(Invited) A New View of Fuel Cell Electrocatalysts through Multimodal Analytical Electron Microscopy

Abstract

Significant advancements have been made in the performance of oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte membrane fuel cells. State-of-the-art Pt-alloy catalysts show exceptionally high mass activity and recent research into ordered nanoparticles shows promising improvements in durability. The activity of platinum group metal-free (PGM-free) catalysts, which will be required as world-wide demand for fuel cell stacks increases, have also improved substantially, although significant durability issues persist. Accelerated materials discovery and development is required to keep pace with ever-changing market and performance demands, such as the shift in focus towards heavy duty transportation where highly durable, cost-effective catalysts are required. Scanning transmission electron microscopy (STEM) has long played an important role in assessing the quality and durability of electrocatalysts and electrode structures, and the increasing demands on catalyst performance require a corresponding improvement in both the resolution and quantitative precision of analytical methods. Fortunately, recent advances in high-speed cameras and spectrometers allow for an unprecedented view of catalyst structure and composition in two, and in some cases, three dimensions. The newly installed JEOL NEOARM atomic-resolution STEM at the Center for Nanophase Materials Sciences combines the latest generation spectrometers with new imaging modes such as electron ptychography. The ability to acquire multiple signals simultaneously at high data acquisition speeds and across a range of accelerating voltages (30-200 kV) allows for a fresh look at the atomic structure of both Pt-based and PGM-free catalysts. The simultaneous application of electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy to study the beam sensitive surfaces of sub-5nm Pt-alloy nanoparticles (<5nm) will be discussed. The combination of dual EELS-EDS analysis using ultra-low voltage (30kV) electron beams under cryogenic cooling will also be demonstrated on atomically dispersed transition metal sites in PGM-free electrodes. Finally, recent developments in pixelated cameras and ptychographic analysis of 2D materials will be extended to both Pt-based and PGM-free electrocatalyst systems to explore the opportunities and limits of low-dose, high-speed imaging. Acknowledgements Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE), as part of the FC-PAD and ElectroCat Consortia, which is part of the Energy Materials Network. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility.