40
5.2 Hydroxyl Adsorption and Oxide Formation
Because of the role in electrochemical energy conversion in fuel cells and other
processes, oxygen reduction is one of the most important and most studied electrochemical reactions. At present, platinum offers as the best catalyst for both the
anodic and cathodic reactions for low-temperature proton exchange membrane fuel
cells (PEMFCs).
It is generally accepted that the oxidation on polycrystalline Pt commences after
the double-layer formation, that is above 0.6 V vs. reversible hydrogen electrode
(RHE). The process is irreversible, as seen in Fig. 5.1. Anodic oxide film formation
on noble metals was summarized by Conway [17].
Pt H O Pt OH H e
V
V
+
→ −
+
+
< <
(
)
+
−
2
0 85
110
.
.
E
(5.1)
Pt OH OH Pt place exchange
–
–
→
(
)
(5.2)
OH Pt Pt O
V
V
–
–
→
< <
(
)
1 10
140
.
.
E
(5.3)
In a sulfuric acid solution, the process proceeds through oxidation of water in
potential region 0.85 V < E < 1.10 V, forming quasi-2D array of OH on the surface.
During such 2D processes, place-exchange between electrosorbed OH or O species
on the surface and Pt atoms within the surface lattice takes place and leads to a
quasi-2D compact film. The film grows ultimately to a multilayer hydrous oxide
film, probably by continuing injection of ions of the substrate metal and their migration through the growing film under the influence of the field. Quasi-3D lattice of
Pt-O proceeds in further positive potential region up to 1.40 V. However, newer
experimental techniques challenge this scheme. The process described by Jerkiewitz
et al. involves strong physisorption of water on Pt surface as early as 0.27 V, but no
current in cyclic voltammetry is visible as the process occurs without electron transfer. The discharge of physisorbed water occurs at 0.85 V < E < 1.10 V, forming a ½
monolayer of Pt-O film in a process that does not involve OH ads as an intermediate: [18].
Pt H O Pt
O
H
strong physisorption
V
V
+
→
−
−
(
)
< <
+
−
2
2
0 27
085
δ
δ
.
.
E
(5.4)
Pt Pt H O
Pt Pt
O
H
e
discharge
V
chem
−
−
δ
δ
(
)−
→(
) −
+
+
(
)
< <
+
−
+
−
2
2
2
0 85
1
.
.
E
1 10V
(5.5)
Pt O
H O
Pt
O
H
e
discharge, p
chem
D lattice
quasi
−
+
→
−
(
)
+
+
+
−
+
−
2
2
2
3
2
2
l lace exchange
V
V
(
)
< <
1 10
140
.
.
E
(5.6)
5 Important Electrosorption Reactions
5.2 Hydroxyl Adsorption and Oxide Formation
Because of the role in electrochemical energy conversion in fuel cells and other
processes, oxygen reduction is one of the most important and most studied electrochemical reactions. At present, platinum offers as the best catalyst for both the
anodic and cathodic reactions for low-temperature proton exchange membrane fuel
cells (PEMFCs).
It is generally accepted that the oxidation on polycrystalline Pt commences after
the double-layer formation, that is above 0.6 V vs. reversible hydrogen electrode
(RHE). The process is irreversible, as seen in Fig. 5.1. Anodic oxide film formation
on noble metals was summarized by Conway [17].
Pt H O Pt OH H e
V
V
+
→ −
+
+
< <
(
)
+
−
2
0 85
110
.
.
E
(5.1)
Pt OH OH Pt place exchange
–
–
→
(
)
(5.2)
OH Pt Pt O
V
V
–
–
→
< <
(
)
1 10
140
.
.
E
(5.3)
In a sulfuric acid solution, the process proceeds through oxidation of water in
potential region 0.85 V < E < 1.10 V, forming quasi-2D array of OH on the surface.
During such 2D processes, place-exchange between electrosorbed OH or O species
on the surface and Pt atoms within the surface lattice takes place and leads to a
quasi-2D compact film. The film grows ultimately to a multilayer hydrous oxide
film, probably by continuing injection of ions of the substrate metal and their migration through the growing film under the influence of the field. Quasi-3D lattice of
Pt-O proceeds in further positive potential region up to 1.40 V. However, newer
experimental techniques challenge this scheme. The process described by Jerkiewitz
et al. involves strong physisorption of water on Pt surface as early as 0.27 V, but no
current in cyclic voltammetry is visible as the process occurs without electron transfer. The discharge of physisorbed water occurs at 0.85 V < E < 1.10 V, forming a ½
monolayer of Pt-O film in a process that does not involve OH ads as an intermediate: [18].
Pt H O Pt
O
H
strong physisorption
V
V
+
→
−
−
(
)
< <
+
−
2
2
0 27
085
δ
δ
.
.
E
(5.4)
Pt Pt H O
Pt Pt
O
H
e
discharge
V
chem
−
−
δ
δ
(
)−
→(
) −
+
+
(
)
< <
+
−
+
−
2
2
2
0 85
1
.
.
E
1 10V
(5.5)
Pt O
H O
Pt
O
H
e
discharge, p
chem
D lattice
quasi
−
+
→
−
(
)
+
+
+
−
+
−
2
2
2
3
2
2
l lace exchange
V
V
(
)
< <
1 10
140
.
.
E
(5.6)
5 Important Electrosorption Reactions