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reaction mechanism (T - V or H - V), and the nature of the hydrogen intermediate
involved, as well as the influence of the structure on the reaction kinetics and the
mechanism. Chen and Kucernak have proposed a novel approach for the investi-
gation of electrochemical reactions using single submicron Pt particles with size
down to about 40 nm grown on the tip of a carbon fiber as ultrasmall electrodes
[Chen and Kucernak, 2004a, b]. Diffusion to the surface of such an ultramicroelec-
trode is hemispherical, with the mass transport coefficient being inversely pro-
portional to the particle radius. Thus, for a 36 nm particle, the apparent mass
transport coefficient was estimated as about 10 cm s
21
. In order to achieve such a
mass transport rate at an RDE, the rotation speed would have to be 4.6
10
8
rev
min
21
[Chen and Kucernak, 2004a, b], which is obviously not feasible. The
approach described allows electrochemical reactions to be studied in the absence
of mass transport limitations. The work by Chen and Kucernak puts into question
a number of previous studies on HOR kinetics and mechanism, where separation
of kinetic and mass transport losses might have been inadequate, and has led to
renewed interest to the mechanism of this important reaction [Chialvo and
Chialvo, 2006; Quaino et al., 2006; Wang et al., 2006]. These recent studies also
challenge previous conclusions on the structural sensitivity of the HOR. For
example, according to Markovic and co-workers, the exchange current density j
0
for Pt(hkl) basal planes falls in the range 0.45 - 0.98 mA cm
22
and the reaction
mechanisms differ between crystal planes, with Pt(111) being the least active
[Markovic et al., 1997b]. In contrast, Seto and co-workers suggested that the
(111) surface has the highest HOR activity, j
0
for Pt(hkl) basal planes being in
the range 1.7 - 3.0 mA cm
22
, while the mechanism (T - V) is the same for all
basal planes [Seto et al., 1987]. Meanwhile, according to the single-particle study
by Chen and Kucernak, j
0
is much higher, amounting to about 20 mA cm
22
[Chen and Kucernak, 2004a, b]. This value is in agreement with j
0
27 mA
cm
22
estimated by Gasteiger and co-workers from MEA studies [Gasteiger et al.,
2004].
Fast HOR kinetics coupled with slow mass transport of H
2
hinder investigation of
the PSEs. The reaction rates reported for small Pt particles by Takasu and co-workers
[Takasu et al., 1989] are several orders of magnitude lower than those deduced from
the work of Chen and Kucernak, and therefore may be questioned. A rare example of a
direct observation of the size dependence of HOR kinetics is the study by Antoine and
co-workers, who investigated the reaction using a GDE approach in the wide Pt par-
ticle size interval from 2.5 to 28 nm by increasing the Pt loading on a Vulcan XC-72
carbon support from 10 wt% to 80 wt% [Antoine et al., 1998]. The authors determined
specific activity (SA) per unit surface area of Pt, and have shown that it increases by
about a factor of ten when the particle size is decreased from about 10 down to 2.8 nm.
The authors applied a mathematical model in order to clarify whether this remarkable
effect is driven by electrochemical kinetics or is related to mass transport, namely tran-
sition from linear to spherical diffusion along with a decrease in metal loading and a
concomitant increase in interparticle separation. After correction for mass transport
effects, Antoine et al. concluded that intrinsic catalytic activity does indeed increase
with decreasing particle size.
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