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19. Electrochemistry at Nanostructured
Diamond Electrodes: Characterization
and Applications
Kensuke Honda and Akira Fujishima
19. 1. Introduction Nanomaterials have many possible applications for
analytical chemistry [1], and for electronic, optical, and
mechanical devices [2]. In particular, nanomaterials and
electrochemistry have a long shared history (e.g., the use of
finely dispersed Pt particles as catalysts in fuel cell
electrodes). This cChapter deals specifically with
electrochemical applications of the template-synthesized
nanostructured diamond. We begin with the basic
electrochemical properties of nanostructured diamond
electrodes. Two possible electrochemical applications are
discussed.
19. 2. Fabrication of Nanostructured Diamond
1
19. 2. 1. Template synthesis of nanostructured materials There are numerous chemical methods for preparing
nanomaterials [2, 3]. A number of researchers have been
studying a method termed “template synthesis” [3].
Traditionally, this method has entailed synthesizing the
nano-ordered structure of a desired compounds or material
by use of a nanoscale template. Recently, the template
method has been used with the pores in a microporous solid
as a nanoscopic mold [3]. Many materials are available for
the template materials [3, 4]. Pore diameter sizes range
from Å to micrometers. Out of the many available template
materials, anodic alumina (Al2O3) has been commonly used
used as a template [5, 6]. When grown on high-purity
aluminum, anodic alumina has a hexagonal pattern of
cylindrical pores. Pore diameters from ~10 to ~400 nm can
be synthesized. Recent improvements in the degree of
ordering obtainable for a hole array has increased the
attractiveness of such materials for nanofabrication.
19.2. 2. Fabrication procedure of nanostructured diamond Figure 19.2.1. shows the procedure for the fabrication of
nano-porous diamond films (diamond nanohoneycomb) by
template synthesis using with porous alumina membranes.
Ordered thorough-hole anodic porous alumina membranes
were laid on the top of the synthetic diamond films, and then
2
deep holes were etched into the film by useing of an oxygen
plasma treatment.
19. 2. 3. Polishing of polycrystalline diamond films Nanohoneycomb structures were fabricated from polished
polycrystalline films. The polishing of the as-deposited films
was carried out by the Namiki Precision Jewel Co., Ltd.,
Tokyo, Japan, by use of a proprietary process. The films
polished by this process are extremely smooth, with height
variations on the order of ca. 1 nm.
Fig. 19.2.1 Schematic diagrams of the fabrication
procedure for the nano-honeycomb diamond electrode
3
Thorough-hole porous anodic alumina mask
Nano-honeycomb diamond
Oxygen Plasma
Polished boron-doped diamond thin film
Thorough-hole porous anodic alumina mask
Nano-honeycomb diamond
Oxygen Plasma
Polished boron-doped diamond thin film
19. 2. 4. Preparation of the anodic alumina maskAnodic porous alumina is formed via the anodization of Al in
an appropriate solution. The preparation of the thorough-
hole porous anodic alumina mask has been described [7].
The pore interval of porous alumina, in other words, the cell
size, was determined by the applied voltage used for
anodization [7]: the cell size has a good linear relationship
with the applied voltage, where the proportionality constant
of cell size per unit applied voltage is approximately 2.5 nm
V-1. In a previous survey, self-ordering has been observed to
occur under limited voltage conditions, which were specific
to the solution used for anodization; self-ordering takes
place at 25 V in sulfuric acid solution with a 65- nm cell size,
at 40 V in oxalic acid solution with a 100- nm cell size, and
at 195 V in phosphoric acid with 500- nm cell size [7].
An aluminum sheet (10 50 30 mm: 99.999%;
Nilaco) was electropolished in a mixed solution of perchloric
acid ([60%)] and ethanol (1:4 in volume) at constant current
conditions of 100 mA cm-2 at a temperature below 10℃ for 4
min. Anodization was conducted under constant voltage
conditions (40 V in a 0.3 M oxialic acid solution for 10 h)
using a DC source (Metronix 410A-350). The temperature of
the electrolyte was maintained at 0 ℃ during anodization
using with a cooling system (EYELA CTP-20). After
anodization the surface was protected against etching using
a coating layer made of a mixture of nitrocellulose and
polyester resin in ethyl acetate, butyl acetate and heptane.
4
The Al layer was removed in a saturated HgCl2 solution.
Then, the bottom part of the anodic porous alumina
membrane was removed in 5 wt% phosphoric acid at 30℃
for 60 min, after which the coating layer was dissolved in
acetone, to form a thorough-hole membrane.
19. 2. 5. Oxygen plasma etching processThe oxygen plasma etching of the diamond films was
conducted with an RF−driven (13.56 GHz) plasma etching
apparatus (Samco BP-1, Japan) [8]. The diamond specimen
with mask was placed on one of the planar electrodes in the
plasma chamber. Oxygen plasma etching was carried out
for 15 min. The operating oxygen pressure was 20.0 Pa, and
the plasma power was 150 W.
19. 3. Impedance Characteristics of the Nanoporous Honeycomb Diamond and Application as an Electrical Double−-Layer Capacitor Fabrication of nanostructured diamond
19. 3. 1. Fabrication of nanostructured diamondNanoporous materials [8-10] have attracted much recent
interest, including that stemming from possible
electrochemical applications [11, 12]. The electrochemical
capacitor [13, 14] is a natural application for nanoporous
structures. Activated carbons have been the most
5
extensively examined capacitor materials over the past
decade [13, 15].
Another possible approach involves improving the
performance of activated carbon-based capacitors through
modification of the electrolyte. In order to increase the
specific energy, organic electrolytes have been examined
due to the larger available operating voltage range (ca. 2.5
V) [13], however, the discharge performance of such
capacitors is much lower than those obtained with aqueous
electrolytes, due to the high resistance of the electrolyte.
The conductivity of aqueous electrolytes is at least one order
of magnitude greater than those of organic electrolytes.
Thus, it would be desirable to have an electrode material
with high capacitance and a wide working potential range in
highly conductive aqueous electrolytes. The most promising
material thus far considered appears to be diamond.
Diamond possesses a wide potential window in
aqueous [16, 17] and nonaqueous [18] media and extreme
electrochemical stability [19]. Although as-deposited
polycrystalline diamond exhibits very low capacitance [17],
here we have demonstrated that the capacitance can be
increased drastically by producing high-aspect-ratio
cylindrical pores in the electrode through oxidative etching.
In the present work, we have carried out the electrochemical
characterization of the diamond honeycomb electrodes using
cyclic voltammetry and impedance measurements.
19.3.2. Film characterization
6
Scanning electron microscopy−Figure 19.3.1 shows SEM
images of the three types of diamond nanohoneycomb films.
Highly uniform, well-ordered arrangements of holes, with a
hexagonal close-packed pattern, are clearly seen in these
figures. Nanoporous boron-doped diamond films with
various pore diameters (30 nm to 400 nm) and pore depths
(50 nm to 3 m) were fabricated by etching polished
polycrystalline diamond films through porous alumina masks
with an oxygen plasma.
Among the three honeycomb films that we have
fabricated, the film with a pore diameter of 60 nm and depth
of 500 nm has the most highly ordered structure, in terms of
both the shapes of the individual pores as well as the overall
arrangement (Fig. 19.3.1B, honeycomb pore dimension type
60 500 nm). The average pore density was 1 1010 cm-2.
Based on the pore dimensions and pore density, the surface
area was estimated to be a factor of 10.5 times larger for the
honeycomb film compared to a flat, polished surface. The
film with 30-nm pores has a lower porosity (i. e., roughness
factor), due to the small diameter of the pores, the larger
7
intervals and the shallower pore depth (Fig.
A-a
B-a
300 nm
A-b
B-b
237nm
1.5 m
300nm
C-a
1.5m
300nm
C-b
A-a
B-a
300nm
A-b
B-b
237 nm
1.5m
300nm
C-a
1.5m
300nm
C-b
Fig. 19.3.1. SEM images of a highly boron-doped nanohoneycomb diamond electrode.(a) top view, (b) oblique view at a 45° tilt angle for pore types (A) 30 50 nm, and (B) 60 nm 500 nm and (C) 400 nm 3 m. Nanohoneycomb films observed by SEM were fabricated from free-standing polished diamond
8
19.3.1A, pore type 30 50 nm). The average pore density
of this film is 2.78 1010 cm-2. Based on the pore
dimensions and pore density, the surface area for this film
was estimated to be only a factor of 2.11 times larger
compared to a flat, polished surface. However, in the case
of the honeycomb with 400-nm diameter pores, the latter
are very closely spaced, and some pores have merged to
form larger ones (Fig. 19.3.1C, pore type 400 nm 3 m).
In this case, due to the larger pore depth, the porosity is
much greater than that of the other two films. Although the
pore density is only 4 108 cm-2, this film has a high
roughness factor, (15.6).
Fig. 19.3.2. Cyclic voltammograms for (a) as-deposited diamond and pore types (b) 30 50 nm, (c) 60 500 nm and (d) 400 nm 3 m;. eElectrolyte,: 1 M H2SO4; sweep
9
-2.5 -1.5 -0.5 0.5 1.5 2.5
Potential (V vs. Ag/AgCl)
Current density (mA cm
-2)
(a) As-deposited diamond
4 mA cm-2
(b) Pore type 30Å~50 nm
-0.5 V 0.4 V
(d) Pore type 400nmÅ~3 m
(c)Poretype60 ~Å 500nm
-2.5 -1.5 -0.5 0.5 1.5 2.5Potential(Vvs.Ag/AgCl)
( Current density mA cm
-2)
( a ) A s - d e p o s i t e d d i a m o n d
4 m A c m-2
(b)Poretype30 ~Å 50nm
- 0.5V 0.4V
(d)Poretype400nm ~Å 3m
(c)Poretype60 ~Å 500nm
rate,: 100 mV s-1. Arrows indicate the potentials at which the impedance measurements were carried out.
Cyclic voltammetry−Because the advantage of diamond in
the double-layer capacitor application is its wide working
potential window, we have examined the current-potential
behavior for the honeycomb films (Figure 19.3.2A).
Interestingly, the working potential window for the
honeycomb films remained essentially the same as that for
the as-deposited film, even after extended oxygen plasma
treatment.
Table 19.3.1. Comparison of double-layer capacitance and specific energy for various types of carbon-based electrodes.
10
Roughnessfactor
Potential windowΔV,Vfromcyclicvoltammetry
(Ered,Eox)a
Cdl,Fcm-2
(geometric)from
impedanceb
Cdl,Fg-1
from
impedancecEdl,mJcm-2
(geometric)d Edl,Jg-1d Edl,Jg
-1e
As-depositeddiamondfilm 4.0 3.04(-1.24,1.80) 12.9 5.94Å~ 10-2
Glassy carbon GC-20 2.47 (-1.03, 1.44) 55.1 1.68 Å~ 10-1
HOPG ZYA 1.93 (-0.64, 1.29) 7.02 1.26 Å~ 10-2
Activated carbon 1.0 (-0.7, 0.3) 100-400 50-200
Pore type 30 ? 50 nm 2.11 2.70 (-1.12, 1.58) 129 9.12 0.469 33.3 150.9
Pore type 60 ? 500 nm 10.9 2.62 (-1.05, 1.57) 1.83 Å~ 103 14.5 6.29 49.9 63.0
Pore type 70 ? 750 nm 16.7 2.61 (-1.05, 1.56) 2.90 Å~ 103 17.9 9.12 61.1 72.8
Pore type 400 nm ? 3 μm 15.6 2.46 (-0.85, 1.60) 3.91 Å~ 103 74.6 11.8 224.8 185.1
Direct etched diamond
(no mask) f4.0 3.17 (-1.34, 1.83) 238 1.20
a Values obtained from cyclic voltammograms measured at 100 mV s-1 . The definition of
potential window is ΔV < 2 mA V-1 cm-2 (data from Fig. 19.3.2).
b Values obtained by AC impedance analysis at 0.4 V vs. Ag/AgCl (data from Fig. 19.3.2).
c The specific capacitance for a hypothetical through-hole diamond membrane.
d The specific energy was estimated from the equation, Edl = 1/2×Cdl×(ΔV)2.
e The specific energy for a thorough-hole membrane estimated from pore parameters
and the differential capacitance of 200 F cm-2.
f Etched for 1 min. SEM showed no significant roughening of the surface.
We have chosen the criterion for the definition of
potential window to be that the slope of the CV at 100 mV s -1
is < 2 mA V-1 cm-2. The potential windows for various
electrodes, estimated in this manner, are summarized in
Table 19.3.1. The potential windows for as-deposited
diamond (3.04 V) and the 30 50-nm pore honeycomb
(2.70 V) are appreciably larger than those for either GC (2.47
V) or HOPG (1.93 V) [17, 20]. The values for the honeycomb
diamond electrodes were somewhat smaller (340 to 580 mV)
than that for as-deposited diamond due in part to the less
negative potential limits (Table 19.3.1). As a result, these
porous structures exhibited wide electrochemical potential
windows (ca. 3.0 V) in aqueous electrolytes, being somewhat
smaller than unetched, as-deposited diamond electrodes,
independent of pore structure. The double layer capacitive
current for the diamond honeycomb was a factor of 18 to 20
larger than that for the as-deposited diamond electrode due
to the surface roughness of the nanohoneycomb structure.
We shall next explore this difference in greater detail using
impedance measurements.
19.3.3. Impedance measurementsImpedance plots−Figure 19.3.3 shows experimental
impedance plots (complex plane representation) obtained
11
for both the as-deposited and the honeycomb diamond
electrodes at 0.4 V. The plots for the pore types, 60 500
nm (Fig. 19.3.3-c), 70 750 nm (not shown), and 400 nm 3 mm (Fig. 19.3.3-d), exhibit two distinct domains: a high
frequency domain, where the impedance behavior is that
expected for a cylindrical pore electrode, with a
characteristic linear portion at a 45° angle, and a low
frequency domain, where the behavior is that expected for a
flat electrode [21].
12
Re Z (105 Ωcm2)
-Im(10Z
5Ωcm
2)
0.0 4.00.0
4.0
8.0
12.00.010
0.025
0.050
0.10105Hz
a
8.0
b
ReZ(104 Ωcm2)0.0 7.5
-Im(10Z
4Ωcm
2)
0.0
7.5
15.00.010
0.025
0.0500.10105Hz
15.0
22.5
R e Z ( 1 0
3 Ωcm2)
0.0 4.0 8.0
-Im(10Z
3Ωcm
2)
0.0
4.0
8.0
12.00.013
0.025
0.0500.10
105Hz
c
ReZ(103 Ωcm2)0.0 3.0
-Im(10Z
3Ωcm
2)
0.0
3.0
6.00.010
0.0250.050
0.10105Hz
d
6.0
9.0
R e Z ( 1 05 Ωcm2)
-Im(10Z
5Ωcm
2)
0.0 4.00.0
4.0
8.0
12.00.010
0.025
0.050
0.10105Hz
a
8.0
b
ReZ(104 Ωcm2)0.0 7.5
-Im(10Z
4Ωcm
2)
0.0
7.5
15.00.010
0.025
0.0500.10105Hz
15.0
22.5b
ReZ(104 Ωcm2)0.0 7.5
-Im(10Z
4Ωcm
2)
0.0
7.5
15.00.010
0.025
0.0500.10105Hz
15.0
22.5
R e Z ( 1 0
3 Ωcm2)
0.0 4.0 8.0
-Im(10Z
3Ωcm
2)
0.0
4.0
8.0
12.00.013
0.025
0.0500.10
105Hz
ReZ(103 Ωcm2)
0.0 4.0 8.0
-Im(10Z
3Ωcm
2)
0.0
4.0
8.0
12.00.013
0.025
0.0500.10
105Hz
c
ReZ(103 Ωcm2)0.0 3.0
-Im(10Z
3Ωcm
2)
0.0
3.0
6.00.010
0.0250.050
0.10105Hz
d
6.0
9.0
Fig. 19.3.3. Complex-plane plots of the impedance for electrodes of (a) as-deposited diamond and pore types (b) 30 50 nm, (c) 60 500 nm, and (d) 400 nm 3 m, at +0.4 V vs. Ag/AgCl. Experimental data points (○) and simulated curves (solid lines), were calculated on the basis of equivalent circuits involving modified transmission line models (see text), are shown. The parameters used in the calculated curves are given in Table 19.3.2.
The impedance plots for the pore type 30 50 nm
electrode, however, exhibit only a high frequency domain,
with a characteristic linear portion at a 45° angle (Fig. 19.
3.3-b). In this case, even at low frequencies, the potential
oscillations have negligible influence beyond a certain depth
(penetration depth).
At cylindrical-pore electrodes, the capacitance tends
to reach an intrinsic limiting value at very low frequencies.
The values were calculated in the low frequency limit (0.01
Hz) from the imaginary component of the impedance with
the relation Z = --i/(C). The results are summarized in
Table 19. 3. 1. The double layer capacitance values per unit
area discussed in this paper are based on the geometric
area, except where explicitly stated otherwise. The
capacitance values were found to increase with increasing
roughness factor, based on the pore dimensions. Among the
electrodes examined, the honeycomb with 400 nm 3 m
pores yielded a maximum capacitance value of 3.91 103
mF cm-2, which is a factor of ca. 400 larger than that for the
as-deposited surface. For the porous film with 30-nm
diameter pores, there was only a very small effect of the
13
pore structure on the capacitance due to the high pore
impedance.
Table 19. 3. 1 shows that the specific capacitance
value (74.6 F g-1) estimated for the 400 nm 3 m pore type
honeycomb is comparable to those typical for activated
carbon electrodes, which range from 100 to 400 F g-1 [22].
In terms of device applications, the ability to store
energy is important, and the larger available potential range
for diamond (> 3.0 V) compared to those for other forms of
carbon (ca. 1.0 V for activated carbon [37]) becomes an
advantage. Energy densities have been calculated for all of
the various types of electrodes examined in the present
work in terms of the geometric areas (Table 19. 3. 1).
Taking the capacitance values (Cdl) from the impedance
measurements and the potential window values (ΔV) from
the CV measurements, the energy densities (per unit
geometric area) for the actual diamond honeycomb double-
layer capacitors for a full cell were calculated by use of the
formula Edl = 0.5 Cdl (ΔV)2.
Assuming that the free-standing diamond honeycomb
films with though-holes were available for the pore
geometries examined here, we have estimated hypothetical
values for the specific capacitance for the various
honeycomb samples (i. e., per unit mass) (Table 19.3.1).
These range from 33.3 to 224.8 J g-1. Due to the large
working potential range, the specific energies for the
honeycomb diamond electrodes fall nearly in the same
range as that for typical activated carbon-based capacitors
14
(50 - 200 F g--
1). Because of the wide electrochemical
potential window in aqueous electrolytes and the high
capacitance, honeycomb diamond electrodes are promising
candidates for electrochemical capacitor applications.
Numerical simulations−The double-layer charging process
for a porous electrode consisting of cylindrical pores can be
simulated with the use of the transmission line model [24-
26]. If the cylindrical pores are characterized by radius r,
length l and number of pores n, the mathematical form for
the transmission line model is
Z = W coth(l)
(19.3.1)
where W and are defined as (RZ)1/2 and (R/Z)1/2,
respectively. Here, 1/Z is jC, and R and C are the
resistance and capacitance per unit pore depth and are
expressed by 1/(nr2) and 2rnCdpore, respectively. is the
electrolyte conductivity and Cdpore is the differential double-
layer capacitance in the pores. The impedance can be
simulated by use of the geometric parameters of the
cylindrical pores observed by SEM.
15
Fig. 19.3.4. Equivalent circuit based on the transmission line model, including both a Faradaic charge-transfer reaction and double-layer charging in the honeycomb diamond electrode
The calculated impedance curves for the various
honeycomb electrodes are shown in Fig. 19. 3. 3, together
with the experimental curves. Figure 19. 3. 4 shows an
equivalent circuit employed to reproduce the impedance
plots for honeycomb diamond electrodes. Table 19. 3. 2
summarizes the values of the fitting parameters and the
average relative errors for the calculated curves. The
calculated curves are in good agreement with the
experimental curves.
The areal capacitances of the pore walls (Cdpore), falling
in the range 120 to 230 mF cm-2, were on the same order as
that of the 1-min direct-etched diamond surface (see Table
19. 3. 1). This capacitance enhancement for the plasma-
etched surfaces is due to contributions from oxygen-
16
Transmission line model
Rspore
Cdpore
Electrolyteconductivity
PoredepthlPorediameter
d
Seriesresistance
Differentialcapacitance
Rsext
Cdext
Rrext Reaction
resistance
Rrpore
ReactionresistanceTransmissionlinemodel
Rspore
Cdpore
Electrolyteconductivity
PoredepthlPorediameter
d
Seriesresistance
Differentialcapacitance
Rsext
Cdext
Rrext Reaction
resistance
Rrpore
Reactionresistance
containing functional groups and various types of defects
generated on the surface during the plasma treatment.
Usually, the electrolyte conductivities inside the honeycomb
pores, as determined by impedance, range from 15 to 180
mS cm-1, which are of the same order of magnitude as the
bulk sulfuric acid conductivity. However, in the case of the
pore type 30 50 nm film, the electrolyte conductivity was
estimated to be only 70 mS cm-1, based on the fitting (Table
19. 3. 2). For the equivalent circuit used for the porous
electrodes, the pore impedance is usually determined only
by the value of the electrolyte conductivity. In the case of
the 30-nm pore diameter nano−honeycomb, the pore
impedance has drastically increased. Using a transmission-
line model for double-layer charging within the pores, we
were able to simulate the experimental impedance curves.
The diamond honeycomb structures appear to be good
approximations to an ideal cylindrical pore-type electrode.
Table 19.3.2. Parameters used for fitting the impedance results in the complex plane (Fig. 19.3.3), based on the modified transmission line model (Fig. 19.3.4).
17
Type of equivalent circuit
Seriesresistance forexternalsurface,
Rsext
, Ωcm2
Differentialcapacitanceforexternal
surface,Cdext,
Fcm-2
Timeconstantforexternalsurface,τext,ms
Reactionresistanceforexternalsurface,
Rrext,Ωcm2
Seriesresistanceforpores,
Rspore
,
Ωcm2
Differentialcapacitanceforpores,
Cdpore
,
Fcm-2
Timeconstantforpore,τpore,s
Reactionresistanceforpore,
Rrpore,
Ωcm2
Porediameter,d,nm
Poredepth,l,nm
Poredensity,
n,cm-2
Electrolyteconductivity,mScm-1
Averagerelativeerror,(%)
As-depositeddiamond 85.2 12.9 1.10
Puretransmissionlinemodel
140 - 60 500 1.0Å~ 107 15
Pore type 30?50 nm 35.5 29 2.69 - 1.42 Å~ 104 120 53.58 - 30 50 2.8 Å~ 1010 0.07 13.1
Pore type 60?500 nm 213 60 9.16 - 71.0 140 5.15 - 60 500 1.0 Å~ 107 15 9.75
Pore type 400 nm?3 μm 639 160 50.8 - 3.20 Å~ 103 230 4.75 - 400 3000 4.8 Å~ 108 180 8.94
19. 4. Electrochemical Properties of Pt−Modified Nanohoneycomb Diamond and Applications as a Size- Selective Sensor Materials
Diamond possesses morphological stability at
extreme anodic and cathodic potentials and corrosion
resistance in both acidic and alkaline conditions, without any
evidence of structural degradation [27]. Polycrystalline
diamond is ideally suited as a current collector for batteries
[28] or as an electrocatalyst support for fuel cells [29] and
for electrosynthesis. Diamond, because of its extremely high
packing density, is almost completely impervious to insertion
of ions. In order to achieve high catalyst loadings and large
surface areas, use of porous diamond supports is
advantageous for applications in electrocatalysis. In this
section, we report the use of conductive nanoporous
honeycomb diamond as a support for Pt nanoparticles for
electrocatalytic applications. In the present work,
nanohoneycomb diamond electrodes with various pore
diameters were modified with Pt nanoparticles and their size-
selective electrocatalytic properties were studied. The
catalytic activity and reaction kinetics for oxygen reduction
and alcohol oxidation were found to be dependent on the
pore dimensions.
19.4.1. Film characterization
18
Scanning electron microscopy−Platinum nanoparticles were
deposited in the pores of the diamond nano-honeycomb film
using the following method. The nanohoneycomb films were
immersed
19
B-a
C-a
300 nm
B-b
C-b
300nm
600 nm600 nm
600 nm3 m
A-a A-b
B-a
C-a
300nm
B-b
C-b
300 nm
600nm600nm
600nm3m
A-a A-b
Fig. 19.4.1. SEM images of Pt-modified highly boron-doped diamond electrodes: (A) top view for Pt-modified as-deposited diamond electrode at (a) low and (b) high magnification;. (a) top view; (b) oblique view at a 45° tilt angle for pore types (B) 60 500 nm, and (C) 400 nm 3 m.
in a 73-mM H2PtCl6 aqueous solution for 8 hours. After
immersion, the film was dried in air, and the Pt ions were
reduced to the metal by a 3-h exposure to flowing H2 gas at
580 °C. This process results in the incorporation of platinum
nanoparticles on the external surface and on the pore walls.
Figure 19.4.1 shows SEM images of three types of Pt-
modified diamond films that were fabricated from as-
deposited diamond and nanoporous diamond films. Figure
19.4.1A (a and b) shows images of the as-deposited diamond
surface with dispersed Pt nanoparticles (as-deposited
diamond / Pt ). The Pt nanoparticles, located mainly at the
grain boundaries, have diameters from 10 to 150 nm. There
are also very small Pt deposits (10-50 nm) on the grain
surface.
The two nanohoneycomb films (Fig. 19.4.1, B and C)
are shown both as top views (left) and oblique views (right).
The top views show highly uniform, well-ordered
arrangements of holes, with a hexagonal close-packed
pattern [26]. The Pt deposits are predominantly present in
the pores rather than on the external surface, as seen by
comparing the top and oblique views. The oblique views of
the edges of the honeycomb films clearly show the well-
20
defined cylindrical pores, with relatively large numbers of Pt
deposits on the pore walls. In the SEM images of both
nanohoneycomb / Pt films (honeycomb pore dimension type
60 500 nm / Pt and 400 nm 3 m / Pt), the
homogeneous distribution of Pt nanoparticles on the inner
walls of the honeycomb pores is clearly evident. For pore
type 60 500 nm, due to the small pores, Pt deposits as
small as 10 to 40 nm were obtained. In contrast, on the as-
deposited diamond surface, the Pt deposits ranged up to 150
nm. Hence, honeycomb films provide better dispersion of Pt
deposits.
Table 19.4.1. Comparison of the number of exposed surface Pt atoms for Pt-modified as-deposited diamond and Pt-modified nano-honeycomb diamond electrodes
Background cyclic voltammetry−Background cyclic
voltammograms were obtained in 1 M H2SO4 solution at a
sweep rate of 50 mV s-1. The voltammetric features of Pt-
modified diamond are characteristic of Pt metal, with Pt
oxide formation in the +0.7 to +1.2 V region, the reduction
of Pt oxide at ca. +0.5 V, and the adsorption and desorption
of hydrogen between 0 and --0.18 V (not shown).
21
Roughnessfactor
Desorption ofhydrogen
/ mC cm-2
(geo.)
Number ofsurface Pt atom
/ 1015
cm-2
(geo.)
Pt surface area
/ cm2 (real)
As-deposited diamond / Pt 3 0.61 3.77 2.88
Pore type 60Å~500 nm / Pt 10.9 1.89 11.8 9.02
Pore type 400 nmÅ~3 m/Pt 15.9 2.84 17.8 13.6
PolycrystallinePt 0.68 4.21 3.21
Integration of the oxidation charge associated with the
desorption of hydrogen between 0 and -0.18 V yielded a
value of 1.89 mC cm-2 for pore type 60 500 nm / Pt. This
charge can be used to calculate the number of exposed
surface Pt atoms, which was estimated to be 1.18 1016 cm-
2 (geometric area) using a standard value of 210 mC cm-2,
which corresponds to a calculated value of 1.30 1015
atoms cm-2 for polycrystalline Pt. The values determined for
the diamond / Pt and polycrystalline Pt from the cyclic
voltammograms are summarized in Table 19.4.1.
Interestingly, the number of exposed surface Pt atoms per
unit geometric area observed on the as-deposited diamond /
Pt was close (ca. 90%) to that for polycrystalline Pt. Thus,
this as-deposited / Pt film was expected to exhibit similar
electrocatalytic activity compared to Pt metal. However, this
was not the case, as discussed later.
19. 4. 2. Electrocatalysis with Pt-modified diamond: cyclic voltammetry
22
-1.2
-0.8
-0.4
0.0
0.4
1.51.00.50.0-0.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.51.00.50.0-0.5
(A)
(B)
Potential / V vs. Ag/AgCl
Current density /
mA
cm-2
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
-1.2
-0.8
-0.4
0.0
0.4
1.51.00.50.0-0.5-1.2
-0.8
-0.4
0.0
0.4
-1.2
-0.8
-0.4
0.0
0.4
1.51.00.50.0-0.5 1.51.00.50.0-0.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.51.00.50.0-0.5-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.51.00.50.0-0.5 1.51.00.50.0-0.5
(A)
(B)
Potential / V vs. Ag/AgCl
Current density /
mA
cm-2
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
Fig. 19.4.2. Cyclic voltammograms for (A) as-deposited diamond before Pt deposition (dotted line), after Pt deposition (solid line) and (B) Pt-nanoparticle-filled nano-honeycomb 60 500 nm (dot-dashed line), 400 nm 3 m (solid line); e. Electrolyte,: oxygen-saturated 1 M H2SO4; sweep rate,: 50 mV s-1; geometric surface area,: 0.071 cm2.
1.2
0.8
0.4
0.0
-0.41.51.00.50.0-0.5
10
8
6
4
2
0
-21.51.00.50.0-0.5
(A)
(B)
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
1.2
0.8
0.4
0.0
-0.4
1.2
0.8
0.4
0.0
-0.41.51.00.50.0-0.5 1.51.00.50.0-0.5
10
8
6
4
2
0
-21.51.00.50.0-0.5
10
8
6
4
2
0
-2
10
8
6
4
2
0
-21.51.00.50.0-0.5 1.51.00.50.0-0.5
(A)
(B)
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
Fig. 19.4.3. Cyclic voltammograms for (A) as-deposited
diamond before Pt deposition (dotted line), after Pt
23
deposition (solid line) and (B) Pt-nanoparticle-filled
nano-honeycomb 60 500 nm (dot-dashed line), 400 nm
3 m (solid line); e. Electrolyte,: 2 M methanol + 1 M
H2SO4; sweep rate,: 50 mV s-1; geometric surface area,:
0.071 cm2.
The effectiveness of the Pt-modified diamond
electrodes for the electrocatalysis of fuel cell reactions was
examined. We have tested their electrocatalytic activities
for O2 reduction and alcohol oxidation. Figure 19.4.2
compares O2 reduction currents for the as-deposited
diamond, the as-deposited diamond / Pt, honeycomb 60 500 nm / Pt and the 400 nm 3 m / Pt electrodes in 1 M
H2SO4 saturated with oxygen at a sweep rate 50 mV s-1. In
the absence of the Pt nanoparticles, essentially no O2
reduction is observed over this potential range, as diamond
is known to have low catalytic activity for O2 reduction. In
contrast, at diamond / Pt composite electrodes, large O2
reduction current is observed at potentials characteristic for
Pt electrocatalysis in this solution.
The cathodic current density for the 400 nm 3 m /
Pt electrode (ca. -1.8 mA cm-2, geometric) was nearly twice
as large as that for the as-deposited diamond / Pt electrode
(ca. -1.0 mA cm-2, geometric), and this is due to the high
surface area. Based on the number of surface Pt atoms per
unit geometric area, which was ca. five times greater for
pore type 400 nm 3 m film than for the as-deposited
diamond, a similar factor could be possible for the peak
24
current, but this is clearly not expected, due to mass
transport limitations.
For methanol oxidation (2 M in 1 M H2SO4), cyclic
voltammograms were obtained at the as-deposited diamond,
the as-deposited diamond / Pt, 60 500 nm / Pt and the 400
nm 3 m / Pt electrodes (Fig. 19.4.3). At an as-deposited
diamond film, no methanol oxidation was observed; diamond
is known to have low activity for methanol oxidation. In the
case of the nonporous diamond / Pt electrode, a large anodic
peak was observed at ca. 0.9
25
V, attributable to methanol oxidation. The Pt-containing film
is known to be electroactive for methanol electrooxidation
[30, 31]. The Pt nanoparticles supported on the diamond
electrode provide the catalytic activity for methanol
oxidation in acid solution. The oxidation current for the 400
nm 3 m / Pt electrode (ca. 7.0 mA cm-2, geometric) was
greatly enhanced compared to the as-deposited diamond / Pt
(ca. 1.1 mA cm-2, geometric) and was found to be ca. 16
times higher than that for the Pt polycrystalline electrode
(ca. 0.44 mA cm-2, geometric) (Fig. 19. 4. 3).
Fig. 19.4.4. Cyclic voltammograms for (A) ethanol
oxidation and (B) 2-propanol oxidation for as-deposited
26
20
15
10
5
0
1.51.00.50.0-0.5
-0.8
-0.4
0.0
0.4
0.8
1.00.50.0-0.5
(A)
(B)
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
20
15
10
5
0
1.51.00.50.0-0.5
20
15
10
5
0
20
15
10
5
0
1.51.00.50.0-0.5 1.51.00.50.0-0.5
-0.8
-0.4
0.0
0.4
0.8
1.00.50.0-0.5-0.8
-0.4
0.0
0.4
0.8
-0.8
-0.4
0.0
0.4
0.8
1.00.50.0-0.5 1.00.50.0-0.5
(A)
(B)
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
Current density /
mA
cm-2
Potential / V vs. Ag/AgCl
diamond / Pt (dotted line), Pt-nanoparticle-filled nano-
honeycomb 60 500 nm (dot-dashed line), and 400 nm
3m (solid line); e. Electrolyte,: 2 M ethanol or 2 M 2-
propanol + 1 M H2SO4; sweep rate,: 50 mV s-1; geometric
surface area,: 0.071 cm2.
At the as-deposited diamond / Pt electrode, the peak
current is proportional to the square root of the scan rate,
indicating that the oxidation of methanol at this electrode is
controlled by diffusion. In contrast, at both the 60 500 nm
/Pt and the 400 nm 3 m / Pt electrodes, the current
densities deviate from the linear curve at higher sweep
rates. This behavior is thought to be caused by the
nanoporous structure effect for methanol mass transport
inside the pores. This effect is expected to be dependent on
the size of the reacting molecules. Therefore, the oxidation
reactions of larger size alcohols were also investigated.
For example, ethanol oxidation was examined. Cyclic
voltammograms were obtained for the as-deposited diamond
/ Pt, 60 500 nm /Pt and 400 nm 3 m / Pt electrodes in 2
M ethanol in 1 M H2SO4 (Fig. 19. 4. 4A). The Pt-modified
diamond electrodes show elecrocatalysis for ethanol
oxidation [32]. It can be seen that the oxidation current for
pore type 400 nm 3 m / Pt was ca. 4 times higher than
that for as-deposited diamond / Pt, but the oxidation current
for pore type 60 500 nm / Pt was suppressed, being only
27
ca. 0.6 times of that for as-deposited diamond / Pt. The
expected current enhancement due to the nanohoneycomb
roughness was not observed for this pore type.
In addition, 2-propanol oxidation was examined.
Figure 19.4.4B shows cyclic voltammograms obtained for as-
deposited diamond / Pt, 60 500 nm /Pt and 400 nm 3
m / Pt electrodes in 2 M 2-propanol in 1 M H2SO4. In this
case [33], it can be seen that the oxidation currents for as-
deposited diamond / Pt, 60 500 nm / Pt and 400 nm 3
mm / Pt electrodes are all similar, and therefore, there was
no enhancement due to the honeycomb roughness for either
nanohoneycomb / Pt electrode.
In order to better illustrate the nanostructure effect
for the electrocatalytic reactions examined here, peak
current ratios were used. (Figure 19.4.5) These values (Rp)
are the ratios of the peak current densities for the
honeycomb diamond / Pt electrodes (Iph) to that for the as-
deposited diamond / Pt (Ipa), normalized by the ratio of
number of surface Pt atoms exposed (Nh and Na,
respectively) using the formula Rp = (Iph / Ipa) (Na / Nh).
This could be considered to be an indicator of the
fraction of surface Pt atoms that are actually actively
involved in the electrocatalytic reaction. In the case of
methanol oxidation, at both honeycomb diamond / Pt
28
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Peak Current Ratio
MethanolOxidation
EthanolOxidation
2-PropanolOxidation
OxygenReduction
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Peak Current Ratio
M e t h a n o l
O x i d a t i o n
E t h a n o l
O x i d a t i o n
2 - P r o p a n o l
O x i d a t i o n
O x y g e n
R e d u c t i o n
electrodes, approximately all of the surface Pt atoms appear
to be available for the catalytic reaction.
Fig. 19.4.5. Relationships of peak current ratios for
oxygen reduction and alcohol oxidation for Pt-
nanoparticles-filled nano-honeycomb (□) 60 500 nm
and (○) 400 nm × 3 m electrodes. The peak current
ratio is defined as the ratio of the peak current density
for the honeycomb / Pt to that for the as-deposited
diamond /Pt and normalized by the ratio of the number
of surface Pt atoms exposed.
In contrast, for ethanol oxidation, the apparent
fraction of active Pt atoms for 60 500 nm / Pt was only 0.2,
which is three times lower than that for pore type 400 nm 3 m / Pt. This result indicates that there is a limitation on
the ability of the ethanol molecules to access the Pt atoms
located within the 60-nm pores. This effect is even more
evident for the larger molecule, 2-propanol, which yielded an
active Pt atom ratio of 0.1, even for 400 nm 3 m / Pt.
These results clearly indicate an effect of molecular size for
the honeycomb / Pt electrodes for the catalytic oxidation of
alcohols. The electrocatalytic activities of the Pt-modified
nanohoneycomb films were found to be dependent on the
structural parameters of the honeycomb pores and the
molecular sizes of the alcohols, indicating that the selectivity
of the electrodes can be controlled by variation of the pore
dimensions.
29
Both nanohoneycomb / Pt electrodes showed high
electrocatalytic activity for oxygen reduction and methanol
oxidation. Hence, these electrodes have potential
application in fuel cell development.
19.4.3. Electrocatalysis with Pt-modified diamond: impedance measurementsIn order to understand the characteristics of the
electrocatalysis reaction inside the nanoporous electrodes,
additional analysis of the ac impedance behaviour was
carried out, and the penetration depths of reactant
molecules in the nanohoneycomb pores for catalytic
reactions and the reaction parameters for different pore
structures were estimated. The ac impedance
measurements for Pt-modified diamond electrodes
30
Fig. 19.4.6. Impedance plots for electrocatalytic reactions: (A) the oxidation of methanol at 0.9 V vs. Ag/AgCl; (B) the oxidation of ethanol at 0.9 V vs. Ag/AgCl; and (C) the reduction of oxygen at 0.4 V vs. Ag/AgCl. Experimental data points are shown as open symbols for (△) as-deposited diamond / Pt, (□) 60 × 500 nm / Pt, and (○) 400 nm × 3 m/ Pt. The simulated curves, calculated on the basis of the equivalent circuit
31
Re Z / 103 Ωcm20.0 1.3 2.6-Im/10Z
3Ωcm
2
0.0
1.3(C)
3.9
ReZ/103 Ωcm20.0 2.0 4.0-Im/10Z
3Ωcm
2
0.0
2.0(A)
0.0
0.3
0.3 0.6 0.90.01
0.050.1
0.250.51
100kHz
0.0200.1
1
0.0200.1
1
100kHz
100kHz
ReZ/103 Ωcm20.0 0.6 1.2-Im/10Z
3Ωcm
2
0.0
0.6
1.8
(B)0.1
0.02
0.5
0.5
1
100kHz
0.05
0.11100kHz
0.020.11
0.1
0.51
0.05
100kHz
0.01
0.5
0.1
0.010.05100kHz
ReZ/103 Ωcm20.0 1.3 2.6-Im/10Z
3Ωcm
2
0.0
1.3(C)(C)
3.9
ReZ/103 Ωcm20.0 2.0 4.0-Im/10Z
3Ωcm
2
0.0
2.0(A)
0.0
0.3
0.3 0.6 0.90.01
0.050.1
0.250.51
100kHz
0.0200.1
1
0.0200.1
1
100kHz
100kHz
ReZ/103 Ωcm20.0 2.0 4.0-Im/10Z
3Ωcm
2
0.0
2.0(A)
0.0
0.3
0.3 0.6 0.9
(A)
0.0
0.3
0.3 0.6 0.90.01
0.050.1
0.250.51
100kHz
0.0200.1
1
0.0200.1
1
100kHz
100kHz
ReZ/103 Ωcm20.0 0.6 1.2-Im/10Z
3Ωcm
2
0.0
0.6
1.8
(B)0.1
0.02
0.5
0.5
1
100kHz
0.05
0.11100kHz
0.020.11
ReZ/103 Ωcm20.0 0.6 1.2-Im/10Z
3Ωcm
2
0.0
0.6
1.8
(B)(B)0.1
0.02
0.5
0.5
1
100kHz
0.05
0.11100kHz
0.020.11
0.1
0.51
0.05
100kHz
0.01
0.5
0.1
0.010.05100kHz
in Fig. 19.3.4, are shown as solid lines. The parameters are summarized in Tables 19.4.2 and 3.
during catalytic reactions were carried out at the peak
potentials obtained in the CV measurements (Figures
19.4.6A-C). The impedance plots for methanol oxidation
(Figures 19.4.6A) consist mainly of parallel RC-type
semicircles whose diameters (and thus the corresponding
resistances) decrease with increasing roughness factor (ca. 3
for as-deposited diamond, 10.9 for the 60-nm pores and 15.9
for the 400-nm pores). The diameters of the semicircles (Ω
cm2, based on the geometric area) decreased in order for the
as-deposited diamond / Pt, pore type 60 500 nm / Pt, and
pore type 400 nm 3 m / Pt, roughly estimated to be 2.9 103 Ω cm2, 6.5 102 Ω cm2, and 5.0 102 Ω cm2,
respectively. These can be related to the charge transfer
resistances (discussed later in detail), which decrease with
increasing effective surface area for the charge transfer
reaction. In contrast, for ethanol oxidation and oxygen
reduction (Figures 19.4.6B and C) the diameters no longer
follow the same order as the roughness. The impedance
plots for the pore type 60 500 nm / Pt electrode trace the
largest semicircles for both ethanol oxidation and oxygen
reduction. The charge-transfer resistance per unit area for
pore type 60 500 nm / Pt is now larger than that for pore
type 400 nm 3 m / Pt due to a mass transfer effect, as
discussed later.
32
The impedance of a porous electrode can be
simulated with the transmission line model, and the
penetration depth can be evaluated [24]. For the non-
porous Pt-modified as-deposited surface, the methanol
oxidation reaction can be simulated as a simple Randles
equivalent circuit comprising a parallel combination of a
double layer capacitance and a semi-infinite Warburg
impedance in series with a charge transfer resistance. For
oxygen reduction, a simple Randles equivalent circuit was
also used, because the reaction mechanism for oxygen
reduction for the Pt electrode can be described by mass
transport-controlled kinetics. The simulated curves are
shown in Figs. 19.4.6(A-C). The fits are reasonably good,
with the charge-transfer resistances (based on geometric
area) Rr values shown in Table 19.4.2.
Table 19.4.2. Parameters used for fitting the impedance results for an as-deposited diamond / Pt electrode in the impedance plots (Fig. 19.4.6), based on the Randles circuit.
The impedance of a charge-transfer reaction at a
porous electrode consisting of cylindrical pores is given in
33
Electroatalytic reaction
Seriesresistance
Rs / Ωcm2
Differentialcapacitance
Cd/Fcm-2
Reactionresistance
Rr / Ω cm2
Diffusionresistance
δ / Ω cm2
Methanol Oxidation 38.2 70 1.90 ? 103 4.60? 102
Ethanol Oxidation 50.2 90 3.60 ? 102 1.45 ? 102
Oxygen Reduction 28.2 80 9.00 ? 102 3.55 ? 102
the previous section by Eq. (19.3.1) [23-26]. To simplify the
calculations, the Faradaic impedance per unit real surface
area was assumed to be potential-independent over a range
of values that would exist along the entire pore, e. g., < 0.25
V, and thus consists only of a parallel combination of the
charge transfer resistance and a double-layer capacitance,
without a Warburg impedance.
For a charge transfer-controlled process, 1/Z = 1/Rct +
jC, and the reaction resistance Rct and capacitance C per
unit pore depth are expressed by Rrpore/(2r) and j2rnCd
pore,
respectively. Here, Rrpore is the charge transfer resistance
with respect to the real surface area on the pore walls. A
distinction between Rr and Rrpore has been made, because we
wish to apply the latter specifically to the pores only, where
most of the Pt particles are located. From the geometric
parameters of the cylindrical pores (i.e., diameter, depth and
number density), which are obtainable from SEM
observation, the impedance can be evaluated.
Table 19.4.3. Parameters used for fitting the impedance results in the impedance plots (Fig. 19.4.6), based on the equivalent circuit in Fig. 19.3.4.
34
Type of Electrodes
Seriesresistanceforexternalsurface
Rsext
/ Ωcm2
Differentialcapacitanceforexternalsurface
Cdext
/Fcm-2
Seriesresistanceforpore
Rspore
/Ωcm2
Differentialcapacitanceforpore
Cdpore
/Fcm-2
Reactionresistancefor pore
Rrpore
/ Ω cm2
Electrolyteconductivityk
/ mS cm-1
Penetrationdepthλ
/ μm
Averagerelativeerror/ %
Pure transmission line 166 1.80 ? 103 80
Methanol 60 ? 500 nm / Pt 400 120 400 166 1.80 ? 103 80 0.46 3.02
Oxidation 400 nm ? 3 μ m / Pt 130 600 200 900 4.50 ? 103 162 2.69 6.75
Pure transmission line 200 3.50 ? 103 0.7
Ethanol 60 ? 500 nm / Pt 350 100 350 200 3.50 ? 103 0.7 0.19 22.48
Oxidation 400 nm ? 3 μ m / Pt 120 480 180 820 1.40 ? 103 830 2.87 12.09
Pure transmission line 400 1.80 ? 104 0.5
Oxygen 60 ? 500 nm / Pt 400 240 400 400 1.80 ? 104 0.5 0.36 8.76
Reduction 400 nm ? 3 μ m / Pt 140 650 100 260 6.20 ? 103 100 2.49 8.01
The calculated impedance curves for the honeycomb
electrodes are shown in Fig. 19.4.6(A-C) together with the
experimental curves. Figure 19.3.4 shows the equivalent
circuit employed to simulate the impedance plots for the
honeycomb / Pt electrodes. Table 19.4.3 summarizes the
values of the fitting parameters and the average relative
errors for the calculated curves. For 60 500 nm / Pt, the
electrolyte conductivity in the pores of 80 mS cm -1,
measured for methanol oxidation, decreased to that for
ethanol oxidation (0.7 mS cm-1, Table 19.4.3). This result
suggests that the conductivity ofassociated with the alcohol
molecule in the 60-nm nanohoneycomb pores decreases
with increasing molecular size.
By use of the transmission line model, the
penetration depth for the reaction can be calculated. The
penetration depth is defined in previous section by Eq.
(19.4.1) [24].
= |Zt|1/2 R-1/2 sec 1/2 (19.4.1)
where |Zt| and are the amplitude and the phase angle for
the impedance of the transmission part, respectively. Table
19.4.3 summarizes the penetration depths for the various
catalytic reactions. For pore type 400 nm 3 m / Pt, the
penetration depths for all of the catalytic reactions were
close to the actual pore depth of 3 m, with the lowest value
being 2.49 m for O2 reduction (ca. 80 % of the pore depth).
For pore type 60 500 nm / Pt, with 460 nm for methanol
oxidation, was also close to the actual depth, indicating
that almost all of the pore surface is available. In contrast,
35
the value for ethanol oxidation was 190 nm, which is only
40 % of the total pore depth. This result suggests that pore
type 60 500 nm is sensitive to the size of the alcohol
molecule, so that decreases with increasing reactant size.
For O2 reduction, decreases due to its low concentration.
However, even so, half of the pore depth for type 60 500
nm was still available for ethanol oxidation and oxygen
reduction.
It is interesting to note that the charge transfer
resistances Rr calculated for methanol oxidation for the Pt-
modified diamond electrodes (ca. 1.8 - 4.5 kΩ cm2) are of the
same order. In contrast, the Rr values calculated for ethanol
oxidation and oxygen reduction for the honeycomb / Pt
electrodes are significantly larger than that for as-deposited
diamond / Pt. Also, an increase in Rr is observed with
decreasing pore size. A possible explanation for the increase
of the reaction resistance could be the relatively low
concentration of the reactant near the active catalytic sites
because of the limitation of mass transport by the
nanoporous structure. In order to clarify the contribution of
the ethanol concentration to the Rr values, we have
examined the concentration dependence of the impedance
behavior.
A series of impedance plots for the Pt-modified as-
deposited diamond electrode in C2H5OH + 1 M H2SO4 solution
were obtained (not shown). The ethanol was varied in
concentration from 0.02 to 2 M. The fact that the Rr value
(1.0 kΩ cm2) obtained for 0.2 M ethanol for the as-deposited
36
diamond / Pt electrode is close to the value (1.4 kΩ cm2) for
400 nm 3 m / Pt in 2 M ethanol (Table 19.4.3) indicates
that the ethanol concentration in the pores for the latter is
one order of magnitude less than that in the bulk. Similarly,
for pore type 60 500 nm / Pt, the Rr value (3.5 kΩ cm2)
was of the same order as that for as-deposited diamond / Pt
(2.8 kΩ cm2) in 0.02 M ethanol. This result suggests that the
concentration inside the 60 500 nm pores is a factor of
200 lower (ca. 0.01 M) than that in the bulk.
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