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[Makharia et al., 2005]. These spectra display a major loop in the Z 00 versus Z 0 plot that
cuts the Z 0 axis at some frequency in the range 0.1 - 1 Hz, followed by an inductive
loop that cuts the Z 0 axis again at a much lower frequency. This frequency response
of the interfacial faradaic process likely reflects variations of ORR current in response
to a cyclic potential perturbation, originating from two effects of the potential on ORR
rate, which are well resolved by their different response times. A relevant expression
describing this behavior is likely of the form
dJ
dE (E, v) ¼
dJ
dE
dJ
du ox
du ox
dE
(E, v) þ
(E, v)
(1 : 7)
u ox
E
This expression suggests that the dependence of the ORR interfacial impedance on
potential at a Pt surface of “frozen coverage,” corresponding to the first term on the
right-hand side, could possibly be measured at perturbation frequencies sufficiently
high that the variation of oxide coverage with potential (appearing in the second
term) is too slow to follow. The additional contribution of du ox /dE to the overall impe-
dance measured under DC conditions can then be derived from the additional changes
of the measured impedance at lower frequencies, i.e., when u ox follows the slow poten-
tial perturbation. Quite strikingly, the variation of the ORR impedance under frozen
coverage conditions, derived from variation with E cath of the diameter of the large
loop in Fig. 1.7, corresponded to a Tafel slope close to 120 mV/decade, whereas
the apparent Tafel slope under DC conditions, corresponding to the value of Z 0 at
the lowest end of the frequency spectrum, was found close to 60 mV (Mark
Mathias, personal communication, 2007).
The other challenging ORR finding reported recently and mentioned above was of
measured apparent reaction order with respect to oxygen of near 0.5 [Neyerlin et al.,
2006]. Added to the suggestion that the measured Tafel slope of 60 mV/decade is
representing “ORR kinetics” at Pt fuel cell catalysts above 0.75 V, the report of a reac-
tion order with respect to O 2 of 0.5 seemed to support the possibility of the slow step in
the ORR belonging further down the ORR sequence, as actually suggested in Nørskov
et al. [2004]. Such a slow step could possibly be the reduction of OH ads according to
OH ads þ H þ þ e ! H 2 O
(1 : 8)
Having explained above why the apparent low Tafel slope is, in fact, hiding an intrin-
sic Tafel slope no smaller than 120 mV/decade, it remains to be seen if the reported
reaction order of 0.5 [Neyerlin et al., 2006] can be understood along similar lines. For
the purpose of this consideration, it is important to note the experimental conditions
employed in Neyerlin et al. [2006] to study the ORR rate dependence on P O 2 .
Unlike previous studies at relevant Pt/ionomer interfaces, which were performed
near room temperature and employed oxygen partial pressures no higher than 1 atm
[Parthasarathy et al., 1992a, b], the most recent study by Neyerlin et al. [2006] used
elevated temperatures typical of an operating PEFC and oxygen pressures between
1 and 4 atm. The likely result of such experimental conditions is that an increase in
the oxygen pressure again has more than the single effect on the rate of the ORR
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