Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 561

of about 0.6 - 0.85 V), mainly as a result of the associated low kinetics and hence high
overpotentials for oxygen reduction at the cathode. The increase in mass activity
gained by dispersion of Pt appears to be limited, and there seems to be a “critical”
particle size (approximately 3 nm in diameter) at which further dispersion leads to a
loss of mass activity and hence an economic penalty [Kinoshita, 1990].
There has been a considerable effort to understand the structural and electrolyte
dependence of the ORR through experiments on single-crystal surfaces. Significant
electrolyte-dependent oxygen reduction activity on Pt was reported and explained
by the effects of specific anion adsorption [Hsueh et al., 1983]. The activity was
found to increase in the order H 2 SO 4 CF 3 SO 3 H , H 3 PO 4 , HClO 4 . Oxygen
reduction on Pt was also found to depend on the structure of the Pt(hkl) surface
[Markovic et al., 1995, 1997]. The activity in weakly adsorbing HClO 4 was found
to increase in the order (100) , (110) (111), with only small differences in the
rate of ORR, while in H 2 SO 4 , where strongly adsorbing HSO 4 2 is present, the differ-
ence between highest and lowest activity was two orders of magnitude, following the
order (111) (100) , (110). Measurements on stepped single-crystal electrodes in
H 2 SO 4 showed higher activity, which was explained by a lowering of (bi)sulfate
adsorption, whereas in HClO 4 , the activities of the stepped surfaces were not found
to be enhanced significantly [Mac´a et al., 2004; Komanicky et al., 2005].
On the basis of the single-crystal observations, one may expect a structure-sensitive
(particle-size-dependent) rate dependence for oxygen reduction in the presence of
strongly adsorbing anions. However a nonlinear dependence of mass activity on dis-
persion that indicates lower specific activity at small particles has been reported not
only in H 2 SO 4 and H 3 PO 4 environments, but also in HClO 4 [Kinoshita, 1990;
Antoine et al., 2001; Guerin et al., 2004]. It has also recently been reported that if
the interspacing of dispersed particles is taken into account, there is in fact no particle
size dependence in HClO 4 electrolyte [Yano et al., 2006]; this is supported by
Stonehart's earlier work [Stonehart, 1994], where it was found that the specific
ORR activity was independent for particles separated by distances at least 10 times
the particle size [Watanabe et al., 1989]. The origin of the electrolyte dependence
or the intrinsic activity of the surface with regard to oxygen reduction has been
described in terms of the influence of surface structure or particle size on the adsorp-
tion of OH. Generally, the OH adsorption strength seems to increase as the particle size
decreases; that is, Pt oxide formation/reduction is found to become more irreversible
[Takasu et al., 1996; Markovic et al., 1997; Antoine et al., 2001; Guerin et al., 2004],
and this can be correlated with a progressively positive shift in binding energy of the
f-states of Pt for smaller particles [Takasu et al., 1996].
16.2.2 Oxygen Reduction on Au
Au has recently received less attention than Pt as a supported catalyst because of its
lower impact in PEMFC energy conversion technology, since the ORR is dominated
by a two-electron reduction process, at what is a high overpotential, in acidic media.
Nevertheless, it is an important oxygen reduction catalyst in alkaline media, and, in
contrast to Pt, is oxide-free in the potential range where oxygen reduction occurs.