JP2008210661A - Direct type fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct type fuel cell system in which hydrogen gas is not exhausted to the outside of the fuel cell system. <P>SOLUTION: The direct type fuel cell system includes: a direct type fuel cell 11 using a solid polymer electrolyte membrane as an electrolyte; a fuel supply means 52 supplying an organic matter or an organic matter solution to an anode of the direct type fuel cell 11; and an oxidant supply means 51 supplying an oxidant to a cathode of the direct type fuel cell 11. Gas exhausted from the anode is supplied to the cathode, hydrogen gas included in the gas is oxidized in the cathode and removed, and then the gas is exhausted from an exhaust port 65 to the outside of the direct type fuel cell system. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a direct fuel cell system capable of generating electricity by directly supplying an organic substance or an organic solution to an anode.

  In recent years, countermeasures against environmental problems and resource problems have become important, and fuel cells have been actively developed as one of the countermeasures. In particular, direct fuel cells that can be used directly for power generation without reforming or gasification using organic substances such as alcohol as fuel, have a simple structure and are easily reduced in size and weight. It is promising as a consumer power source for portable power sources, computers, portable electronic devices, mobile phones and the like.

  These direct fuel cells are configured by laminating a number of unit cells each having a membrane electrode assembly (MEA) in which a cathode electrode and an anode electrode are bonded to both sides of an electrolyte membrane and sandwiched between the cathode electrode separator and the anode electrode separator. A device in which a plurality of MEAs are arranged on a plane and each MEA is electrically connected has been developed. In particular, the latter form in which a plurality of MEAs are arranged on a plane is promising as a power source for portable electronic devices such as laptop computers and mobile phones.

  In the direct type fuel cell, when an organic solution is supplied to the anode side, carbon dioxide gas is generated by the cell reaction, and waste fuel and carbon dioxide gas are discharged on the fuel exhaust side. On the other hand, when air is supplied as an oxidant on the cathode side, water is generated by the cell reaction and is discharged from the air outlet.

  Among these direct fuel cells, active type using a power unit such as a pump for supplying air to the cathode and fuel to the anode, and passive type direct methanol type fuel cell not using the power unit are considered. It has been.

  Such a passive type fuel cell has a disadvantage that it is difficult to obtain an output as compared with an active type fuel cell. On the other hand, no power is required for driving the pump, and the power generation efficiency of the cell can be increased. There is an advantage. In addition, there is a feature that a simple and compact system can be configured, and that it becomes a quiet generator without pump drive noise. Therefore, a simpler system configuration is possible, and there is a possibility that the fuel cell is optimal for small consumer applications such as a power source for mobile phones and a power source for laptop computers.

In the process of research and development of a direct fuel cell, we have discovered a phenomenon in which hydrogen is generated from the anode of the direct fuel cell under specific conditions, as shown in Patent Document 1. The hydrogen generation conditions are determined by the amount of oxidant supplied to the cathode and the cell voltage, and whether hydrogen is generated when the open circuit voltage of the unit cell is in the range of 300 to 800 mV, and whether or not power is supplied from the fuel cell to the load. I know it does n’t matter.
Japanese Patent No. 3812581

  This indicates that in a direct fuel cell system, hydrogen may be unintentionally discharged from the direct fuel cell system under certain conditions. Since hydrogen is a flammable gas and produces an explosive mixture with air at a very wide mixing ratio, the generation of hydrogen in a direct fuel cell system may cause an explosion accident. There is a need for a means to avoid this danger.

In addition, in the case of a direct methanol fuel cell (DMFC), when an open circuit is in an oxygen-deficient state, the battery reaction and the electrolytic reaction coexist in a single cell, and CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e at the cathode. ^- the reaction, an anode in ^6H + + 6e ^- the → 3H ₂ reaction occurs, hydrogen is also known to occur from the anode side (see non-Patent documents 1 and 2).
The paper of Non-Patent Document 1 states that “the generation of hydrogen not only reduces the power output in the operating cell, but also consumes fuel continuously in the open circuit, so the DMFC is in operation and on standby. At any time, it is important to keep supplying oxygen to the cathode sufficiently and consistently, ”the paper of Non-Patent Document 2 also states that“ DMFCs with large MEA areas are subject to system shutdown and startup. It is necessary to pay attention to the hydrogen accumulation that is caused ", so it can be said that the technical idea that the amount of hydrogen generated from the anode side should be reduced or removed is shown. However, none of the papers describes a means for removing hydrogen.
Electrochemical and Solid-State Letters, 8 (1) A52-A54 (2005) Electrochemical and Solid-State Letters, 8 (4) A211-A214 (2005)

Furthermore, Patent Document 2 describes that in a direct methanol fuel cell, gas discharged from the anode is sent to the cathode, and vaporized methanol contained in the gas is oxidized at the cathode. There is no suggestion of oxidizing the gas at the cathode.
JP 2006-32209 A

In a fuel cell using hydrogen gas as a fuel, the hydrogen off-gas discharged from the anode is mixed with the cathode off-gas discharged from the cathode, and brought into contact with the oxidation catalyst, thereby reducing the hydrogen concentration in the gas to the atmosphere. However, it is necessary to separately provide a device for oxidizing the hydrogen off-gas.
JP 2002-289237 A

  The present invention is intended to solve the problem of hydrogen generation in the direct fuel cell system as described above, and provides a direct fuel cell system in which hydrogen gas is not discharged outside the fuel cell system. This is the issue.

The present invention employs the following means in order to solve the above problems.
(1) A direct fuel cell using a solid polymer electrolyte membrane as an electrolyte, a fuel supply means for supplying an organic substance or an organic solution to the anode of the direct fuel cell, and an oxidant for the cathode of the direct fuel cell And a direct fuel cell system comprising: an oxidant supply means configured to supply a gas discharged from the anode to the cathode, and oxidize the hydrogen gas contained in the gas at the cathode to remove the hydrogen gas. The direct fuel cell system is characterized in that it is discharged outside the direct fuel cell system.
(2) The direct fuel cell system according to (1), wherein a hydrogen permeable membrane is provided in a pipe for sending the gas discharged from the anode to the cathode.
(3) A control device for detecting a condition for generating hydrogen gas from the anode is provided, and when the control device detects a hydrogen gas generation condition, gas discharged from the anode is sent to the cathode. The direct fuel cell system according to (1) or (2).
(4) A three-way cock is provided in a pipe for discharging the gas discharged from the anode to the outside of the direct fuel cell system, and when the control device detects a hydrogen gas generation condition, the three-way cock is switched to switch the anode The direct fuel cell system according to (3), wherein the gas discharged from the fuel cell is sent to the cathode.
(5) The direct fuel cell system according to any one of (1) to (4), wherein the gas discharged from the anode is discharged out of the direct fuel cell system through a fuel tank. .

According to the present invention, the hydrogen gas contained in the gas discharged from the anode of the direct fuel cell system is oxidized at the cathode and converted to water, and then the anode exhaust gas is released outside the fuel cell system. Without providing a separate device to oxidize the hydrogen gas contained in the exhaust gas, the hydrogen gas in the anode exhaust gas can be oxidized and changed to water. Accidents can be prevented.
In addition, when a hydrogen permeable membrane is provided in the pipe that is fed to the cathode, only hydrogen gas can be efficiently removed.
Furthermore, when a control device that detects hydrogen gas generation conditions is used, the gas discharged from the anode can be sent to the cathode only when hydrogen gas is included. It can be used efficiently, and the life of the cathode oxidation catalyst can be extended.

  In the direct fuel cell system of the present invention, a direct fuel cell using a solid polymer electrolyte membrane as an electrolyte is used. The solid polymer electrolyte membrane only needs to be permeable to organic matter or organic solution, and usually has proton conductivity. The solid polymer electrolyte membrane having proton conductivity is preferably a perfluorocarbon sulfonic acid membrane having a sulfonic acid group such as a Nafion membrane manufactured by DuPont.

  The organic substance to be supplied to the anode of the direct type fuel cell may be any liquid or gaseous fuel that passes through a solid polymer electrolyte membrane having proton conductivity and is electrochemically oxidized to generate protons. Liquid fuels containing alcohols such as ethylene glycol and 2-propanol, aldehydes such as formaldehyde, carboxylic acids such as formic acid, and ethers such as diethyl ether are preferred. As the organic solution, a solution containing these liquid fuel and water, and among them, an aqueous solution containing alcohol, particularly methanol, is preferable.

  As the oxidant supplied to the cathode of the direct fuel cell, a gas containing oxygen or oxygen is preferable, and air can be used.

In a direct fuel cell using methanol as a fuel and an oxygen-containing gas as an oxidizer, the reaction (A) on the anode side and the reaction (B) on the cathode side usually occur as described below to generate power. Has been done.
(A) CH ₃ OH + H ₂ O → 6H ⁺ + 6e ⁻ + CO ₂
(B) 6H ⁺ + 6e ⁻ + 3 / 2O ₂ → 3H ₂ O

On the other hand, when a proton-conducting solid electrolyte membrane such as Nafion is used, a crossover phenomenon in which CH ₃ OH permeates from the anode to the cathode is known, and when the air flow rate decreases, the cathode oxygen supply is insufficient. In the region, the reaction (B) does not occur, the crossover methanol is electrolytically oxidized, and the reaction (D) occurs. On the other hand, the reaction (C) occurs on the anode side to generate hydrogen.
(C) 6H ⁺ + 6e ⁻ → 3H ₂
(D) CH ₃ OH + H ₂ O → 6H ⁺ + 6e ⁻ + CO ₂

  In the present invention, if the hydrogen gas generated by the reaction (C) on the anode side is directly discharged out of the direct fuel cell system, there is a danger as described above. Therefore, the gas discharged from the anode is sent to the cathode. The hydrogen gas contained in the gas is oxidized at the cathode to remove the hydrogen gas. In this case, the cathode catalyst serves as an oxidation catalyst to oxidize hydrogen gas.

  In order to efficiently remove only hydrogen gas, it is preferable to provide a hydrogen permeable membrane in a pipe for sending the gas discharged from the anode to the cathode. Although the hydrogen permeable membrane is not limited, use a hydrogen permeable metal membrane having a thickness of 5 to 50 μm and capable of selectively permeating hydrogen formed on the inorganic porous layer. Can do. The inorganic porous layer is a carrier for holding a hydrogen permeable metal film, and is formed of a porous stainless steel nonwoven fabric, ceramics, glass or the like in a thickness range of 0.1 mm to 1 mm. As the hydrogen permeable metal film, an alloy containing Pd, an alloy containing Ni, or an alloy containing V can be used, but an alloy containing Pd is preferable. Examples of the alloy containing Pd include a Pd / Ag alloy, a Pd / Y alloy, and a Pd / Ag / Au alloy.

Further, in the present invention, in order to efficiently use the cathode as a hydrogen gas removing means and extend the life of the oxidation catalyst of the cathode, a control device for detecting a condition for generating hydrogen gas from the anode is provided. When the hydrogen gas generation conditions are detected, it is preferable to send the gas discharged from the anode to the cathode.
As shown in Patent Document 1, it is known that the generation of hydrogen gas depends on the open circuit voltage, and when the air flow rate decreases and the open circuit voltage becomes 300 to 800 mV, hydrogen gas is generated. A voltage measuring means for measuring the voltage of the unit cell of the direct fuel cell is provided, and when the open circuit voltage of the unit cell measured by the voltage measuring means is 800 mV or less, the gas discharged from the anode is used as the cathode. Means for feeding can be employed. For example, a three-way cock is provided in a pipe that discharges the gas discharged from the anode to the outside of the direct fuel cell system, and the gas discharged from the anode by switching the three-way cock when the control device detects a hydrogen gas generation condition. May be sent to the cathode.

  Hereinafter, the present invention will be described in detail with reference to the drawings, taking a direct methanol fuel cell using a methanol aqueous solution as a fuel as an example.

(Comparative example)
A fuel electrode paste prepared by mixing a Teflon (registered trademark) dispersion and a Nafion (registered trademark) solution with a fuel electrode catalyst in which platinum and ruthenium are supported on activated carbon is applied onto carbon paper to form an anode. 1 is applied to an air electrode catalyst obtained by supporting platinum on activated carbon and a Teflon (registered trademark) dispersion and a Nafion (registered trademark) solution mixed with carbon paper. Got. For both the anode and cathode, the electrode area was 64 cm ² , and the catalyst loading was 1 mg / cm ² .

  Next, as shown in FIG. 1, the anode 1 and the cathode (not shown) produced as described above were joined to both surfaces of the electrolyte membrane (2) made of Nafion 117 (registered trademark) by hot pressing. Ten membrane electrode assemblies thus obtained were laminated via a silicon rubber packing (3) and a graphite bipolar separator (4). Further, as shown in FIG. 2, the laminate (5) held between two metal current collector plates (6) is joined to two metal plates via an insulating plate (7) made of silicon rubber. The end plate (8) was held, and as shown in FIG. 3, these end plates were joined together with bolts (9) and nuts (10) to produce a direct methanol fuel cell stack (11).

  The bipolar separator (4) has a fuel supply groove (12) on one side [not shown. In FIG. 1, it is formed on the back surface of the separator (4). ] And a manifold hole (13) for supplying fuel to the fuel supply groove (12) is formed through the separator (4). The fuel supply manifold holes (13) of each separator (4) communicate with each other in the assembled state of the stack (11), as well as the current collector plate (6), the laminate (5), the insulating plate (7), and the end. It communicates with a fuel supply manifold hole provided in the plate (8). The bipolar separator (4) is formed with a manifold hole (14) for discharging fuel from the fuel supply groove (12) so as to penetrate the separator (4). The fuel discharge manifold holes (14) of each separator (4) communicate with each other in the assembled state of the stack (11), as well as the current collector plate (6), the laminate (5), the insulating plate (7), and the end. It also communicates with a fuel discharge manifold hole provided in the plate (8). Thus, the fuel supplied from the outside of the fuel cell stack (11) to the fuel supply manifold hole of the end plate (8) flows through the fuel supply groove (12) of each bipolar separator (4). It is supplied to the anode (1) of each unit cell. The fuel not consumed at the anode (1) is discharged from the fuel discharge manifold hole of the end plate (8) together with the reaction product generated at the anode (1).

  The bipolar separator (4) is provided with an oxidant supply groove (15) on the surface opposite to the surface on which the fuel supply groove is provided, and an oxidant supply groove ( A manifold hole (16) for supplying an oxidant to 15) is formed through the separator (4). The oxidant supply manifold holes (16) of the separators (4) communicate with each other in the assembled state of the stack (11), and the current collector plate (6), the laminate (5), the insulating plate (7), It communicates with an oxidant supply manifold hole provided in the end plate (8). The bipolar separator (4) is formed with a manifold hole (17) for discharging the oxidant from the groove (15) for supplying the oxidant so as to penetrate the separator (4). The oxidant discharge manifold holes (17) of the separators (4) communicate with each other in the assembled state of the stack (11), and the current collector plate (6), the laminate (5), the insulating plate (7), It communicates with an oxidant discharge manifold hole provided in the end plate (8). As a result, the oxidant supplied from the outside of the fuel cell stack (11) to the oxidant supply manifold hole of the end plate (8) becomes the oxidant supply groove (15) of each bipolar separator (4). Is supplied to the cathode of each unit cell. The oxidant not consumed at the cathode is discharged from the oxidant discharge manifold hole of the end plate (8) together with the reaction product generated at the cathode.

  A fuel cell system shown in FIG. 4 was constructed using the fuel cell stack (11). The blower (51) sends air as an oxidant into the fuel cell stack (11). The pump (52) sends about 3 weight percent methanol aqueous solution as fuel from the fuel tank (56) to the fuel cell stack (11). In the fuel cell stack (11), oxygen in the air and methanol react to generate electric power. The generated electric power is supplied to the load from the power terminals (63) and (64). The water generated by the reaction and unreacted air are discharged from the oxidant discharge manifold of the fuel cell stack (11), cooled by the radiator (53), and then discharged from the exhaust port (65) to the outside of the fuel cell system. Is discharged. The water condensed by the radiator (53) is collected in the water tank (54).

  During operation, methanol in the fuel is consumed in the fuel cell stack (11), so that the concentration of methanol in the fuel in the fuel tank (56) gradually decreases. A signal (92) from a methanol concentration sensor (not shown) and a fuel temperature sensor (not shown) installed in the fuel tank (56) is sent to the control device (60), and the control device (60) The methanol concentration in the fuel is estimated from the signal (92). If the methanol concentration in the fuel is estimated to be 2 percent by weight or less, the controller (60) sends a signal (91) to the valve (62) and opens the valve (62). Thereby, 50 to 60 weight percent methanol aqueous solution stored in the high concentration fuel tank (55) is replenished to the fuel tank (56), and the methanol concentration of the fuel in the fuel tank (56) is adjusted to about 3 weight percent. To do.

  Further, when a liquid level sensor (not shown) installed in the fuel tank (56) detects that the liquid level in the fuel tank (56) has decreased, a signal (92) is sent from the liquid level sensor to the control device (60). ) Is sent. Upon receiving the signal (92), the control device (60) sends a signal (90) to the valve (61) to open the valve (61). Thereby, the water stored in the water tank (54) is replenished to the fuel tank (56), and the amount of fuel liquid in the fuel tank (56) is recovered. Further, when the control device (60) estimates that the methanol concentration in the fuel exceeds 4 weight percent, the control device (60) sends a signal (90) to the valve (61) and turns the valve (61) on. open. Thereby, the water stored in the water tank (54) is replenished to the fuel tank (56), and the methanol concentration of the fuel in the fuel tank (56) is adjusted to about 3 weight percent.

  When this fuel cell system was used to supply 5 liters of air per minute and 1 liter of fuel per minute to the fuel cell stack (11), 70 W of power was generated. When gas discharged from the vent (66) attached to the fuel tank (56) was collected and analyzed by gas chromatography, hydrogen was not detected.

  Subsequently, when the air flow rate was reduced to 500 milliliters per minute in this state, the voltage between the electrodes of the fuel cell stack (11) was about 4 V, and 5 W of power was generated. The gas discharged from the vent (66) attached to the fuel tank (56) was collected and analyzed by gas chromatography, and generation of hydrogen was confirmed. The hydrogen generation rate was about 10 milliliters per minute.

  A function is added to the fuel cell system of the comparative example, and a pipe connecting the blower (51) and the fuel cell stack (11) and an exhaust port (65) of the fuel tank (56) are connected via a check valve (67). Connected. The check valve (67) is for preventing air supplied from the blower (51) to the fuel cell stack (11) from flowing into the fuel tank (56). The connecting portion (68) has a venturi structure, and exhaust gas from the fuel tank (56) is drawn into the flow of air discharged from the blower (51). Therefore, the anode exhaust gas discharged from the anode of the fuel cell stack (11) is sent to the cathode of the fuel cell stack (11) via the fuel tank (56). Hydrogen contained in the anode exhaust gas is oxidized by the cathode catalyst in the cathode of the fuel cell stack (11) to become water, and part of it is recovered by the radiator (53) and sent to the water tank (54). The rest is discharged out of the fuel cell system through the exhaust port (65). The configuration of the fuel cell system of this example is shown in FIG.

  When the above fuel cell system was used to supply 5 liters of air per minute and 1 liter of fuel per minute to the fuel cell stack (11), 70 W of power was generated.

  Subsequently, in this state, when the air flow rate was reduced to 500 milliliters per minute, the voltage between the electrodes of the fuel cell stack (11) was about 3 V, and 4 W of power was generated. When gas discharged from the exhaust port (65) was collected and analyzed by gas chromatography, hydrogen was not detected.

  In Example 1, compared with the comparative example, the cause of the decrease in voltage and power is that the anode exhaust gas is sent to the cathode, so that an oxidation reaction of hydrogen and methanol vapor occurs at the cathode, and It is thought that the oxygen partial pressure at the cathode decreases due to carbon dioxide in the cathode exhaust gas. The latter phenomenon occurs even under conditions in which hydrogen generation does not occur because sufficient oxygen is supplied during normal operation, thereby reducing the efficiency of the fuel cell system. In order to avoid this, as shown in FIG. 6, the exhaust from the fuel tank (56) is switched by the three-way cock (69), and the signal is generated only when the control device (60) detects the hydrogen generation condition. The three-way cock (69) is switched by (93) to guide the exhaust gas from the fuel tank (56) to the cathode of the fuel cell stack (11).

  In addition, said Example is only what was given as an example in order to make an understanding to this invention easy, and does not limit the scope of the present invention at all. In the embodiments, a direct methanol fuel cell system called an active type in which fuel or air is sent to a fuel cell stack by a power device such as a pump has been described as an example. However, a power device such as a pump is used to supply fuel or air. The present invention is also suitably used for a direct methanol fuel cell system of a type called a passive type that does not use the.

  According to the present invention, since the release of hydrogen from the direct methanol fuel cell system to the external environment is suppressed, a highly safe direct methanol fuel cell system can be obtained. In particular, since the safety of a direct methanol fuel cell that is brought into a transportation system such as an airplane or railroad or carried in a pocket of clothes can be improved, the present invention is greatly applied to the practical application of a direct methanol fuel cell. Can contribute.

It is a block diagram of the cell of an Example and a comparative example. It is a block diagram of the fuel cell stack of an Example and a comparative example. It is a perspective view of the fuel cell stack of an Example and a comparative example. It is a block diagram of the fuel cell system of a comparative example. It is a block diagram of the fuel cell system of Example 1. It is a block diagram of the fuel cell system of Example 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Anode 2 Electrolyte membrane 3 Packing 4 Separator 5 Laminate 6 Current collecting plate 7 Insulating plate 8 End plate 9 Bolt 10 Nut 11 Fuel cell stack 12 Groove for fuel supply 13 Manifold hole for fuel supply 14 Manifold hole for fuel discharge 15 Oxidation Groove for supplying agent 16 Manifold hole for supplying oxidant 17 Manifold hole for discharging oxidant 51 Blower 52 Pump 53 Radiator 54 Water tank 55 High concentration fuel tank 56 Fuel tank 60 Controller 61, 62 Valve 63, 64 Power terminal 65 Exhaust Port 66 Vent 67 Check valve 68 Connection part 69 Three-way cock 90-93 Signal

Claims (5)

  A direct fuel cell having a solid polymer electrolyte membrane as an electrolyte, a fuel supply means for supplying an organic substance or an organic solution to the anode of the direct fuel cell, and an oxidant for supplying an oxidant to the cathode of the direct fuel cell A direct fuel cell system comprising: a supply means; a gas discharged from the anode is sent to the cathode, and hydrogen gas contained in the gas is oxidized at the cathode to remove the hydrogen gas; A direct fuel cell system, wherein the direct fuel cell system is discharged outside the fuel cell system.   2. The direct fuel cell system according to claim 1, wherein a hydrogen permeable membrane is provided in a pipe for sending the gas discharged from the anode to the cathode.   A control device for detecting a condition for generating hydrogen gas from the anode is provided, and when the control device detects a hydrogen gas generation condition, gas discharged from the anode is sent to the cathode. Item 3. The direct fuel cell system according to Item 1 or 2.   A three-way cock is provided in a pipe for discharging the gas discharged from the anode to the outside of the direct fuel cell system, and when the control device detects a hydrogen gas generation condition, the three-way cock is switched to be discharged from the anode. The direct fuel cell system according to claim 3, wherein a gas is sent to the cathode.   The direct fuel cell system according to any one of claims 1 to 4, wherein the gas discharged from the anode is discharged out of the direct fuel cell system via a fuel tank.

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