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.
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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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
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.
0. 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.
0. 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.
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.
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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.
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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.
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25. The electrochemical gas sensor as defined in
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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.
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37. The electrochemical gas sensor as defined in
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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.
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55. The electrochemical gas sensor as defined in
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0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 81. The electrochemical gas sensor as defined in claim 80, wherein the film is substantially composed of a noble metal.
0. 82. The electrochemical gas sensor as defined in claim 81, wherein the noble metal is platinum.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 103. The electrochemical gas sensor as defined in claim 75, wherein the gas is CO.
0. 104. The electrochemical gas sensor as defined in claim 75, wherein the gas is NOx.
0. 105. The electrochemical gas sensor as defined in claim 75, wherein the gas is hydrogen.
0. 106. The electrochemical gas sensor as defined in claim 75, wherein the gas is H2S.
0. 107. The electrochemical gas sensor as defined in claim 75, wherein the gas is H2O vapor.
0. 108. The electrochemical gas sensor as defined in claim 75, wherein the gas is alcohol vapor.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 116. The electrochemical gas sensor as defined in claim 115, wherein the film is substantially composed of a noble metal.
0. 117. The electrochemical gas sensor as defined in claim 116, wherein the noble metal is platinum.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
0. 124. The electrochemical gas sensor of claim 123 in which the sensing electrode comprises a mixed protonic-electronic conductive electrode.
0. 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.
0. 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.
0. 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.
0. 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.
0. 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.
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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.
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° to 500° 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 CO2 in an oxidation reaction of CO.
CO+H2O→CO2+2H++2e− (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++2e−+2O2→H2O (2)
These two reactions are the basis of prior art room temperature zero power electrochemical gas sensors utilizing a proton conductor.
The current generated by the reactions depicted in
Whether the transport processes shown in
The sensor of
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™ 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.
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 H2, H2S, H2O 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.
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:
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 Rbulk of the inventive sensor is equal to
where Ro 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 Rbulk is not a performance limit. Electronic resistivity of the electrodes is in order of 105 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
In
A sensor 10 in
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
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
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
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™ 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
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 Sb2O5.4H2O as a composition of materials.
Microporous hydrophobic membrane 204 will preferably be an organic material such as CELGARD 2400™ supplied by Celanese Corporation of the United States of America. Alternatively, microporous hydrophobic membrane 204 may also be a GORETEX™ membrane supplied by W. L. Gore & Associates, Inc. of the United States. Alternatively, microporous hydrophobic membrane 204 may also be a ZITEX™ 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
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
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.
MATERIALS
1. H3Mo12PO40•29H2O
6. NAFION ™
DuPont . (US)
2. H3W12PO4•29H2O
7. C membrane
Chlorine
Engineer's (Japan)
3. HUO2PO4•4H2O
8. XUS-1304.10
Dow (US)
4. Zr(HPO4)2•3H2O
9. R4010-55
PALL RAI
Manufacturing
Co. (US)
5. Sb2O5•4H2O
Protonic conductive membrane 12 is preferably constructed of materials 6, 7, 8, or 9 which are unreinforced film of perfluroinated copolymers.
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
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.
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
As can be seen from
The mixed conductor material found in the electrode seen in
Sensor 10 in
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
The inventive CO gas sensor using the mixed protonic-electronic conductive materials in the electrodes with high surface area of 100 to 1000 M2/g shows a shorting current as high as 150 μA/cm2 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.
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 ™
25 wt % NAFION ™
Other compositions of such electrodes are as follows:
Pd 20 wt %
Pd 20 wt %
Carbon 60 wt %
Carbon 60 wt %
Sb2O5•4H2O 20 wt %
Sb2O5•4H2O 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 ™ 25 wt %
NAFION ™ 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 ™
25 wt % NAFION ™
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
While the inventive gas sensor can be used to measure CO concentration, it is also capable of measuring other gases such as H2, H2S, H2O vapor alcohol, and NOx 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:
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.
Shen, Yousheng, Consadori, Franco, Field, D. George
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