| AVS 56th International Symposium & Exhibition | |
| Applied Surface Science | Friday Sessions |
| Session AS-FrM |
| Session: | Practical Surface Analysis |
| Presenter: | H.M. Meyer III, Oak Ridge National Laboratory |
| Authors: | K.S. Reeves, Oak Ridge National Laboratory K.L. More, Oak Ridge National Laboratory R.G. White, Thermo Fisher Scientific, UK T.S. Nunney, Thermo Fisher Scientific, UK A.E. Wright, Thermo Fisher Scientific, UK H.M. Meyer III, Oak Ridge National Laboratory |
| Correspondent: | Click to Email |
Membrane electrode assemblies (MEA) were examined before and after use in a hydrogen fuel cell. MEAs consisted of anode and cathode (both comprised of carbon and Pt-catalyst) separated by a polymeric proton exchange membrane (PEM) made from Nafion®. Of particular interest is through-depth compositional uniformity of the membranes after use. PEMs are typically 20-50 microns thick and as such, the use of surface sensitive techniques to probe their through-depth composition requires unique sample preparation. Traditional Ar-ion depth profiling is not possible since even low energy (200-500 eV) Ar ions disrupt the polymer structure and chemical bonding. Polyatomic sputter systems developed for etching polymer surfaces without imparting damage are also of little use due to low sputter rates (~0.1 nm/s). Even with a carefully prepared cross-section of the MEA (routinely done for TEM sample prep), only limited information is obtained using the relatively large x-ray spots (10’s-to-100’s of microns) of most XPS instruments. To probe the inner region of the membrane film, we have developed an ultra-low-angle microtomy (ULAM) sample preparation technique. The ULAM technique is similar to metallographic low-angle lapping and is based on similar methods described by Watts and co-workers [1]. The ULAM-derived taper through a ~25 micron thick membrane effectively extends the cross-section to greater than 400 microns. With this exaggerated cross-section, the 30 micron X-ray spot of our Thermo Fisher Scientific K-Alpha XPS instrument was able to evaluate composition as a function of depth from the anode, across the membrane to the cathode. Core level and VB spectra from the anode, cathode, and membrane components will be presented for a fresh MEA and for the same MEA after 500 hrs of use. To demonstrate the effectiveness of the ULAM technique, data from samples prepared using ULAM will be compared to surfaces of free-standing membrane films and from standard cross-sections.
Research supported by the U.S. Dept. of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cell, and Infrastructure Technologies Program. Research at the ORNL SHaRE User Facility was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Dept. of Energy.
[1] S.J. Hinder, C. Lowe, J.T. Maxted, J.F.Watts, Journal of Materials Science 40 (2005) 285-293.