Environmental Engineering Reference
In-Depth Information
†
Assuming that the electrode has not been left at potentials over 0.75 V for hours,
the variation of u
ox
with electrode potential can be derived from the voltammetric
charge associated with bringing the Pt electrode up from a chemisorbed
oxygen-free state to some given potential above 0.75 V. A further assumption
required here is on the number of electrons passed per surface oxygen species
deposited, and this number was assumed by Uribe et al. [1992] to be 1, corre-
sponding to an OH
ads
intermediate formed on a single Pt surface site by the
Reaction (1.3).
The rate expression considering the blocking of active sites by surface oxide can
then be written as
E
o
E
b
int
J
ORR
¼
J
0
(1
u
ox
) exp
(1
:
5)
where E is the cathode potential, E
o
is the redox potential for the O
2
/H
2
O couple under
the relevant operation conditions, u
ox
is the coverage by chemisorbed oxygen at poten-
tial E, and b
int
is the “intrinsic” value of the ORR Tafel slope that would be measured at
a Pt metal surface free of any blocking surface species. The variation of ORR current
with Pt cathode overpotential to be expected from (1.5) is given by
d(log J
ORR
)
d(E
o
E)
¼
1
1
1
u
ox
du
ox
dE
b
int
þ
(1
:
6)
where the dependence of u
ox
on cathode potential E appears as an additional source
of ORR rate variation with potential. The qualitative result of the dependence of sur-
face oxygen coverage on E
cath
can be seen from (1.6): the apparent Tafel slope for
ORR is expected to be smaller than the intrinsic slope b
int
, i.e., to be smaller than
120 mV per decade of current density near ambient conditions. Furthermore, when
b
int
is constant through the relevant potential range, the apparent Tafel slope will
not be constant necessarily, as can be realized from the nature of the second term
on the right-hand side of (1.6).
It is not difficult to understand the physical picture behind the apparent low Tafel
slope expected for the ORR according to (1.5). The two terms on the right-hand side of
(1.6) correspond to two contributions to ORR rate enhancement with increase in cath-
ode overpotential: (a) lowering of the activation energy for the ORR process at Pt
metal surface sites and (b) generation of more active (metal) surface sites by reducing
the coverage u
ox
of the ORR-blocking chemisorbed oxygen species. It should be also
easy to understand why interpretation of an apparent low Tafel slope, resulting from
two different contributions to ORR rate enhancement with cathode overpotential,
cannot be presented and further analyzed as if it were the intrinsic Tafel slope for
the process at the metal surface. The suggestion by Neyerlin et al. [2006] that a
Tafel slope of 60 mV/decade describes well the ORR kinetics at a Pt electrode in
the fuel-cell-relevant potential range, and is therefore the value to use in fuel cell cath-
ode diagnostics, is not defensible. A measured, practically constant slope of 60 mV per
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