Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 301

Figure 9.24 Amounts of dissolved platinum, normalized to 1 cm 2 and 1 mL of solution,
plotted against the dissolution potentials for all the investigated surfaces. (Reproduced with per-
mission from Komanicky et al. [2006].)
Zhou [2007] have recently suggested that electronically conductive supports, such as
carbon black, are critical for Pt re-precipitation. Therefore, particle growth by Ostwald
ripening involves coupled transport, for example, Pt ions and electron transport via
water/ionomer and conductive supports, respectively [Virkar and Zhou, 2007].
Yasuda and co-workers performed an interesting study using double-layer cathodes.
Pt particles were not observed in the membrane with a Pt black layer sandwiched
between the membrane and a normal Pt/C catalyst layer, while no substantial differ-
ence was found between cathode layers with and without sandwiching an extra carbon
black layer. The authors concluded that dissolved Pt species were easier to deposit on
unsupported Pt black, which has a much higher surface energy than carbon [Yasuda
et al., 2006a, b]. If these results are correct, then a new door to minimizing the ECA
loss may be opened through capturing the dissolved Pt species by materials with
higher surface energy than Pt [Tseung and Dhara, 1975].
However, minimization of the surface energy can drive crystallite migration/
coalescence as well as Ostwald ripening, independent of an electric potential. There
were indeed reports that ECA loss during the operation of PAFCs was insensitive to
the potential [Blurton et al., 1978; Gruver et al., 1980]. The crystallite migration
and coalescence mechanism would result in the particle size distribution tailing
toward large particles, which in fact has been observed in studies of both PAFCs
[Aragane et al., 1988; Bett et al., 1976; Blurton et al., 1978] and PEMFCs [Wilson
et al., 1993]. Bett and co-workers found that particle coalescence did not become
more severe with higher Pt loading on carbon supports, as would be expected from