3 The Measurements of the Oxygen Reduction Reaction
41
In the formula I d O 2 = 4Fr O 2 C
√
ω, is the limiting current of the four-electron
reaction of oxygen molecules, r = 0.62D2/3v−1/6, D is the diffusion coefficient, v
is the dynamic viscosity coefficient, and ω is the electrode rotation speed.
Taking
I D
I R
or
I d O 2 −I D
I R
as the X axis and ω(−1/2) as the y axis, the above two
equations can be drawn as two straight lines. The two slopes and the two intercepts
of the two lines can list four equations with five rate constants as variables. However,
an equation is still needed to completely solve these five variables. Other additional
conditions can be introduced. For example, it is assumed that the change in potential
does not affect the catalytic decomposition of hydrogen peroxide (k 4 ), or that k 2 and
k 2 are linked by the standard equilibrium potential of the O 2 /H 2 O 2 pair.
Therefore, the determination steps of the oxygen reduction reaction can be studied
by analyzing the ratio of the ring current to the disk current at a fixed potential, I D /I R ,
(I d −I R )/I R , and the rotational speed ω (−1/2), and solve the rate constant for each
reaction step.
3.2.2.2 Film Rotating Round (Ring) Disk Electrode
The simplest catalyst activity test method is membrane electrode test, but due to the
mass transfer of the oxygen reduction reactant to the membrane electrode, the incomplete utilization of the electrocatalyst and the influence of water during the reaction,
it is difficult to obtain the intrinsic electrocatalysis of the high specific surface area
catalyst performance. The rotating ring (ring) disk electrode can eliminate the mass
transfer effect, obtain the catalytic kinetic limit, and can quantitatively analyze the
intermediate products of the reaction, and then analyze the reaction mechanism.
In 2001, the research team of Schmidt and Gasteiger proposed using a thin-film
rotating disk electrode method (TF-RDE) to simulate a fuel cell membrane electrode
test to study the kinetic limit of the catalyst without mass transfer and no ohmic drop.
This method is based on predecessors, dating back to 1976 by Stonehart and Ross,
who used a rotating thin-layer electrode (RTLE) to directly test the electrochemical
rate constant of a high specific surface area catalyst. The preparation method of
coating the porous catalyst on the surface of the rotating disk electrode at that time
was similar to the preparation of a gas diffusion electrode of Teflon platinum thinfilm electrode (PTFE). Therefore, the treatment analysis method of the rotating thin
layer electrode is similar to the Teflon platinum thin-film electrode model, and is
established on the full penetration model, that is, the void of the porous film layer is
also sufficiently wetted. In order to explain the mass transfer efficiency of the porous
catalyst layer, the model introduces an effective mass transfer coefficient 1, which
is the ratio of the measured current density to the net current density without any
mass transfer, and is affected by the exchange current density, the diffusivity of the
reactant in the porous liquid region of the porous layer, the ratio of the active material
of the catalytic layer, and the thickness of the catalytic layer. For slow reaction or thin
catalytic layer electrodes, the effective mass transfer coefficient 1 is approximately
equal to 1, and the 1 of each region of the catalytic layer is substantially the same,
41
In the formula I d O 2 = 4Fr O 2 C
√
ω, is the limiting current of the four-electron
reaction of oxygen molecules, r = 0.62D2/3v−1/6, D is the diffusion coefficient, v
is the dynamic viscosity coefficient, and ω is the electrode rotation speed.
Taking
I D
I R
or
I d O 2 −I D
I R
as the X axis and ω(−1/2) as the y axis, the above two
equations can be drawn as two straight lines. The two slopes and the two intercepts
of the two lines can list four equations with five rate constants as variables. However,
an equation is still needed to completely solve these five variables. Other additional
conditions can be introduced. For example, it is assumed that the change in potential
does not affect the catalytic decomposition of hydrogen peroxide (k 4 ), or that k 2 and
k 2 are linked by the standard equilibrium potential of the O 2 /H 2 O 2 pair.
Therefore, the determination steps of the oxygen reduction reaction can be studied
by analyzing the ratio of the ring current to the disk current at a fixed potential, I D /I R ,
(I d −I R )/I R , and the rotational speed ω (−1/2), and solve the rate constant for each
reaction step.
3.2.2.2 Film Rotating Round (Ring) Disk Electrode
The simplest catalyst activity test method is membrane electrode test, but due to the
mass transfer of the oxygen reduction reactant to the membrane electrode, the incomplete utilization of the electrocatalyst and the influence of water during the reaction,
it is difficult to obtain the intrinsic electrocatalysis of the high specific surface area
catalyst performance. The rotating ring (ring) disk electrode can eliminate the mass
transfer effect, obtain the catalytic kinetic limit, and can quantitatively analyze the
intermediate products of the reaction, and then analyze the reaction mechanism.
In 2001, the research team of Schmidt and Gasteiger proposed using a thin-film
rotating disk electrode method (TF-RDE) to simulate a fuel cell membrane electrode
test to study the kinetic limit of the catalyst without mass transfer and no ohmic drop.
This method is based on predecessors, dating back to 1976 by Stonehart and Ross,
who used a rotating thin-layer electrode (RTLE) to directly test the electrochemical
rate constant of a high specific surface area catalyst. The preparation method of
coating the porous catalyst on the surface of the rotating disk electrode at that time
was similar to the preparation of a gas diffusion electrode of Teflon platinum thinfilm electrode (PTFE). Therefore, the treatment analysis method of the rotating thin
layer electrode is similar to the Teflon platinum thin-film electrode model, and is
established on the full penetration model, that is, the void of the porous film layer is
also sufficiently wetted. In order to explain the mass transfer efficiency of the porous
catalyst layer, the model introduces an effective mass transfer coefficient 1, which
is the ratio of the measured current density to the net current density without any
mass transfer, and is affected by the exchange current density, the diffusivity of the
reactant in the porous liquid region of the porous layer, the ratio of the active material
of the catalytic layer, and the thickness of the catalytic layer. For slow reaction or thin
catalytic layer electrodes, the effective mass transfer coefficient 1 is approximately
equal to 1, and the 1 of each region of the catalytic layer is substantially the same,