Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 298

catalysts for the ORR. In this section, we will discuss the ECA loss mechanisms. The
carbon corrosion mechanism is outside the scope of this review.

9.4.1 Platinum Dissolution

The thermodynamic behavior of Pt bulk is described by potential - pH diagrams
(Pourbaix diagrams) [Pourbaix, 1974]. The main pathways for Pt dissolution involve
either direct dissolution of the metal,

Pt ! Pt 2 þ þ 2e E 0 ¼ 1 : 19 þ 0 : 029 log [Pt þ ]

(9 : 5)

or formation of an oxide film and subsequent chemical reaction,

Pt þ H 2 O ! PtO þ 2H þ þ 2e

E 0 ¼ 0 : 98 0 : 59 pH (9 : 6)

PtO þ 2H þ ! Pt 2 þ þ H 2 O log [Pt 2 þ ] ¼ 7 : 06 2pH (9 : 7)

The dissolution through Pt oxides is expected to be slow owing to the self-
passivation effect. The solubility of Pt at different potentials, pH, and temperatures
has been extensively studied. In acidic solutions, Pt solubility increases with decreas-
ing pH, suggesting an acidic dissolution mechanism [Mitsushima et al., 2007a, b].
Pt solubility depends strongly on temperature, following an Arrhenius relationship
[Dam and de Bruijn, 2007; Mitsushima et al., 2007a, b]. The effect of potential on
Pt solubility has been extensively studied in acidic solutions [Bindra et al., 1979;
Dam and de Bruijn, 2007; Ferreira et al., 2005; Mitsushima et al., 2007a, b; Wang
XP et al., 2006] and summarized in [Borup et al., 2007]. The equilibrium concen-
tration of dissolved Pt increases with potential up to 1.1 V [Bindra et al., 1979;
Ferreira et al., 2005; Wang XP et al., 2006] and decreases thereafter, which is ascribed
to the formation of a PtO film [Wang XP et al., 2006]. At higher potential, Wang and
co-workers found that Pt dissolution was alleviated by the formation of a PtO 2 film.
From the slope of the logarithmic plot of Pt solubility against potential in hot concen-
trated H 3 PO 4 solution, Bindra and co-workers concluded that Pt dissolution under-
went a two-electron reaction pathway (9.5) [Bindra et al., 1979], which is consistent
with the Pourbaix diagram [Pourbaix, 1974]. However, in H 2 SO 4 (80 8C) [Ferreira
et al., 2005] and HClO 4 (23 8C) [Wang X et al., 2006] solutions and lower tempera-
tures, the slopes are much smaller, indicating that Pt dissolution involves the formation
of dissolved species other than Pt 2 þ .
Potential cycling has been found to accelerate Pt dissolution compared with poten-
tiostatic conditions. The dissolution mechanisms and dissolved species involved in
this process are unclear [Johnson et al., 1970; Kinoshita et al., 1973; Ota et al.,
1988; Rand and Woods, 1972]. Darling and Meyers have developed a mathematical
model based on (9.5) - (9.7) to study Pt dissolution and movement in a PEMFC
during potential cycling from 0.87 to 1.2 V [Darling and Meyers, 2003, 2005].
Severe Pt dissolution occurs when the potential switches to the upper limit potential
(1.2 V), and then stops once a monolayer of PtO has formed. The charge difference
between the anodic and cathodic cycles was found to be consistent with the amount