U.S. patent application number 11/110083 was filed with the patent office on 2005-09-15 for electrically conductive polycrystalline diamond and particulate metal based electrodes.
This patent application is currently assigned to MICHIGAN STATE UNIVERSITY. Invention is credited to Swain, Greg M., Wang, Jian.
Application Number | 20050200260 11/110083 |
Document ID | / |
Family ID | 33422712 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050200260 |
Kind Code |
A1 |
Swain, Greg M. ; et
al. |
September 15, 2005 |
Electrically conductive polycrystalline diamond and particulate
metal based electrodes
Abstract
An electrically conducting and dimensionally stable diamond (12,
14) and metal particle (13) electrode produced by electrodepositing
the metal on the diamond is described. The electrode is
particularly useful in harsh chemical environments and at high
current densities and potentials. The electrode is particularly
useful for generating hydrogen, and for reducing oxygen and
oxidizing methanol in reactions which are of importance in fuel
cells.
Inventors: |
Swain, Greg M.; (East
Lansing, MI) ; Wang, Jian; (Westmont, IL) |
Correspondence
Address: |
MCLEOD & MOYNE, P.C.
2190 COMMONS PARKWAY
OKEMOS
MI
48864
US
|
Assignee: |
MICHIGAN STATE UNIVERSITY
East Lansing
MI
48824-1046
|
Family ID: |
33422712 |
Appl. No.: |
11/110083 |
Filed: |
April 20, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11110083 |
Apr 20, 2005 |
|
|
|
10338318 |
Jan 8, 2003 |
|
|
|
6884290 |
|
|
|
|
60347675 |
Jan 11, 2002 |
|
|
|
Current U.S.
Class: |
313/311 |
Current CPC
Class: |
H01M 4/926 20130101;
C23C 16/274 20130101; C25B 11/091 20210101; C25D 3/50 20130101;
C02F 2001/46133 20130101; Y02E 60/50 20130101; C02F 1/46109
20130101; C23C 16/278 20130101; H01M 4/92 20130101; H01M 4/8853
20130101; C02F 2001/46142 20130101 |
Class at
Publication: |
313/311 |
International
Class: |
H01J 001/30 |
Goverment Interests
[0002] The research disclosed in this application was supported by
the Department of Energy Grant No. DE-FG02-01ER15120. The U.S.
government has certain rights to this invention.
Claims
1-12. (canceled)
13. A diamond electrode which comprises: (a) a first
polycrystalline diamond support, doped with at least one element so
as to be electrically conductive; (b) particles of a conductive
metal which have been electrodeposited as a coating on the diamond
support; and (c) a polycrystalline diamond film deposited on the
diamond support and around the particles of the conductive metal to
surround and anchor the particles to thereby provide the conductive
electrode.
14. The electrode of claim 13 wherein the element is boron.
15. The electrode of claim 13 wherein the first and second diamond
films are deposited by chemical vapor deposition.
16. The electrode of claim 12 wherein the chemical vapor deposition
is microwave-assisted in the presence of methane and hydrogen at
reduced pressures.
17. A diamond electrode which comprises: (a) a first
polycrystalline diamond film doped with at least one element so as
to be electrically conductive; (b) particles of a conductive metal
which have been electrodeposited as a coating on the first diamond
film; and (c) a second polycrystalline diamond film deposited on
the first diamond film and around the particles of the conductive
metal to surround and anchor the particles to thereby provide the
conductive electrode.
18. The electrode of claim 17 wherein the element is boron.
19. The electrode of claim 17 wherein the first and second diamond
films are deposited by chemical vapor deposition.
20. The electrode of claim 19 wherein the chemical vapor deposition
is microwave-assisted in the presence of methane and hydrogen at
reduced pressures.
21. The electrode of any one of claim 17, 18, 19 or 20 wherein the
metal has been electrodeposited by electrolyzing a metal halide
salt which is reduced to the metal.
22. The process of any one of claims 17, 18, 19 or 20 wherein the
second polycrystalline diamond film is also doped.
23. The electrode of any one of claims 17, 18, 19 or 20 wherein the
second polycrystalline film is also doped.
24. The electrode of any one of claims 17, 18, 19 or 20 wherein the
conductive metal particles are irregularly spherical in shape.
25. The electrode of any one of claims 17, 18, 19 or 20 wherein the
metal has been electrodeposited by electrolyzing a platinum halide
salt in the presence of perchloric acid.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to provisional application
Ser. No. 60/347,675, filed Jan. 11, 2002.
BACKGROUND OF THE INVENTION
[0003] (1) Field of the Invention
[0004] The present invention relates to a polycrystalline
conductive diamond electrode with a particulate metal
electrolytically deposited on and anchored in the diamond
particles. In particular, the present invention relates to
particulate platinum (Pt) or ruthenium (Ru) or rhodium (Rh) and
nobel metal alloys thereof based diamond film electrodes. The
diamond electrode can be used in fuel cells, electrosynthesis or
electrochemical-based chemical contaminant remediation.
[0005] (2) Description of Related Art
[0006] The present invention uses a deposition process similar to
that described by Gruen et al. See for example U.S. Pat. Nos.
5,989,511; 5,849,079 and 5,772,760. The patents to Gruen et al.
describe processes for synthesizing relatively smooth
polycrystalline diamond films starting with the mixing of
carbonaceous vapors, such as methane or acetylene gas, with a gas
stream consisting of mostly an inert or noble gas, such as argon,
with, if necessary, also small fractional (1-3%) additions of
hydrogen gas. This gas is then activated in, for example, in a
microwave plasma environment, and under the appropriate conditions
of pressure, gas flow, microwave power, substrate temperature and
reactor configuration, nanocrystalline diamond films are deposited
on a substrate.
[0007] Other related patents relating to diamond deposition are
U.S. Pat. Nos. 5,209,916 to Gruen; 5,328,676 to Gruen; 5,370,855 to
Gruen; 5,462,776 to Gruen; 5,620,512 to Gruen; 5,571,577 to Zhang
et al; 5,645,645 to Zhang et al; 5,897,924 to Ulczynski et al and
5,902,640 to Krauss, as well as numerous patents to Asmussen which
are all incorporated by reference herein.
[0008] U.S. Pat. Nos. 6,106,692 to Kunimatsu et al; and 5,900,127
to Iida et al describe conductive diamond electrodes. G. M. Swain
(Wang, J., et al., J. New Mater. Electrochem. Syst. 3 75 (2000) and
Wang, J., et al., Electrochem. Solid-State Lett., 3 286 (2000))
describe electrodes with embedded platinum particles produced by
magnetron sputtering.
[0009] Electrodes consisting of supported metal catalysts are used
in a number of industrial processes (e.g., electrosynthesis) and
electrochemical energy conversion devices (e.g., fuel cells). The
metal catalysts are typically impregnated into the porous structure
of several types of sp.sup.2 bonded carbon materials; chemically or
physically activated carbon, carbon black, and graphitized
carbons..sub.1 Activated carbon is the most common type of support,
at least in part because of the material's chemical stability in
acidic and alkaline environments. The primary role of the support
is to finely disperse and stabilize small metallic particles, and
thus provide access to a much larger number of catalytically active
atoms than in the bulk metal even when the latter is ground into a
fine powder (Auer, W., et al., Appl. Catal., A, 173 259 (1998)).
Several properties of the support are important; among them
porosity, pore size distribution, crush strength, surface
chemistry, and microstructural and morphological stability.
[0010] The present invention uses electrically conducting diamond
thin films (Wang, J., et al., J. New Mater. Electrochem. Syst., 3
75 (2000); Wang, J., et al., Electrochem. Solid-State Lett., 3, 286
(2000); and Witek, M., et al., J. Wide Bandgap Mater. Vol. 8 No.
3-4 171-188 (January/April 2001)). The use of electrically
conducting microcrystalline and nanocrystalline diamond electrodes
in electrochemistry is a relatively new field of research (Xu, J.,
et al., Anal. Chem. 69, 591A (1997); Swain, G. M., et al., MRS
Bull., 23, 56 (1998); Tenne, R., et al., Isr. J. Chem. 38 57
(1998); Pleskov, Y. V., Russian Chemical Reviews 68 381 (1999);
Vinokur, N., et al., J. Electrochem. Soc. 143 L238 (1996); and Rao,
T. N., et al., Anal. Chem. 71 2506 (1999)). The properties of this
new electrode material make it ideally suited for electrochemical
applications, particularly demanding ones (i.e., complex matrix,
high current density, and potential, high temperature, extremes in
pH, and the like). Recent work has shown that nanometer-sized
dispersions of Pt can be incorporated and anchored into the surface
microstructure of boron-doped microcrystalline diamond thin film
electrodes (Wang, J., et al., J. New Mater. Electrochem. Syst., 3,
75 (2000); Wang, J., et al., Electrochem. Solid-State Lett., 3 286
(2000); and Witek, M., et al., J. Wide Bandgap Mater. Vol. 8 No.
3-4 171-188 (January/April 2001)). The diamond film serves as a
host for the catalyst particles providing electrical conductivity
(est., 0.1 .OMEGA. cm), thermal conductivity, and dimensional
stability. The microstructure and morphology of the diamond, as
well as the electrocatalytic activity of the Pt particles, were
observed to be very stable during extended electrolysis as no
degradation of either was detected after 2000 potential cycles
between the hydrogen and oxygen evolution regimes in 0.1 M
HClO.sub.4 at room temperature (1-6 mA/cm.sup.2) (Wang, J., et al.,
Electrochem. Solid-State Lett., 3 286 (2000)). Importantly, the
metal catalyst exposed at the surface is in electronic
communication with the current collecting substrate through the
boron-doped diamond film, and is electroactive for the
underpotential deposition of hydrogen (Wang, J., et al., J. New
Mater. Electrochem. Syst., 3, 75 (2000); Wang, J., et al.,
Electrochem. Solid-State Lett., 3 286 (2000); and Witek, M., et
al., J. Wide Bandgap Mater. Vol. 8 No. 3-4 171-188 (January/April
2001)), the reduction of oxygen, and the oxidation of methanol
(Wang, J., et al., J. New Mater. Electrochem. Syst., 3 75 (2000);
and Wang, J., et al., Electrochem. Solid-State Lett., 3 286
(2000)).
[0011] Given the corrosion susceptibility of conventional carbon
support materials, there is a technological need for advanced
support materials that are morphologically and microstructurally
stable during exposure to aggressive chemical and electrochemical
environments.
OBJECTS
[0012] It is therefore an object of the present invention to
provide a novel process and diamond electrode produced thereby.
These and other objects will become increasingly apparent by
reference to the following description.
DESCRIPTION OF FIGURES
[0013] FIG. 1 are AFM images showing 10 to 500 nm diameter
(spherical) Pt particles incorporated into the diamond surface
microstructure with smaller diamond particles on the triangular
diamond microcrystallite surfaces and larger particles in the grain
boundaries between the microcrystallites.
[0014] FIG. 2 is a schematic diagram showing the steps in the
process to form the diamond electrode containing particles of
platinum anchored by diamond particles.
[0015] FIG. 2A is a schematic diagram of the apparatus 10 used in
the process of FIG. 2. In a particular example, C.sub.M (methane)
was 0.3%, P was 60 torr, T.sub.s (stage) was 875.degree. C., and
B/C was .about.0.1% by volume.
[0016] FIG. 3 is a graph showing CV i-E curves for a Pt/diamond
composite electrode in 0.1 M HClO.sub.4 before (dashed line) and
after two 1 h polarizations (solid lines) in 85 wt %
H.sub.3PO.sub.4 at 170.degree. C., and an anodic current density of
0.1 A/cm.sup.2.
[0017] FIGS. 4A and 4B are images (in air) of a Pt/diamond
composite electrode (FIG. 4A) before and (FIG. 4B) after anodic
polarization in 85 wt % H.sub.3PO.sub.4 at 170.degree. C. and an
anodic current density of 0.1 A/cm.sup.2.
[0018] FIGS. 5A and 5B are optical micrographs of a commercial
sp.sup.2 carbon cloth electrode impregnated with Pt (FIG. 5A)
before and (FIG. 5B) after a 1 h anodic polarization in 85 wt %
H.sub.3PO.sub.4 at 170.degree. C. and an anodic current density of
0.1 A/cm.sup.2. Images of a treated (left) and untreated electrode
(right) are shown in FIG. 5A.
[0019] FIG. 6 (prior art) is a graph showing CV i-E curves for the
sp carbon cloth electrode impregnated with Pt in 0.1 M HClO.sub.4
before and after a 1 h polarization in 85 wt % H.sub.3PO.sub.4 at
170.degree. C. and an anodic current density of 0.1 A/cm.sup.2.
[0020] FIG. 7 is a schematic front view of the cell used in the
example.
SUMMARY OF THE INVENTION
[0021] The present invention relates to a process for the
production of a diamond electrode which comprises: providing a
first diamond support, doped with at least one element so as to be
electrically conductive; electrodepositing particles of a
conductive metal as a coating on the diamond support; and
depositing a diamond film on the diamond support and around the
particles of the conductive metal to surround and anchor the
particles and to produce the diamond electrode wherein the
particles are conductive through the support.
[0022] The present invention relates to a process for the
production of a diamond electrode which comprises:
[0023] providing a first diamond film, doped with at least one
element, such as boron, so as to be electrically conductive, on an
electrically conductive substrate;
[0024] electrodepositing particles of a conductive metal, such as
platinum, as a coating on the diamond film; and
[0025] depositing a second diamond film on the first diamond film
and around the particles of the conductive metal for the purpose of
surrounding the metal particles to anchor the particles and to
produce the diamond electrode wherein the metal particles are in
good electrical communication with the conductive substrate through
the electrically conducting diamond film. It was unexpected that
the electrolytically deposited particles of the metal would be
anchored securely by this method. A representative diamond film is
shown in FIG. 1.
[0026] The present invention also relates to a diamond electrode
which comprises:
[0027] a first polycrystalline diamond support, doped with at least
one element so as to be electrically conductive;
[0028] particles of a conductive metal which have been
electrodeposited as a coating on the diamond support; and
[0029] a polycrystalline diamond film deposited on the diamond
support and around the particles of the conductive metal to
surround and anchor the particles to thereby provide the conductive
electrode.
[0030] The present invention also relates to a diamond electrode
which comprises:
[0031] a first polycrystalline diamond film doped with at least one
element so as to be electrically conductive;
[0032] particles of a conductive metal which have been
electrodeposited as a coating on the first diamond film; and
[0033] a second polycrystalline diamond film deposited on the first
diamond film and around the particles of the conductive metal to
surround and anchor the particles to thereby provide the conductive
electrode.
[0034] The "electrically conductive substrate" can have any shape
(such as planar or curved) and be in the form of a low surface area
planar substrate or a high surface area substrate as a mesh, foam
or particle substrate. The substrate can also be a composite of
multiple electrically conductive substrate.
[0035] The electrically conductive diamond can be a composite of
various layers or forms of diamond alone (such as nanocrystalline
or single crystal diamond) or with carbon in different forms. All
of these forms of diamond (including diamond-like carbons) are well
known to those skilled in the art.
[0036] Preferably the particles of metal are platinum. Also
preferably the doping element is boron. The diamond films are
preferably deposited by chemical vapor deposition. The chemical
vapor deposition is preferably accomplished by microwave activation
in the presence of methane and hydrogen at reduced pressures.
Preferably electrodeposition is of a metal halide salt which is
reduced to the metal. Preferably the conductive metal particles are
irregularly spherical in shape. Most preferably the
electrodeposition is from a platinum halide salt in the presence of
perchloric acid.
[0037] Thus the objective of this invention is a new
electrocatalytic electrode with extreme microstructural and
morphological stability to be used for electrosynthesis,
electrochemical-based toxic waste remediation and energy conversion
devices. This dimensional stability allows the electrode to be
stably operated under extreme conditions (e.g., acidic or caustic
solutions, high current density (>0.1 A/cm.sup.2) and high
temperature (>150.degree. C.)). The platform for the invention
is an electrically conducting diamond thin film in which
nanometer-sized particles of Pt have been incorporated. The
dispersed metal particles are incorporated into the surface
microstructure of the diamond and exposed such that the electrodes
are active for the generation of hydrogen gas, the reduction of
oxygen gas and the oxidation of methanol. The preferred embodiment
is referred to as a Pt/diamond composite electrode. An atomic force
microscope image of the composite electrode is shown in FIG. 1.
[0038] Electrodes consisting of supported metal catalysts are used
in a number of industrial processes (e.g., electrosynthesis) and
electrochemical energy conversion devices (e.g., fuel cells). The
metal catalysts are typically impregnated into the porous structure
of several types of sp.sup.2 bonded carbon materials: chemically or
physically activated carbon, carbon black and graphitized carbons.
The advantage of the Pt/diamond composite electrode is the extreme
dimensional stability of the diamond host/support. The composite
electrode can operate stably under harsh electrochemical
conditions, such as extremes in solution pH, high temperature and
high current density; conditions under which commercial sp.sup.2
carbon supports fail catastrophically. The metal catalyst particles
are physically anchored within the diamond such that they do not
agglomerate or come detached during high density electrolysis (0.1
A/cm.sup.2).
[0039] Several markets benefit from this invention, in particular,
companies manufacturing electrolyzers to generate chlorine or
ozone, and reactors to electrochemically remediate toxic waste.
Companies marketing dimensionally stable electrodes for
electrosynthesis could also benefit from this technology. Finally,
companies manufacturing and marketing small-scale fuel cells would
be interested in this technology.
[0040] The composite electrodes are fabricated by a multistep
process that is illustrated in the second attached FIG. 2. A boron
doped first diamond film 12 is grown on a P--Si or platinum
substrate 11. Platinum (Pt) is electrolytically deposited on the
film 12 as irregular microspheres. A second boron doped diamond
film 14 is then grown around the Pt to anchor the Pt on the first
film 12. FIG. 2A shows a side view of a microwave CVD reactor 10.
In FIG. 2A the following elements are present:
[0041] 10--reactor
[0042] 11--microwave generator and antenna
[0043] 12--quartz chamber
[0044] 13--plasma
[0045] 14--substrate
[0046] 15--substrate holder
[0047] 16--vacuum pump
[0048] 17--gas inlet
[0049] 18A to 18D--mass flow controllers
[0050] 19A to 19D--gas cylinders
[0051] Such a reactor 10 is well known to those skilled in the
art.
[0052] Working forms of the Pt/diamond composite electrodes were
fabricated. The electrodes fabricated all used electrically
conducting silicon substrates. The diamond particle size ranges
from 30 to 300 nm with a particle distribution of about
2.5.times.10.sup.8/cm.sup.2. The electrode response toward hydrogen
evolution, oxygen reduction and methanol oxidation was evaluated,
as were several aspects of the dimensional stability during
exposure to harsh electrochemical conditions and found to be
satisfactory.
[0053] The nominal diamond particle size is preferably between 10
and 50 nm. The present invention contemplates fabricating the
composite electrodes in a cost effective manner, and advancing the
technology so as to coat high surface area metal mesh supports.
Finally, the incorporation of Pt/Ru and Pt/Os metal alloy particles
can be accomplished.
[0054] In summary, dimensionally stable Pt/diamond composite
electrodes have been developed for use in electrosynthesis,
electrochemical-based toxic waste remediation and energy conversion
devices. The dimensionally stable and corrosion resistant
electrodes consist of well-faceted microcrystallites with dispersed
Pt particles incorporated into the surface region. The metal
particles are well anchored and range from 30 to 300 nm with a
distribution of about 2.5.times.10.sup.8/cm.sup.2. Importantly, the
Pt particles at the surface are in communication with the current
collecting substrate through the boron-doped diamond matrix, and
they are electroactive for the underpotential deposition of
hydrogen, the reduction of oxygen and the oxidation of methanol.
The dispersed Pt particles are extremely stable as no loss in
activity is observed after 2000 potential cycles between the
hydrogen and oxygen evolution regimes in 0.1 M HClO.sub.4 (1-6
mA/cm.sup.2). The composite electrode is also extremely stable
during anodic polarization in 85% H.sub.3PO.sub.4 at 170.degree. C.
and 0.1 A/cm.sup.2. The composite electrodes exhibit no evidence of
any morphological or microstructural damage, and more importantly,
no evidence of any catalyst activity loss for hydrogen evolution or
oxygen reduction during exposure to the extreme conditions.
[0055] The diamond films are conductive because they are doped with
a conductive element (such as boron). The diamond is doped at a
level of 0.1% by atomic concentration (B/C) or higher, where p-Si
is used as a substrate rather than platinum, then the p-Si is doped
at a level of about 0.05 to 0.1% by atomic concentration boron.
[0056] The metal particles are preferably comprised of a metal in
Group VIIIB. Particularly preferred are Pt, Rh and/or Ru and alloys
thereof. The metal particles are generally irregularly spherical in
shape since the metal particles are formed by an isolated
nucleation and growth mechanism. Obviously the particles have
irregular spherical shapes.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0057] The morphological and microstructural stability, as well as
the catalytic activity of a Pt/diamond composite electrode during
two 1 h periods of anodic polarization in 85% H.sub.3PO.sub.4 at
170.degree. C. and 0.1 A/cm.sup.2, were investigated. The composite
electrode consisted of an electrically conducting diamond thin film
support with Pt metal particles entrapped in the surface
microstructure by diamond particles. The Pt particles range in
diameter from 30 to 300 nm with a distribution of about
2.times.10.sup.8 cm.sup.-2. No evidence of morphological of
microstructural damage, and, more importantly, no loss of catalyst
activity for hydrogen evolution or oxygen reduction was observed
after the harsh electrolysis. A Pt-impregnated sp.sup.2 carbon
cloth electrode was observed to catastrophically fail during the
first hour of electrolysis.
[0058] A stability test was conducted of the Pt/diamond composite
electrode more demanding than any which has been published (Swain,
G. M., J. Electrochem. Soc., 141 3382 (1994); and Chen, Q., et al.,
J. Electrochem. Soc., 144 3806 (1997)), exposure to 85% phosphoric
acid at 170.degree. C. for 2 h at an anodic current density of 0.1
A/cm.sup.2. The electrode morphology and microstructure were
evaluated before and after the electrolysis using optical
microscopy, atomic force microscopy (AFM), and Raman spectroscopy.
The electrocatalytic activity of the metal catalyst was examined
before and after using cyclic voltammetry (CV) in 0.1 M HClO.sub.4.
A commercial Pt-impregnated sp.sup.2 carbon cloth electrode having
a loading of 0.5 mg/cm.sup.2 and a nominal catalyst size of 2 nm
was exposed to the same electrolysis conditions. The purpose for
this was to compare the performance of the Pt/diamond composite
electrode with that of a "real world," sp carbon-supported
catalytic electrode.
EXAMPLE
[0059] Experimental
[0060] The boron-doped diamond thin films were deposited on p-Si
(100) substrates (<0.01 .OMEGA. cm) using microwave-assisted
chemical vapor deposition (CVD) (1.5 kW, 2.54 GHz, Astex, Inc.,
Lowell, Mass.). Details of the deposition procedure have been
presented elsewhere (Wang, J., et al., J. New Mater. Electrochem.
Syst., 3 75 (2000); and Wang, J., et al., Electrochem. Solid-state
Lett., 3 286 (2000)). The composite electrodes were prepared by
initially depositing a ca. 3 .mu.m thick boron-doped film for 12 h
using a CH.sub.4/H.sub.2 volumetric ratio of 0.35%. The microwave
power was 1000 W, the pressure was ca. 40 to 60 Torr and the
substrate temperature was ca. 875.degree. C. The diamond growth was
then stopped and the substrates cooled to less than 300.degree. C.
in the presence of atomic hydrogen. After cooling to room
temperature, the film-coated substrates were removed from the
reactor and a discontinuous layer of Pt particles was
electrodeposited. The metal was electrodeposited from 1 mM
KP.sub.2PtCl.sub.6+0.1M HClO.sub.4 using a constant current of 100
.mu.A (500 .mu.A/cm.sup.2) and a variable deposition time from 100
to 500 s. The Pt-coated films were then placed back in the CVD
reactor and boron-doped diamond was deposited for an additional 3 h
using the same conditions as described above. This second
deposition results in diamond film growth around the metal
particles securely anchoring them into the surface microstructure.
The final Pt particles range in diameter from 30 to 300 nm with a
distribution of about 2.times.10 cm.sup.-2. These particles are
larger than desired for a catalytic electrode (.about.5 nm diam
optimum). The control of the metal particle size to less than 50 nm
is easily within the skill of the art.
[0061] The film morphology was investigated with AFM using a
Nanoscope II instrument (Digital Instruments, Santa Barbara,
Calif.) operated in the contact mode. Pyramidal-shaped
Si.sub.3N.sub.4 tips mounted on gold cantilevers (100 .mu.m legs,
0.38 N/m spring constant) were used to acquire topographical images
in air.
[0062] The film microstructure was assessed with Raman
spectroscopy. The spectra were obtained at room temperature with a
Chromex 2000 spectrometer (Chromex, Inc., Albuquerque, N.Mex.)
using laser excitation at 532 nm, a monochromator slit width of 5
.mu.m, and integration time of 10 s. The spectrometer was equipped
with a 1026.times.200 element charge-coupled device (CCD) detector.
A white light spectrum was collected under the same conditions and
used to ratio the spectra. The laser power at the sample was ca. 30
mW, as measured with a thermopile detector.
[0063] The anodic polarization was performed in 85% H.sub.3PO.sub.4
(ultrapure grade, Aldrich Chemical) at 170.degree. C. A partially
sealed single compartment, three-electrode cell, as shown in FIG.
7, was placed inside an oven to regulate the temperature. The
entire oven was placed inside a fume hood to exhaust any released
acid vapors. An anodic current density of 0.1 A/cm.sup.2 was
applied for two 1 h periods. The same anodic polarization was
performed using a Pt-impregnated commercial sp.sup.2 carbon cloth
electrode. The new electrode had a 0.5 mg/cm.sup.2 Pt loading (2 nm
diam particles) with 0.2 cm.sup.2 geometric area exposed to the 85%
H.sub.3PO.sub.4 solution at 170.degree. C. Significant gas
evolution (i.e., oxygen evolution) occurred at both electrodes
during the electrolysis. The potential of the Pt/diamond composite
electrode was stable at ca. 2.52 V vs. the carbon rod counter
electrode during the 2 h electrolysis. The potential for the carbon
cloth electrode progressively increased from 2.42 to 3.92 V during
the first hour of electrolysis. For reference, the equilibrium
potential of the carbon rod vs. Ag/AgCl in the electrolysis
solution at room temperature was 0.080 V. These two observations
reflect the stability of the Pt/diamond composite electrode and the
instability of the sp.sup.2 carbon cloth electrode, as discussed
below. Background cyclic voltametric i-E curves in 0.1 M HClO.sub.4
(ultrapure grade, Aldrich Chemical) were recorded for each
electrode, before and after the anodic polarization, to check for
changes in the catalyst activity. All solutions were prepared with
ultrapure water (Barnstead E-Pure, 18 M.OMEGA.-cm).
[0064] The Pt electrodeposition, anodic polarization, and CV were
performed with a CS-2000 digital potentiostat/galvanostat (Cypress
Systems Inc., Lawence, Kans.). A Ag/AgCl (saturated KCl) electrode
was used as the reference and a large-area carbon rod served as the
counter electrode. The Pt/diamond composite electrodes were pressed
against the bottom of the glass cell using an Al plate current
collector with the fluid being contained by a Viton O-ring. A small
section of the back side of the Si substrate was scratched, cleaned
with isopropanol, and coated with Ag paste before making ohmic
contact with the Al plate. The exposed geometric area was 0.2
cm.sup.2 and all currents are normalized to this area. While
mounted in the cell, the composite electrodes were sequentially
rinsed with ultrapure water, soaked for 20 min in distilled
isopropanol, and then rinsed with ultrapure water. The electrolyte
solution was deoxygenated with nitrogen (BOC Gases) for 20 min
prior to any of the voltametric measurements. All the voltametric
characterizations were done at room temperature (22-24.degree.
C.).
[0065] Results and Discussion
[0066] FIG. 3 shows CV i-E curves for a Pt/diamond composite
electrode in 0.1 M HClO.sub.4 before and after two 1 h periods of
anodic polarization. As stated above, the polarization was
performed in 85% H.sub.3PO.sub.4 at 170.degree. C. and a current
density of 0.1 A/cm.sup.2. The curve for the electrode prior to the
polarization (dashed line) reveals the presence of Pt with the
expected features; Pt oxide formation, Pt oxide reduction, the
adsorption and desorption of underpotential deposited hydrogen and
hydrogen evolution. Well-resolved and symmetrical features are
observed for hydrogen ion adsorption and desorption between 100 and
-100 mV. The current in the Pt oxide formation region, beginning at
ca. 700 mV, is flat and featureless indicative of a clean and
contaminant-free surface, at least for electroactive contaminants
at these potentials. The reduction of Pt oxide occurs at ca. 550
mV. These voltametric features were stable with multiple scans.
[0067] After the two 1 h polarizations, the voltametric features
are unchanged and clearly reveal that there is no loss of catalyst
activity due to degradation of the diamond microstructure and
morphology. All the characteristic Pt voltametric features are
present. Importantly, there is no loss in the charge associated
with hydrogen ion adsorption and desorption. Such loss would be
expected if the Pt catalyst particles were detached from the
surface due to an oxidizing and corroding diamond support. In fact,
the charge associated with the hydrogen ion adsorption actually
increased after the electrolysis. The cathodic charge between 100
and -100 mV was 355 .mu.C/cm.sup.2 before and increased to 420 and
455 .mu.C/cm.sup.2 after the two 1 h polarizations, respectively.
The increased charge was attributed to minor surface cleaning and
crystallographic changes in the deposits that occur during the
vigorous gas evolution.
[0068] One type of minor cleaning that is possible is the oxidative
removal of residual carbon deposits formed during the diamond
deposition. These deposits do not affect the stability of the metal
particles but, rather, influence their surface activity toward
faradaic electron transfer processes. There is no significant
change in the particle size and coverage after polarization, at
least as revealed by AFM. Some representative images are shown in
FIGS. 4A and 4B. The most significant change in the voltammograms
is the reduced overpotential for oxygen evolution after the
polarizations. The current associated with the reduction of this
oxygen is superimposed on the Pt-oxide reduction current at ca. 550
mV, and this causes the current maximum to shift to slightly more
negative potentials. There is also a minor decrease in the
overpotential for hydrogen evolution after the polarization.
[0069] FIGS. 4A and 4B show ex situ AFM images of the Pt/diamond
composite electrode before and after the two 1 h polarizations. A
well-faceted, polycrystalline morphology is observed before and
after electrolysis. The crystallites are randomly oriented with
spherical Pt dispersions decorating both the facets and grain
boundaries. Clearly, there is no evidence of any morphological or
microstructural damage, such as film delamination, grain
roughening, or pitting. The similarity of the image features before
and after polarization is consistent with the CV data.
[0070] Raman measurements were also made on the composite
electrodes before and after polarization. No significant spectral
changes were observed consistent with a stable microstructure and
near-surface optical properties. The diamond line position was
upshifted from that observed for a piece of high pressure, high
temperature diamond by 1 cm.sup.-1 or less. The line position,
linewidth, line intensity, and photoluminescence background were
unchanged after the polarization.
[0071] Optical micrographs of a Pt-impregnated sp.sup.2 carbon
cloth electrode after a 1 h polarization are presented in FIGS. 5A
and 5B. FIG. 5A shows images of the treated electrode on the left
and the untreated electrode on the right. FIG. 5B shows a larger
area of the treated electrode. The physical integrity of the
electrode was catastrophically damaged due to the oxidation of the
carbon support. The physical evidence for major morphological and
microstructural damage was loose pieces of the electrode floating
in the solution, lost portions of the electrode, especially at the
edges due to oxidation and gasification reactions, and lost
catalyst activity.
[0072] FIG. 6 shows background CV i-E curves for the carbon cloth
electrode in 0.1 M HClO.sub.4 before and after a 1 h polarization.
It was previously reported that diamond electrodes exhibit superior
dimensional stability to other sp.sup.2 carbon electrodes (e.g.,
glassy carbon, Grafoil, and highly oriented pyrolytic graphite)
during less vigorous electrolysis conditions than those employed
herein, for example, in acidic fluoride media (Swain, G. M., J.
Electrochem. Soc. 141 3382 (1994)). The well-resolved features
characteristic of Pt were not observed prior to the polarization
even with extensive cycling. However, the characteristic low
overpotentials for oxygen and hydrogen evolution are evident as the
onset potentials for the anodic and cathodic current are 1300 and
300 mV, respectively. There is little evidence of any catalyst
activity after the polarization. The currents for oxygen and
hydrogen evolution decrease substantially and the electrode
response resembles that expected for a pure resistance. Consistent
with this loss of catalytic activity is the observation that the
electrode potential progressively increases from 2.5 to 4.0 V
during electrolysis. Therefore, it can be concluded that the
polarization causes oxidation and corrosion of the carbon support
to such an extent that the catalyst is lost and the electrode's
electrical resistance is increased.
[0073] The above example shows the Pt/diamond composite electrode
exhibited superb morphological and microstructural stability during
vigorous electrolysis in acidic media at high temperature and
current density. There was no degradation of the diamond electrode,
nor was there any loss in catalytic activity for hydrogen evolution
or oxygen reduction. The Pt catalyst dispersions are physically
entrapped within the dimensionally surface microstructure of the
diamond lattice and are not detached during the high current
density electrolysis in hot phosphoric acid.
[0074] This technology allows deposition of the films in an
economic and cost effective manner, deposition of the films on
higher surface area metal meshes, and by incorporation of other
interesting metal catalyst particles, like Pt/Ru alloys.
[0075] It is intended that the foregoing description be only
illustrative of the present invention and that the present
invention be limited only by the hereinafter appended claims.
* * * * *