Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 543

† Dependence of the electric field distribution in the double layer on particle size
[Zhdanov and Kasemo, 2002; Chen and Kucernak, 2004a, b], which, according
to Zhdanov and Kasemo, should result in an increase in the rates of electro-
chemical reactions on nanometer-sized metal particles.

Further studies are needed in order to better understand the origins of the PSEs in
electrocatalysis.
The strong sensitivity of reaction kinetics to point and extended defects requires that
studies of size and structural effects be performed with materials whose size and struc-
ture are characterized using a variety of complementary imaging and spectroscopic tech-
niques, preferably in situ. This would greatly aid in reducing controversies and
misinterpretations in the electrocatalysis of nanoparticulate systems. Although size
and structural effects are closely related, it is worth separating effects inherent to
small systems from structural effects that are common also to extended surfaces. A note-
worthy example concerns monoatomic steps, which may be present on single-crystalline
surfaces and likely also on nanoparticle facets. Another example concerns intergrain
boundaries—extended defects whose presence strongly affects electrocatalytic activity,
as confirmed for CO and methanol electro-oxidation on Pt and Pt-Ru materials.
Multigrained materials with a high density of intergrain boundaries are generic for
fuel cell electrocatalysts containing metals in high weight percentage. Unfortunately,
their formation is often ignored, and this may lead to misinterpretation of PSEs.
Understanding of PSEs is complicated by a lack of understanding of the intimate
mechanisms of electrocatalytic reactions, for example for the ORR and the
MOR. Further insights into the reaction mechanisms and identification of the rate-
determining steps will help to elucidate PSEs.
Interpretation of published data is often complicated by the fact that rather complex
catalytic materials are utilized, namely, polydisperse nonuniform metal particles,
highly porous supports, etc., where various secondary effects may influence or even
submerge PSEs. These include mass transport and discrete particle distribution effects
in porous layers, as confirmed by Gloaguen, Antoine, and co-workers [Gloaguen et al.,
1994, 1998; Antoine et al., 1998], and diffusion - readsorption effects, as shown by
Jusys and co-workers for the MOR and by Chen and Kucernak for the ORR [Jusys
et al., 2003; Chen and Kucernak, 2004a, b]. Novel approaches to the design of ordered
nanoparticle arrays where nanoparticle size and interparticle distances can be varied
independently

are expected to shed light on PSEs in complex multistep

multielectron processes such as the MOR and the ORR.
As the reader might have noticed, many conclusions in electrocatalysis are based on
results obtained with electrochemical techniques. In situ characterization of nanopar-
ticles with imaging and spectroscopic methods, which is performed in a number of
laboratories, is invaluable for the understanding of PSEs. Identification of the types
of adsorption sites on supported metal nanoparticles, as well as determination of the
influence of particle size on the adsorption isotherms for oxygen, hydrogen, and
anions, are required for further understanding of the fundamentals of electrocatalysis.
Understanding the physical origins of PSEs requires a coordinated effort from
experimentalists and theoreticians, and will proceed together with the development