Electron Beam Induced Radiation Damage in the Catalyst Layer of a Proton Exchange Membrane Fuel Cell QIANPING HE, 1 JIHUA CHEN, 2 DAVID J. KEFFER, 3 AND DAVID C. JOY 2,3 1 Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 2 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee Summary: Electron microscopy is an essential tool for the evaluation of microstructure and properties of the catalyst layer (CL) of proton exchange membrane fuel cells (PEMFCs). However, electron microscopy has one unavoidable drawback, which is radiation damage. Samples suffer temporary or permanent change of the surface or bulk structure under radiation damage, which can cause ambiguity in the characterization of the sample. To better understand the mechanism of radiation damage of CL samples and to be able to separate the morphological features intrinsic to the material from the consequences of electron radiation damage, a series of experiments based on high-angle annular dark-field–scanning transmission scanning microscope (HAADF-STEM), energy filtering trans- mission scanning microscope (EFTEM), and electron energy loss spectrum (EELS) are conducted. It is observed that for thin samples (0.3–1 times l), increasing the incident beam energy can mitigate the radiation damage. Platinum nanoparticles in the CL sample facilitate the radiation damage. The radiation damage of the catalyst sample starts from the interface of Pt/C or defective thin edge and primarily occurs in the form of mass loss accompanied by atomic displacement and edge curl. These results provide important insights on the mechanism of CL radiation damage. Possible strategies of mitigating the radiation damage are provided. SCANNING 36:338–346, 2014. © 2013 Wiley Periodicals, Inc. Key words: STEM, EELS, radiation damage, catalyst layer, PEMFC Introduction Proton exchange membrane fuel cells (PEMFCs) are considered to be one of the most promising energy conversion devices for applications such as mobile and stationary power supply due to their high energy conversion efficiency, fast start up and low/zero emission level (Cheng et al., 2004; Yu et al., 2012). However, there are several technological challenges impeding their broader commercialization. For exam- ple, the functionality and structure of the catalyst layer (CL) located in the membrane electrode assembly (MEA) need to be optimized for an effective utilization of the precious metal catalysts (Xie et al., 2005) and better water management in MEAs (Eikerling, 2006). The CL is the component where electrochemical reaction, ionic/electronic conduction, and reactant gas transport are taking place simultaneously during operation (Sun et al., 2010). Characterizing the microstructure of the CL helps to elucidate microstruc- ture-related process occurring during operation and degradation mechanisms contributing to PEMFC per- formance loss. Transmission electron microscopy (TEM) and scan- ning electron microscopy (SEM) are essential and powerful analytical and imaging techniques for the evaluation of microstructural and microchemical changes that determine fuel cell performance stability. Many research groups take advantage of TEM and SEM in investigating the microstructural changes in the MEA of PEMFC (Akita et al., 2006; Ma et al., 2009; Ficicilar et al., 2010; Mamat et al., 2010; Choi et al., 2011). However, while providing useful nanoscale informa- tion, the electron beam used in SEM and TEM can cause temporary or permanent change in the surface or bulk structure of the specimen (Egerton et al., 2004), especially for soft materials such as Nafion (Mauritz Contract grant sponsor: National Science Foundation; contract grant numbers: DGE-0801470, OCI 07-11134.5. Address for reprints: David C. Joy, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996 E-mail: djoy@utk.edu Received 23 May 2013; Accepted with revision 28 June 2013 DOI: 10.1002/sca.21117 Published online 29 July 2013 in Wiley Online Library (wileyonlinelibrary.com). SCANNING VOL. 36, 338–346 (2014) © Wiley Periodicals, Inc.