Study On Electrocatalysts Of Oxygen Reduction Reaction Based On Mesoporous Carbon Materials

Polymer electrolyte membrance fuel cells (PEMFCs) as a kind of fuel cells have attracted great attention due to their high power density, low operating temperature, low emission and potential applications in automotive and portable power sources. Cathodic oxygen reduction catalysts, as the key component of PEMFC still show poor long-term stability and relatively slow dynamics process in oxygen reduction reaction (ORR), though the research of PEMFC has made great progress. These become the major barriers for PEMFCs commercialization.In this paper, on the basis of the excellent properties of mesoporous carbon materials, composites of platinum and mesoporous carbon, nitrogen doped hollow carbon microspheres@platinum nanoparticles (HNCMS@Pt NPs) and Pt/high-level porous carbon (HPC) (Pt/HPC) composites, have been synthesized. The catalytic performances of HNCMS@Pt NPs and Pt/HPC composites for ORR were investigated in acid media. In addition, we have synthesized the phosphorus and nitrogen co-doped mesoporous carbon materials (PNDCS) by using zeolite imidazolate framework-8 (ZIF-8) as precursors and studied its oxygen reduction performance in alkaline condition. The details of this thesis are as follows:(1) Pt NPs were firstly loaded on the surface of NH2-functionalized SiO2 microspheres (Pt NPs/SiO2). Then, the Pt NPs/SiO2 hybrids were wrapped by polydomine (PDA) film. By direct carbonization of PDA-wrapped Pt NPs/SiO2 hybrids in nitrogen atmosphere and further treatment in hydrofluoric acid solution, Pt NPs embedded within nitrogen-doped hollow carbon microsphere (HNCMS) were obtained and labeled as HNCMS@Pt NPs. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, specific surface area analyzer and X-ray photoelectron spectroscopy were used to characterize the morphology,structure and compositeof the HNCMS@Pt NPs hybrids. The electrochemical properties of the HNCMS@Pt NPs hybrids for oxygen reduction reaction have also been investigated by cyclic voltammetry and linear sweep voltammetry. The results show that the Pt loading mass in HNCMS@Pt NPs hybrids is up to 11.9 wt.%. Furthermore, the as-prepared HNCMS@Pt NPs catalyst exhibits good electrocatalytic activity, high stability and excellent methanol-tolerant property towards oxygen reduction reaction.(2) High-level porous carbon materials (HPC) were obtained by a high temperature carbonization of Al-PCP and Pt NPs were deposited on HPC uniformly via microwave assisted reduction (Pt/HPC). The morphology, structure and pore size distribution of the Pt/HPC catalyst have been characterized by SEM, TEM, Raman, XRD and BET, respectively. The results show that the Pt NPs with small particle are dispersed on HPC. Furthermore, CV, LSV and a standard accelerated durability testing (ADT) technology were used to examine the oxygen reduction performances of the Pt/HPC composite material in 0.5 M H2SO4 solution. The onset potential of Pt/HPC catalyst for oxygen reduction is more positive than commercial E-TEK Pt/C and the limiting current density is 1.6 times of that of the commercial E-TEK Pt/C. A small negative shift of the LSV curve occurs on the Pt/HPC catalyst and the half-wave potential of ORR on the Pt/HPC catalyst shifts negatively about 11 mV after ADT. However, for the commercial E-TEK Pt/C catalyst, a big negative shift of the LSV curve occurs after ADT and the half-wave potential shifts negatively about 52 mV. These results demonstrate that Pt/HPC catalyst has excellent properties and long-term stability for ORR.(3) Nitrogen doped mesoporous carbon materials (NDCS) were prepared by high-temperature carbonization of zeolite imidazolate framework-8 (ZIF-8), then NDCS was treated with ethylene diphosphonic acid, further carbonized to obtain phosphorus and nitrogen co-doped mesoporous carbon materials (PNDCS). SEM, Raman, XRD and BET were used to characterize the morphology and structure of the PNDCS catalyst. The electrocatalytic properties of the PNDCS catalyst for ORR also have been investigated through CV, LSV and I-t in 0.1 M KOH. The results show that the PNDCS material is typical rhombic dodecahedron morphology and the size is about 300 nm; the onset potential of the PNDCS catalyst for oxygen reduction is close to that of commercial E-TEK Pt/C and the limiting current density is higher than that of the commercial E-TEK Pt/C. Compared to the commercial E-TEK Pt/C, PNDCS catalyst has more excellent long-term stability and methanol-tolerant property.