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 26

process, as the proximity to E surface redox couple determines the population of active,

reduced sites, according to

1
Z þ 1

N red ¼ N total (1 : 15)

where Z is a function of E E surface redox couple .
Along the same lines, the population of the active metal sites on a Pt/Pt - Ox
catalyst surface can be seen to be well described by (1.15), with the parameter Z
being given as Z ¼ exp (F = RT) E E Pt - OH 2 = Pt - OH , and with the ORR process pro-
ceeding along conceptual steps similar to those in the process (1.14), according to

4Pt-OH surface þ 4H þ þ 4e ! 4Pt surface þ 2H 2 O
(active reduced site generation)

(a)

4Pt surface þ O 2 þ 4e þ 4H þ ! 4Pt-OH surface
(faradaic reaction of O 2 at = with the reduced site followed by
repeat of (A), and so on)

(b) (1 : 16)

This analogy to a surface redox mediated process is significant. In a way very similar to
the reaction sequence (1.14), the standard potential of the redox surface system
Pt(H 2 O)/Pt-OH ads (0.80 V with respect to RHE) determines the active (reduced)
site population at any cathode potential E, and consequently is the critical parameter
in determining the “ignition potential” for the ORR process.
The critical role of the M/M22OH redox system in determining the population of
the surface active metal sites is, with high probability, the actual reason for the strong
predictive power of the M22Ox bond strength with regard to the relative rates of ORR
at different “metal” surfaces. In fact, a better presentation of the “volcano plot” would
be obtained by using, for the ordinate of the plot the value (1 = Z þ 1) exp( DH act = RT),
at a constant cathode potential of interest for fuel cell technology, e.g., 0.85 V, where
Z for a metal M is given by

F
RT

E E o M(H 2 O) = M - OH ads

Z ¼ exp (1 : 17)

For the ascending branch of the volcano plot, the term (1/Z þ 1) could serve by itself
as an effective ORR activity predictor, whereas, for the descending branch, (1/Z þ 1)
becomes close to unity at 0.85 V, and the exponential factor exp( DH act = RT ), then
determines the ORR rate based on the residual interaction of dioxygen with the (exces-
sively) noble metal catalyst surface.
The expression (1/Z þ 1)] exp[2DH act /RT ] at 0.85 V, better reflects the reality of a
partially oxidized Pt surface and the critical effect of active site availability on the rate
of the ORR. Effects of site availability were not considered in the calculations in
Nørskov et al. [2004] of ORR “activity” for various metals. The expression used to
calculate “activity” defined the ordinate parameter in the ORR volcano plots pre-
sented. This parameter was defined in Nørskov et al. [2004] as kT min i log(k i /k o ),