JP3887815B2 - Fuel cell system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fuel cell system. More specifically, the present invention relates to a fuel cell system in which a liquid fuel such as methanol is directly supplied to a negative electrode and an oxidant gas such as air is supplied to a positive electrode.
[0002]
[Prior art]
In recent years, countermeasures to environmental problems and resource problems have been emphasized, and fuel cells have been actively developed as one of the countermeasures. In particular, direct methanol fuel cells that are directly used for power generation without reforming or gasifying the alcohol of the fuel are portable and distributed power sources because they have a simple structure and are easy to reduce in size and weight. It is attracting attention as.
[0003]
A direct methanol fuel cell is formed by stacking a large number of unit cells in which a separator is sandwiched between a negative electrode and a positive electrode and sandwiched by a separator, and fuel supplied to the negative electrode in the separator. As a oxidant gas to be supplied to the positive electrode, a flow path groove through which methanol aqueous solution flows and a flow channel through which air flows.
[0004]
In the direct methanol fuel cell, when an aqueous methanol solution is supplied to the negative electrode and air is supplied to the positive electrode, the negative electrode generates carbon dioxide and releases hydrogen ions and electrons by an electrochemical reaction in which methanol and water react. Then, water is generated by an electrochemical reaction that takes in the hydrogen ions and electrons that have passed through the electrolyte, and electric energy is obtained in an external circuit.
[0005]
In addition, the flow path groove of the separator not only serves to supply a methanol aqueous solution to the negative electrode and supply air to the positive electrode, but also a part of the methanol aqueous solution not used for the reaction with the carbon dioxide gas generated in the negative electrode and It also plays a role of discharging the water produced at the positive electrode and a part of the air that was not used for the reaction to the outside.
[0006]
However, the electrolyte used in the direct methanol fuel cell described above is a proton conductive solid polymer membrane and plays a role as an electrolyte. The methanol that did not pass through the electrolyte reaches the positive electrode, which reacts with oxygen at the positive electrode to produce carbon dioxide and water, which can cause a reduction in fuel utilization, or that methanol exists on the catalyst of the positive electrode This causes a decrease in the positive electrode potential.
[0007]
On the other hand, in the direct methanol fuel cell described above, from the viewpoint of output characteristics, it is preferable to increase the fuel concentration. However, if the fuel concentration is increased, methanol permeation (crossover) increases. It is necessary to determine the output characteristics after taking into account the decrease in efficiency due to the increase in the amount of permeation, in other words, the constraint that the output characteristics and efficiency greatly depend on the operating conditions such as the operating temperature and the supply amount of fuel and oxidant gas was there.
[0008]
Conventionally, there has been known a structure in which an optimal aqueous methanol solution is supplied to the fuel electrode side to reduce such restrictions. For example, Japanese Patent Publication No. 11-510311 (International Publication No. WO97 / 21256) discloses an unused fuel that is separated from unused fuel that has permeated from the anode to the cathode and carbon dioxide gas produced at the anode. A structure has been proposed in which methanol or water is added from a pure methanol tank or water tank by a liquid feed pump while mixing with water generated at the cathode and the concentration sensor so that the concentration becomes the optimum value. . Japanese Patent Laid-Open No. 2000-21426 discloses that water and carbon dioxide generated by an electrochemical reaction are supplied to a mixer and reacted with water stored in the mixer in advance to generate carbonic acid. A configuration is described in which a reaction product is effectively used and a decrease in fuel utilization efficiency is suppressed by mixing methanol as fuel. Japanese Laid-Open Patent Publication No. 9-161860 discloses a configuration in which water produced by an electrochemical reaction, carbon dioxide gas, and carbon dioxide removed from unused methanol are circulated by a pump while controlling to an optimum concentration. ing.
[0009]
[Problems to be solved by the invention]
What is described in each of the above publications is preferable in terms of effectively using unused methanol, but using a pump for the circulation or mixing water and carbon dioxide generated by an electrochemical reaction. Since it requires a mechanism such as providing a mixer for stirring, the structure is complicated, and the features of the direct methanol fuel cell are the simple structure, and it is suitable for miniaturization and weight reduction. There was a problem that could not be demonstrated.
[0010]
[Means for Solving the Problems]
In order to solve the above problems, the invention described in claim 1 is a fuel cell system including a cell stack in which a large number of single battery cells are stacked and a storage tank, and the single battery cell has a high proton conductive solid content. A structure in which a negative electrode and a positive electrode are opposed to each other via an electrolyte composed of a molecular film, and a flow channel for supplying liquid fuel to the negative electrode and supplying an oxidant gas to the positive electrode The separator has a reaction product produced by an electrochemical reaction in the cell stack, an unreacted liquid fuel and an oxidant gas, and an unused liquid used for the electrochemical reaction. Fuel is configured to be introduced, and the reaction product generated by the electrochemical reaction is liquidated from the storage tank to the cell stack by a pressure at which the reaction product is introduced into the storage tank. Fuel is characterized in that it has to be supplied.
[0011]
That is, according to the invention described in claim 1, unreacted fuel and carbon dioxide gas are introduced into the storage tank from the negative electrode by the pressure at the time of discharge, and unreacted oxidant gas and water are introduced from the positive electrode. Introduced by the pressure at the time of discharge, these are mixed with unused liquid fuel in the storage tank, and the liquid fuel is supplied from the storage tank to the cell stack by this pressure, so the liquid fuel is not provided separately. Can be supplied to the cell stack.
[0012]
Further, the invention according to claim 2 is the fuel cell system according to claim 1, wherein the reaction product generated by the electrochemical reaction in the cell stack, the unreacted liquid fuel, and the oxidant gas are supplied from the bottom to the inside of the storage tank. And is configured to be mixed with unused liquid fuel.
[0013]
That is, according to the invention described in claim 2, unreacted fuel and carbon dioxide gas are introduced into the storage tank from the bottom by the pressure at the time of discharge, and unreacted oxidant gas and water are discharged from the positive electrode. Are introduced into the storage tank from the bottom by the pressure at the time of discharge, and these and the unused liquid fuel can be uniformly mixed in the storage tank.
[0014]
According to a third aspect of the present invention, in the fuel cell system according to the first or second aspect, the storage tank detects the concentration of the stored liquid fuel and controls the concentration to be a predetermined value. A concentration sensor and a pressure sensor for detecting the pressure in the tank and controlling the pressure in the tank to be a predetermined value are provided, and the amount of unused liquid fuel introduced is controlled by the concentration sensor, The pressure in the storage tank is controlled by the pressure sensor.
[0015]
That is, according to the invention described in claim 3, the amount of unused liquid fuel introduced can be appropriately controlled by the concentration sensor, and the pressure in the storage tank can be appropriately controlled by the pressure sensor.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described based on the embodiments.
[0017]
FIG. 1 is a schematic diagram of a fuel cell system according to an embodiment of the present invention.
[0018]
The fuel cell system shown in FIG. 1 includes a cell stack 10 in which a large number of unit cells are stacked, and a storage tank 11 for storing an aqueous methanol solution as a fuel to be supplied to the cell stack 10. The fuel and carbon dioxide gas are introduced by the pressure at the time of discharge, and unreacted oxidant gas and water are introduced from the positive electrode by the pressure at the time of discharge, and these are mixed with the unused liquid fuel in the storage tank. In addition, the liquid fuel is supplied from the storage tank to the cell stack by this pressure.
[0019]
The unit cell includes a structure in which a negative electrode and a positive electrode are opposed to each other via an electrolyte composed of a proton conductive solid polymer membrane, and a separator that sandwiches the structure. It has a channel groove for supplying an aqueous methanol solution as a liquid fuel, and a channel groove for supplying air as an oxidant gas to the positive electrode.
[0020]
A path through which reaction products (water and carbon dioxide), unreacted liquid fuel, and oxidant gas generated by an electrochemical reaction in the cell stack 10 flows into the storage tank 11 from the cell stack 10 to the radiator 17. A path through which unused liquid fuel used for the electrochemical reaction flows is provided from the liquid fuel tank 13 through the liquid feed pump 14. And the path | route supplied in order to use the liquid fuel which flowed in through each path | route mentioned above for the electrochemical reaction in the said cell stack 10 is provided with respect to the said cell stack 10 from the said storage tank 11. .
[0021]
In the above-described embodiment, the reaction product, unreacted liquid fuel, and oxidant gas are routed from the cell stack 10 through the radiator 17 to the inside of the storage tank 11 from the bottom to the inside of the storage tank 11. If these are introduced, they can be uniformly mixed with unused liquid fuel in the storage tank 11, and there is no need to provide a separate mixer.
[0022]
In the above-described embodiment, the storage tank 11 is provided with the concentration sensor 12 and the pressure sensor 15 so that the concentration of the liquid fuel in the storage tank 11 becomes a predetermined value by the concentration sensor 12. If the operation of the relief valve 16 provided in the storage tank 11 is controlled by the pressure sensor 15 so that the pressure in the storage tank 11 becomes a predetermined value by the pressure sensor 15, the amount of unused liquid fuel introduced The pressure in the storage tank can be appropriately controlled according to the operating conditions of the fuel cell system without providing a separate pump.
[0023]
That is, the reaction product (water and carbon dioxide gas), unreacted liquid fuel, and oxidant gas flowing into the storage tank 11 from the cell stack 10 enter the radiator 17 in a mixed state and are cooled here. Although stored in the storage tank 11, the concentration sensor 12 provided in the storage tank 11 detects the concentration of the stored liquid fuel and controls the operation of the liquid feed pump 14 so that the concentration becomes a predetermined value. At the same time, the pressure sensor 15 detects the pressure in the tank, and the operation of the relief valve 16 provided in the storage tank 11 is controlled so that the pressure becomes a predetermined value. As a result, the liquid fuel stored in the storage tank 11 is maintained at a predetermined concentration by appropriately introducing unused liquid fuel from the fuel tank 13 through the liquid feed pump 14, and the relief valve 16 operates appropriately. By doing so, a predetermined pressure can be maintained, so that an unused liquid fuel is supplied to the cell stack 10 at a constant pressure according to the operating conditions of the fuel cell system without providing a separate means such as a pump. be able to.
[0024]
According to the fuel cell system according to the above-described embodiment, when the operation is performed under a pressure higher than the atmospheric pressure, the characteristics of the fuel cell whose output characteristics are improved are adjusted by adjusting the pressure sensor 15 of the relief valve 16. Since the pressure condition of the entire system can be controlled by adjusting the operating point, the fuel cell can be easily operated under the optimum pressurizing condition.
[0025]
Further, air, water, carbon dioxide gas, unreacted liquid fuel, and oxidant gas generated in the cell stack 10 are introduced into the storage tank 11, and the liquid introduced from the liquid fuel tank 13 to the storage tank 11 by the concentration sensor 12. Since the amount of fuel introduced can be controlled, it is possible to easily realize the operation of the fuel cell under the optimum liquid fuel concentration.
[0026]
When the fuel cell system having such a configuration supplies a methanol aqueous solution as a liquid fuel from the storage tank 11 to the negative electrode of the cell stack 10 and supplies air as an oxidant gas from the blower 18 to the positive electrode of the cell stack 10, the negative electrode Then, carbon dioxide gas is generated by the electrochemical reaction and hydrogen ions and electrons are released. In the positive electrode, water is generated by the electrochemical reaction that takes in the hydrogen ions and electrons that have passed through the electrolyte, and electric energy is obtained in the external circuit. Can do.
[0027]
【The invention's effect】
As described above, in the fuel cell system of the present invention, the advantages of the direct methanol type fuel cell, which are the simple structure, the small size and the light weight, can be exhibited to the maximum. This can contribute to the configuration of a fuel cell system that can be miniaturized.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a fuel cell system according to an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Cell stack 11 ... Storage tank 12 ... Concentration sensor 13 ... Liquid fuel tank 14 ... Liquid feed pump 15 ... Pressure sensor 16 ... Relief valve 17 ... Radiator 18 ... Blower

Claims (3)

A fuel cell system comprising a cell stack in which a large number of single battery cells are stacked and a storage tank, wherein the single battery cell has a negative electrode and a positive electrode facing each other through an electrolyte composed of a proton conductive solid polymer membrane And a separator having a flow channel for supplying liquid fuel to the negative electrode and supplying an oxidant gas to the positive electrode. A reaction product generated by an electrochemical reaction in a cell stack, an unreacted liquid fuel and an oxidant gas, and an unused liquid fuel used for the electrochemical reaction are introduced. A fuel characterized in that liquid fuel is supplied from the storage tank to the cell stack by a pressure at which a reaction product generated by an electrochemical reaction is introduced into the storage tank. Battery system. 2. The fuel cell system according to claim 1, wherein the reaction product generated by the electrochemical reaction in the cell stack, the unreacted liquid fuel and the oxidant gas are introduced into the storage tank from the bottom, and the unused liquid fuel and A fuel cell system characterized by being mixed. 3. The fuel cell system according to claim 1 or 2, wherein the storage tank detects a concentration of the stored liquid fuel, and detects a concentration sensor for controlling the concentration to a predetermined value and a pressure in the tank. And a pressure sensor for controlling the pressure in the tank to be a predetermined value, the amount of unused liquid fuel introduced is controlled by the concentration sensor, and the pressure in the storage tank is controlled by the pressure sensor. A fuel cell system characterized by being controlled.

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