Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-06-29
2002-08-20
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000
Reexamination Certificate
active
06436566
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fuel cell including a hydrogen electrode and an oxygen electrode disposed across an electrolyte layer, which hydrogen ion permeates, as well as a polymer electrolyte membrane that forms an electrolyte layer of a polymer electrolyte fuel cell.
2. Description of the Related Art
Fuel cells generally have a hydrogen electrode and an oxygen electrode disposed across an electrolyte layer, which hydrogen ion permeates. In the fuel cells, reactions expressed by Equations (1) and (2) given below proceed respectively on an anode (hydrogen electrode) and a cathode (oxygen electrode).
Anode (Hydrogen Electrode)
H
2
→2H
+
+2
e
(1)
Cathode (Oxygen Electrode)
(½)O
2
+2H
+
+2
e→
H
2
O (2)
The hydrogen ion produced on the hydrogen electrode is hydrated to form hydroxonium ion (xH
2
O)H
+
and shifts to the oxygen electrode through the electrolyte layer.
A diversity of fuel cells with various electrolyte layers have been proposed: phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, and alkali fuel cells. Much attention has been drawn to polymer electrolyte fuel cells using a hydrogen ion-conductive polymer membrane as the electrolyte layer, because of the potential for high output density and size reduction. Various techniques have been studied to improve the properties of such fuel cells.
The fuel cells with any electrolyte layers generate electricity, based on the above principle. The theoretical electromotive force, that is, the theoretical potential difference between the hydrogen electrode and the oxygen electrode, is approximately 1.23 V. In the actual conditions, the output voltage is lowered to approximately 0.95 to 1 V, due to a variety of losses. One of the main factors to decrease the output voltage is the internal resistance, that is, the resistance caused by the low mobility of hydrogen ions in the electrolyte layer.
A diversity of techniques have been proposed to reduce the internal resistance in the polymer electrolyte fuel cells; for example, the techniques disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 6-231781, No. 8-171920, and No. 7-135004. The techniques disclosed in the former two applications vary the water content of the polymer electrolyte membrane formed as the electrolyte layer in such a manner that the water content on the side of the hydrogen electrode is higher than the water content on the side of the oxygen electrode. As mentioned previously, the hydrogen ions are hydrated or combined with water molecules to form the hydroxonium ions, while shifting through the electrolyte layer. With a progress in reaction, water molecules become insufficient on the side of the hydrogen electrode that supplies the hydrogen ions, while becoming excess on the side of the oxygen electrode. The proposed techniques give a difference in water content between the two electrodes, so as to cancel the shortage of water molecules and facilitate the smooth shift of the hydrogen ions.
The technique disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 7-135004 increases the concentration of the ion exchange group contained in the electrolyte layer. The hydrogen ions and the hydroxonium ions shift through the electrolyte layer with the aide of the ion exchange groups. The increase in concentration of the ion exchange group accordingly decreases the internal resistance. JAPANESE PATENT LAID-OPEN GAZETTE No. 7-135004 also discloses the technique that makes the concentration of the ion exchange group on the side of the hydrogen electrode higher than that on the side of the oxygen electrode. The higher concentration of the ion exchange group generally improves the water absorption capacity. The higher concentration of the ion exchange group on the side of the hydrogen electrode than that on the side of the oxygen electrode accordingly increases the water content on the side of the hydrogen electrode. This ensures the similar effects to those attained by the techniques disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 6-231781 and No. 8-171920 described above.
These proposed techniques aim to reduce the internal resistance to improve operation efficiency of the fuel cells, but not to enhance the electromotive force of the fuel cells. The reduction of the internal resistance slightly enhances the output voltage. But the improved level still remains at about 1 V against the theoretical, maximum electromotive force of approximately 1.23 V.
In the event that fuel cells are used as the power source of various apparatuses, the fuel cells are expected to output the required voltage according to each apparatus. The low electromotive force per unit cell causes an increase in the number of unit cells connected to output the required voltage. The greater number of unit cells undesirably makes the whole power source system bulky and increases the manufacturing cost. From this point of view, the enhancement of the electromotive force of the fuel cells is very important. The proposed techniques have been mainly directed to the reduction of the internal resistance to improve the operation efficiency, but there has been no fully discussion on the enhancement of the electromotive force.
These problems arise not only in polymer electrolyte fuel cells but in other types of fuel cells.
SUMMARY OF THE INVENTION
An object of the present invention is thus to provide a technique that enhances electromotive force of a fuel cell.
Another object of the invention is to provide an electrolyte membrane that is applied for a polymer electrolyte fuel cell having an enhanced electromotive force.
At least part of the above and the other related objects is attained by a fuel cell including a hydrogen electrode and an oxygen electrode disposed across an electrolyte layer, which hydrogen ion permeates. The electrolyte layer has a first contact area, where the electrolyte layer is in contact with the oxygen electrode, and a second contact area, where the electrolyte layer is in contact with the hydrogen electrode. The hydrogen ion concentration in the first contact area is higher than the hydrogen ion concentration in the second contact area. It is preferable that the difference of the ion concentration is a predetermined value corresponding to a target electromotive force on an occasion of power generation.
The fuel cell of this arrangement has the enhanced electromotive force, due to the difference in hydrogen ion concentration between the side of the hydrogen electrode and the side of the oxygen electrode. The fuel cell of the present invention is preferably used as the unit cell of a power source system. This desirably decreases the required number of unit cells to output the required voltage, thereby reducing the size and the manufacturing cost of the whole power source system.
The following describes the relationship between the variation in hydrogen ion concentration and the electromotive force. The electromotive force of the fuel cell represents the potential difference between the hydrogen electrode and the oxygen electrode. The reactions expressed by Equations (1) and (2) given above proceed on the respective electrodes. The reactions occurring at the respective electrodes are in an equilibrium state in the process of power generation. The potentials at the respective electrodes in the equilibrium state are generally expressed by the Nernst equation. According to the Nernst equation, the equilibrium electrode potential E
H
at the hydrogen electrode and the equilibrium electrode potential E
O
at the oxygen electrode are expressed respectively by Equations (3) and (4) given below.
E
H
=
E
H0
+
(
RT
/
F
)
×
ln
(
aH
)
=
E
H0
-
(
RT
/
F
)
×
pH
(
3
)
E
O
=
E
O0
+
(
RT
/
F
)
×
ln
(
aH
)
=
E
O0
-
(
RT
/
F
)
×
pH
(
4
)
where R denotes the gas constant, T denotes the absolute temperature or Kelvin temperature, F denotes the Faraday constant, aH denotes the activit
Kalafut Stephen
Kenyon & Kenyon
Toyota Jidosha & Kabushiki Kaisha
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