2 Mechanism of Oxygen Reduction Reaction
25
in a very small amount of H 2 O 2 detected on the ring. This is also direct evidence that
the four-electron reaction is considered to be the main pathway for oxygen reduction,
but from the current point of view, the two-electron continuous reaction is greatly
underestimated, and which may be the main pathway. The same conclusion was also
used by Inaba [5] to reduce the loading of the PDE/C on the RRDE surface. They
found that H 2 O 2 molecule can be greatly increased by carrying a very low Pt/C
electrocatalyst on the RRDE. The catalytic activity of oxygen reduction on the Pt
particles of the cube was improved, but after repeated cyclic voltammetry scanning
in 0.05–1.4 V versus RHE for a period of time, the activity of oxygen reduction
was significantly decreased and the production of H 2 O 2 molecules was increased.
The main reason is that the Pt (100) plane on the Pt particles surface is changed
to polycrystalline Pt. This conclusion can clearly indicate that the particle size and
surface state of the catalyst have an important influence on the catalytic principle of
oxygen reduction.
A large amount of work has shown that the decrease in the particle size of the
Pt catalyst leads to lower activity of the ORR [7, 10], that is, “negative particle
size function.” That discovered when H 3 PO 4 was used as an electrolyte. Later, this
discipline was also applied to acidic electrolytes such as H 2 SO 4 and HClO 4 . When
the Pt particle is less than 4–5 nm, SA will decrease with the decrease of the Pt
particle size.SA will reach the maximum value on Pt particles of 3–4 nm and reduce
the Pt particle size, SA will decrease the ratio of active sites on the surface of the
large Pt particles decreases which is the reason why increasing particle size also
leads to decrease of SA. Therefore, reducing the Pt particles continuously does not
necessarily increase the activity of the catalyst.
The effect of Pt different crystal plane on the catalytic activity of ORR is obvious.
There are some regular patterns in different electrolytes [1]: sulfuric acid, Pt(110)
> (100) > (111); perchloric acid, Pt(100) > (110) > (111); and alkaline potassium
hydroxide Pt(100) > (110) > (111). These all indicate that the Pt(111) crystal plane
has the lowest catalytic activity in different electrolytes. The reason for this law is that
the different anions included HSO4
− have the strongest adsorption on the Pt(111)
crystal plane, resulting in a decrease in the active catalytic position.
In addition to the adsorption of anions in the above solution, another factor that
may explain the decrease in the particle size of the Pt catalyst leading to lower
activity of the ORR is the adsorption of oxygen-containing molecules on the Pt
particle surface [10]. As the atomic proportion of the low coordination number on
the surface of the metal particles increases, surface free energy increasing makes it
easier to adsorb atoms and other molecules. O 2 forms strong chemical adsorption on
the surface of Pt particles below 3 nm. In solution, OH ads are more readily adsorbed
on the surface of smaller Pt particles. The reduction of the active sites on the surface
of the Pt catalyst by these oxygen atoms or oxygen-containing groups is also one of
the reasons why smaller Pt particle leads to a decrease in ORR performance.
In addition, the decrease of O 2 diffusion coefficient on the Pt particles surface is
also one of the possible factors for the effect of Pt particle reduction on the catalyst
activity decrease [4]. Since the Pt particles reduce the diffusion coefficient of O 2 and
its intermediates on the surface of Pt, leading to the catalytic performance is lowered.
25
in a very small amount of H 2 O 2 detected on the ring. This is also direct evidence that
the four-electron reaction is considered to be the main pathway for oxygen reduction,
but from the current point of view, the two-electron continuous reaction is greatly
underestimated, and which may be the main pathway. The same conclusion was also
used by Inaba [5] to reduce the loading of the PDE/C on the RRDE surface. They
found that H 2 O 2 molecule can be greatly increased by carrying a very low Pt/C
electrocatalyst on the RRDE. The catalytic activity of oxygen reduction on the Pt
particles of the cube was improved, but after repeated cyclic voltammetry scanning
in 0.05–1.4 V versus RHE for a period of time, the activity of oxygen reduction
was significantly decreased and the production of H 2 O 2 molecules was increased.
The main reason is that the Pt (100) plane on the Pt particles surface is changed
to polycrystalline Pt. This conclusion can clearly indicate that the particle size and
surface state of the catalyst have an important influence on the catalytic principle of
oxygen reduction.
A large amount of work has shown that the decrease in the particle size of the
Pt catalyst leads to lower activity of the ORR [7, 10], that is, “negative particle
size function.” That discovered when H 3 PO 4 was used as an electrolyte. Later, this
discipline was also applied to acidic electrolytes such as H 2 SO 4 and HClO 4 . When
the Pt particle is less than 4–5 nm, SA will decrease with the decrease of the Pt
particle size.SA will reach the maximum value on Pt particles of 3–4 nm and reduce
the Pt particle size, SA will decrease the ratio of active sites on the surface of the
large Pt particles decreases which is the reason why increasing particle size also
leads to decrease of SA. Therefore, reducing the Pt particles continuously does not
necessarily increase the activity of the catalyst.
The effect of Pt different crystal plane on the catalytic activity of ORR is obvious.
There are some regular patterns in different electrolytes [1]: sulfuric acid, Pt(110)
> (100) > (111); perchloric acid, Pt(100) > (110) > (111); and alkaline potassium
hydroxide Pt(100) > (110) > (111). These all indicate that the Pt(111) crystal plane
has the lowest catalytic activity in different electrolytes. The reason for this law is that
the different anions included HSO4
− have the strongest adsorption on the Pt(111)
crystal plane, resulting in a decrease in the active catalytic position.
In addition to the adsorption of anions in the above solution, another factor that
may explain the decrease in the particle size of the Pt catalyst leading to lower
activity of the ORR is the adsorption of oxygen-containing molecules on the Pt
particle surface [10]. As the atomic proportion of the low coordination number on
the surface of the metal particles increases, surface free energy increasing makes it
easier to adsorb atoms and other molecules. O 2 forms strong chemical adsorption on
the surface of Pt particles below 3 nm. In solution, OH ads are more readily adsorbed
on the surface of smaller Pt particles. The reduction of the active sites on the surface
of the Pt catalyst by these oxygen atoms or oxygen-containing groups is also one of
the reasons why smaller Pt particle leads to a decrease in ORR performance.
In addition, the decrease of O 2 diffusion coefficient on the Pt particles surface is
also one of the possible factors for the effect of Pt particle reduction on the catalyst
activity decrease [4]. Since the Pt particles reduce the diffusion coefficient of O 2 and
its intermediates on the surface of Pt, leading to the catalytic performance is lowered.