Carbon monoxide sensor

Details Carbon monoxide sensor Carbon monoxide sensor

2001-08-09

2003-12-30

Larkin, Daniel S. (Department: 2856)

Measuring and testing

Gas analysis

C073S023310, C073S031050, C204S421000, C204S424000

Reexamination Certificate

active

06668616

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a carbon monoxide sensor for detecting the a concentration of carbon monoxide in fuel gas having a high content of hydrogen used, for example, in a fuel cell.
BACKGROUND ART
Recently, a fuel cell using a solid high polymer proton conductive film is intensively developed for a home appliance and an automobile. The fuel cell uses hydrogen gas as fuel gas for operation, and hence requires a reformer for producing hydrogen gas by reforming liquid fuel such as methanol or utility gas. However, a fuel gas mainly composed of hydrogen produced by the reformer contains very little carbon monoxide of about scores of ppm. Carbon monoxide which is adsorbed in a platinum catalyst composing electrodes of the fuel cell (this phenomenon is called poisoning) lowers an electromotive force of the fuel cell. Consequently, it is necessary to monitor concentration of carbon monoxide in the fuel gas and to control the fuel cell accordingly.
A carbon monoxide sensor for detecting carbon monoxide in fuel gas for a fuel cell has been disclosed, for example, in Japanese Laid-open Patent No. 8-327590.
A sectional view of a schematic structure of this carbon monoxide sensor is shown in FIG.
13
. An electrolyte film
110
is composed of a high polymer having proton conductivity. On both surfaces of the film, electrodes
112
,
114
having a carbon cloth containing kneaded carbon powder and a platinum catalyst carried thereon are bonded by performing hot pressing. Mesh metal plates
116
,
118
are disposed on surfaces of the electrodes
112
,
114
, on which the electrolyte film
110
is not disposed, respectively. The electrolyte film
110
, electrodes
112
,
114
, and metal plates
116
,
118
are held by flanges
120
a
,
122
a
provided inside of metal cylindrical holders
120
,
122
. An o-ring
126
for sealing gas is disposed at an electrolyte film
110
side at an end of the holder
122
.
On an outer circumference of the holders
120
,
122
, threaded portions
120
b
,
122
b
are formed. The holders
120
,
122
are fixed by being driven into threaded portions
124
a,
124
b formed inside of an insulating member
124
made of polytetrafluoroethylene, such as TEFLON (a trademark of Du Pont).
At one end of the holder
120
, one end of a gas influent passage
128
is connected, from which object gas (fuel gas in this case) is introduced into the carbon monoxide sensor. One end of the holder
122
is not connected to the gas influent passage
128
and opens to an atmosphere.
Another end of the gas influent passage
128
is connected to a branch port
140
a
, provided at a part of a fuel gas passage
140
, to allow gas to be introduced into the fuel cell.
Detecting terminals
120
T,
122
T are provided at the holders
120
,
122
, and an electric circuit
130
is connected to these terminals. The electric circuit
130
is composed of a voltmeter
132
, and a resistor
134
, for adjusting the load current, connected in parallel with the voltmeter. The detecting terminals
120
T and
122
T are connected to negative and positive electrodes, respectively.
An operation of the carbon monoxide sensor will be explained. An object gas (fuel gas) containing much hydrogen gas reaches the electrode
112
through the gas influent passage
128
. The electrode
114
always contacts oxygen gas in the atmosphere. Therefore, on a surface of the electrolyte film
110
contacting the electrodes
112
,
114
, hydrogen gas and oxygen gas react similarly to that as in the fuel cell to generate an electromotive force between the electrodes
112
and
114
. The resistor
134
connected between the electrodes
112
and
114
creates a specified load current flow, and the voltmeter
132
detects a voltage between the electrodes
112
and
114
.
In this situation, if carbon monoxide is mixed in the object gas, the carbon monoxide is adsorbed into the platinum catalyst at the electrode
112
and poisons the electrode
112
. As a result, hydrogen gas and oxygen gas are prevented from reacting and voltage between the electrodes
112
and
114
is lowered. Since concentration of carbon monoxide relates to a degree of poisoning, by measuring voltage between the electrodes
112
and
114
, a concentration of carbon monoxide in the object gas can be detected.
In this carbon monoxide sensor, to the extremely thin electrolyte film
110
made of high polymer sealing the object gas from the atmosphere, a differential pressure between object gas and atmosphere (usually several atmospheres) is always applied. In such circumstance, an abnormally high pressure, or an unexpected large pressure due to vibration or the like, is applied, especially at a portion designated by the circle in FIG.
13
. This pressure may breaks the electrolyte film
110
, and object gas with a high content of hydrogen gas may leak out to the atmosphere. To avoid such breakage, provided is a structure having a safety valve for releasing object gas in case of abnormal pressure. In any case, leakage of object gas into the atmosphere cannot be avoided.
Japanese Patent Laid-open No.11-219716 discloses another carbon monoxide sensor for detecting carbon monoxide in fuel gas containing much hydrogen gas supplied in a fuel cell.
A perspective exploded view of a schematic structure of this carbon monoxide sensor is shown in FIG.
14
. An electrolyte film
50
is an electron exchange film composed of a high polymer having proton conductivity, for example, NAFION (a trademark of Du Pont). On opposite sides of the film, an anode
42
and cathode
44
containing catalyst particles are disposed. Conductive diffusion portions
43
and
45
made of carbon paper contact the anode
42
and cathode
44
, respectively. The conductive diffusion portion
43
contacts a housing
54
having an object gas inlet
59
, an anode flow channel
46
in which an object gas flows, and an object gas outlet
51
. The cathode
44
is exposed to ambient air through an opening
52
of the housing
54
. A metal current collector plate
49
in which a plurality of holes are formed contacts the conductive diffusion portion
45
and transfers current to a terminal
47
. The terminal
47
projects outwardly from the housing
54
through a slot
55
.
An operation of the carbon monoxide sensor will be explained. An object gas (fuel gas) having much hydrogen gas reaches the anode flow channel
46
through the object gas inlet
59
. From here, the gas passes through the conductive diffusion portion
43
and is exhausted from the outlet
51
. The cathode
44
always contacts oxygen gas in an atmosphere. Therefore, on a surface of the electrolyte film
50
contacting the anode
42
and cathode
44
, hydrogen gas and oxygen gas chemically react, similarly to how they react in the fuel cell, so as to generate electricity by using hydrogen gas and oxygen gas, and thereby generates an electromotive force between the anode
42
and cathode
44
. Current and voltage at this time are detected by a current detecting device and a voltage detecting device (not shown) connected between the terminal
47
and housing
54
.
In this situation, if carbon monoxide is mixed in the object gas, the carbon monoxide is adsorbed into catalyst particles in the anode
42
, and hence poisons the anode
42
. As a result, hydrogen gas and oxygen gas are prevented from reacting, and the electromotive force between the anode
42
and cathode
44
is lowered. Since concentration of carbon monoxide varies depending on a degree of poisoning, by measuring a current change or voltage change due to drop of an electromotive force, a concentration of carbon monoxide in the object gas can be detected.
A resultant measurement of an output characteristic of the carbon monoxide sensor is indicated with a broken line in FIG.
10
. It takes several minutes from introducing object gasp containing 50 ppm of carbon monoxide, into the carbon monoxide sensor for sensor output or electromotive force to change. During this period, voltage or current from the carbon monoxide sensor does not change, and therefore volta

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