(420k) Electron Beam Induced Radiation Damage In Nafion and the Lifetime of Fuel Cells

Polymer electrolyte membrane fuel cells
(PEMFCs) are a potentially viable, clean, and renewable source of energy. 
There are numerous technical challenges that must be overcome in order to
achieve significant market penetration.  The interface between the
electrode and electrolyte remains one of the areas of greatest ignorance. 
A molecular-level understanding of fuel cell structure will be very helpful to
understand the electron/proton transportation mechanism at the electrode/electrolyte
interface, see Fig. 1. SEM and TEM are two powerful tools commonly used to gain
molecular level information about the electrode/electrolyte interface structure. 
However, Schneider [1], has reported that Nafion (used as electrolyte in the
PEMFC) is susceptible to radiolysis during SEM imaging   (Fig. 2.),
and suggested that such damage was dependent on the microstructure of the
substance being viewed. Because beam induced damage could have major
implications for transport phenomena occurring at the electrolyte/electrode
interface of PEMFC and could possibly suggest one of the possible mechanisms of
fuel cell aging, we have investigated the response of Nafion to electron beam
irradiation.

In the work described here, high angle
annular dark field (HAADF) STEM images of our interface samples were recorded
as a function of the integrated beam dose using a Zeiss MERLIN SEM operated in
scanning transmission (STEM) mode. The magnitude of the HAADF signal is
linearly proportional to the mass thickness of the specimen [2]. So thicker
regions of the sample or areas with higher density will appear bright, whilst a
hole through the sample in the beam path will appear darker. To make
measurements the probe beam, containing a known beam current, is scanned at TV
rates in a square raster a few micrometers in size across the Nafion sample for
time periods varying from a few seconds up to of the order of one minute. An
image of the exposed area is then recorded and, during the photo-recording, the
beam is blanked for one or two seconds to provide a zero-signal ("black level")
reference. The recorded STEM image is then analyzed by a histogram which
indentifies both the zero signal baseline value and the signal level in the
irradiated region (Fig. 3.), which permits the brightness of the irradiated
area to properly determined. This procedure is then repeated, as required, to
increase the deposited beam dose while simultaneously measuring the change in
STEM image brightness. The relative change in sample thickness with irradiation
can then be found by plotting the signal intensity as a function of the beam
dose deposited. An absolute measurement of the damage rate versus dose can be produced
by using a mass standard, such as ferritin, to calibrate the sensitivity of the
HAADF signal under the given experimental conditions.