Studies On Polypyrrole Modified Carbon-supported Catalysts

Proton exchange membrane fuel cells (PEMFCs) have attracted much attention due to their high efficiency and environmental friendliness. However, commercialization of PEMFCs suffers from their high cost caused by the use of platinum as cathode catalyst. The development of non-platinum catalysts is one of the critical issues in the recent research of PEMFCs. This work has studied the use of polypyrrole-modified carbon supported non-platinum composites as the oxygen reduction reaction catalysts; discussed their catalytic mechanism and tested their performance in fuel cells.Polypyrrole-modified carbon supported transition metal based composites (M-PPy/C) were prepared by a chemical method using pyrrole, carbon and Fe (or Co) nitrate as the raw materials. The surface composition, micro structure and electrochemical performance were characterized by powder X-ray diffraction, X-ray photoemission spectroscopy, field emission scanning electron microscopy, as well as electrochemical analysis methods such as cyclic voltammetry. The key factor that influences the catalytic activity has been identified, and a possible structure of the active site has been suggested in this work. This may help, theoretically and practically, to further understand the catalytic mechanism in this kind of material, and to develop novel non-platinum catalysts with high activity and durability.The cyclic structure of pyrrole has retained during chemical polymerization. And based on the XPS data, when used as a dopant to modify the carbon supports, PPy will not change the chemical state of carbon atoms in the carbon support, as well as the electrochemical properties of the carbon material. Carbon support plays an important role in the catalytic activity of PPy-modified carbon supported Fe based composite. The fuel cell using BP2000as the catalyst support demonstrates a highest power density of375mW cm-2when operated at60℃, which makes BP2000an ideal carbon support in the catalyst.The transition metal also plays a critical role in the catalytic activity of PPy-modified carbon supported transition metal based composite. Compared to Fe based material. Co based composite demonstrates a higher catalytic activity. By studying the influence of different components on the activity of the catalyst, it can be concluded that carbon material is a necessary composite to form the active site. It is believed that the active site has a triple structure which consists of carbon support, N-containing materials and transition metal. The interaction between Co and N must have an effect on the electronic configuration of N atom, which further influence the chemical state of the C atom close to the N atom. And that change in the C atom gives a promotion of O2adsorption and electrochemical reduction activity. Based on the study above, a series of fuel cell test has been performed using the synthesized composites as cathode materials. And the results reveals that, PPy modified carbon supported Co-based composite shows similar performance and stability compared to Pt/C catalyst. After studying the polarization curves before and after the lifetime test, it can be concluded that the anode polarization is also a critical problem that needs to be considered. By improving the catalyst structure of the anode, the fuel cell shows little degradation after1000h, when operated at65℃and a discharge current density of40mA cm-2.