Electrocatalysis for the Direct Alcohol Fuel Cell - Fuel Cell Catalysis: A Surface Science Approach - page 361

likely that the structure of b-FePc in perpendicular stacks in the unit cell [Kirner et al.,
1976] leads either to an increase in the metal - metal distance, which favors the adsorp-
tion of oxygen as FePc - O 2 peroxo species rather than as PcFe - O 2 - FePc m-oxo
species, or to a decrease in the accessibility of molecular oxygen to the catalytic cen-
ters. a-FePc appears then as the better of the catalysts for fuel cell applications in terms
of higher current densities achieved at higher cathode potentials. Moreover, rotating
ring - disk electrode experiments (Fig. 11.18) indicated that a-FePc catalyzes a four-
electron process of oxygen reduction to form water only for potentials higher than
0.8 V vs. RHE (which correspond to the cathode potentials of fuel cells under operat-
ing conditions) and with an efficiency not less than 96% for higher overpotentials
[Baranton et al., 1995, 1996]. H 2 O 2 formation at b-FePc was not evaluated; however,
considering the limited current density achieved (which is lower than that obtained
with a-FePc), it can reasonably be assumed that water and hydrogen peroxide are
formed at this catalyst.
11.6 CONCLUSIONS
This chapter has presented the main features concerning electrocatalysts for DAFCs in
acidic media. To achieve the best electrical efficiency, it is necessary to develop elec-
trocatalysts able to oxidize the alcohol completely to CO 2 and to reduce oxygen com-
pletely to water with high reaction rates. For the ORR, Pt-based bimetallic catalysts
and non-noble metal catalysts have shown higher activity than pure Pt, and, moreover,
exhibit a higher tolerance or even total insensitivity to the presence of alcohol. On the
other hand, in a strongly acidic medium as in a DEFC using a proton exchange mem-
brane, Pt is necessary to realize the dissociative adsorption of the alcohol, but is easily
poisoned by adsorbed species such as CO. To reduce the effect of poisoning, it is
necessary to add to the Pt anode other metals, such as Sn, Ru, or Mo. For example,
Pt-Ru catalysts exhibit higher activity for methanol electro-oxidation, whereas Pt-Sn
catalysts exhibit higher activity for ethanol electro-oxidation, both in an electrochemi-
cal half-cell as well as in a DAFC. However, in the case of ethanol, selectivities with
regard both to CO 2 and to acetaldehyde are lowered, whereas selectivity with regard to
acetic acid is increased. Therefore, it seems that the possibility of obtaining a total far-
adaic efficiency 1 F (selectivity of the catalyst) and a higher potential efficiency 1 E
(poisoning of Pt) has to be abandoned. Still, one goal could be to achieve a total selec-
tivity of the electrocatalyst with regard to the production of acetic acid, which is a
liquid and easy to manage, but it would provide only one-third of the faradaic effi-
ciency (1 F ¼ 0.33), and thus leads to a decrease in the overall efficiency and power
density by two-thirds.
Solid alkaline membrane fuel cells (SAMFCs) can be a good alternative to
PEMFCs. The activation of the oxidation of alcohols and reduction of oxygen occur-
ring in fuel cells is easier in alkaline media than in acid media [Wang et al., 2003;
Yang, 2004]. Therefore, less Pt or even non-noble metals can be used owing to the
improved electrode kinetics. For example, Ag/C catalytic powder can be used as an
efficient cathode material [Demarconnay et al., 2004; Lamy et al., 2006]. It has also