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 25

that it is the value of E E Pt(H 2 O) = Pt - OH ads that could be the key for the “ignition” of the
ORR current on a gradual increase of the cathode overpotential. The simple-case
expression used here for Z yields a ratio of number of metal sites to number of
chemisorbed oxygen covered sites of 1 : 100 at E E Pt(H 2 O) = Pt - OH ads ¼ 0 : 12V, increas-
ing to 1 : 10 at E E Pt(H 2 O) = Pt - OH ads ¼ 0 : 06 V. That “ignition” of the ORR process is
indeed observed about 0.1 V positive of E Pt(H 2 O) = Pt - OH ads clearly confirms the criticality
of cathodic generation of active metal sites and, consequently, of proper representation
of site availability by the (1/1 þ Z ) term in (1.12).

1.7 POSSIBLE DESCRIPTION OF ORR AT A Pt / Pt-Ox CATALYST
SURFACE AS A REDOX-MEDIATED PROCESS

The notion of a surface redox system that determines an “ignition potential” for a far-
adaic process, including for the ORR, is well recognized in cases where such a surface
redox system is added onto a metal or a carbon substrate. For example, to activate
the ORR on various high surface area carbon structures, Co and/or Fe ion centers
are attached, using appropriate ligation, which affects the standard potential of the
redox couple and assists in surface bonding. Redox mediation in cases of such
ORR catalysts can be conceptually divided into three steps:

1. A significant steady-state population of reduced surface sites is generated by the
cathodic overpotential, as set by the fuel cell load.
2. The reduced sites donate electrons to the reactant oxygen molecule and to inter-
mediates formed, this electron transfer being coupled with bond breaking and
making involved in the ORR process.
3. Electrons donated to the surface oxygen species are instantaneously replenished
from the ohmic contact to maintain a steady state population of electron-filled
(reduced) surface states determined by E cath .

A description of the above sequence in terms of equations for the electrochemical
steps takes the form exemplified by the following specific case:

4Co þ 3

surface þ 4e ! 4Co þ 2

(a)

surface

4Co þ 2

surface þ O 2 þ 4H þ ! 4Co þ 3

(b)

surface þ 2H 2 O

(1 : 14)

4Co þ 3

surface þ 4e ! 4Co þ 2

(c) [followed by repeat of step (a)]

surface

In such cases of redox mediation, it seems clear why step (a) would maintain a steady
state population of reduced sites according to the value of E E surface redox couple ,
whereas the rate of step (b) will be enhanced with E E O 2 = H 2 O , the cathode over-
potential driving the faradaic four-electron reduction of oxygen. And it is also clear
why approaching E surface redox couple could be the prerequisite for “igniting” the ORR