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 27

where k i ¼ n i exp(2E i /kT), with n i a frequency factor and E i the activation energy of
the slower step, i, in the ORR sequence at the metal surface. Nørskov et al. [2004], in
fact, explain that such a calculation of “activity” yields a maximum value for the rate
expected when the maximum possible surface sites required for the rate-limiting pro-
cess are all available. The latter means that, for the case of the Reaction (1.2) being the
slow ORR step, all surface sites are oxide-free metal sites—an unrealistic assumption
even in the case of Pt, not to mention more electropositive metals. It is the site avail-
ability effect that dominates the actual activity, rather than the activation energy of the
slow step in the faradaic process.
So why then do volcano plots in Nørskov et al. [2004] generate reasonable fits to
experiment while completely disregarding the major factor of metal site availability?
This is because of the assumption made there that the reduction of surface oxygen is
the slow step in the faradaic ORR process. The result is the appearance of the potential
associated with the M/M - OH couple in the activation energy of the assumed slow
step of the ORR process occurring at metal sites. In reality, however, the significance
of the potential of the M/M - OH couple is in determining the active (metal) site
availability near 0.9 V, not in determining the rate of the faradaic ORR process at
metal sites. The latter sites are, in fact, completely unavailable at the relevant
potentials on all metals less noble than Pt appearing on the ascending branch of the
ORR volcano plots.
Using (1/Z þ 1) as a gauge of ORR activity therefore also addresses a critical
remark frequently raised regarding the applicability of volcano plots based on
metal - oxygen bond strengths to those metals that are fully covered by a surface
oxide in the relevant cathode potential range, i.e., metals that cannot actually form
new M - Ox bonds when interacting with dioxygen. Once the value of the pre-
exponential factor (1/Z þ 1) appears as part of or as the complete ORR activity
yardstick, metals strongly covered by surface oxide at E cath . 0.75 V are clearly pre-
dicted to exhibit negligible ORR activity, as a direct result of their excessive coverage
by surface oxide.
1.8 SOME CONCLUSIONS
A description of ORR at “Pt” that disregards the potential-dependent blocking of
active metal surface sites is fundamentally unsatisfactory, because it neglects a central
physical/chemical feature of the real interfacial system. Furthermore, such descrip-
tions can lead to numerical values and mechanistic conclusions in apparent conflict
with understandings of the ORR mechanism that have been established and widely
accepted to date. Indeed, such conflict may, at least in some cases, be the result of a
“pragmatic approach,” targeting system parameterization while knowingly detaching
the result from any physical or mechanistic meaning. Unfortunately, it does not take
much for such reported apparent parameters to be considered relevant to real inter-
facial kinetics and to be of mechanistic significance, leaving uncertainty regarding
the key reported ORR kinetic parameters and proposed ORR mechanisms.
Once the key role of the overpotential with respect to E Pt(H 2 O) = Pt - OH ads in securing
active surface sites has been realized, new light is shed on the Mn22OorM22OH