An electrochemical three-electrode flow-cell is presented for in situ small-angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS) experiments in transmission mode at synchrotron X-ray sources. The cell also allows for in situ XAS performed in fluorescence mode. Constant experimental conditions, even under moderate gas evolution, are provided by the electrolyte flow with controlled gas saturation. A special configuration of working and counter electrode, respectively, yields low residual ohmic resistance in three-electrode measurements that enables the study of thick porous electrodes of active high surface area materials. The cell proved its functionality and reliability in two studies: First, an in situ anomalous SAXS experiment for the high-potential degradation properties of a Pt/IrO 2 -TiO 2 catalyst for the oxygen reduction reaction at polymer electrolyte fuel cell cathodes; and second, an in situ XAS study of the electronic state of Ir centers inside an IrO 2 -TiO 2 catalyst under oxygen evolution conditions. © The Author Modern research in electrocatalysis makes extensive use of in situ X-ray techniques that provide information about the structure and the electronic state of catalyst materials under electrochemical potential control. The reason for this is the limited, merely indirect information about the state of the catalyst that can be deduced from purely electrochemical testing like cyclic voltammetry (CV) which often does not allow for an unambiguous interpretation of the data. In order to develop an understanding at a more fundamental level, additional information is required about the potential-dependent state of electrocatalyst materials that can be provided by synchrotron-based techniques like X-ray scattering or X-ray absorption spectroscopy.One example is the investigation of polymer electrolyte fuel cell (PEFC) Pt cathode catalyst degradation. Different mechanisms have been proposed for the loss of electrochemically active Pt surface area (ECSA) that occurs most severely at transient high-potential spikes during PEFC start and stop.1,2 Processes like agglomeration of primary Pt particles due to migration or carbon support corrosion, Pt loss due to dissolution, and growth of primary Pt nanoparticles due to dissolution/redeposition cycles have been considered 3,4 and quantified for different operation conditions and electrochemical environments. The most common technique applied for this purpose is transmission electron microscopy (TEM), which has the convenient advantage that changes of the Pt nanoparticle structure can be directly visualized, especially with the use of identical location TEM (IL-TEM).5 Although successfully demonstrated, 6 in situ TEM remains limited to certain electrochemical systems. Whereas the strength of TEM lies in the direct imaging of individual catalyst particles, it is challenging to extract quantitative statistical information about the entire catalyst sample from TEM analysis. Finally, the distinction of Pt nanoparticles from the support material p...

Small-angle x-ray scattering (SAXS) is a powerful technique for the investigation of catalyst materials at the nanoscale. We present results of an anomalous SAXS study on metal-oxide-supported platinum particles used as electrocatalysts for oxygen reduction. The scattering interferences between catalyst particles and support material are taken into account qualitatively and quantitatively by a mathematical model for the data-fitting procedure. Our results clearly demonstrate the fundamental importance of these catalyst-particle-support-material interferences in the analysis of SAXS data from supported catalysts.

A unique electrochemical three-electrode flow-cell design is presented for in situ small-angle X-ray scattering experiments on a multi-purpose laboratory X-ray diffractometer. An electrolyte layer thickness of 2 mm enables sufficient photon transmission to acquire in situ scattering curves at high signal-to-noise ratio within less than one hour despite the restricted photon flux from a standard Cu X-ray tube. Complete tightness of the cell allows electrolyte flow with controlled gas saturation in order to guarantee constant experimental conditions even for long experimental protocols. Good electrochemical performance is achieved by a special arrangement of working and counter electrodes that are deposited on the opposing X-ray transmission windows of the cell. The functionality and reliability of the cell are demonstrated in an in situ small-angle X-ray scattering study of the degradation properties of carbon-supported Pt nanoparticles during electrochemical high-potential cycling. Careful subtraction of background scattering and absolute normalization of the scattering curves yield absolute quantitative structural information about the Pt nanoparticle phase at different stages of the degradation protocol, bringing insights into the real-time evolution during electrochemical characterization. The need for high-efficiency and non-polluting energy conversion technologies for automotive applications has resulted in an increasing interest in polymer electrolyte fuel cells (PEFCs). One critical issue facing the commercialization of PEFCs is the gradual decline in performance resulting in a severe limitation of PEFC lifetime. 1,2 One important cause of PEFC performance degradation is the loss of electrochemically active surface area (ECSA) of the supported platinum nanoparticles (Pt-NPs) at the cathode. This ECSA decrease of the cathode catalyst can arise from different degradation mechanisms which take place simultaneously, especially during PEFC start and stop events, when the cathode potential can reach very high transient values up to 1.4-1.5 V RHE . 3 In situ analytical tools are required to gain a better understanding of the phenomena involved and to distinguish between different degradation mechanisms like platinum loss due to dissolution, nanoparticle growth resulting from electrochemical Ostwald ripening or platinum nanoparticle agglomeration due to support material corrosion. [4][5][6][7][8] Electrochemical in situ small angle X-ray scattering (in situ SAXS) offers the unique opportunity not only to observe the evolution of the platinum nanoparticle size distribution during an electrochemical degradation experiment, but also to simultaneously monitor the evolution of the absolute platinum mass content of the electrode. In situ SAXS experiments can be conveniently performed at synchrotron X-ray sources. [9][10][11][12][13][14][15][16] The high photon flux from these radiation sources yields a very good scattering signal-to-noise ratio in a short acquisition time despite the strong X-ray absorption of the elec...

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 LIII and the Ir LIII 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) [2] H.G. Haubold, X.H. Wang, H. Jungbluth, G. Goerigk, and W. Schilling, J. Mol. Struct. 383, 283-289 (1996) [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)