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over 0.75 V but under 0.95 V, such that coverage by the OH
ads
formed by the
Reaction (1.3) is still incomplete, enabling ORR activity (1.4a) at some non-oxidized
(water-covered) Pt metal sites.
The lesson to be taken from this report by Paik et al. [2004] is that a Pt catalyst in
contact with a hydrous electrolyte is so active in forming chemisorbed oxygen at temp-
eratures and potentials relevant to an operating PEFC, that the description of the cath-
ode catalyst surface as “Pt,” implying “Pt metal,” is seriously flawed. Indeed, that a
Reaction (1.4) actually takes place at a Pt catalyst surface, “exposes,” Pt to be less
noble than usually considered (although it remains a precious metal nevertheless
...). Such a surface oxidation process, taking place on exposure to O
2
and water
and driven by electronically shorted ORR cathode site and metal anode site, is ordina-
rily associated with surface oxidation (and corrosion) of the less noble metals.
Turning next to the effect of these catalyst surface oxidation processes on the rate of
ORR at “Pt”, it has been well established that the chemisorbed oxygen species is an
inhibitor of the ORR at a “Pt” catalyst surface, as would be expected for any process
that requires Pt metal sites to proceed. This has been repeatedly demonstrated exper-
imentally and should be readily understood from theoretical considerations, including
the notion of optimized M22Ox bond strength as a yardstick for the rate of ORR where
Misametal surface site. One direct experimental demonstration of this inhibiting
effect is the observed continuous PEFC performance reduction on extended operation
time scales, shown to be caused by the continuous build-up of chemisorbed oxygen
[Eickes et al., 2006], likely formed by both Reactions (1.3) and (1.4). The chemi-
sorbed oxygen formed practically instantaneously on a Pt surface by Reaction (1.3),
for example during a potential sweep at several millivolts per second into the “Pt
oxide region,” is itself well documented to be an ORR inhibitor. This is readily
seen from higher ORR currents measured in the fuel-cell-relevant potential domain
(.0.75 V) during the anodic half-cycle of a triangular potential scan (1.2 V -
0.4 V - 1.2 V) [Eickes et al., 2006]. During the cathodic half-cycle, the Pt surface is
still covered at V . 0.75 V by (irreversibly reduced) chemisorbed oxygen species
formed by the Reaction (1.3) at the high end potential (1.2 V), whereas the anodic
half-cycle starts from a low potential of 0.4 V where all the chemisorbed oxygen
has been reductively removed, and, consequently, the coverage by this inhibiting
species is quite small when the fuel-cell-relevant potential domain is traversed, result-
ing in higher observed ORR currents.
With these documented experimental findings and the clear understanding of the
criticality of metal sites free of chemisorbed oxygen for ORR catalysis, it remains
to be explained why any analysis of the ORR process at Pt catalysts would consider
a purely metallic Pt catalyst surface in contact with surface water molecules as the rel-
evant interface. In some cases, this could have been a choice made consciously, aiming
to reduce the complexity of the system addressed by leaving full consideration of the
actual state of the Pt catalyst surface for later. However, one other factor could have
been insufficient knowledge of earlier literature, covering significant efforts to charac-
terize and study the formation of surface and subsurface oxygen on Pt as function of Pt
electrode potential, temperature, and time. The latter work was done to a large degree
by Conway and co-workers in the 1970s and 1980s. Conway et al. reported that the
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