Journal of
Electroanalytical
Chemistry
Journal of Electroanalytical Chemistry 583 (2005) 69–76
www.elsevier.com/locate/jelechem
Effect of loading level in platinum-dispersed carbon black
electrocatalysts on oxygen reduction activity evaluated by
rotating disk electrode
Eiji Higuchi a, Hiroyuki Uchida b, Masahiro Watanabe
b
a,*
a
Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan
Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan
Received 2 November 2004; received in revised form 25 December 2004; accepted 10 January 2005
Available online 6 July 2005
Abstract
The dependence of oxygen reduction reaction (ORR) activity on the loading level of Pt electrocatalysts highly dispersed on carbon black (Pt/CB) has been investigated. We present a standard method for the evaluation of ORR activity at Pt/CB by a rotating
disk electrode (RDE), in which Pt/CB powder was attached on a glassy carbon (GC) disk uniformly, followed by coating a thin
Nafion film. We describe the optimized protocols for preparing a suspension of the Pt/CB in an ethanol aqueous solution, controlling vapor pressure during drying process, and coating very thin (0.05 lm) Nafion film. The area-specific activity for the ORR in airsaturated 0.1 M HClO4 electrolyte solution was found not to be influenced by the Pt-loading level on CB from 19.2 to 63.2 wt%,
when the amount of Pt/CB on GC and the dispersion state were optimized.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Rotating disk electrode; Highly dispersed platinum electrocatalyst; Oxygen reduction reaction
1. Introduction
Polymer electrolyte fuel cells (PEFCs) have attracted
great attention as a primary power source for electric
vehicles or residences due to their high energy conversion efficiency and low emission of pollutants. Nanosized platinum particles dispersed on high surface area
carbon substrates have been used as the electrocatalyst
for both the anode and the cathode. When pure hydrogen is used as the fuel, the anodic hydrogen oxidation
reaction (HOR) overpotential is negligibly small even
at high current density of 1 A/cm2. Because a sluggish
oxygen reduction reaction (ORR) causes a large overpotential at the low operating temperature, the develop-
*
Corresponding author. Tel.: +81 55 220 8620; fax: +81 55 254 0371.
E-mail address: [email protected] (M. Watanabe).
0022-0728/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jelechem.2005.01.041
ment of a high performance cathode catalyst is very
important.
At the first screening for the electrocatalyst, planar
electrodes with well-defined characteristics (surface area,
surface and bulk compositions, and crystal structure)
have often been examined in aqueous acidic electrolyte
solutions, for example, at platinum electrodes of polycrystalline or single crystal at around room temperature.
Use of the rotating disk electrode (RDE) has a great
advantage to eliminate a mass-transfer problem and
evaluate precisely a kinetically controlled ORR activity
[1–9]. We reported previously an effect of thickness of
Nafion film covered on the Pt disk electrode on the
activities for both ORR and HOR [1]. It was found that
the kinetically controlled activity can be achieved as the
thickness is thin enough (ca. <0.2 lm). Schmidt et al.
[3,4] proposed the use of RDE to evaluate activities of
Pt nanoparticles supported on carbon black (Pt/CB) in
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E. Higuchi et al. / Journal of Electroanalytical Chemistry 583 (2005) 69–76
both ORR and HOR. Commercial Pt/CB (20 wt% Pt/
Vulcan XC72) powder was attached on a glassy carbon
(GC) disk electrode by means of a thin Nafion film (ca.
0.05 lm in thickness). This method has also been applied to the activity evaluation of Pt or Pt-alloy highly
dispersed on carbon substrates [10–12].
Recently, commercial Pt/CB catalysts with high Pt
loading (>60 wt%) have been developed with keeping
the particle size dPt < 3 nm. Such a highly loaded
Pt/CB makes a thickness of the anode or cathode
catalyst layers in PEFCs thinner than the present level
at a constant Pt loading. The thin layer structure may,
in principle, enhance mass-transfer rates for O2 as well
as H+ when it is optimized. An evaluation of the intrinsic activity of such a catalyst before the application into
practical gas-diffusion electrodes is very important for
designing high performance electrodes, because one cannot eliminate effects of the utilization of catalyst particles and/or the mass-transfer rate on the performance
of the practical electrodes. The intrinsic activity is considered as a clear target which one can achieve in the
ideal case. For the purpose, we tried to use the evaluation technique proposed by Schmidt and co-workers
[4]. However, we found the necessity of the modification
of their recipe, taking into account a dispersing state of
the carbon black particles and the amount of them on
the GC disk electrode to ensure sufficient reactant-gas
diffusion to Pt catalyst particles supported on them.
In the present paper, we propose a standard method
for the evaluation of ORR activity applicable to the Pt/
CB with various Pt loading amounts by using RDE
method. That is the optimized protocols for preparing
a suspension of the Pt/CB in an ethanol aqueous solution, controlling the atmosphere during drying the suspension on GC disk electrode, and coating a very thin
(0.05 lm) Nafion film on the Pt/CB–GC. By using the
optimized recipe, we, for the first time, demonstrate that
the area-specific ORR activity for Pt/CB is quite constant over wide range of Pt-loading level on CB in airsaturated 0.1 M HClO4 electrolyte solution.
2. Experimental
2.1. Preparation of Pt/CB-dispersed GC disk electrodes
Fig. 1 shows the preparation protocol of the working
electrode, Pt/CB-dispersed GC-disk covered with thin
Nafion film. GC disks (diameter = 10 mm, geometric
area = 0.785 cm2) were polished with an alumina paste
containing the particles in 1 lm diameter for 30 min,
0.3 lm alumina for 10 min, and finally 0.05 lm alumina
for 10 min. They were washed ultrasonically in hot
water, followed by degreasing with ethanol. After washing sufficiently in pure water with ultrasonic bath, we
confirmed the mirror finish by an optical microscope
Polishing and washing of GC electrode
Pipetting catalyst suspension on GC
Drying under ethanol vapor pressure
Pipetting Nafion solution
Drying under ethanol vapor pressure
Heat treatment (120oC)
Fig. 1. Preparation method of Pt/CB highly dispersed on GC disk
electrode.
(VH-Z250, Keyence Co. Ltd.) and a laser microscope
(VK9510, Keyence Co. Ltd.).
Platinum-dispersed high-surface-area carbon black
catalysts (Pt/CB, Tanaka Kikinzoku Kougyo Co. Ltd.)
with various Pt loading of 19.2 to 63.2 wt% were used
as-received. Scanning transmission electron microscope
(STEM, HD-2000, Hitachi High-Technologies Corp.)
was employed to observe the catalysts with an acceleration voltage of 200 kV. Table 1 shows the physical properties of these catalysts. To make ultra-thin and uniform
dispersion of Pt/CB on the GC disk, we examined various conditions for preparing Pt/CB suspension in an
ethanol aqueous solution and for the drying process of
the suspension on GC. Typical conditions are summarized in Table 2. A constant volume (40 lL) of the suspension was pipetted onto the GC. As the first
approach, we adjusted the amount of Pt/CB in the
suspension so as to keep the weight of CB support constant at wCB = 5.70 ± 0.07 lg/cm2 (Nos. 1, 4, 6, and 7 in
Table 2) irrespective of Pt-wt%, i.e., the volume of CB
framework is constant in the Pt/CB layer on the GC
disk. In this recipe, the weight of Pt loaded on the GC
(wPt) increases with Pt-wt% in the catalyst. Here, for
example, we explain the procedure for the case of No.
4 in Table 2 (46.3 wt%-Pt/CB). Pt/CB powder of 8.4
mg was ultrasonically dispersed in 40 mL of 40 vol%
ethanol aqueous solution for 10 min (CPt/CB = 0.21
g/L) by using an ultrasonic homogenizer (US-300T,
Table 1
Typical properties of commercial Pt/CB catalysts employed
Catalyst No.
Pt loading (wt%)
dPt (estimated by XRD) (nm)
TEC10E20E
TEC10E30E
TEC10E50E
TEC10E70E
19.2
29.3
46.3
63.2
2.7
2.2
2.6
2.4
E. Higuchi et al. / Journal of Electroanalytical Chemistry 583 (2005) 69–76
Table 2
Preparation conditions of catalyst suspensions and resulting wPt and
wCB by pipetting the suspension with 51 lL/cm2 on GC disk
Sample
No.
Pt loading
(wt%)a
CPt/CB
(g/L)b
CEtOH
(vol%)c
wPt
(lg/cm2)d
wCB
(lg/cm2)e
1
2
3
4
5
6
7
63.2
63.2
63.2
46.3
46.3
29.3
19.2
0.30
0.22
0.15
0.21
0.16
0.16
0.14
40
40
40
40
40
35
35
9.66
7.08
4.83
4.95
3.83
2.39
1.37
5.63
4.13
2.81
5.75
4.45
5.76
5.76
a
b
c
d
e
Pt-weight percent in Pt/CB catalysts (see Table 1).
Amount of Pt/CB catalyst in the suspension.
Ethanol concentration in the suspension.
Amount of Pt attached in the catalyst layer.
Amount of CB attached in the catalyst layer.
Nihon Seiki Seisakujo Co. Ltd.). A 40 lL aliquot of the
suspension was pipetted onto the mirror-finished GC
disk substrate (0.785 cm2), giving wPt = 4.95 lg/cm2
and wCB = 5.75 lg/cm2. This was dried at room temperature in a glass Petri dish containing ethanol. The dish
was covered by a lid with a small gap to evaporate the
solvent droplet slowly under the saturated vapor pressure of ethanol. An 18 lL of 0.05 wt% Nafion (diluted
with ethanol) solution was put on top of the dried catalyst layer to yield the film thickness of 0.05 lm. This was
dried at room temperature and under ethanol vapor
pressure in the similar manner as described above. Finally, Nafion-coated catalyst layer on the GC was
heat-treated at 120 °C for 1 h in air.
2.2. Electrochemical measurements
The rotating disk electrode (RDE) equipment (RED-1,
Nikko Keisoku Co. Ltd.) with a gas-tight Pyrex glass
cell was employed to examine the ORR activity of the
electrocatalyst. A platinum wire and a reversible hydrogen electrode (RHE) were used as the counter electrode
and the reference electrode, respectively. All electrode
potentials are reported with respect to the RHE. The
electrolyte solution of 0.1 M HClO4 was prepared from
reagent grade chemicals (Kanto Chemical Co., Japan)
and Milli-Q water (Milli-pore Japan Co. Ltd.) and purified in advance with conventional pre-electrolysis methods [13–15].
Prior to the ORR experiments, the potential of the
working disk electrode was cycled 10 times between
0.05 and 1.0 V at 0.5 V/s in 0.1 M HClO4 deaerated with
N2 gas to clean the surface. The electrochemical active
area of Pt, SPt, was evaluated from an electric charge
for the hydrogen desorption QH in the positive-going
potential scan from 0.05 to 0.40 V in cyclic voltammetry
at the sweep rate of 0.1 V/s at 25 °C, supposing
QH = 0.21 mC/cm2 for a smooth polycrystalline platinum [13,14].
71
After bubbling air in 0.1 M HClO4 solution for 30
min, hydrodynamic voltammograms for the ORR at
the working disk electrode were recorded by sweeping
the potential from 1.0 to 0.05 V at 10 mV/s under the
rotating rate of 1000–2750 rpm. All the electrochemical
experiments were performed at 25 °C.
3. Results and discussion
3.1. Preparation of Pt/CB thin layer uniformly dispersed
on GC disk electrode
One of the important subjects in this research is to
clarify the dependence of the specific ORR activity on
the amount of Pt loaded on a given CB (high-surfacearea carbon black, HSA-CB) support. To avoid socalled ‘‘particle–crystallite size effect’’ [16–18], we have
chosen Pt/CB catalysts with almost the same Pt particle size, irrespective of Pt-loading level. Fig. 2 shows
typical STEM photographs of 19.2 wt% and 63.2
wt%-Pt/CB catalysts, which are the lowest and highest
loading level among the catalysts examined, respectively. It is clearly seen that Pt particles are highly dispersed on the carbon support for both samples. The Pt
particle size ranged from 2 to 3 nm. The crystallite size
of Pt (dPt), estimated from X-ray diffraction (XRD),
for each catalyst is shown in Table 1. The values of
dPt are 2.2–2.7 nm, in accord with that observed by
STEM. Thus, these Pt/CB catalysts are suitable for
our objective.
Next, we present how to disperse Pt/CB catalyst uniformly on the GC disk electrode. Schmidt and co-workers [4] reported a recipe for preparing the thin-film Pt/
CB (20 wt%-Pt supported on Vulcan XC 72) catalyst
layer on the GC; an aliquot of 20 lL catalyst suspension
(CPt/CB = 1 g/L in pure water) was pipetted onto the GC
(0.283 cm2, yielding wPt = 14.1 lg/cm2 and wCB = 56.5
lg/cm2), followed by drying in argon stream. Unfortunately, as shown in Fig. 3(a), an application of this recipe to our 19.2 wt%-Pt supported on HSA-CB resulted
in aggregation (stacking) of the catalyst, especially at
the circumference part of the GC disk. We have judged
that the amount of the catalyst loaded on the GC in [4]
is too much because the thickness of CB framework
(density = 2 g/cm3) is estimated to be 0.28 lm even if
the catalysts can uniformly be dispersed over the GC.
Schmidt et al. [3] claimed that this thickness value is
small enough compared with the hydrodynamic masstransfer boundary layer thickness (ca. 10 lm). However,
we have clearly shown that the catalytic activity becomes controlled by the mass-transfer process in the
Nafion film, covering Pt catalyst surface, thicker than
0.2 lm [1]. The Nafion film with the thickness of 0.1
lm in their recipe [4] may fill the space of the above
CB frameworks of 0.28 lm, where the utilization of
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E. Higuchi et al. / Journal of Electroanalytical Chemistry 583 (2005) 69–76
Fig. 2. STEM photograph of commercial Pt/CB catalysts: (a) 19.2 wt%-Pt (TEC10E20E); (b) 63.2 wt%-Pt (TEC10E70E).
Fig. 3. Optical microscope photograph of Pt/CB (19.2 wt%-Pt) layer on GC disk after drying: (a) prepared by the method in [4], wPt = 14.1 lg/cm2
and wCB = 56.5 lg/cm2; (b) by our recipe No. 7 in Table 2, wPt = 1.37 lg/cm2 and wCB = 5.76 lg/cm2.
the catalyst for the ORR should be different at the top
and bottom of the stacked layer.
To make uniform dispersion of Pt/CB particles on the
GC disk, where they are separated each other ideally as
a single particle, we examined various conditions regarding the solvent of the catalyst suspension, the amount of
catalyst, and the evaporation rate of the solvent. For
each run, we observed the catalyst layer by an optical
microscope with a magnification of 250 from end to
end along the diameter (typically 12 continuous pictures). Fig. 3(b) shows one of microscope photographs
for 19.2 wt%-Pt/CB dispersed on the GC by our optimized recipe. Four important points to obtain such a
uniformly dispersed Pt/CB layer on the GC are: (1)
adjustment of amount of Pt/CB (CPt/CB) in the suspension, (2) addition of ethanol (CEtOH = 35 or 40 vol%)
in water to prepare the suspension, (3) pipetting suitable
volume of the suspension on the GC, and (4) evaporation of the solvent droplet on the GC slowly under saturated vapor pressure of ethanol. Regarding the amount
of Pt/CB in the thin film layer on the GC, it is noted that
the major framework (or volume) is constructed by the
CB support, considering the density of CB (2 g/cm3)
and Pt (21.4 g/cm3). The amount of CB in our catalyst
layer is about 1/10 of that recommended in [4], which
prevents the catalyst from the aggregation. The addition
of ethanol in the suspension reduces a surface tension
(or a contact angle) of the droplet on the GC, leading
to uniform spreading of the suspension over whole surface and easy evaporation of the solvent. This also reduces the volume of the suspension put on the GC,
i.e., 51 lL/cm2 in our recipe is about 70% of 71 lL/cm2
in [4]. Because the dispersion state of the catalysts on
the GC somewhat changed depending on the
E. Higuchi et al. / Journal of Electroanalytical Chemistry 583 (2005) 69–76
Pt-loading level, we tuned up the concentration of ethanol at CEtOH = 35 and 40 vol% for low Pt loading (19.2
and 29.3 wt%-Pt) and higher Pt-loading catalysts,
respectively. Thus, as shown in Figs. 3(b) and 4, we succeeded to prepare thin Pt (19.2–63.2 wt%)/CB layers uniformly dispersed on the GC with a good reproducibility
by the recipe shown in Table 2.
3.2. Effect of Nafion film thickness on the ORR current
On top of the dried Pt/CB catalyst layer prepared
above, a thin Nafion layer was coated. The Nafion film
thickness L (cm) was calculated by Eq. (1)
L ¼ ðV d l C N Þ=ðd N A 100Þ;
ð1Þ
3
where V (cm ) is the volume of Nafion solution coated
on the catalyst layer, dl (0.874 g/cm3) is the density of
Nafion solution, CN is weight percent of Nafion in the
solution (0.05 wt% in our standard recipe), dN (2 g/cm3)
73
is the density of dried Nafion, and A (0.785 cm2) is projected surface area of the GC disk electrode. We varied
the Nafion film thickness from 0.05 to 0.8 lm in order to
find out the optimum condition for the RDE
experiments.
Fig. 5 shows hydrodynamic voltammograms recorded at 2000 rpm for the ORR at Pt/CB (wPt = 2.39
lg/cm2, wCB = 5.76 lg/cm2, No. 6 in Table 2) on GC
disk electrodes coated with Nafion film of various thickness in air-saturated 0.1 M HClO4 at 25 °C. The ORR
currents at the disk electrode commence at ca. 0.95 V
and reach to a limiting current around 0.6 V. It is evident that the limiting current decreases with increasing
L. As reported for bulk Pt disk [1] or Pt/CB dispersed
on GC disk [4] electrodes, such a decrease in the ORR
current with increasing L is ascribed to a mass-transport
resistance in Nafion film. Then, in the same manner as in
literature [1], the ORR current I is expressed by the following equation:
1 1
1 1
¼ þ þ ;
I Ik Id If
ð2Þ
I d ¼ 0.62nFSD2=3 Cm1=6 x1=2 ;
ð3Þ
where Ik, Id, and If are the kinetically controlled current,
diffusion-limited current through the solution phase (socalled Levich current), and diffusion-limited current
through the Nafion film, respectively. Parameters in
Eq. (3) have their usual meanings in the conventional
Levich equation. However, it should be careful to distinguish S from A; S is an effective projected area covered
by Pt catalysts, whereas A is the (geometric) projected
area of the disk electrode. Because the ORR activities
of both CB itself and GC uncovered by Pt are negligibly
Fig. 4. Optical microscope photograph of Pt/CB layer on GC disk
after drying. (a) No. 6, (b) No. 4, and (c) No. 1.
Fig. 5. Hydrodynamic voltammograms for the ORR at Pt/CB coated
with Nafion of the thickness L attached on GC electrodes at 2000 rpm
in air-saturated 0.1 M HClO4 at 25 °C. Scan rate, 10 mV/s; rotation
rate, 2000 rpm; L, the thickness of Nafion film on GC electrodes;
catalyst, 29.3 wt%-Pt/CB (2.39 lg-Pt/cm2, 5.76 lg-CB/cm2, No. 6 in
Table 2).
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E. Higuchi et al. / Journal of Electroanalytical Chemistry 583 (2005) 69–76
low at E > 0.6 V, S is smaller than A, and usually cannot
be defined. Similar to the conventional Levich–Koutecky plots, I1 at a given potential was plotted against
x1/2. From the slope of the plot, the number of electrons transferred n cannot be determined for the Pt/CB
dispersed GC disk electrode due to a lack of S value
as described above. The value of intercept (denoted as
1/2
I 1
to zero
inf ) was determined by extrapolating x
1
(I d to zero). Then, the current density at the infinite
rotation rate jinf was obtained by dividing the value of
Iinf by the electrochemical active area of Pt (SPt).
Fig. 6 shows plots of jinf as a function of L1. The value
of jinf increases almost linearly in proportion to L1 for
L > 0.2 lm, suggesting that a planar diffusion of the reactant oxygen may be dominant in the Nafion film [1]. With
decreasing L, the value of jinf reaches a maximum constant value in the region less than 0.1 lm. By coating such
a thin Nafion film on top of the Pt/CB layer, the contribution of the film diffusion resistance [the third term in Eq.
(2)] to the measured current I becomes negligible. Hence,
we can judge that jinf obtained at L 6 0.1 lm equals the
kinetically controlled current density jk. This critical
thickness is just the same as that for Pt/CB on GC [4]
but is smaller than the value of 0.2 lm for a smooth Pt
electrode [1]. In the case of Nafion-coated smooth Pt disk
electrode, the diffusion of the reactant O2 through Nafion
film to Pt surface is well expressed by planar-type diffusion, i.e., the O2 flux is perpendicular to the Nafion or
Pt surface. In contrast, for Nafion-coated Pt/CB layer
on the GC disk, the diffusion of O2 within the Nafion
toward Pt catalyst particles cannot be regarded as a simple planar-type. Supposing that the Pt/CB is dispersed
uniformly on the GC, the thickness of the catalyst layer
is estimated to be about 0.03 lm at wCB = 5.76 lg/cm2.
Although the planar-type O2 diffusion may be dominant
toward the Pt catalysts near the top of the catalysts layer,
Fig. 6. Dependence of jinf for the ORR on thickness of Nafion coating
(L).
O2 is supplied to the Pt particles near the GC substrate by
a spherical diffusion, i.e., longer diffusion path. Therefore, for the evaluation of the catalytic activity without
any effect of mass transfer, the Nafion thickness must
be very thin, i.e., L 6 0.1 lm, compared with a smooth
Pt electrode. Hereinafter, we performed all the experiments with L = 0.05 lm.
3.3. Effect of Pt loading on the mass or specific activities
evaluated for the ORR
The last important issue for the precise evaluation of
the mass or specific ORR activity at Pt/CB layer by the
RDE is the control of the Pt weight loaded in the layer
(wPt) so as to utilize all of the Pt particles evenly. Fig. 7
shows typical examples of cyclic voltammograms (CVs)
at various working disk electrodes without rotation in
deaerated 0.1 M HClO4 solution. The shape of the CV
is similar to that of polycrystalline bulk Pt, and considerable change in the shape is not seen among catalysts
with 19.2–46.3 wt%-Pt. The value of hydrogen desorption charge QH in the positive-going potential scan at
each electrode is plotted as a function of wPt in Fig. 8.
Besides the regular sample with wCB = 5.70 ± 0.07
lg/cm2, we also prepared samples No. 2, 3, and 5 with less
CB loading to check the effect of wPt. The value of QH
increases linearly in proportion to wPt from 0 to 7.08
lg/cm2, irrespective of kind of Pt/CB catalysts. However, at sample No. 1 in Table 2 (wPt = 9.66 lg/cm2
and wCB = 5.63 lg/cm2, prepared with 63.2 wt%-Pt/
CB), the value of QH deviates from the regression line.
Since any aggregation of Pt/CB was not seen by an optical microscope at the magnification of 250, the reason
why the sample No. 1 with high wPt exhibited almost
saturated QH value cannot be explained well. However,
some fraction of Pt particles, probably inside the
Fig. 7. Cyclic voltammograms for the Pt/CB–GC disk electrodes
(samples No. 1, 4, 6, and 7) in deaerated 0.1 M HClO4 at 25 °C. Scan
rate, 100 mV/s. Thickness of Nafion film, 0.05 lm.
E. Higuchi et al. / Journal of Electroanalytical Chemistry 583 (2005) 69–76
75
a bulky smooth Pt RDE measured at 0.80 V in air-saturated 0.5 M H2SO4 at 25 °C [9]. To our knowledge,
the present research is the first to demonstrate that the
specific ORR activity per SPt (active Pt surface area)
of Pt/CB is quite constant over wide range of Pt-loading
level from 19.2 to 63.2 wt%.
We can expect that the use of highly loaded Pt/CB
can provide an advantage of reducing the catalyst layer
thickness without losing the kinetic activity, if the catalyst layer is properly designed to achieve a good balance
between the catalyst utilization and the gas diffusivity.
4. Conclusions
Fig. 8. Relation between QH and wPt for various Pt/CB–GC disk
electrodes in deaerated 0.1 M HClO4 at 25 °C. Pt-loading level: 63.2
wt% (d), 46.3 wt% (h), 29.3 wt% (m), and 19.2 wt% (.).
catalyst layer, might not be electrochemically utilized. In
contrast, the values of QH at samples No. 2 and 3 with
low wPt prepared with the same 63.2 wt%-Pt/CB are reasonably located on the regression line.
Fig. 9 shows plots of the ORR activities jk [mA (Ptcm2)] at 0.76 and 0.80 V as a function of wPt for all
the RDE experiments examined at 25 °C. It is striking
that the area-specific ORR activity at 0.76 V or 0.80 V
is constant irrespective of the Pt-loading level from
19.2 to 63.2 wt% in Pt/CB at wPt 6 7.08 lg/cm2. However, at wPt = 9.66 lg/cm2 prepared from 63.2 wt%-Pt/
CB, the value of jk at each potential is apparently low
probably due to the low Pt utilization as described
above. The present value of jk = ca. 0.7 mA/(Pt-cm2)
for wPt 6 7.08 lg/cm2 at 0.80 V in air-saturated 0.1 M
HClO4 is in good accord with jk = ca. 0.8 mA/cm2 for
We have demonstrated the optimized method to prepare a Pt/CB supported RDE for the precise evaluation
of ORR activity. Important points to disperse Pt/CB uniformly on the GC disk are: (1) adjustment of amount of
Pt/CB so as to keep the weight of CB support constant
at wCB = 5.70 ± 0.07 lg/cm2 (except 63.2 wt%-Pt) in 35
or 40 vol% ethanol aqueous suspension, (2) pipetting a
suitable volume 51 lL/cm2 of the suspension on the
GC, and (3) evaporation of the solvent droplet on the
GC slowly under saturated vapor pressure of ethanol.
Additional care must be taken for highly-loaded catalyst
(63.2 wt%-Pt) so as to attach wPt 6 7 lg/cm2 on the GC
for high Pt utilization. Nafion film coating on top of the
catalyst layer must be very thin (<0.1 lm) in order to
minimize the film diffusion resistance. We have clearly
shown that the area-specific ORR activity is independent
of the Pt-loading level on CB from 19.2 to 63.2 wt% in airsaturated 0.1 M HClO4 electrolyte solution when the
RDE is prepared by the optimized recipe.
Acknowledgements
This work was supported by CREST of Japan Science and Technology (JST) Corporation, and also was
supported by the fund for ‘‘Leading Project’’ of Ministry of Education, Science, Culture, Sports and Technology of Japan.
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Fig. 9. Dependence of kinetically controlled current density jk (per
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GC RDE measured in air-saturated 0.1 M HClO4 at 25 °C. Thickness
of Nafion film; 0.05 lm. wPt is amount of Pt per geometric surface area
of GC disk.
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