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determinations of the reaction order with respect to O 2 and the Tafel slope for the ORR
at Pt in acid electrolytes. A reaction order of (or slightly under) 1.0 and a Tafel slope
near 120 mV/decade, measured in the potential range where the Pt surface is practi-
cally free of chemisorbed oxygen, are both in agreement with the Reaction (1.2) as the
slow step in the ORR sequence. Such measured Tafel slope is expected for a first,
one-electron-transfer process like (1.2), when a, the “symmetry factor” in the electro-
chemical rate equation, has a value of 0.5. Such a value of ameans that the change in
activation energy in the Reaction (1.2) per some increase in overpotential is 50% of the
corresponding change in the reaction free energy, i.e., f d(DG # )/dV g / f d(DG)/dV g ¼
0.5. There is no a priori reason to expect this specific value for a; however, a value of
0.5 for what is basically a “Brønsted factor” that describes d(DG # ) as some constant
fraction of d(DG) is commonly found in electrochemical reactions. Consequently,
the reported Tafel slope of 120 mV/decade of current density seems to be in accord-
ance with the first, one-electron-transfer step being the slow step in the ORR sequence,
as does the first order in oxygen partial pressure documented for the ORR at the Pt/
hydrated ionomer interface [Parthasarathy et al., 1992a, b].
It would seem that to test the predictive power of recent theoretical work with
regard to the ORR mechanism at Pt in acid electrolytes, the Tafel slope predicted
by theory should be compared with a reported value of 120 mV/decade measured
for ORR at Pt metal, whereas the reaction order predicted by theory be compared
with the reported value of, or slightly below 1.0 [Parthasarathy et al., 1992a, b].
However, testing of the recent theoretical predictions against these two experimentally
measured parameters has just become complicated by recent reports suggesting strong
disagreement between Tafel slopes and reaction orders reported previously from
measurements of ORR kinetics at rotating disk or other, ionomer-filmed bulk Pt elec-
trodes and the slope and reaction order measured for cathodes in an operating PEFC. In
one such recent experimental report [Neyerlin et al., 2006], the conclusion offered was
that, for a fuel cell cathode employing a Pt/C catalyst, the full description of
ORR kinetics in the potential range relevant to fuel cell operation should be based
on a constant Tafel slope of 60 mV/decade of current density and on a reaction
order with respect to oxygen of 0.5. Such reports of a measured constant slope of
60 mV/decade for ORR at the temperatures of an operating PEFC and in the poten-
tial range relevant to fuel cell operation have been identified by theorists [Nørskov
et al., 2004] as experimental support for a conjecture that the rate-determining step
in the ORR may be a single electron and proton transfer to an OH ads or O ads surface
intermediate. According to that conjecture, this step is likely to be rate-determining
in light of the significant energy sink from which these intermediates need to be
lifted to complete the four-electron process [Nørskov et al., 2004]. The latter authors
did not address, however, one consequence expected of a rate-determining step
located lower in the sequence of one-electron steps in the ORR process, namely
that the reaction order with respect to O 2 should then be 0.5, rather than 1.0 consist-
ently reported from ORR investigations at model, Pt/electrolyte interfaces, includ-
ing the Pt/hydrated Nafion interface [Parthasarathy et al., 1992a, b]. As if to
strengthen the growing question mark regarding widely accepted values of key
ORR kinetic parameters, subsequent to the 2004 ORR theoretical paper [Nørskov
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