Advances in Electron Microscopy Methods to Evaluate Thin Ionomer Layers in PEM Fuel Cell Catalyst Layers

Abstract

The ability to characterize the thin ionomer films within the catalyst layers in membrane electrode assemblies (MEAs) at the nm-scale via analytical electron microscopy presents many challenges. The ionomer degrades rapidly during analysis with high-current electron beams, which can be mitigated to some extent when experiments are conducted using higher operating voltages and under cryogenic temperatures.1 These microscopy operational parameters have proven most useful for imaging, mapping, and analyzing ionomer aggregates that are typically tens-to-hundreds of nms in size, have not been particularly useful for understanding the nature of the thin ionomer films that percolate through the thickness of the catalysts layers. Recently, Oak Ridge National Laboratory has acquired a new aberration-(probe)-corrected analytical scanning transmission electron microscope (STEM) that can be operated under conditions significantly more favorable for interrogating “soft matter” at very high resolution, e.g., structure and chemistry of the nm-scale thin ionomer films within intact catalyst layers. This new STEM, the JEOL-NEOARM, can be operated between 30kV-200kV at low-electron-dose and is equipped with state-of-the-art analytical capabilities for the simultaneous acquisition of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS). Most notably, the new STEM is equipped with a novel pixelated detector that enables ultrafast image/diffraction (ptychographic) data acquisition that will minimize exposure of the ionomer films to the electron beam, preventing damage accumulation, and allowing for complete data reconstruction following data collection. We will present imaging and spectroscopic results obtained by varying the STEM operating conditions on the NEOARM (voltage, electron dose, sample temperature, ptychographic reconstruction methods) and exploiting its unique combination of ultrafast capabilities. The high-resolution structural and compositional information provided will provide new insight regarding thin ionomer films within MEA catalyst layers Research supported by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, as part of the Fuel Cell Performance and Durability (FC-PAD) Consortium. Microscopy research performed as part of a user project at the Center for Nanophase Materials Sciences, which is a U. S. Department of Energy, Office of Science User Facility. References D.A. Cullen, R. Koestner, R.S. Kukreja, Z.Y. Liu, S. Minko, O. Trotsenko, A. Tokarev, L. Guetaz, H.M. Meyer, C.M. Parish, and K.L. More, “Imaging and Microanalysis of Thin Ionomer Layers by STEM,” Journal of The Electrochemical Society 161[10] F1111-F1117 (2014).