Page 101 - MODERN ASPECTS OF ELECTROCHEMISTRY

Claude LamyAet al.
84
structural effect was confirmed by in situ infrared spectroscopy which
showed a high coverage of adsorbed CO on a Pt( 110) plane: conversely,
on the other single-crystal planes, a distribution of different species is
clearly visible. 36,42,43 Further, strong lateral interactions between the dif-
ferent adsorbed species on Pt(100) lead to very low activity of this
electrode at low potentials. 36

(ii) Effect of Particle Size and Carbon Support
The effects of dispersion of the electrocatalyst and of particle size on
the kinetics of electrooxidation of methanol have been the subject of
numerous studies because of the utilization of carbon support in DMFC
anodes. The main objective is to determine the optimum size of the
platinum anode particles in order to increase the effectiveness factor of
platinum. Such a size effect, which is widely recognized in the case of the
reduction of oxygen, 44,45 is still a subject of discussion for the oxidation
46
of methanol. According to some investigators, an optimum of 2 nm for
the platinum particle size exists, but studying particle sizes up to 1.4 nm,
48
other authors observed no size effect. According to a recent study, the
47
rate of oxidation of methanol remains constant for particles greater than
4.5 nm, but decreases with size for smaller particles (up to 2.2 nm).
Such information can be obtained from cyclic voltammetric measure-
49
ments. It is possible to determine the quantity of electricity involved in
the adsorption of hydrogen, or for the electrooxidation of previously
adsorbed CO, and then to estimate the real surface area and the roughness
factor (γ) of a Pt-C electrode. From the real surface area and the Pt loading,
it is possible to estimate the specific surface area, S (in m g ), as follows:
2
-1
S = 100 γ/W (30)
-2
where W (in µg cm ) is the Pt loading of the carbon powder (referred to
the geometric surface area) and γ is the roughness factor, i.e., the ratio of
the real surface area A to the geometric surface area A . This specific
r
g
surface area is useful for easily characterizing the dispersed catalyst. Using
the simple idea that all the particles can be assumed to be hemispherical,
the particle diameters din nanometers can be obtained from the value of
S by:

d = 60W = 6000 (31)
γ ρ Sρ