Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 541

CO 2 increased from 50% to 80%. It would be highly desirable to investigate MOR
on model nonporous substrates with DEMS in order to determine the influence of
particle size on various reaction steps and the partitioning between the “direct” and
“indirect” pathways.
Finally, it should be pointed out that physical and chemical properties of carbon
supports and catalyst preparation methods may influence methanol electro-oxidation
kinetics [Attwood et al., 1980; Gloaguen et al., 1997; Gojkovic and Vidakovic,
2001]. For example, Shukla and co-workers established a correlation between electro-
catalytic activity for the MOR and the concentration of surface functional groups
[Shukla et al., 1994]. Pt nanoparticles were supported on Ketjenblack carbons
with different concentrations of acid/base functional groups. Shukla et al. claimed
higher electrocatalytic activities for electrodes with low concentrations of surface
functional groups (pH zpc values between 6 and 7). On the other hand, Frelink,
Gloaguen, and co-workers arrived at the opposite conclusion, namely, that the pre-
sence of acidic functionalities on carbon slightly enhances electrocatalytic activity
for the MOR [Frelink et al., 1995; Gloaguen et al., 1997]. It should also be pointed
out that Rao and co-workers demonstrated a pronounced influence of carbon porosity
on electrocatalytic activity for the MOR in MEAs [Rao et al., 2005]. They prepared
Pt-Ru/Sibunit particles with constant size of metal particles but different specific sur-
face areas of carbon supports, S BET . Both MA and SA increased with decreasing S BET .
This observation highlights the importance of the catalyst support, and suggests that
the latter may obscure PSEs. For example, catalyst utilization and mass transport
losses may depend on the porosity of the support.
The above discussion emphasizes the limitations imposed by the use of metal
particles on porous substrates, and calls for further efforts in designing model systems
for better understanding of PSEs in complex multistep electrochemical reactions.
15.5.3.4 ORR in the Presence of Methanol We have seen in the preceding
sections that Pt particle size has significant effects on the kinetics of both the ORR and
the MOR. On the other hand, “crossover” of methanol from the anode to the cathode is
a serious practical problem limiting the performance of DMFCs. Although some
methanol-tolerant electrocatalysts have been developed during the last decade,
Pt-based electrocatalysts are still the materials of choice, because of their relatively
high stability in acidic and oxidizing media. Studies of oxygen reduction in the pre-
sence of methanol have confirmed that the ORR and the MOR occur simultaneously
at a Pt electrode [Bittins-Cattaneo et al., 1993; Chu and Gilman, 1994]. For carbon-
supported Pt nanoparticles, the potential loss DE with respect to the ORR in a
methanol-free electrolyte increases significantly with increasing size of Pt particles
[Maillard et al., 2002]. This occurs simultaneously with the positive shift of the metha-
nol electro-oxidation peak. SA for oxygen electro-reduction in a methanol-containing
electrolyte at 0.85 V vs. RHE was reported to decrease with increasing Pt particle size
[Maillard et al., 2002]. Since for both the ORR and the MOR, SA decreases with
decreasing particle size (i.e., it exhibits “negative” particle size effects), the enhanced
tolerance of smaller Pt particles to methanol may be explained by a stronger decrease
in SA for the MOR with particle size.