 XPS Investigations of a Fuel Cell Membrane Surface
Fuel Cells are regarded as efficient and clean
converters for the conversion of chemically
bonded energy into electrical current. As this
technology is predicted to have a high
potential a lot of research is dedicated to a
number of different cell types like Solid
Oxide Fuel Cells (SOFC) or Polymer Electrolyte
Fuel Cells1 (PEFC).
TAS-AN-X2E
the insufficient proton conductivity of the
iomer.
Figure 1: Scheme of a Polymer Electrolyte Fuel Cell (PEFC)
In PEFC fuel cells polymer membranes are used
as an electrolyte, i.e. as a transport medium for
catalytically generated protons. Beside Nafion
sulphonated Polyetheretherketone (sPEEK,
structure see Fig. 2 top) has turned out to be a
good iomer for membrane manufacturing.
During investigations for the optimisation of
PEFCs considerable fluctuations of the
electrolytic sPEEK membrane properties were
observed. Photoelectron Spectroscopy (XPS,
ESCA) 2 should be used to find the reason for
1
2
also known as:
- Proton Exchange Membrane Fuel Cell (PEMFC)
- Solid Polymer Fuel Cell (SPFC)
XPS: X-ray PhotoelectronSpectroscopy
ESCA: Electron Spectroscopy for Chemical Analysis
Figure 2: C1s Photoelectron spectra of sPEEK (top), a PET
reference (centre) and a PET contaminated sPEEK fuel cell
membrane (bottom).
Figure 2 (top) shows the C1s signal of a sPEEK
reference. According to the chemical structure
of sPEEK three different C1s bonding states are
detected. Comparison of this fit data with the
C1s signal of the defective membrane (fig. 2
bottom) shows significant differences in peak
shape. This was the crucial evidence for the
presence of an organic contamination on the
membrane surface. For the exact identification
of the nature of the contamination several
materials were analysed that were used for
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membrane manufacturing and storing. The
results of a Polyethylene Terephthalate (PET)
foil that was used as a supporting substrate
showed a noticeable signal (C5), coming from
the COO bonding of the PET (see. fig. 2 centre).
This signal also was detected on the defective
membrane but not on the reference membrane.
The exact fit of the PET and sPEEK reference
data showed a perfect accordance with the C1s
spectrum of the contaminated membrane. On
the basis of these fits a PET coverage of approx.
10% on the membrane could be determined.

ToF-SIMS characterisation of
membrane layers – identification of
polymer type and additive composition

identification and quantification of
catalytic active compounds (e.g. Pt) in
the top atomic layer (LEIS)

stoichiometry, contamination and
catalytic active components of metal
oxide membranes in Solid Polymer Fuel
Cells (XPS)

localisation of contaminants on
metallic surfaces (LEIS)

analysis of diffusion along grain
boundaries of Metal Oxide Membranes
(ToF-SIMS)
werden.
Figure 3: Cross section of a fuel cell membrane
(left: XPS-Overlay of P (red) and Pt (green); right: SEM
image)
Another example for the analysis of PEFCs is
shown in figure 3. The exact composition of the
catalytic active Pt electrodes that were applied
onto the phosphoric membrane should be
analysed. Because of the different information
depths of XPS and SEM/EDX both techniques
were used for this comparative analysis.
Already the material contrast in the SEM image
(fig. 3 right) indicates a higher content of
heavy elements in the left Pt electrode. This
qualitative indication could be quantified by
the detailed data interpretation of the XPS and
EDX results.
Further analytical options:

XPS line scans for monitoring the
quantitative composition as a function
of position
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