Oxygen reduction catalysts based on alloys of Pt with base metals such as Fe, Co, and Ni have demonstrated high performance in ex situ aqueous studies and in situ fuel cell studies, but excessive rates of base metal leaching continue to hinder deployment of these catalysts. Recently, ordered intermetallic structures have been demonstrated to be less susceptible to base metal leaching. 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]. These tests have been confirmed with good durability in the fuel cell environment [2].

Several factors can affect base metal stability, including chemical nature of the base metal, degree of ordering, particle size, and Pt skin formation. By carefully tuning these parameters, extremely high durability can be achieved (Fig. 1 Top), with diffraction data demonstrating that high degrees of ordering can be maintained even after the DOE accelerated stress test (30,000 trapezoidal cycles between 0.6 and 0.95 V with 0.5 s rise time) (Fig. 1 Bottom). Ongoing work seeks to focus on optimizing the surface structure of fct-FePt and fct-CoPt intermetallic nanoparticle catalysts such that a synergistic effect between the surface and core can be established, i.e., a stable Pt surface that protects the fct-core that is thin enough to benefit from the geometric and electronic effects from the core.

Fig. 1 Top. Fuel cell polarization behavior of fct-CoPt/C before and after the 30,000 cycle AST. H₂/air, 100%/20% RH, 80°C, 150 kPa_abs, 0.1 mgPt/cm².

Fig. 1 Bottom. X-ray diffraction of fct-CoPt/C before and after the 30,000 cycle AST.

References

1. Q. Li et al., "New Approach to Fully Ordered fct-FePt Nanoparticles for Much Enhanced Electrocatalysis in Acid," Nano Lett. 2015, 15, 2468−2473.
2. J. Li et al., “Fe Stabilization by Intermetallic L10-FePt and Pt Catalysis Enhancement in L10-FePt/Pt Nanoparticles for Efficient Oxygen Reduction Reaction in Fuel Cells,” JACS 2018, 140, 2926−2932.

Acknowledgements

This research is supported by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy.