Page 92 - MODERN ASPECTS OF ELECTROCHEMISTRY

Direct Methanol Fuel Cells
2. Identification of the Reaction Products and the AdsoáedA 75
Intermediates
The electrocatalytic oxidation of methanol has been thoroughly investi-
gated during the past three decades (see reviews in Refs. 21-27), particu-
larly in regard to the possible development of DMFCs. 21,22,25,27 The
oxidation of methanol, the electrocatalytic reaction, consists of several
steps, which also include adsorbed species. The determination of the
mechanism of this reaction needs two kinds of information: (1) the
electrode kinetics of the formation of partially oxidized and completely
oxidized products (main and side products) and (2) the nature and the
distribution of intermediates adsorbed at the electrode surface.

(i) Experimental Methods

(a) Analysis of reaction products
The quantitative analyses of reaction products due to partial or
complete oxidation can be performed by different methods. This type of
determination is essential to improve electrode composition. Apart from
a decrease in the Coulombic efficiency (see Section II.2), the formation
of partially oxidized products can be deleterious for the DMFC application
because some of these products (e.g., formic acid) may be in liquid form
and are corrosive.
Quantitative analysis can be carried out by chromatography (in gas
or liquid phase) during prolonged electrolysis of methanol. 24,28,29 The
main product is carbon dioxide, 24,28,29 which is the only desirable oxida-
tion product in the DMFC. However, small amounts of formic acid and
formaldehyde have been detected, mainly on pure platinum electrodes.
The concentrations of partially oxidized products can be lowered by using
platinum-based alloy electrocatalysts; for instance, the concentration of
carbon dioxide increases significantly with Pt-Ru and Pt-Ru-Sn elec-
22
trodes, which thus shows a more complete reaction with alloy electro-
catalysts.
In situ analysis of the reaction products can also be carried out by
mass spectrometry, using the differential electrochemical mass spectrome-
try (DEMS) technique. This technique permits the detection of gaseous
30
products since they are produced and captured through a porous electrode.
It has been confirmed that carbon dioxide is the main reaction product.
With this technique, it is also possible to determine the production of CO 2