePyP/PMo₁₂ was well dispersed on MWCNTs with a tunable loading.Interestingly,the hybrid electrocatalysts demonstrated a much higher ORR activity(half-wave potential,E_1/2?0.55V vs.RHE)than individual FeNMePyP and PMo₁₂ in acidic media.It is speculated that the excellent proton conduction ability of PMo₁₂ promotes O₂ to combine with protons more easily at the FeNMePyP active site,which enhances ORR activity of FeNMePyP.(2)By controlling pH(2-4.6),positively charged aniline monomers can be adsorbed on the negatively charged surface of SiO₂ nanospheres through electrostatic interactions.After insitu polymerization of aniline monomers,polyaniline(PANI)coated SiO₂(PANI@SiO₂)composite was formed.Then,Fe?meso-tetra(4-methoxyphenyl)porphyrin(FeTMPP)was deposited on the surface of PANI@SiO₂.After pyrolysis and removal of the template,FeTMPP-modified porous PANI-based electrocatalysts were synthesized.Remarkably,the resultant 40%FeTMPP/PANI-18-700 electrocatalyst demonstrated a high ORR activity,in terms of an E_1/2 of 0.843 V(vs.RHE)in alkaline medium,which is comparable to that of commercial Pt/C(0.878V vs.RHE).The improvement of the ORR activity likely originates from the abundant pore structure(18 nm average pore diameter,pore volume of 1.1cm³ g^-1),large surface area(687.5m² g^-1),and high N content(6.4 N%).Only 25mV degradation of the E_1/2 was observed for 40%FeTMPP/PANI-18-700 during accelerated durability test,in contrast to a 74mV negative shift of the E_1/2 for commercial Pt/C.Additionally,a hydroxide exchange membrane fuel cell fabricated with 40%FeTMPP/PANI-18-700 as the cathode approached a peak power density of 42mW cm^-2.(3)A series of capsule-like PCN-222 metal organic frameworks(MOFs)with tunable length and width of 75 to 1200nm and 50 to 450nm were synthesized via the self-assembly of Fe? meso-tetra(4-carboxyphenyl)porphyrin(FeTCPP)and benzoic acid with Zr?.Interestingly,at 700 ?,the PCN-222 nanocapsules all evolved into uniform carambola-like electrocatalysts.The nanocarambola electrocatalysts exhibited efficient ORR activity in both acidic(E_1/2 of 0.788V vs.RHE)and alkaline(E_1/2 of 0.873V vs.RHE)medium.As evidenced by X-ray absorption fine structure(XAFS)spectra,aberration-corrected high-angle annular dark-field scanning transmission electron microscope(HAADF-STEM)and 57Fe M?ssbauer spectroscopy,the excellent ORR activity likely originates from the atomically dispersed highspin(O/N)2-Fe?-N4 active sites.In addition,the nanocarambola electrocatalysts also exhibited an efficient durability with only 53 and 18mV decrease of the E_1/2 after 2500 cycles of accelerate durability test in acidic and alkaline medium,respectively(the decay of commercial Pt/C electrocatalyst is 32 and 74mV in acidic and alkaline medium).A proton exchange membrane fuel cell fabricated with nanocarambola electrocatalysts as the cathode approached a peak power density of 164mW cm^-2.