U.S. patent application number 15/862374 was filed with the patent office on 2019-07-04 for solvent independent reference electrodes for use with non-aqueous electrolytes.
The applicant listed for this patent is Lawrence Livermore National Security, LLC. Invention is credited to Corie HORWOOD, Michael STADERMANN.
Application Number | 20190204257 15/862374 |
Document ID | / |
Family ID | 67059456 |
Filed Date | 2019-07-04 |
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United States Patent
Application |
20190204257 |
Kind Code |
A1 |
HORWOOD; Corie ; et
al. |
July 4, 2019 |
SOLVENT INDEPENDENT REFERENCE ELECTRODES FOR USE WITH NON-AQUEOUS
ELECTROLYTES
Abstract
The present disclosure relates to a reference electrode
apparatus for use in electrochemical testing applications. The
apparatus may have a hollow tube and a frit disposed adjacent one
end of the hollow tube. A layer of material may secure the frit to
the one end of the hollow tube to close off the one end. A silver
wire is included which has a coating that helps to prevent a
voltage potential change in an output of the apparatus during a
test in which the silver wire is in contact with an ionic liquid.
The silver wire with the coating is disposed at least partially in
the hollow tube. An ionic liquid is disposed in the hollow tube in
which at least a portion of the sulfide coated wire is
disposed.
Inventors: |
HORWOOD; Corie; (Livermore,
CA) ; STADERMANN; Michael; (Pleasanton, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lawrence Livermore National Security, LLC |
Livermore |
CA |
US |
|
|
Family ID: |
67059456 |
Appl. No.: |
15/862374 |
Filed: |
January 4, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01N 27/301 20130101;
C23C 14/0623 20130101 |
International
Class: |
G01N 27/30 20060101
G01N027/30; C23C 14/06 20060101 C23C014/06 |
Goverment Interests
STATEMENT OF GOVERNMENT RIGHTS
[0001] The United States Government has rights in this invention
pursuant to Contract No. DE-AC52-07NA27344 between the U.S.
Department of Energy and Lawrence Livermore National Security, LLC,
for the operation of Lawrence Livermore National Laboratory.
Claims
1. A reference electrode apparatus for use in electrochemical
testing applications, the apparatus comprising: a hollow tube; a
frit disposed adjacent one end of the hollow tube; a layer of
material securing the frit to the one end of the hollow tube to
close off the one end; a silver wire coated with a coating that
helps to prevent a voltage potential change in an output of the
apparatus during a test in which the silver wire is in contact with
an ionic liquid, the silver wire with the coating thereon being
disposed in the hollow tube; and an ionic liquid disposed in the
hollow tube in which at least a portion of the silver wire is
disposed.
2. The apparatus of claim 1, wherein the coating comprises a silver
sulfide coating.
3. The apparatus of claim 1, wherein the hollow tube comprises a
hollow glass tube.
4. The apparatus of claim 1, wherein the frit comprises a porous
glass frit.
5. The apparatus of claim 1, wherein the layer of material securing
the frit to the one end of the hollow tube comprise a layer of
shrink wrap material.
6. A reference electrode apparatus for use in electrochemical
testing applications, the apparatus comprising: a hollow tube; a
frit disposed adjacent one end of the hollow tube; a layer of
material securing the frit to the one end of the hollow tube to
close off the one end; a silver sulfide coated silver wire disposed
within the hollow tube; and an ionic liquid solution disposed in
the hollow tube in which at least a portion of the silver sulfide
coated silver wire is disposed.
7. The apparatus of claim 6, wherein the hollow tube comprises a
hollow glass tube.
8. The apparatus of claim 7, wherein the hollow glass tube
comprises a diameter of no more than about 4 mm.
9. The apparatus of claim 7, wherein the frit comprises a glass
frit.
10. The apparatus of claim 7, wherein the layer of material
comprises a shrink wrap.
11. A method for forming a reference electrode for use in
electrochemical testing applications, the method comprising:
positioning a glass frit against one end of a hollow glass tube;
arranging a portion of shrink wrap over portions of the glass frit
and the glass tube; heating the portion of shrink wrap, the glass
tube and the glass frit to shrink the shrink wrap onto the portions
of the glass frit and the glass tube to form an assembly; boiling
the assembly in a solution to remove organic contaminants; boiling
the assembly in water; drying the assembly; forming a silver
sulfide coated silver wire; filling the glass tube with an ionic
liquid; and inserting at least a portion of the silver sulfide
coated silver wire into the ionic liquid in the glass tube.
12. The method of claim 11, wherein boiling the assembly in a
solution comprises boiling the assembly in hydrogen peroxide.
13. The method of claim 11, wherein drying the assembly comprises
drying the assembly in an oven at a temperature of about
140.degree. C.
14. The method of claim 11, wherein heating the portion of shrink
wrap comprises heating the portion of shrink wrap to a temperature
of about 350.degree. C. for two hours.
15. The method of claim 11, wherein forming a silver sulfide coated
silver wire comprises: placing a quantity of sulfur on a hot plate;
arranging a silver wire over the sulfur; and using the hot plate to
heat the sulfur to cause sublimation of the sulfur and subsequent
reaction with the silver wire, to thus form the silver sulfide
coated silver wire.
16. A reference electrode apparatus for use in electrochemical
testing applications, the apparatus comprising: a silver wire; and
a silver sulfide coating formed on the wire to produce a silver
sulfide coated silver wire; the silver sulfide coating helping to
prevent a voltage potential change in an output of the apparatus
during a test in which the silver sulfide coated silver wire is in
contact with an ionic liquid for some period of time during a
testing application.
17. The apparatus of claim 16, further comprising a glass tube in
which the silver sulfide coated silver wire is positioned during
the testing application, the glass tube further containing the
ionic liquid.
18. The apparatus of claim 17, further comprising a glass frit
secured to an end of the glass tube.
19. The apparatus of claim 18, further comprising a shrink wrap
which secures the glass frit to the glass tube.
Description
FIELD
[0002] The present disclosure relates to electrodes used with
non-aqueous solutions, and more particularly to a reference
electrode constructed such that its output is largely independent
of the solution both within the electrode and outside the
electrode.
BACKGROUND
[0003] This section provides background information related to the
present disclosure which is not necessarily prior art.
[0004] Reference electrodes are used in experimental
electrochemistry to provide a constant calibration point to which
all other potentials, either applied in the case of potential
control, or observed in the case of current control, are related.
The potential of common reference electrodes for aqueous
electrochemistry, such as silver/silver chloride or saturated
calomel electrodes, is well known for a variety of temperatures,
pH, electrolytes, etc. These universal electrodes allow for
comparison and reproducibility between different experimental
conditions and different labs.
[0005] When experiments are operated with controlled potential
(i.e., potentiostatic or voltammetry), the potential between the
working and counter electrode varies with the electrodes
(materials, coatings, etc.) and the properties of the electrolyte
(e.g., concentration, pH, etc.), which can change throughout the
experiment. When a reference electrode is used, the potential of
the working electrode is controlled relative to the reference
electrode, allowing the counter electrode to drift in potential as
required to mirror the current flowing through the working
electrode. Unlike the counter electrode, for a well-made reference
electrode, the potential should not drift throughout the
experiment, allowing for precise control of potential at the
working electrode. FIGS. 1 and 2 illustrate the undesirable
potential drift of a quasi-reference electrode making use of a
platinum wire.
[0006] There is currently no universal reference electrode for
electrochemical work with ionic liquids. The most commonly used
reference system involves a silver or platinum wire reference
electrode placed directly in the electrolyte, referred to as a
quasi-reference electrode. The potential is determined by oxides or
other surface coatings on the wire, and therefore varies with the
type, quality and cleanliness of the wire. To calibrate the wire
reference electrode, a redox couple, such as ferrocene/ferrocenium,
is added to the electrolyte. The reduction and oxidation potentials
of this redox couple is measured versus the reference electrode
wire, and the experimental potentials are corrected to the midpoint
of the redox couple reduction/oxidation potentials.
[0007] Accordingly, it would be highly desirable to provide a
stable and reproducible reference electrode for use with ionic
liquids that does not require frequent calibration with an added
redox couple. The potential of the desired reference electrode
ideally should be nearly constant regardless of the electrolyte.
The reproducibility between labs should also be better than the
current standard for reference electrodes, where the potential
depends on an unknown reaction which can change throughout the
experiment (i.e., if a small amount of metal oxide on the wire
surface is reduced to metal, the reaction will change to involve
other species on the wire).
SUMMARY
[0008] This section provides a general summary of the disclosure,
and is not a comprehensive disclosure of its full scope or all of
its features. In one aspect the present disclosure relates to a
reference electrode apparatus for use in electrochemical testing
applications. The apparatus may comprise a hollow tube and a frit
disposed adjacent one end of the hollow tube. A layer of material
may be used to secure the frit to the one end of the hollow tube to
close off the one end. A silver wire coated with a coating may be
included which helps to prevent a voltage potential change in an
output of the apparatus during a test in which the silver wire is
in contact with an ionic liquid, and where the silver wire with the
coating thereon is disposed in the hollow tube. An ionic liquid may
be disposed in the hollow tube in which at least a portion of the
silver wire is disposed.
[0009] In another aspect the present disclosure comprises a
reference electrode apparatus for use in electrochemical testing
applications. The apparatus may comprise a hollow tube, a frit
disposed adjacent one end of the hollow tube and a layer of
material securing the frit to the one end of the hollow tube to
close off the one end. A silver sulfide coated silver wire may be
disposed within the hollow tube. An ionic liquid solution may also
be disposed in the hollow tube in which at least a portion of the
silver sulfide coated silver wire is disposed.
[0010] In another aspect the present disclosure relates to a method
for forming a reference electrode for use in electrochemical
testing applications. The method may comprise positioning a glass
frit against one end of a hollow glass tube, and arranging a
portion of shrink wrap over portions of the glass frit and the
glass tube. The method may further involve heating the portion of
shrink wrap, the glass tube and the glass frit to shrink the shrink
wrap onto the portions of the glass frit and the glass tube to form
an assembly. The method may further involve boiling the assembly in
a solution to remove organic contaminants. The method may further
involve boiling the assembly in water and then drying the assembly.
The method may further involve forming a silver sulfide coated
silver wire, filling the glass tube with an ionic liquid, and
inserting at least a portion of the silver sulfide coated silver
wire into the ionic liquid in the glass tube.
[0011] In still another aspect the present disclosure relates to a
reference electrode apparatus for use in electrochemical testing
applications. The apparatus may comprise a silver wire and a silver
sulfide coating formed on the silver wire to produce a silver
sulfide coated silver wire. The silver sulfide coating helps to
prevent a voltage potential change in an output of the apparatus
during a test in which the silver sulfide coated silver wire is in
contact with an ionic liquid for some period of time during a
testing application.
[0012] Further areas of applicability will become apparent from the
description provided herein. The description and specific examples
in this summary are intended for purposes of illustration only and
are not intended to limit the scope of the present disclosure.
DRAWINGS
[0013] The drawings described herein are for illustrative purposes
only of selected embodiments and not all possible implementations,
and are not intended to limit the scope of the present
disclosure.
[0014] FIG. 1 is a prior art voltage vs. current graph illustrating
an example of the current drift of a platinum quasi reference
electrode ("QRE") in a non-aqueous solution of
butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (BM.sub.3N
NTf.sub.2), where both the working electrode ("WE") and the counter
electrode ("CE") are made from platinum;
[0015] FIG. 2 is a prior art voltage vs. current graph illustrating
the current drift of the quasi reference electrode of FIG. 1 but in
a different non-aqueous solution (1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide, EMIM NTf.sub.2);
[0016] FIG. 3 is a side view of one embodiment of a reference
electrode constructed in accordance with the present
disclosure;
[0017] FIG. 4 is a high level flowchart of various operations that
may be performed in constructing the reference electrode of FIG.
3;
[0018] FIG. 5 is a high level flowchart of various operations that
may be performed in constructing the silver sulfide coated silver
wire used in the reference electrode of FIG. 3; and
[0019] FIG. 6 is a graph illustrating the general absence of
voltage drift of the reference electrode of FIG. 3, once
stabilized, over a large number of test cycles (e.g., several
thousand test cycles, example shown in inset, in which the voltage
is ramped at a specified rate from the lower to the upper voltage
limit shown and back).
[0020] Corresponding reference numerals indicate corresponding
parts throughout the several views of the drawings.
DETAILED DESCRIPTION
[0021] Example embodiments will now be described more fully with
reference to the accompanying drawings.
[0022] In one embodiment the present invention involves a reference
electrode formed by a wire, for example a silver wire, having an
excess of silver sulfide thereon. The excess of silver sulfide may
be visible as a black coating on the silver wire. The silver
sulfide coating helps to prevent the reaction and potential change
which would otherwise would occur throughout an experiment.
[0023] The potential of the reaction occurring at the reference
electrode is the standard to which the potential at other
electrodes in the cell is measured. Therefore, the reaction
occurring at the reference electrode must be constant throughout an
experiment, and the concentration of reactants and products
involved in the reference electrode reaction must not significantly
change throughout the experiment. In this invention, the reference
electrode reaction is the reduction of silver ions to silver
metal:
Ag.sup.++e.sup.-->Ag Eo*==0.799 V
[0024] Silver ions (Ag.sup.+) are present in the solution of the
reference electrode due to the dissolution of silver sulfide
(Ag.sub.2S) according to the reaction:
Ag.sub.2S->2Ag.sup.++S.sup.2-
[0025] Silver sulfide is effectively insoluble in water, however,
it has been shown to have some limited solubility in several ionic
liquids. Electrochemistry in ionic liquids is characterized by
small current densities at the working electrode, typically
microamps per cm.sup.2. Normally, a reference electrode is
connected to the cell through a high input impedance potentiostat.
This results in very little current flowing through the reference
electrode, very little silver ion reduction, and very little
sulfide production. It is therefore expected that the small
increase in sulfide concentration in the reference electrode tube
will not significantly change the solubility of silver sulfide, and
therefore, will not affect the reduction reaction potential.
[0026] The silver sulfide coated silver wire is responsible for the
stable reference electrode potential reported here, however, to
further improve the reference electrode, it is suggested that a
tube with a porous frit be included. Using a tube to contain the
silver sulfide coated silver wire reference electrode allows the
electrode to stabilize more quickly, and prevents the
electrochemical cell from becoming contaminated with silver and
sulfide ions from the reference electrode.
[0027] Referring to FIG. 3, a high level illustration of an
apparatus in the form of a reference electrode 10 in accordance
with one embodiment of the present disclosure is shown. The
reference electrode 10 (hereinafter simply "electrode" 10) in this
example may be constructed for virtually any size electrochemical
cell, but in one specific implementation is constructed for use
with a small-volume (.about.2 mL) electrochemical cell, where the
electrode diameter needs to be about 4 mm or less. In this example
the electrode 10 may be constructed by using a shrink tubing 12,
for example a TEFLON.TM. shrink tubing, to attach a frit, which in
this example is an ultrafine porous glass frit 14 to a hollow,
small diameter glass tube 16. The glass frit 14 is positioned
adjacent one end of the glass tube 16 so as to block off the one
end of the glass tube once the shrink wrap tubing 12 is secured
over adjacent portions of the glass frit and the glass tube. In
this example the porous glass frit 14 and the glass tube 16 are
each about 4 mm in diameter. However, it will be appreciated that
these dimensions may vary somewhat depending on a specific
application. Also, it will be appreciated that the glass frit 14
may instead be placed inside one end of the glass tube 16 and then
secured with epoxy, or by melting the tube, or by any other
suitable means of securing. Still further, it will be appreciated
that the glass tube 16 may instead be a tube made from some other
suitable material and coated with a TEFLON.TM. coating, or a tube
made from plastic or from any other suitable material.
[0028] With further reference to FIG. 3 and additional reference to
flowchart 100 of FIG. 4, the assembly of the glass frit 14 and
glass tube 16, using the shrink wrap tubing 12, may be accomplished
by placing the shrink wrap tubing 12 over a portion 16a of the
glass tube 16 and a portion 14a of the glass frit 14 (FIG. 4,
operation 102), and baking the assembly in an oven for a time
sufficient to shrink the shrink wrap tubing 12 onto the glass frit
14 and the glass tube 16 (FIG. 4, operation 104). For example, the
baking may be performed at 350.degree. C. for three or more hours
to sufficiently shrink the shrink wrap tubing 14 onto the glass
tube 16 and the glass frit 14.
[0029] The glass frit 14, glass tube 16 and shrink wrap assembly 12
may then be boiled in a suitable solution, for example hydrogen
peroxide, to remove organic contamination in the glass frit 14
(FIG. 4, operation 106), and more particularly until an initial
brown coloration of the glass frit disappears. The glass frit 14,
glass tube 16 and shrink wrap 12 assembly may then be boiled in
deionized water for about 10-20 minutes (FIG. 4, operation 108),
then dried in an oven at a suitable temperature (e.g., about
140.degree. C.) until thoroughly dried. The drying typically may
take two or more hours (FIG. 4, operation 110). The glass tube 16
may then be filled with an ionic liquid, represented in FIG. 3 by
dashed line 18. Finally, a silver sulfide coated silver wire 20 may
be placed in the ionic liquid 18 within the glass tube 16 (FIG. 4,
operation 112) preferably such that the wire extends generally
parallel along a substantial portion of the longitudinal length of
the glass tube.
[0030] FIG. 5 shows a high level flowchart 200 for constructing the
silver sulfide coated silver wire 20. At operation 202 a quantity
of sulfur may be placed on a hot plate. At operation 204 a silver
wire having a desired gauge or diameter (for example, 0.5 mm
diameter) may be supported in a stationary manner over the sulfur.
At operation 206 the hot plate may be turned on to cause
sublimation of sulfur onto the exterior surface of the silver wire.
This produces a silver sulfide coating on the silver wire which
appears as a black coating. Alternatively, a silver sulfide coating
could also be formed by exposing the silver wire to hydrogen
sulfide gas for a period of approximately 10 minutes, or by
covering the silver wire with sulfur (powder) for 24 hours before
rinsing the silver wire (coated with a black silver sulfide layer)
with water.
[0031] FIG. 6 illustrates a graph 300 showing an example of the
half-wave potential (E.sub.1/2) of the oxidation and reduction of
ferrocene/ferrocenium in curve 302, measured with respect to the
silver wire electrode reference electrode coated with silver
sulfide (Ag.sub.2S) during a test. The curve 302 illustrates how
after an initial break in period for a newly made reference
electrode, denoted by portion 302a and covering typically 50-75 or
so cycles, the potential stabilizes (portion 302b). Once
stabilized, the potential drift is less than 10 mV for one week of
continuous use, or at least one month of non-continuous use. Inset
graph 304 illustrates how the voltage was ramped at a specified
rate from the lower to the upper voltage limit shown, and back,
during this test.
[0032] The reference electrode 10 of the present disclosure is
especially useful with non-aqueous solutions, where the potential
of the reference electrode 10 is largely independent of the
solution both within the electrode (filling solution) and outside
the electrode (electrolyte). The potential of the reference
electrode 10 is fixed by the reduction reaction of silver ions
(Ag.sup.+) to silver metal (Ag), which occurs at the interface of a
silver wire that is coated with silver sulfide. The potential of
electrode 10 is determined by the Ag.sup.+ concentration, which is
a function of the solubility of the silver sulfide coating. These
coatings are chosen for their very low solubility, thus the
Ag.sup.+ and S.sup.2- concentrations remain low and stable
throughout the experiment. The filling solution of the reference
electrode can be matched to the electrolyte in the system, thereby
avoiding the junction potential that would be present if the
filling solution was different from the electrolyte.
[0033] The reference electrode 10 of the present disclosure
provides a number of significant advantages over heretofore
manufactured reference electrodes used with ionic liquids. The
electrode 10 is relatively easy to manufacture and highly
reproducible. It can be made in a normal air atmosphere, has no
detrimental sensitivity to air or water, and is not sensitive to
light. Additional advantages of the reference electrode 10 are that
the same ionic liquid may be used inside and outside the electrode
glass tube 16. Therefore, there is no liquid junction potential (or
possibly only an exceedingly small liquid junction potential). The
silver ion concentration is held constant by the equilibrium of the
sparingly soluble silver sulfide coating on the silver wire, which
results in virtually no voltage potential drift during extended
periods of use.
[0034] An important use of the electrode 10 is expected to be as a
reference electrode for use in experiments involving ionic liquids.
The stability or solubility of silver salts in each ionic liquid
does not need to be known beforehand, as the silver ion
concentration is determined by the solubility of the silver sulfide
coating. Furthermore, electrode 10 can be assembled in a normal air
atmosphere (no glovebox required), making it well suited to
industrial processes.
[0035] Applications for the reference electrode 10 may include
development and monitoring of energy storage systems with
non-aqueous solvents (i.e., Lithium ion batteries), as well as
plating, coating and electroforming applications involving
non-aqueous solvents. It is possible that the reference electrode
10 may also find application with organic solvents such as
acetonitrile, or deep eutectic solvents. The reference electrode 10
may of course also be used in aqueous solvents.
[0036] The foregoing description of the embodiments has been
provided for purposes of illustration and description. It is not
intended to be exhaustive or to limit the disclosure. Individual
elements or features of a particular embodiment are generally not
limited to that particular embodiment, but, where applicable, are
interchangeable and can be used in a selected embodiment, even if
not specifically shown or described. The same may also be varied in
many ways. Such variations are not to be regarded as a departure
from the disclosure, and all such modifications are intended to be
included within the scope of the disclosure.
[0037] Example embodiments are provided so that this disclosure
will be thorough, and will fully convey the scope to those who are
skilled in the art. Numerous specific details are set forth such as
examples of specific components, devices, and methods, to provide a
thorough understanding of embodiments of the present disclosure. It
will be apparent to those skilled in the art that specific details
need not be employed, that example embodiments may be embodied in
many different forms and that neither should be construed to limit
the scope of the disclosure. In some example embodiments,
well-known processes, well-known device structures, and well-known
technologies are not described in detail.
[0038] The terminology used herein is for the purpose of describing
particular example embodiments only and is not intended to be
limiting. As used herein, the singular forms "a," "an," and "the"
may be intended to include the plural forms as well, unless the
context clearly indicates otherwise. The terms "comprises,"
"comprising," "including," and "having," are inclusive and
therefore specify the presence of stated features, integers, steps,
operations, elements, and/or components, but do not preclude the
presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof. The
method steps, processes, and operations described herein are not to
be construed as necessarily requiring their performance in the
particular order discussed or illustrated, unless specifically
identified as an order of performance. It is also to be understood
that additional or alternative steps may be employed.
[0039] When an element or layer is referred to as being "on,"
"engaged to," "connected to," or "coupled to" another element or
layer, it may be directly on, engaged, connected or coupled to the
other element or layer, or intervening elements or layers may be
present. In contrast, when an element is referred to as being
"directly on," "directly engaged to," "directly connected to," or
"directly coupled to" another element or layer, there may be no
intervening elements or layers present. Other words used to
describe the relationship between elements should be interpreted in
a like fashion (e.g., "between" versus "directly between,"
"adjacent" versus "directly adjacent," etc.). As used herein, the
term "and/or" includes any and all combinations of one or more of
the associated listed items.
[0040] Although the terms first, second, third, etc. may be used
herein to describe various elements, components, regions, layers
and/or sections, these elements, components, regions, layers and/or
sections should not be limited by these terms. These terms may be
only used to distinguish one element, component, region, layer or
section from another region, layer or section. Terms such as
"first," "second," and other numerical terms when used herein do
not imply a sequence or order unless clearly indicated by the
context. Thus, a first element, component, region, layer or section
discussed below could be termed a second element, component,
region, layer or section without departing from the teachings of
the example embodiments.
[0041] Spatially relative terms, such as "inner," "outer,"
"beneath," "below," "lower," "above," "upper," and the like, may be
used herein for ease of description to describe one element or
feature's relationship to another element(s) or feature(s) as
illustrated in the figures. Spatially relative terms may be
intended to encompass different orientations of the device in use
or operation in addition to the orientation depicted in the
figures. For example, if the device in the figures is turned over,
elements described as "below" or "beneath" other elements or
features would then be oriented "above" the other elements or
features. Thus, the example term "below" can encompass both an
orientation of above and below. The device may be otherwise
oriented (rotated 90 degrees or at other orientations) and the
spatially relative descriptors used herein interpreted
accordingly.
* * * * *