U.S. patent application number 11/520961 was filed with the patent office on 2007-06-07 for reduction of the loss of zinc by its reaction with oxygen in galvanized steel and batteries.
This patent application is currently assigned to Board Of Regents, The University Of Texas System. Invention is credited to Adam Heller, Woonsup Shin.
Application Number | 20070125644 11/520961 |
Document ID | / |
Family ID | 37889337 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070125644 |
Kind Code |
A1 |
Heller; Adam ; et
al. |
June 7, 2007 |
Reduction of the loss of zinc by its reaction with oxygen in
galvanized steel and batteries
Abstract
A non-porous Zn.sup.2+ conducting inorganic lamellar layer is
formed on the zinc coating of galvanized steel or on a zinc anode
of an electrochemical cell. The layer reduces the rate of the
unwanted chemical reaction of zinc and oxygen but allows desired
electrochemical reactions underlying the cathodic protection of the
steel and the efficient utilization of zinc anodes in
electrochemical cells, e.g., a physiological buffer solution or
serum as their electrolytes. The ion conducting non-porous lamellar
layer having a hopeite phase Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O may
be formed spontaneously on, e.g., NAFION.RTM. coated zinc anodes
discharged in neutral pH saline phosphate solutions.
Inventors: |
Heller; Adam; (Austin,
TX) ; Shin; Woonsup; (Kyungki-do, KR) |
Correspondence
Address: |
CHALKER FLORES, LLP
2711 LBJ FRWY
Suite 1036
DALLAS
TX
75234
US
|
Assignee: |
Board Of Regents, The University Of
Texas System
Austin
TX
|
Family ID: |
37889337 |
Appl. No.: |
11/520961 |
Filed: |
September 14, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60718158 |
Sep 15, 2005 |
|
|
|
Current U.S.
Class: |
204/290.01 |
Current CPC
Class: |
H01M 4/244 20130101;
Y02E 60/10 20130101; B32B 15/018 20130101; H01M 8/16 20130101; C22C
18/00 20130101; C25D 17/10 20130101; H01M 4/12 20130101; Y02E 60/50
20130101; H01M 4/366 20130101; H01M 12/06 20130101 |
Class at
Publication: |
204/290.01 |
International
Class: |
C25C 7/02 20060101
C25C007/02 |
Goverment Interests
[0002] The U.S. Government may own certain rights in this invention
pursuant to the terms of the Naval Research Grant
(00014-021-1-0144).
Claims
1. A composition comprising: a phosphate anion and one or more
metal cations that form a non-porous inorganic lamellar layer on a
substrate surface, wherein the lamellar layer allows the transport
of at least one ion at 25.degree. C.
2. The composition of claim 1, wherein the non-porous lamellar
layer comprises hopeite.
3. The composition of claim 1, wherein the non-porous lamellar
layer is substantially oxygen impermeable.
4. The composition of claim 1, wherein the non-porous lamellar
layer conducts Zn.sup.2+ ions.
5. The composition of claim 1, wherein the non-porous lamellae
layer further comprises nickel, magnesium, manganese, calcium,
cobalt, copper, magnesium or mixtures and combinations thereof.
6. A substrate comprising: a first inner metal and a second outer
metal, the surface of the second outer metal reacted to form a
non-porous lamellar layer that comprises a compound formed of a
phosphate anion and one or more metal cations, the lamellar layer
allowing the transport of at least one ion at 25.degree. C.
7. The composition of claim 6, wherein the non-porous lamellar
layer is substantially oxygen impermeable.
8. The composition of claim 6, where the first inner metal is an
alloy of iron or copper.
9. The composition of claim 8, where the alloy of iron is
steel.
10. The composition of claim 6, where the second outer metal is
zinc of galvanized steel.
11. A method of surface treating a corrodible metal comprising the
steps of: coating a corrodible metal with a non-porous lamellar
film comprising an inorganic phosphate of one or more metal cations
to form a substantially impermeable film that provides an
anticorrosion effect on the corrodible metal.
12. The method of claim 11, wherein the non-porous lamellar film
comprises hopeite.
13. An electrical power generating electrochemical cell comprising:
an electrolyte providing for ion transport between a cathode and a
zinc anode, wherein the zinc anode comprises a non-porous inorganic
lamellar film of a compound of phosphate and one or more metal
cations, the film preventing or reducing non-Faradaic corrosion of
the zinc anode.
14. The cell of claim 13, wherein the lamellar film prevents, or
reduces the rate of, the reaction of oxygen of with the zinc of the
zinc anode.
15. The cell of claim 13, wherein the substantially non-porous
lamellar film is hopeite.
16. The cell of claim 13, wherein the substantially non-porous
lamellar film conducts Zn2+ ions.
17. The cell of claim 13, wherein the electrolyte comprises a body
fluid.
18. The cell of claim 13, wherein the cathode and the zinc anode
are implanted in an animal.
19. A zinc anode electrode protected against non-Faradaic corrosion
by an ion conducting inorganic lamellar layer that is formed by
passing a current through the electrode immersed in a phosphate
containing solution to form a non-porous hopeite lamellar layer on
the zinc electrode.
20. The electrode of claim 19, wherein the lamellar layer comprises
hopeite.
21. A composition comprising: a cation exchanger coated on a metal
substrate; and an inorganic lamellar layer formed on or in the
cation exchanger that is substantially oxygen impermeable and that
allows the transport of one or more metal ions, wherein the
inorganic lamellar layer comprises one or more phosphate anions and
one or more metal cations, whereby the cation exchanger influences
relative growth of the inorganic lamellar layer.
22. The composition of claim 21, wherein the lamellar layer
comprises hopeite and the metal substrate comprises zinc.
23. A method of surface treating a corrodible metal comprising the
steps of: coating a corrodible metal with a cation exchanger;
forming an inorganic lamellar layer on or in the cation exchanger
that allows the transport of one or more metal ions and that
provides an anticorrosion effect on the corrodible metal, wherein
the inorganic lamellar layer comprises one or more phosphate anions
and one or more metal cations, whereby the cation exchanger
influences relative growth of the inorganic lamellar layer.
24. The method of claim 23, wherein the lamellar film comprises
hopeite and the corrodible metal comprises zinc.
25. A method of forming a metal electrode that is protected against
non-Faradaic corrosion comprising the steps of: immersing a cation
exchanger coated metal electrode in a phosphate containing
solution; and forming an inorganic lamellar layer on or in the
cation exchanger that allows the transport of one or more metal
ions, whereby the cation exchanger influences relative growth of
the inorganic lamellar layer.
26. The method of claim 25, wherein the non-porous lamellar film
comprises hopeite and the cation exchanger coated metal electrode
comprises zinc.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application Ser. No. 60/718,158, filed Sep. 15, 2005, the contents
of which are incorporated by reference herein in its entirety.
TECHNICAL FIELD OF THE INVENTION
[0003] The present invention relates in general to the prevention
or alleviation of loss of zinc in anodes by its reaction with
oxygen. The prevention of such loss is of essence for high the
utilization efficiency of the zinc in galvanized steel. Here the
zinc cathodically protects the steel against corrosion. Protection
of metal corrosion is of particular use in batteries with zinc
anodes, particularly batteries having small, high surface to volume
ratio, zinc anodes.
BACKGROUND OF THE INVENTION
[0004] Without limiting the scope of the invention, its background
is described in view of utilization of the zinc anodes in
galvanized steel and in batteries.
[0005] Galvanized steel is one of the most widely used structural
metals. For example, bodies of automobiles are made of galvanized
steel. Galvanized steel includes, in addition to its steel, which
provides strength and other desired mechanical characteristics, a
layer of zinc, which protects the structural steel against
corrosion. The zinc is usually phosphated. In the phosphating
process, hydrated zinc phosphates are formed on the surface of the
zinc. Galvanized steels are generally made by dipping the steel in
molten zinc, a method assuring excellent bonding of the zinc to the
steel. Alternatively, galvanized steels are made by electroplating
zinc on the steel. Other methods, such as high rate evaporation,
sputtering or electroless plating are also possible.
[0006] Generally, the process of steel corrosion is initiated by an
electrical potential difference between regions of the surface that
come in contact with an electrolyte (e.g., moisture condensing from
the air). An electrical current begins to flow and iron ions are
formed that result in loose flaky rust (i.e., iron (III) oxide).
The electrons generated in the Faradaic reaction of zinc keep the
iron reduced to the metal, i.e., prevent its oxidation to iron
oxide, by shifting the initial corrosion reaction
FeFe.sup.2++2e.sup.- to the left. Techniques currently used in the
art include a zinc coating that is metallurgically bonded to the
surface and protects against corrosion by shielding the base from
the atmosphere and acting as a sacrificial protection layer that is
gradually consumed (e.g., the zinc is more electronegative than
iron or steel and provides cathodic protection).
[0007] The zinc coating may be applied by hot dip galvanizing,
electroplating, evaporating or other methods known in the art. For
example, one process of zinc coating is a hot dip galvanizing
process that involves: cleaning (i.e., generally an alkaline
solution) and pickling (i.e., a pickling solution is generally
hydrochloric or sulfuric acid) at free the surface of dirt, grease,
rust and scale; a preflux step to dissolve oxides formed on the
surface after pickling and to prevent further rust formation; a
coating step of immersion into molten zinc; and a quenching step. A
patina of zinc oxide, zinc hydroxide, and zinc carbonate will form
on the zinc surface.
[0008] The zinc coating of galvanized steel cathodically protects
the steel against corrosion. When the steel is cathodically
protected, the oxidative corrosion of steel, evident, for example,
when the steel is rusting, is slowed or prevented, while zinc of
the coating is sacrificially oxidized to Zn.sup.2+. The Zn.sup.2+
is usually precipitated, for example, as a carbonate, oxide, or,
when an anion is present, as a zinc salt of the anion.
[0009] After the steel is galvanized, it is often phosphated. The
phosphating process involves the immersion of the steel in a
phosphate containing bath. Phosphating, or phosphatizing, are known
to produce films containing, or having, hopeite; however, as was
shown, for example by Miles in U.S. Pat. No. 4,330,345, the
structure, morphology, and size of the crystallites of different
phases in the films affect their properties.
[0010] It is well known that phosphating often produces a film
containing, or having, hydrated zinc phosphate, which protects the
protective zinc. The hydrated zinc phosphate often is, or may
include, some hopeite, the chemical composition of which is
Zn.sub.3(PO.sub.4).sub.2. 4 H.sub.2O.
[0011] Numerous methods and formulations of compositions for
phosphating the zinc layer of galvanized steel and forming on it
hopeite containing films are known. A few, such as those of T.
Sugama, U.S. Pat. No. 5,604,040 and T. Nakayama et al., U.S. Pat.
No. 6,478,860, disclose adding to the phosphating solution, about
0.5 to 5.0% by weight of water soluble polycarboxylates, such as
polyacrylates acid, polymethacrylate and nucleating components;
however, the polyanions were not applied as films on the zinc
anodes, unlike in the method disclosed here.
[0012] When Zn cathodically protects steel, the electrons generated
in the anodic reaction Zn.fwdarw.Zn.sup.2++2e.sup.- reverse the
initial step of the corrosion reaction of iron,
Fe.fwdarw.Fe.sup.2++2e.sup.-, the iron being poised now at the
thermodynamic potential of the zinc anode, where it could, instead,
be plated, Fe.sup.2++2e.sup.-.fwdarw.Fe. The necessary
electrochemical half cell reaction, Zn.fwdarw.Zn.sup.2++2e.sup.-,
can not proceed unless microscopic charge neutrality was maintained
in the electrooxidation of the Zn. Maintenance of charge neutrality
was thought to require either transport of electrolytic solution
phase anions to the zinc anode or electrolytic solution phase
transport of Zn.sup.2+ from the anode. An electrolytic solution
could not access, however, the anode surface in absence of defects,
such as pores, in the film on the zinc anode, which was mostly
hopeite in galvanized steel, and zinc oxide, or hydrated zinc
oxide, in battery anodes. When pores, or other solution access
providing defects are present in a film, dissolved oxygen also
reaches the electrochemically active metallic zinc surface. Because
oxygen reacts non-Faradaically with the zinc, it reduces the
utilization efficiency of the zinc that cathodically protects the
steel in galvanized steel, or is available for the desired cell
reaction in a battery.
[0013] The foregoing problems have been recognized for many years
and while numerous solutions have been proposed, none of them
adequately addresses all of the problems in a single device.
SUMMARY OF THE INVENTION
[0014] The inventors discovered materials and methods for
maintaining microscopic electrical neutrality at an operating zinc
anode, without providing access of an aqueous electrolytic solution
to the surface of the zinc anode. Usually, the maintenance of
microscopic electrical neutrality requires access of liquid
electrolyte to the zinc anode through pores in the film on the zinc
anode passivating it against rapid reaction with dissolved oxygen
and/or water. After the zinc is phosphated, access of ions to or
from the zinc must be provided. Most commonly, the access is
through pores or other defects in the phosphated zinc layer, filled
with the electrolytic solution. The solution, including that in the
pores, dissolves oxygen, which oxidizes the zinc. The oxidation of
zinc by oxygen is an undesired non-Faradaic reaction, reducing the
coulombic efficiency of zinc utilization in the cathodic protection
of steel. The inventors increased the zinc utilization efficiency
of zinc anodes operating in oxygen containing environments by
forming in their phosphating process a substantially non-porous
Zn.sup.2+ ion conducting hopeite film allowing the discharge of
zinc anodes in absence of contact between the aqueous solution and
the electroactive zinc. Specifically, the inventors discovered that
pore-free crystals of hopeite can be solid state conductors of
Zn.sup.2+ and possibly also of other, ions. The transport of
Zn.sup.2+ through solid hopeite allows the desired Faradaic
reaction Zn.fwdarw.Zn.sup.2++2e.sup.-. At the same time, because
hopeite does not dissolve oxygen and because oxygen does not
permeate through hopeite, the film prevents the undesired
non-Faradaic reactions 2 Zn+O.sub.2.fwdarw.2 ZnO and/or 2
Zn+O.sub.2+H.sub.2O.fwdarw.2 Zn(OH).sub.2.
[0015] It was found that the coulombic efficiency of zinc
utilization is highest when the zinc anodes are coated with
non-porous or minimally porous hopeite. Minimal porosity and good
Zn.sup.2+ conductivity are best achieved when the hopeite crystals
on the Zn anodes are large but thin, which is the case when the
crystals are lamellar. In general, the widths and the lengths of
the hopeite lamellae are greater than about 2 micrometers, more
preferably they are greater than about 5 micrometers and most
preferably they are greater than about 10 micrometers. The ratio of
their height, or thickness, to their width or length is less than
about 1/3, is preferably less than about 1/5 and is most preferably
less than about 1/10.
[0016] The inventors further discovered that the Zn.sup.2+
conductive hopeite lamellae are produced on Zn anodes discharged in
safe and easy to handle, environmentally friendly, solutions. The
preferred solutions are dilute, neutral or near neutral pH,
phosphate solutions, containing alkali metal or tetraalkyl ammonium
salts, of which simple Na.sup.+ or/and K.sup.+ salts are preferred,
and the simplest and least expensive Na.sup.+ salts, NaCl or
Na.sub.2SO.sub.4, are most preferred.
[0017] To produce the densely packed, non-porous film of large
lamellar hopeite crystals, the inventors pre-coated their Zn anodes
with a thin film of a cation exchanger. They hypothesized, that in
combination with the Na.sup.+ ions in the solution, which compete
with the Zn.sup.2+ ions for the anionic sites of the cation
exchanging film, the cation exchanger controls the relative rates
of hopeite nucleation and growth of the hopeite crystals, which
depend on the local Zn.sup.2+ concentration. However, it should be
understood that particular embodiments described herein are shown
by way of illustration and not as limitations of the invention.
[0018] The pre-coated cation exchangers can be polycarboxylates,
such as polyacrylates or polymethacrylates, polysulfonates or
polyphosphonates. Of these, polysulfonates such as NAFION.RTM. and
other thermostable and chemically stable polysulfonates, useful in
membranes of acid electrolyte fuel cells, described, for example,
in U.S. Pat. Nos. 6,423,784, 6,559,237, 6,649,295, 6,833,412,
7,060,738 or 7,060,756, are preferred.
[0019] After the inventors discovered that the non-Faradaic
corrosion of a fluorinated sulfonic acid polymer (e.g.,
NAFION.RTM.) coated zinc anodes is drastically reduced at neutral
pH both in a physiological saline buffer solution (pH 7.4, 0.15 M
NaCl, 20 mM phosphate) under air and also in serum under air, when
the anodes are overgrown by large non-porous lamellae of hopeite
[Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O] which are impermeable to
O.sub.2, they demonstrated the operation of a hopeite protected low
rate zinc anode in a potentially implantable in-vivo power
supplying electrochemical cell. The cell, using as an exemplary
cathodic reactant silver chloride, also uses, optionally, a body
fluid as its electrolyte. The use of a body fluid as the
electrolyte obviates the requirement of a case, and allows the
formation of in-vivo electrochemical cells having merely of an
implanted, miniature, zinc anode and a miniature silver chloride,
or other, cathode.
[0020] In the context of potentially implantable biofuel cells, the
inventors observed that the Zn.sup.2+ conducting hopeite protected
against non-Faradaic corrosion Zn fiber anodes in calf serum, an
exemplary body fluid of an animal. A miniature Zn |serum| Ag/AgCl
cell, made with a fine, about 100 micrometer diameter and about 2
cm long Zn fiber anode, operated at about 1.00 V and about 13 .mu.A
cm.sup.-2 for about 2 weeks, at about 60% zinc utilization
efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] For a more complete understanding of the features and
advantages of the present invention, reference is now made to the
detailed description of the invention along with the accompanying
figures and in which:
[0022] FIGS. 1A, 1B and 1C are schematics of the preparation of the
coated zinc anodes of the present invention;
[0023] FIGS. 2A and 2B are optical micrograph images of the tips of
the Zn anodes of the present invention;
[0024] FIG. 3 is a plot showing the identity of X-ray diffraction
patterns of the present invention and a commercial
Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O;
[0025] FIGS. 4A, 4B and 4C are images of the showing the dimensions
and morphologies of the coatings formed at the completion of the
discharge on the zinc anodes;
[0026] FIG. 5 is an electron micrograph of the non-porous hopeite
lamellae overgrowing the coated zinc fiber anode;
[0027] FIG. 6 is a plot illustrating the time dependence of the
potential of the discharged Zn anode discharged at a current
density of about 13 micoamperes per square cm;
[0028] FIG. 7 is a cyclic voltammogram that is indicative of the
blockage of O.sub.2 transport to the Zn anodes;
[0029] FIG. 8 is a structural model of hopeite
Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O; and
[0030] FIG. 9 is a schematic diagram of a case-less miniature
Zn(NAFION.RTM.)--AgCl battery operating in the subcutaneous
interstitial fluid.
DETAILED DESCRIPTION OF THE INVENTION
Glossary of Terms
[0031] Terms such as "a", "an" and "the" are not intended to refer
to only a singular entity, but include the general class of which a
specific example may be used for illustration. The terminology
herein is used to describe specific embodiments of the invention,
but their usage does not delimit the invention, except as outlined
in the claims.
[0032] Bath refers to a solution that directly contacts the
substrate. The term "Bath" is not intended as a limitation of the
manner of application of coating which generally can be applied to
the substrate by various techniques, e.g., nonexclusive examples
include immersion, dipping, spraying, placing the substrate into
the bath, intermittent spraying, flow coating, and combined methods
such as spraying-dipping-spraying, spraying-dipping,
dipping-spraying or combinations thereof.
[0033] Electrolyte denotes a usually electronically insulating
substance through which an electric current is carried through the
motion of ions.
[0034] Steel: An alloy of iron, containing carbon, and, optionally,
other, usually metallic or semiconducting, elements.
[0035] Cathodic protection means the reduction of the rate of
oxidative corrosion of a metal, such as steel, by the application
of a reducing potential. Although the potential may be applied
using a source of DC or rectified AC power, it is more convenient
to apply it by electrically contacting the protected metal, such as
steel, with another metal having a more reducing electrovhemical
potential in the solution in which both metals are immersed, such
as zinc or an alloy of zinc. When the zinc is in electrical contact
with the steel, the steel is cathodically protected by the
zinc.
[0036] Zinc or Zn: In addition to the metal itself, which is
generally most preferred, the term includes also alloys and
mixtures of zinc, where the atomic fraction of zinc is not less
than about 0.5, is more preferably not less than about 0.7, and is
most preferably greater than about 0.9. Exemplary metals in alloys
or mixtures of Zn are Fe, Mn, Co, Ni, Ca, Mg, and/or Ti.
[0037] Galvanized steel: Steel, preferably steel sheet, coated with
zinc. The coating is, optionally, by dipping the steel in molten
zinc. Alternatively, the zinc can be deposited on the steel by
other means, for example by electroplating.
[0038] Zinc anode: A zinc comprising electrode, some or all of its
zinc capable of undergoing the reaction Zn.fwdarw.Zn.sup.2++2
e.sup.-. The zinc coating of galvanized steel and zinc electrodes
of batteries are examples of zinc anodes.
[0039] Faradaic reaction: An electron flux producing or consuming
electrochemical process. Because microscopic charge neutrality is
maintained in Faradaic reactions, in addition to the flow of
electrons, Faradaic reactions also require transport of ions. In
contrast to a Faradaic reaction, a non-Faradaic reaction does not
produce or consume an electron flux.
[0040] Corrosion of zinc means the usually undesired non-Faradaic
reaction of zinc with oxygen and/or water. Such corrosion reduces
the zinc utilization efficiency.
[0041] Zinc utilization efficiency, coulombic efficiency, and
coulometric efficiency are used interchangeably to denote the ratio
of the generated charge (e.g., by a discharging Zn anode of a
battery) to the consumed charge, which is the charge passed through
an electrode in the electroplating of zinc. It is the ratio,
optionally expressed as a percentage, of the electrical charge
stored in a charged electrode that is recovered during its
discharge. Generally, current inefficiencies arise from reactions
other than the intended electrochemical reactions taking place, or
side reactions consuming the electrodes. The three terms all mean
the ratio of the charge produced in the Faradaic reaction whereby
an electrode is consumed and the sum of the of the Faradaic and
non-Faradaic reactions by which the same electrode is consumed. For
example, a zinc anode is consumed by the Faradaic reaction
Zn.fwdarw.Zn.sup.2++2 e.sup.- and by the non-Faradaic reactions 2
Zn+O.sub.2.fwdarw.2 ZnO or 2 Zn+O.sub.2+2 H.sub.2O.fwdarw.2
Zn(OH).sub.2.
[0042] When the current efficiency, electrode utilization
efficiency or coulombic efficiency is 1.00 than about 2 Faradays,
or about 2.times.96485 Coulombs, are produced per about 65.4 grams
of zinc. When only the non-Faradaic reaction takes place, the
charge produced is nil. In galvanized steel, the zinc utilization
efficiency defines, for example, the period over which the steel is
cathodically protected when in absence of cathodic protection the
steel would corrode at a given rate.
[0043] Zinc, Zn, and zinc anode are synonymous and include alloys
in which the fraction of Zn atoms exceeds 0.5, preferably 0.7 and
most preferably 0.9.
[0044] Oxygen means the dioxygen molecule, O.sub.2.
[0045] Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O and hopeite are
synonymous and include compounds or phases
Zn.sub.[3-x]M.sub.x(PO.sub.4).sub.2.4H.sub.2O that conduct
Zn.sup.2+ and/or M.sup.2+ ions, where M.sup.2+ is Fe.sup.2+,
Co.sup.2+, Mn.sup.2+, Ni.sup.2+, Ca.sup.2+ and/or Mg.sup.2+.
[0046] Lamallae and lamellar refer to hopeite crystals the
thickness of which is smaller than their width or length.
Typically, the thickness of the lamellar crystals is less than
about 1/3.sup.rd of their width or length. Preferably, it is less
than about 1/5.sup.th of their width or length; and most preferably
it is less than about 1/10.sup.th of their width or length.
[0047] While the making and using of various embodiments of the
present invention are discussed in detail below, it should be
appreciated that the present invention provides many applicable
inventive concepts that can be embodied in a wide variety of
specific contexts. The terminology used and specific embodiments
discussed herein are merely illustrative of specific ways to make
and use the invention and do not delimit the scope of the
invention.
[0048] The electrodes of the present invention comprise an at least
partially conductive substrate having a Zn layer deposited onto the
at least partially conductive substrate. The at least partially
conductive substrate then has a sulfur or phosphorus containing
polymer coating and a layer of hopeite deposited thereon. The
electrodes of the present invention also include a Zn anode on an
at least partially conductive substrate, having an external layer
of hopeite crystals.
[0049] The present invention also provides a method of preparing an
electrode by depositing a metal layer onto a substrate, applying a
cation exchanging polymer-coating layer onto the metal layer and
depositing a layer of hopeite onto the polymer coating layer. The
invention further provides a solid electrolyte film, formed
generally of non-porous hopeite lamellae that are impermeable to
molecules, particularly to oxygen. The solid film conducts
Zn.sup.2+ and/or other ions. The film is impermeable to oxygen
because oxygen is insoluble in hopeite. Therefore, the present
invention provides numerous uses for the protection of functional
electrode surfaces from oxygen.
[0050] The invention additionally provides a non-porous film having
or including hopeite lamellae on a zinc surface, preventing the
zinc from rapidly reacting with oxygen and/or moisture in air. The
zinc may be the on steel or any other metal surface that could
corrode by reacting with oxygen, e.g. cast iron, wrought iron, or
copper and its alloys. The invention also provides for efficient
use of zinc following its treatment with a phosphate containing
solution, such that a non-porous lamellar hopeite film is formed on
the zinc surface. The inorganic hopeite crystals conduct Zn.sup.2+
ions and unlike other films on galvanized steel are non-porous and
impermeable to liquid electrolytes and/or oxygen.
[0051] The inventors consider as a particularly important the
application of their discovery of Zn.sup.2+ ion conduction in an
oxygen impermeable inorganic solid, such as hopeite, the
improvement of the coulombic efficiency of zinc utilization in the
cathodic protection of structural alloys, particularly of
galvanized steel.
[0052] An exemplary process leading to greater utilization
efficiency of the zinc in galvanized steel, would start with
coating of the zinc surface with a polymer. The polymer is
preferably a cation exchanger, and it is most preferably a cation
exchanger with sulfonate and/or sulfonic acid functions. Cation
exchangers of which acidic fuel cell membranes are made are
preferred. These include, for example, NAFION.RTM. and other
perfuorinated, fluorinated and non-fluorinated cation exchangers,
including, for example, those described in U.S. Pat. Nos.
6,423,784, 6,559,237, 6,649,295, 6,833,412, 7,060,738 or
7,060,756.
[0053] A solution or dispersion cation of the cation exchanger,
when necessary containing an added crosslinker, could then be
applied to the galvanized steel, for example, by spraying, hot
spraying, brushing or dipping. Following the application, the film
is cured or dried in such a way that the film is rendered
substantially insoluble in water at ambient temperature. It is
advantageous for forming the desired Zn anodes to coat these with a
film of a cation exchanging polymer, such as NAFION.RTM.. It is
preferred that the resulting film, formed after drying or curing,
be of about uniform thickness. The thickness of the useful films is
usually greater than about 10 nm and is less than about 100
micrometers. The preferred thickness of the dry films is between
about 50 nm and about 1 micrometer. Optionally, after their drying,
the films may be baked or otherwise heated at a temperature greater
than about 90.degree. C. and less than about 200.degree. C.,
preferably greater than about 110.degree. C. and less than about
150.degree. C. for a period that is typically not less than about
30 seconds and is not greater than about 24 hours.
[0054] The now polymer coated galvanized steel would then be
exposed to a solution containing a phosphate and an alkali metal
salt. The source of phosphate ion may be any material or compound
known to those skilled in the art to ionize in solutions to form
phosphate anions such as phosphoric acid, alkali metal phosphates
such as monosodium phosphate, monopotassium phosphate, disodium
phosphate, divalent metal phosphates and the like, as well as
mixtures thereof. The preferred alkali metal salts are salts with
anions the zinc salts of which are readily water soluble, their
solubility exceeding about 0.1 M at 25.degree. C. Examples of
useful salts are salts with Na.sup.+ or K.sup.+ or tetraalkyl
ammonium, e.g. tetramethyl ammonium, cations and anions like
Cl.sup.-, Br.sup.-, SO.sub.4.sup.2- or NO.sub.3.sup.-. The most
preferred salt is NaCl. The concentration of the salt is generally
greater than about 50 mM and less than about 3 M, a concentration
greater than about 0.1 M and less than about 1 M being preferred.
The pH range of the phosphate containing solution is generally less
than about 10 and greater than about 4. The phosphate concentration
in the solution is generally greater than 5 mM and less than about
500 mM. The preferred phosphate concentration range is between
about 10 mM and about 200 mM. The phosphate may be added as an
alkali metal salt, such as sodium or a potassium salt, optionally
as a mono or dibasic salt.
[0055] Although the desired hopeite film is expected to form
spontaneously upon immersion of the galvanized steel in the aerated
phosphate and salt containing solution, the formation of hopeite
could be accelerated by applying a constant potential or passing a
constant current through the zinc to cause electrooxidation of a
small fraction of the zinc to Zn.sup.2+ and more rapid formation of
the hopeite film. The preferred current density of zinc
electrooxidation is not more than about 100 microamperes per square
centimeter (cm) and not less than about 1 microamperes per square
cm. Alternatively, a chemical oxidant may be added to the
phosphating solution. Examples of chemical oxidants include alkali
metal chlorates and hydrogen peroxide.
[0056] The inventors also consider as important the improvement of
the coulombic efficiency of the utilization of zinc in anodes of
electrochemical cells, particularly of zinc anodes in
electrochemical cells utilizing body fluids as their
electrolytes.
[0057] The rates, meaning the current densities, of Zn the anodes,
for example of those operating in fluids of the body of animals,
are generally about 100 microamperes per square cm or less. The
electrodes and batteries that may be implanted into a human,
mammal, animal, fish, and so forth provide therefore low power for
a long period of time, rather than high power for a short period.
Generally, the zinc anodes are discharged, when they are in
continuous use, over periods longer than about three days.
Preferably, the zinc anodes are discharged over periods longer than
one week.
[0058] When the aqueous electrolyte is a body fluid, the battery
case is obviated as long as the materials constituting the anode
and the cathode, as well as the reaction products of the two
electrodes, are neither toxic nor otherwise harmful. This is so,
for example, when the battery is formed of a zinc anode and a
silver chloride based cathode. The silver chloride based cathode
can be an Ag and/or carbon and AgCl containing paste printed on a
plastic film, such as a polyester film, or a chlorided silver wire,
overcoated with a harmless, ion conducting, immobile, bioinert
hydrogel, as described, for example, by Feldman, et al., in United
States Patent Application No. 20050173245.
[0059] The zinc anode may, optionally, be subcutaneously implanted,
or placed on the skin. It can be used in combination with a cathode
that may be also implanted, or it may be on the skin, like an ECG
electrode, which usually comprises either silver chloride or an
oxide of nickel. Alternatively, an air (oxygen) cathode can be used
in the body or on the skin. Such a cathode may include an
electrocatalyst for the reduction of oxygen to water, for example,
a copper enzyme like bilirubin oxidase, which may be electrically
connected through a redox hydrogel to the cathode, typically made
of carbon.
[0060] The implanted or skin surface cells of the present invention
are much smaller than any commercially available battery because
they do not require an electrolyte or a case. Their electrolyte can
be, for example, interstitial fluid between cells of tissues of the
body, peritoneal fluid, blood, plasma or sweat. In one embodiment,
the two electrodes (e.g., the zinc anode and the cathode) are
inserted in the body, under the skin, in electrolytic contact with
a body fluid. In absence of a case or an added electrolyte, the
batteries can have volumes smaller than 1 mm.sup.3, and even
smaller than 0.1 mm.sup.3.
[0061] In one of its embodiments, the present invention discloses
an ion conducting hopeite coated electrode. Additionally, the
present invention provides an electrochemical cell, having a
cathode and an electrolyte, comprising phosphate and oxygen, in
contact with the cathode. The electrochemical cell has a polymer
coated zinc anode and a hopeite layer in contact with the
electrolyte. The electrolyte may be a body fluid that has phosphate
and oxygen, e.g., interstitial fluid. For example, the polymer may
be a sulfonate function comprising polymer, exemplified by
commercially available NAFION.RTM. or by one of the polymers
useful, like NAFION.RTM., in membranes separating anode and cathode
compartments in acid electrolyte fuel cells, described, for example
in U.S. Pat. Nos. 6,423,784, 6,559,237, 6,649,295, 6,833,412,
7,060,738 or 7,060,756. However, the skilled artisan will recognize
the layer of hopeite may be formed in a variety of ways, e.g., by a
discharge of the zinc anode in a physiological saline buffer. Salts
of alkali metal cations, preferably Na.sup.+ or K.sup.+, and anions
that do not precipitate or strongly complex at about 0.1 M
concentration Zn.sup.2+ are preferred. Examples of useful anions
are Cl.sup.-, Br.sup.-, SO.sub.4.sup.2- and NO.sub.3.sup.-. The
useful concentration range of the salts is greater than about 50 mM
and less than about 3 M, the range greater than about 0.1 M and
less than about 1 M being preferred.
[0062] One example of the present invention includes an
electrochemical cell that is a miniature case-less cell and may be
of a variety of sizes depending on the application, e.g., the cell
may be smaller than about 30 mm.sup.3, between about 20 and about
30 mm.sup.3, between 10 and 20 mm.sup.3 or smaller than about 10
mm.sup.3. The present invention also includes a cell having a
hopeite coated anode.
[0063] Cation exchanging polymer overcoating of the zinc anodes is
of essence for controlling the nucleation underlying the formation
of the large non-porous hopeite lamellae (leaves). Other than the
exemplary thin a fluorinated sulfonic acid polymer (e.g.,
NAFION.RTM.) film, typically thinner than about 100 micrometers,
preferably thinner than about 10 micrometers and most preferably
about 1 micrometer in thickness when not swollen in water, films of
other cation exchanging polymers may be used to coat the battery
anode or the galvanized steel.
[0064] The zinc anodes of the body fluid comprising cells may have
a variety of different sizes and shapes depending on the
application. Other than the exemplary fine (e.g., about 100
micrometer diameter) zinc wire anode, zinc containing films may be
printed, painted, screen-printed, evaporated, adhered or sprayed a
surface, e.g., plastics, polyester, Nylon sheets, ceramic sheets,
woven carbon sheets, non-woven carbon sheets or a combinations
thereof. The zinc paints and inks may include a conductive binder,
often comprising a polymer with fine graphite or carbon black
particles.
[0065] Electrolytes may include body fluids (e.g., interstitial
fluid, peritoneal fluid, sweat, mucus or blood), and non-toxic,
environmentally friendly aqueous solutions. The electrolytes which
comprise phosphate (PO.sub.4.sup.-), may also comprise sodium
(Na.sup.+), potassium (K.sup.+), calcium (Ca.sup.2+), magnesium
(Mg.sup.2+), chloride (Cl.sup.-), and bicarbonate
(HCO.sub.3.sup.-). Electrolytes are well known in the art and the
skilled artisan will recognize the numerous electrolytes that may
be used in conjunction with the present invention.
[0066] The pH range of the phosphate containing solution is
generally less than about 10 and greater than about 4. It is
preferably more than about 5 and less than about 8; most preferably
it is between about pH 7.0 and about 7.5. The phosphate
concentration range is between at least about 5 mM and less than
about 500 mM. Specifically in one example, the phosphate
concentration range is between about 10 mM and about 200 mM. The
phosphate may be added as an alkali metal salt, such as sodium or
potassium salt, optionally a mono or dibasic salt.
[0067] Exemplary useful electrolytes include (a) pH 7.4, 20 mM
phosphate, 0.05-1 M NaCl; (b) pH 7.4, 20 mM phosphate, 0.5 M NaCl;
(c) pH 7.4, 1-100 mM phosphate; 0.03-0.5 M Na.sub.2SO.sub.4; (d) pH
7.4, 20 mM phosphate, 0.05-1 M NaBr; (e) pH 7.4, 20 mM phosphate,
0.05-1.0 M NaNO.sub.3. The importance of solution pH is well known
in the art and the skilled artisan will recognize the range of pHs
that may be used in conjunction with the present invention.
[0068] The present invention includes an electrode having an at
least partially conductive substrate having a metal layer on the at
least partially conductive substrate. The at least partially
conductive substrate then has a polymer coating and a layer of
hopeite deposited thereon.
[0069] Membranes may be used for controlling the nucleation and
morphology of hopeite growth. The useful thickness of the cation
exchange membrane, such as NAFION.RTM., is generally greater than
10 nm and is less 500 micrometers. Preferably it is greater than
about 0.5 micrometers and is less than about 20 micrometers.
[0070] The present invention provides a Zn anode coated with a
cation exchanging polymer film, e.g., NAFION.RTM.. The coating can
formed, for example, by brushing, spraying spin-coating or
dip-coating using a solution or suspension or dispersion of the
cation exchanger. It is preferred that the resulting film, formed
after drying, be of about uniform thickness. The thickness of the
useful films is usually greater than about 10 nm and is less than
about 100 micrometers. The preferred thickness of the dry films is
between about 50 nm and about 1 micrometer. Optionally, the cation
exchanging polymer films may be baked on after their drying. When
baked, baking at a temperature greater than about 90.degree. C. and
less than about 200.degree. C. is preferred, the baking temperature
range above about 110.degree. C. and below about 150.degree. C.
being most preferred.
[0071] In some instances, the at least partially conductive
substrate is a metal, (e.g., platinum); however other metals and
alloys may be used. The metal layer deposited onto the at least
partially conductive substrate and may be zinc, zinc alloys or
other metals or metals alloys. The anodes protected can be of
different elemental metals and alloys. The protective metal is Zn
or an alloy of Zn, for example a Zn alloy with Mg, Al, Ti, Ni, Co,
Fe, Y, Yb or Zr. The Zn or its alloy can be applied to the
protected metal, such as steel, for example by hot rolling, cold
rolling, extrusion, hot pressing, electroplating or dipping in
molten Zn or a molten alloy of Zn.
[0072] The skilled artisan will recognize the layer of hopeite may
be formed in a variety of ways, e.g., by a discharge in a
physiological saline buffer. The present invention includes a layer
of hopeite deposited in and/or on a polymer coating. Furthermore,
the coating may be of a Zn.sup.2+ conducting inorganic substance
other than hopeite. The coating may be used with other subsequently
applied films, such as epoxies, enamels and other paints.
[0073] Furthermore, the hopeite of the present invention may have
lattice substituting ions; the properties of the basic Zn.sup.2+
ion conductive hopeite phase might be modulated by substituting
Zn.sup.2+ in the Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O lattice. For
example, the modifications may increase the conductivity of
Zn.sup.2+. Potential lattice substituents of Zn.sup.2+ are
divalent, trivalent or tetravalent cations, typically of ionic
radii within about .+-.0.03 nm of the ionic radius of Zn.sup.2+.
They can be, for example, chosen from the group Ni.sup.2+,
Co.sup.2+, Fe.sup.2+, Ga.sup.3+, Y.sup.3+, Yb.sup.3+, Bi.sup.3+, or
Zr.sup.4+. In producing the zinc anodes, it might be advantageous
to alloy the zinc with one or more of the corresponding metals.
[0074] The present invention includes an anode having an at least
partially conductive substrate and an external layer of inorganic
crystals and in some instances, the inorganic crystals are hopeite.
In addition, the present invention provides a method of preparing
an electrode by depositing a metal layer onto a substrate, applying
a polymer coating layer onto the metal layer and growing a layer of
hopeite onto the polymer coating layer.
[0075] In some instances, the conductive substrate is a metal
(e.g., platinum); however alloys and other combinations metals may
be used. The metal layer (e.g., zinc or zinc alloys) is deposited
onto the substrate; however, the skilled artisan will recognize
that other metals may be used depending on the particular
application. For example, the present invention provides a method
of preparing an electrode by depositing a zinc layer onto a
platinum substrate. A fluorinated sulfonic acid polymer layer is
applied to the Zn layer and a layer of hopeite is deposited onto
the fluorinated sulfonic acid polymer layer. However, the present
invention may be used with other fluorinated polymers (e.g.,
NAFION.RTM.) and a variety of other sulfonic acid copolymers.
Although, fluorinated sulfonic acid polymers are used as an example
the skilled artisan will recognize that a variety of coatings of
polymers and copolymers may be used.
[0076] Generally, FIG. 1 shows schematically the process by which
the fluorinated sulfonic acid polymer coated zinc electrode 10 was
prepared. FIGS. 1A, 1B and 1C are cross-sectional views of a
schematic that illustrates the steps of the preparation of the
fluorinated sulfonic acid polymer coated zinc electrode 10. FIG. 1A
indicates about a 2 cm long and about 76 .mu.m thick (0.031
cm.sup.2) platinum wire 12. FIG. 1B illustrates a Zn
electrodeposition. The Zn layer 16 is electrodeposition deposited
onto the platinum wire 12. Zinc anode 14 was prepared by
electro-plating about a 20 .mu.m thick Zn layer 16 on about a 2 cm
long and about a 76 .mu.m diameter platinum wire 12 at about 2 mA
constant current, in about 1,000 seconds, from about 0.6 M
ZnCl.sub.2--2.8 M KCl--0.32 M H.sub.3BO.sub.3--0.3% polystyrene
sulfonate and about 2 coulombs being passed. The initial surface
area was about 0.075 cm.sup.2. FIG. 1C illustrates a fluorinated
sulfonic acid polymer coating 18 of the electrodeposited Zn 16 on
the platinum wire 12. The anodes were coated with a 16 .mu.m thick
fluorinated sulfonic acid polymer film, by dipping in about 0.5%
(10:1 isopropanol diluted) about 5% fluorinated sulfonic acid
polymer and drying.
[0077] Cells of about 1.01 V open circuit voltage were formed of
the hopeite coated zinc anodes and Ag/AgCl cathodes with
physiological saline pH 7.4 buffer containing 20 mM phosphate
buffer and 0.15 M NaCl as the electrolyte. The cathodes were much
larger than the anodes, making the cell characteristics
anode-controlled. To test the cells under conditions of high
corrosive loss of anodic capacity, the cells were discharged
slowly, at about 1 .mu.A (13 .mu.A cm.sup.-2), across about a 1
M.OMEGA. resistor at about 1 V. When the fluorinated sulfonic acid
polymer coated Zn anodes 10 were discharged in the physiological
saline buffer, the fluorinated sulfonic acid polymer coated Zn
anodes 10 were overgrown by colorless crystals and the diameter of
the fluorinated sulfonic acid polymer coated Zn anodes 10 increased
from about 152 .mu.m (e.g., Pt wire, about 76 .mu.m; NAFION.RTM.
about 2.times.16 .mu.m; Zn about 2.times.20 .mu.m) to about 485
.mu.m at completion of the discharge.
[0078] FIGS. 2A, 2B and 2C are images of the overgrowth on the
fluorinated sulfonic acid polymer coated Zn anodes 10 at different
stages of the discharge. FIG. 2A shows optical micrographs of the
tips of fluorinated sulfonic acid polymer coated about 2 cm long Zn
anode 10 in the pH=about 7.4.+-.0.1 physiological buffer (e.g.,
about 20 mM phosphate, about 0.15 M NaCl) after passage of a charge
of about 4 mC. FIG. 2B shows optical micrographs of the tips of
fluorinated sulfonic acid polymer coated 2 cm long Zn anode about
10 in the pH=about 7.4.+-.0.1 physiological buffer (e.g., about 20
mM phosphate, about 0.15 M NaCl) after passage of a charge of about
250 mC. FIG. 2C shows optical micrographs of the tips of
fluorinated sulfonic acid polymer coated 2 cm long Zn anode about
10 in the pH=7.4.+-.0.1 physiological buffer (e.g., about 20 mM
phosphate, about 0.15 M NaCl) after passage of a charge of about
430 mC.
[0079] FIG. 3 shows the X-ray diffraction patterns of the
precipitate formed in the fluorinated sulfonic acid polymer
membrane of the operating Zn anode (bottom trace) and the
commercial Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O (top trace). The
X-ray powder diffraction pattern of the crystals overgrowing the
fluorinated sulfonic acid polymer shown in FIG. 3 are identical to
that of Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O (Aldrich, Milwaukee,
Wis.) and agreed with the data on PDF card #33-1474 for hopeite
(International Centre for Diffraction Data, 12 Campus Blvd. Newton
Square, Pa. 19073) and with that reported for synthetic
hopeite.sup.5.
[0080] FIGS. 4A, 4B and 4C are images of the fluorinated sulfonic
acid polymer coated zinc electrode 10. The top images of FIGS. 4A,
4B and 4C are compositions of optical images (e.g., 40.times.
magnification) and the bottom images of the FIGS. 4A, 4B and 4C are
electron micrographs (e.g., about 5000.times. magnification)
showing the dimensions and morphologies of the coatings formed at
the completion of the discharge on the fluorinated sulfonic acid
polymer coated zinc electrode 10. FIG. 4A shows the dimensions and
morphologies of the films formed at the completion of the discharge
on the fluorinated sulfonic acid polymer coated zinc electrode 10
in about 0.15 M NaCl without phosphate in about 0.15 M NaCl without
phosphate. FIG. 4B shows the dimensions and morphologies of the
films formed at the completion of the discharge on the fluorinated
sulfonic acid polymer coated zinc electrode 10 in pH about 7.4
about 20 mM phosphate buffer without NaCl. FIG. 4C shows the
dimensions and morphologies of the films formed at the completion
of the discharge on the fluorinated sulfonic acid polymer coated
zinc electrode 10 in about 20 mM pH about 7.4 phosphate buffer with
about 0.15 M NaCl.
[0081] FIG. 5 is an image that shows details of the electron
micrograph at 5000.times. magnification of the non-porous hopeite
Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O lamellae overgrowing the
fluorinated sulfonic acid polymer coated zinc fiber anode upon its
discharge in pH about 7.4 physiological (e.g., about 20 mM
phosphate, about 0.15 M NaCl) buffer.
[0082] FIG. 6 is a plot illustrating the time dependence of the
potential of the fluorinated sulfonic acid polymer coated zinc
electrode 10. The fluorinated sulfonic acid polymer coated zinc
electrode 10 was discharged against an Ag/AgCl cathode at a
constant current of about 1 .mu.A at about 25.degree. C. in serum.
FIG. 6 trace (a) shows the curve for the cell with serum. The
fluorinated sulfonic acid polymer coated zinc electrode 10 was
discharged against an Ag/AgCl cathode in pH about 7.4, about 0.15 M
NaCl, about 20 mM phosphate buffer. FIG. 6 trace (b) shows the
discharge curve at about 1 .mu.A for the cell with physiological
saline buffer as an electrolyte. The fluorinated sulfonic acid
polymer coated zinc electrode 10 was discharged against an Ag/AgCl
cathode in about 0.15 M NaCl FIG. 6 trace (c). The fluorinated
sulfonic acid polymer coated zinc electrode 10 was discharged
against an Ag/AgCl cathode in pH about 7.4, about 20 mM phosphate
buffer, without NaCl as seen in FIG. 6 trace (d). With serum, the
about 1 V output was steady for two weeks, during which the charge
passed was about 1.2 C, corresponding to about 60% efficiency of
utilization of the about 2 C utilized in forming the Zn plate. In
the physiological buffer, the about 1 V output was steady at for
three weeks, and the charge passed was about 1.73 C, corresponding
to about 86% Zn utilization. Growth of the non-porous lamellae and
high anode utilization efficiency required fluorinated sulfonic
acid polymer, phosphate and NaCl. Without phosphate as in FIG. 6
trace (c), the hopeite film could not form and in the absence of
NaCl the film was porous and the anode corroded rapidly as seen in
FIG. 6 trace (d).
[0083] At the physiological pH of about 7.4 the dominant
non-Faradaic reaction of Zn is that with dissolved O.sub.2..sup.6
The extent to which the flux of O.sub.2 to the surface of the
operating anode is reduced defines the current efficiency. That the
high anode utilization efficiency resulted from blockage of
permeation of O.sub.2 to the active Zn by the hopeite film was
confirmed as follows. A set of anodes was prepared by
electrodepositing different amounts of Zn on the Pt wires. Upon
completion of discharge, the cyclic voltammograms were measured,
the potential scanned to about -0.5 V vs. Ag/AgCl, where O.sub.2 is
electroreduced even on non-catalytic conductors.
[0084] FIG. 7 is a cyclic voltammogram that is indicative of the
blockage of O.sub.2 transport to the Zn anode. The fluorinated
sulfonic acid polymer coated zinc electrode 10 is about 0.075
cm.sup.2 anodes with fluorinated sulfonic acid polymer coated and
about 2 cm long. The voltammograms of FIG. 7 confirmed that the
hopeite film was non-porous and blocked the O.sub.2 permeation. The
voltammograms of FIG. 7 are for a set of anodes, differing in their
amount of electrodeposited Zn, i.e., (a) 0.00 C; (b) 0.05 C; (c)
0.12 C; (d) 0.20 C, after completion of their discharge, and were
obtained in air-equilibrated pH about 7.4, about 0.15 M NaCl, about
20 mM phosphate buffer.
[0085] O.sub.2 transport-blocking films are rarely solid
electrolytes and are usually highly resistive to passage of ionic
currents. Thus, with few exceptions, anodes with O.sub.2 transport
blocking films discharge only at high overpotentials. The
non-porous lamellar hopeite-overgrown Zn anodes were, nevertheless,
discharged with little polarization, even when their overgrowth was
100 .mu.m thick. Measurement of the polarization of a set of
identical Zn anodes after their discharge to 0, 10, 25 and 50%
depth in the physiological saline buffer solution (Table 1) showed
that at a current density of about 0.13 mA cm.sup.-2 the
polarization of the half-discharged anodes differed by less than
about 50 mV from their polarization at the start of discharge. At
about 0.26 mA cm.sup.-1, the polarization of the half-discharged
anodes exceeded their initial polarization only by about 110 mV,
showing that the ionic conductance was at least about
2.times.10.sup.-3 S. TABLE-US-00001 TABLE 1 illustrates the
dependence of the polarization on the state of discharge of the Zn
anode* J 0% 10% 25% 50% 0.013 0.01 0.00 0.01 0.01 0.026 0.01 0.00
0.01 0.02 0.065 0.02 0.00 0.04 0.04 0.13 0.04 0.01 0.08 0.09 0.26
0.05 0.06 0.19 0.16 Where *J in mA cm.sup.-2; half cell potential
about 1.01 .+-. 0.01 V vs. Ag/AgCl; discharged at about 13 .mu.A
cm.sup.-2; all values in V .+-. about 0.01 V.
[0086] FIG. 8 is a structural model of hopeite
Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O. Hopeite, an open-framework
hydrated zinc phosphate with intersecting channels, is a known host
of neutral organic molecules, as well as cations, resembling in
this aspect zeolites. The overgrowth of the fluorinated sulfonic
acid polymer coated zinc anode by hopeite lamellae continues
throughout the entire discharge period, the size reaching about
about 20 .mu.m.times.about 20 .mu.m.times.about 1 .mu.m. (e.g.,
FIG. 5). The continued growth, as long as the supply of Zn.sup.2+
lasts, implies permeation of either phosphate and water or
Zn.sup.2+. The hopeite crystal .sup.1,7 consists of layers of
ZnO.sub.4 tetrahedra, linked by PO.sub.4 groupings. FIG. 8 is a
structural model of hopeite Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O with
water channels in which Zn.sup.2+ can diffuse are seen. Selected
hydrogen bonds are shown between H14--O4, H35--O5, H36--O6. Between
these two sheets, a crystallographically separate Zn atom, Zn1 is
present in octahedrally coordinated units of
ZnO.sub.2(H.sub.2O).sub.4. Two oxygen atoms are shared with two
separate PO.sub.4 tetrahedra. The four water molecules form a
hydrogen-bonded network. This Zn.sup.2+ atom site contains
vacancies,.sup.8 is replaceable by Ni.sup.2+9 and by
Co.sup.2+10,11, and is ion exchanged by Li.sup.+ or Cs.sup.+. These
properties suggest that the lattice permeating ion is Zn.sup.2+,
not phosphate.
[0087] FIG. 9 is a schematic diagram of a case-less miniature
Zn(NAFION.RTM.)--AgCl battery operating in the subcutaneous
interstitial fluid. The case-less miniature Zn |electrolyte|
Ag/AgCl cell battery 22 includes a Zn anode 24 and an Ag/AgCl
cathode 26 that are implanted into the skin 28 of a subject and are
in electrical communication through the subcutaneous interstitial
fluid. The Zn anode 24 and an Ag/AgCl cathode 26 are connected to
contact pads 30 and 32 respectively.
[0088] The Zn anode 24 and Ag/AgCl cathode 26 may be implanted
individually into the skin 28 at a variety of depths and a variety
of distances may separate the Zn anode 24 and a cathode 26 provided
electrical communication through the subcutaneous interstitial
fluid is maintained. For example, the Zn anode 24 and Ag/AgCl
cathode 26 may extend about 0.5 cm below the surface of the skin
28, however, the Zn anode 24 and Ag/AgCl cathode 26 may
independently be positioned at other depths, e.g., surface mounted,
or penetrate 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0,
11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0 cm or
more into the skin and any fraction thereof, e.g., 0.1, 0.2, 0.3,
0.4, 0.5, 0.6, 0.7, 0.8 or 0.9 cm. In addition, the Zn anode 24 and
Ag/AgCl cathode 26 may be separated by any distance, provided
electrical communication through the subcutaneous interstitial
fluid is maintained, e.g., 0.25 cm; however, other separation
distances may also be used, e.g., 1.0, 2.0, 3.0, 4.0, 5.0, 6.0,
7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0,
18.0, 19.0, 20.0 cm or more and any fraction thereof, e.g., 0.1,
0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9 cm.
[0089] The Zn anode 24 includes an at least partially conductive
layer of Zinc, one or more fluorinated sulfonic acid polymer films
(not shown) and an external layer of inorganic crystals (not
shown). The inorganic crystals may be hopeite,
Zn.sub.3(PO.sub.4).sub.2.4H.sub.2O, which is an ambient temperature
solid phase that conducts Zinc ion (Zn.sup.2+) and is nonporous and
impermeable to oxygen to provide protection of the Zn anode 24 from
oxygen and other diffusants.
[0090] Hopeite allows about 86% efficient discharge of Zn anode 24
in a physiological saline buffer. In addition, the skilled artisan
will recognize that the Zn anode 24 may contain a variety of
different elemental metals and alloys, e.g., Zn, an alloy of Zn
with Mg, Al, Ti, Ni, Co, Fe, Y, Yb or Zr.
[0091] The contact pads 30 and 32 are connected to the Zn anode 24
and a cathode 26 respectively. Although, the connections are
illustrated as external contact pads 30 and 32, it is not necessary
for the contact pads 30 and 32 to be on the epidermal layer, but
contact pads 30 and 32 may be on or in the dermal layer, basal
layer, externally positioned on or about an organ, internally
positioned to an organ and so forth.
[0092] The Zn anode 24 and a cathode 26 are connected through the
subcutaneous interstitial fluid of the skin 28. The subcutaneous
interstitial fluid may include body fluids (e.g., mucus, blood,
sweat, and tears) and/or aqueous solutions. The subcutaneous
interstitial fluid acts as an electrolytes and may include sodium
(Na.sup.+), potassium (K.sup.+), calcium (Ca.sup.2+), magnesium
(Mg.sup.2+), chloride (Cl.sup.-), phosphate (PO.sub.4.sup.-), and
bicarbonate (HCO.sub.3.sup.-) and salts (e.g., KCl, NaCl and so
forth). The pH range of the subcutaneous interstitial fluid is
generally less than about 10 and greater than about 4. In some
instances the pH may be as low as about 5 and as high as about 8,
e.g., between about pH 7.0 and about 7.5.
[0093] For example, case-less miniature Zn |electrolyte| Ag/AgCl
cell battery 910, made include a fine Zn fiber anode 24 and an
Ag/AgCl cathode 26 may be implanted into the skin 28 to provide
about 1.00 V and about 13 .mu.A cm.sup.-2 for about 2 weeks, at
about 60% current efficiency.
[0094] In combination with the Ag/AgCl cathode, the
hopeite-overgrown zinc fiber anode forms a cell with an open
circuit voltage of about 1.01 V. Its polarization is merely about
10 mV at about 13 .mu.A cm.sup.-2, and the zinc utilization
efficiency is as high as about 86%. With a chlorided silver fiber
cathode, overcoated with crosslinked hydrated polyethylene oxide or
with another bioinert chloride-permeable hydrogel, .sup.12 the
anode could form a miniature in-vivo cell about a hundredfold
smaller than the smallest available battery and could operate at
about 1 V for about two weeks.
[0095] Alternatively, in combination with an oxygen cathode having
a copper enzyme like bilirubin oxidase, the zinc anode would form a
cell with an about 1.5 V open circuit voltage operating at about
1.2 V-1.4 V in a buffer with about neutral pH having phosphate and
an alkali halide, e.g., NaCl. In combination with an oxygen cathode
having a copper enzyme like laccase the zinc anode would form a
cell operating at about pH 4-6 with an open circuit voltage of
about 1.6 V operating at about 1.3-1.5 V.
[0096] The present invention provides a non-porous inorganic
lamellar composition including a phosphate anion and one or more
metal cations formed on a substrate surface, wherein the non-porous
inorganic lamellar composition forms a layer that allows the
transport of at least one ion at 25.degree. C.
[0097] The present invention also provides a substrate having a
first inner metal and a second outer metal. The surface of the
second outer metal reacts to form a non-porous lamellar layer that
allows the transport of at least one ion at 25.degree. C. The
non-porous lamellar layer includes a compound formed of a phosphate
anion and one or more metal cations.
[0098] A method of surface treating a corrodible metal by coating a
corrodible metal with a non-porous lamellar film is also provided
by the present invention. The non-porous lamellar film includes an
inorganic phosphate of one or more metal cations to form a
substantially impermeable film that provides an anticorrosion
effect on the corrodible metal.
[0099] In addition the present invention provides an electrical
power generating electrochemical cell. The electrochemical cell
includes an electrolyte providing for ion transport between a
cathode and a zinc anode. The anode includes a non-porous inorganic
lamellar film of a compound of phosphate and one or more metal
cations for the preventing or reducing non-Faradaic corrosion of
the anode. In one embodiment the substantially non-porous lamellar
film is hopeite and conducts Zn.sup.2+ ions.
[0100] The present invention provides a composition having a cation
exchanger coated on a metal substrate and an inorganic lamellar
layer formed on the cation exchanger. The inorganic lamellar layer
is substantially oxygen impermeable and allows the transport of one
or more metal ions. The inorganic lamellar layer includes one or
more phosphate anions and one or more metal cations and the cation
exchanger influences relative growth of the inorganic lamellar
layer. Generally, the inorganic lamellar layer is hopeite; however
other inorganic lamellar layers may be used. The metal substrate
includes zinc, a zinc alloy and in some instances may be a coating
of zinc or zinc alloy on another substrate.
[0101] In addition the present invention provides a method of
surface treating a corrodible metal by coating a corrodible metal
with a cation exchanger and forming an inorganic lamellar layer on
the cation exchanger. The inorganic lamellar layer allows the
transport of one or more metal ions and that provides an
anticorrosion effect on the corrodible metal. The inorganic
lamellar layer includes one or more phosphate anions and one or
more metal cations and the cation exchanger influences relative
growth of the inorganic lamellar layer.
[0102] In addition, the present invention provides a method of
forming a metal electrode that is protected against non-Faradaic
corrosion by immersing a cation exchanger coated metal electrode in
a phosphate containing solution and forming an inorganic lamellar
layer on the cation exchanger. The inorganic lamellar layer allows
the transport of one or more metal ions, with the cation exchanger
influencing the relative growth of the inorganic lamellar layer.
Generally, the inorganic lamellar layer is hopeite; however other
inorganic lamellar layers may be used. The metal substrate includes
zinc, a zinc alloy and in some instances may be a coating of zinc
or zinc alloy on another substrate.
[0103] In addition, the phosphate containing solution may contain
other salts including NaCl, KCl and combinations thereof.
Furthermore, in animal or human implantable embodiments NaCl, KCl,
phosphate and metal ions may be supplied by the fluids and cells of
the animal or human implanted therewith.
[0104] As used in this specification and claim(s), the words
"comprising" (and any form of comprising, such as "comprise" and
"comprises"), "having" (and any form of having, such as "have" and
"has"), "including" (and any form of including, such as "includes"
and "include") or "containing" (and any form of containing, such as
"contains" and "contain") are inclusive or open-ended and do not
exclude additional, unrecited elements or method steps.
[0105] The term "or combinations thereof" as used herein refers to
all permutations and combinations of the listed items preceding the
term. For example, "A, B, C, or combinations thereof" is intended
to include at least one of: A, B, C, AB, AC, BC, or ABC, and if
order is important in a particular context, also BA, CA, CB, CBA,
BCA, ACB, BAC, or CAB. Continuing with this example, expressly
included are combinations that contain repeats of one or more item
or term, such as BB, AAA, MB, BBC, AAABCCCC, CBBAAA, CABABB, and so
forth. The skilled artisan will understand that typically there is
no limit on the number of items or terms in any combination, unless
otherwise apparent from the context.
[0106] It will be understood that particular embodiments described
herein are shown by way of illustration and not as limitations of
the invention. The principal features of this invention can be
employed in various embodiments without departing from the scope of
the invention. Those skilled in the art will recognize, or be able
to ascertain using no more than routine experimentation, numerous
equivalents to the specific procedures described herein. Such
equivalents are considered to be within the scope of this invention
and are covered by the claims.
[0107] All of the compositions and/or methods disclosed and claimed
herein can be made and executed without undue experimentation in
light of the present disclosure. While the compositions and methods
of this invention have been described in terms of preferred
embodiments, it will be apparent to those of skill in the art that
variations can be applied to the compositions and/or methods and in
the steps or in the sequence of steps of the method described
herein without departing from the concept, spirit and scope of the
invention. All such similar substitutes and modifications apparent
to those skilled in the art are deemed to be within the spirit,
scope and concept of the invention as defined by the appended
claims.
[0108] References:
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Structural Crystallography and Crystal Chemistry 1975, B31,
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[0110] (2) Mano, N.; Kim, H.-H.; Zhang, Y.; Heller, A. An oxygen
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