Environmental Engineering Reference
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electro-oxidation kinetics. This is due to the formation of strongly bonded species
limiting the number of active sites. In order to improve the reaction kinetics, a deep
understanding of the mechanisms of the electrocatalytic reactions is a key issue. As
several different steps are generally necessary to carry out the complete electro-
catalytic reaction, an optimized catalyst should be multifunctional.
At Pt catalysts, methanol is adsorbed, with the formation of poisoning species
(adsorbed carbon monoxide) [Beden et al., 1987]. On the other hand, owing to the
chemical structure of ethanol, its electro-oxidation is more difficult because of the
necessary cleavage of the C - C bond to form the final product (carbon dioxide).
However, even if such bond breaking is difficult, adsorbed CO is also observed by
in situ infrared reflectance spectroscopy [Vigier et al., 2004a]. In both cases, the for-
mation of such poisoning species leads to poor activity, and the challenge is to enhance
the activity of Pt. Because of the different steps likely involved in methanol and ethanol
adsorption and oxidation at Pt, the design of multi-metallic electrocatalysts is essential.
The composition of the catalysts (nature and proportion of the metals involved) and the
structure (size of particles, atomic arrangement, superficial structure, etc.) are crucial,
and their tolerance to ageing should also be taken into account.
11.2 THERMODYNAMICS AND KINETICS OF REACTIONS
IN A DAFC: ETHANOL OXIDATION
Considering as an example a DEFC, electro-oxidation of ethanol takes place at the
anode (negative pole of the cell),
CH 3 CH 2 OH þ 3H 2 O ! 2CO 2 þ 12H þ þ 12e
E 1 ¼ 0 : 085 V vs. SHE
(11 : 1)
while at the cathode (positive pole) oxygen undergoes electro-reduction,
O 2 þ 4H þ þ 4e
E 2 ¼ 1 : 229 V vs. SHE
! 2H 2 O
(11 : 2)
where E 1 and E 2 are the electrode potentials versus the standard hydrogen (reference)
electrode (SHE). This corresponds to the overall combustion reaction of ethanol
in oxygen:
CH 3 CH 2 OH þ 3O 2 ! 2CO 2 þ 3H 2 O
(11 : 3)
with the thermodynamic data, under standard conditions:
DG o ¼ 1325 kJ mol 1 ,
DH o ¼ 1366 kJ mol 1
(11 : 4)
This gives a standard electromotive force (EMF) at equilibrium
E eq ¼ DG o
¼ 1325 10 3
12 96,485 ¼ E 2 E 1 ¼ 1 : 144 V
(11 : 5)
nF
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