Electrocatalysis for the Direct Alcohol Fuel Cell - Fuel Cell Catalysis: A Surface Science Approach - page 354

As soon as acetaldehyde is formed, it can adsorb on Pt sites, leading to a Pt - CO - CH 3
species:

Pt þ CHO-CH 3 ! Pt (CO-CH 3 ) ads þ e þ H þ

(11 : 17)

Because Sn is known to activate water at lower potentials than Pt, OH species can be
formed at low potentials on Sn sites according to the reaction

Sn þ H 2 O ! Sn (OH) ads þ e þ H þ

(11 : 18)

and the adsorbed acetaldehyde species can react with adsorbed OH species to produce
acetic acid according to the reaction

Pt (CO-CH 3 ) ads þ Sn (OH) ads ! Pt þ Sn þ CH 3 COOH (11 : 19)

At this stage, it should be pointed out that modification of a Pt-Sn catalyst by Ru
atoms increases cell performance (and hence catalytic activity with regard to ethanol
electro-oxidation), but has no effect on the OCV or on product distribution
[Rousseau et al., 2006]. It seems, then, that the oxidation mechanism is the same on
Pt-Sn and Pt-Sn-Ru, which supports the proposition that Ru allows OH species to be
produced when the anode potential is increased and noncatalytically active tin
oxides are formed.

11.5 ELECTROCATALYTIC ASPECTS OF OXYGEN REDUCTION

One of the main issues concerning the improvement of DAFC efficiency is to limit or
avoid the effect of crossover of the alcohol, i.e., crossing of the electrolytic membrane
by alcohol from the anode side to the cathode side due to intrinsic permeability of the
membrane and electro-osmosis. This phenomenon causes a great depolarization of the
cathode and hence a decrease in fuel cell electrical performance [Oedergaard et al.,
2004; Hogarth and Ralph, 2002; Yang, 2004]. Several cathode catalysts have been
studied in order to improve the oxygen reduction reaction (ORR) in the absence or
in the presence of methanol, including Pt-alloy particles [Paffett et al., 1988; Beard
and Ross, 1990; Toda et al., 1999a, b; Neergat et al., 2001]; transition metal chalco-
genides [Alonso-Vante et al., 2000]; macrocycles, either heat-treated [Lalande et al.,
1995, 1996] or not [Zagal et al., 1980; Coutanceau et al., 1994, 1995]; and N 2 -Co or
N 2 -Fe based catalysts [Bashyam and Zelenay, 2006]. Other non-Pt catalysts have
recently been investigated for acid membrane electrolyte fuel cell application, such
as chalcogenides (CoSe or FeSe [Sidik and Anderson, 2006]; CoS or FeS
[Campbell, 2006]), and CoN 2 -type catalysts [Bashyam and Zelenay, 2006]. These
catalysts displayed relatively good activity, although lower than that of Pt, and good
tolerance to the presence of alcohol. However, for most of these catalysts, their
structure, and therefore the reason for their activity and stability, is still unknown.
Numerous studies have shown that Pt-based binary alloy electrocatalysts such as
Pt-Fe, Pt-Co, Pt-Ni, and Pt-Cr exhibit a higher catalytic activity for the ORR in an