A Study of Platinum Alloy Fuel Cell Catalyst Degradation in Aqueous and Membrane Electrode Assembly Environments Using in Situ Anomalous Small-Angle X-Ray Scattering

Numerous ex-situ and in-situ spectroscopic and microscopic techniques have aided in advancing research efforts aimed at improving the oxygen reduction reaction activity and durability of the platinum-based cathode electrocatalysts for polymer electrolyte fuel cells. One such technique is small angle X-ray scattering (SAXS), which can be used to determine, in situ, mean particle size and particle size distributions (PSDs) on the nanometer scale.¹ When scattering patterns are acquired at multiple energies near the absorption edge of the element of interest, a technique termed anomalous SAXS (ASAXS), the PSD of just this element can be resolved.²

 In this study, we are using ASAXS to determine PSDs of nanoparticle platinum alloy electrocatalysts while under potential control in aqueous and membrane-electrode assembly (MEA) environments. The information from this technique is correlated with electrochemically-active surface area (ECA) losses to investigate the mechanisms of catalyst degradation.  Our studies have focused on the effects of environment (i.e., MEA or aqueous) and potential profile on the extent and mechanisms of particle growth and corresponding ECA loss.  The catalyst studied was 40 wt% Pt₃Co nanoparticles supported on an Akzo Nobel Ketjen EC300J carbon black support.  The as-prepared Pt₃Co catalyst had a mean particle size of ~5.6 nm as found through transmission electron microscopy (TEM) analysis. 

 As an example of the results that will be discussed, we show a comparison of the ASAXS results for the Pt₃Co catalyst for two potential profiles in the aqueous environment.  ASAXS patterns were taken as a function of triangle wave cycling between 0.6 and 1.0 V at 50 mV/s and square wave cycling between 0.4 and 1.05 V (20 s/cycle).  Fitting of the ASAXS patterns in the ~0.02 to ~0.3 Å^-1 Q range resulted in evolution of the PSDs, shown in Figure 1, for both cycling protocols. Figure 2 shows the change in the mean particle diameter as a function of the number of potential cycles for both the triangle and square wave protocols. After 1,500 potential cycles the mean diameter of the Pt₃Co catalyst increased 0.35 nm for the triangle wave case and 0.45 nm for the square wave case. The triangle wave potential cycling experiment also showed lower losses in the geometric surface area (GSA), which has been shown to be directly related to ECA,³ and mass as calculated from the PSDs. The triangle wave experiment resulted in a GSA loss of 6% and mass loss of 7% over the first 1,500 potential cycles. Meanwhile, the square wave experiment resulted in a GSA loss of 9% and a mass loss of 8%. This increased degradation for the square wave cycling is similar to previously reported results showing increased loss of ECA with increasing anodic sweep rate.⁴

 These results along with the results from potential cycling experiments performed in the MEA environment will be compared and analyzed in order to investigate Pt₃Co catalyst degradation mechanisms in both environments.

Acknowledgements: The authors acknowledge funding from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program (Nancy Garland, Technology Development Manager) and the U.S. Department of Energy, Office of Basic Energy Sciences for support of the Advanced Photon Source.

References

1. T. Narayanan, Soft Matter: Scattering, Imaging, and Manipulation. 1st ed.; Springer: Berlin, 2007.
2. C. Yu, S. Koh, J. Leisch, M.T. Toney, and P. Strasser, 2008, Faraday Discuss., 140, 283.
3. E. F. Holby, W. C. Sheng, Y. Shao-Horn, D. Morgan, 2009, Energy Environ. Sci. 2, 865.
4. M. Uchimura, S. Kocha, 2007, ECS Trans, 11, 1215.