Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 273

energy of an O atom on Pt(111) was estimated to be 23.6 eV experimentally
[Campbell et al., 1981; Gland et al., 1980].

9.2.1.2 Inhibition Effect of Adsorbed OH and Pt Oxidation The ques-
tion about the equivalence of adsorbed OH species (OH ads ) formed through dissocia-
tive adsorption of molecular oxygen followed by protonation and via water oxidation
has been recurring, although a consensus has emerged that high OH ads coverage can
reduce the ORR rate. In the potential region of 0.75 - 1.10 V, OH ads is thought to form
from the oxidation of H 2 O:
Pt þ H 2 O ! Pt OH þ H þ þ e (9 : 1)

OH ads formation has a clear voltammetric signature on a number of surfaces, including
the (111)-oriented surfaces of platinum group metals, Pt(111) in alkaline and acid
electrolytes of non-adsorbing anions [Markovic and Ross, 2002], and Au(111),
Au(100), and Ag(111) in neutral and alkaline electrolytes [Savinova et al., 2002].
On these surfaces, the reaction has a reversible character. Anderson and co-workers
calculated the reversible potential of Reaction (9.1) on Pt to be 0.62 V with respect
to a reversible hydrogen electrode (RHE) [Anderson, 2002]. The Pt(111) - OH bond
energy has been estimated to be about 21.4 eV in an alkaline electrolyte [Markovic
and Ross, 2002].
Wang and co-workers have analyzed the site-blocking and electronic effects of
OH ads (in HClO 4 ) and bisulfate (in H 2 SO 4 ) with the adsorption isotherms incorporated
in the model for a Pt(111) surface [Wang et al., 2004]. The best fits yielded the intrinsic
Tafel slope in the range from 2118 to 2130 mV/dec, supporting the interpretation
of single electron transfer in the rate-determining step, with the corresponding transfer
coefficients equal to 0.50 and 0.45, respectively. In addition to site blocking, a
negative electronic effect on ORR kinetics was found for both OH and bisulfate,
with the effect of the latter being much stronger. The deviation of the apparent
Tafel slope in HClO 4 from its intrinsic value can be fully accounted for by the site-
blocking and electronic effects of OH ads , which vary with coverage over the mixed
kinetic - diffusion-controlled region. Since the anion adsorption effects are pro-
portional to the coverage, they affect the slope of the polarization curve only when
the coverage changes in the mixed kinetic - diffusion-controlled region of ORR.
That appears to be the reason why the apparent Tafel slope deviates from its intrinsic
value in HClO 4 but not in H 2 SO 4 .
The quantitative evaluation of the effects of adsorbed species on the ORR kinetics
was obtained from the following equation:

j k (E) ¼ j 0 [1 gu(E)] m exp 2 : 3 E E o 1u(E)
b

(9 : 2)

where j 0 and b are the intrinsic exchange current and Tafel slope for an adsorbate-free
Pt(111) surface. The (1 2 gu) m term accounts for the geometric site-blocking effect,
whereas the electronic effect of anion adsorption on ORR kinetics is described by a