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acid electrolyte than pure Pt [Paffett et al., 1988; Beard and Ross, 1990; Toda et al.,
1999a, b; Neergat et al., 2001]. In that case, to achieve economic metal loadings,
fuel cell electrodes must be prepared from Pt-based nanoparticles (d , 5 nm) sup-
ported on high surface area carbon black. For the ORR, the specific activity SA (in
mAcm 22
Pt ) decreases with decreasing particle size, while it remains approximately
the same as on Pt electrodes for particles larger than 5 nm [Kinoshita, 1992]. As a
result, plots of the mass activity MA (in A g P 21 ) versus particle size for oxygen
reduction in a sulfuric or phosphoric acid electrolyte exhibit a marked maximum
for particle sizes close to 3.5 nm [Kinoshita, 1992].
As an example, Fig. 11.11 shows the oxygen reduction polarization curves
obtained at Pt 12x Cr x /C catalysts prepared using the carbonyl route [Yang et al.,
2004]. The presence of Cr leads to a shift in the onset of the reduction wave towards
higher potential, whereas the limiting current density is the same as that at Pt nanopar-
ticles, indicating that the final product is likely the same (in agreement with a four-
electron process of oxygen electro-reduction). The observed electrocatalytic enhance-
ment was interpreted either by an electronic factor, namely, the change in the d-band
vacancy in Pt upon alloying, and/or geometric effects (Pt coordination number
and Pt - Pt distance). On the basis of X-ray absorption near-edge structure
(XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopic inves-
tigations of the ORR at Pt/C and Pt-X/C catalysts, Mukerjee and co-workers showed
that Pt/C exhibited a significant increase in the number of Pt d-vacancies per atom
compared with Pt alloys [Mukerjee et al., 1995]. They showed that this increase
was due to the adsorption of OH species from the electrolyte, resulting in a decrease
in electrocatalytic activity with regard to the ORR compared with Pt alloys. Both
Figure 11.11 Linear cyclic voltammograms of carbon-supported nanosized Pt and Pt-Cr
alloy catalysts with different atomic ratios (prepared using the carbonyl route [Yang et al.,
2004]) recorded in 0.5 M HClO 4 saturated with pure oxygen at a scan rate of 5 mV s 21 and a
rotation speed of 2000 rev min 21 . Current densities are normalized to the geometric surface
area. ——, Pt/C;----,Pt 0.7 Cr 0.3 /C;—-—-,Pt 0.5 Cr 0.5 /C.
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