Study On Electrocatalysts For Oxygen Reduction Reaction Based On Pyrolyzed Metal-organic Frameworks

Polymer electrolyte membrance fuel cells (PEMFCs) have attracted great attention due to their advantages such as high power density, low operating temperature and low emission, and have potential applications in automotive and portable power sources. As an important part in fuel cell, the performances of oxygen reduction reaction at the cathode have great influence on electrochemical reaction rate, overpotential of electrode reaction, and battery voltage. The carbon black supporting platinum-based nanoparticles (NPs) are widely used as the catalyst for ORR. However, the widespread use of PEMFCs is still hindered by several factors, including high cost resulting from progressive increase of Pt price, poor durability of cathodic electrocatalysts, and methanol crossover from anode to cathode through the proton exchange membranes.Using metal-organic framework as precursors(MOF), cubic Co3O4 composites with the carbon nanotube, nitrogen-doped mesoporous graphitic carbons materials and nitrogen-doped nanopolyhedrons mesoporous carbon with Pt NPs embedded in framework were obtained by heat treatment MOF in air atmosphere or nitrogen atmosphere. The morphology,structure and the electrocatalytic properties of the catalysts have been studied in detail. The main points of this thesis are as follows:(1) Pt NPs were successfully inlaid into the matrix of ZIF-8 by continuously synchronous adsorption Pt NPs during the process of coordination growth of zeolitic imidazolate framework-8 (ZIF-8). Then, nitrogen doped nanopolyhedrons carbons-encapsulated Pt NPs (Pt NPs@NMCs) have been prepared successfully by direct carbonization of ZIF-8-encapsulated Pt NPs (Pt NPs@ZIF-8). The obtained Pt NPs/NMCs hybrids were characterized by SEM?TEM?XRD?XPS and Raman. The results show that the Pt NPs@NMCs hybrids possess high surface area (1226 m2 g-1), abundant mesopores with narrow pore-size distribution (centered at 3.9 nm), nitrogen doping (5.13 At.%), and small and well-dispersed Pt NPs (3.7 nm) encapsulated in NMCs. Furthermore, the as-prepared Pt NPs@ NMCs catalyst exhibits high electrocatalytic activity, high stability and excellent methanol-tolerance.(2) Nitrogen-doped porous graphitic carbons (NMGCs) have been synthesized by a simple carbonization process with a cobalt-containing ZIF-67 as self-sacrificing template and the carbon and nitrogen sources. Cobalt can act as a catalyst to induce carbon graphitization. The morphology and structure of NMGCs have been characterized by XRD, TEM, SEM, Raman, and nitrogen adsorption isotherm. The results show that NMGCs not only retain the morphology of the parent ZIF-61, but also possess rich nitrogen(6.14 wt.%) and high surface area (576 m2 g-1) with uniform size(300 nm). Furthermore, the result showe that NMGCs are highly graphitized. The promising potential of NMGCs as metal-free electrocatalysts for oxygen reduction reactions in 0.1 M KOH is demonstrated. Compared with the commercial Pt/C, the optimized NMGCs (carbonized at 800? for 5 h) catalyst exhibits comparable electrocatalytic activity via an efficient four-electron-dominant ORR process coupled with superior methanol tolerance as well as cycling stability in alkaline media.(3) By direct oxidation of ZIF-67 and ZIF-67/Ao-CNT in air atmosphere, cubic Co3O4 and composites cubic Co3O4 with the acide oxidized carbon nanotube (Co3O4/Ao-CNT) have been prepared. SEM?XRD?TGA were used to characterize the catalysts. After combining with acide oxidized carbon nanotube, the electrochemical impedance of Co3O4 was obviously reduced. The electrocatalysts properties of Co3O4/Ao-CNT was improved. In spite of the onset potention was slightly negative than the Pt/C catalyst, Co3O4/AO-CNT catalyst exhibits higher stability and more excellent methanol tolerance.