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co-workers in H 2 SO 4 [Giordano et al., 1991; Gamez et al., 1996]. According to
Gamez et al. [1996], the local current density at a Pt/C particle is much smaller
than the local limiting current density expected for spherical diffusion, and suggests
that in the interval of low currents/low overpotentials, diffusion limitations
occur only when the interparticle distance is less than five times the size of the Pt
particles. Recently, a diffusion-related effect has been reported for model Pt
nanoparticle arrays supported on flat indium - tin oxide (ITO)-coated glass substrates
[Kumar and Zou, 2006]. O 2 electro-reduction on Pt particle arrays with the same
particle size of 4 nm shows a negative shift in the peak potential and a significant
increase in current density with increasing interparticle spacing from 40 to 80 nm.
These changes have been explained by the change of the extent of O 2 diffusion
field overlap.
Yano and co-workers have recently undertaken an RDE study of the ORR on
carbon-supported Pt nanoparticles combined with 195 Pt electrochemical nuclear
magnetic resonance (NMR) spectroscopy [Yano et al., 2006]. They conclude that
there is a negligible difference in the surface electronic properties of these Pt/CB
(carbon black) catalysts owing to size variations and therefore that ORR activities
are not affected by differences in particle size. It is unfortunate that the authors of
this otherwise very careful study have chosen to determine the kinetic currents
at rather low electrode potentials of 0.70, 0.76, and 0.80 V vs. RHE. Under these
conditions, the measured current is approaching the mass-transport-limiting value,
and proper evaluation of the intrinsic catalytic activity is hardly feasible. Note that
according to Chen and Kucernak, even at 0.9 V the measured current is strongly
influenced by mass transport, not to speak of more negative electrode potentials
[Chen and Kucernak, 2004a, b]. Higuchi and co-workers put special effort into prepar-
ing a very thin layer (,0.1 mm) film of carbon particles on a solid support and disper-
sing the catalyst very evenly [Higuchi et al., 2005]. Unfortunately, they did not take
into consideration the fact that even for these very thin CLs, mass transport limitations
are overwhelming and the effectiveness factor may be well below 1 at the high over-
potentials utilized in their work. Thus, the absence of the PSE claimed by Yano and
co-workers is likely related to severely underestimated SAs due to mass transport
limitations [Yano et al., 2006]. Our concerns are confirmed by the values of the kinetic
currents furnished by these authors. For example, for bulk Pt, the apparent
rate constant k app ¼ j k /(4F[H þ ][O 2 ]) plotted in Fig. 15.6 of [Yano et al., 2006]
amounts to about 10 2 cm 4 mol 21 s 21 at about 0.8 V vs. RHE and 60 8C. Assuming
a Tafel slope of 60 mV dec 2 , this translates into about 10 2 mAcm 22 at 0.9 V vs.
RHE. This is an order of magnitude lower than the value reported by Gasteiger and
co-workers for polycrystalline Pt under relevant conditions [Gasteiger et al., 2005].
Note that for Pt/CB samples, along with the external mass transport losses, internal
losses in the pores of carbon materials cannot be ruled out. Unfortunately, Yano et al.
did not supply details of the specific surface areas of the carbons utilized. Low
values for kinetic currents are reported also in Higuchi et al. [2005].
In concluding this discussion, we would like to point out that understanding PSEs
in the ORR is a very important issue, and special care should be taken to determine
appropriate experimental conditions.
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