24
Z. Zhao and P. K. Shen
with the oxygen partial pressure. These indicating that the double bond of O 2 will be
destroyed after the control step of the ORR. Second, the concentration of H
+ is also
an important influence factor. While the electrode potential is higher than 0.8 V, the
slope of the Tafel curve is from −120 mV
−1 to −60 mV
−1 .
The reaction rate of the ORR on the Pt/C electrocatalyst can be expressed by the
following formula [4, 9].
j k = −k 0 n F P O 2 [H
+
](1 − θ) exp(−
G θ
RT
) exp(−
a c F E
RT
)
(2.15)
K 0 is the reaction constant of the control step, n is the number of electrons participating in the reaction, F is the Faraday constant, P O2 is the oxygen pressure, (1-θ ) is
the ratio of the Pt surface without covered by the oxide, and G θ is the free energy
of oxygen adsorption, E Indicates the electrode potential.
Since the exchange current density is very low, the activity of oxygen reduction
is determined by the reaction current density of a specific voltage (generally 0.9 V)
in the power control region as catalytic activity (SA, A cm
−2 ), or for the dispersed
catalyst material determined by catalytic activity (MA, A g
−2 ).
The intermediate product of the ORR reaction was calculated by this method, but
research showed that the particle size of the catalyst and the spacing between the
particles have a great influence on the detection of the oxygen reduction intermediate.
Chen and Kucernak et al. developed a novel method to study the mechanism of oxygen
reduction [4]. They used carbon fiber with implanted submicron Pt (40 nm) to prepare
microelectrodes. For single Pt particle, the transmission radius is hemispherical, the
distance equaling to the radius of the particle. Therefore, for a 36-nm particle, the
diffusion coefficient is 10 cm s
−1 . If a rotating disk electrode is used to achieve the
same diffusion coefficient, the rotational speed should reach 4.6*108 r min
−1 which
is obviously unachievable. Under this condition, the Pt particles can be regarded
as a polycrystalline Pt disk. However, it was found that the number of electrons
involved in the transfer reaction decreased from 4 to 3.5 by using a diffusion-enhanced
method. It shows that only 75% of the oxygen molecules are reduced to water, and
the remaining oxygen molecules are only reduced to hydrogen peroxide. This shows
that the traditional four-electron reaction proposed by the small amount of hydrogen
peroxide molecules obtained by the RRDE test prevails because the slow rate of
H 2 O 2 causes the H 2 O 2 escaping from the catalyst surface, readsorption to the Pt
surface and further reduced to the water.
This important experiment led us to find that many of the conclusions of the
RRDE test need to be reconsidered, especially regarding the mechanism of the
oxygen reduction reaction, whether it is the direct four-electron reaction or the twoelectron reaction, or the two reaction pathways ratio calculation in the total reaction.
H 2 O 2 molecule is more difficult to escape from the catalytic layer surface due to
the experimental conditions using the high specific surface area carbon-supported Pt
nanoparticle as a catalyst compared to the polycrystalline Pt disk surface. After H 2 O 2
molecule is produced, it is readsorbed to the adjacent Pt metal surface or the porous
carbon material surface, thereby further reduced to the final product water, resulting
Z. Zhao and P. K. Shen
with the oxygen partial pressure. These indicating that the double bond of O 2 will be
destroyed after the control step of the ORR. Second, the concentration of H
+ is also
an important influence factor. While the electrode potential is higher than 0.8 V, the
slope of the Tafel curve is from −120 mV
−1 to −60 mV
−1 .
The reaction rate of the ORR on the Pt/C electrocatalyst can be expressed by the
following formula [4, 9].
j k = −k 0 n F P O 2 [H
+
](1 − θ) exp(−
G θ
RT
) exp(−
a c F E
RT
)
(2.15)
K 0 is the reaction constant of the control step, n is the number of electrons participating in the reaction, F is the Faraday constant, P O2 is the oxygen pressure, (1-θ ) is
the ratio of the Pt surface without covered by the oxide, and G θ is the free energy
of oxygen adsorption, E Indicates the electrode potential.
Since the exchange current density is very low, the activity of oxygen reduction
is determined by the reaction current density of a specific voltage (generally 0.9 V)
in the power control region as catalytic activity (SA, A cm
−2 ), or for the dispersed
catalyst material determined by catalytic activity (MA, A g
−2 ).
The intermediate product of the ORR reaction was calculated by this method, but
research showed that the particle size of the catalyst and the spacing between the
particles have a great influence on the detection of the oxygen reduction intermediate.
Chen and Kucernak et al. developed a novel method to study the mechanism of oxygen
reduction [4]. They used carbon fiber with implanted submicron Pt (40 nm) to prepare
microelectrodes. For single Pt particle, the transmission radius is hemispherical, the
distance equaling to the radius of the particle. Therefore, for a 36-nm particle, the
diffusion coefficient is 10 cm s
−1 . If a rotating disk electrode is used to achieve the
same diffusion coefficient, the rotational speed should reach 4.6*108 r min
−1 which
is obviously unachievable. Under this condition, the Pt particles can be regarded
as a polycrystalline Pt disk. However, it was found that the number of electrons
involved in the transfer reaction decreased from 4 to 3.5 by using a diffusion-enhanced
method. It shows that only 75% of the oxygen molecules are reduced to water, and
the remaining oxygen molecules are only reduced to hydrogen peroxide. This shows
that the traditional four-electron reaction proposed by the small amount of hydrogen
peroxide molecules obtained by the RRDE test prevails because the slow rate of
H 2 O 2 causes the H 2 O 2 escaping from the catalyst surface, readsorption to the Pt
surface and further reduced to the water.
This important experiment led us to find that many of the conclusions of the
RRDE test need to be reconsidered, especially regarding the mechanism of the
oxygen reduction reaction, whether it is the direct four-electron reaction or the twoelectron reaction, or the two reaction pathways ratio calculation in the total reaction.
H 2 O 2 molecule is more difficult to escape from the catalytic layer surface due to
the experimental conditions using the high specific surface area carbon-supported Pt
nanoparticle as a catalyst compared to the polycrystalline Pt disk surface. After H 2 O 2
molecule is produced, it is readsorbed to the adjacent Pt metal surface or the porous
carbon material surface, thereby further reduced to the final product water, resulting