Electrocatalysis for the Direct Alcohol Fuel Cell - Fuel Cell Catalysis: A Surface Science Approach - page 348

Figure 11.6 Arrhenius plots obtained at 0.5 V on Pt þ Ru/C colloid catalysts with different
atomic compositions (0.5 M H 2 SO 4 , 1.0 M MeOH; sweep rate 1 mV s 21 ).
before adding the fuel (HCHO or CH 3 OH) resulted in a decrease of 0.3 V in the oxi-
dation onset potential with respect to pure Pt [Shropshire, 1965]. Moreover, when
added to Pt-Ru catalysts, molybdenum led to higher activity with regard to methanol
electro-oxidation at low potentials [Lima et al., 2001; Jusys et al., 2002]. Such anodes
(prepared by electrodeposition of metals on a carbon gas diffusion layer [Coutanceau
et al., 2004]) led to an improvement in the electrical performance of a single DMFC at
90 8C (with the electrode characteristics given in the caption of Fig. 11.7). The tri-
metallic Pt 0.7 Ru 0.26 Mo 0.04 /C catalyst led to a higher cell performance than a
Pt 0.8 Ru 0.2 /C anode prepared in the same way (Fig. 11.7a). For ethanol electro-
oxidation, a trimetallic Pt 0.86 Sn 0.1 Ru 0.04 /C catalyst made by the colloidal route
[Rousseau et al., 2006] leads to a cell performance twice as high as that obtained
with a Pt 0.9 Sn 0.1 /C catalyst under the same experimental conditions (Fig. 11.7b). In
the case of molybdenum, the increase in the open circuit voltage (OCV) of the
single cell of about 100 mV can be explained by a decrease in surface poisoning or
by an effect on the composition of chemisorbed species [Podlovchenko et al., 1966;
Smirnova et al., 1988]. Moreover, the higher capacity of this third metal to form
oxy-hydroxyl species at low potentials was often proposed as an explanation for the
enhancement of methanol oxidation. In the case of ethanol electro-oxidation at
PtSnRu and PtSn, the OCV is similar for both catalysts, and it was shown that the pre-
sence of Ru does not lead to a noticeable change in the product distribution in the
anode outlet [Rousseau et al., 2006]. Here, the role of Ru seems more difficult to
explain. Its main role may be to limit strong adsorption of surface poisons by diluting
adsorption sites [Adˇiˇ, 1984], or to provide adsorbed OH species as soon as tin starts
to form higher oxide species that are not catalytically active [Rousseau, 2004].