The performance of polymer electrolyte membrane fuel cells (PEMFCs) is primarily controlled by the materials within membrane electrode assemblies (MEAs), which consist of an anode and cathode separated by a proton-conducting membrane.  Developing highly stable MEAs is one key to wide-spread commercialization of this clean energy technology.  The electrodes consist of a complex hierarchal structure, which must be capable of simultaneously transporting hydrogen, protons, electrons, oxygen, and water to and way from the critical electrocatalytic active sites.  In conventional PEMFCs, Pt serves as the active catalyst, and is sometimes alloyed with a transition metal such as Co or Ni to enhance activity or lower cost.[1]  In non-platinum group metal (non-PGM) electrodes, the catalytically active site takes the form of Fe or Co metal-nitrogen-carbon (M-N-C) complexes.[2] In either case, understanding the evolution of these catalysts, and the complex support structures in which they reside, in relation to observed performance losses is essential for furthering fuel cell development.

Scanning transmission electron microscopy (STEM) is an ideal analytical characterization tool for performing both pre- and post-mortem nano to mesoscale compositional and structural characterization of the individual material components in MEAs.  STEM has been used to quantify key changes responsible for performance gains and losses during fuel cell conditioning and cycling.[3,4]  These structural changes can vary widely in both extent and scale, from carbon-corrosion-related electrode thinning at the micron scale to dealloying-induced reordering on the surfaces of advanced Pt-alloy catalysts at the atomic scale.

This presentation will focus on three topics spanning multiple length scales in the MEA.  Beginning at the atomic scale, strain effects and skeleton structures arising from in situ Ni dealloying in Pt-alloy nanostructured thin films (NSTF) will be presented.  The surface structure and composition is critical to electrocatalytic activity, and, as shown by the aberration-corrected Z-contrast STEM image in Fig. 1a, these structures can be resolved with atomic resolution.  Changes in surface structure/strain will be studied as a function of fuel cell cycling and electrochemical activity.  At the nanoscale, carbon support effects on Pt anchoring and agglomeration will be presented, highlighting the critical role that catalyst-support interactions and corrosion-resistant supports play in MEA durability (Fig. 1b).  Finally, at the mesoscale, changes in electrode structure with aging in non-PGM MEAs will be presented, highlighting the key role that hierarchal pore structures (spanning the micro to macro) play in fuel cell performance.  Complementary imaging and compositional mapping by energy dispersive X-ray spectroscopy, as shown in Fig. 1c, provide unique insight into how both changes in mechanical and chemical structure impact durability.   

 References

1. V. R. Stamenkovic et al., Nat. Mater. 6 (2007) 241.
2. G. Wu, K. L. More, C. M. Johnston, P. Zeleany, Science 332 (2011) 443.
3. D. A. Cullen et al., J. Mater. Chem. A, 3 (2015) 1660. 
4. B. Han et al., Energy Environ. Sci., 8, 258 (2015).

 Acknowledgements

Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE) and ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.