U.S. patent application number 14/174459 was filed with the patent office on 2015-08-06 for lead-free galvanic oxygen sensor.
The applicant listed for this patent is HONEYWELL INTERNATIONAL INC.. Invention is credited to Christa DUMSCHAT.
Application Number | 20150219583 14/174459 |
Document ID | / |
Family ID | 52432699 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150219583 |
Kind Code |
A1 |
DUMSCHAT; Christa |
August 6, 2015 |
LEAD-FREE GALVANIC OXYGEN SENSOR
Abstract
A lead-free galvanic oxygen sensor having an aqueous electrolyte
and a bismuth anode is disclosed. The electrolyte contains a polyol
in addition to water and a salt. Surprisingly, a sensor with such
an electrolyte has an increased resistance to passivation.
Inventors: |
DUMSCHAT; Christa; (Wismar,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HONEYWELL INTERNATIONAL INC. |
Morristown |
NJ |
US |
|
|
Family ID: |
52432699 |
Appl. No.: |
14/174459 |
Filed: |
February 6, 2014 |
Current U.S.
Class: |
204/431 |
Current CPC
Class: |
G01N 27/404 20130101;
G01N 27/30 20130101 |
International
Class: |
G01N 27/26 20060101
G01N027/26 |
Claims
1. An oxygen sensor comprising: a housing, a cathode, a bismuth
anode, and an aqueous electrolyte comprising a polyol and a salt
wherein the anode is substantially free of lead.
2. The oxygen sensor of claim 1 wherein the cathode comprises an
electrically conductive material selected from the group consisting
of platinum, gold, silver, palladium, rhodium, iridium and carbon
plated with platinum, gold, silver, palladium, rhodium, or
iridium.
3. The oxygen sensor of claim 1 wherein the cathode is carbon
plated with platinum.
4. The oxygen sensor of claim 1 wherein the salt is an alkaline
salt.
5. The oxygen sensor of claim 1 wherein the salt is selected from
the group consisting of potassium hydroxide, sodium hydroxide,
potassium acetate, and sodium acetate.
6. The oxygen sensor of claim 1 wherein the salt is potassium
hydroxide.
7. The oxygen sensor of claim 1 wherein the salt is an ammonium
quaternary hydroxide, R.sub.4N.sup.+OH.sup.-.
8. The oxygen sensor of claim 7 wherein R is an alkyl group
selected from the group consisting of methyl, ethyl, propyl, butyl,
and mixtures thereof.
9. The oxygen sensor of claim 1 wherein the polyol is selected from
the group consisting of glycerol, erythritol, sorbitol, ethylene
glycol, and mixtures thereof.
10. The oxygen sensor of claim 1 wherein the polyol is
glycerol.
11. The oxygen sensor of claim 1 wherein the polyol comprises about
20% to about 30% by volume.
12. An oxygen sensor comprising: a housing, a cathode, a bismuth
anode, and an aqueous electrolyte that includes a salt and a
polyol, wherein the anode is substantially free of lead.
13. The oxygen sensor of claim 12 wherein the cathode comprises an
electrically conductive material selected from the group consisting
of platinum, gold, silver, palladium, rhodium, iridium and carbon
plated with platinum, gold, silver, palladium, rhodium, or
iridium.
14. The oxygen sensor of claim 12 wherein the salt is selected from
the group consisting of potassium hydroxide, sodium hydroxide,
potassium acetate, and sodium acetate.
15. The oxygen sensor of claim 12 wherein the salt is potassium
hydroxide.
16. The oxygen sensor of claim 12 wherein the salt is an ammonium
quaternary hydroxide, R.sub.4N.sup.+OH.sup.-.
17. The oxygen sensor of claim 16 wherein R is an alkyl group
selected from the group consisting of methyl, ethyl, propyl, butyl,
and mixtures thereof.
18. The oxygen sensor of claim 12 wherein the polyol is selected
from the group consisting of glycerol, erythritol, sorbitol and
ethylene glycol.
19. The oxygen sensor of claim 12 wherein the polyol is
glycerol.
20. An electrochemical sensor comprising a bismuth anode and
aqueous electrolyte comprising a polyol and a salt.
Description
FIELD
[0001] This application pertains to a lead-free galvanic oxygen
sensor. More particularly, the application pertains to a lead-free
galvanic oxygen sensor having a housing, a cathode, a bismuth-based
anode, and an aqueous electrolyte including a salt and a
polyol.
[0002] BACKGROUND
[0003] Galvanic oxygen sensors based on consumable lead anodes are
well-known. These instruments are generally reliable and have good
sensitivity. The presence of lead, however, is undesirable in such
an instrument due to environmental and health concerns associated
with lead contamination.
[0004] Thus, there is a need for a reliable, sensitive galvanic
oxygen sensor, which avoids the use of lead anodes.
[0005] Recently, there have been attempts to replace lead anodes
with those made of zinc, aluminum, and tin. It appears that those
types of anodes have a very limited lifetime though, due to the
self-corrosion and passivation of the anode surface.
[0006] There is thus a continuing need for a lead-free galvanic
oxygen sensor. It was surprising and unexpected that the addition
of a polyol to an aqueous electrolyte prevents passivation of a
bismuth anode in a galvanic oxygen sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic illustration of a lead-free galvanic
oxygen sensor.
DETAILED DESCRIPTION
[0008] While disclosed embodiments can take many different forms,
specific embodiments hereof are shown in the drawings and will be
described herein in detail with the understanding that the present
disclosure is to be considered as an exemplification of the
principles hereof, as well as the best mode of practicing same, and
is not intended to limit the claims hereof to the specific
embodiment illustrated.
[0009] The present invention relates to a lead-free galvanic oxygen
sensor. FIG. 1 depicts a lead-free galvanic oxygen sensor 10 shown
generally in accordance with one illustrated embodiment. The sensor
10 can be constructed with a plastic or metal housing 12. Included
within the housing 12 are a cathode 14, a bismuth anode 16, an
aqueous electrolyte including a salt and a polyol 18, and a barrier
20 (permeable membrane or capillary). The cathode 14 and anode 16
can be coupled to an external load resistor 24 by a set of wire
collectors 22.
[0010] The cathode 14 is an electrically conductive material
selected from the group consisting of platinum, gold, silver,
palladium, rhodium, iridium and carbon plated with platinum, gold,
silver, palladium, rhodium, or iridium or any other suitable
material. In one embodiment, the cathode 14 is made of a
polytetrafluoroethylene (PTFE) membrane impregnated with a high
surface area platinum catalyst embedded in a carbon matrix.
[0011] The anode 16 is made of bismuth, which is thermodynamically
stable in water. The standard potential for the following reaction:
Bi.sub.2O.sub.3+3H.sub.2O.sub.(I)+6e.sup.-2Bi.sub.(s)+6OH.sup.- is
-0.46 volt . There is no self-corrosion due to hydrogen evolution
at the anode and no self-corrosion due to hydrogen evolution at the
cathode. Importantly, bismuth is commercially available and is not
toxic.
[0012] The aqeuous electrolyte in the sensor contains a salt and a
polyol. The salt is selected from the group consisting of potassium
hydroxide, sodium hydroxide, potassium acetate, and sodium acetate.
In yet another embodiment, the salt is potassium hydroxide. The
concentration of potassium hydroxide ranges from about 1 M to
saturation.
[0013] The salt can also be ammonium quaternary hydroxide, such as
R.sub.4N.sup.+OH.sup.-wherein R is an alkyl group selected from the
group consisting of methyl, ethyl, propyl, butyl, and mixtures
thereof. The electrolyte is not consumed by the oxygen sensing
reaction. Furthermore, water is not involved in the overall
electrochemical reaction so that the water level of the sensor will
be governed by external factors.
[0014] In other sensors, the electrochemical oxidation of bismuth
leads to the formation of a Bi.sub.2O.sub.3 layer on the surface of
the electrode in the presence of water in a neutral or an alkaline
electrolytic environment. That layer passivates the electrode,
which means no current can flow through the surface at moderate
voltages.
[0015] Surprisingly, the addition of a polyol to the aqueous
electrolyte suppresses passivation of the bismuth anode. The polyol
can be glycerol, erythritol, sorbitol, ethylene glycol, and
mixtures thereof. In one embodiment, the polyol is glycerol. The
glycerol can be present in the range of about 5% to about 70% by
volume. In yet another embodiment, the glycerol is present at about
20% to about 30% by volume.
[0016] The permeable barrier 20 is selectively permeable to oxygen.
For example, a tetrafluoroethylene resin membrane or a
tetrafluoroethylene-hexafluoropropylene copolymer membrane can be
used. It is also possible to use a capillary as diffusion
barrier.
[0017] As for the wire collectors 22, these can be made of nickel
or platinum.
[0018] It was an unexpected and surprising benefit that the
addition of a polyol to the aqueous electrolyte would lead to
prevention of passivation of the bismuth anode that leads to
decreased sensor life. Such a sensor has an excellent linearity for
the oxygen partial pressure. The signal at oxygen is 4.7 times
higher than that in air.
[0019] In addition, an accelerated lifetime test of a lead-free
oxygen sensor having a bismuth anode was carried out in pure oxygen
with an electrolyte containing either 7 M KOH in water or 7 M KOH
with a mixture of 30% by volume glycerol in water as solvent. The
sensor having the KOH alone stopped working after 7 days due to
passivation of the bismuth anode. The expected lifetime in air is
one month. Surprisingly, the sensor having the KOH and glycerol
mixture worked for 167 days in oxygen. The expected lifetime in air
was two years.
[0020] In another test, galvanostatic measurements were done on the
different polyols to determine their suitability in the oxygen
sensor. Electrolyte solutions were prepared having 3 g of the
desired polyol dissolved/mixed in 7 ml of 10 M KOH, with the
exception of ethanol in which the solvents did not mix completely.
The current on the bismuth electrode was kept constant at 30 mA and
the time of a sudden rise in potential was noted.
TABLE-US-00001 TABLE 1 ethylene D- meso- H.sub.2O glycol sorbitol
erythritol glycerol ethanol Time to sudden 0.75 8.5 2.5 14.5 36.3 0
rise in potential (min.) no. of 2 2 2 2 3 1 measurements
[0021] Thus, these experiments demonstrate the suitability of the
claimed polyols in the oxygen sensor described herein. Such a
bismuth electrode and aqueous electrolyte comprising a polyol and a
salt can be useful in other electrochemical sensors and galvanic
cells for other purposes, as in batteries.
[0022] From the foregoing, it will be observed that numerous
variations and modifications may be effected without departing from
the spirit and scope of the invention. It is to be understood that
no limitation with respect to the specific apparatus illustrated
herein is intended or should be inferred. It is, of course,
intended to cover by the appended claims all such modifications as
fall within the scope of the claims.
* * * * *