Environmental Engineering Reference
In-Depth Information
bond energy as a yardstick for the activity of a metal or metal alloy surface in the ORR.
Because the generation of active metal sites is likely the important key to achieving
practical ORR rates at high, relevant cathode potentials, it can be understood why
an optimized M22Ox bond strength may indeed become an effective yardstick,
almost irrespective of the details of the mechanism of the ORR at an exposed active
metal site.
The new analysis of the ORR offered here helps to reconcile experimental findings,
particularly those reported recently for ORR at dispersed Pt catalysts in the PEFC cath-
odes [Neyerlin et al., 2006], and the likely molecular level mechanism of the ORR at
Pt metal. The apparent Tafel slope of 60 mV/decade through the fuel-cell-relevant
potential domain can be fully explained by an intrinsic “classical” Tafel slope of
120 mV/decade for ORR at Pt metal, modified by the added effect of the cathode
overpotential in generating more active metal surface sites (see Fig. 1.6). True, a
Tafel slope cannot by itself serve to establish the overall mechanism of a complex
process such as the ORR. However, it is certainly important, on the other hand, to
examine why the prevalent assignment of the first electron and proton transfer step
as rate-determining in the ORR is in apparent conflict with results of measurements
on a fuel cell cathode catalyst [Neyerlin et al., 2006]. The elucidation that the value
of the intrinsic Tafel slope for the ORR on Pt metal is, in fact, not different than
120 mV/decade at the dispersed fuel cell Pt cathode catalyst [see Equation (1.6)
and Fig. 1.6], whereas the apparent smaller reaction order of around 0.5 could be
the result of enhanced Pt surface oxidation (i.e., enhanced ORR catalyst deactivation)
with increase in oxygen pressure at elevated temperatures [see Equation (1.9b)],
restores confidence in the assignment of the Reaction (1.2) as rate-determining in
the ORR at a Pt cathode catalyst. The analysis used here also brings home the full
impact of the complex dependence of the ORR process on the nature of the actual
Pt catalyst surface in the fuel-cell-relevant potential range, this catalyst surface
having a composition and structure that vary with potential, temperature, and time,
resulting in a behavior that can be explained only when such surface modification
effects are considered in the rate expressions.
ACKNOWLEDGMENTS
Thanks are due to the organizers of the Leiden meeting for a very pleasant and stimulating event.
The idea that the cathode potential with respect to E
Pt(H
2
O)
=
Pt
-
OH
ads
determines the value of the
pre-exponential factor in the ORR rate expression was inspired by a comment by Andy
Gewirth (Urbana) in his talk in Leiden, pointing to the value of Pourbaix diagrams for under-
standing ORR electrocatalysis. Indeed, the information on these ORR-mediating and facil-
itating M/M - OH surface redox systems is to be found in Pourbaix's Atlas.
Extensive discussions with Hubert Gasteiger of the GM Fuel Cell Project (and now with ACTA)
and with Jan Rossmeisl of Lyngby, Denmark, are gratefully acknowledged. It would be fair
to add that these discussions did not resolve key disagreements; however, they were certainly
conducted in and with good spirit.
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