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 17

[Conway et al., 1990] as “reversible OH groups,” implying a fast OH ads formation/
reduction cycle—a property not shared by the majority of surface oxygen on Pt,
which requires a significant cathodic overpotential to be electroreduced. Such surface
oxygen groups (the majority) have either been anodically deposited directly on surface
sites providing higher coordination (e.g., steps) or have achieved a state of lower
energy by surface migration and/or place exchange. The presence of a much more
“reversible component of OH ads ” was concluded by Conway and co-workers from
measurements of frequency-dependent pseudocapacitance at Pt at potentials above
0.75 V [Conway et al., 1990], and Pt surface coverage by this “reversible” type of
OH ads was determined to be near 10% of the total surface oxygen species. This insight,
provided back in the 1980s, reveals that chemisorbed oxygen, or chemisorbed OH on
Pt, can exhibit a wide range of bond strengths to metal surface atoms, with only a small
population of (likely “newly formed”), surface OH groups being associated with a
relatively high rate of reduction, i.e., having the characteristics expected of an active
surface intermediate in a faradaic process.
A distribution of adsorption energies of chemically identical surface species
formed on a metal electrocatalyst, with a large fraction playing “spectator” and/or
“site blocking” roles and only a small fraction being active intermediates in a faradaic
process, is not a situation unique to chemisorbed oxygen species and the ORR process
at Pt. A recent analysis of the hydrogen oxidation process on Pt reveals a similar
situation, as described by Wang et al. [2006].
1.5 IMPACTS OF THE ACTUAL STATE OF THE CATALYST
SURFACE ON THE RATE OF THE ORR AT “Pt”
To evaluate the effects of the actual nature of a “Pt” catalyst surface on measured ORR
characteristics, one has to abandon the prevailing assumption that measured changes
of the ORR rate with potential, or with oxygen pressure, can be properly analyzed
based on a conceptual model of a catalyst surface of invariable composition, ordinarily
assumed to be Pt metal. To evaluate the Tafel slope expected for the ORR at “Pt” in the
potential range 0.90 - 0.75 V, the variation with potential of the surface coverage by
blocking chemisorbed oxygen species must be considered, in addition to the “classical”
effect of enhancement of the rate of the ORR at Pt metal sites with increased cathode
overpotential [Uribe et al., 1992; Wang et al., 2004]. In an early treatment of the expected
combined effects of cathodic polarization in (i) removing blocking surface oxide species
and (ii) enhancing the rate of ORR at “free” Pt metal sites available on the surface, Uribe
et al. [1992] used the following assumptions:
† The ORR at Pt metal sites has a rate dependence on potential described by a Tafel
slope of 120 mV/decade (as determined for Pt model systems at high cathodic
overpotential, where Pt is practically surface oxide-free [Parthasarathy et al.,
1992a, b]).
† The effect of the surface oxide species on the rate of the ORR is explained
by metal site blocking, and can be described mathematically by including a
1 2 u ox term in the pre-exponential factor of the rate expression.