Journal of Power Sources 185 (2008) 734–739 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Non-destructive transmission electron microscopy study of catalyst degradation under electrochemical treatment Karl J.J. Mayrhofer a,∗ , Sean J. Ashton a , Josef C. Meier a , Gustav K.H. Wiberg a , Marianne Hanzlik b , Matthias Arenz a,∗∗ a Institut für Physikalische Chemie, Technische Universität München, Garching, D-85748, Germany b Zentrum für Elektronenmikroskopie, Technische Universität München, Garching, D-85748, Germany article info Article history: Received 13 June 2008 Received in revised form 13 July 2008 Accepted 3 August 2008 Available online 13 August 2008 Keywords: Fuel cell Catalyst degradation Transmission electron microscopy Accelerated electrocatalyst testing abstract A novel, non-destructive transmission electron microscopy technique is introduced, which enables the observation of the identical locations of a catalyst before and after electrochemical treatment (IL-TEM). The significance of this method is exemplified by the analysis of a standard, commercially available carbon supported platinum catalyst. We demonstrate that the observed changes of the catalyst particles are a direct consequence of the applied electrochemical treatment; and that selected catalyst regions are representative for the catalyst as a whole. Different electrochemical treatments were applied in order to discuss the potential of the method for studying processes of catalyst degradation. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Fundamental investigations of electrocatalysts are essential for the further development of practical applications such as fuel cells or batteries [1]. Besides the activity and selectivity for certain elec- trochemical reactions, the long term stability of the catalyst is of major interest. Whereas plenty of advances in the performance of catalysts have been recently reported [2–6], improvements in the durability are scarce. This is partially due to the lack of inves- tigative techniques that enable an effective analysis of degradation processes in electrolyte solutions. Commonly, the loss of active surface area is determined in-situ by electrochemical means [7,8]. Additionally, X-ray diffraction (XRD) and transmission electron microscopy (TEM) are applied to obtain the average crystallite size and complete size distributions, respectively. Latter techniques are, however, considered destructive for the study of electrocatalysts [9], since the catalyst has to be removed from the working electrode after the treatment in electrolyte. Consequently, only one single measurement can be conducted on a certain sample, in contrast to in-situ ultra-high vacuum TEM studies of catalysts [10,11]. ∗ Corresponding author. Tel.: +49 8928913294; fax: +49 8928913389. ∗∗ Corresponding author. E-mail addresses: karl.mayrhofer@mytum.de (K.J.J. Mayrhofer), matthias.arenz@mytum.de (M. Arenz). The information on the surface area loss and/or particle growth is often not sufficient for a detailed description of occurrences on the catalyst particles in the nanometer scale. As a consequence, sev- eral theories are proposed in order to explain the loss in the active surface area of catalysts. The four primary mechanisms believed to be of relevance to low temperature fuel cell catalysts are [12–15]: (i) Ostwald-ripening, metal ions dissolve from smaller particles, dif- fuse, and re-deposit onto larger particles, resulting in reduced metal surface area via a minimization in surface energy; (ii) reprecip- itation, Pt dissolves into the ionomer phase within the cathode and then precipitates again as newly formed Pt particles via the reduction of hydrogen; (iii) particle coalescence, Pt particles that are in close proximity sinter together to form larger particles; (iv) corrosion of the carbon support that anchors the Pt particles and provides electrical contact. These particle growth mechanisms and their rates may vary as a function of electrode potential, cell volt- age cycling conditions, current density, particle size and shape, the hydration state of the membrane, and operating conditions. In order to obtain an improved understanding of the degra- dation mechanism of fuel cell catalysts, we developed a novel, non-destructive method based on TEM, which enables the inves- tigation of identical locations on a catalyst before and after electrochemical treatments (IL-TEM). The results from studies on a standard, commercially available Pt catalyst presented in this work should demonstrate the high potential of this method. 0378-7753/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2008.08.003