Environmental Engineering Reference
In-Depth Information
direct pathway is dominant is usually made on the basis of RRDE studies. Low
amounts of H
2
O
2
detected at the ring of a Pt-disk RRDE is often taken as support
for a direct four-electron mechanism of the ORR on Pt.
The ORR has been most thoroughly investigated on Pt electrodes of various
structures and morphologies. The reaction rate is first order with respect to the
oxygen partial pressure P
O
2
, which implies that the O
2
bond break occurs after or
during the ORR rate-determining step (RDS). The reaction order with respect to H
þ
concentration is equal to 1
2
at low and 1 at high current densities [Zinola et al.,
1994]. The Tafel slopes for Pt electrodes in acidic solutions have been found to
change from about 2120 mV dec
21
in the potential interval below about 0.8 V vs.
RHE, where the Pt surface is free from oxide, to about 260 mV dec
21
at higher
electrode potentials, where the surface is oxide-covered. Based on the experimental
observations, the RDS on Pt is usually believed to be the first electron transfer coupled
with the proton transfer:
(O
2
)
ads
þ
H
þ
þ
e
!
(O
2
H)
ads
(15
:
16)
The reaction rate can therefore be expressed as [Markovic and Ross, 2002; Chen and
Kucernak, 2004a, b]:
exp
a
c
FE
RT
j
k
¼
k
0
nFP
O
2
[H
þ
]
ð
1
u
Þ
exp
DG
u
RT
(15
:
17)
where k
0
is the heterogeneous rate constant of the RDS, [H
þ
] is the concentration of
protons near the electrode surface, 1 -uis the free surface site coverage, and DG
u
is
the coverage-dependent free energy of oxygen adsorption, the other terms having
their usual meanings.
ORR kinetics is usually investigated using an RDE or RRDE, and the kinetic
current density is isolated from the measured current via Levich - Koutecky analysis.
Since the exchange current density is very low, the activity is usually characterized by
the kinetic current density at specified electrode potential in the kinetic region (usually
at 0.9 V) either as specific activity, SA (in A cm
22
), or, for dispersed materials, as
mass activity, MA (in A g
21
). Chen and Kucernak [2004a, b] have studied the
ORR on Pt under enhanced mass transport conditions using the single-particle
approach described above. Particles with a radius of several micrometers demonstrated
behavior similar to that of polycrystalline Pt, with the effective number of electrons
transferred in the overall reaction, n
eff
, being close to 4. However, decrease of the par-
ticle radii below about 1 mm resulted in a systematic decrease of n
eff
to 3.5, which
suggests that only about 75% of the reactant oxygen molecules are reduced to
water, with the rest being only reduced as far as hydrogen peroxide. This means
that the small amount of H
2
O
2
usually detected on Pt electrodes in a conventional
RRDE or thin-layer cell configuration is not necessarily due to the four-electron
direct pathway, but should rather be attributed to the slow diffusion of H
2
O
2
from
the electrode surface, its readsorption, and further reaction via the series pathway.
Search WWH ::
Custom Search