Environmental Engineering Reference
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for example, in Parthasarathy et al. [1992a, b], under much milder conditions with
respect to Pt surface oxidation.
1.6 A COMPREHENSIVE EXPRESSION FOR J
ORR
WITH
CONSIDERATION OF THE REAL NATURE OF THE CATHODE
CATALYST SURFACE
The ORR rate expression used in most previous ORR studies—theoretical as well
as experimental—has not included proper consideration of the actual, potential-
dependent catalyst surface composition under relevant PEFC cathode operating con-
ditions. From the discussion above, inclusion of the effect of chemisorbed oxygen
species in terms of site blocking, i.e., in terms of lowering of the number of active
metal sites available for the ORR, can be seen to be essential for the proper description
of ORR in a PEFC cathode. This effect of site blocking belongs in the pre-exponential
factor of the rate expression, which includes, in general, a frequency factor that is
multiplied by the probability of the surface reaction to generate a fruitful encounter
of the reactant molecule with the surface containing the catalytically active sites.
Accordingly, an expression that considers the site blocking effect under discussion
here will have the form
exp
E
E
O
2
=
H
2
O
b
int
J
ORR
(E)
¼
kN
total
P
O
2
(1
u
ox
) exp
DH
act
(1
:
10)
RT
where k is a frequency factor, N
total
is the total number of active sites, and DH
act
is the
activation energy for the (slow step in the) ORR process at E
O
2
=
H
2
O
with respect to
an RHE. In Reaction (1.10), the 1 2 u
ox
term is included in the pre-exponential
factor to describe the effect of site blocking on the probability of fruitful encounters,
as has been done previously in Uribe et al. [1992] and Wang et al. [2004]. But the
effect of the electrode potential in (1.10) is still confined to the exponential factor,
i.e., to the effect of cathode potential on the ORR activation energy at metal sites.
In reality, as discussed above, u
ox
is a function of E
cath
, and this dependence can be
described more explicitly.
As is well documented, formation of chemisorbed oxygen species on a Pt surface at
V . 0.75 V occurs in an inert atmosphere on Pt in contact with an aqueous, or hydrous
polymer electrolyte, by “anodic discharge” of water molecules to form OH
ads
on metal
sites, according to the Reaction (1.3). It is this chemisorbed oxygen species, derived
from “water discharge,” that will be considered in the following discussion.
Significantly, the Reaction (1.3) is associated with a redox potential E
Pt(H
2
O)
=
Pt
-
OH
ads
which is quite different from the redox potential for the faradaic ORR process,
E
o
2
=
H
2
O
. In fact, E
Pt(H
2
O)
=
Pt
-
OH
ads
was determined from recent DFT calculations of
formation energies for various oxygen species adsorbed on a Pt metal surface. The
reported result was E
Pt(H
2
O)
=
Pt
-
OH
ads
¼
0
:
80 V with respect to an RHE, with u
OH
¼
3
selected to define the standard state [Nørskov et al., 2004]. This calculated value of
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