U.S. patent application number 10/660450 was filed with the patent office on 2004-07-22 for method for operating a metal particle electrolyzer.
Invention is credited to Des Jardins, Stephen R., Golovin, M. Neal, Smedley, Stuart I..
Application Number | 20040140222 10/660450 |
Document ID | / |
Family ID | 31999860 |
Filed Date | 2004-07-22 |
United States Patent
Application |
20040140222 |
Kind Code |
A1 |
Smedley, Stuart I. ; et
al. |
July 22, 2004 |
Method for operating a metal particle electrolyzer
Abstract
A method for operating an electrolyzer to produce metal
particles by electrolysis of an electrolyte solution while
optimizing electrolyzer service life and particle quality. An
electrolyzer immersed in a body of electrolyte including dissolved
metal is energized by a power supply and cell voltage across an
anode and cathode is monitored. Power supply output is adjusted
responsive to the monitored voltage to maintain current density
within a preferred range to promote high-quality particle growth.
In a growth cycle, particles fully grown are removed from the
cathode, and cell voltage polarity is reversed to dissolve
unremoved particles. Peak cathode current may be monitored during
polarity reversal to indicate a cathode surface condition, and the
surface reconditioned if peak current exceeds an operating
limit.
Inventors: |
Smedley, Stuart I.;
(Escondido, CA) ; Des Jardins, Stephen R.;
(Encinitas, CA) ; Golovin, M. Neal; (San Marcos,
CA) |
Correspondence
Address: |
HOWREY SIMON ARNOLD & WHITE, LLP - OC
301 RAVENSWOOD AVENUE
BOX 34
MENLO PARK
CA
94025
US
|
Family ID: |
31999860 |
Appl. No.: |
10/660450 |
Filed: |
September 10, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10660450 |
Sep 10, 2003 |
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10424539 |
Apr 24, 2003 |
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60410561 |
Sep 12, 2002 |
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60410426 |
Sep 12, 2002 |
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60410548 |
Sep 12, 2002 |
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60410565 |
Sep 12, 2002 |
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60410590 |
Sep 12, 2002 |
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Current U.S.
Class: |
205/337 |
Current CPC
Class: |
C25C 1/16 20130101; C25D
1/00 20130101; C25C 1/00 20130101; C25C 5/02 20130101; H01M 12/06
20130101; C25C 7/06 20130101 |
Class at
Publication: |
205/337 |
International
Class: |
C25C 005/00 |
Claims
What is claimed is:
1. A method for operating an electrolyzer for producing metal
particles through electrolysis, said electrolyzer having anode and
cathode surfaces at least partially immersed in an electrolyte
solution including dissolved metal, the method comprising:
determining an operating range for a cell voltage across the anode
and cathode surfaces; supplying electrical current flow through the
solution between the anode and cathode to create a cell voltage
within the operating range; monitoring the cell voltage; adjusting
the current responsive to the monitored voltage to maintain the
cell voltage within the operating range, thereby forming metal
particles on the cathode surface by electrolysis of the dissolved
metal.
2. The method of claim 1 wherein the operating range is selected to
produce a current density in the cathode greater than about 5
kA/m.sup.2.
3. The method of claim 1 wherein the dissolved metal is in a form
of one or more oxides of the metal.
4. The method of claim 1 wherein the solution is a reaction product
of an electrochemical reaction in a metal/air fuel cell.
5. The method of claim 1 wherein the dissolved metal is zinc.
6. The method of claim 1 wherein the determining step further
comprises determining an optimal cell voltage and limiting the
operating range to within 20% of the optimal cell voltage.
7. The method of claim 6 wherein a cell voltage within the
operating range can produce a current density in the cathode
between about 10 kA/m.sup.2 and about 40 kA/m.sup.2.
8. The method of claim 7 wherein a cell voltage within the
operating range can produce a current density in the cathode of
about 34,340 A/m.sup.2.
9. A method for operating an electrolyzer, comprising: immersing an
electrolyzer having anode and cathode surfaces into an electrolyte
solution, the solution including dissolved metal; applying DC
voltage to the electrolyzer to produce metal particles on the
cathode surface through electrolysis; removing metal particles from
the electrolyzer when the particles achieve a desired size;
reducing the applied voltage to a value below about 1.65 volts; and
reversing polarity of the applied voltage thereby dissolving
unremoved metal particles from the cathode surface.
10. The method of claim 9 wherein the reversing step further
comprises reversing polarity a first time for a time period, then
reversing polarity a second time and repeating the method beginning
with the applying step.
11. The method of claim 10 wherein the time period continues until
embedded particles are substantially dissolved from the cathode
surface.
12. The method of claim 10 wherein the time period is between about
60 and about 300 seconds.
13. The method of claim 9 further comprising determining an
operating range for a cell voltage across the anode and cathode
surfaces, monitoring the cell voltage, and adjusting the DC voltage
responsive to the monitored voltage to maintain the cell voltage
within the operating range.
14. The method of claim 13 wherein the operating range is selected
to produce a current density in the cathode greater than about 5
kA/m.sup.2.
15. The method of claim 13 wherein the determining step further
comprises determining an optimal cell voltage and limiting the
operating range to within 20% of the optimal cell voltage.
16. The method of claim 15 wherein a cell voltage within the
operating range can produce a current density in the cathode
between about 10 kA/m.sup.2 and about 40 kA/m.sup.2.
17. The method of claim 16 wherein a cell voltage within the
operating range can produce a current density in the cathode of
about 34,340 A/m.sup.2.
18. The method of claim 9 wherein the dissolved metal is in a form
of one or more oxides of the metal.
19. The method of claim 9 wherein the solution is a reaction
product of an electrochemical reaction in a metal/air fuel
cell.
20. The method of claim 18 wherein the dissolved metal is zinc.
21. A method for operating an electrolyzer having anode and cathode
surfaces, the method comprising: immersing an electrolyzer at least
partially within a body of electrolyte solution, the solution
including dissolved metal; supplying electrical current to the
electrolyzer to create a cell voltage across the anode and cathode
surfaces thereby forming metal particles on the cathode surface
through electrolysis; monitoring the cell voltage; adjusting the
current responsive to the monitored voltage to maintain the cell
voltage within a predetermined range; removing metal particles from
the cathode surface when the particles achieve a desired size;
reducing the applied voltage to a value that precludes oxgen
evolution on the cathode surface; and reversing polarity of the
applied voltage thereby dissolving unremoved metal particles from
the cathode surface.
22. The method of claim 21 wherein the predetermined range is
selected to produce a current density in the cathode between about
10,000 A/m.sup.2 and about 40,000 A/m.sup.2.
23. The method of claim 21 wherein the adjusting step further
comprises determining an optimal cell voltage and adjusting the
current responsive to the monitored voltage to maintain cell
voltage within 20% of the optimal cell voltage.
24. The method of claim 23 wherein a cell voltage within 20% of the
optimal cell voltage can produce a current density in the cathode
of about 34,340 A/m.sup.2.
25. The method of claim 21 further comprising monitoring peak
current through the electrolyzer during the reversing step.
26. The method of claim 25 further comprising shutting off power to
the electrolyzer if peak current exceeds an operating limit.
27. The method of claim 26 further comprising mechanically
reconditioning the cathode surface.
28. The method of claim 21 wherein the reversing step further
comprises reversing polarity a first time for a fixed time period,
then reversing polarity a second time and repeating the method
beginning with the supplying step.
29. The method of claim 28 wherein the time period continues until
embedded particles are substantially dissolved from the cathode
surface.
30. The method of claim 28 wherein the time period is between about
60 and about 300 seconds.
31. The method of claim 28 further comprising monitoring the peak
current through the electrolyzer during the first reversing step,
and if peak current exceeds an operating limit, mechanically
reconditioning the cathode surface.
32. The method of claim 21 wherein the dissolved metal is in a form
of one or more oxides of the metal.
33. The method of claim 21 wherein the solution is a reaction
product of an electrochemical reaction in a metal/air fuel
cell.
34. The method of claim 21 wherein the dissolved metal is zinc.
35. A method for operating an electrolyzer having anode and cathode
surfaces, the method comprising: a step for immersing an
electrolyzer at least partially within a body of electrolyte
solution, the solution including dissolved metal; a step for
supplying electrical current to the electrolyzer to create a cell
voltage across the anode and cathode surfaces; a step for
monitoring the cell voltage; a step for adjusting the current
responsive to the monitored voltage to maintain the cell voltage
within a predetermined range; a step for removing metal particles
from the cathode surface when the particles achieve a desired size;
a step for reducing the applied voltage to a value below about 1.65
volts; and a step for reversing polarity of the applied voltage
thereby dissolving unremoved metal particles from the cathode
surface; a step for monitoring peak current through the
electrolyzer during the reversing step; and a step for
reconditioning the cathode surface if peak current exceeds an
operating limit.
Description
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/424,539 filed Apr. 24, 2003, which is
hereby incorporated by reference herein as though set forth in
full.
[0002] This application claims the benefit of U.S. Provisional
Application No. 60/410,561 filed Sep. 12, 2002, U.S. Provisional
Application No. 60/410,426 filed Sep. 12, 2002, U.S. Provisional
Application No. 60/410,548 filed Sep. 12, 2002, U.S. Provisional
Application No. 60/410,565 filed Sep. 12, 2002, and U.S.
Provisional Application No. 60/410,590 filed Sep. 12, 2002, each of
which is hereby fully incorporated by reference herein as though
set forth in full.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates generally to production of metal
particles through electrolysis, and more specifically, to a method
for operating a metal particle electrolyzer to optimize
electrolyzer service life and metal particle quality.
[0005] 2. Related Art
[0006] There are many applications for metal particles produced
through electrolysis including, for example, for use as feedstock
for laboratory and industrial processes, and for use in refuelable
and regenerative metal/air fuel cells. In these fuel cells, the
metal particles function as the fuel for replenishing discharged
fuel cells, and this fuel can be regenerated from the spent
reaction solution which results from fuel cell discharge. In
applications such as this, it is desirable to be able to regenerate
the metal particles in a space efficient and self contained manner
so that the regeneration of the metal particles can take place at
the same location as the power source or cell stack within the fuel
cell. For additional information on metal/air fuel cells, the
reader is referred to the following patents and patent
applications, which disclose a particular embodiment of a metal/air
fuel cell in which the metal is zinc: U.S. Pat. Nos. 5,952,117;
6,153,328; and 6,162,555; and U.S. patent application Ser. Nos.
09/521,392; 09/573,438; and 09/627,742, each of which is
incorporated herein by reference as though set forth in full. The
term "fuel cell" as used throughout this disclosure is synonymous
with the terms "battery" and "refuelable metal/air battery."
[0007] Unfortunately, known methods of producing metal through
electrolysis are all unsatisfactory for these applications. Some
methods, e.g., electroplating, do not produce metal in the required
particulate form, and require expensive and cumbersome mechanical
processing to put the metal in the required form.
[0008] For example, a method disclosed in U.S. Pat. No. 4,164,453
forms zinc dendrites on cathode tips that protrude into an anodic
pipe carrying a flow of zincate solution. The cathode protrusions
are specially formed in a curved configuration. Dendrites form on
the cathode tips during low flow in one direction, and are then
dislodged during high flow in the opposite direction. This
technique is not suitable for particle production because it yields
dendritic zinc that requires further processing to make pellets.
Also, the curved cathodic protrusions are expensive to manufacture,
and spatially inefficient.
[0009] Another method, represented by U.S. Pat. No. 5,792,328,
involves electro-depositing dendritic or mossy zinc onto the
surface of a planar cathode plate, and then scraping the zinc from
the surface of the cathode. Since the recovered metal is in the
form of mossy dendrites, and cannot be easily put into the desired
particulate form absent expensive and complicated mechanical
processing steps, this method is likewise not suitable.
[0010] A third method, in U.S. Pat. No. 3,860,509, uses a cathodic
surface that consists of many small conductive areas in the hundred
micron range spaced apart by an insulating matrix. These areas are
exposed to a high temperature metal bearing electrolyte solution
which, by electrolysis, deposits metal dendrites on the cathode.
The metal is recovered by mechanically scraping the cathode which
produces a powdery metal dust composed of particles so small that
they are not suitable for use in a metal/air fuel cell.
[0011] A fourth method, known as electrowinning, represented by
U.S. Pat. Nos. 5,695,629 and 5,958,210, involves immersing seed
particles in an electrolyte, and causing metal to form over the
seed particles through electrolysis. However, because of the risk
that metal particles will get caught in a porous separator between
the anode and cathode, and cause a disastrous short between the
anode and cathode, this method is unsatisfactory. Another factor
weighing against this method is the burden and expense of
maintaining a supply of seed particles.
[0012] Another method, represented by U.S. Pat. No. 5,578,183,
involves forming dendritic or mossy metal on a cathode through
electrolysis, removing the metal, and then pressing the metal into
pellets through mechanical forming steps such as extrusion. This
technique is unsuitable for the applications mentioned earlier
because the required mechanical forming steps are expensive, and do
not permit a space-efficient and self-contained particle recovery
process.
SUMMARY
[0013] A method for operating a metal particle electrolyzer is
described. The method extends the service life of the electrolyzer,
and controls the quality of metal particles produced by the
electrolyzer by maintaining current density through the
electrolyzer cathode within a preferred range. An operating range
for cell voltage across the anode and cathode of the electrolyzer
is determined based on cathode configuration. A particle growth
cycle is initiated by a power supply energizing the electrolyzer
immersed in an electrolyte solution containing dissolved metal.
Cell voltage is monitored and power supply output is adjusted
responsive to the monitored voltage to maintain the current density
in the preferred range. In one embodiment, current density is
maintained in a range between about 10,000 A/m.sup.2 and 40,000
A/m.sup.2. In another embodiment, power supply output is adjusted
to maintain cell voltage within 20% of an optimal value. In another
embodiment, the electrolyte solution is a reaction product of an
electrochemical reaction in a metal/air fuel cell in which the
metal may be zinc.
[0014] After particles are grown to a desired size, the particles
are removed from the cathode, and cell voltage polarity is reversed
to dissolve particles embedded in the cathode surface. Reverse
polarity voltage is maintained at a value sufficiently low to
preclude oxygen evolution on the electrode surface that was
formerly the cathode. After a period of time has elasped to allow
for dissolving of embedded particles, the growth cycle is
completed, and another cycle may begin. During application of
reverse polarity voltage, peak current through the electrolyzer may
be monitored to indicate a cathode surface condition. When peak
current exceeds an operating limit, the electrolyzer may be
de-energized, and the cathode surface may be mechanically
reconditioned to extend the service life of the electrolyzer.
[0015] A means for electrolyzing the dissolved metal using a
discrete particle electrolyzer is also described. An anode and
cathode spaced from one another are at least partially immersed in
a solution of dissolved metal. The surface of the cathode is
configured with one or more active zones separated from one another
by an insulator. The active zones are made of a material which is
electrically conductive. An electric potential is applied between
the anode and the cathode while the solution containing the
dissolved metal is caused to flow along the surface of the cathode.
The electric potential causes an electric current to flow through
the solution. The current density is sufficient to allow metal
particles to form on the active zones of the cathode through
electrolysis. When the metal particles are of sufficient size, they
are removed from the surface of the cathode through a scraper or
other suitable means integral to the cathode structure, and applied
to the surface of the cathode.
[0016] Other systems, methods, features and advantages of the
invention will be or will become apparent to one with skill in the
art upon examination of the following figures and detailed
description. It is intended that all such additional systems,
methods, features and advantages be included within this
description, be within the scope of the invention, and be protected
by the accompanying claims.
BRIEF DESCRIPTION OF THE FIGURES
[0017] The invention can be better understood with reference to the
following figures. The components in the figures are not
necessarily to scale, emphasis instead being placed upon
illustrating the principles of the invention. Moreover, in the
figures, like reference numerals designate corresponding parts
throughout the different views.
[0018] FIG. 1 illustrates a first embodiment of a system for
producing metal particles through electrolysis, including a lateral
view of electrolyte flow across the cathode surface.
[0019] FIG. 2a shows the front view of an embodiment of a
cathode.
[0020] FIG. 2b shows a magnified side view of the cathode of FIG.
2a.
[0021] FIG. 3a shows a side view of a second embodiment of a
cathode.
[0022] FIG. 3b shows a top view of the cathode of FIG. 3a.
[0023] FIG. 4 illustrates an embodiment of a system for producing
metal particles through electrolysis in which a cylindrical cathode
is mounted within an anode pipe.
[0024] FIG. 5 illustrates an embodiment of a system for producing
metal particles through electrolysis configured with a double-sided
planar cathode.
[0025] FIG. 6 illustrates an embodiment of a system for producing
metal particles through electrolysis configured with multiple
double-sided planar cathodes in a series configuration and
dielectric material separating adjacent anodes.
[0026] FIG. 7 illustrates an embodiment of a system for producing
metal particles through electrolysis configured with multiple dual
cathodic/anodic plates in a series configuration.
[0027] FIG. 8 shows an embodiment of a system for producing metal
particles through electrolysis in which particle removal is
achieved through movement of a moveable planar cathode past a
stationary scraper.
[0028] FIG. 9 is a partial view of an embodiment of a system for
producing metal particles through electrolysis in which particle
removal is achieved through rotation of a cylindrical cathode past
a stationary scraper.
[0029] FIG. 10 is an exploded view of a system for producing metal
particles through electrolysis in which particle removal from a
planar cathode is achieved by means of a rotary scraper.
[0030] FIG. 11 illustrates an embodiment of a system for producing
metal particles through electrolysis configured with means for
collecting particles and automatically refueling a fuel cell with
the collected particles.
[0031] FIG. 12 illustrates an apparatus for conducting simulations
of systems for producing zinc particles.
[0032] FIG. 13 plots regions of zinc particle quality as a function
of current density and ZnO molarity, based on simulations conducted
using the experimental testing apparatus of FIG. 12.
[0033] FIG. 14a shows a magnified cross sectional view of metal
particle formation on a cathode surface according to one embodiment
of the invention.
[0034] FIG. 14b shows a magnified view of the first phase of metal
particle formation on an active zone of a cathode according to one
embodiment of the invention.
[0035] FIG. 14c shows a magnified view of the second phase of metal
particle formation on an active zone of a cathode according to one
embodiment of the invention.
[0036] FIG. 15a shows a sliced planar cross section of a cathode
surface formed from an insulated bundle of wire according to one
embodiment of the invention.
[0037] FIG. 15b shows a side view of a fixture for spacing an
insulated bundle of wire for manufacturing cathodes.
[0038] FIG. 16a shows a top view of a metal plate machined to form
active zones in hexagonal array to form a planar cathode surface
according to one embodiment of the invention.
[0039] FIG. 16b shows a side view of the plate of FIG. 16a.
[0040] FIG. 16c shows a magnified side view of the plate of FIG.
16a.
[0041] FIG. 16d shows a magnified side view of the plate of FIG.
16a after the addition of a layer of insulating material to the
surface of the plate.
[0042] FIG. 16e shows a magnified side view of the plate of FIG.
16d after finishing the surface.
[0043] FIG. 17a illustrates a minimal number of active zones
arranged in a hexagonal array.
[0044] FIG. 17b shows a plurality of active zones arranged in a
hexagonal array.
[0045] FIG. 17c depicts active zones in hexagonal array on coins of
different shapes that form a portion of a planar cathode surface
formed according to one embodiment of the invention.
[0046] FIG. 17d shows a top view of a metal strip for preparing
coins shown in FIG. 17c.
[0047] FIG. 17e shows a magnified side view of the metal strip of
FIG. 17d after coatings have been applied to its top and bottom
surfaces.
[0048] FIG. 17f illustrates the metal strip of FIG. 17d punched to
form hexagonal coins of FIG. 17c.
[0049] FIG. 17g depicts a hexagonal coin having a plurality of
active zones stamped on its surface.
[0050] FIG. 17h illustrates a portion of a planar cathode surface
configured from a plurality of hexagonal coins of FIG. 17g.
[0051] FIG. 18a shows a top view of an metal plate etched to define
active zones that form a planar cathode surface according to one
embodiment of the invention.
[0052] FIG. 18b shows a magnified side view of the plate of FIG.
18a after adding a layer of insulating material to the etched
surface.
[0053] FIG. 18c shows magnified views of an active zone comprising
multiple layers of metal formed according to one embodiment of the
invention.
[0054] FIG. 19 is a flow chart of a method according to one
embodiment of the invention of manufacturing a cathode by
chemically etching a metal substrate.
[0055] FIG. 20 illustrates an embodiment of a method according to
the invention for producing metal particles.
[0056] FIG. 21a illustrates an embodiment of a method according to
the invention for removing particles from a cathode.
[0057] FIG. 21b illustrates a second embodiment of a method
according to the invention for removing particles from a
cathode.
[0058] FIG. 22a shows a magnified cross-sectional view of an
electrolyzer having irregular areas on the active zones of a
cathode.
[0059] FIG. 22b shows a magnified cross-sectional view of the
cathode of FIG. 22a after reconditioning.
[0060] FIG. 23 illustrates an embodiment of a method according to
the invention for operating an electrolyzer.
[0061] FIG. 24 illustrates another embodiment of a method according
to the invention for operating an electrolyzer using
reverse-polarity cleansing.
[0062] FIG. 25 illustrates another embodiment of a method for
operating an electrolyzer using reverse-polarity cleansing and
mechanical reconditioning.
DETAILED DESCRIPTION
[0063] FIG. 1 illustrates a system 100 configured to produce metal
particles by electrolysis of a reaction solution 110 that contains
dissolved metal. Solution 110 may be aqueous or non-aqueous, and
may be an electrolyte, acid, or organic solvent. It may contain
metal ions in the form of one or more oxides or salts of the metal.
In one implementation, the solution 110 contains reaction products,
such as zincate, of an electrochemical reaction occurring in a
metal/air fuel cell. Examples of the metal include zinc, copper,
nickel, and potassium. In one implementation example, the reaction
solution comprises potassium hydroxide (KOH) containing zincate,
Zn(OH).sub.4.sup.2-, or dissolved zinc oxide, ZnO, a white
non-toxic powder which is soluble in the reaction solution. The
zincate in this implementation example may be produced through the
following electrochemical reaction which occurs in one embodiment
of a zinc/air fuel cell:
Zn+4OH.sup.-.fwdarw.Zn(OH).sub.4.sup.2-+2e.sup.- (1)
[0064] Zinc oxide may then be formed through precipitation of the
zincate in accordance with the following reaction:
Zn(OH).sub.4.sup.2-.fwdarw.ZnO+H.sub.2O+2OH.sup.- (2)
[0065] The system 100 produces metal particles through electrolysis
which occurs between the rightmost surface of anode 104 and the
leftmost surface of cathode 106. Anode 104 and cathode 106 are
electrodes at least partially immersed in solution 110, and are
coupled, respectively, to the positive and negative terminals of
power supply 112. The solution 110 is contained within container
114.
[0066] In the previously discussed implementation example of system
100, the following reaction may take place at the cathode:
Zn(OH).sub.4.sup.2-+2e.sup.-.fwdarw.Zn+40H.sup.- (3)
[0067] The two electrons in this equation originate from the
cathode where the following reaction takes place: 1 2 OH - 1 2 O 2
+ H 2 O + 2 e - ( 4 )
[0068] Pump 108 provides a means for circulating solution 110 into
and out of container 114. The solution flows into container 114
through conduit 116, and flows out of container 114 through conduit
118. By pumping solution into and out of container 114, a flow path
120 of solution along the surface of cathode 106 is created.
Cathode 106 includes on its surface a plurality of active zones 102
that are exposed to the solution 110 flowing along flow path 120.
As pump 108 causes solution 110 to flow past the active zones 102,
while power supply 112 energizes anode 104 and cathode 106, metal
particles are formed on the active zones 102 by electrolysis. Once
formed, the particles may be removed from the active zones 102 by a
scraper or other suitable means. The active zones 102 may be formed
of a material with easy release surface properties to facilitate
removal of the metal particles. These surface properties may be
imbued by a suitable coating added to the surface of the active
zones, or through oxidation of the surface of the active zones.
Materials capable of forming active zones having oxide layers
include magnesium, nickel, chromium, niobium, tungsten, titanium,
zirconium, vanadium, and molybdenum.
[0069] The active zones 102 are formed of a conductive material and
are electrically coupled to conductor 122 within the cathode 106.
The active zones 102 are electrically isolated from one another at
the cathode surface by an insulator. The design of the conductor,
insulator, and active zones may be tailored to suit a particular
application, thus, the surface of the cathode may take on a variety
of forms. It may be flat or curved, and have a general shape that
is planar, cylindrical, spherical, or any combination thereof. The
cathode may have a single surface with active zones, or may have
multiple surfaces with active zones. The size and number of the
active zones on the surface of the cathode determine, generally,
the size and number of metal particles that the system will produce
in a single operation.
[0070] An active zone, considered separately, may itself have a
flat or curved surface, may assume any regular geometric shape, or
may have an irregular shape. The separation distance between the
nearest points of any two active zones is between about 0.1 mm and
about 10 mm, preferably between about 0.4 mm and about 0.8 mm, and
the surface area of each active zone is no less than about 0.02
square mm. The active zones, considered collectively, may comprise
multiple shapes, sizes and placement patterns. The active zones may
be formed from the conductor, or may be separate parts connected
thereto.
[0071] A perforated insulator that covers the conductor, exposing
areas of the conductor to the cathode surface, may form the active
zones. It is also possible to form the insulator by creating an
oxide layer on the surface of the conductor that separates the
active zones. A skilled artisan will appreciate from a reading of
this disclosure that the conductor, insulator, and active zones may
be composed from a variety of materials, and be configured in a
variety of ways. Accordingly, many variations in the design of the
cathode are possible.
[0072] One embodiment of a cathode 200 is illustrated in FIG. 2a.
In this embodiment, cathode 200 has a generally planar form, with a
plurality of active zones 202 occupying one of the planar surfaces
212. A plurality of pins 204 extend from and are electrically
coupled to conductor 206 within the cathode 200, as shown in
magnified form in FIG. 2b. The ends of the rods 204 at the surface
212 of the cathode form the active zones 202. The rods 204 may be
machined from conductor 210, or may be separately attached to
conductor 210 by threaded connection, welding, or other means. Both
the rods 204 and conductor 210 are electrically conductive, but
need not be made from the same material. An insulator 208 fills the
gaps between rods 204 to maintain separation and electrical
isolation between the active zones 202 and create a generally flat
surface 212. It also coats the remaining surfaces of conductor 210
sufficiently such that the active zones 202 are the only conductive
portion of cathode 200 which is immersed in solution 110 in the
system 100. The insulator 208 may be formed from a potting
compound, a molded plastic, or any other dielectric material.
[0073] A second embodiment of a cathode 300 is illustrated in FIG.
3a. In this embodiment, cathode 300 is generally cylindrical in
form, with a plurality of active zones 302 spaced around the outer
surface 312 of the cylinder. As illustrated in FIG. 3b, rods 304
extend radially outward from conductor 306, and the ends thereof at
the outer surface 312 of the cylinder form the active zones 302.
Conductor 306 includes a center terminal 310 that extends axially
through the cylinder, and acts as a means for external electrical
connection. Insulator 308 fills the interstices between active
zones 302 to achieve electrical isolation of the active zones from
at each other at surface 312, and also to complete the surface
312.
[0074] FIG. 4 illustrates a second embodiment of system for metal
particle production. In system 400, the electrolysis occurs inside
a metal pipe 404 that functions as the anode. Metal pipe 404 has a
first portion 414 and a second portion 416, and cathode 406 is
situated within the second portion 416 of the pipe 404 as shown.
Solution 410 flows through pipe 404, entering the first portion 414
and exiting the second portion 416 as shown. At the same time,
power supply 412 creates an electric potential between pipe 404 and
cathode 406. In one embodiment, the cathode 406 is cylindrical in
shape and is configured as shown in FIG. 3a. The active zones of
cathode 406 are identified with numeral 402. A bus bar 408 couples
electrical energy from power supply 412 to the cathode 406 through
a penetration 418 in pipe 404, while maintaining a watertight seal
for pipe 404 at the point of penetration 418, and maintaining
electrical insulation between cathode 406 and pipe 404 at the point
of penetration 418.
[0075] A third embodiment of a system 500 for metal particle
production is illustrated in FIG. 5. In system 500, a double-sided
planar cathode 506 is situated between planar anodes 504a and 504b.
In a manner similar to system 100 illustrated in FIG. 1, pump 508
circulates solution 510 into container 514 through conduit 518, and
out of container 514 through conduit 520. The solution 510 is
caused to flow past the surfaces 506a and 506b of cathode 506 by
means respectively of flow paths 516a and 516b. Flow paths 516a and
516b in turn are created through the circulation of the solution
510 through the container 514. While solution 510 flows along flow
paths 516a and 516b, power supply 512 energizes anodes 504 and
cathode 506 to cause formation of metal particles on active zones
502 of the surfaces 506a and 506b of cathode 506. Once formed, the
metal particles may be removed as in the previous embodiments. In
this embodiment, anodes 504 are electrically connected in
parallel.
[0076] A fourth embodiment of a system 600 for metal particle
production is illustrated in FIG. 6. System 600 comprises a
plurality of the systems of FIG. 5 coupled in series. In FIG. 6,
four such systems are shown, identified with numerals 624a, 624b,
624c, and 624d, but it should be appreciated that embodiments are
possible in which fewer or more than four such systems are
provided.
[0077] The series connection is achieved as follows: Coupler 616
connects the positive terminal of power supply 602 to the anode
pair in the first system 624a. Coupler 618 connects the cathode in
the first system 624a to the anode pair in the second system 624b.
Similar couplers respectively connect the cathode in the second
system 624b to the anode pair in the third system 624c, and the
cathode in the third system to the anode pair in the fourth system
624d. The cathode in the fourth system 624d is then coupled to the
negative terminal of power supply 602 through coupler 620. A
dielectric material 622 may be placed between the anode plates in
adjacent systems that may be at different electric potentials to
prevent electrolysis between anodes.
[0078] A pump 626 pumps solution to each of the system 624a, 624b,
624c, and 624d through conduit 628 in the manner shown. The
solution flows through each of the systems 624a, 624b, 624c, and
624d, through flow paths which cause solution to flow across the
two surfaces of the cathode in each system. After flowing through
the individual system, the solution then collects in the bottom 632
of the overall system 600, and is then returned to pump 626 by
means of conduit 630. Each of the systems 624a, 624b, 624c, and
624d are configured as previously described in relation to the
system of FIG. 5.
[0079] FIG. 7 illustrates a fifth embodiment of a system 700 for
producing metal particles. In system 700, each of electrodes 706 is
a bipolar cathodic-anodic plate, having an anode plate on one
surface, and having on the other surface a plurality of active
zones electrically coupled to the anode plate and separated from
each other by an insulator. The series connection is made by
coupling the positive terminal of power supply 702 to electrode
704, which is a plain anode plate, and by coupling the negative
terminal of 702 to electrode 708, which is a plain cathode plate.
This creates a path for the flow of electric current from anode 704
through the sequence of bipolar electrodes 706 to cathode 708.
Thus, a series configuration of bipolar electrodes 706 is formed in
which the dielectric material and coupling devices included in the
system of FIG. 6 are eliminated.
[0080] Pump 712 pumps solution through conduit 714 into the system
700 such that individual flow paths are created to cause the
solution to flow past the cathode 708, and the cathodes in each of
the electrodes 706. The solution is then returned to pump 712 by
means of conduit 716.
[0081] Various means are possible form removing particles from the
active zones of the cathode when they have reached the desired
size. For example, particles may be removed by scraping the cathode
surface, by vibrating the cathode, by delivering a mechanical shock
to the cathode, or by increasing the flow velocity of the solution.
One embodiment of a scraping means is illustrated in FIG. 8. In
this embodiment, cathode 806 has outward facing active zones 802
and is movable relative to stationary scraper 804. After particles
have accumulated on the active zones 802, cathode 806 is moved from
position A to position B (or vice versa) such that the outer
surface thereof passes against scraper 804, thus dislodging the
particles. Scraper 804 may be composed of any material of a
hardness sufficient to dislodge the metal particles from the active
zones. In addition, as previously discussed, the material making up
the active zones may have easy release surface properties to
facilitate removal of the particles.
[0082] A second embodiment of a particle-removal system 900 is
illustrated in FIG. 9, which shows a cut-away view of a cylindrical
anode 908 enclosing cylindrical cathode 906. A scraper 904 is
situated against the active zones 902 on the surface of cathode
906. Cathode 906 is configured so that it can be moved relative to
scraper 904. Cathode 906 is then rotated, causing particles to be
scraped from active zones 902. Scraper 902 may be mounted directly
to anode 908, or may be independently mounted.
[0083] FIG. 10 shows an exploded view of a system according to one
embodiment of the invention comprising a planar anode plate 1002
and cathode plate 1014 configured with a rotary scraper 1012. Anode
plate 1002 is mounted to a back plate 1004 that provides a mounting
location for drive motor 1006. Back plate 1004 also provides a
fluid manifold 1008 for the passage of electrolyte solution. Drive
motor 1006 is mechanically coupled to a scraper driver 1010 that is
centrally located in anode plate 1002, as shown. A scraper 1012 is
coupled to scraper driver 1010 such that the scraper contacts, or
nearly contacts, the surface of cathode plate 1014. Cathode plate
1014, configured with a plurality of active zones, mounts to anode
plate 1002 and back plate 1004 to complete the assembly and form a
narrow channel (not shown) to conduct solution from fluid manifold
1008 down through the channel between anode plate 1002 and cathode
plate 1014.
[0084] System 1000 operates generally as previously discussed to
form metal particles on the surface of the active zones of cathode
plate 1014. When the particles have grown to a desired size, drive
motor 1006 is energized to rotate scraper 1012 against the
particles with a minimal force required to dislodge the particles.
In one embodiment, scraper 1012 may be rotated through one or more
complete revolutions, as required to dislodge particles. In another
embodiment, scraper 1012 may be rotated through one half of a
complete revolution, thereby dislodging about half of the
particles, then reversed and rotated in the opposite direction
through a complete revolution to dislodge the remaining
particles.
[0085] In another embodiment, scraper 1012 may be oscillated like
an inverted pendulum with an increasing amplitude. Initially,
scraper 1012 is positioned vertically in a twelve o'clock position.
Scraper 1012 then rotates through an initial angle comprising a
partial revolution, then rotates in the opposite direction through
an angle greater than the initial angle to dislodge more particles,
then reverses direction again. As particles are dislodged, they
fall from the cathode plate 1014 by means of gravity or entrainment
in fluid flow. With each reversal, scraper 1012 is rotated through
an angle greater than the previous one in order to cover unscraped
areas of the cathode. This process is continued until the entire
cathode surface is sufficiently scraped. In another embodiment, the
initial position of scraper 1012 is at a position other than twelve
o'clock, for example, six o'clock. At the six o'clock position,
scraper 1012 oscillates as described above, causing any dislodged
particles that accumulate on scraper 1012 to fall from the cathode
plate 1014 with each reversal of direction. The advantage to the
pendulum movement is that it prevents excessive accumulation of
dislodged particles on the scraper, thereby allowing the drive
motor to deliver a minimal force and reduce the risk of particle
disintegration.
[0086] FIG. 11 illustrates the system of FIG. 5 equipped with a
particle-collection means. When particles have reached the
appropriate size, a scraper or other means (not shown) dislodges
the particles. The dislodged particles then fall by gravity,
through the flow of solution, or by some other suitable means into
hopper 1104, where they are funneled into collection tube 1106 and
entrained in fluid flow. Pump 1108 then draws the fluid borne
particles through conduit 1110 for transport to a storage device or
to a fuel cell for a metal/air battery.
[0087] In order to ensure consistent shape and quality of the metal
particles, it may be necessary to maintain several operational
parameters within certain ranges. The flow rate and temperature of
the solution, the molarity of the dissolved metal, the electrolyte
concentration, the Reynolds number of the flow path past the
cathode surface, flow turbulence, the electric current through the
solution, and the current density at the active zones are all
parameters that may need to be controlled in order to produce good
quality, crystalline particles that are free of dendritic
formations. The Reynolds number Re is defined as follows: 2 Re = UD
h ( 5 )
[0088] where .rho. is the solution density, U is the solution
velocity, .mu. is the solution viscosity, and D.sub.h is a length
dimension defined as 3 D h = 4 ( WG 2 ( W + G ) ) ( 6 )
[0089] For a substantially rectangular flow channel, such as that
depicted in FIG. 1 for flow path 120, G is the gap between the
anode and cathode plates. W is the width of the channel across the
anode or cathode surface, measured as shown in FIG. 2. In other
words, W and G are the cross-sectional rectangular dimensions of
the flow channel normal to the direction of flow. Thus, for a given
.rho., .mu., and D.sub.h, the Reynolds number may be controlled by
controlling the velocity of the solution flow. Generally, in
particle-free fluid flow, Re greater than about 2000 promotes
turbulent flow.
[0090] An apparatus for determining appropriate ranges for these
parameters for a zinc particle production system configured to
produce zinc particles through electrolysis of a potassium
hydroxide solution containing zincate is illustrated in FIG. 12.
The configuration and operation of the apparatus is generally
similar to that of FIG. 1. Cathode 1206 in this apparatus comprises
a 100 mm square Mg plate (W=100 mm) with 4900 circular active
zones, each about 0.4 mm in diameter, evenly spaced in a square
array. The cathode is formed from magnesium because this metal
avoids a strong bond with electro-deposited zinc. The insulation
separating the active zones on the cathode is formed from
commercial epoxy adhesive. Cathode 1206 is partially submerged in
the potassium hydroxide solution containing zincate and spaced 3 mm
(G=3 mm) from anodic surface 1202. The anodic surface 1202 consists
of a nickel mesh coated with oxygen evolution catalysts attached to
the surface of a stainless steel plate 1204. Both cathode 1206 and
anode 1202 are electrically connected to a constant current DC
power supply 1212. The current from power supply 1212 can be varied
from 0 to 300 A. Electrolyte 1210, consisting of aqueous solution
of 45% KOH with different concentrations of ZnO, was pumped into
and out of container 1214 (and through a flow path between the
spacing between cathode 1206 and anode 1202) by a 100 W centrifugal
pump 1208. Preferably, electrolyte concentration should be kept
within the 25 to 55 weight percent range. Electrolyte temperature
was maintained between a preferred range of 40 and 55 degrees
Centigrade, although satisfactory results may be achieved over a
much wider temperature range of between 0 and about 100 C. Reynolds
numbers were calculated according to equations (5) and (6) under
different flow conditions. Using the apparatus described above,
various flow rates, molarities and current densities were tested
for their impact on particle consistency and quality.
[0091] The results of these tests are summarized in the graph of
FIG. 13. The graph plots regions of zinc particle quality as a
function of current density and ZnO molarity, with the Reynolds
number of the flow kept constant at approximately 3200, well within
the turbulent flow range. Current density is calculated as the
total load current divided by the sum of the active zone surface
areas. Good quality crystalline particles were produced while
operating the apparatus with ZnO concentrations in the preferred
range of about 0.1 M<[ZnO]<about 4.5 M, and current densities
in the preferred range of about 5,000 A/m.sup.2<I<about
40,000 A/m.sup.2. Current densities below about 10,000 A/m2
produced poor quality zinc. At very high current densities,
I>about 55,000 A/m.sup.2, the apparatus produced crystalline
zinc too brittle to maintain particle integrity. Current densities
I>about 30,000 A/m.sup.2 with low zincate concentrations, i.e.,
[ZnO]<0.4 M, also produced poor quality brittle, crystalline
particles. Dendritic formations occurred only at very low
concentrations, [ZnO]<about 0.2 M, and very high current
densities I>about 15,000 A/m.sup.2. Also, under laminar flow
conditions, where the Reynolds number, Re, was low, i.e.,
Re<about 1500, the system yielded zinc formations that were
dendritic and amorphous, regardless of molarity and current
density. These tests indicate that within the preferred ranges of
temperature, molarity, and current density, maintaining a velocity
of the solution sufficient to promote turbulent flow. For purposes
of this disclosure, terms such as "about" or "approximately" or
"substantially" or "near" are in intended to allow some leeway in
numeral exactness which is acceptable in the trade. Generally
speaking, these terms refer to variations of .+-.25% or less. Also,
for purposes of this disclosure, "turbulent" means sufficient
agitation or fluctuation to achieve the condition where there is
substantially no boundary layer between the solution and the
growing metal particles at the one or more active zones of the
cathode. Under this condition, transfer of dissolved metal atoms to
the surface of the growing particle is not mass transfer controlled
and the growth process is under kinetic control which provides a
particle morphology suitable for fuel cell applications. In one
embodiment, a turbulent flow is one where Re exceeds a transition
value in the range of between about 1,000 and about 10,000. In a
second embodiment, a turbulent flow is one where Re>about
1500.
[0092] Metal particle quality may also be enhanced by certain
chemical additives in the electrolyte. For example, adding bismuth
in the proportion 400 ppm Bi2O.sub.3 to 40 liters of electrolyte,
or adding indium in the proportion of 250 ppm In(OH).sub.3 to 40
liters of electrolyte, was found to generally improves particle
form and consistency.
[0093] Additionally, the force required to remove the particles
from the active zones was tested. For zinc particle formation on Mg
zones, it was determined that minimal force was required to
dislodge the particles.
[0094] Metal particle shape and quality also depends on the
construction of the cathode. For example, the morphology of the
metal particles may be affected by the surface area of the active
zones, and also by the spacing between active zones. To illustrate
the formation of particles on active zones, FIG. 14a shows a
magnified cross sectional view of one embodiment of a cathode
surface. Pins 1402 are shown protruding from conductive substrate
1404, and insulated from each other at surface 1406 by insulating
material 1408. A system operating as discussed above causes metal
particles 1410 to form on active zones generally outwardly and
upwardly, as shown.
[0095] A closer view of particle formation on the surface of an
active zone is shown in FIGS. 14b and 14c. In the absence of the
parametric constraints discussed above, and with active zones
having large surface areas widely spaced from one another, zinc
particle composition has been experimentally determined to result
from three phases of growth. These experiments were conducted using
cathodes having magnesium active zones approximately 0.5 mm in
diameter. In the initial phase, metal deposits form as individual
grains 1412 on the active zone 1414. The grains adhere weakly to
the active zone, and tend to develop weak bonds between other
grains. In the second phase, the metal deposits grow outwardly in
the form of six to eight crystalline lobes 1416, forming a total
diameter of about 0.6 to 0.8 mm. These lobes are anchored weakly to
one or more grains previously deposited, and do not bond to other
outwardly growing lobes. In the third phase, the lobes grow
upwardly in the form of columns 1418 as shown in FIG. 14a. The
columns are generally not joined to each other, but bond weakly to
the grain foundation, forming the general structure of particle
1410.
[0096] Metal particles that grow from grain foundations in this
fashion are not suitable for use in anode beds of metal/air fuel
cells. When these particles are generally subjected to mechanical
scraping or anodic dissolution, the weak adhesive forces between
the grains which make up the foundation of the particle are quickly
broken, and the particle disintegrates into many small grains of
about 200 microns in size, and into lobes of about 100 to 200
microns in diameter and 500 microns in length. In a fuel cell,
these fine particles tend to accumulate in the flow channels or at
the bottom of the anode bed. This leads to a reduction in
electrolyte flow and premature cell failure.
[0097] In order to eliminate grain foundations from metal particles
and promote the production of stronger particles, the surface area
of the active zones and the spacing between active zones should be
maintained within certain limits. To determine these limits,
cathodes having different active zone geometries were configured to
produce different batches of zinc particles. The particles were
then sieved to remove particles smaller than 0.38 mm. The remaining
particles were then subjected to a collision test by placing a 150
ml sample of the particles within a 45 wt % KOH solution and
circulating the mixture through a hydraulic circuit consisting of a
pump, a test cylinder, a ball valve, and conduit. After 4 hours of
operation, the particles were collected and again sieved. The
volume of particles smaller than 0.38 mm passing through the sieve
were recorded as a percentage of the initial volume. The results
showed that cathodes having active zones less than about 0.04
square mm spaced apart by less than about 2.0 mm (most preferably
less than about 1.0 mm) produced zinc particles that were most
resistant to disintegration. If circular, in one embodiment the
diameter of the active zones should be less than about 0.2 mm. In
one example, the diameter is about 0.15 mm. These are high quality
particles that tend to grow initially from lobes rather than from
grains. In addition, the lobes of these particles tend to bond
together, creating a metal particle that is coherent and
mechanically strong, but also of low superficial density and high
surface area. As a result, these particles have a high
electrochemical reactivity, and are therefore most suitable for use
in metal/air fuel cells and other industrial and chemical
processes.
[0098] A skilled artisan will recognize from a reading of this
disclosure that there are many ways to construct a cathode
according to the invention that is within the preferred limits for
active zone geometry. In one embodiment, illustrated in FIGS. 15a
(top view) and 15b (side view), the cathode surface comprises a
sliced planar cross section 1502 of an insulated bundle of wire
conductors 1504. Wires 1506 having the proper diameter may be held
at the appropriate spacing by a special fixture 1508, and then
surrounded by an insulating material 1510, for example, a thermoset
epoxy compound. The bundle is then cured and sliced perpendicular
to the axes of the wires to form wafers 1502 having the desired
active zone geometry in cross section. A wafer may then be
mechanically attached to a metal support plate by soldering,
conductive adhesive, or other means, thereby electrically coupling
the plate to the active zones 1512. Optionally, the assembly may
then be coated with another layer of insulation so that the only
exposed metal components are a bus connector and the active zones.
The cathode surface having the active zones may be machined to
create a smooth planar surface.
[0099] In another embodiment, the bundle may be produced by
combining successively larger bundles of partially cured insulated
wire. Multiple partially cured insulated wires, along with uncured
insulator, are grouped together and pulled through a heat and
pressure die to form a larger bundle with the proper cross
sectional geometry. Multiple bundles can be combined in similar
fashion with additional uncured insulator to form a single, larger
bundle. The final bundle is cured and sliced into wafers as
described above.
[0100] FIGS. 16a to 16c illustrate another embodiment of a method
for constructing a cathode according to the invention. This method
involves machining a metal plate 1602 to form a plurality of pins
1604 that protrude from the surface of plate 1602. FIG. 16a shows a
top view of one example of a plate 1602 machined to form pins 1604
in a generally square array, that is, a "pegboard" pattern of pins
regularly spaced in rows at right angles to columns, where each pin
is separated an equal distance from adjacent pins at its top,
bottom, left and right. FIG. 16b shows a side view of plate 1602.
FIG. 16c is a magnified side view of a portion of plate 1602,
showing pins 1604 protruding above the surface of plate 1602, and
separated by gaps 1606 machined between pins 1604. In one
embodiment, plate 1602 is machined mechanically. In another
embodiment, plate 1602 is machined by electric discharge machining.
After machining pins 1604, plate 1602 is coated with a curable
insulating material 1608, as shown in the magnified side view FIG.
16d. After curing, insulating material 1608 and pins 1604 are
further machined to form a smooth cathode surface 1610 having a
plurality of active zones (the ends of pins 1604) separated by an
insulator (cured and finished insulating material 1608). A
magnified side view of the finished cathode surface 1610 is shown
in FIG. 16e. In a preferred embodiment, for the production of zinc
particles, the pins are machined from magnesium plate in a
generally hexagonal array, and coated with a commercial epoxy
sealant to form the insulator. Zinc particles deposited on a
generally circular cathode may be dislodged effectively by means of
the rotary scraper described above.
[0101] Another embodiment of a method for constructing a cathode
according to the invention involves coining a pattern of active
zones onto a metal substrate, such as magnesium. In general, a
plate comprising the substrate is stamped using a closed die set
configured to impress the desired active zone geometry onto the
surface of the plate. The cathode surface is then coated with an
insulator and finished as described in previous embodiments. FIGS.
17a to 17g illustrate an alternative embodiment of the coining
method in which individual coins 1702 having active zones 1704 are
produced. In this embodiment, active zones 1704 are arranged in
hexagonal array. FIG. 17a illustrates a minimal number of active
zones in hexagonal array, where the nearest active zones 1704
surrounding any particular active zone 1706 form a hexagonal
pattern. That is, any single zone 1706 (except for zones on the
perimeter of the array) is centrally spaced among six adjacent
surrounding zones 1704 that are spaced equally from each other.
This pattern is maintained as more zones are added to the array, as
shown in FIG. 17b.
[0102] As shown in FIG. 17c, coins 1702 may generally comprise any
geometric shape that can regularly divide a plane, for example,
hexagonal, rectangular, or triangular shapes. Coins 1702 may be
manufactured from metal stock in the form of strips 1708 having
appropriate width (shown in FIG. 17d) and thickness (shown in FIG.
17e). For example, a strip 1708 may be prepared by covering one
surface with a masking material 1710, and plating the opposite
surface with a solderable or silver compatible metal 1712. After
plating, masking material 1710 is removed, and coining blanks 1714
are punched out of the strip in a desired shape, as shown in FIG.
17f. Each coining blank 1714 is then stamped with a pattern of
active zones 1704, to produce a finished coin 1702, for example,
the hexagonal coin depicted in FIG. 17g. The plated sides of coins
1702 are then assembled to a conductive support plate 1716 by
soldering or by means of a conductive bonding agent. When fully
assembled, coins 1702 and comprise a planar cathode surface as
depicted, for example, in FIG. 17h.
[0103] FIG. 18 illustrates another embodiment of a method for
constructing a cathode according to the invention. In this method,
an insulating area 1802 is chemically etched into the surface of a
metal plate 1804 to define raised active zones 1806, as shown in
FIG. 18a. A layer of insulating film 1808 is then added between
active zones 1806, as shown in a magnified side view in FIG. 18b.
In one embodiment, for the production of zinc particles, metal
plate 1804 is composed of a magnesium alloy about 0.25 inch thick.
By experimentation, this method yielded good results using
magnesium alloy KIA having about 0.7% Zr. In another embodiment,
active zones 1802 may be formed on an etched copper plate. The
copper areas 1810 that form the active zones may then be plated
with an additional layer of chromium 1812. Optionally, the copper
may be plated first with a layer of nickel 1814, followed by a
final layer of chromium 1812, as shown in FIG. 18c.
[0104] A flowchart of an implementation of this method is
illustrated in FIG. 19. First, in step 1902, KIA Mg plates about
310 mm square by 3 mm thick are prepared by grinding flat to a
tolerance of about 0.002 in. and polishing to an 8 micro inch
finish. Next, in step 1904, one side of the plate is plated with
tin by any conventional plating method. The side plated is the side
intended for eventual attachment to a conductive backing plate.
Next, in step 1906, the desired pin pattern is etched into the
surface of the Mg plate by a conventional etching technique. The
pattern advantageously defines the desired geometric spacing and
surface area for the active zones. In step 1908, the etched pin
pattern is laminated with a coating of insulating film. In the next
step, 1910, a partially cured conductive epoxy or the like is
laminated to the plated side of the Mg plate. This step may
optionally include attachment of a protective sheet to the
laminated layer of conductive epoxy. Then in step 1912, a planar
cathode is formed from the plate by punching or machining to
achieve a desired cathode shape. In step 1914, a hot platen press
is used to laminate the cathode form to the support plate and to
fully cure the epoxy. Next, in step 1916, a final coating of
curable insulation is used to encapsulate the entire assembly. The
insulator is cured in step 1918. Finally, step 1920 is performed to
remove a portion of the cured insulation by machining, sanding, or
polishing the cathode surface to expose the active zones and finish
the assembly.
[0105] Another implementation of a method of manufacturing a
cathode according to the invention comprises forming active zones
on a metal substrate by deposition of titanium nitride by means of
chemical vapor deposition. Titanium nitride is desirable for its
low surface energy which discourages other materials from bonding
to it. Metal particles forming on titanium nitride by
electrodeposition are therefore easily removable by application of
minimal force. The substrate may be composed of any metal suitable
for the purpose, for example, copper, nickel, stainless steel,
magnesium, or aluminum. Active zones formed in this manner yield
titanium nitride sites in the range of 0 to 1000 micrometers in
height. In one embodiment, an insulating film of tantalum oxide,
about 20 to 100 micrometers in height, is formed between the active
zones and bonded to the substrate to complete the cathode
surface.
[0106] A flowchart of an embodiment of a method of operation of a
system for producing metal particles according to the invention is
illustrated in FIG. 20. In step 2002, a solution including
dissolved metal is contained, for example, within a container as
described in any of the above figures. Next, in step 2004, the
temperature of the solution is maintained between 0 and about 100
degrees C. An anode as described above is at least partially
immersed in the solution in step 2006. Similarly, in step 2008, a
cathode configured with one or more active zones is at least
partially immersed in the solution. The cathode may comprise any of
the aforedescribed, or similar, embodiments that is complimentary
to the anode of step 2006.
[0107] With the anode and cathode immersed in solution within the
container, step 2010 is performed to effect and maintain a
turbulent flow of the solution past one or more active zones of the
cathode. The velocity of the flow is at a level sufficient to avoid
dendrite formation on the active zones. In one embodiment, the flow
achieves a Reynolds number greater than about 1500. In another
embodiment, the flow velocity is any velocity sufficient to produce
turbulent flow that promotes good quality particle growth, i.e.
non-brittle crystalline particles free of dendrite formations. In
another embodiment wherein the solution comprises dissolved metal
in electrolyte, the flow velocity is maintained between about 15
and about 20 gallons per minute.
[0108] Next, in step 2012, an electric potential is applied across
the anode and cathode sufficient to create a current density in the
active zones greater than about 5 kA/m.sup.2. In one embodiment,
the current density is maintained in the range between about 10
kA/m.sup.2 and 40 kA/m.sup.2. Through the foregoing steps, metal
particles of a desired size are allowed to form on the active zones
of the cathode in step 2014. In one embodiment, this step occurs by
predetermining a time period which is sufficient to allow particles
of a desired size to form in a particle production system according
to the invention, loading the predetermined time period into a
timer, and then operating a metal production system according to
the invention until a time out condition is detected, at which
point, particle growth is ceased.
[0109] In another embodiment of a method according to the
invention, step 2002 may further comprise containing a solution
having a molarity sufficient to promote good quality particle
formation. For zinc particle formation from potassium zincate
solution, the molarity should be in the range of about 0.1 M to
about 4.5 M. In another embodiment, this step further comprises
maintaining the molarity within the desired range during an entire
operating cycle of the system.
[0110] FIG. 21a is a flowchart of an embodiment of a method
according to the invention for removing metal particles from the
active zones of a cathode by scraping. In step 2102, it s
determined when the particles have grown to a desired size. In one
implementation, this can be accomplished visually, or by expiration
of a time out condition as previously discussed. Next, step 2104 is
performed. In step 2104, the scraper and cathode are relatively
positioned so that the scraper effectively engages the surface of
the cathode for purposes of particle removal. This step may be
accomplished by positioning a cathode relative to a stationary
scraper, by positioning a scraper relative to a stationary cathode,
or both. Step 2106 then occurs. In step 2106, the particles are
dislodged by relative motion between the scraper and the cathode
surface.
[0111] FIG. 21b is a flowchart of a second embodiment of a method
according to the invention for removing particles from the cathode
surface. This embodiment is applicable to a cathode in which the
non-conductive material forms a perforated layer of insulation on
the surface of the conductive material, and in which relative
motion between the cathode conductive material and the conductive
material is permitted. In step 2108, it is determined whether metal
particles of a desired size have grown. Again, in one
implementation, this step may occur through visual observation and
through detection of a time out condition. Step 2108 is followed by
step 2110. In step 2110, the particles are dislodged by relative
motion between the conductive and non-conductive portions of the
cathode surface.
[0112] Referring again to FIGS. 21a and 21b, other embodiments of a
method according to the invention may further comprise additional
steps for directing the particles dislodged in either step 2106 or
2110 into a collection area. One such embodiment comprises
directing the particles by entraining them within a flow of the
solution. Another embodiment comprises directing the dislodged
particles by means of gravity. In either of these methods, the
dislodged particles may then be collected or allowed to accumulate
in a collection area, and eventually recovered for transport to a
storage device or injected directly into a metal/air fuel cell
thereby recharging the cell.
[0113] The various apparatus and methods presented throughout the
foregoing discussion provide means for producing metal particles
under controlled conditions. Operating systems such as those shown
in FIGS. 1, 5 and 11 may yield high quality metal particles
provided that parameters such as flow velocity, current density,
temperature, and molarity are maintained within the preferred
ranges. In addition, manufacturing tolerances for the cathode
surface may be controlled to minimize variations in current
densities in the active zones. Variations in the size of active
zones of one cathode, which may range between 10 and 20%, can lead
to substantial variations in the aggregate electrochemically active
surface area of the one cathode as compared to other cathodes
produced by the same manufacturing method. As a consequence, a
power supply operating at a fixed output voltage, or otherwise
configured to maintain current density within a preferred range for
one cathode may be unable to maintain current density within the
same range for another cathode. Thus, in order to counteract the
effects of manufacturing variations among cathodes, additional
system controls may be necessary for a system according to the
invention to optimize its yield of high quality particles.
[0114] As an example, consider a 14.5 cm.times.30 cm standard
cathode having about 163,000 pins, which may be produced according
to any of the above methods. Using a constant-current power supply,
an optimal current density has been determined experimentally using
the standard cathode in a system as described herein for
zinc-particle production. Under conditions that maintain process
parameters within the preferred ranges, the standard cathode will
produce zinc particles of highest quality at a current density of
about 34,340 A/m.sup.2. However, a second cathode manufactured by
the same process and coupled to the same constant-current power
supply may not produce particles of like quality if the second
cathode has an electroactive area that varies substantially from
the area of the standard cathode. For example, if the second
cathode comprises an electroactive area 20% greater than standard,
the current density will be accordingly reduced. As illustrated in
the graph of FIG. 13, low current densities can result in poorer
particle quality, especially at high zincate concentrations where
zinc deposits tend to form dendritic bridges between active zones.
These dendrites cause the particles to fuse together, making the
particles more difficult to remove from the cathode surface.
Particles that remain fixed to the cathode surface continue to grow
and eventually lead to system failure by short circuiting the
cathode to the anode. If the short cannot be removed by repeated
action of the scraper, then the electrolyzer unit must be
disassembled for cleaning.
[0115] The short-circuiting problem may be exacerbated by the
presence of irregular areas on the active zones. This phenomenon is
illustrated in FIG. 22a, which is a magnified cross-sectional view
of an electrolyzer 2200 showing cathode surface 2206 separated a
distance G from anode 2216. Cathode surface 2206 comprises active
zones 2202 on pins 2204 separated by insulator 2208. Active zones
2202 may contain one or more irregular areas 2240 comprising tiny
cracks or notches that penetrate a pin 2204. Areas 2240 may occur
during the normal course of manufacturing due to physical stresses
on pins 2204. During electrolysis, areas 2240 provide anchoring
points for metal particles 2210 growing on active zones 2202. When
particles 2210 are removed from surface 2206 by scraping, a small
amount of metal from a pin 2204 may be torn away from the site of
an area 2240. After repeated cycles of electrodeposition and
scraping, areas 2240 enlarge, allowing subsequently deposited
particles 2210 to become further embedded within active zones 2202.
Eventually, active zones 2202 become so damaged that embedded
particles can no longer be removed by scraping. These particles
become larger with each growth cycle and eventually short circuit
cathode 2206 to anode 2216, thereby rendering the electrolyzer
inoperable.
[0116] A cathode having an overabundance of irregular areas 2240
may be mechanically reconditioned to remove the outermost layer of
the cathode surface and thereby extend the service life of the
electrolyzer. FIG. 22b shows a magnified cross-sectional view of a
cathode surface 2206(b) of electrolyzer 2200 after reconditioning.
Reconditioning a cathode surface such as 2206 may comprise
grinding, sanding, buffing, polishing, or other suitable means,
until a sufficient layer of material has been removed to produce a
surface substantially free of irregular areas. In the example of
FIG. 22b, cathode surface 2206 has been reconditioned to remove an
outer layer of thickness d to produce a smooth surface 2206(b)
comprising pins 2204(b) separated by insulator 2208(b). Removal of
cathode material of a thickness d on the order of about 0.01 mm to
0.001 mm should adequately recondition the cathode. Reconditioning
may be accomplished using an accessory attachable to the particle
scraping means. In other cases, reconditioning may require
disassembly of the electrolyzer and removal of the cathode. The
cathode may then be reconditioned by a separate process before
reassembly into the electrolyzer.
[0117] FIG. 23 illustrates a method 2300 of the present invention
for operating of a metal particle electrolyzer to reduce the
probability of short circuit failure. Method 2300 provides a
consistent current density through the electrolyzer by monitoring
the cell voltage developed across the anode and cathode when the
electrolyzer is energized. In the first step 2302, an operating
range for the cell voltage of an electrolyzer is determined. This
range may be determined empirically for a particular electrolyzer
design. For example, an optimal cell voltage may be selected based
on operating experience, and an operating range selected to be
within 20% of the optimal value. Alternatively, an operating range
may be calculated as a function of the preferred range of current
density. In step 2304, electrical current is supplied to the
electrolyzer by means of a power supply, and the process of
particle production commences, as previously described. Ideally,
energizing the electrolyzer in this fashion will produce a cell
voltage within or near the operating range that was determined in
the previous step. In step 2306, the cell voltage is monitored.
This may be accomplished, for example, by connecting low-resistance
test leads to test points 2250 and 2260 of FIG. 22a to measure the
potential difference between anode surface 2216 and a conductive
section of cathode surface 2206. Referring again to FIG. 23, step
2308 is performed. In step 2308, current output from the power
supply is adjusted responsive to the monitored cell voltage, as
necessary to maintain the cell voltage within the operating range.
This step may be achieved by configuring the power supply as a
voltage-controlled current source by a variety of techniques well
known in the art. After performing step 2308, method 2300 loops
back to step 2304 and may continue as desired.
[0118] FIG. 24 illustrates a method 2400 according to the invention
for operating a metal particle electrolyzer to reduce the
probability of short circuit failure, and to extend the service
life of the electrolyzer. Method 2400 extends electrolyzer service
life by dissolving metal particles that become embedded on the
cathode surface before the commencement of another growth cycle.
The initial step 2402 comprises immersing an electrolyzer into a
solution that includes dissolved metal. As described herein in
previous embodiments, the electrolyzer comprises anode and cathode
surfaces, and is configured to produce metal particles through
electrolysis. In the next step 2404, a DC voltage is applied to the
electrolyzer, the electrolysis process begins, and metal particles
begin to form on the cathode surface. During this step, some of the
particles may become embedded within irregularities in the cathode
surface. In the next step 2406, when the particles achieve a
desired size, particles that are not embedded are removed from the
cathode surface by any of the removing means previously disclosed.
The embedded particles are removed in the next two steps. First, in
step 2408, the DC voltage being applied to the electrolyzer is
reduced to a value, less than about 1.65 volts, that will preclude
oxygen from evolving on the active zones. In an embodiment where
the electrolyzer produces zinc particles from an electrolyte
containing zincate ions according to equation (3), the DC voltage
may be reduced in step 2408 to about 1.0 volt. Next, in step 2410,
the polarity of the applied voltage is reversed, such that the
electrode that was formerly the cathode achieves a higher
electrical potential then the electrode that was formerly the
anode. Reversing the polarity causes zinc particles that are
embedded in the active zones to dissolve into positively charged
zinc ions which migrate toward, and eventually plate to, the
negatively charged electrode that was formerly the anode. Reverse
polarity is maintained for a time period of about 60 to about 300
seconds, or until embedded particles are substantially dissolved.
After performing step 2410, method 2400 loops back to step 2404 and
another growth cycle may begin. Thus, during each growth cycle,
method 2400 dissolves particles that become embedded in the active
zones, thereby preventing excessive particle formation that may
cause premature failure of the electrolyzer.
[0119] Another embodiment of the invention for operating an
electrolyzer is illustrated in the flow chart of FIG. 25. Method
2500 begins with step 2502, which comprises immersing a metal
particle electrolyzer within a body of electrolyte solution that
includes a dissolved metal. For example, the electrolyte may
comprise a KOH solution including zincate ions and dissolved ZnO.
Next, in step 2506, electric current is supplied to the
electrolyzer using a power supply configured as in embodiments
previously discussed. The flow of current through the electrolyzer
creates a voltage potential across the electrolyzer cell.
Preferably, the initial power supplied to the electrolyzer is
accurately estimated to create a cell voltage within a
predetermined range that is consistent with a preferred value for
current density through the electrolyzer cathode. Next, in step
2506, the cell voltage is monitored across the electrolyzer anode
and cathode, and fed back to the power supply. In step 2508, the
current output of the power supply is adjusted responsive to the
monitored voltage to maintain the cell voltage within the
predetermined range. In the next step 2510, metal particles are
removed from the cathode surface after the particles have achieved
a desired size. Particle removal may be accomplished by scraping,
or by any other suitable method. After particles are removed, step
2512 is performed, wherein the cell voltage is reduced to a value
below a standard cell potential. Next, in step 2514, the polarity
of the cell voltage is reversed a first time. Reversing the
polarity causes metal particles embedded within irregular areas of
the active zones to dissolve and plate out on the surface of the
electrode that was formerly the anode. Reverse polarity is
maintained for a time period until embedded particles are
substantially dissolved from the active zones. In step 2516, during
application of reverse polarity voltage, peak current through the
electrolyzer is monitored. As the size and number of irregular
areas in the cathode surface increase, the volume of particles
embedded within the surface will also increase, thereby reducing
the initial electrical resistance to reverse-polarity current flow.
The value of peak current is therefore an indication of the
condition of the cathode surface. The measured value of peak
current is then compared to empirical data in decision block 2518.
If the peak current exceeds a selected operating limit (i.e. a
predetermined value of current based on empirical data) that
indicates excessive build-up of embedded particles, the method
proceeds to step 2520. In step 2520, power to the electrolyzer is
shut off, and in step 2522, the electrolyzer cathode is
mechanically reconditioned. If in decision block 2518, the peak
current does not exceed the operating limit, the method loops back
to step 2504, the polarity is reversed a second time to restore the
original polarity, and another growth cycle begins.
[0120] From the foregoing, it will be seen that embodiments of the
invention are possible in which particles are produced having a
size that is related to the size of the surface area of the active
zones of a cathode. This factor in turn promotes consistent
production of particles within a predetermined size range. In
addition, embodiments are possible in which 1) the particles which
are produced can be used directly in a metal/air fuel cell without
first having to sort the particles by size; 2) seed particles are
not required to initiate particle growth; 3) operation thereof
occurs at high current densities, thereby enabling construction of
a compact, efficient device with a high rate of particle output; 4)
operation thereof occurs at high current density and high liquid
flow rate, thereby producing high quality crystalline metal
particles over a wide range of reaction solution/dissolved metal
concentrations; or 5) the metal particles that are produced are
coherent and mechanically strong but also of low density and high
surface area and therefore of high electrochemical reactivity.
[0121] Skilled artisans will appreciate that the aforedescribed
method is not limited to the recovery of zinc from alkaline
solution. By appropriately adjusting the various process
parameters, the method may be exploited for the recovery of other
metals, for example, magnesium, aluminum, calcium, nickel, copper,
cadmium, tin, or lead dissolved in a suitable electrolytic
solvent.
[0122] While various embodiments of the invention have been
described, it will be apparent to those of ordinary skill in the
art that many more embodiments and implementations are possible
that are within the scope of this invention. Accordingly, the
invention is not to be restricted except in light of the attached
claims and their equivalents.
* * * * *