Synthesis And Evaluation Of PGM-free Catalysts For Proton Exchange Membrane Fuel Cell Cathodes

Proton exchange membrane fuel cell (PEMFC) is a promising technology which can be used in transportation and other fields, due to its high zero emission,high power density and high efficiency. Oxygen reduction reaction (ORR), the kinetically sluggish cathode reaction, is a more important research topic in PEMFC that requires more attention and efforts. So far, Pt and Pt-based alloys are the most active catalysts for ORR. However, the high cost and scarcity of Pt limit the application of PEMFC. To overcome this, it is of great significance to develop highly active Platinum group metal-free (PGM-free) catalysts for ORR. Among all kinds of PGM-free catalysts, transition metal-nitrogen-carbon (M-N-C) catalysts obtained by pyrolysis of transition metals, nitrogen, and carbon precursors have been viewed as the promising PGM-free catalysts for PEMFC cathodes. However, they have not yet met all performance characteristics required for real application in terms of activity and durability. To develop active PGM-free ORR catalysts, the research in this thesis was carried out as follows:(1) Based on the proposal that FeNx is the most possible active site for Fe-N-C type catalyst. We hoped to obtain a Fe-N-C catalyst through synthesis of precursors with high FeN4 coordination structure ratio. 11,11'-bis(dipyrido[3,2-a:2',3'-c]phenazinyl) (bidppz) was selected as a ligand for the formation of a nitrogen-rich iron-coordinated coordination polymer (Fe-bidppz) which forms a self-supporting catalyst containing high densities of nitrogen and iron doping by pyrolysis. The catalyst pyrolyzed at 800 ? (Fe-N/C-800) shows the highest ORR activity in both alkaline and acidic electrolytes. The optimal Fe-N/C-800 catalyst displays much greater durability and tolerance of methanol than Pt/C. Fe-N/C-800 catalyst has a considerably high density of surface active sites due to its high ORR activity achieved with low specific surface area. Electrochemical results show that ORR on Fe-N/C-800 catalyst follows the effective 4e pathway.(2) Based on previous reports suggesting that M-N-C catalysts containing two different metals show better activity than that containing only single metal. We synthesized a bimetallic (Fe,Mo)-N/C catalyst based on our previous work through a simple ion exchange process. The random ion-exchange occurs in the network of the Mo-bidppz precursor, leading to a good distribution of iron in the catalyst, the presence of Mo prevents Fe from aggregation, and enhances the electric conductivity of the carbon support as well. Rotating disk electrode (RDE) experiments verify that the ORR on optimized (Fe,Mo)-N/C catalyst follows a 4e pathway. FeNx and MoNx moieties are possible active sites in (Fe,Mo)-N/C, and FeNx moieties probably act as the major active site.(3) Besides the strategy to increase active site density to enhance the catalytic activity of catalysts, tuning the surface area and porosity of a catalyst is thus an more straightforward way to enhance the ORR activity of M-N-C type ORR catalysts,since high surface area is of great significance to obtain more accessible active sites and good mass transport property. We used a zinc salt (zinc chloride) as a pore-forming agent responsible for the formation of a highly microporous catalyst. The resulting (CM+PANI)-Fe-C(Zn) catalyst had higher surface area more porosity than the (CM+PANI)-Fe-C catalyst obtained using just cyanamide as pore-former under the same conditions. Excellent activity towards the ORR in PEMFC, 75 mA/cm2 at 0.8 V cell voltage in PEMFC were achieved. In order to improve the performance of the mass transport region of the (CM+PANI)-Fe-C(Zn) cathode in the H2-air PEMFC,the porosity of the catalyst layer and the composition of the catalyst layer, including different ionomer types (different equivalent weight) and contents in catalyst layers were systematically optimized. The optimized catalyst layer exhibited improved kinetic region performance in PEMFC and much better mass transport region performance. The optimized MEA shows excellent performance in H2-O2 fuel cell,reaching the 2018 target of DOE.