Fuel Cell Testing of Intermetallic Oxygen Reduction Catalysts

Abstract

Platinum-based alloy catalysts for the oxygen reduction reaction (ORR) continue to be the industry standard for fuel cell applications, but improvements are needed to increase ORR activity and durability. Despite extensive study, the effects of structure and composition on real world performance and durability in fuel cells are poorly understood. Most research to date has examined random alloy structures, but preliminary results from intermetallic catalysts, especially intermetallic tetragonal structured catalysts, have indicated that superior performance and durability can be achieved through creation of ordered intermetallic structures. Results so far indicate that the degree of ordering is a critical parameter in determining intermetallic catalyst activity and durability, with fully-ordered face-centered tetragonal (fct) FePt catalysts exhibiting substantially improved activity and durability compared to partially ordered phases [1]. Most work on intermetallic nanocatalysts reported so far has been based on ex situ testing methods that use an idealized aqueous electrochemistry environment. While these ex situmethods are inexpensive and convenient, they do not reliably predict performance and durability in real fuel cells. Therefore, rigorous catalyst testing in fuel cells is needed to build understanding of the relationship between catalyst properties and fuel cell performance and durability. In this work, fct-FePt intermetallic nanoparticle catalysts have been subjected to the DOE catalyst AST (30,000 trapezoidal cycles between 0.6 and 0.95 V with 0.5 s rise time) in operating fuel cells. Comparison of characterization results from before and after testing serves to illustrate the changes in structure and composition that occur during the AST. Initial results depicted in Fig. 1 indicate that fct-FePt can have performance comparable to a baseline commercial Pt/C, despite the use of much larger particles (8-9 nm for fct-FePt/C vs. 2-3 nm for Pt/C). Notably, the fct-FePt exhibits lower loss of performance following the 30,000 cycle AST. Ongoing work seeks to reduce the fct-FePt particle size while maintaining fully-ordered structures, which could lead to substantial improvements in performance and durability. Fig. 1. Fuel cell polarization behavior of Pt/C and fct-FePt/C before and after the 30,000 cycle AST. References Q. Li et al., "New Approach to Fully Ordered fct-FePt Nanoparticles for Much Enhanced Electrocatalysis in Acid," Nano Lett. 2015, 15, 2468−2473. Acknowledgements This research is sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy. Figure 1