Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 527

Thus, this work challenges the validity of some earlier concepts on the mechanism of
the ORR, and suggests that the serial pathway may be equally important, if not domi-
nant. This shows how little, in fact, we understand about this practically important
reaction.
Note that for metal nanoparticles supported on porous carbon materials, it is
even more difficult to establish the mechanism of the ORR. Indeed, for the above-
described thin layer or porous RRDE (Section 15.3), H 2 O 2 has very little chance
to escape from the CL and be detected at the ring. H 2 O 2 can readsorb either on
Pt particles or on the carbon support, and undergo chemical decomposition or further
electrochemical reduction, while diffusing out of the CL. This implies great difficul-
ties in establishing the detailed ORR mechanism on nanometer-sized metal

nanoparticles.
The effect of Pt particle size on electrocatalytic activity in the ORR has been
the subject of many controversial studies. The so-called “negative” PSEs, with SA
decreasing with decreasing particle size, were first discovered in H 3 PO 4 in connection
with the development of cathode catalysts for phosphoric acid fuel cells (see
Kinoshita [1992] and references therein). Similar dependences were then found in
H 2 SO 4 , as well as in electrolytes containing weakly adsorbing anions, such as
HClO 4 , with SA declining when the particle size decreased below about 4 - 5 nm
[Bregoli, 1978; Sattler and Ross, 1986; Kinoshita, 1988, 1990; Mukerjee, 1990;
Giordano et al., 1991; Gloaguen et al., 1994; Kabbabi et al., 1994; Gamez et al.,
1996; Takasu et al., 1996; Genies et al., 1998; Maillard et al., 2002; Guerin et al.,
2004; Gasteiger et al., 2005]. The MA decreases below about 3 - 4 nm, where it
reaches its maximum, and decreases again at larger particle sizes owing to the decrease
in specific surface area [Bregoli, 1978; Sattler and Ross, 1986; Kinoshita, 1990;
Gloaguen et al., 1994; Gamez et al., 1996; Genies et al., 1998; Antoine et al., 2001;
Maillard et al., 2002; Gasteiger et al., 2005]. This means that it is not advantageous
to use very small particles, because the increase in specific surface area is counter-
balanced by the loss of SA. PSEs are weakest in HClO 4 [Maillard et al., 2002],
much stronger in NaOH [Genies et al., 1998], and very strong in H 2 SO 4 and H 3 PO 4
[Gamez et al., 1996; Gloaguen et al., 1994; Kabbabi et al., 1994; Mukerjee, 1990].
This trend is illustrated in Fig. 15.7, which shows SA from Maillard et al. [2002];
Gasteiger et al. [2005]; Guerin et al. [2004] as a function of the inverse Pt
particle diameter.
The greater influence of particle size in the presence of strongly adsorbing anions is
in agreement with the stronger structural sensitivity for Pt single crystals in H 2 SO 4 and
H 3 PO 4 as compared with HClO 4 . For example, El-Kadiri and co-workers, and later
Markovic and co-workers, reported that ORR kinetic currents in H 2 SO 4 decrease
strongly in the order (110) . (100) . (111) [El-Kadiri et al., 1991; Markovic et al.,
1994, 1997a]. The same order of catalytic activity was observed in H 3 PO 4
[El-Kadiri et al., 1991; Adzic, 1998]. Meanwhile, in HClO 4 , the differences are rela-
tively small, with SA increasing in the order (100) , (110) (111) [Markovic et al.,
1997a]. Similar structure sensitivity was observed in KOH, with SA increasing in the
order (100) , (110) , (111), but with larger differences [Markovic et al., 1997a].
These effects may be rationalized by considering the very strong adsorption of