Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 297

the other still adsorbed on the hollow site near Co after the dissociative adsorption of
the O 2 molecule [Fernandez et al., 2006]. The second O 2 could dissociate on Co with
an O atom pre-bound on the hollow site near it. Balbuena and co-workers proposed a
similar thermodynamic guideline for designing Pd alloy catalysts [Wang and
Balbuena, 2005a, b]. For Pd with fully occupied valence d-orbitals, alloying with tran-
sition metals such as Co with unoccupied valence d-orbitals reduces significantly the
Gibbs free energy both for the first charge-transfer step and for the steps involving the
reduction of intermediates. However, it is not clear whether O 2 can still easily dis-
sociate after the reactive metal centers are fully occupied by O; besides, the reactive
metals on the alloy surface are unstable and leach out rapidly during
electrochemical measurements. Thus, these arguments for ORR electrocatalysis on
bimetallic surfaces cannot explain the relatively good stability of Pd-M alloys in
acidic media [Tarasevich et al., 2007]. The Pd-enriched skin, on the other hand, can
account for both the good activity and the stability [Lamas and Balbuena, 2006;
Shao et al., 2007a; Suo et al., 2007].
Pd ternary alloys, including Pd-Co-Au [Fernandez et al., 2005a, b] and Pd-Co-Mo
[Raghuveer et al., 2005] have been developed to further improve the stability of the
catalyst. The addition of 10% Au to the Pd-Mo mixture improved catalyst
stability. Another promising way to improve the activity and durability of Pd-M
alloys is to deposit a Pt monolayer on them. Recently, a Pt monolayer
deposited on Pd 3 Fe/C was found to possess higher activity than that of Pt/C [Shao
et al., 2007b].
Methanol tolerance is a very important property of Pd-M alloys. In particular,
methanol tolerance was demonstrated for Pd 3 Fe/C and for Pd-Co based alloys
[Mustain et al., 2007; Raghuveer et al., 2005; Shao et al., 2006c; Zhang L et al.,
2007]. The high ORR activity in the presence of a high concentration of methanol
indicates that the Pd-Co and Pd-Fe electrocatalysts are not active for methanol
oxidation.
9.4 CATALYST STABILITY
One of the critical issues with regard to low temperature fuel cells is the gradual loss of
performance due to the degradation of the cathode catalyst layer under the harsh oper-
ating conditions, which mainly consist of two aspects: electrochemical surface area
(ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon sup-
port itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid
fuel cells (PAFCs) have shown that ECA loss is mainly caused by three mechanisms:
† Pt dissolution and redeposition (Ostwald ripening)
† Pt particle agglomeration due to crystallite migration on carbon supports
† Pt particle agglomeration due to carbon corrosion
Similar ECA loss phenomena have been observed in PEMFCs. Understanding ECA
loss and carbon corrosion mechanisms may help with designing more durable