Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-11-21
2003-11-25
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S425000, C204S426000
Reexamination Certificate
active
06652723
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydrogen gas sensor, and more particularly, to a hydrogen gas sensor suitable for measuring the hydrogen concentration of a fuel gas used for fuel cells.
2. Description of the Related Art
In view of the issue of global-scale environmental deterioration, fuel cells, which are clean and efficient power sources, have recently become the subject of active studies. Among fuel cells, a polymer electrolyte fuel cell (PEFC) is expected to be suitable for vehicle use due to its advantages, including low operation temperature and high output density. In this case, a reformed gas obtained from methanol or the like is advantageously used as a fuel gas. Further, in order to improve efficiency and other parameters of performance, a gas sensor capable of directly measuring hydrogen concentration of the reformed gas is needed.
Since such a hydrogen gas sensor is used in a hydrogen-rich atmosphere, the operation temperature of the gas sensor must be low (about 100° C. or less). Such a low-operation-temperature-type sensor is disclosed in Japanese Patent Publication (kokoku) No. 7-31153. In the sensor, a working electrode, a counter electrode, and a reference electrode are disposed on an insulating substrate, and the three electrodes are integrally covered with a gas-permeable, proton-conductive film; more specifically, “NAFION®” (trademark, product of Dupont), which is a type of fluororesin. NAFION® is a proton-conductive material capable of operating at low temperature and is used at portions of polymer electrolyte fuel cells.
The present Inventors found that when NAFION® is used as a proton-conductive layer as in the gas sensor disclosed in Japanese Patent Publication No. 7-31153, the sensor output varies depending on the H
2
O concentration partial pressure of a gas under measurement (hereinafter referred to as a measurement gas atmosphere), so that accurate measurement becomes difficult. Further, the present Inventors found that the above phenomena occurs because protons pass through NAFION® together with H
2
O molecules, and therefore, the proton conductivity varies with the H
2
O concentration of the measurement gas atmosphere. That is, when the proton-conductive layer is formed of NAFION®, the sensor output depends on the H
2
O concentration of the measurement gas atmosphere, so that the sensor output decreases greatly, especially when the H
2
O concentration is low.
The present Inventors further found that although porous Pt electrodes (catalysts) are generally known to exhibit high activity at low temperature (porous Pt electrodes are used, for example, in fuel cells), when such a Pt electrode is exposed to an atmosphere having a high CO concentration, CO is adsorbed on the Pt electrode, or the Pt electrode is CO-poisoned, so that the sensor output is greatly decreased.
Since many fuel cells use pressurized fuel gas in order to improve power generation efficiency, sensors used in the fuel gas are required to have a small pressure dependency. However, in the sensor described in the above-mentioned Japanese Patent Publication No. 7-31153, a gas under measurement is diffused to the working electrode via the gas-permeable, proton-conductive film, so that the sensor exhibits a great pressure dependency, depending on the structure of the proton-conductive film itself, and therefore high measurement accuracy cannot be obtained.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a hydrogen gas sensor capable of accurately measuring hydrogen concentration in the presence of a variety of interfering gasses.
In the hydrogen gas sensor of the present invention, the rate of conduction of protons from a first electrode to a second electrode is rendered greater than the rate at which protons are derived from hydrogen introduced onto the first electrode via a diffusion-rate limiting portion.
That is, because the rate of conduction of protons from the first electrode to the second electrode is sufficiently greater than the rate at which protons are derived from hydrogen introduced from the measurement gas atmosphere onto the first electrode via the diffusion-rate limiting portion, the sensor can accurately measure hydrogen concentration without causing a great decrease in sensor output. That is so even when the measurement gas atmosphere has a low H
2
O concentration or a high CO concentration.
The present invention is applicable to both a hydrogen gas sensor not having a reference electrode and to a hydrogen gas sensor having a reference electrode. In the latter gas sensor, the voltage applied between the first and second electrodes can be variably controlled such that a constant voltage is produced between the first electrode and the reference electrode, or such that the hydrogen concentration on the first electrode becomes constant. Therefore, for any given hydrogen concentration an optimal voltage can be applied between the first and second electrodes, so that a more accurate measurement of hydrogen concentration can be obtained within a wide range of concentration.
The hydrogen gas sensor according to the present invention is advantageously used for measuring an atmosphere in which hydrogen H
2
O, and other components coexist, especially for measuring the hydrogen concentration of a fuel gas for polymer electrolyte fuel cells.
In a preferred mode of the present invention, the diffusion-rate limiting portion preferably has a relatively high gas-diffusion resistance, so as to render the proton-conducting performance excessive. In this case, the rate of conduction of protons through the proton-conductive layer becomes greater than the rate at which protons are derived from hydrogen introduced onto the first electrode. The gas-diffusion resistance of the diffusion-rate limiting portion is increased, for example, by increasing the length (thickness) of the diffusion-rate limiting portion in the gas diffusion direction or by decreasing the cross sectional area perpendicular to the gas diffusion direction (hereinafter referred to as a “flow sectional area”). Alternatively, when the diffusion-rate limiting portion is formed of a porous material, the gas-diffusion resistance of the diffusion-rate limiting portion is increased by decreasing the porosity (pore diameter, apparent porosity, etc.) of the porous material.
The gas-diffusion resistance of the diffusion-rate limiting portion is preferably set as follows in order to render the rate of conduction of protons from the first electrode to the second electrode greater than the rate at which protons derived from hydrogen are introduced onto the first electrode via the diffusion-rate limiting portion.
(1) Proton Conduction Condition A
A proton-conducting rate under severe conditions is measured. That is, a current (a) flowing between the first and second electrodes is measured upon applying a sufficiently high voltage between the first and second electrodes in a state in which the gas-diffusion resistance of the diffusion-rate limiting portion is rendered sufficiently small (e.g., about 0.9 mA/mm
2
or more of the first electrode (3), with current conversion, at H
2
=40%) in order to introduce a sufficiently large amount of hydrogen onto the first electrode, but under the severest conditions for proton conduction; e.g., conditions such that the measurement gas atmosphere has a very low H
2
O concentration (specifically, 10% or less at 80° C.) or a very high CO concentration (specifically, 1000 ppm or greater). Although the above-described current (a) need not be a saturation current, the applied voltage (specifically, 50 mV or higher) is preferably equal to or higher than the voltage applied in the case of condition B described below.
(2) Proton Conduction Condition B
Next, a proton-conducting rate under favorable conditions is measured. That is, a saturation current (b) flowing between the first and second electrodes is measured upon application of a sufficiently high voltage between the first and second electrodes in a stat
Inoue Ryuji
Ishida Norobu
Kondo Tomonori
Nadanami Norihiko
Oshima Takafumi
NGK Spark Plug Co. Ltd.
Nguyen Nam
Olsen Kaj K.
Sughrue & Mion, PLLC
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