(Invited) In Situ Small-Angle X-Ray Scattering for the Analysis of Electrochemical Degradation of Metal Oxide Supported Pt Nanoparticles

In current research on oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cell (PEFC) cathodes, metal oxide supported Pt nanoparticles are considered to be one promising alternative to the standard carbon supported Pt, which offers the possibility of higher durability in the oxidizing environment at the PEFC cathode [1]. Detailed studies of the stability of Pt/metal oxide catalysts require the determination of the structure and surface area of the Pt nanoparticles during electrochemical durability testing. In situsmall-angle X-ray scattering (SAXS) offers the unique opportunity not only to monitor the evolution of the Pt nanoparticle size distribution in an electrochemical environment, but also to observe the evolution of the total Pt mass loading of the electrode at the same time. Furthermore, information about the dispersion/agglomeration of the Pt nanoparticles on the support surface can be inferred. Thus, the overall observed degradation of the electrochemically active surface area (ECSA) can be further analyzed in terms of different degradation phenomena like Pt loss due to dissolution, Pt nanoparticle growth due to electrochemical Ostwald ripening, and Pt nanoparticle migration and agglomeration.

We have performed electrochemical in situ anomalous SAXS experiments, for the first time, on metal oxide supported Pt nanoparticles. The support consisted of a mixed iridium oxide-titanium oxide (IrO₂-TiO₂). The anomalous changes of the elemental scattering cross sections around the Pt L_III and the Ir L_III absorption edges were used to separate the Pt scattering contribution from the support scattering. This approach of anomalous SAXS was previously used for in situ studies of conventional carbon supported Pt [2,3]. Our results not only show that it is possible in this way to obtain a precise net Pt scattering signal even in the presence of a strongly scattering support material, but they also demonstrate clearly the existence of a previously neglected scattering interference effect due to the spatial correlations between Pt nanoparticles and support particles. This effect can become very strong for support materials containing high-Z elements like the IrO₂-TiO₂ support investigated in this study. However, we could also show that these particle-support interferences are non-negligible even for weakly scattering carbon supports.  We developed a novel analytical tool to take the particle-support interferences into account in the data fitting procedure with high accuracy [4]. This approach highly improves the quantitative analytical power of SAXS studies on supported catalyst materials.

We applied the novel method to the analysis of in situ SAXS data from Pt/IrO₂-TiO₂ to study the degradation properties of this ORR catalyst during harsh electrochemical potential cycling between 0.5 V and 1.5 V vs. RHE in 0.1 M HClO₄. This protocol simulates the oxidative conditions during PEFC start/stop cycles [5]. The results not only reveal a Pt particle growth due to electrochemical Ostwald ripening and a concomitant Pt mass loss due to dissolution, but also indicate a possible influence of the X-ray beam on the catalyst degradation. Hence, on the one hand, the X-ray exposure time of the catalyst must be chosen long enough in order to achieve a good signal-to-noise ratio, but on the other hand, it must be short enough to prevent X-ray induced catalyst degradation. In summary, our findings pave the way to a detailed and quantitative in situ analysis of Pt nanoparticle degradation on various metal oxide support materials.

Acknowledgments

This work was supported by CCEM Switzerland and Umicore AG & Co KG within the project DuraCat. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland, for provision of synchrotron radiation beamtime at the cSAXS beamline of the SLS.

References

[1] A. Rabis, P. Rodriguez, and T.J. Schmidt, ACS Catal. 2, 864-890 (2012)

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[3] J.A. Gilbert, N.N. Kariuki, R. Subbaraman, A.J. Kropf, M.C. Smith, E.F. Holby, D. Morgan, and D.J. Myers, J. Am. Chem. Soc. 134, 14823 (2012)

[4] T. Binninger, M. Garganourakis, J. Han, A. Patru, E. Fabbri, O. Sereda, R. Kötz, A. Menzel, and T.J. Schmidt, submitted to Phys. Rev. Lett. (2014)

[5] H. Tang, Z. Qi, M. Ramani, and J.F. Elter, J. Power Sources 158, 1306–1312 (2006)