41
The place exchange process occurs at 1.10 < E < 1.20 V due to the strong dipole–
dipole lateral repulsive interactions between the half-monolayer of Pt-O on the surface and the second half-monolayer that is being formed. In order to minimize these
repulsions, the initial half-monolayer of O chem adatoms undergoes an interfacial
place-exchange process with the Pt surface atoms, which eventually, upon further
potential excursion to 1.40 V, leads to the development of a quasi-3D surface lattice
composed of Pt
2+
and O
2−
ions.
Some researchers proposed that the chemisorption of OH with partial discharge
starts as early as 0.3 V in the hydrogen adsorption region, so that the voltammetry
profile of single-crystal electrodes, particularly Pt(100), needs to be deconvoluted to
separate the two processes [19].
It was found out that the catalytic activity of PEMFC degrades following oxidation of the surface. Potential cycling induces dissolution of Pt, not only anodically
during positive-going sweeps but also cathodically during negative-going sweeps in
the acid electrolyte [20, 21]. As seen, the oxygen adsorption region is a highly complex reaction, and there is still no agreement among various researchers.
Conway et al. developed a related technique of superimposing a rapid, small
amplitude “triangular” modulation signal on a much slower linear potential sweep,
with the resulting modulated current response being displayed on an oscilloscope.
The regions of “kinetically reversible” response along the potential scale are then
easily observable [22].
Kozlowska, Conway, and Hamelin, by making cyclic voltammetry at Pt down to
low temperatures (−60 °C in strong HClO 4 solution), showed how a clear distinction could be demonstrated between the “reversible” and the “irreversible” states of
oxide formation and reduction, the former arising at low OH or O coverages, up to
15% in θ [23].
Pt nanoparticle surface is also prone to surface oxidation, as shown by timeresolved high-energy X-ray diffraction. Results indicate severe surface reconstruction
of the nanoparticle surface showing at least three types of Pt-O bonds (adsorbed
OH, adsorbed atomic O, and amorphous PtOx) under oxidative conditions [24].
The shape of voltammetry profiles in the oxide region is sensitive to the nature of
anions in the solution. As seen in Fig. 5.4, the strength of specific adsorption of
anions on the stepped surface Pt(332) = 6(111) × (111) is reflected in both H UPD and
in the oxide formation. The feature at around 0.55 V in H 2 SO 4 probably corresponds
to the adsorption/desorption process of (bi)sulfate. This feature is shifted to more
positive potentials in carbonate and bicarbonate solutions, and its irreversibility
indicates that it partially overlaps with the place-exchange region, as the high-limit
of the voltammetry profile is chosen to 0.9 V for picture clarity.
A Pt ML supported on Pd/annealed-Pd 3 Fe(111) shows the highest oxygen reduction reaction (ORR) kinetics compared to Pt(111) and Pt ML /Pd(111) surfaces, as
demonstrated by their corresponding specific activities at 0.9 V vs RHE. DFT studies suggest that the observed enhancement of ORR activity originates mainly from
weaker Pt–OH interaction on Pt ML /Pd 3 Fe (111) compared to Pt ML /Pd(111) [25].
5.2 Hydroxyl Adsorption and Oxide Formation
The place exchange process occurs at 1.10 < E < 1.20 V due to the strong dipole–
dipole lateral repulsive interactions between the half-monolayer of Pt-O on the surface and the second half-monolayer that is being formed. In order to minimize these
repulsions, the initial half-monolayer of O chem adatoms undergoes an interfacial
place-exchange process with the Pt surface atoms, which eventually, upon further
potential excursion to 1.40 V, leads to the development of a quasi-3D surface lattice
composed of Pt
2+
and O
2−
ions.
Some researchers proposed that the chemisorption of OH with partial discharge
starts as early as 0.3 V in the hydrogen adsorption region, so that the voltammetry
profile of single-crystal electrodes, particularly Pt(100), needs to be deconvoluted to
separate the two processes [19].
It was found out that the catalytic activity of PEMFC degrades following oxidation of the surface. Potential cycling induces dissolution of Pt, not only anodically
during positive-going sweeps but also cathodically during negative-going sweeps in
the acid electrolyte [20, 21]. As seen, the oxygen adsorption region is a highly complex reaction, and there is still no agreement among various researchers.
Conway et al. developed a related technique of superimposing a rapid, small
amplitude “triangular” modulation signal on a much slower linear potential sweep,
with the resulting modulated current response being displayed on an oscilloscope.
The regions of “kinetically reversible” response along the potential scale are then
easily observable [22].
Kozlowska, Conway, and Hamelin, by making cyclic voltammetry at Pt down to
low temperatures (−60 °C in strong HClO 4 solution), showed how a clear distinction could be demonstrated between the “reversible” and the “irreversible” states of
oxide formation and reduction, the former arising at low OH or O coverages, up to
15% in θ [23].
Pt nanoparticle surface is also prone to surface oxidation, as shown by timeresolved high-energy X-ray diffraction. Results indicate severe surface reconstruction
of the nanoparticle surface showing at least three types of Pt-O bonds (adsorbed
OH, adsorbed atomic O, and amorphous PtOx) under oxidative conditions [24].
The shape of voltammetry profiles in the oxide region is sensitive to the nature of
anions in the solution. As seen in Fig. 5.4, the strength of specific adsorption of
anions on the stepped surface Pt(332) = 6(111) × (111) is reflected in both H UPD and
in the oxide formation. The feature at around 0.55 V in H 2 SO 4 probably corresponds
to the adsorption/desorption process of (bi)sulfate. This feature is shifted to more
positive potentials in carbonate and bicarbonate solutions, and its irreversibility
indicates that it partially overlaps with the place-exchange region, as the high-limit
of the voltammetry profile is chosen to 0.9 V for picture clarity.
A Pt ML supported on Pd/annealed-Pd 3 Fe(111) shows the highest oxygen reduction reaction (ORR) kinetics compared to Pt(111) and Pt ML /Pd(111) surfaces, as
demonstrated by their corresponding specific activities at 0.9 V vs RHE. DFT studies suggest that the observed enhancement of ORR activity originates mainly from
weaker Pt–OH interaction on Pt ML /Pd 3 Fe (111) compared to Pt ML /Pd(111) [25].
5.2 Hydroxyl Adsorption and Oxide Formation