(295b) The Impact of Pt Oxide Coverage on the Measurement of Durability Diagnostic Parameters

There are many durability challenges
that need to be overcome for the successful commercialization of automotive
PEMFCs such as load cycling, cold-start, start-up shut-down, and dry
operation/durability of membranes. The catalyst layer which is ~10um thick porous
layer (composed of Pt/C, ionomer) applied to a proton conducting membrane is
the heart of the fuel cell.  Power is generated, water and heat are produced
and hydrogen and oxygen consumed in this thin layer. The durability of this
layer in the MEA is one of the primary challenges facing PEMFCs.  In order to characterize,
understand and find solutions to mitigate the degradation of the catalyst layer,
accurate and robust in-situ diagnostic techniques are necessary.  Some of the
most fundamental parameters that can be used as a measure and benchmark of the
degradation rate include the change in electrochemical area (ECA, m²/g),
specific activity (i_s, uA/cm²) and mass activity (i_m,
mA/mg). In this paper, we report on the significant effect of PtO_x surface
coverage on the measured values of fundamental parameters such as surface area,
specific activity and mass activity.  Further, the effect of voltage cycling
parameters such as cycle profile, and operating conditions on the degradation
of the catalyst layer of membrane electrode assemblies (MEAs) in subscale fuel
cells are reported using our proposed measurement methods.

There exists a wide range of
values of intrinsic ORR parameters of specific activity (i_s), mass
activity (i_m) and exchange current density (i₀) as
measured in both liquid electrolyte cells using rotating disc electrodes (RDE),
as well as in-situ, in MEAs of subscale single cells.^1,2  Some of
the variations can be attributed to the different operating conditions under
which these parameters have been measured and the difficulty in converting them
to a set of standard conditions.  The conversion to standard conditions (such
as 100 kPa, 80^oC) depends on the accurate knowledge of the effect of
temperature on kinetics and the reaction order ?m' for the effect of partial pressure. 
Besides errors induced by these factors, another important factor often ignored
is the effect of pre-conditioning (immediate history) of the MEA in the steps
prior to measurement of activity (typically @ 0.90V).  The pre-conditioning
step can affect the oxide coverage of Pt depending on whether the step was a
hold at high potentials above 0.80V or held at potentials below 0.70V in a regime
where the surface is predominantly free of oxides.  In our work, we will report
for the first time on the effects of such pre-conditioining steps on the
measured specific and mass activity of the catalyst.

Using the reproducible diagnostic
measurement techniques thus developed, we apply them to the measurement of potential
cyclic durability of MEAs in subscale fuel cells.  Under normal automotive
operation load cycles in the range between idling and peak load where the
voltage range is about 0.95-0.60V is common.  This range involves a change in
Pt surface from one covered with oxides to one that is not and thus is also of
interest in terms of fundamental understanding of Pt and PtO_x
dissoultion.  Bulk Pt as well as Pt/C dissolution is known to take place when
held at high potentials.^3-6  In addition, the severe effects of
potential cycling on Pt has been the subject of several reports in recent
literature.^7-11  In this study, we investigate and compare the
effect of cycle profile under different operating conditions and report on the degradation
of surface area (ECA) as well as specific and mass activity over ~15,000 cycles,
besides discussing the possible mechanisms for the degradation.

References

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S.S. Kocha, Principles of MEA preparation, in: W. Vielstich, A.Lamm, H.
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Shigenori Mitsushima, Shuya Kawahara, Ken-ichiro Ota, and Nobuyuki Kamiya J.
Electrochem. Soc, 154 2 B153-B158 2007.