U.S. patent application number 11/928114 was filed with the patent office on 2008-03-27 for catalyst and a method for manufacturing the same.
Invention is credited to Robert Greenberg, Dao Min Zhou.
Application Number | 20080076007 11/928114 |
Document ID | / |
Family ID | 37895883 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080076007 |
Kind Code |
A1 |
Zhou; Dao Min ; et
al. |
March 27, 2008 |
Catalyst and a Method for Manufacturing the Same
Abstract
An improved platinum and method for manufacturing the improved
platinum wherein the platinum having a fractal surface coating of
platinum, platinum gray, with a increase in surface area of at
least 5 times when compared to shiny platinum of the same geometry
and also having improved resistance to physical stress when
compared to platinum black having the same surface area. The
process of electroplating the surface coating of platinum gray
comprising plating at a moderate rate, for example at a rate that
is faster than the rate necessary to produce shiny platinum and
that is less than the rate necessary to produce platinum black.
Platinum gray is applied to manufacture a fuel cell and a
catalyst.
Inventors: |
Zhou; Dao Min; (Saugus,
CA) ; Greenberg; Robert; (Los Angeles, CA) |
Correspondence
Address: |
Tomas Lendvai;Second Sight Medical Products, Inc.
Builsing 3
12744 San Fernando Road
Sylmar
CA
91342
US
|
Family ID: |
37895883 |
Appl. No.: |
11/928114 |
Filed: |
October 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11260002 |
Oct 26, 2005 |
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11928114 |
Oct 30, 2007 |
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11198361 |
Aug 4, 2005 |
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11260002 |
Oct 26, 2005 |
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10226976 |
Aug 23, 2002 |
6974533 |
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11198361 |
Aug 4, 2005 |
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60372062 |
Apr 11, 2002 |
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Current U.S.
Class: |
502/300 ;
205/264; 429/524; 429/535 |
Current CPC
Class: |
B01J 23/44 20130101;
H01M 4/8853 20130101; H01M 4/92 20130101; C25D 5/16 20130101; B01J
35/002 20130101; H01M 8/00 20130101; H01M 4/921 20130101; B01D
2255/1021 20130101; H01M 8/1007 20160201; Y02E 60/50 20130101; B01J
23/42 20130101; B01J 23/08 20130101; B01J 23/40 20130101; B01J
37/348 20130101; Y02P 70/50 20151101; B01D 53/945 20130101; B01J
23/468 20130101; H01M 4/90 20130101; Y02T 10/12 20130101; B01J
37/0215 20130101; H01M 4/94 20130101; C25D 3/50 20130101; C25D
21/12 20130101; H01M 4/8807 20130101 |
Class at
Publication: |
429/040 ;
205/264 |
International
Class: |
H01M 4/86 20060101
H01M004/86; C25D 3/50 20060101 C25D003/50 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under grant
No. R24EY12893-01, awarded by the National Institutes of Health.
The Government has certain rights in the invention.
Claims
1. An anode and/or a cathode comprising a substrate wherein the
surface coating is electroplated to the surface of said conductive
substrate at a rate such that the metal particles form on the
conductive substrate faster than necessary to form shiny platinum
and slower than necessary to form platinum black.
2. A method for manufacturing of a catalyst by electroplating a
surface coating such that surface has a rough surface coating
comprising: electroplating the surface of a conductive substrate at
a rate such that the particles are form on the conductive substrate
faster than necessary to form shiny platinum and slower than
necessary to form platinum black.
3. The method of claim 2 wherein said step of electroplating is
accomplished at a rate of more than 0.05 microns per minute, but
less than 1 micron per minute.
4. The method of claim 2 wherein said electroplating is
accomplished at a rate of greater or equal to 1 micron per minute,
but less than 10 microns per minute.
5. The method of claim 2 wherein said electroplating is controlled
by the electrode voltage.
6. The method of claim 5 wherein said voltage is a constant
voltage.
7. The method of claim 5 wherein the controlled voltage causes at
least a partially diffusion limited plating reaction.
8. The method of claim 2 wherein the voltage of said electroplating
is less than 0.2 Volts and greater than -1 Volts vs. Ag/AgCl
reference electrode.
9. The method of claim 2 wherein the electroplating solution is at
least 3 mM but less than 30 mM ammonium hexachloroplatinate in
about 0.4 M disodium hydrogen phosphate.
10. A method of using at least one catalyst of claim 1 in a fuel
cell.
11. A method of using at least one catalyst of claim 1 in an
autocatalyst.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of application Ser. No.
11/260,002, entitled "A Catalyst and a Method for Manufacturing the
Same", filed Oct. 26, 2005, which is a continuation in part of
application Ser. No. 11/198,361, "Platinum Surface Coating and
Method for Manufacturing the Same", filed Aug. 4, 2005, the
disclosure of which is incorporated herein by reference, which is a
divisional of application Ser. No. 10/226,976, "Platinum Electrode
and Method for Manufacturing the Same" filed Aug. 23, 2002, the
disclosure of which is incorporated herein by reference, which
claims the benefit of U.S. Provisional Application No. 60/372,062,
"Platinum Deposition for Electrodes", filed Apr. 11, 2002, the
disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The field of the invention relates to a catalyst especially
for a fuel cell and autocatalyst and electrode surface coating and
electroplating processes for deposition of surface coating,
especially in a fuel cell and a catalyst.
[0005] 2. Description of Related Art
[0006] Platinum has often been used as a preferred catalyst
material for electrodes in fuel cells, especially fuel stacks for
cars, and in autocatalysts. A catalyst, especially a platinum
catalyst is a crucial part in a fuel cell. The catalytic reaction
takes place on the surface of the electrodes. The electricity is
created by the catalytic reaction when a fuel such as Hydrogen is
electrochemically oxidized to protons on the surface of the anode.
Platinum is known to be an excellent catalyst for fuel cells;
however it is a very expensive material.
[0007] Since platinum has a smooth surface and its surface area is
limited by the geometry of the electrode, it is not efficient for
transferring electrical charge. The platinum with a smooth surface
is hereinafter called shiny platinum.
[0008] An electrode which is intended for long term use with a
nonrenewable energy source must require minimal energy--a high
electrode capacitance and correspondingly low electrical impedance
is of great importance.
[0009] It is known that a catalyst applied on an electrode surface
accelerates the electrode reactions and that the transfer current
is proportional to the surface area of the electrode. Many attempts
are reported trying to improve the ability of a catalyst converting
fuel to electricity. Those attempts try to increase the surface
area of the electrode without increasing the amount of the
expensive platinum catalyst material.
[0010] One approach to increase the surface area of a platinum
electrode without increasing the electrode size is to electroplate
platinum rapidly such that the platinum molecules do not have time
to arrange into a smooth, shiny surface. The rapid electroplating
forms a platinum surface which is commonly known as platinum black.
Platinum black has a porous and rough surface which is less dense
and less reflective than shiny platinum. U.S. Pat. No. 4,240,878 to
Carter describes a method of plating platinum black on
tantalum.
[0011] Platinum black is more porous and less dense than shiny
platinum. Platinum black has weak structural and physical strength
and is therefore not suitable for applications where the electrode
is subject to even minimal physical stresses. Platinum black also
requires additives such as lead to promote rapid plating. Finally,
due to platinum black's weak structure, the plating thickness is
quite limited. Thick layers of platinum black simply fall
apart.
[0012] Fuel stacks for cars use about 2 oz of platinum group metals
per unit. Pure platinum catalysts are used for hydrogen fueled fuel
cells, while alloys of platinum with ruthenium are typically used
for reformed hydrocarbon fuel cells to improve the tolerance of the
catalyst to carbon monoxide.
[0013] The fuel cell research estimates that loadings can be
reduced to about 1 oz per unit through better utilization of
platinum and thinner deposition layer. Other estimates show that
when fuel cells are commercially produced each engine will require
between 0.2 and 0.3 oz platinum per unit.
[0014] The main consumer of world platinum group metal supply is
the automobile industry. 41% of platinum demand in 2001 was
accounted for autocatalyst use. Platinum group metals are used in
autocatalysts to facilitate the removal of three of the main
combustion byproducts CO, hydrocarbons, and NO.sub.x. The use of
platinum is increased due to strong growth in production and sales
for diesel cars. Diesel autocatalysts only use platinum rather than
the mixture of platinum and palladium commonly used in gasoline
catalysts.
[0015] Platinum is the most common catalyst for fuel cells.
However, due to its high cost it is often doped with palladium,
ruthenium, cobalt, or more recently iridium or osmium. In addition
to its high cost, platinum is also quite rare. In fact, there is
not enough platinum in the world to equip every vehicle in use
today with a traditional platinum catalyst proton exchange membrane
fuel cell. For this reason, there is a high desire to develop new
catalysts, and new platinum deposition techniques to reduce the
amount of platinum needed for fuel cell catalysts and autocatalysts
in general.
[0016] For the foregoing reasons there is a need for an improved
platinum surface coating and process for coating the surface to
obtain an increased surface area for a given geometry and at the
same time the coating is structurally strong enough to be used in
applications where the platinum surface coating is subject to
physical stresses.
SUMMARY OF THE INVENTION
[0017] The present invention is directed in part to a catalyst
which comprises at least one substrate; and a surface coating of
said substrate of at least one of the following metals platinum,
palladium or iridium or an alloy of two or more metals, or a
combination of two or more alloys or metal layers having an
increase in the surface area of 5 times to 500 times of the
corresponding surface area resulting from the basic geometric
shape.
[0018] The present invention is further directed to a fuel cell
comprising at least one catalyst.
[0019] The present invention is further directed to a catalyst to
facilitate the removal of three of the main combustion byproducts
CO, hydrocarbons and NO.sub.x.
[0020] The present invention is further directed to an anode and/or
a cathode comprising a substrate wherein the surface coating is
electroplated to the surface of a said conductive substrate at a
rate such that the particles of metal form on the conductive
substrate faster than necessary to form shiny platinum and slower
than necessary to form platinum black.
[0021] The present invention is further directed to a method for
manufacturing of a catalyst by electroplating a surface coating
such that surface has a rough surface coating comprising
electroplating the surface of a conductive substrate at a rate such
that the particles are form on the conductive substrate faster than
necessary to form shiny platinum and slower than necessary to form
platinum black.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 depicts a platinum gray surface magnified 2000
times.
[0023] FIG. 2 depicts a shiny platinum surface magnified 2000
times.
[0024] FIG. 3 depicts a platinum black surface magnified 2000
times.
[0025] FIG. 4 depicts color density (D) values and lightness (L*)
values for several representative samples of platinum gray,
platinum black and shiny platinum.
[0026] FIG. 5 depicts a three electrode electroplating cell with a
magnetic stirrer.
[0027] FIG. 6 depicts a three electrode electroplating cell in an
ultrasonic tank.
[0028] FIG. 7 depicts a three electrode electroplating cell with a
gas dispersion tube.
[0029] FIG. 8 depicts an electroplating system with constant
voltage control or constant current control.
[0030] FIG. 9 depicts an electroplating system with pulsed current
control.
[0031] FIG. 10 depicts an electroplating system with pulsed voltage
control.
[0032] FIG. 11 depicts an electroplating system with scanned
voltage control.
[0033] FIG. 12 depicts the electrode capacitance for both plated
and unplated electrodes of varying diameter.
[0034] FIG. 13 depicts a representative linear voltage sweep of a
representative platinum electrode.
[0035] FIG. 14 depicts a schematic drawing of a fuel cell.
[0036] FIG. 15 depicts a side view of an autocatalyst.
DETAILED DESCRIPTION OF THE INVENTION
[0037] Referring to FIG. 1, an illustrative example of a platinum
gray surface coating for an electrode according to the present
invention is shown having a fractal surface with a surface area
increase of greater than 5 times the surface area over that of a
shiny platinum surface of the same geometry, shown in FIG. 2, and
an increase in strength over a platinum black surface, shown in
FIG. 3. FIGS. 1 to 3 are images produced on a Scanning Electron
Microscope at 2000.times. magnification taken by a JSM5910
microscope (JEOL, Tokyo, Japan). Under this magnification level it
is observed that the platinum gray is of a fractal configuration
having a cauliflower shape with particle sizes ranging from 0.5
.mu.m to 15 .mu.m. Each branch of such structure is further covered
by smaller and smaller particles of similar shape. The smallest
particles on the surface layer may be in the nanometer range. This
rough and porous fractal structure increases the electrochemically
active surface area of the platinum surface when compared to an
electrode with a smooth platinum surface having the same geometric
shape.
[0038] Because no impurities or other additives such as lead need
to be introduced during the plating process to produce platinum
gray, the surface can be pure platinum. It is very advantageous of
not using lead in view of the environment. Lead is likely to lover
the catalyst activity in an autocatalyst. This is another advantage
of not using any lead in the platinum catalyst. Alternatively,
other materials such as iridium, rhodium, gold, tantalum, titanium
or niobium could be introduced during the plating process if so
desired but these materials are not necessary to the formation of
platinum gray.
[0039] Platinum gray can also be distinguished from platinum black
and shiny platinum by measuring the color of the material on a
spectrodensitometer using the Commission on Illumination L*a*b*
color scale.
[0040] L* defines lightness,
[0041] a* denotes the red/green value, and
[0042] b*, the yellow/blue value.
[0043] The lightness value, called L* Value, can range from 0 to
100, where white is 100 and black is 0--similar to grayscale. The
a* value can range from +60 for red and -60 for green, and the b*
value can range from +60 for yellow and -60 for blue. All samples
measured have very small a* and b* values, they are colorless or in
the so called neutral gray zone, which suggests that the lightness
value can be used as grayscale for Platinum coatings.
[0044] Another example of a platinum surface coating for an
electrode yields a rough surface with a surface area increase of
greater than 5 times the surface area for a platinum surface of the
same geometry having a regular shape with particle sizes ranging
from 0.1 .mu.m to 2.0 .mu.m and has an average size of 0.4 .mu.m to
0.6 .mu.m, preferably about 0.5 .mu.m. The thickness of the coating
is 0.1 .mu.m to 5.0 .mu.m, preferably 1.0 .mu.m to 4.0 .mu.m, more
preferably 3.3 .mu.m to 3.8 .mu.m. Some rough features with a scale
in the nanometer range were present on each particle. The plated
platinum layer is not believed to be porous. The bead shaped
platinum particles with nanometer rough features on the particles
increase the electrode's electrochemical active surface. The
electrochemical capacitance of the electrode array with the surface
coating of rough platinum is about 1300 .mu.F/cm.sup.2 to 1500
.mu.F/cm.sup.2, measured in a 10 mM phosphate buffered saline
solution. The relation of the platinum surface area to the
thickness of the platinum surface coating is of 4.0 F/cm.sup.3 to
5.0 F/cm.sup.3. The thin-film platinum disks have an average
capacitance of less than 20 .mu.F/cm.sup.2 before plating, measured
at the same condition. The electrochemical active surface area
increase is 70 to 80, preferably about 71 to 75 fold.
[0045] The electroplating process with platinum can be preferably
performed in an agues solution containing sodium dihydrogen
phosphate (NaH.sub.2PO.sub.4) and/or disodium hydrogen phosphate
(Na.sub.2HPO.sub.4) and platinum tetra chloride (PtCl.sub.4) at
20.degree. C. to 40.degree. C. Different concentrations of platinum
can be used and the range of platinum salt concentrations can be
from 1 to 30 mM. Other Pt salts will also produce similar
results.
[0046] Another example of a platinum surface coating yields an
electrode having a rough surface with a surface area increase of
greater than 5 times the surface area for a platinum surface of the
same geometry having a regular shape with particle sizes ranging
from 0.1 .mu.m to 2.0 .mu.m and has an average size of 0.4 .mu.m to
0.6 .mu.m, preferably about 0.5 .mu.m. The thickness of the coating
is 0.1 .mu.m to 4.0 .mu.m, preferably 2.0 .mu.m to 3.0 .mu.m, more
preferably 2.3 .mu.m to 2.8 .mu.m. Some rough features with a scale
in the nanometer range were present on each particle. The plated
platinum layer is not believed to be porous. The bead shaped
platinum particles with nanometer rough features on the particles
increased electrode's electrochemical active surface. The
electrochemical capacitance of the electrode array with the surface
coating of rough platinum is about 1150 .mu.F/cm.sup.2 to 1680
.mu.F/cm.sup.2, measured in a 10 mM phosphate buffered saline
solution. The relation of the platinum surface area to the
thickness of the platinum surface coating is of 5.0 F/cm.sup.3 to
6.0 F/cm.sup.3. The thin-film platinum disks have an average
capacitance of less than 20 .mu.F/cm.sup.2 before plating, measured
at the same condition. The electrochemical active surface area
increase is 65 to 75, preferably about 68 to 72 fold.
[0047] The relation of the platinum surface area to the thickness
of the platinum surface coating is of 4.0 F/cm.sup.3 to 5.0
F/cm.sup.3 as referred to in FIG. 4 and of 5 5.0 F/cm.sup.3 to 6.0
F/cm.sup.3 as referred to in FIG. 5. FIGS. 4 and 5 both depict a
rough surface platinum coating according to the present invention.
This value is calculated by dividing the electrochemically active
platinum surface area (.mu.F/cm.sup.3) by the thickness of the
platinum coating (.mu.m). In comparison to the rough surface
platinum coating the fractal platinum coating as referred to in
FIG. 1 has a relation of the platinum surface area to the thickness
of the platinum surface coating of 0.8 F/cm.sup.3 to 1.5
F/cm.sup.3. The rough platinum coating of the present invention
yields on the same electrochemically active surface area a thinner
coating with a higher capacitive volume compared with platinum
gray.
[0048] Another example is a palladium surface coating for an
electrode having a rough surface with a surface area increase of
greater than 5 times the surface area for a palladium surface of
the same geometry having a regular shape with particle sizes
ranging from 0.1 .mu.m to 3.0 .mu.m, preferably from 0.5 .mu.m to
1.5 .mu.m. The thickness of the coating is 0.1 .mu.m to 5.0 .mu.m,
preferably 0.5 .mu.m to 2.0 .mu.m.
[0049] Some rough features with a scale in the nanometer range were
present on each particle. The plated palladium layer is not
believed to be porous. The bead shaped palladium particles with
nanometer rough features on the particles increase the electrode's
electrochemical active surface. The electrochemical capacitance of
the electrode array with the surface coating of rough palladium is
about 100 .mu.F/cm.sup.2 to 300 .mu.F/cm.sup.2, measured in a 10 mM
phosphate buffered saline solution. The thin-film platinum disks
have an average capacitance of less than 20 .mu.F/cm.sup.2 before
plating, measured at the same condition. The relation of the
palladium surface area to the thickness of the palladium surface
coating is of 1.5 F/cm.sup.3 to 3.5 F/cm.sup.3. The electrochemical
active surface area increase is 50 to 70, preferably about 52 to 60
fold.
[0050] The smallest particles on the surface layer may be in the
nanometer range. This rough structure increases the
electrochemically active surface area of the palladium surface when
compared to an electrode with a smooth palladium surface having the
same geometric shape.
[0051] The surface is pure palladium because no impurities or other
additives such as lead need to be introduced during the plating
process to produce this palladium.
[0052] Another example is an iridium surface coating for an
electrode having a rough surface with a surface area increase of
greater than 5 times the surface area for an iridium surface of the
same geometry having an irregular shape with particle sizes ranging
from 0.01 .mu.m to 2.0 .mu.m, preferably from 0.1 .mu.m to 1.0
.mu.m. The coating has a thickness from 0.01 .mu.m to 10 .mu.m,
preferably from 0.8 .mu.m to 3.0 .mu.m.
[0053] The plated iridium layer is not believed to be porous. The
bead shaped iridium particles with nanometer rough features on the
particles increase the electrode's electrochemical active surface.
The electrochemical capacitance of the electrode array with the
surface coating of rough palladium is about 1000 .mu.F/cm.sup.2 to
1300 .mu.F/cm.sup.2, measured in a 10 mM phosphate buffered saline
solution. The thin-film platinum disks have an average capacitance
of less than 20 .mu.F/cm.sup.2 before plating, measured at the same
condition. The relation of the palladium surface area to the
thickness of the palladium surface coating is of 4.5 F/cm.sup.3 to
6.5 F/cm.sup.3. The electrochemical active surface area increase is
55 to 70, preferably about 60 to 65 fold. The smallest particles on
the surface layer may be in the nanometer range. This rough
structure increases the electrochemically active surface area of
the iridium surface when compared to an electrode with a smooth
iridium surface having the same geometric shape.
[0054] The surface is pure iridium because no impurities or other
additives such as lead need to be introduced during the plating
process to produce this iridium.
[0055] The electroplating process with iridium can be preferably
performed in an agues solution containing sodium dihydrogen
phosphate (NaH.sub.2PO.sub.4) and/or disodium hydrogen phosphate
((Na.sub.2HPO.sub.4) and ammonium hexachloroiridate
((NH.sub.4).sub.2IrCl.sub.6) at 20.degree. C. to 40.degree. C.
Different concentrations of ((NH.sub.4).sub.2IrCl.sub.6 can be used
and the range of iridium salt concentrations can be from 3 to 30
mM. Other iridium salts such as (NH.sub.4).sub.3IrCl.sub.6 or
IrCl.sub.4 also produce similar, good results.
[0056] Referring to FIG. 4, the L*, a*, and b* values for
representative samples of platinum gray, platinum black and shiny
platinum are shown as measured on a color reflection
spectrodensitometer, X-Rite 520. Platinum gray's L* value ranges
from 25 to 90, while platinum black and shiny platinum both have L*
values less than 25.
[0057] Referring to FIG. 4, color densities have also been measured
for representative samples of platinum gray, platinum black and
shiny platinum. Platinum gray's color density values range from 0.4
D to 1.3 D while platinum black and shiny platinum both have color
density values greater than 1.3 D.
[0058] Platinum gray can also be distinguished from platinum black
based on the adhesive and strength properties of the thin film
coating of the materials. Adhesion properties of thin film coatings
of platinum gray and platinum black on 500 .mu.m in diameter
electrodes have been measured on a Micro Scratch Tester (CSEM
Instruments, Switzerland). A controlled micro scratch is generated
by drawing a spherical diamond tip of radius 10 .mu.m across the
coating surface under a progressive load from 1 millinewton to 100
millinewtons with a 400 .mu.m scratch length. At a critical load
the coating will start to fail. Using this test it is found that
platinum gray can have a critical load of over 60 millinewtons
while platinum black has a critical load of less than 35
millinewtons.
[0059] Referring to FIGS. 5 to 8, a method to produce platinum gray
according to the present invention is described comprising
connecting a platinum electrode 2, the anode, and a conductive
substrate to be plated 4, the cathode, to a power source 6 with a
means of controlling and monitoring 8 either the current or voltage
of the power source 6. The anode 2, cathode 4, a reference
electrode 10 for use as a reference in controlling the power source
and an electroplating solution are placed in an electroplating cell
12 having a means 14 for mixing or agitating the electroplating
solution. Power is supplied to the electrodes with constant
voltage, constant current, pulsed voltage, scanned voltage or
pulsed current to drive the electroplating process. The power
source 6 is modified such that the rate of deposition will cause
the platinum to deposit as platinum gray, the rate being greater
than the deposition rate necessary to form shiny platinum and less
than the deposition rate necessary to form platinum black.
[0060] Referring to FIGS. 5 to 7, the electroplating cell 12, is
preferably a 50 ml to 150 ml four neck glass flask or beaker, the
common electrode 2, or anode, is preferably a large surface area
platinum wire or platinum sheet, the reference electrode 10 is
preferably a Ag/AgCl electrode, the conductive substrate to be
plated 4, or cathode, can be any suitable material depending on the
application and can be readily chosen by one skilled in the art.
Preferable examples of the conductive substrate to be plated 4
include but are not limited to platinum, iridium, rhodium, gold,
tantalum, titanium or niobium.
[0061] The stirring mechanism is preferably a magnetic stirrer 14
as shown in FIG. 5, an ultrasonic tank 16, such as the VWR
Aquasonic 50D, as shown in FIG. 6, or gas dispersion 18 with Argon
or Nitrogen gas as shown in FIG. 7. The plating solution is
preferably 3 to 30 mM (milimole) ammonium hexachloroplatinate in
disodium hydrogen phosphate, but may be derived from any
chloroplatinic acid or bromoplatinic acid or other electroplating
solution. The preferable plating temperature is approximately in
the range of 24.degree. C.-26.degree. C.
[0062] Electroplating systems with pulsed current and pulsed
voltage control are shown in FIGS. 9 and 10 respectively. While
constant voltage, constant current, pulsed voltage or pulsed
current can be used to control the electroplating process, constant
voltage control of the plating process has been found to be most
preferable. The most preferable voltage range to produce platinum
gray has been found to be in the range of -0.45 V to -0.85 V.
Applying voltage in this range with the above solution yields a
plating rate in the range of about 1 .mu.m per minute to about 0.05
.mu.m per minute, the preferred range for the plating rate of
platinum gray. Constant voltage control also allows an array of
electrodes in parallel to be plated simultaneously achieving a
fairly uniform surface layer thickness for each electrode.
[0063] The optimal potential ranges for platinum gray plating are
solution and condition dependent. Linear voltage sweep can be used
to determine the optimal potential ranges for a specific plating
system. A representative linear voltage sweep is shown in FIG. 13.
During linear voltage sweep, the voltage of an electrode is scanned
cathodically until hydrogen gas starts developing revealing plating
rate control steps of electron transfer 20 and diffusion 22. For a
given plating system, it is preferable to adjust the electrode
potential such that the platinum reduction reaction has a limiting
current under diffusion control or mixed control 24 between
diffusion and electron transfer but that does not result in
hydrogen evolution 26.
[0064] Furthermore, it has been found that because of the physical
strength of platinum gray, it is possible to plate surface layers
of thickness greater than 30 .mu.m. It is very difficult to plate
shiny platinum in layers greater than approximately several microns
because the internal stress of the dense platinum layer will cause
the plated layer to peel off and the underlying layers cannot
support the above material. The additional thickness of the plate's
surface layer allows the electrode to have a much longer usable
life.
[0065] The following example is illustrative of electroplating
platinum on a conductive substrate to form a surface coating of
platinum gray according to the present invention.
[0066] Electrodes with a surface layer of platinum gray are
prepared in the following manner using constant voltage plating. An
electrode platinum silicone array having 16 electrodes where the
diameter of the platinum discs on the array range from 510 to 530
.mu.m is first cleaned electrochemically in sulfuric acid and the
starting electrode impedance is measured in phosphate buffered
saline or acid solution. Referring to FIG. 5, the electrodes are
arranged in the electroplating cell such that the plating electrode
2 is in parallel with the common electrode 4. The reference
electrode 10 is positioned next to the electrode array 4. The
plating solution is added to the electroplating cell 12 and the
stirring mechanism 14 is activated.
[0067] A constant voltage is applied on the plating electrode 2 as
compared to the reference electrode 10 using an EG&G PAR M273
potentiostat 6. The response current of the plating electrode 2 is
recorded by a recording means 8. The response current is measured
by the M273 potentiostat 6. After a specified time, preferably 1
minute-90 minutes, and most preferably 30 minutes, the voltage is
terminated and the electrode array 4 is thoroughly rinsed in
deionized water.
[0068] The electrochemical impedance of the electrode array 4 with
the surface coating of platinum gray is measured in a saline or
acid solution. The charge/charge density and average plating
current/current density are calculated by integrating the area
under the plating current vs. time curve. Energy Dispersed Analysis
by X-ray (EDAX.TM.) can be performed on selected electrodes.
Scanning Electron Microscope (SEM) Micrographs of the plated
surface can be taken showing its fractal surface. Energy Dispersed
Analysis demonstrates that the sample is pure platinum rather than
platinum oxide or some other materials.
[0069] From this example it is observed that the voltage range is
most determinative of the formation of the fractal surface of
platinum gray. For this system it observed that the optimal voltage
drop across the electrodes to produce platinum gray is
approximately -0.55 to -0.65 Volts vs. Ag/AgCl reference electrode.
The optimal platinum concentration for the plating solution is
observed to be approximately 8 to 18 mM ammonium
hexachloroplatinate in 0.4 M (mole) disodium hydrogen
phosphate.
[0070] FIG. 12 shows the increase in electrode capacitance of
several electrodes of varying diameter for a polyimide array plated
according to the above example at -0.6 V vs. Ag/AgCl reference
electrode for 30 minutes compared with unplated electrodes of the
same diameters. Because the electrode capacitance is proportional
to its surface area, the surface area increase, calculated from
electrode capacitance, is 60 to 100 times that of shiny platinum
for this array. Shiny platinum exhibits some roughness and has a
surface area increase up to 3 times that of the basic geometric
shape. While it is simple to measure a surface area change between
two sample using capacitance, it is difficult to compare a sample
with the basic geometric shape.
[0071] The present invention provides a very effective method for
plating on smaller substrates by retaining the same or better
effectiveness compared with known plating methods. This method is
very useful for manufacturing miniaturized or micro fuel cells
especially in the applications of hand-held electronics and some
medical implants.
[0072] A change of conditions, including but not limited to the
plating solution, surface area of the electrodes, pH, platinum
concentration and the presence of additives, will also result in
change of the optimal controlling voltage and/or other controlling
parameters according to the basic electroplating principles.
Platinum gray will still be formed so long as the rate of
deposition of the platinum particles is slower than that for the
formation of platinum black and faster than that for the formation
of shiny platinum.
[0073] A fuel cell 60 is an electrochemical device as shown in FIG.
14 that combines hydrogen fuel and oxygen from the air to produce
electricity, heat and water. Fuel cells operate without combustion,
so they are virtually pollution free. Since the fuel is converted
directly to electricity, a fuel cell can operate at much higher
efficiencies than internal combustion engines, extracting more
electricity from the same amount of fuel.
[0074] The fuel cell itself has no moving parts, which makes it a
quiet and reliable source of power.
[0075] The fuel cell 60 is composed of an anode 62, which is the
negative electrode of the fuel cell 60, an electrolyte 64 in the
center, which can be a liquid or a solid for example a membrane,
and a cathode 66 which is the positive electrode of the fuel cell
60. As hydrogen flows into the fuel cell anode 62, a platinum
catalyst in the anode facilitates the reaction from hydrogen gas
into protons and electrons as shown in the equation 1 below. 1.
.times. .times. 2 .times. H 2 .times. Pt anode .times. 4 .times. H
+ + 4 .times. e - ##EQU1##
[0076] The reaction (1) is facilitated by the anode due to the
catalytic reaction performed by the electroplated platinum as
catalyst. The electrons (e.sup.-) generated in the reaction (1) at
the anode 62 flow through an external circuit 68 in the form of
electric current.
[0077] The electrolyte 64 in the center allows only the protons to
pass through the electrolyte 64 to the cathode 66 side of the fuel
cell 60.
[0078] As oxygen flows into the fuel cell cathode 66 another
platinum catalyst helps the oxygen, protons, and electrons combine
to produce pure water and heat as shown in the equation 2 below. 2.
.times. .times. O 2 + 4 .times. H + + 4 .times. e - .times. Pt
cathode .times. 2 .times. H 2 .times. O + .DELTA. heat ##EQU2##
[0079] The overall chemical reaction which is performed in a fuel
cell is shown in the equation 3 below. 3. .times. .times. 2 .times.
H 2 + O 2 .times. .times. 2 .times. H 2 .times. O ##EQU3##
[0080] Individual fuel cells 60 can be combined into a fuel cell
stack to obtain desired amount of electrical power. The number of
fuel cells in the stack determines the total voltage, while the
surface area of the cell determines its current.
[0081] According to the present invention platinum gray can be
applied by electroplating for the coating of the anode 62 and
cathode 66. Platinum gray provides an enlarged service area by
applying less platinum for the coating and an improved adhesion to
the substrate of the electrode. Platinum gray has the same or
improved performance of the fuel cell 60 and at the same time a
large amount of platinum can be saved. The fuel cell production is
economical and ecological by using platinum gray for the coating of
the anode 62 and the cathode 66.
[0082] A catalyst, especially an autocatalyst 70 is a cylinder of
circular or elliptical cross section as shown in FIG. 15, made from
ceramic or metal formed into a fine honeycomb and coated with a
solution of chemicals and platinum group metals. It is mounted
inside a stainless catalytic converter, and is installed in the
exhaust line of the vehicle between the engine and a small muffler.
Vehicle exhaust contains a number of harmful elements which can be
controlled by the platinum group metals in autocatalysts.
[0083] The major exhaust pollutants are carbon monoxide, which is a
poisonous gas, oxides of nitrogen, which contribute to acid rain,
low level ozone and smog formation and which exacerbate breathing
problems, hydrocarbons, which are involved in the formation of smog
and have an unpleasant smell, and particulate, which contains known
cancer causing compounds.
[0084] Autocatalysts convert over 90 percent of hydrocarbons as
shown in the equation 4 below: 4. .times. .times. CH 4 + 2 .times.
O 2 .times. Pt catalyst .times. CO 2 + 2 .times. H 2 .times. O
##EQU4## carbon monoxide as shown in the equation 5 below: 5.
.times. .times. 2 .times. CO + O 2 .times. Pt catalyst .times. 2
.times. CO 2 ##EQU5## oxides of nitrogen from gasoline engines as
shown in equations 6 and 7 below: 6. .times. .times. 2 .times. NO 2
.times. Pt catalyst .times. N 2 + 2 .times. O 2 ##EQU6## 7. .times.
.times. 2 .times. NO 3 .times. Pt catalyst .times. N 2 + 3 .times.
O 2 ##EQU6.2## and ozone as shown in equation 8 below: 8. .times.
.times. 2 .times. O 3 .times. Pt catalyst .times. 3 .times. O 2
##EQU7## into less harmful carbon dioxide, nitrogen, water vapor
and oxygen as shown above in the equations 3 to 7.
[0085] The dominant material of ceramic substrate for catalytic
converters is porous cordierite, which can be used at temperatures
up to 1300.degree. C. Because of its nature of crystallization,
chemical composition, cordierite has an extremely low thermal
expansion coefficient. A low pressure drop, chemical inertness,
fast heat up time, and structural stability at high temperatures
make a ceramic honeycomb an ideal catalyst substrate for both
oxidation and reduction catalysts.
[0086] According to the present invention metal can be applied by
electroplating for the coating of the inside of a cylinder of
circular or elliptical cross section made from ceramic or metal.
Metal applied according to the present invention provides an
enlarged surface area by applying less platinum for the coating and
an improved adhesion to the substrate of the catalyst. Metal of the
present invention has the same or improved performance of the
catalyst. At the same time a large amount of metal, like platinum,
palladium, or iridium can be saved. The catalyst production is
economical and ecological by using metal of the present invention
for the coating of the catalyst substrate.
[0087] The present invention will be further explained in detail by
the following examples.
EXAMPLE 1
Electroplating Palladium on a Conductive Substrate Palladium
Plating Solution Preparation
[0088] 2.7 g sodium dihydrogen phosphate (NaH.sub.2PO.sub.4) and
3.2 g disodium hydrogen phosphate (Na.sub.2HPO.sub.4) [Fluka] were
dissolved in 100 ml deionized water, and stirred by magnetic
stirring for 30 minutes. The concentrations for NaH.sub.2PO.sub.4
and Na.sub.2HPO.sub.4 were equally 225 mM. Then 0.18 g Palladium
chloride (PdCl.sub.2) [Aldrich] was added to the phosphate
solution. The solution was then stirred for 30 minutes and filtered
to black solids. The PdCl.sub.2 concentration was about 5 mM. The
pH of the solution was measured at 6.8. The color of the solution
was brown. The solution was deaerated before the plating process by
bubbling nitrogen through the solution.
Preparation of the Substrate
[0089] A thin-film platinum polyimide array was used for palladium
plating. The array included 16 electrodes with 200 .mu.m thin-film
Pt disk as exposed electrode surface. All the electrodes in the
array were shorted to common contact points for the plating. The Pt
disk electrodes were first electrochemically cleaned by bubbling
the surface with oxygen at +2.8V vs Ag/AgCl in 0.5 M
H.sub.2SO.sub.4 for 10 sec. Then the surface was cleaned by
bubbling with hydrogen at -1.2 V vs Ag/AgCl in 0.5 M
H.sub.2SO.sub.4 for 15 sec. This removes surface contaminations and
polymer residues.
Electroplating Cell
[0090] A classical Pyrex glass three-electrode reaction cell was
used for the electroplating. The reference electrode compartment
was separated from the reaction compartment by a Vicor porous frit,
in order to avoid the migration of concentrated KCl and AgCl from
the inner filling solution of the reference electrode to the
plating bath. The counter electrode was a platinized-platinum sheet
of a real surface area equal to 1.8 cm.sup.2.
[0091] A digital magnetic stirrer (Dataplate PMC720) was used to
agitate the solution during plating. The solution temperatures were
from 15.degree. C. to 80.degree. C. and were controlled by a VWR
circulating water bath with a digital temperature controller (VWR
1147P).
[0092] The potential was controlled by using an EG&G PARC model
273 potentiostat-galvanostat and the response current, current
density and charge were recorded by EG&G PARC M270 software.
The charge/charge density and average plating current/current
density were calculated by integrating the area under the plating
current vs. time curve. The plating time was from 1 minute to 120
minutes.
Palladium Plating
[0093] A platinum polyimide electrode array having 16 electrodes
(FIG. 14) having a diameter of 200 .mu.m platinum disc on the array
was cleaned electrochemically in 0.5 M H.sub.2SO.sub.4. The
electrode array was placed in an electroplating cell containing a
plating solution having a concentration of 5 mM palladium chloride
in 0.025 M sodium dihydrogen phosphate and 0.425 M disodium
hydrogen phosphate. The plating bath temperature was at 22.degree.
C. A constant voltage of -1.0 V vs Ag/AgCl reference electrode was
applied on the electrode and terminated after 10 minutes. The
electrode array was thoroughly rinsed in deionized water. The
charge/charge density and average plating current/current density
were calculated by integrating the area under the plating current
vs. time curve. The current density was near linearly increased
from initial 0.96 A/cm.sup.2 to final 3.5 A/cm.sup.2. The
electrochemical capacitance of the electrode array with the surface
coating of rough palladium was 190 .mu.F/cm.sup.2, measured in a 10
mM phosphate buffered saline solution. The smooth thin-film Pt
disks measured at the same condition before plating an average
capacitance of less than 20 .mu.F/cm.sup.2. The electrochemical
active surface area increase is about 10 fold in this case. The
optimal voltage drop across the electrodes for producing rough
iridium was from -0.8 to -1.3 Volts vs. Ag/AgCl reference
electrode. The plated palladium surface coating thickness was about
1.0 .mu.m.
[0094] Example 1 yields a palladium surface coating having a rough
surface as shown in FIG. 4. The electrochemical active surface area
increase is about 10 fold. The relation of surface area to the
thickness of the platinum surface coating is 1.90 F/cm.sup.3
[surface coating of rough platinum 190 .mu.F/cm.sup.2 per thickness
of the platinum coating of 1.0 .mu.m.] The palladium surface
coating adhesive strength was 54 mN. The palladium coating contains
particles with very regular particle shape and regular average
size. The coating is thinner than known coatings and has a rough
surface which is mainly not porous with a large surface area. The
coating provides a good adherence between the substrate and the
platinum coating. The palladium coated electrode is biocompatible
and therefore implantable and provides less tissue reaction.
EXAMPLE 2
Electroplating Iridium on a Conductive Substrate Iridium Plating
Solution Preparation
[0095] 0.3 g sodium dihydrogen phosphate (NaH.sub.2PO.sub.4) and
6.03 g disodium hydrogen phosphate (Na.sub.2HPO.sub.4) [Fluka] were
dissolved in 100 ml deionized water, magnetic stirring for 30
minutes. The concentrations for NaH.sub.2PO.sub.4 and
Na.sub.2HPO.sub.4 were 25 mM and 425 mM, respectively. Then 0.882 g
of Ammonium hexachloroiridate ((NH.sub.4).sub.2IrCl.sub.6) from
[Alfa Aesar] was added to the phosphate solution to form the
iridium salt concentrations of 20 mM. The solution was stirred for
30 minutes prior to plating. The pH of the solution was measured at
7.9. The initial color of the solution is brown and changed to dark
blue after overnight aging.
Preparation of the Substrate
[0096] A thin-film platinum polyimide array was used for iridium
plating. The array had 16 electrodes with 200 .mu.m thin-film Pt
disk as exposed electrode surface. All electrodes in the array were
shorted to a common contact points for the plating. The Pt disk
electrodes were first electrochemically cleaned by bubbling the
surface with oxygen at +2.8 V vs Ag/AgCl in 0.5 M H.sub.2SO.sub.4
for 10 sec. Then the surface was cleaned by bubbling with hydrogen
at -1.2 V vs Ag/AgCl in 0.5 M H.sub.2SO.sub.4 for 15 sec. This
removes surface contaminations and polymer residues.
Electroplating Cell
[0097] A classical Pyrex glass three-electrode reaction cell was
used for the electroplating. The reference electrode compartment
was separated from the reaction compartment by a Vicor porous frit,
in order to avoid the migration of concentrated KCl and AgCl from
the inner filling solution of the reference electrode to the
plating bath. The counter electrode was a platinized-platinum sheet
of a real surface area equal to 1.8 cm.sup.2.
[0098] A digital magnetic stirrer (Dataplate PMC720) was used to
agitate the solution during plating. The solution temperatures were
from 15.degree. C. to 80.degree. C. and were controlled by a VWR
circulating water bath with a digital temperature controller (VWR
1147P).
[0099] The potential was controlled by using an EG&G PARC model
273 potentiostat-galvanostat and the response current, current
density and charge were recorded by EG&G PARC M270 software.
The charge/charge density and average plating current/current
density were calculated by integrating the area under the plating
current vs. time curve. The plating time was from 1 minute to 120
minutes.
Iridium Plating
[0100] A platinum polyimide electrode array having 16 electrodes
(FIG. 14) having a diameter of 200 .mu.m platinum disc on the array
was cleaned electrochemically in 0.5 M H.sub.2SO.sub.4. The
electrode array was placed in an electroplating cell containing a
plating solution having a concentration of 28 mM ammonium
hexachloroiridate in 0.025 M sodium dihydrogen phosphate and 0.425
M disodium hydrogen phosphate. The plating bath temperature was at
32.degree. C. A constant voltage of -2.5 V vs Ag/AgCl reference
electrode was applied on the electrode and terminated after 60
minutes. The electrode array was thoroughly rinsed in deionized
water. The charge/charge density and average plating
current/current density were calculated by integrating the area
under the plating current vs. time curve. The current density was
near linearly increased from initial 1.6 A/cm.sup.2 to final 2.2
A/cm.sup.2. The electrochemical capacitance of the electrode array
with the surface coating of rough iridium was 1115 .mu.F/cm.sup.2,
measured in a 10 mM phosphate buffered saline solution. The
thin-film Pt disks measured before plating at the same conditions
an average capacitance of lower than 20 .mu.F/cm.sup.2. The
electrochemical active surface area increase is about 56 fold in
this case. The optimal voltage drop across the electrodes for
producing rough iridium was from -1.5 to -3.0 Volts vs. Ag/AgCl
reference electrode. The plated iridium surface coating thickness
was about 2.0 .mu.m. The electrochemical active surface area
increase is about 56 fold. The relation of surface area to the
thickness of the iridium surface coating is 5.58 F/cm.sup.3
[surface coating of rough iridium 1115 .mu.F/cm.sup.2 per thickness
of the iridium coating of 2.0 .mu.m.] The platinum surface coating
adhesive strength was 62 mN.
[0101] Example 2 yields an iridium surface coating having a rough
surface as shown in FIG. 5. The iridium coating contains particles
with very regular particle shape and regular average size. The
coating is thinner than known coatings and has a rough surface
which is mainly not porous with a large surface area. The coating
provides a good adherence between the substrate and the coating.
The iridium coated electrode is biocompatible and therefore
implantable and provides less tissue reaction.
EXAMPLE 3
Electroplating Platinum on a Conductive Substrate Platinum Plating
Solution Preparation
[0102] 0.3 g sodium dihydrogen phosphate (NaH.sub.2PO.sub.4) and
6.03 g disodium hydrogen phosphate (Na.sub.2HPO.sub.4) [Fluka] were
dissolved in 100 ml deionized water, and stirred by magnetic
stirring for 30 minutes. The concentrations for NaH.sub.2PO.sub.4
and Na.sub.2HPO.sub.4 were 25 mM and 425 mM. Then 0.5 g of Platinum
chloride (PtCl.sub.4) [Alfa Aesar] was added to the phosphate
solution to form the platinum salt concentrations of 15 mM. The
solution was then stirred for 30 minutes. Different concentrations
of (PtCl.sub.4) were used in the experiments and the range of Pt
salt concentrations was from 3 to 30 mM. The pH of the solution was
measured at 7.9. The color of the solution was amber. The solution
was deaerated before the plating process by bubbling nitrogen
through the solution.
Preparation of the Substrate
[0103] A thin-film platinum polyimide array was used for platinum
plating. The array included 16 electrodes with 200 .mu.m thin-film
Pt disk as exposed electrode surface. All the electrodes in the
array were shorted to common contact points for the plating. The Pt
disk electrodes were first electrochemically cleaned by bubbling
the surface with oxygen at +2.8V vs Ag/AgCl in 0.5 M
H.sub.2SO.sub.4 for 10 sec. Then the surface was cleaned by
bubbling with hydrogen at -1.2 V vs Ag/AgCl in 0.5 M
H.sub.2SO.sub.4 for 15 sec to remove surface contaminations and
polymer residues.
Electroplating Cell
[0104] A classical Pyrex glass three-electrode reaction cell was
used for the electroplating. The reference electrode compartment
was separated from the reaction compartment by a Vicor porous frit,
in order to avoid the migration of concentrated KCl and AgCl from
the inner filling solution of the reference electrode to the
plating bath. The counter electrode was a platinized-platinum sheet
of a real surface area equal to 1.8 cm.sup.2.
[0105] A digital magnetic stirrer (Dataplate PMC720) was used to
agitate the solution during plating. The solution temperatures were
from 15.degree. C. to 80.degree. C. and were controlled by a VWR
circulating water bath with a digital temperature controller (VWR
1147P).
[0106] The potential was controlled by using an EG&G PARC model
273 potentiostat-galvanostat and the response current, current
density and charge were recorded by EG&G PARC M270 software.
The charge/charge density and average plating current/current
density were calculated by integrating the area under the plating
current vs. time curve. The plating time was from 1 minute to 60
minutes.
Platinum Plating
[0107] A platinum polyimide electrode array having 16 electrodes
(FIG. 14) having a diameter of 200 .mu.m platinum disc on the array
was cleaned electrochemically in 0.5 M H.sub.2SO.sub.4. The
electrode array was placed in an electroplating cell containing a
plating solution having a concentration of 15 mM platinum chloride
in 0.025 M sodium dihydrogen phosphate and 0.425 M disodium
hydrogen phosphate. The plating bath temperature was at 22.degree.
C. A constant voltage of -0.525 V vs Ag/AgCl reference electrode
was applied on the electrode and terminated after 10 minutes. The
electrode array was thoroughly rinsed in deionized water. The
charge/charge density and average plating current/current density
were calculated by integrating the area under the plating current
vs. time curve. The current density was near linearly increased
from initial 11.1 A/cm.sup.2 to final 15.2 A/cm.sup.2. The
electrochemical capacitance of the electrode array with the surface
coating of rough platinum was 1462 .mu.F/cm.sup.2, measured in a 10
mM phosphate buffered saline solution. The thin-film Pt disks only
have an average capacitance of less than 20 .mu.F/cm.sup.2 before
plating measured at the same condition. The optimal voltage drop
across the electrodes for producing rough platinum was from -0.4 to
-0.7 Volts vs. Ag/AgCl reference electrode. The plated platinum
surface coating thickness is about 3.5 .mu.m. The electrochemical
active surface area increase is about 73 fold. The relation of
surface area to the thickness of the platinum surface coating is
4.18 F/cm.sup.3 [surface coating of rough platinum 1462
.mu.F/cm.sup.2 per thickness of the platinum coating of 3.5 .mu.m.]
The platinum surface coating adhesive strength was 55 mN.
[0108] The platinum coating contains particles with very regular
particle shape and regular average size. The coating is thinner
than known platinum coatings and has a rough surface which is
mainly not porous with a large surface area. The coating provides a
good adherence between the substrate and the platinum coating. The
platinum coated electrode is biocompatible and therefore
implantable and provides less tissue reaction.
EXAMPLE 4
Electroplating Platinum on a Conductive Substrate Platinum Plating
Solution Preparation
[0109] 0.3 g sodium dihydrogen phosphate (NaH.sub.2PO.sub.4) and
6.03 g disodium hydrogen phosphate (Na.sub.2HPO.sub.4) [Fluka] were
dissolved in 100 ml deionized water, and stirred by magnetic
stirring for 30 minutes. The concentrations for NaH.sub.2PO.sub.4
and Na.sub.2HPO.sub.4 were 25 mM and 425 mM. Then 0.5 g of Platinum
chloride (PtCl.sub.4) [Alfa Aesar] was added to the phosphate
solution to form the platinum salt concentrations of 15 mM. The
solution was then stirred for 30 minutes and filtered to black
solids. Different concentrations of (PtCl.sub.4) were used in the
experiments and the range of Pt salt concentrations was from 3 to
30 mM. The pH of the solution was measured at 7.9. The color of the
solution was amber. The solution was deaerated before the plating
process by bubbling nitrogen through the solution.
Preparation of the Substrate
[0110] A thin-film platinum polyimide array was used for platinum
plating. The array included 16 electrodes with 200 .mu.m thin-film
Pt disk as exposed electrode surface. All the electrodes in the
array were shorted to common contact points for the plating. The Pt
disk electrodes were first electrochemically cleaned by bubbling
the surface with oxygen at +2.8V vs Ag/AgCl in 0.5 M
H.sub.2SO.sub.4 for 10 sec. Then the surface was cleaned by
bubbling with hydrogen at -1.2 V vs Ag/AgCl in 0.5 M
H.sub.2SO.sub.4 for 15 sec to remove surface contaminations and
polymer residues.
Electroplating Cell
[0111] A classical Pyrex glass three-electrode reaction cell was
used for the electroplating. The reference electrode compartment
was separated from the reaction compartment by a Vicor porous frit,
in order to avoid the migration of concentrated KCl and AgCl into
the inner filling solution of the reference electrode. The counter
electrode was a platinized-platinum sheet of a real surface area
equal to 1.8 cm.sup.2.
[0112] A digital magnetic stirrer (Dataplate PMC720) was used to
agitate the solution during plating. The solution temperatures were
from 15.degree. C. to 80.degree. C. and were controlled by a VWR
circulating water bath with a digital temperature controller (VWR
1147P).
[0113] The potential was controlled by using an EG&G PARC model
273 potentiostat-galvanostat and the response current, current
density and charge were recorded by EG&G PARC M270 software.
The charge/charge density and average plating current/current
density were calculated by integrating the area under the plating
current vs. time curve. The plating time was from 1 minute to 60
minutes.
Platinum Plating
[0114] A platinum polyimide electrode array having 16 electrodes
(FIG. 14) having a diameter of 200 .mu.m platinum disc on the array
was cleaned electrochemically in 0.5 M H.sub.2SO.sub.4. The
electrode array was placed in an electroplating cell containing a
plating solution having a concentration of 15 mM platinum chloride
in 0.025 M sodium dihydrogen phosphate and 0.425 M disodium
hydrogen phosphate. The plating bath temperature was at 22.degree.
C. A constant voltage of -0.5 V vs Ag/AgCl reference electrode was
applied on the electrode and terminated after 10 minutes. The
electrode array was thoroughly rinsed in deionized water. The
charge/charge density and average plating current/current density
were calculated by integrating the area under the plating current
vs. time curve. The current density was near linearly increased
from initial 10.8 A/cm.sup.2 to final 14.6 A/cm.sup.2. The
electrochemical capacitance of the electrode array with the surface
coating of rough platinum was 1417 .mu.F/cm.sup.2, measured in a 10
mM phosphate buffered saline solution. The thin-film Pt disks only
have an average capacitance of less than 20 .mu.F/cm.sup.2 before
plating measured at the same condition. The optimal voltage drop
across the electrodes for producing rough platinum was from -0.4 to
-0.7 Volts vs. Ag/AgCl reference electrode. The plated platinum
surface coating thickness is about 2.5 .mu.m. The electrochemical
active surface area increase is about 70 fold. The relation of
surface area to the thickness of the platinum surface coating is
5.67 F/cm.sup.3 [surface coating of rough platinum 1417
.mu.F/cm.sup.2 per thickness of the platinum coating of 2.5 .mu.m.]
The platinum surface coating adhesive strength was 58 mN.
[0115] The platinum coating contains particles with very regular
particle shape and regular average size. The coating is thinner
than known platinum coatings and has a rough surface which is
mainly not porous with a large surface area. The coating provides a
good adherence between the substrate and the platinum coating. The
platinum coated electrode is biocompatible and therefore
implantable and provides less tissue reaction.
EXAMPLE 5
Electroplating Platinum Gray on a Conductive Substrate
[0116] A platinum polyimide electrode array having 16 electrodes
(FIG. 14) having a diameter of 200 .mu.m platinum disc on the array
was cleaned electrochemically in 0.5 M H.sub.2SO.sub.4. The
electrode array was placed in an electroplating cell containing a
plating solution having a concentration 20 mM ammonium
hexachloroplatinate, 0.025 M sodium dihydrogen phosphate and 0.425
M disodium hydrogen phosphate. The voltage of -0.65 V was
terminated after 30 minutes. The electrode was thoroughly rinsed in
deionized water. The electrochemical capacitance of the electrode
with the surface coating of platinum gray was 1200 .mu.F/cm.sup.2,
measured in a 10 mM phosphate buffered saline solution. The
charge/charge density and average plating current/current density
were calculated by integrating the area under the plating current
vs. the time curve. The optimal voltage drop across the electrodes
for producing platinum gray was from -0.55 to -0.75 Volts vs.
Ag/AgCl reference electrode.
[0117] The platinum coating showed the following properties:
[0118] platinum surface coating thickness: 11.0 .mu.m;
[0119] electrochemical active surface area increase: 60 fold;
[0120] platinum surface coating adhesive strength: 50 mN; and
[0121] platinum surface coating color density: 1.0 D.
[0122] Example 5 yields a platinum surface coating having a fractal
surface as shown in FIG. 1. The relation of surface area to the
thickness of the platinum surface coating is 1.09 F/cm.sup.3
[surface coating of rough platinum 1200 .mu.F/cm.sup.2 per
thickness of the platinum coating of 11.0 .mu.m.] The coating
provides a good adherence between the substrate and the platinum
coating. The platinum coated electrode is biocompatible and
therefore implantable and provides less tissue reaction.
[0123] The plating conditions and properties of the platinum
coatings performed in Examples 1 to 5 are summarized in the
following tables 1 to 3. TABLE-US-00001 TABLE 1 Conditions of the
Plating Reactions Conditions Plating Agent Temp. Voltage Time
Example 1 5 mM PdCl.sub.2 22.degree. C. -1.0 V 10 min Rough Pd
Example 2 28 mM NH.sub.4[PtIr.sub.6] 32.degree. C. -2.5 V 60 min
Rough Ir Example 3 15 mM PtCl.sub.4 22.degree. C. -0.525 V 10 min
Rough Pt Example 4 15 mM PtCl.sub.4 22.degree. C. -0.5 V 10 min
Rough Pt Example 5 20 mM NH.sub.4[PtCl.sub.6] 22.degree. C. -0.6 V
30 min Fractal Pt
[0124] TABLE-US-00002 TABLE 2 Properties of the Coatings Final Area
Coating/ Properties Capacitance Thickness Increase Thickness
Example 1 190 .mu.F/cm.sup.2 1.0 .mu.m 10 fold 1.90 F/cm.sup.3
Rough Pd Example 2 1115 .mu.F/cm.sup.2 2.0 .mu.m 56 fold 5.58
F/cm.sup.3 Rough Ir Example 3 1462 .mu.F/cm.sup.2 3.5 .mu.m 73 fold
4.18 F/cm.sup.3 Rough Pt Example 4 1417 .mu.F/cm.sup.2 2.5 .mu.m 70
fold 5.67 F/cm.sup.3 Rough Pt Example 5 1200 .mu.F/cm.sup.2 11.0
.mu.m 60 fold 1.09 F/cm.sup.3 Fractal Pt
[0125] TABLE-US-00003 TABLE 3 Properties of the Coatings Adhesive
Current Density Initial Voltage Drop Across Properties Strength to
Final the Electrodes Example 1 54 mN 0.96 A/cm.sup.2 to -0.8 V to
-1.3 V Rough Pd 3.5 A/cm.sup.2 Example 2 60 mN 1.6 A/cm.sup.2 to
-1.5 V to -3.0 V Rough Ir 2.2 A/cm.sup.2 Example 3 55 mN 11.1
A/cm.sup.2 to -0.4 V to -0.7 V Rough Pt 15.2 A/cm.sup.2 Example 4
58 mN 10.8 A/cm.sup.2 to -0.4 V to -0.7 V Rough Pt 14.6 A/cm.sup.2
Example 5 50 mN 15.1 A/cm.sup.2 to -0.55 V to -0.75 V Fractal Pt
24.5 A/cm.sup.2
[0126] While the invention has been described by means of specific
embodiments and applications thereof, it is understood that
numerous modifications and variations could be made thereto by
those skilled in the art without departing from the spirit and
scope of the invention. It is therefore to be understood that
within the scope of the claims, the invention may be practiced
otherwise than as specifically described herein.
* * * * *