Environmental Engineering Reference
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of Pt dissolved. Thus, an anodic dissolution mechanism was suggested either via (9.5)
or via (9.6) and (9.7). A cathodic dissolution mechanism is also possible according to
Johnson and co-workers, who detected Pt 2 þ in a rotating ring - disk electrode study
during the negative-going potential scan in 0.1 M HClO 4 solution [Johnson et al.,
1970]. The Pt 2 þ species was formed as a result of the reduction of PtO 2 [Johnson
et al., 1970; Mitsushima et al., 2007a, b]:
PtO 2 þ 4H þ þ 2e ! Pt 2 þ þ 2H 2 O
E 0 ¼ 0 : 84 þ 0 : 12 pH þ 0 : 029 log[Pt 2 þ ](9 : 8)
The dissolution rate during potential cycling was reported to be around 3.0 - 5.5
ng/cm 2 per cycle, with the upper potential limit between 1.2 and 1.5 V and various
potential scanning rates [Johnson et al., 1970; Kinoshita et al., 1973; Rand and
Woods, 1972; Wang XP et al., 2006]. The predominant forms of dissolved Pt were
Pt 2 þ species [Johnson et al., 1970; Wang XP et al., 2006] and Pt 4 þ species
[Mitsushima et al., 2007a, b; Rand and Woods, 1972] in HClO 4 and H 2 SO 4 solution,
respectively, which may be due to the ability of H 2 SO 4 to form a more stable complex
with Pt 4 þ [Rand and Woods, 1972].
Dissolved Pt species (Pt z þ ) at the cathode can either redeposit on existing Pt par-
ticles, resulting in particle growth, or diffuse into the membrane (PEMFC) or
matrix (PAFC). The concentrations of these species increased upon the aging of the
membrane electrode assembly (MEA), suggesting that mobile Pt species move into
the membrane [Guilminot et al., 2007a, b]. A Pt band of large particles forms in the
membrane near the interface of membrane/cathode during cycling with H 2 /N 2
[Ferreira et al., 2005; Ferreira and Shao-Horn, 2007; Yasuda et al., 2006a, b], and
somewhere away from the cathode with H 2 /O 2 (air) [Bi et al., 2007; Patterson,
2002; Yasuda et al., 2006a, b; Zhang J et al., 2007a]. Studies combining experimental
data and mathematical modeling suggested that the location of the Pt band in the mem-
brane under open circuit voltage and cycling conditions depends on the partial press-
ures of H 2 and O 2 and on the permeability of those species and Pt z þ through the
membrane [Bi et al., 2007; Zhang J et al., 2007a] (Fig. 9.23). These results suggest
that Pt z þ in the membrane is chemically reduced by H 2 . Pt can also move into the
anode in the absence of H 2 [Yasuda et al., 2006a, b].
The driving force for the crossover of dissolved Pt species into the membrane/
matrix can be electro-osmotic drag and/or concentration gradient diffusion
[Guilminot et al., 2007a, b]. The identity of the counteranions of Pt z þ , however, is
not yet clear [Johnson et al., 1970; Kinoshita et al., 1973; Ota et al., 1988; Rand
and Woods, 1972]. Membrane degradation products, such as fluoride [Healy et al.,
2005; Xie et al., 2005a, b] and sulfate [Teranishi et al., 2006; Xie et al., 2005a, b]
anions, have been detected during the operation of PEMFCs, and may be the complex-
ing ligands for Pt z þ . In fact, Guilminot and co-workers presented strong evidence that
the concentration of fluoride around the Pt nanoparticles in an aged membrane was
higher than that in a new one [Guilminot et al., 2007a, b]. Other halide ions, such
as chloride and bromide, left on carbon and Pt surfaces during catalyst synthesis rep-
resent another possibility [Guilminot et al., 2007a, b].
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