Magnetic Annealing of Pt-Alloy Nanostructured Thin Film Catalysts

High field magnetic processing, in which materials are exposed to magnetic fields in excess of 9 Tesla (T), is a powerful technique for modifying microstructure and facilitating phase transformations [1,2].  In this work, the potential of high magnetic field annealing to produce highly active surface structures on Pt-alloy oxygen reduction reaction (ORR) catalysts is explored.  The most active ORR catalysts consist of Pt alloyed with Fe, Co, and Ni, which are also the main ferromagnetic metals [3].  This innovative approach aims to exploit the magnetic properties of these alloys through high magnetic field annealing to modify catalyst surface structures, compositions, and properties under conditions which are both scalable and commercially viable.  3M’s Pt₃Ni₇ nanostructured thin films (NSTF) have been selected as the primary test structure for these experiments.

Oak Ridge National Lab magnetic processing facility houses a horizontal superconducting magnet with an 5” diameter warm bore and uniform 9 T field strength over a 12” work zone that is coupled with a 9 kW commercial-scale induction heating system capable of heating to 2200^oC.  Figure 1 demonstrates the setup for these high magnetic field annealing experiments.  Sections of Pt₃Ni₇ NSTF were heated to 400^oC in either an inert (Ar) or reducing (H₂) environment in the presence of a 9T field.  

The modified structures produced by magnetic annealing are under investigation by rotating disk electrode (RDE), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning transmission electron microscopy (STEM).  Complementary secondary electron and Z-contrast STEM images are presented in Fig. 2 for various annealing conditions.  These same structures will be studied following RDE characterization, as it has been shown that electrode conditioning/break-in in either RDE or in membrane electrode assemblies (MEA) can have significant impact on alloy catalyst morphology and composition [4]. 

The experimental thrust is accompanied by modeling efforts using screened Korringa-Kohn-Rostoker (SKKR) method, a density functional theory method ideal for modeling disordered alloy system.  This fully relativistic, all-electron model will be used to calculate a wide range of physical properties, including d-band shifts, stability of new structures with respect to surface/bulk composition, and magnetic phase diagrams.

Figure 1.  (a) Schematic of magnetic annealing setup.  Photographs of (b) 9T horizontal superconducting magnet, (c) gas delivery system for annealing in H₂ and Ar gases

Figure 2.  (top) Secondary electron and (bottom) Z-contrast STEM images of Pt₃Ni₇ NSTF submitted to different annealing conditions, including high-field magnetic annealing. 

References

1. Z. H. I. Sun, M. Guo, J. Vleugels, O. Van der Biest, B. Blanpain, Curr. Opin. Solid St. M., 16, 254 (2012).
2. Y. Ma, L. Xiao, L. Yan, Chinese Sci. Bull., 51, 2944 (2006).
3. V. R. Stamenkovic et al., Nat. Mater., 6, 241 (2007).
4. B. Han et al., Energy Environ. Sci., 8, 258 (2015).

Acknowledgements

This work was supported by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy under the 2015 Lab Call and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.  Special thanks to A.J. Steinbach and D. van der Vliet of 3M for providing NSTF material and for helpful discussions.