Electrocatalysis of Oxygen Reduction in Polymer Electrolyte Fuel Cells: A Brief History and a Critical Examination of Present Theory and Diagnostics - Fuel Cell Catalysis: A Surface Science Approach - page 13

described for the purposes of either theory or diagnostics as “Pt metal.” While Pt sur-
face coverage by chemisorbed oxygen species at cathode potentials higher than 0.75 V
has been recognized, at least to a degree, in ORR studies, in most examinations of
ORR kinetics at “Pt,” it has been considered mostly as an afterthought, rather than
the major factor it really is in determining the ORR rate and, in fact, in defining the
documented dependence of the ORR rate on the M22Ox bond strength. As ORR
takes place at “Pt,” parallel interfacial processes that need highlighting in ORR diag-
nostics are driven by the substantial reactivity of the Pt surface with water and with
dioxygen. Such processes cause potential- and time-dependent modification of the
interfacial composition and structure of an ORR cathode. A recent study by Paik
et al. [2004] highlights the significant reactivity of a Pt metal surface in a PEFC cath-
ode under fuel cell cathode operation conditions, bringing about slow build-up of Pt
surface OH ads and/or O ads , above and beyond that measured under ordinary cyclic vol-
tammetry conditions in an inert atmosphere. The coverage of a Pt catalyst surface by
chemisorbed oxygen species under a set, constant cathode potential can be estimated
from the charge passed during a cathodic potential scan following some timed exposure
of the catalyst surface to an oxidizing gaseous atmosphere as shown in Fig. 1.3. As
amply documented, Pt is covered by a chemisorbed oxygen species through the poten-
tial domain relevant for a fuel cell cathode, even in the complete absence of dioxygen,
and this chemisorbed species which is formed by water discharge is quite irreversibly
reduced. As seen in Fig. 1.3, the surface oxygen coverage at some cathode potential
could actually grow substantially further when dioxygen is introduced into the
system. The extra growth in oxide coverage following introduction of O 2 is most sub-
stantial at 0.85 V and significantly lower at 0.95 V. This suggests that the extra chemi-
sorbed oxygen species probably forms on Pt metal by a mechanism different than the
one operative in the absence of dioxygen. The extra “chemical” surface oxidation pro-
cess is apparently inoperative when the surface population of metal sites is too low, as
would be the case when the applied potential is 0.95 V or above.
The Pt surface electro-oxidation process observed in the absence of dioxygen
to form chemisorbed OH from water is driven by the potential difference at the Pt/
electrolyte interface, according to the reaction

H 2 O ads ! OH ads þ H þ þ e (1 : 3)

whereas the additional surface oxygen collected on adding dioxygen at an electrode
potential around 0.85 V, is likely driven by O 2 reduction at surface metal sites, accord-
ing to the reaction

2 O 2 þ 2H þ þ 2e ! H 2 O

(a)

1

þ

(b) 2H 2 O ads ! 2OH ads þ 2H þ þ 2e

1

2 O 2 þ H 2 O ads ! 2OH ads

(1 : 4)