U.S. patent number RE45,186 [Application Number 10/621,637] was granted by the patent office on 2014-10-14 for low cost room temperature electrochemical carbon monoxide and toxic gas sensor with humidity compensation based on protonic conductive membranes.
This patent grant is currently assigned to Atwood Mobile Products Inc.. The grantee listed for this patent is Franco Consadori, D. George Field, Yousheng Shen. Invention is credited to Franco Consadori, D. George Field, Yousheng Shen.
United States Patent |
RE45,186 |
Shen , et al. |
October 14, 2014 |
Low cost room temperature electrochemical carbon monoxide and toxic
gas sensor with humidity compensation based on protonic conductive
membranes
Abstract
A low cost room temperature electrochemical gas sensor with
humidity compensation for sensing CO, alcohol vapors and other
toxic analyte gases has a solid protonic conductive membrane with a
low bulk ionic resistance. A sensing electrode and a counter
electrode, optionally a counter electrode and a reference
electrode, which are separated by the membrane, can be made of
mixed protonic-electronic conductors, or can be made of a thin
electrically conducting film such as platinum. A reservoir of water
maintain the solid protonic conductive membrane at constant 100
percent relative humidity to compensate for ambient humidity
changes. Embodiments of the inventive sensor also include an
electrochemical analyte gas pump to transport the analyte gas away
from the counter electrode side of the sensor. Analyte gas pumps
for the inventive sensor include dual pumping electrodes situated
on opposite sides of the membrane, and include a means for applying
a DC power across the membrane to the sensing and counter
electrodes. Another embodiment of the inventive sensor has first
and second solid protonic conductive membranes, one of which has a
sensing electrode and a counter electrode separated by the first
membrane, and the other of which has dual pump electrodes situated
on opposite sides of the second membrane.
Inventors: |
Shen; Yousheng (Draper, UT),
Consadori; Franco (San Juan Capistrano, CA), Field; D.
George (Pleasant Grove, UT) |
Applicant: |
Name |
City |
State |
Country |
Type |
Shen; Yousheng
Consadori; Franco
Field; D. George |
Draper
San Juan Capistrano
Pleasant Grove |
UT
CA
UT |
US
US
US |
|
|
Assignee: |
Atwood Mobile Products Inc.
(Elkhart, IN)
|
Family
ID: |
24083025 |
Appl.
No.: |
10/621,637 |
Filed: |
July 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
08381718 |
Jan 31, 1995 |
5573648 |
|
|
Reissue of: |
08522946 |
Sep 1, 1995 |
5650054 |
Jul 22, 1997 |
|
|
Current U.S.
Class: |
204/412; 205/784;
205/788; 204/424; 205/781; 204/421; 205/786.5; 205/783.5 |
Current CPC
Class: |
G01N
27/4074 (20130101); G01N 33/004 (20130101); G01N
27/4045 (20130101) |
Current International
Class: |
G01N
27/407 (20060101) |
Field of
Search: |
;204/412,421-429
;205/781,783.5,784,786.5,788 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Mahlon S. Wilson, Fernando H. Garzon, Kurt E. Sickafus, and
Shimshon Gottesfeld, Surface Area Loss of Supported Platinum in
Polymer Electrolyte Fuel Cells, vol. 140, No. 10, Oct. 1993. cited
by examiner .
"Humidity Dependence of Carbon Monoxide Oxidation Rate in a
Nafion-Based Electrochemical Cell," Lee et al., Technical Papers,
Solid-State Science and Technology, J. Electrochem. Soc., vol. 142,
No. 1, Jan. 1995; pp. 157-160. cited by applicant .
"Recent Developments in Electrochemical Solid Polymer Electrolyte
Sensor Cells for Measuring Carbon Monoxide and Oxides of Nitrogen,"
La Conti et al., Chemical Hazards in the Workplace: Measurement and
Control, ACS Symposium, series 149, 1981; pp. 551-573. cited by
applicant .
"Effects of Surface Roughening of Nafion.sup.r on Electrode
Plating, Mechanical Strength, and Cell Performances for SPE Water
Electrolysis," Sakai et al., J. Electrochem. Soc., 137:3777-3783,
1990. cited by applicant .
"Chemical Sensing with Solid State Devices," Madou and Morrison,
Academic Press, Inc., pp. 448-459, 1989. cited by applicant .
"Fuel Cell Systems," Edited by Blomen and Mugerwa, Plenum Press,
Srinivasan et al., pp. 48-52 and 63-67, 1993. cited by applicant
.
Notice of Opposition in European Patent No. 762117B, including
Opponent's statement of grounds (entitled "Reasons") (18 pages
total). cited by applicant .
Sung B. Lee, Anthony Cocco, Darioush Keyvani and G. Jordan Maclay,
Humidity Dependence of Carbon Monoxide Oxidation Rate in a
Nafion-Based Electrochemical Cell, vol. 142, No. 1, Jan. 1995.
cited by applicant.
|
Primary Examiner: Olsen; Kaj K
Attorney, Agent or Firm: Banner & Witcoff, Ltd.
Parent Case Text
This is a continuation-in-part of U.S. Pat. application Ser. No.
08/381,718 Filed on Jan. 31, 1995, now U.S. Pat. No. 5,573,648,
titled "Gas Sensor Based on Protonic Conductive Membranes", which
is incorporated herein by reference.
Claims
What is claimed and desired to be secured by United States Letters
Patent is:
1. An electrochemical gas sensor for quantitative measurement of a
gas in a ambient atmosphere comprising: a sensing electrode
permeable to water vapor and comprised of an electrical conducting
material and having a surface exposed to the ambient atmosphere; a
counter electrode permeable to water vapor and comprised of an
electrical conducting material; a first protonic conductive
electrolyte membrane permeable to water vapor and situated between
and in contact with the sensing and counter electrodes, the sensing
electrode reacting with the gas to produce a change in electrical
characteristic between the sensing electrode and the counter
electrode; means for electrical measurement electrically connected
to said sensing and counter electrodes; means, containing a volume
of water vapor, for exposing a surface of said counter electrode to
said water vapor, wherein the electrical conducting material of at
least one of said sensing and counter electrodes is a
proton-electron mixed conductive material having 10-50 wt % of a
proton conductor material and 50-90 wt % of a first and a second
electrical conductor material; whereby, in a positive ambient
atmosphere concentration of said gas, said electrical measurement
means detects changes in said electrical characteristic.
2. The electrochemical gas sensor as defined in claim 1, wherein
said water vapor containing means contains a volume of water and an
antifreeze additive.
3. The electrochemical gas sensor as defined in claim 1, wherein
the surface of said sensing electrode that is exposed to the
ambient atmosphere has a surface area that is smaller than the
surface area of the surface of the counter electrode that is
exposed to said water vapor, whereby the first protonic conductive
electrolyte membrane is exposed to substantially 100 percent
relative humidity, and a positive pressure of said water vapor
exists from the surface of said counter electrode exposed to said
water vapor to the surface of said sensing electrode exposed to the
ambient atmosphere.
4. The electrochemical gas sensor as defined in claim 3, wherein
the surface area of the surface of the counter electrode that is
exposed to said water vapor is separated from said means for
exposing a surface of said counter electrode to said water vapor by
a hydrophobic membrane permeable to water vapor and substantially
impervious to water.
5. The electrochemical gas sensor as defined in claim 1, wherein
the first protonic conductive electrolyte membrane has opposing
surfaces, each of said opposing surfaces being in contact with one
of the sensing and counter electrodes, wherein at least one of the
opposing surfaces of said first protonic conductive electrolyte
membrane in contact with one of the sensing and counter electrodes
is substantially nonplanar.
6. The electrochemical gas sensor as defined in claim 1, wherein at
least one of the sensing and counter electrodes is comprised of
film having a thickness in the range of about 50 Angstroms to
10,000 Angstroms.
7. The electrochemical gas sensor as defined in claim 6, wherein
the film is substantially composed of a noble metal.
8. The electrochemical gas sensor as defined in claim 7, wherein
the noble metal is platinum.
9. The electrochemical gas sensor as defined in claim 1, wherein
the first protonic conductive electrolyte membrane is substantially
composed of a solid, perfluorinated, ion-exchange polymer.
10. The electrochemical gas sensor as defined in claim 1, wherein
the first protonic conductive electrolyte membrane is a hydrated
metal oxide protonic conductor electrolyte membrane.
11. The electrochemical gas sensor as defined in claim 1, wherein
the proton conductor material for said at least one of the sensing
and counter electrodes is a copolymer having a tetrafluoroethylene
backbone with a side chain of perfluorinated monomers containing at
least one of a sulfonic acid group or a carboxylic acid group.
12. The electrochemical gas sensor as defined in claim 1, wherein
one of the first and second electrical conductor materials for said
at least one of the sensing and counter electrodes is about 50-99
wt % of carbon black, and the other of the first and second
electrical conductor materials for said at least one of the sensing
and counter electrodes is about 1-50 wt % of platinum.
13. The electrochemical gas sensor as defined in claim 1, wherein
one of the first and second electrical conductor materials for said
at least one of the sensing and counter electrodes is about 50-99
wt % of carbon black, and the other of the first and second
electrical conductor materials for said at least one of the sensing
and counter electrodes is about 1-50 wt % of Ru oxide.
14. The electrochemical gas sensor as defined in claim 1, wherein
the electrochemical gas sensor further comprises: first and second
pump electrodes comprised of an electrical conducting material
permeable to water vapor, separate from said sensing and counter
electrodes, and situated on opposite sides of and in contact with
said first protonic conductive electrolyte membrane, said second
pump electrode being situated on the same side of said first
protonic conductive membrane as the counter electrode and having a
surface thereon exposed to the water vapor in said means for
exposing a surface of said counter electrode to said water vapor;
and means for applying a DC power across the first protonic
conductive electrolyte membrane, said first and second pump
electrodes having in electrical connection therebetween said means
for applying DC power across the first protonic conductive
electrolyte membrane; whereby the gas is transported away from the
counter electrode when the DC power means applies a DC power to the
first and second pump electrodes.
15. The electrochemical gas sensor of claim 14, wherein the
electrical conducting material of the first and second pump
electrodes is substantially composed of carbon.
16. The electrochemical gas sensor as defined in claim 14, wherein
the electrical conducting material of the first and second pump
electrodes is substantially composed of noble metals.
17. The electrochemical gas sensor as defined in claim 14, wherein
the electrical conducting material of the first and second pump
electrodes is substantially composed of conductive hydrated metal
oxides.
18. The electrochemical gas sensor as defined in claim 14, wherein
at least one of the first and second pump electrodes is comprised
of a film having a thickness in the range of about 50 Angstroms to
10,000 Angstroms.
19. The electrochemical gas sensor as defined in claim 14, wherein
the electrical conducting material of said first and second pump
electrodes is a proton-electron mixed conductive material having
10-50 wt % of a proton conductor material and 50-90 wt % of a first
and a second electrical conductor material.
20. The electrochemical gas sensor as defined in claim 19, wherein
the proton conductor material for both the first and second pump
electrodes is a copolymer having a tetrafluoroethylene backbone
with a side chain of perfluorinated monomers containing at least
one of a sulfonic acid group or a carboxylic acid group.
21. The electrochemical gas sensor as defined in claim 19, wherein
one of the first and second electrical conductor materials for the
first pump electrode is about 50-99 wt % of carbon black, and the
other of the first and second electrical conductor materials for
the first pump electrode is 1 to 50 wt % of platinum.
22. The electrochemical gas sensor as defined in claim 19, wherein
one of the first and second electrical conductor materials for the
second pump electrode is about 50-99 wt % of carbon black, and the
other of the first and second electrical conductor materials for
the second pump electrode is 1 to 50 wt % of Ru oxide.
23. The electrochemical gas sensor as defined in claim 1, wherein
the electrochemical gas sensor further comprises: a second protonic
conductive electrolyte membrane permeable to water vapor; first and
second pump electrodes permeable to water vapor and comprised of an
electron conductive material, and being separate from said sensing
and counter electrodes and situated on opposite sides of and in
contact with said second protonic conductive electrolyte membrane,
said means for exposing a surface of said counter electrode to said
water vapor exposing a surface of said second pump electrode to
said water vapor, and said first pump electrode having a surface
exposed to the ambient atmosphere; and means for applying a DC
power across said second protonic electrolyte membrane, said first
and second pump electrodes having in electrical connection
therebetween said means for applying DC power across said second
protonic electrolyte membrane; whereby the gas is transported away
from the counter electrode when the DC power means applies a DC
power to the first and second pump electrodes.
24. The electrochemical gas sensor as defined in claim 23, wherein
the second protonic conductive electrolyte membrane is
substantially composed of a solid, perfluorinated, ion-exchange
polymer.
25. The electrochemical gas sensor as defined in claim 23, wherein
the second protonic conductive electrolyte membrane is a hydrated
metal oxide protonic conductor electrolyte membrane.
26. The electrochemical gas sensor as defined in claim 23, wherein
the surface area of the surface of said first pump electrode that
is exposed to the ambient atmosphere is smaller than the surface
area of the surface of the second pump electrode that is exposed to
said water vapor, whereby the second protonic conductive
electrolyte membrane is exposed to substantially 100 percent
relative humidity, and a positive pressure of said water vapor
exists from the surface of said second pump electrode that is
exposed to said water vapor to the surface of said first pump
electrode that is exposed to the ambient atmosphere.
27. The electrochemical gas sensor as defined in claim 26, wherein
the surface area of the surface of the second pump electrode that
is exposed to said water vapor is separated from said means for
exposing a surface of said counter electrode to said water vapor by
a hydrophobic membrane permeable to water vapor and substantially
impervious to water.
28. The electrochemical gas sensor as defined in claim 1, further
comprising: means for applying a DC pulse power source across the
first protonic conductive membrane, said sensing and counter
electrodes having in electrical connection therebetween said means
for applying DC pulse power across the first protonic conductive
membrane; and switch means for alternating the connection between
the sensing and counter electrodes from the electrical measurement
means to the DC pulse power means; whereby, in a positive ambient
atmosphere concentration of said gas, said electrical measurement
means detects changes in said electrical characteristic when said
switch means connects said electrical measurement means to the
sensing and counter electrodes; and whereby said DC pulse power
means moves the gas away from a side of the gas sensor where the
counter electrode is placed when said switch means connects said DC
pulse power means to the sensing and counter electrodes.
29. The electrochemical gas sensor as defined in claim 1, wherein
the gas is CO.
30. The electrochemical gas sensor as defined in claim 1, wherein
the gas is NO.sub.x.
31. The electrochemical gas sensor as defined in claim 1, wherein
the gas is hydrogen.
32. The electrochemical gas sensor as defined in claim 1, wherein
the gas is H.sub.2S.
33. The electrochemical gas sensor as defined in claim 1, wherein
the gas is H.sub.2O vapor.
34. The electrochemical gas sensor as defined in claim 1, wherein
the gas is alcohol vapor.
35. An electrochemical gas sensor for quantitative measurement of a
gas in an ambient atmosphere comprising: a sensing electrode
permeable to water vapor and comprised of an electrical conducting
material and having a surface exposed to the ambient atmosphere; a
counter electrode permeable to water vapor and comprised of an
electrical conducting material; a first protonic conductive
electrolyte membrane permeable to water vapor and situated in
between and in contact with the sensing and counter electrodes, the
sensing electrode reacting with the gas to produce a change in
electrical characteristic between the sensing electrode and the
counter electrode; a second protonic conductive electrolyte
membrane permeable to water vapor; first and second pump electrodes
permeable to water vapor and comprised of an electrical conductive
material, and being separate from said sensing and counter
electrodes and situated on opposite sides of and in contact with
said second protonic conductive electrolyte membrane; means,
containing a volume of water vapor, for exposing a surface of said
second pump electrode to said water vapor, and said first pump
electrode having a surface exposed to the ambient atmosphere, said
second pump electrode being separated from said counter electrode
by said means for exposing a surface of said second pump electrode
to said water vapor, and said counter electrode having a surface
exposed to said water vapor by said means for exposing a surface of
said second pump electrode to said water vapor; means for
electrical measurement in electrical communication with said
sensing electrode and said counter electrode; and means for
applying a DC power across said second protonic electrolyte
membrane in electrical contact with said first and second pump
electrodes; whereby the gas is transported away from the counter
electrode when the DC power means applies a DC power across said
second protonic electrolyte membrane; and whereby, in a positive
ambient concentration of said gas, said electrical measurement
means detects changes in said electrical characteristic.
36. The electrochemical gas sensor as defined in claim 35, wherein
at least one of said first and second protonic conductive
electrolyte membranes is substantially comprised of a solid,
perfluorinated, ion-exchange polymer.
37. The electrochemical gas sensor as defined in claim 35, wherein
at least one of the first and second protonic conductive
electrolyte membranes is a hydrated metal oxide protonic conductor
electrolyte membrane.
38. The electrochemical gas sensor as defined in claim 35, wherein
the surface of said first pump electrode that is exposed to the
ambient atmosphere has a surface area smaller than the surface area
of the surface of the second pump electrode that is exposed to said
water vapor, and wherein the surface of said sensing electrode that
is exposed to the ambient atmosphere has a surface area smaller
than the surface area of the surface of the counter electrode that
is exposed to said water vapor, whereby the first protonic
conductive electrolyte membrane is exposed to substantially 100
percent relative humidity, a positive pressure of said water vapor
exists from the surface of said counter electrode that is exposed
to said water vapor to the surface of said sensing electrode that
is exposed to the ambient atmosphere, the second protonic
conductive electrolyte membrane is exposed to substantially 100
percent relative humidity, and a positive pressure of said water
vapor exists from the surface of said second pump electrode that is
exposed to said water vapor to the surface of said first pump
electrode that is exposed to the ambient atmosphere.
39. The electrochemical gas sensor as defined in claim 38, wherein
the surface area of each of the surfaces of the second pump and
counter electrodes that are exposed to said water vapor by said
means for exposing a surface of said second pump electrode to said
water vapor are each separated from said means for exposing a
surface of said second pump electrode to said water vapor by a
hydrophobic membrane permeable to water vapor and substantially
impervious to water.
40. The electrochemical gas sensor as defined in claim 35, wherein
said means for exposing a surface of said second pump electrode to
said water vapor further contains an antifreeze additive.
41. The electrochemical gas sensor as defined in claim 35, wherein
at least one of the surfaces of said first protonic conductive
electrolyte membrane in contact with one of the sensing and counter
electrodes is substantially nonplanar, and wherein at least one of
the surfaces of said second protonic conductive electrolyte
membrane in contact with one of the first and second pump
electrodes is substantially nonplanar.
42. The electrochemical gas sensor as defined in claim 35, wherein
at least one of the sensing, counter, first pump, and second pump
electrodes is comprised of film having a thickness in the range of
about 50 Angstroms to 10,000 Angstroms.
43. The electrochemical gas sensor as defined in claim 42, wherein
the film is substantially composed of a noble metal.
44. The electrochemical gas sensor as defined in claim 43, wherein
the noble metal is platinum.
45. The electrochemical gas sensor as defined in claim 35, wherein
the at least one of the sensing, counter, first pump, and second
pump electrodes is substantially comprised of proton conductive
material.
46. The electrochemical gas sensor as defined in claim 35, wherein
at least one of the first and second protonic conductive
electrolyte membranes is substantially comprised of a solid,
perfluorinated, ion-exchange polymer.
47. The electrochemical gas sensor as defined in claim 35, wherein
at least one of the first and second protonic conductive
electrolyte membranes is a hydrated metal oxide protonic conductive
electrolyte membrane.
48. The electrochemical gas sensor as defined in claim 35, wherein
the electrical conducting material of at least one of said sensing,
counter, first pump, and second pump electrodes is a
proton-electron mixed conductive material having 10-50 wt % of a
proton conductor material and 50-90 wt % of a first and a second
electrical conductor material.
49. The electrochemical gas sensor as defined in claim 48, wherein
the proton conductor material for said at least one of the sensing,
counter, first pump, and second pump electrodes is a copolymer
having a tetrafluoroethylene backbone with a side chain of
perfluorinated monomers containing at least one of a sulfonic acid
group or a carboxylic acid group.
50. The electrochemical gas sensor as defined in claim 48, wherein
one of the first and second electrical conductor materials for said
at least one of the sensing, counter, first pump, and second pump
electrodes is about 50-99 wt % of carbon black, and the other of
the first and second electrical conductor materials for said at
least one of the sensing, counter, first pump and second pump
electrodes is about 1-50 wt % of platinum.
51. The electrochemical gas sensor as defined in claim 48, wherein
one of the first and second electrical conductor materials for said
at least one of the sensing, counter, first pump, and second pump
electrodes is about 50-99 wt % of carbon black, and the other of
the first and second electrical conductor materials for said at
least one of the sensing, counter, first pump, and second pump
electrodes is about 1-50 wt % of Ru oxide.
52. An electrochemical gas sensor for quantitative measurement of a
gas in an ambient atmosphere comprising: a sensing electrode
permeable to water vapor and comprised of an electrical conducting
material and being exposed to the ambient atmosphere; a reference
electrode permeable to water vapor and comprised of an electrical
conducting material; a counter electrode permeable to water vapor
and comprised of an electrical conducting material and being
separate from both said sensing and reference electrodes, and being
exposed to the ambient atmosphere; a protonic conductive
electrolyte membrane permeable to water vapor, having top and
bottom sides, said bottom side of said protonic conductive membrane
being in contact with the counter electrode, and the top side of
said protonic conductive membrane being in contact with the sensing
and reference electrodes; means, containing a volume of water
vapor, for exposing a surface of said counter electrode to said
water vapor, the sensing electrode reacting with the gas to produce
a change in electrical characteristic between the sensing electrode
and the counter electrode; and means for electrical measurement in
electrical contact between the sensing electrode and the counter
electrode, wherein the electrical conducting material of at least
one of said sensing, counter, and reference electrodes is a
proton-electron mixed conductive material having 10-50 wt % of a
proton conductor material and 50-90 wt % of a first and a second
electrical conductor material; whereby, in a positive ambient
concentration of said gas, said electrical measurement means
detects changes in said electrical characteristic.
53. The electrochemical gas sensor as defined in claim 52, further
comprising: means for applying a DC power across said protonic
electrolyte membrane in electrical contact between the sensing
electrode and said counter electrode, whereby the gas is
transported away from the counter electrode when the DC power means
applies a DC power across said protonic electrolyte membrane.
54. The electrochemical gas sensor as defined in claim 52, wherein
said means for exposing a surface of said counter electrode to said
water vapor further contains an antifreeze additive.
55. The electrochemical gas sensor as defined in claim 52, wherein
the surface of said sensing electrode that is exposed to the
ambient atmosphere has a surface area smaller than the surface area
of the surface of the counter electrode that is exposed to said
water vapor, whereby the first protonic conductive electrolyte
membrane is exposed to substantially 100 percent relative humidity,
and a positive pressure of said water vapor exists from the surface
of said counter electrode that is exposed to said water vapor to
the surface of said sensing electrode that is exposed to the
ambient atmosphere.
56. The electrochemical gas sensor as defined in claim 55, wherein
the surface area of the surface of the counter electrode that is
exposed to said water vapor is separated from said means for
exposing a surface of said counter electrode to said water vapor by
a hydrophobic membrane permeable to water vapor and substantially
impervious to water.
57. The electrochemical gas sensor as defined in claim 52, wherein
at least one of the surfaces of said protonic conductive
electrolyte membrane in contact with one of the sensing, counter,
and reference electrodes is substantially nonplanar.
58. The electrochemical gas sensor as defined in claim 52, wherein
at least one of the sensing, counter, and reference electrodes is
comprised of film having a thickness in the range of about 50
Angstroms to 10,000 Angstroms.
59. The electrochemical gas sensor as defined in claim 58, wherein
the film is substantially composed of a noble metal.
60. The electrochemical gas sensor as defined in claim 59, wherein
the noble metal is platinum.
61. The electrochemical gas sensor as defined in claim 52, wherein
the protonic conductive electrolyte membrane is substantially
comprised of a solid, perfluorinated, ion-exchange polymer.
62. The electrochemical gas sensor as defined in claim 52, wherein
the protonic conductive electrolyte membrane is a hydrated metal
oxide protonic conductor electrolyte membrane.
63. The electrochemical gas sensor as defined in claim 52, wherein
the proton conductor material for said at least one of the sensing,
counter, and reference electrodes is a copolymer having a
tetrafluoroethylene backbone with a side chain of perfluorinated
monomers containing at least one of a sulfonic acid group or a
carboxylic acid group.
64. The electrochemical gas sensor as defined in claim 52, wherein
one of the first and second electrical conductor materials for said
at least one of the sensing, counter, and reference electrodes is
about 50-99 wt % of carbon black, and the other of the first and
second electrical conductor materials for said at least one of the
sensing, counter, and reference electrodes is about 1-50 wt % of
platinum.
65. The electrochemical gas sensor as defined in claim 52, wherein
one of the first and second electrical conductor materials for said
at least one of the sensing, counter, and reference electrodes is
about 50-99 wt % of carbon black, and the other of the first and
second electrical conductor materials for said at least one of the
sensing, counter, and reference electrodes is about 1-50 wt % of Ru
oxide.
.Iadd.66. A two-electrode electrochemical gas sensor for
quantitative measurement of a gas in an ambient atmosphere
comprising: a sensing electrode permeable to water vapor and
comprised of an electrical conducting material and having a surface
exposed to the ambient atmosphere; a counter electrode permeable to
water vapor and comprised of an electrical conducting material; a
first protonic conductive electrolyte membrane permeable to water
vapor and situated between and in contact with the sensing and
counter electrodes, the sensing electrode and the counter electrode
being the only two electrodes in contact with the first protonic
conductive electrolyte membrane and the sensing electrode reacting
with the gas to produce a change in electrical characteristic
between the sensing electrode and the counter electrode; means for
electrical measurement electrically connected to said sensing and
counter electrodes; means, containing a volume of water vapor, for
exposing a surface of said counter electrode to said water vapor,
wherein the electrical conducting material of at least one of said
sensing and counter electrodes is a proton-electron mixed
conductive material having 10-50 wt % of a proton conductor
material and 50-90 wt % of a first and a second electrical
conductor material; whereby, in a positive ambient atmosphere
concentration of said gas, said electrical measurement means
detects changes in said electrical characteristic..Iaddend.
.Iadd.67. An electrochemical gas sensor for quantitative
measurement of a gas in an ambient atmosphere comprising: a sensing
electrode permeable to water vapor and comprised of an electrical
conducting material and having a surface exposed to the ambient
atmosphere; a counter electrode permeable to water vapor and
comprised of an electrical conducting material; a first protonic
conductive electrolyte membrane permeable to water vapor and
situated between and in contact with the sensing and counter
electrodes, the sensing electrode reacting with the gas to produce
a change in electrical characteristic between the sensing electrode
and the counter electrode in the absence of an applied voltage to
the sensing electrode; means for electrical measurement
electrically connected to said sensing and counter electrodes;
means, containing a volume of water vapor, for exposing a surface
of said counter electrode to said water vapor, wherein the
electrical conducting material of at least one of said sensing and
counter electrodes is a proton-electron mixed conductive material
having 10-50 wt % of a proton conductor material and 50-90 wt % of
a first and a second electrical conductor material; whereby, in a
positive ambient atmosphere concentration of said gas, said
electrical measurement means detects changes in said electrical
characteristic..Iaddend.
.Iadd.68. A non-biased electrochemical gas sensor for quantitative
measurement of a gas in an ambient atmosphere comprising: a sensing
electrode permeable to water vapor and comprised of an electrical
conducting material and having a surface exposed to the ambient
atmosphere; a counter electrode permeable to water vapor and
comprised of an electrical conducting material; a first protonic
conductive electrolyte membrane permeable to water vapor and
situated between and in contact with the sensing and counter
electrodes, the sensing electrode reacting with the gas to produce
a change in electrical characteristic between the sensing electrode
and the counter electrode; means for electrical measurement
electrically connected to said sensing and counter electrodes;
means, containing a volume of water vapor, for exposing a surface
of said counter electrode to said water vapor, wherein the
electrical conducting material of at least one of said sensing and
counter electrodes is a proton-electron mixed conductive material
having 10-50 wt % of a proton conductor material and 50-90 wt % of
a first and a second electrical conductor material; whereby, in a
positive ambient atmosphere concentration of said gas, said
electrical measurement means detects changes in said electrical
characteristic in the absence of any biasing voltage..Iaddend.
.Iadd.69. The non-biased electrochemical gas sensor of claim 68 in
which the sensing electrode and the counter electrode are the only
two electrodes in contact with the first protonic conductive
electrolyte membrane..Iaddend.
.Iadd.70. A two-electrode electrochemical gas sensor for
quantitative measurement of a gas in an ambient atmosphere
comprising: a sensing electrode permeable to water vapor and
comprised of an electrical conducting material and having a surface
exposed to the ambient atmosphere; a counter electrode permeable to
water vapor and comprised of an electrical conducting material; a
first protonic conductive electrolyte membrane permeable to water
vapor and situated between and in contact with the sensing and
counter electrodes, the sensing electrode and the counter electrode
being the only two electrodes in contact with the first protonic
conductive electrolyte membrane, and the sensing electrode reacting
with the gas to produce a change in electrical characteristic
between the sensing electrode and the counter electrode in the
absence of an applied voltage to the sensing electrode; means for
electrical measurement electrically connected to said sensing and
counter electrodes; means, containing a volume of water vapor, for
exposing a surface of said counter electrode to said water vapor,
wherein the electrical conducting material of at least one of said
sensing and counter electrodes is a proton-electron mixed
conductive material having 10-50 wt % of a proton conductor
material and 50-90 wt % of a first and a second electrical
conductor material; whereby, in a positive ambient atmosphere
concentration of said gas, said electrical measurement means
detects changes in said electrical characteristic..Iaddend.
.Iadd.71. An electrochemical gas sensor for quantitative
measurement of a gas in an ambient atmosphere comprising: a sensing
electrode permeable to water vapor and comprised of an electrical
conducting material and having a surface exposed to the ambient
atmosphere; a counter electrode permeable to water vapor and
comprised of an electrical conducting material; a first protonic
conductive electrolyte membrane permeable to water vapor and
situated between and in contact with the sensing and counter
electrodes, the sensing electrode reacting with the gas to produce
a change in electrical characteristic between the sensing electrode
and the counter electrode, the sensing electrode and the counter
electrode being on opposite sides of the first protonic conductive
electrolyte membrane; means for electrical measurement electrically
connected to said sensing and counter electrodes; means, containing
a volume of water vapor, for exposing a surface of said counter
electrode to said water vapor, wherein the electrical conducting
material of at least one of said sensing and counter electrodes is
a proton-electron mixed conductive material having 10-50 wt % of a
proton conductor material and 50-90 wt % of a first and a second
electrical conductor material; whereby, in a positive ambient
atmosphere concentration of said gas, said electrical measurement
means detects changes in said electrical
characteristic..Iaddend.
.Iadd.72. The electrochemical gas sensor of claim 71 in which the
sensing electrode and the counter electrode are the only two
electrodes in contact with the first protonic conductive
electrolyte membrane..Iaddend.
.Iadd.73. The electrochemical gas sensor of claim 72 in which the
sensing electrode reacts with the gas to produce a change in
electrical characteristic between the sensing electrode and the
counter electrode in the absence of an applied voltage to the
sensing electrode..Iaddend.
.Iadd.74. The electrochemical gas sensor of claim 71 in which the
sensing electrode reacts with the gas to produce a change in
electrical characteristic between the sensing electrode and the
counter electrode in the absence of an applied voltage to the
sensing electrode..Iaddend.
.Iadd.75. An electrochemical gas sensor for quantitative
measurement of a gas in an ambient atmosphere at room temperature
comprising: a sensing electrode permeable to water vapor and
comprised of an electrical conducting material and having a surface
exposed to the ambient atmosphere; a counter electrode permeable to
water vapor and comprised of an electrical conducting material; a
first protonic conductive electrolyte membrane permeable to water
vapor and situated between and in contact with the sensing and
counter electrodes, the sensing electrode reacting with the gas to
produce a change in electrical characteristic between the sensing
electrode and the counter electrode; means for electrical
measurement electrically connected to said sensing and counter
electrodes, said means for electrical measurement being capable of
detecting changes in said electrical characteristic in a positive
ambient atmosphere concentration of said gas; means, containing a
volume of water vapor, for exposing a surface of said counter
electrode to said water vapor, wherein the electrical conducting
material of at least one of said sensing and counter electrodes is
a proton-electron mixed conductive material having 10-50 wt % of a
proton conductor material and 50-90 wt % of a first and a second
electrical conductor material..Iaddend.
.Iadd.76. The electrochemical gas sensor as defined in claim 75,
wherein said water vapor containing means contains a volume of
water and an antifreeze additive..Iaddend.
.Iadd.77. The electrochemical gas sensor as defined in claim 75,
wherein the surface of said sensing electrode that is exposed to
the ambient atmosphere has a surface area that is smaller than the
surface area of the surface of the counter electrode that is
exposed to said water vapor, whereby the first protonic conductive
electrolyte membrane is exposed to substantially 100 percent
relative humidity, and a positive pressure of said water vapor
exists from the surface of said counter electrode exposed to said
water vapor to the surface of said sensing electrode exposed to the
ambient atmosphere..Iaddend.
.Iadd.78. The electrochemical gas sensor as defined in claim 77,
wherein the surface area of the surface of the counter electrode
that is exposed to said water vapor is separated from said means
for exposing a surface of said counter electrode to said water
vapor by a hydrophobic membrane permeable to water vapor and
substantially impervious to water..Iaddend.
.Iadd.79. The electrochemical gas sensor as defined in claim 75,
wherein the first protonic conductive electrolyte membrane has
opposing surfaces, each of said opposing surfaces being in contact
with one of the sensing and counter electrodes, wherein at least
one of the opposing surfaces of said first protonic conductive
electrolyte membrane in contact with one of the sensing and counter
electrodes is substantially nonplanar..Iaddend.
.Iadd.80. The electrochemical gas sensor as defined in claim 75,
wherein at least one of the sensing and counter electrodes is
comprised of film having a thickness in the range of about 50
Angstroms to 10,000 Angstroms..Iaddend.
.Iadd.81. The electrochemical gas sensor as defined in claim 80,
wherein the film is substantially composed of a noble
metal..Iaddend.
.Iadd.82. The electrochemical gas sensor as defined in claim 81,
wherein the noble metal is platinum..Iaddend.
.Iadd.83. The electrochemical gas sensor as defined in claim 75,
wherein the first protonic conductive electrolyte membrane is
substantially composed of a solid, perfluorinated, ion-exchange
polymer..Iaddend.
.Iadd.84. The electrochemical gas sensor as defined in claim 75,
wherein the first protonic conductive electrolyte membrane is a
hydrated metal oxide protonic conductor electrolyte
membrane..Iaddend.
.Iadd.85. The electrochemical gas sensor as defined in claim 75,
wherein the proton conductor material for said at least one of the
sensing and counter electrodes is a copolymer having a
tetrafluoroethylene backbone with a side chain of perfluorinated
monomers containing at least one of a sulfonic acid group or a
carboxylic acid group..Iaddend.
.Iadd.86. The electrochemical gas sensor as defined in claim 75,
wherein one of the first and second electrical conductor materials
for said at least one of the sensing and counter electrodes is
about 50-99 wt % of carbon black, and the other of the first and
second electrical conductor materials for said at least one of the
sensing and counter electrodes is about 1-50 wt % of
platinum..Iaddend.
.Iadd.87. The electrochemical gas sensor as defined in claim 75,
wherein one of the first and second electrical conductor materials
for said at least one of the sensing and counter electrodes is
about 50-99 wt % of carbon black, and the other of the first and
second electrical conductor materials for said at least one of the
sensing and counter electrodes is about 1-50 wt % of Ru
oxide..Iaddend.
.Iadd.88. The electrochemical gas sensor as defined in claim 75,
wherein the electrochemical gas sensor further comprises: first and
second pump electrodes comprised of an electrical conducting
material permeable to water vapor, separate from said sensing and
counter electrodes, and situated on opposite sides of and in
contact with said first protonic conductive electrolyte membrane,
said second pump electrode being situated on the same side of said
first protonic conductive membrane as the counter electrode and
having a surface thereon exposed to the water vapor in said means
for exposing a surface of said counter electrode to said water
vapor; and means for applying a DC power across the first protonic
conductive electrolyte membrane, said first and second pump
electrodes having in electrical connection therebetween said means
for applying DC power across the first protonic conductive
electrolyte membrane; whereby the gas is transported away from the
counter electrode when the DC power means applies a DC power to the
first and second pump electrodes..Iaddend.
.Iadd.89. The electrochemical gas sensor of claim 88, wherein the
electrical conducting material of the first and second pump
electrodes is substantially composed of carbon..Iaddend.
.Iadd.90. The electrochemical gas sensor as defined in claim 88,
wherein the electrical conducting material of the first and second
pump electrodes is substantially composed of noble
metals..Iaddend.
.Iadd.91. The electrochemical gas sensor as defined in claim 88,
wherein the electrical conducting material of the first and second
pump electrodes is substantially composed of conductive hydrated
metal oxides..Iaddend.
.Iadd.92. The electrochemical gas sensor as defined in claim 88,
wherein at least one of the first and second pump electrodes is
comprised of a film having a thickness in the range of about 50
Angstroms to 10,000 Angstroms..Iaddend.
.Iadd.93. The electrochemical gas sensor as defined in claim 88,
wherein the electrical conducting material of said first and second
pump electrodes is a proton-electron mixed conductive material
having 10-50 wt % of a proton conductor material and 50-90 wt % of
a first and a second electrical conductor material..Iaddend.
.Iadd.94. The electrochemical gas sensor as defined in claim 93,
wherein the proton conductor material for both the first and second
pump electrodes is a copolymer having a tetrafluoroethylene
backbone with a side chain of perfluorinated monomers containing at
least one of a sulfonic acid group or a carboxylic acid
group..Iaddend.
.Iadd.95. The electrochemical gas sensor as defined in claim 93,
wherein one of the first and second electrical conductor materials
for the first pump electrode is about 50-99 wt % of carbon black,
and the other of the first and second electrical conductor
materials for the first pump electrode is 1 to 50 wt % of
platinum..Iaddend.
.Iadd.96. The electrochemical gas sensor as defined in claim 93,
wherein one of the first and second electrical conductor materials
for the second pump electrode is about 50-99 wt % of carbon black,
and the other of the first and second electrical conductor
materials for the second pump electrode is 1 to 50 wt % of Ru
oxide..Iaddend.
.Iadd.97. The electrochemical gas sensor as defined in claim 75,
wherein the electrochemical gas sensor further comprises: a second
protonic conductive electrolyte membrane permeable to water vapor;
first and second pump electrodes permeable to water vapor and
comprised of an electron conductive material, and being separate
from said sensing and counter electrodes and situated on opposite
sides of and in contact with said second protonic conductive
electrolyte membrane, said means for exposing a surface of said
counter electrode to said water vapor exposing a surface of said
second pump electrode to said water vapor, and said first pump
electrode having a surface exposed to the ambient atmosphere; and
means for applying a DC power across said second protonic
electrolyte membrane, said first and second pump electrodes having
in electrical connection therebetween said means for applying DC
power across said second protonic electrolyte membrane; whereby the
gas is transported away from the counter electrode when the DC
power means applies a DC power to the first and second pump
electrodes..Iaddend.
.Iadd.98. The electrochemical gas sensor as defined in claim 97,
wherein the second protonic conductive electrolyte membrane is
substantially composed of a solid, perfluorinated, ion-exchange
polymer..Iaddend.
.Iadd.99. The electrochemical gas sensor as defined in claim 97,
wherein the second protonic conductive electrolyte membrane is a
hydrated metal oxide protonic conductor electrolyte
membrane..Iaddend.
.Iadd.100. The electrochemical gas sensor as defined in claim 97,
wherein the surface area of the surface of said first pump
electrode that is exposed to the ambient atmosphere is smaller than
the surface area of the surface of the second pump electrode that
is exposed to said water vapor, whereby the second protonic
conductive electrolyte membrane is exposed to substantially 100
percent relative humidity, and a positive pressure of said water
vapor exists from the surface of said second pump electrode that is
exposed to said water vapor to the surface of said first pump
electrode that is exposed to the ambient atmosphere..Iaddend.
.Iadd.101. The electrochemical gas sensor as defined in claim 100,
wherein the surface area of the surface of the second pump
electrode that is exposed to said water vapor is separated from
said means for exposing a surface of said counter electrode to said
water vapor by a hydrophobic membrane permeable to water vapor and
substantially impervious to water..Iaddend.
.Iadd.102. The electrochemical gas sensor as defined in claim 75,
further comprising: means for applying a DC pulse power source
across the first protonic conductive membrane, said sensing and
counter electrodes having in electrical connection therebetween
said means for applying DC pulse power across the first protonic
conductive membrane; and switch means for alternating the
connection between the sensing and counter electrodes from the
electrical measurement means to the DC pulse power means; whereby,
in a positive ambient atmosphere concentration of said gas, said
electrical measurement means detects changes in said electrical
characteristic when said switch means connects said electrical
measurement means to the sensing and counter electrodes; and
whereby said DC pulse power means moves the gas away from a side of
the gas sensor where the counter electrode is placed when said
switch means connects said DC pulse power means to the sensing and
counter electrodes..Iaddend.
.Iadd.103. The electrochemical gas sensor as defined in claim 75,
wherein the gas is CO..Iaddend.
.Iadd.104. The electrochemical gas sensor as defined in claim 75,
wherein the gas is NO.sub.x..Iaddend.
.Iadd.105. The electrochemical gas sensor as defined in claim 75,
wherein the gas is hydrogen..Iaddend.
.Iadd.106. The electrochemical gas sensor as defined in claim 75,
wherein the gas is H.sub.2S..Iaddend.
.Iadd.107. The electrochemical gas sensor as defined in claim 75,
wherein the gas is H.sub.2O vapor..Iaddend.
.Iadd.108. The electrochemical gas sensor as defined in claim 75,
wherein the gas is alcohol vapor..Iaddend.
.Iadd.109. An electrochemical gas sensor for quantitative
measurement of a gas in an ambient atmosphere at room temperature
comprising: a sensing electrode permeable to water vapor and
comprised of an electrical conducting material and being exposed to
the ambient atmosphere; a reference electrode permeable to water
vapor and comprised of an electrical conducting material; a counter
electrode permeable to water vapor and comprised of an electrical
conducting material and being separate from both said sensing and
reference electrodes, and being exposed to the ambient atmosphere;
a protonic conductive electrolyte membrane permeable to water
vapor, having top and bottom sides, said bottom side of said
protonic conductive membrane being in contact with the counter
electrode, and the top side of said protonic conductive membrane
being in contact with the sensing and reference electrodes; means,
containing a volume of water vapor, for exposing a surface of said
counter electrode to said water vapor, the sensing electrode
reacting with the gas to produce a change in electrical
characteristic between the sensing electrode and the counter
electrode; and means for electrical measurement in electrical
contact between the sensing electrode and the counter electrode,
wherein the electrical conducting material of at least one of said
sensing, counter, and reference electrodes is a proton-electron
mixed conductive material having 10-50 wt % of a proton conductor
material and 50-90 wt % of a first and a second electrical
conductor material; whereby, in a positive ambient concentration of
said gas, said electrical measurement means detects changes in said
electrical characteristic..Iaddend.
.Iadd.110. The electrochemical gas sensor as defined in claim 109,
further comprising: means for applying a DC power across said
protonic electrolyte membrane in electrical contact between the
sensing electrode and said counter electrode, whereby the gas is
transported away from the counter electrode when the DC power means
applies a DC power across said protonic electrolyte
membrane..Iaddend.
.Iadd.111. The electrochemical gas sensor as defined in claim 109,
wherein said means for exposing a surface of said counter electrode
to said water vapor further contains an antifreeze
additive..Iaddend.
.Iadd.112. The electrochemical gas sensor as defined in claim 109,
wherein the surface of said sensing electrode that is exposed to
the ambient atmosphere has a surface area smaller than the surface
area of the surface of the counter electrode that is exposed to
said water vapor, whereby the first protonic conductive electrolyte
membrane is exposed to substantially 100 percent relative humidity,
and a positive pressure of said water vapor exists from the surface
of said counter electrode that is exposed to said water vapor to
the surface of said sensing electrode that is exposed to the
ambient atmosphere..Iaddend.
.Iadd.113. The electrochemical gas sensor as defined in claim 112,
wherein the surface area of the surface of the counter electrode
that is exposed to said water vapor is separated from said means
for exposing a surface of said counter electrode to said water
vapor by a hydrophobic membrane permeable to water vapor and
substantially impervious to water..Iaddend.
.Iadd.114. The electrochemical gas sensor as defined in claim 109,
wherein at least one of the surfaces of said protonic conductive
electrolyte membrane in contact with one of the sensing, counter,
and reference electrodes is substantially nonplanar..Iaddend.
.Iadd.115. The electrochemical gas sensor as defined in claim 109,
wherein at least one of the sensing, counter, and reference
electrodes is comprised of film having a thickness in the range of
about 50 Angstroms to 10,000 Angstroms..Iaddend.
.Iadd.116. The electrochemical gas sensor as defined in claim 115,
wherein the film is substantially composed of a noble
metal..Iaddend.
.Iadd.117. The electrochemical gas sensor as defined in claim 116,
wherein the noble metal is platinum..Iaddend.
.Iadd.118. The electrochemical gas sensor as defined in claim 109,
wherein the protonic conductive electrolyte membrane is
substantially comprised of a solid, perfluorinated, ion-exchange
polymer..Iaddend.
.Iadd.119. The electrochemical gas sensor as defined in claim 109,
wherein the protonic conductive electrolyte membrane is a hydrated
metal oxide protonic conductor electrolyte membrane..Iaddend.
.Iadd.120. The electrochemical gas sensor as defined in claim 109,
wherein the proton conductor material for said at least one of the
sensing, counter, and reference electrodes is a copolymer having a
tetrafluoroethylene backbone with a side chain of perfluorinated
monomers containing at least one of a sulfonic acid group or a
carboxylic acid group..Iaddend.
.Iadd.121. The electrochemical gas sensor as defined in claim 109,
wherein one of the first and second electrical conductor materials
for said at least one of the sensing, counter, and reference
electrodes is about 50-99 wt % of carbon black, and the other of
the first and second electrical conductor materials for said at
least one of the sensing, counter, and reference electrodes is
about 1-50 wt % of platinum..Iaddend.
.Iadd.122. The electrochemical gas sensor as defined in claim 109,
wherein one of the first and second electrical conductor materials
for said at least one of the sensing, counter, and reference
electrodes is about 50-99 wt % of carbon black, and the other of
the first and second electrical conductor materials for said at
least one of the sensing, counter, and reference electrodes is
about 1-50 wt % of Ru oxide..Iaddend.
.Iadd.123. A residential electrochemical gas sensor for
quantitative measurement of carbon monoxide gas in an ambient
atmosphere comprising: a sensing electrode permeable to water vapor
and comprised of an electrical conducting material and having a
surface exposed to the ambient atmosphere; a counter electrode
permeable to water vapor and comprised of an electrical conducting
material; a first protonic conductive electrolyte membrane
permeable to water vapor and situated between and in contact with
the sensing and counter electrodes, the sensing electrode reacting
with the carbon monoxide gas to produce a change in electrical
characteristic between the sensing electrode and the counter
electrode; means for electrical measurement electrically connected
to said sensing and counter electrodes; means, containing a volume
of water vapor, for exposing a surface of said counter electrode to
said water vapor, wherein the electrical conducting material of at
least one of said sensing and counter electrodes is a
proton-electron mixed conductive material having 10-50 wt % of a
proton conductor material and 50-90 wt % of a first and a second
electrical conductor material; whereby, in a positive ambient
atmosphere concentration of the carbon monoxide gas at room
temperature, said electrical measurement means detects changes in
said electrical characteristic..Iaddend.
.Iadd.124. The electrochemical gas sensor of claim 123 in which the
sensing electrode comprises a mixed protonic-electronic conductive
electrode..Iaddend.
.Iadd.125. A two-electrode residential electrochemical gas sensor
for quantitative measurement of carbon monoxide gas in an ambient
atmosphere comprising: a sensing electrode permeable to water vapor
and comprised of an electrical conducting material and having a
surface exposed to the ambient atmosphere; a counter electrode
permeable to water vapor and comprised of an electrical conducting
material; a first protonic conductive electrolyte membrane
permeable to water vapor and situated between and in contact with
the sensing and counter electrodes, the sensing electrode and the
counter electrode being the only two electrodes in contact with the
first protonic conductive electrolyte membrane, and the sensing
electrode reacting with the carbon monoxide gas to produce a change
in electrical characteristic between the sensing electrode and the
counter electrode in the absence of an applied voltage to the
sensing electrode; means for electrical measurement electrically
connected to said sensing and counter electrodes; means, containing
a volume of water vapor, for exposing a surface of said counter
electrode to said water vapor, wherein the electrical conducting
material of at least one of said sensing and counter electrodes is
a proton-electron mixed conductive material having 10-50 wt % of a
proton conductor material and 50-90 wt % of a first and a second
electrical conductor material; whereby, in a positive ambient
atmosphere concentration of the carbon monoxide gas at room
temperature, said electrical measurement means detects changes in
said electrical characteristic; wherein each of the sensing
electrode and the counter electrode comprise a mixed
protonic-electronic conductive electrode including platinum, carbon
and a copolymer having a tetrafluoroethylene backbone with a side
chain of perfluorinated monomers containing a sulfonic acid group;
and wherein the protonic conductive solid electrolyte membrane is
substantially comprised of a solid, perfluorinated, ion-exchange
polymer..Iaddend.
.Iadd.126. The two-electrode electrochemical gas sensor as defined
in claim 125, wherein the sensing and counter electrodes have a
diameter in a range of 1 mm to 15 mm, and the protonic conductive
electrolyte membrane has a thickness in a range of 0.1 mm to 1
mm..Iaddend.
.Iadd.127. The two-electrode electrochemical gas sensor as defined
in claim 125, wherein the sensing and counter electrodes have a
diameter of about 15 mm, and the protonic conductive electrolyte
membrane has a thickness of about 0.17 mm..Iaddend.
.Iadd.128. The two-electrode electrochemical gas sensor as defined
in claim 125, wherein the electrical conducting material of at
least one of said sensing and counter electrodes is a
proton-electron mixed conductive material having at least
approximately 25 wt % of a proton conductor material..Iaddend.
.Iadd.129. The two-electrode electrochemical gas sensor as defined
in claim 125, wherein said counter electrode is exposed to said
water vapor at a 100% relative humidity..Iaddend.
.Iadd.130. The two-electrode electrochemical gas sensor as defined
in claim 125, wherein the surface of said sensing electrode that is
exposed to the ambient atmosphere has a surface area that is
smaller than the surface area of the surface of the counter
electrode that is exposed to said water vapor, whereby a positive
pressure of said water vapor exists from the surface of said
counter electrode exposed to said water vapor to the surface of
said sensing electrode exposed to the ambient atmosphere..Iaddend.
Description
1. The Field of the Invention
The invention relates to electrochemical gas sensors, and
particularly relates to humidity compensated electrochemical gas
sensors having a sensing electrode, a counter electrode, and a
solid proton conductor for room temperature detection of the
concentration of carbon monoxide (CO) in the ambient.
2. Background of the Invention
In most prior art solid state commercial gas sensors, it is
necessary to heat the sensor element to elevated temperatures in
order to acquire both fast response time and high sensitivity to
objective gases. For example, N-type semiconductor tin oxide gas
sensors and catalytic combustion type Pd/Pt gas sensors must
usually be operated in a temperature range of ca. 200.degree. to
500.degree. C. These sensors must be equipped with heaters
connected to external power sources. Therefore, room temperature CO
gas sensors, which use less power, are desirable.
It is well known that CO reacts with moisture in air at room
temperature, and forms protons, electrons, and CO.sub.2 in an
oxidation reaction of CO.
CO+H.sub.2O.fwdarw.CO.sub.2+2H.sup.++2e.sup.- (1)
It is also known that there is a moisture formation reaction by
combining protons, electrons, and oxygen in a reduction reaction of
oxygen: 2H.sup.++2e.sup.-+2O.sub.2.fwdarw.H.sub.2O (2)
These two reactions are the basis of prior art room temperature
zero power electrochemical gas sensors utilizing a proton
conductor. FIG. 1 shows the transport processes of such a CO gas
sensor. A protonic conductor 12 conducts ionized hydrogen atoms
from a sensing electrode 16 where the sensor signal originates from
the oxidation reaction of carbon monoxide at sensing electrode 16.
Ionized hydrogen atoms, each of which constitutes a single proton,
are conducted through protonic conductor 12 to a counter electrode
14. Electrons that are liberated in the oxidation of carbon
monoxide at sensing electrode 16 are conducted through an
electrical lead 22 to voltage meter 18, through an electrical lead
20, and to counter electrode 14 for a reduction reaction of oxygen.
In a steady state reaction, the hydrogen ions are transported from
sensing electrode 16 to counter electrode 14 in the depicted
potentiometric CO gas sensor.
The current generated by the reactions depicted in FIG. 1 can also
be measured by an amp meter 24 having a resistor R.sub.L 26, which
circuit represents a transport process of an amperometric CO
sensor. Absent amp meter 24, resistor R.sub.L 26, the leads thereto
which are shown in phantom, transport processes of a potentiometric
CO gas sensor are shown for voltage meter 18 and leads 20, 22.
Whether the transport processes shown in FIG. 1 are for
potentiometric CO gas sensor or for an amperometric CO sensor,
electrons from the process of the oxidation reaction of carbon
monoxide travel as seen in arrow 21 in FIG. 1 through leads 20,
22.
The sensor of FIG. 1 is operated in a current mode when the sensing
and counter electrodes 16, 14 are connected to each other through
load resistor R.sub.L, or are connected to a DC power source (not
shown) which electrically drives the protons across proton
conductor 12.
A prior art room temperature proton conductor sensor developed by
General Electric using a polymer porous support material saturated
by a liquid proton conductor, has been constructed as an
electrochemical amperometric CO gas sensor (the G. E. Sensor). In
the G. E Sensor, a liquid reservoir was used to provide the liquid
proton conductor to the porous support material. Protons, which are
indicative of the ambient CO concentration, were driven across the
porous support material through the liquid conductor by a DC
voltage. Electrical current response of the sensor to ambient CO
concentration was linear. The cost of the sensor with such a
complicated design, however, is high and is thus not be suitable
for practical consumer applications.
In U.S. Pat. No. 4,587,003, a room temperature CO gas sensor using
a liquid proton conductor is taught. Basically, the mechanism and
design of the sensor were similar to the G. E. sensor, except that
the outside surfaces of the sensing and counter electrodes of the
sensor in this patent were coated by porous NAFION.TM. layers. The
CO room temperature gas sensor taught in the patent currently costs
about $200.00. The lifetime of such a sensor is about 6-12 months
due to the rapid drying of the liquid of the electrolytes. In
addition, the sensor requires maintenance due to leakage and
corrosion of liquid electrolyte.
Other types of gas sensors incorporating a liquid proton conductive
electrolyte are also known. In particular, U.S. Pat. No. 5,228,974
discloses a liquid proton conductive electrolyte of an aqueous
solution of calcium nitrate and lithium nitrate. Additionally, U.S.
Pat. No. 5,126,035 discloses another liquid proton conductive
electrolyte.
It is necessary in prior art gas sensors to routinely re-calibrate
the gas sensor so that the drift of its output signal can be
corrected. This drift in the signal is due to the fact that such
sensors must be exposed to the ambient in order to sample the
desired target gases and, through such exposure, the liquid
electrolyte slowly dries during the sensor service time, as
described above. As a result of this process, the proton
conductivity of the liquid electrolyte changes and the change in
relationship between electrical current and gas concentration
results in the need to re-calibrate these gas sensors. The
re-calibration requirement of such gas sensors limits their
applications. For instance, it would be impractical to install a
carbon monoxide sensor in a residential home that required
re-calibration. As such, re-calibratable gas sensors may be
suitable for industrial lab applications and would be practically
excluded from consumer applications.
Liquid proton conductive electrolytes used in current amperometric
room temperature electrochemical gas sensors are known to have long
term stability problem. One such source of problems threatening
stability of such sensors is the use of fine noble metal particles
in such gas sensors to catalyze the electrochemical reactions with
the target gas.
It is known that an electrical or an electrochemical driving force
such as DC power for a period of time, such as a month or longer,
will cause the size of the noble metal particles used in such
centers to increase. Particle size increase will cause a reduction
in performance of the sensor so as to affect its long term sense of
stability in accurate sensor signal output. Additionally, in prior
art gas sensors a large gap is needed between the sensing electrode
and the counter electrode to ensure that a liquid electrolyte can
mix and maintain a uniform distribution in prior art gas sensors.
As a consequence of the large gap between electrodes, the internal
conductivity of the sensor becomes too small to generate a
detectable current signal therebetween. As such, a DC power source
is required to act upon the gas sensor as a driving force so as to
generate a detectable proton current. Additionally, while DC power
is being supplied to the gas sensor, water moisture or vapor in the
ambient decomposes into protons, electrons, and oxygen.
The background proton electrical current is a function of both
humidity and voltage of the DC power source. Should there be a
change in either humidity or DC bias voltage, then the background
proton electrical current also changes with the resulting error in
the signal output from the sensor. Thus, gas measurements using
such prior art gas sensors are inaccurate.
From the above, it can be seen that there is a need for a
practical, electrochemical, room temperature gas sensor that does
not require periodic re-calibration, that does not experience
background electrical current drift, and does not experience long
term stability problems.
The discovery of room temperature solid proton conductors aroused
considerable efforts to investigate low cost, all-solid
electrochemical room temperature CO gas sensors. One such sensor
that was developed was a room temperature CO gas sensor with a
tubular design using proton conductors, electronically conductive
platinum or the like as the sensing electrode, and electronically
conductive silver, gold, graphite or the like as the counter
electrode. The sensing electrode decomposed carbon monoxide gas to
produce protons and electrons, whereas the counter-reference
electrode exhibited no activity to decompose carbon monoxide with
the result that a Nernst potential occurred between the two
electrodes. Thus, carbon monoxide gas was detected.
In detecting carbon monoxide with the tubular design sensor,
protons and electrons are generated at the sensing electrode. For
the reaction to be continued, protons and electrons must be removed
from the reaction sites, and CO and moisture must be continuously
provided from the gaseous phase to the reaction sites. Therefore,
the CO reaction only occurs at three-phase contact areas. The
three-phase contact areas consist of the proton membrane phase, the
platinum electron phase, and the gas phase. Due to the limited
three-phase contact areas in the tubular design sensor, the CO
reaction was slow. Additionally, as a result of this slow reaction,
the response signal was weak. Further, the Nernst potential was not
zero in clean air.
A modified electrochemical CO room temperature gas sensor using a
planar or tubular sensor design was a subsequent development to the
earlier tubular design CO sensor. In order to overcome the problem
that the Nernst potential is not zero in clean air experienced with
the earlier tubular design CO sensor, the improved design proposed
a four probe measurement method for CO gas detection. The improved
design achieved a zero reading in clean air, and the improved
sensor was insensitive to variations in relative humidity.
Theoretical analysis based on electrochemistry, however, indicates
that there is no difference between the four probe method and the
normal two probe method of the earlier tubular design CO sensor.
The improved sensor still used electronic conductors for both the
sensing and counter electrodes, and showed slow and weak response
signals to CO gas.
A still further improved design of a CO sensor is a room
temperature electrochemical gas sensor using a solid polymer proton
conductor with a planar sensor design. Response of this further
improved sensor to CO was very weak, and was in the nA range even
as a DC power source was applied. Apparently, the internal
resistance of the sensor was too large. Calculations based on this
further improved sensor dimensions indicates that the ionic
resistance of the proton conductor membrane is about 400 K-ohm,
which is too large to generate a useable strong signal. Further
development and improvement of the planar CO gas sensor, which
incorporated a sensing mechanism, resulted in performance that was
still in nA range of sensor response.
From the above, it can be seen that it would be desirable to
provide a solid proton conductor in a gas sensor that had a strong
response signal and was relatively rapid in response. Additionally,
it would be desirable to provide humidity compensation with both a
strong and rapid signal response from the gas sensor.
SUMMARY AND OBJECTS OF THE INVENTION
It is an object of this invention to provide a low cost room
temperature, electrochemical gas sensor, for carbon monoxide and
other toxic gases, having a low ionic resistance, a rapid response,
and a strong signal to the detection of gaseous CO in the ambient.
The alcohol and toxic gases that can be sensed by the inventive
sensor, each of which is referred to herein as an analyte gas,
include methyl alcohol, ethyl alcohol, etc, and may also include
H.sub.2, H.sub.2S, H.sub.2O vapor, Nox, etc.
It is a further object of the present invention to provide an
electrochemical, room temperature gas sensor that does not require
periodic re-calibration, that eliminates background electrical
current drift, and that does not experience long term instability
problems known to prior art gas sensors.
It is a further object of this invention to provide a solid proton
conductive thin membrane electrolyte, the conductivity of which is
time independent, and has a large enough conductivity so that no DC
power is needed to drive the electrical proton current.
The inventive electrochemical sensor has both a sensing electrode
and a counter electrode, or optionally a sensing electrode, a count
electrode, and a reference electorode. Each of the sensing and
counter electrodes can be made of a thin layer of a noble metal or,
alternatively, a thicker layer of mixed protonic-electronic
conductors so as to encourage a high surface area for reactions at
the electrodes, either of which cause fast analyte gas reaction
kinetics and a continuity in the transport of electrical charges so
as to avoid polarization effects at the electrodes, thus achieving
a fast and strong signal response by the sensor in the presence of
the analyte gas.
A further aspect of the inventive gas sensor is that either two
electrodes or three electrodes are required, whereas prior art gas
sensors always require three electrodes and a DC power supply.
These objects have been achieved by an electrochemical room
temperature gas sensor having a solid proton conductive electrolyte
membrane. The gas sensor has a lower cap having a water reservoir
therein, a perforated washer covered over by a hydrophobic
microporous lower membrane, a proton conductive membrane coating
with catalysts on both sides serving as electrodes thereto, a
sealing ring that electrically insulates the lower cap containing
the water from a perforated upper cap. Optionally, the gas sensor
may be also provided with a dust filter covering over the
perforations to the upper cap to prevent the contamination of the
inside of the gas sensor from ambient dust. The novel sensor
design, may include thin film noble metal or mixed proton-electron
conductive electrodes, various embodiments of which may also
include an electrochemical analyte gas pump to transport analyte
gas away from the counter electrode side of the gas sensor. While
the inventive sensor is referred to herein as a CO sensor, it is
contemplated that the inventive sensor is also capable of sensing
alcohol gases and other toxic analyte gases disclosed herein.
These and other objects and features of the present invention will
become more fully apparent from the following description and
appended claims, or may be learned by the practice of the invention
as set forth hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the manner in which the above-recited and other
advantages and objects of the invention are obtained, a more
particular description of the invention briefly described above
will be rendered by reference to specific embodiments thereof which
are illustrated in the appended drawings. Understanding that these
drawings depict only a typical embodiment of the invention and are
not therefore to be considered to be limiting of its scope, the
invention will be described and explained with additional
specificity and detail through the use of the accompanying drawings
in which:
FIG. 1 is an electrochemical gas sensor showing the transport
processes of both a potentiometric CO sensor and an amperometric CO
sensor, where hydrogen protons are conducted through a protonic
conductive membrane situated between sensing and counter
electrodes, where electrons travel between said electrodes away
from the protonic conductive membrane, where the sensing electrode
is the locus of the oxidation reaction of carbon monoxide
instigated by a catalyst, and the counter electrode is the locus of
the synthesis of water from the products of the electrochemical
reaction of the sensor.
FIG. 2 shows an embodiment for the inventive electrochemical gas
sensor that is contained in a water containing can having a cap
which encloses a counter electrode and a sensing electrode, where a
protonic conductor is situated between the electrodes, which
electrodes are separated by insulated packing material within the
can and cap container arrangement.
FIG. 3 shows an alternative embodiment of the inventive
electrochemical sensor, further featuring a water reservoir and a
CO pump structure. The electrochemical sensor depicted in FIG. 4
has four electrodes attached to a protonic conductive membrane, two
of which are normal sensing and counter electrodes, and the other
two electrodes are used to pump permeated CO out of the counter
electrode side of the electrochemical cell. In this alternative
embodiment of the inventive CO sensor, DC power can be applied in
either a pulse mode or a constant mode. The electrochemical sensor
is enclosed within an electrically insulated cap and can
design.
FIG. 4 shows a further embodiment of the inventive electrochemical
CO sensor, having two protonic conductive membranes, the first
membrane being used to sense CO, and the second membrane being used
to pump permeated CO out of the counter electrode side of the
electrochemical CO sensor. A de-ionized water reservoir in the
middle of the sensor is separated on each side thereof from the
first and second membranes by a microporous hydrophobic filter and
an electrode.
FIG. 5A depicts the inventive sensor electrical current output over
time in an atmosphere of constant temperature and relative humidity
with increasing CO concentration.
FIG. 5B depicts the inventive sensor electrical current output in
increasing CO concentration at dual relative humidity.
FIG. 6 shows an electrically conductive thin electrically
conductive film electrode, permeable to water vapor, in contact
with a current collector, and a protonic conductive membrane having
either a planar or nonplanar interface, including electron
conductive phases, gas phases, and three-phase contact areas.
FIG. 7 shows a mixed protonic-electronic conductive electrode,
permeable to water vapor, the electrode being shown in contact with
a current collector, and a protonic conductive membrane having
either a planar or a nonplanar interface in amplified view of the
materials therein having protonic and electronic conductive phases,
gas phases, and three-phase contact areas.
FIG. 8 is an alternative embodiment of the inventive gas sensor
having a water reservoir and three electrodes which are a sensing
electrode, a count electrode, and a reference electrode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The inventive CO sensor features a solid protonic conductive
membrane operable at room temperature and having a fast and high
signal response. To achieve a fast detection time and a high signal
response, it is desirable to provide a CO sensor having a low bulk
ionic resistance. Bulk ionic resistance R.sub.bulk of the inventive
sensor is equal to
.times. ##EQU00001## where R.sub.o is the ionic specific
resistivity of the protonic conductive membrane, S is the cross
section area of the protonic conductive membrane between the two
electrodes, and d is the thickness of the protonic conductive
membrane.
Resistance of an electrochemical cell includes at least three
components: 1) bulk ionic resistance of the membrane, 2) interface
resistance between the membrane and electrodes, and 3) electronic
resistance of the electrodes. The bulk ionic resistance of the
sensor is reduced to about 1 ohm by the inventive sensor design,
such that R.sub.bulk is not a performance limit. Electronic
resistivity of the electrodes is in order of 10.sup.5 ohm-cm and is
not a performance limit. Therefore, the interface resistance, which
is relative to the available three-phase contact area, becomes the
performance limit. The interface resistance of the sensor according
to this invention has been reduced by introducing mixed
proton-electronic conductor, or alternatively, a thin film electron
conductor electrode.
Reference is now made to FIGS. 1-8 wherein like reference numerals
between different embodiments of the inventive sensor designate
like features.
In FIG. 1, electrical leads 22, 20 are in electrical communication,
respectively, with sensing electrode 16 and counter electrode 14.
Measurement of signals output by the sensor seen in FIG. 1 is seen
in two alternative embodiments. In a first embodiment, a voltage
meter 18 measures potential differences between electrical leads
20, 22 in a potentiometric CO gas sensor embodiment. In a second
embodiment, an amp meter 24, in combination with a resistor R.sub.L
26 provides an amperometric CO sensor embodiment. Electrical
circuitry, as seen in FIG. 1, can be used in other CO sensor
embodiments depicted and described elsewhere to measure the output
thereof. Additionally, such electrical circuity serves as an
example and illustration of a means for electrical measurement that
is electrically connected to the sensing and counter-reference
electrodes.
A sensor 10 in FIG. 2 is shown with an electrically conductive can
30 having a reservoir 200 filled with de-ionized water. Can 30 is
an example and illustration of a means, containing a volume of
water vapor, for exposing a surface of a counter electrode to the
water vapor. Upon such exposure to an atmospheric concentration of
a target gas, such as CO, electrical measurements can be made to
detect changes in electrical characteristics of the electrodes.
The de-ionized water in can 30 may contain an antifreeze additive,
such as glycol or other known antifreeze. Can 30 may have a
sealable opening therein to replenish water in reservoir 200.
Reservoir 200 is separated from counter electrode 14 by a large
hole 202 in an electrically conductive washer 36 covered over by a
microporous hydrophobic filter 204. Can 30 has an opening which is
covered by a metallic cap 32. Cap 32 has a small air sampling hole
206 therein so as to provide a venting of sensing electrode 16 to
the ambient. Hole 206 is optionally covered over with a dust filter
212 to prevent contamination of sensing electrode 16 by ambient
dust and other air borne particulate matter. An insulation packing
material 34 electrically insulates cap 32 from can 30. Cap 32, can
30, and washer 36 need not be electrically conductive when
electrical leads 50, 22 are embedded, respectively, within
electrodes 14, 16.
In FIG. 2, electrical leads 50, 22 are connected to a switching
mechanism for sensor 10 made up of a switch 40 that is opened and
closed by unit 42 so as to alternatively provide a power source 44
in electrical communication with cap 32 and can 30 of sensor 10.
Unit 42, and related circuitry, serves as an example and
illustration of a means for applying a DC power across the protonic
conductive electrolyte membrane, and switch 40 and related
circuitry serves as an example and illustration of a switch means
for alternating the connection between the sensing and counter
electrodes from the electrical measurement means to the DC pulse
power means.
The purpose of the foregoing electrical switching circuitry is to
provide a switchable CO pump to sensor 10 so as to direct CO away
from counter electrode 14 before and after sensing and measuring CO
concentration with sensor 10. If switchable CO pump circuitry is
not included in the embodiment of sensor 10 shown in FIG. 2, then
continuous sensing without CO pumping is performed by sensor
10.
The amperometric sensor also can be combined with an
electrochemical CO pump, as defined hereinafter, and an accurate
response will be achieved in such combined sensors.
The presence of the water vapor from the de-ionized water in
reservoir 200 assures a 100% relative humidity exposure to the
sensing electrode, protonic conductor and the sensing electrode,
protonic conductor, and the counter electrode at all times. This is
particularly important in that the resistance of the solid proton
conductive electrolyte membrane is a function of the water vapor
pressure to which the electrolyte membrane is exposed. As such, the
proton conductivity of the electrolyte membrane is constant
throughout the sensor's life and does not require re-calibration,
where such life is dependent upon the presence of water in
reservoir 200.
Hole 202 in washer 36 is preferably larger than hole 206 in cap 32.
Preferably, hole 202 will have a diameter of approximately 3 mm and
hole 206 will have a diameter of approximately 0.2 mm. The relative
diametric differences between holes 202, 206 ensure that water
vapor pressure at protonic conductive membrane 12, sensing
electrode and counter electrode will be sufficient to saturate the
same and be constant as long as reservoir 200 contains water
therein. Over a period of the useful service life of sensor 10, the
water and reservoir 200 will be evaporated. The proton conductivity
of protonic conductive membrane 12, however, will remain constant
and sensor 10 will not require re-calibration while water is
present.
While FIG. 2 is depicted with a DC power source, it should be
understood that the embodiment shown in FIG. 2 of the sensor may
also be operated without a DC power source. Operation of the sensor
seen in FIG. 2 without a DC power source is desirable in that such
a power source will cause noble metal catalysts in protonic
conductive membrane 12 to coalesce into larger particles which
tends to reduce long term sensor response and detract from the
stability of the sensor. Consequently, the small proton resistance
of the relatively thin protonic conductive membrane 12 enables the
embodiment seen in FIG. 2 to be operated without a DC power source
in the measurement of CO gas by sensor 10.
Protonic conductive membrane 12 will preferably be a solid proton
conductive electrolyte membrane coated with a catalyst on both
sides thereof. The solid proton conductive electrolyte membrane is
composed of an organic material, such as a polymer material, or may
also be composed of an inorganic material such as a metal oxide.
Where the solid protonic conductive electrolyte membrane is an
organic membrane, the organic membrane will preferably be a polymer
proton conductive material such as NAFION.TM. 117, or XUS-1304.10
membrane. NAFION is manufactured by DuPont and XUS is manufactured
by Dow Chemical Co. of the United States of America. Alternatively,
the organic material may be a R4010-55 membrane supplied by PALL
RAI Co., also of the United States of America.
In the inventive sensor design as shown in FIG. 2, it is desirable
that both area and thickness parameters are optimized. It is
beneficial for CO sensor 10 to have a 0.1 mm-1 mm thick NAFION.TM.
protonic membrane, and that the diameter of sensing and counter
electrodes 16, 14 be approximately 1 mm to 15 mm. Preferably, CO
sensor 10 has a 0.17 mm thick NAFION.TM. protonic membrane or the
like with 10 mm diameter sensing and counter electrodes 16, 14,
which results in a bulk ionic resistance of 1.0 ohm. The proton
conductor for both the sensing and counter electrodes is preferably
a copolymer based on a tetrafluoroethylene backbone with a side
chain of perfluorinated monomers containing sulfonic or carboxylic
acid groups, especially a NAFION.TM. 117 material from DuPont, or a
R4010-55.TM. material from Pall RAI Manufacture Co., or the
like.
Where the solid proton conductor of electrolyte membrane is
composed substantially of inorganic materials, such as a metal
oxide proton conductive material, then it is preferable that the
metal oxide proton conductive material be Sb.sub.2O.sub.5.4H.sub.2O
as a composition of materials.
Microporous hydrophobic membrane 204 will preferably be an organic
material such as CELGARD 2400.TM. supplied by Celanese Corporation
of the United States of America. Alternatively, microporous
hydrophobic membrane 204 may also be a GORETEX.TM. membrane
supplied by W. L. Gore & Associates, Inc. of the United States.
Alternatively, microporous hydrophobic membrane 204 may also be a
ZITEX.TM. membrane supplied by Norton Performance Plastics
Corporation, also of the United States of America.
Preferably, counter electrode 14 and sensing electrode 16 will have
a thickness of approximately 0.1 mm and a diameter of approximately
13 mm. Also preferably, hole 202 will have a diameter of 3 mm and
hole 206 will have a diameter of 0.2 mm, each hole being in the
center of its respective piece. Preferably, microporous hydrophobic
membrane 204 will have a thickness of 0.1 mm and a diameter of 10
mm. Preferably, protonic conductive membrane 12 will have a
thickness of 0.1 mm and a diameter of 20 mm. It is preferable that
protonic conductive membrane 12 be less than 1 mm in thickness so
that the resistance of the same will be desirably low. It is
desirable to have a small proton resistance so that the proton
electrical current that is generated as a result of pressure
differences of the target gas CO across protonic conductive
membrane 12 without applying a DC power. The background electrical
current will be preferably in the nA range or less, which is
negligible.
As an alternative to manufacturing counter electrode 14 and sensing
electrode 16 from mixed protonic electronic conductive materials, a
thin film of electrically conductive film, such as noble metal film
which is deposited upon protonic conductive membrane 12, may also
be used in replacement for such electrodes. Preferably, the thin
metal film will be deposited by sputtering or physical vapor
deposition, or other known methods of depositing a thin metallic
film, where the film is permeable to water vapor. Such a film is
seen in FIG. 6, discussed below.
The purpose of the noble metal thin film is to generate a chemical
reaction by acting as a catalyst and for conducting electrons
through a very thin distance provided by the deposited layer of
thin noble metal film. The thin noble metal film is in contact with
the protonic conductive membrane which conducts protons
therethrough. By minimizing the distance through which electrons
are conducted by the thin noble metal film, and selecting the thin
noble metal film to be an excellent electron conductor, then the
sensor is made most efficient in transferring both electrons and
protons therethrough in the process of detecting CO concentration
in the ambient. Preferably, the noble metal thin film will be in
the range of about 50 Angstroms to about 5,000 Angstroms. By way of
example, platinum or palladium are suitable noble metal films. It
is preferable that the catalyst be a good electron conductor while
the protonic conductive membrane be a good proton conductor.
The sensor depicted in FIG. 2 may also be constructed and used
without applying a DC power. This is particularly true in that a
sensor constructed according to that depicted in FIG. 2 absent the
electrical circuitry, has a small proton resistance. The proton
current generated as a result of the pressure difference of the
target gas across the membrane is strong enough to be detected
without applying a DC power. In such an embodiment of the inventive
sensor, the background current is in the nA range or less, which is
basically negligible. Operating the sensor of FIG. 2 without
applying a DC power is advantageous in that it lends long term
stability in the performance of the inventive sensor. This is true
in that the particle size of the noble metal catalyst of the
electrodes to the inventive sensor do not coalesce when there is no
DC power applied. As such, the maintenance of the small particle
size of the noble metal catalyst in the electrode prevents the
reduction of the long term functionality and accuracy of the
inventive sensor response.
Protonic conductors membranes are usually slightly permeable to CO
gas. When a membrane is under a carbon monoxide partial pressure
difference, a very small amount of carbon monoxide will permeate
across the membrane into the counter electrode side.
Influence of the CO permeation to sensor response usually is
insignificant because this very small amount of permeated CO is
instantly converted into carbon dioxide at the reference electrode.
If a precision CO concentration detection is needed, CO
concentration in the counter electrode can be minimized by
attaching an electrochemical CO pump to the sensor according to
this invention. The purpose of an electrochemical pumping circuitry
is to prevent the buildup of CO gas at the counter electrode side
of the sensor so that a precision CO detection is achieved.
Protonic conductive membrane 12 may be substantially composed of a
solid, perfluorinated ion-exchange polymer, or a metal oxide
protonic conductor electrolyte material. The following table serves
as a further example of solid state protonic conductor which can be
used at room temperature in the inventive gas sensor.
TABLE-US-00001 MATERIALS 1.
H.sub.3Mo.sub.12PO.sub.40.cndot.29H.sub.2O 6. NAFION .TM. DuPont .
(US) 2. H.sub.3W.sub.12PO.sub.4.cndot.29H.sub.2O 7. C membrane
Chlorine Engineer's (Japan) 3. HUO.sub.2PO.sub.4.cndot.4H.sub.2O 8.
XUS-1304.10 Dow (US) 4. Zr(HPO.sub.4).sub.2.cndot.3H.sub.2O 9.
R4010-55 PALL RAI Manufacturing Co. (US) 5.
Sb.sub.2O.sub.5.cndot.4H.sub.2O
Protonic conductive membrane 12 is preferably constructed of
materials 6, 7, 8, or 9 which are unreinforced film of
perfluroinated copolymers.
FIG. 3 features counter numerals similar to FIG. 2, with identical
counter numerals referring to similar structures performing similar
functions. FIG. 3 shows an alternative embodiment of sensor 10. An
amperometric measuring unit 44 is in electrical communication with
electrodes 14, 16, and DC power switching circuity is shown in
electrical communication with a pair of pump electrodes 15, 17. All
of the electrodes 14-17 are interfacing with protonic conductive
membrane 12. The purpose of pump electrodes 15, 17 is to
continuously pump CO away from counter electrode 14 side of sensor
10 while continuously sensing the presence of CO gas in the
ambient. This continuous pumping of CO away from the side of sensor
10 where counter electrode 14 is located serves to give stability
to the sensor signal response to CO concentration in the ambient.
The DC power source can be operated in either "pulse" mode to pump
CO, or in the "on" mode to sense CO concentration. In sensor 10,
depicted in FIG. 3, both can 30 and cap 32 are preferably made of
electrically insulative materials.
Large holes 38a in washer 36 expose a larger surface area of
reference electrode 16 to water vapor in reservoir 200 than the
surface area of sensing electrode 14 and count electrode 15 exposed
to ambient atmosphere. As such, protonic conductive membrane 12 and
all electrodes are exposed to substantially 100 percent relative
humidity, and a positive pressure of the water vapor in reservoir
200 exists from each hole 38a to each hole 38b due to the partial
pressure difference therebetween.
A further embodiment of the inventive CO sensor is seen in FIG. 4
as a sensor 110. Sensor 110 has two protonic conductive membranes
112, 122 that prevent interference with the response of sensor 110
due to the detection of CO concentrations. Sensor 110 features a
can 130 having water reservoir 200 therein.
Can 130 is an example and illustration of a means, containing a
volume of water vapor, for exposing a surface of a counter
electrode to the water vapor. Upon such exposure to an atmosphere
concentration of a gas, such as CO, electrical measurements can be
made to detect changes in electrical characteristics of the
electrodes.
Material 134 and washer 136 retains first and second protonic
conductive membranes 122, 112 within can 130. A sensor electrode
116 is on an opposite side of first protonic conductive membrane
122 from a counter electrode 114. First and second pump electrodes
115, 117 are in contact with opposite sides of second protonic
conductive membranes 112. A bottom cap 132A and a top cap 132B have
holes therein, respectively, having holes 206a, 206b. Can 130 has
large holes 210, 208 therein.
Due to the geometric of holes 206a, 206b, 208, and 210, there will
be a greater exposed surface area of first pump electrode 115 to
the ambient than the surface area of sensing electrode 116 exposed
to the ambient. Consequently, protonic conductive membrane 112 is
exposed to substantially 100 percent relative humidity.
A DC power source 140 is in electrical contact with first pump
electrode 115 and metallic can 130 through electrical contacts 146
and 144. DC power source 140, and related circuity, serve as an
example of a means for applying a DC power across the protonic
electrolyte membranes. Sensing electrode 116 is in contact with an
electrical measurement means 142 through electrical leads 146, 144.
DC power supply 140 serves as a CO pump to sensor 110. By way of
example and illustration of an electrical sensing means, a meter
142 is used to measure the response of sensor 110 to concentrations
of CO.
Sensing electrode 116 is exposed to the ambient through hole 206a
via microporous hydrophobic dust filter 212. First pump electrode
115 is exposed to the ambient atmosphere holes 206b, where the area
between reservoir 200 and membrane 122 serves as a counter
environment for electrode 114.
Sensing electrode 116, exposed to the ambient atmosphere through
holes 206a, with first protonic conductive membrane 122 performs
the function, in combination with counter electrode 114 of sensing
CO concentration through the conduction therethrough of protons.
Second protonic conductive membrane 112, in combination with first
and second pump electrodes 115, 117, performs the function of
pumping CO out of the side of sensor 110 associated with counter
electrode 114 so as to stabilize the sensor response of sensor 110
upon the detection of a concentration of CO in the ambient.
FIG. 5A shows sensor voltage response with respect to time of the
inventive one protonic conductive membrane gas sensor seen in FIG.
2. Reference point 90 shows zero time with a negligible CO
concentration. Reference point 92 shows an environment of 100 ppm
CO after a period of less than one minute. At reference point 93 on
FIG. 5A, an injection of 200 ppm CO is made into the environment
such that sensor responses maximizes at reference point 94a on FIG.
5A. At reference point 94b on FIG. 5A, the atmosphere is seen to be
opened up to clean air and the sensor response decreases by a
slight under shoot to reference point 94c on FIG. 5A after a period
of about one minute. The senor response levels back to zero amps as
time goes on from reference point 94c. FIG. 5A reflects
environmental parameters of 23.degree. C. and 35% relative
humidity. Such a sensor current response is seen in a nonlogrithmic
scale in FIG. 5B.
FIG. 5B shows the characteristic of the inventive CO sensor with
respect to its independence of varying relative humidity
environments. In an amperometric embodiment of the inventive CO gas
sensor, as can be seen from FIG. 5B, relative humidity does not
interfere with the linear nature of the sensor response in
increasing environments of CO concentration. The ability of the
inventive amperometric CO sensor to avoid interference with
relative humidity is that, because of the water vapor contained in
the housing of sensor and the exposure of the protonic conductive
membrane to the water vapor, the protonic conductive membrane is
constantly in a state of saturation regardless of ambient
atmospheric relative humidity. Thus, bulk ionic resistance of the
inventive CO sensor is constant as relative humidity changes.
Electrical current, which is the measurement of sensor response,
also remains constant.
In the inventive CO sensor, the sensing electrode is exposed to an
environment containing CO, whereas the sensing electrode, counter
electrode side and proton conductor are exposed to a 100 percent
relative humidity environment. The protonic conductive membrane can
have the thickness so that the reactant oxygen and the produced
water permeate the membrane. A small part of CO gas also permeates
through the membrane, but the permeated CO is consumed by the
reaction with oxygen electrochemically and catalytically at the
counter electrode.
Alternative embodiments of the inventive sensor are depicted in
FIGS. 6 and 7, where the protonic conductive membrane is seen in
two parts. Particularly a nonplanar surface of protonic conductor
membrane 12 is seen at portion 12a thereof. A planar surface of
protonic conductor membrane 12 is seen in portion 12b thereof. By
increasing the surface area of portion 12a via a nonplanar surface,
greater contact with the materials of the catalyzing and electrical
conducting electrode thereover is possible. A greater contact will
ensure a greater conductivity of protons and electrons therethrough
as well as a greater surface upon which to catalyze. The creation
of such a nonplanar surface may be accomplished by a chemical or
physical abrasion process, or by other known method.
FIG. 6 illustrates an example of an electrode made of an
electrically conductive thin film situated upon a protonic
conductive membrane. The film has an average thickness in the range
of about 50 Angstroms to 10,000 Angstroms, and will preferably be
in the range of about 4,000 Angstroms to 6,000 Angstroms. The
preferable film is substantially composed of a noble metal, such as
platinum. The film may be deposited on protonic conductive membrane
12 by sputter, or by vapor deposition techniques, or by other known
film layering techniques.
FIG. 6 is an amplified view of an electrically conductive electrode
having protonic conductive membrane 12, a current collector
electrical lead 22, and an electron conductive phase material 82
therebetween. Electron conductive phase material 82 has a plurality
of gaps 80 interstitially placed between particles of electron
conductive phase material 82. A plurality of three-phase contact
areas 86 exists and interfaces between protonic conductive membrane
12 and electron conductive phase material 82. CO gas in the ambient
coming in contact with electron conductive phase material 82
produces electrons which are drawn to current collector electrical
lead 22. CO gas in the ambient coming in contact with the interface
of electron conductive phase material 82 and protonic conductive
membrane 12 at three-phase conductive contact area 86 will produce
hydrogen ions, or protons, which are conducted through protonic
conductive membrane 12. As can be seen from FIG. 6, the creation of
hydrogen ions occurs only at the surface of protonic conductive
membrane 12 at three-phase contact area 86. Thus, there is limited
surface at which the creation of hydrogen ions can take place in
the embodiment of the electronically conducted electrode shown in
FIG. 6.
FIG. 7 shows a mixed protonic-electronic conductive electrode
having a protonic conductive membrane 12, a current collector
electrical lead 22, and a variety of amplified particles
therebetween and consisting of an electronic conductive phase
material 82, and a protonic conductive phase material 84. Between
particles of protonic conductive phase material 84 and electronic
conductive phase material 82, there are gaps 80 which represent the
pores between the particles situated between current collector
electrical lead 22 and protonic conductive membrane 12. Electrons
are transmitted to current collector electrical lead 22 when CO gas
in the ambient comes in contact with three-phase contact area 86.
Hydrogen ions are transported to protonic conductor membrane 12
when CO gas in the ambient comes in contact with three-phase
contact area 86. The creation of both hydrogen ions and electrons
occurs at each of the plurality of three-phase contact areas 86
shown in FIG. 7. Neither electrons nor hydrogen ions are created at
interface 88 which is situated between protonic conductive membrane
12 and protonic conductive phase material 84. Similarly, no
reaction to create electrons or hydrogen ions occurs at an
interface 88 between current collector electric lead 22 and
electronic conductor phase material 82.
As can be seen from FIG. 7, the creation of hydrogen ions occurs in
the three-dimensional area between current collector electrical
lead 22 and protonic conductive membrane 12. Thus, the surface area
available to create hydrogen ions is greater in the electrodes seen
in FIG. 7 as compared to the electrode seen in FIG. 6. This
additional surface area for creation of hydrogen ions is due to the
presence of protonic conductive phase material 84 in the electrode
above protonic conductive membrane 12. Conversely, FIG. 6 does not
contain any protonic conductive phase material situated on and
above protonic conductive membrane 12.
The mixed conductor material found in the electrode seen in FIG. 7
has desirable benefits, such as provision of a high surface area
for the CO oxidation reaction in the sensing electrode side, and
providing a high surface area for the H.sub.2O formation reaction
in counter electrode side. The thin film of the electrode seen in
FIG. 6, while being an efficient conductor of electrons, also
provides a short path for protonic conduction, which tends to be
faster than a thicker electrically conductive electrode that does
not conduct protons efficiently. The inventive alcohol sensor based
on FIG. 7 shows large response to alcohol, and the inventive CO
sensor based on FIG. 6 shows almost zero interference by other
gases. As can be seen either electrode embodiment of FIGS. 6 or 7
can be beneficial. Proton conductive membrane 12 is indicated in
both FIGS. 6 and 7 as having either a substantially non-planar
interface 12a, or a substantially planar interface 12b. As such, it
is intended that any of the electrodes of the inventive sensor, in
any of the disclosed embodiments herein, may be either a mixed
proton electron conductor material electrode or may be a thin film
electron conductor material electrode. Further, mixed conductor and
thin film electrodes may been used in any combination thereof
within any embodiment of the inventive sensor.
FIG. 8 depicts an embodiment of the inventive sensor having three
electrodes. While the foregoing embodiments of the inventive sensor
used only two electrodes, and thereby resulted in cost savings, a
three-electrode embodiment of the invention is seen as sensor 10 in
FIG. 8. Reference numerals in FIG. 8 identical to reference
numerals in FIG. 2, represent similar structures performing similar
functions.
Sensor 10 in FIG. 8 has a reference electrode 15 and a counter
electrode 14 and on an cap 32 on an opposite side of a protonic
conductive membrane 12 from a sensing electrode 16. Sensing
electrode 16 is vented to the ambient through a small hole 206a
that is smaller than a large hole 202 vented to reservoir 200
having water vapor therein. A microporous hydrophobic membrane 204
is positioned in between hole 202 and counter electrode 14. Gasket
36 and a material 34 retain protonic conductive membrane 12 in can
30.
Those of ordinary skill in electrical measurement circuitry may
incorporate such circuitry to obtain sensor response in ambient
concentrations of a target gas with sensor 10 in FIG. 8, including
variations of circuitry disclosure herein.
The inventive CO gas sensor using the mixed protonic-electronic
conductive materials in the electrodes with high surface area of
100 to 1000 M.sup.2/g shows a shorting current as high as 150
.mu.A/cm.sup.2 to 1,000 ppm CO, which is at least two orders of
magnitude higher compared to the sensors with electronic conductive
electrodes according to prior art.
TABLE-US-00002 COUNTER ELECTRODE SENSING ELECTRODE A preferred
composition of such electrodes is as follows: 7.5 wt % Ru oxide 20
wt % Pt-black 67.5 wt % carbon 55 wt % carbon 25 wt % NAFION .TM.
25 wt % NAFION .TM. Other compositions of such electrodes are as
follows: Pd 20 wt % Pd 20 wt % Carbon 60 wt % Carbon 60 wt %
Sb.sub.2O.sub.5.cndot.4H.sub.2O 20 wt %
Sb.sub.2O.sub.5.cndot.4H.sub.2O 20 wt % Rb 25 wt % Pd 25 wt %
Carbon 50 wt % Ni 50 wt % R4010-55 25 wt % R4010-55 25 wt % 10 wt %
Pt on vulcan carbon 10 wt % Pt on vulcan carbon XC72 25 wt % XC72
25 wt % NAFION .TM. 25 wt % NAFION .TM. 25 wt % Ti 50 wt % Ni 50 wt
% 20 wt % Pt-Black 20 wt % Pt-black 55 wt % carbon 55 wt % carbon
25 wt % NAFION .TM. 25 wt % NAFION .TM.
The role of platinum in the sensing electrode is to favor the CO
decomposition reaction (1) whereas Ru oxide in the counter
electrode is to favor the water formation reaction (2). According
to this invention, the Ru oxide, instead of expensive platinum and
the like, as reported in prior art, shows excellent CO sensing
performance.
It is also contemplated that the electrodes disclosed herein can be
composed substantially of carbon, noble metals, or conductive metal
oxides. The electrical conducting material in electrodes disclosed
here is preferably a proton-electron mixed conductive material
having 10-50 wt % of a proton conductor material and 50-90 wt % of
a first and a second electrical conductor material. The proton
conductor material for the electrodes disclosed herein is
preferably a copolymer having a tetrafluoroethylene backbone with a
side chain of perfluorinated monomers containing at least one of a
sulfonic acid group or carboxylic acid group. Preferably, one of
the first and second electrical conductor materials for the sensing
electrodes disclosed herein is 50-99 wt % of carbon black, and the
other of the first and second electrical conductor materials for
the sensing electrodes disclosed herein is 1-50 wt % of platinum.
Also preferably, one of the first and second electrical conductor
materials for the counter electrode is 50-99 wt % of carbon black,
and the other of the first and second electrical conductor
materials for the counter electrode is 1-50 wt % of Ru oxide.
In a composition of 25 wt % protonic conductor in electrodes, which
is a physically continuous phase, there is proton conduction,
whereas the rest of the phases in electrodes provide electronic
conduction as well as catalytic activity. If without 25 wt % proton
conductor in electrodes, the electrodes were only an electronic
conductor, and the reactions (1) and (2), above, would only occur
at three-phase contact area 86 seen in FIG. 6, which is a very
limited small area. When the electrodes are made of mixed
conductors according to this invention, the reactions (1) and (2)
will occur on all surface of the electrodes. Therefore, by using
high surface area mixed conductive electrodes (100 to 1,000
M.sup.2/g) seen in FIG. 7, fast CO reaction kinetics at the
interface are achieved and strong signal response is obtained.
While the inventive gas sensor can be used to measure CO
concentration, it is also capable of measuring other gases such as
H.sub.2, H.sub.2S, H.sub.2O vapor alcohol, and NO.sub.x
concentrations.
Various protonic conductors, including organic protonic conductors
and inorganic protonic conductors, can be used in the sensor
according to this invention. In what follows, a copolymer protonic
conductive membrane based on a tetrafluoroethylene backbone with a
side chain of perfluorinated monomers containing sulfonic acid
group is used herein as an example of the fabrication of the
inventive sensor.
To prevent deterioration of the polymer membrane in the subsequent
wetting/drying steps, the membrane must be first converted from the
proton form to the sodium form by the following steps A: A. The
polymer membrane is soaked in lightly boiling dilute NaOH solution
for 1-3 hours. It is then rinsed first in tap water for 0.5-3
hours, then in deionized water for 10-30 minutes, and is then laid
out on a rack to air dry. B. The materials for the preferred mixed
conduction electrodes are as follows: Pt/carbon powder, carbon
powder, Ru oxide powder, solubilized polymer solution, Glycerol,
NaOH solution, and deionized water. C. The steps for fabrication
are as follows: 1. Pre-mix deionized water and glycerol in 20-30%
weight ratio, and store the mixture in a container; 2. Weigh an
appropriate amount of Pt/carbon powder into a clean container; 3.
Weigh an appropriate amount of 5% wt polymer solution, and add to
material in step C.2, and then mix. Typically, add 1-3 parts 5% wt
NAFION.TM. solution (on a dry polymer basis) to 3-5 parts Pt/carbon
powder; 4. Weigh and add an appropriate amount of water/glycerol
mixture to mixture in step C.3, and then mix. Typically, add 25-35
parts water/glycerol mixture to one part Pt/carbon powder; 5. Weigh
and add an appropriate amount of 1-2 Moles NaOH to the mixture in
step C.4, and then mix. Typically, add 1-2 parts 1-2 Moles NaOH to
9-15 parts 5% wt polymer solution; and further mix the wet
electrode mixture ultrasonically for 60 minutes. D. For Carbon/Ru
Oxide electrode preparation, the following steps are taken: 1.
Pre-mix the deionized water and glycerol in 20-30% weight ratio,
store the mixture in a container, and set aside; 2. Weigh an
appropriate amount of carbon powder and Ru oxide into a clean
container; 3. Weigh an appropriate amount of 5% wt polymer
solution, and add to the material in step D.2, and then mix.
Typically, add 1-3 parts 5% wt polymer solution (on a dry polymer
basis) to 3-5 parts carbon/Ru oxide powder; 4. Weigh and add an
appropriate amount of water/glycerol mixture to mixture in step
D.3, and then mix. Typically, add 25-35 parts water/glycerol
mixture to 1 part carbon/Ru oxide powder; 5. Weigh and add an
appropriate amount of 1-2 Moles NaOH to the mixture in step C.4,
and then mix. Typically, add 1 part 1-2 Moles NaOH to 9-15 parts 5%
wt polymer solution; and further mix the wet electrode mixture
ultrasonically for 60 minutes. E. For Pt/Carbon Electrode
application drying, the following steps are taken: 1. Re-mix the
wet electrode mixture ultrasonically for at least 30 minutes prior
to use; 2. Fill the dispensing machine tubing with the Pt/carbon
wet electrode mixture; 3. Dispense the wet electrode mixture to the
surface of the membrane at the desired location; and 4. Place the
membrane/electrode in an oven at 100.degree.-170.degree. C. for
10-60 minutes. F. For Carbon/Ru Oxide Electrode application drying,
the following steps are taken: Repeat step A on the opposite side
of the membrane. G. For acidification, the following steps are
taken: 1. For Ion-Exchange, soak membrane/electrodes in lightly
boiling dilute MH2S04 solution for 1-3 hours. 2. For cleaning,
rinse the membrane/electrodes in deionized water; 3. For drying,
dry the membrane/electrodes in air, or air dry then desiccate
overnight, or place in a 30.degree.-50.degree. C. oven for 1-3
hours before cutting to the final dimensions.
The present invention may be embodied in other specific forms
without departing from its spirit or essential characteristics. The
described embodiments are to be considered in all respects only as
illustrated and not restrictive. The scope of the invention is,
therefore, indicated by the appended claims rather than by the
foregoing description. All changes which come within the meaning
and range of equivalency of the claims are to be embraced within
their scope.
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