New Approaches to Non-PGN Catalysts through Rational Design

摘要

The catalytic oxygen reduction reaction (ORR) at the cathode is a critical process to proton exchange membrane fuel cell (PEMFC) operation. At present, the cathode catalyst materials of choice are dominated by platinum group metals (PGMs). The high costs and limited reserves of PGMs, however, create a major barrier for large scale commercialization of PEMFCs.To meet U. S. DOE's 2015 target of $30/kW_e for the automotive fuel cell system, a substantial reduction in the electrode catalyst cost is needed. Intensive efforts have been dedicated to the search for non-PGM ORR catalysts which led to several major advancements in this field [1-5]. For example, Bashyam and Zelenay reported ORR activity for a cobalt-polypyrrole composite, which was attributed to Co ligated by pyrrolic nitrogen as the catalytic site [3]. Activation in an inert atmosphere of the similar polymer composite through pyrolysis further improved catalytic activity [4]. More recently, significant enhancement in ORR activity was demonstrated in a carbon-supported iron-based catalysts, and it was suggested that micropores (width < 20 A) have critical influence on the formation of the active site with a structure of cationic ions coordinated by four pyridinic nitrogens [5,6]. These studies identified the importance of the TM-N4 entities as the active centers for the catalytic ORR process. Critical challenges for non-PGM ORR catalysts include their relatively low turn-over-frequencv and ORR onset potential compared to Pt [7]. To compensate low activity without using excessive amounts of catalyst resulting in thick electrode layers, it is desirable to have the highest possible active site density accessible to gas diffusion through a uniformly decorated catalytic framework. The catalyst should have high specific surface area to ensure the maximum exposure of the active site to the gas reactants. It should also have a high fraction of micropore volume which is considered to have direct correlation with the catalytic activity [6]. Furthermore, the non-PGM catalysts should have sufficient stability in the oxidative and acidic operating environments such as that found inside of cathode of PEMFC.