Support and Particle Size Effects in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 560

gold catalysts designed to investigate the effect of particle size and substrate on
electrocatalytic activity.
We describe the application of a high throughput synthesis and screening method to
investigate the effect of substrate and particle size on electrocatalytic activity. Au sup-
ported on carbon and titania surfaces has been used to investigate these effects because
of the strong influence of particle size and support on the heterogeneous counterparts.
Combinatorial, or high throughput, approaches have been increasingly applied to the
optimization of the composition and structure of materials for specific physical or
chemical properties, not least in the fields of electrocatalyst and battery materials
[Reddington et al., 1998; Chen et al., 2001; Sun et al., 2001; Morris and Mallouk,
2002; Spong et al., 2003; Guerin et al., 2004]. Recently, Guerin and Hayden described
a physical vapor deposition system that has been developed for the high throughput
synthesis of thin-film materials [Guerin and Hayden, 2006]. It employs source shutters
to achieve controlled gradients of the depositing elements across a substrate or an array
of electrodes [Guerin and Hayden, 2006]. This allows, for example, the synthesis of
compositional libraries of alloys [Guerin et al., 2006a] and particle centers supported
on homogeneous supports [Guerin et al., 2006b, c; Hayden et al., 2007a, c]. This com-
binatorial approach has been applied to the optimization of Pd/Pt/Au ORR catalysts
[Guerin et al., 2006a] and the investigation of particle size and substrate effects in elec-
trocatalysis by Au and Pt particles [Guerin et al., 2006b, c; Hayden et al., 2007a, c,
2009]. The work described here centers on how the electrocatalyzed ORR and CO oxi-
dation reaction are influenced by particle size and support, and we emphasize the close
relationship in some cases between the activities of structures supporting electrocata-
lytic and heterogeneous catalyzed reactions.
16.2 PARTICLE SIZE AND GEOMETRIC EFFECTS
IN ELECTROCATALYSIS
16.2.1 Oxygen Reduction on Pt
The ORR has been widely studied in recent years because of its role in the cathode
reaction of low temperature polymer electrolyte membrane fuel cells (PEMFCs).
Even if a precious metal such as Pt is used as the cathode catalyst, which is known
to promote almost exclusively the four-electron reduction of oxygen to water, rela-
tively high overpotentials and low exchange current densities are observed.
Nevertheless, Pt appears to be one of the most effective catalysts for the ORR in
acidic media, but, because of the high price and scarcity of the element, it must be sig-
nificantly dispersed in the catalyst to be viable economically [Gasteiger et al., 2004,
2005]. State-of-the-art fuel cell catalysts are precious metal nanoparticles supported
on high surface area carbons, where the metal is commonly Pt or a Pt alloy
[Gasteiger et al., 2004]. The aim of the support and the alloying is to maximize the
weight activity of Pt. This approach has yielded a significant cost reduction over the
last three decades of fuel cell catalyst development, although there remains a signifi-
cant potential loss in PEMFCs (under load, cell potentials are commonly in the range