DEGRADATIONS AND IMPROVEMENTS IN PEM FUEL CELL MATERIALS: A COMPUTATIONAL STUDY

Abstract

The advantages of Proton Exchange Membrane (PEM) fuel cells include lower operating temperature than other fuel cells and size small enough to fit into a car. Improving the cost and durability of PEM fuel cell materials is a hot topic of research today. The Nafion membrane and cathode catalysts are two areas where PEM fuel cells have issues of cost, durability, and efficiency. In order to improve these materials, researchers need a better understanding of the detailed mechanisms for basic operation and degradation. Computational quantum mechanics has improved in recent years to the point where it can provide accurate potential energy maps of reactions that are difficult to determine by laboratory experiments alone. With the basic understanding of mechanisms, experimentalists can make educated predictions of ways to improve fuel cell materials. Experimental studies suggest that Nafion degradation is caused by generation of trace radical species (such as OH●, H●) when in the presence of H2, O2, and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H2 (or H+) and O2 in the presence of the Pt catalyst. We find that OH● can be generated in trace amounts on the Pt surface from HOOH and OOHad. Next, we look at various ways in which the OH● can attack the Nafion sidechains or endgroups on the backbone. Researchers are looking for ways to replace the Pt cathode catalyst, due to the preciousness of Pt and the low efficiency of the oxygen reduction reaction (ORR) on Pt, among other things. Alloying Pt with non-precious Co greatly increases the ORR efficiency. However, Pt3Co was reported to not withstand long-cycle testing due to the migration of Co metals onto the catalyst surface and leaching of Co into the electrolyte. To overcome these challenges, we first study Pt3Co to find out what makes these alloys so special in improving fuel cell efficiency, as well as what causes degradation to occur. Then, we apply the principles we learned in proposing improved fuel cell alloy catalysts.