JP2010050037A - Direct fuel cell system, and method for controlling the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct fuel cell system capable deterring unintended hydrogen generation, and to provide a method for controlling the direct fuel cell system. <P>SOLUTION: The direct fuel cell system is controlled so that fuel does not practically come in contact with an anode when starting the supply of an oxidant to a cathode at start of operation or when stopping the supply of the oxidant to the cathode at stop of operation. <P>COPYRIGHT: (C)2010,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, and a control method thereof.

  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. An object, an object 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 doesn't matter.
This indicates that in a direct fuel cell system, hydrogen may be unintentionally discharged from the direct fuel cell system under specific conditions. Since hydrogen is a flammable gas and produces an explosive mixture with air in a very wide mixing ratio, the generation of hydrogen in a direct fuel cell system can cause an explosion accident, etc. There is a need for means to avoid 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, the anode in ^6H + + 6e ^- → reaction 3H ₂ occurs, the hydrogen from the anode side is generated, it is also known that the open circuit voltage in the case is about 400～500MV (non-Patent Document 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 sufficiently and consistently to the cathode. " Non-Patent Document 2 concluded that "DMFC with large MEA area needs to be careful about hydrogen accumulation caused by system shutdown and startup", so hydrogen generation from the anode side Although it can be said that the technical idea that it is better to suppress hydrogen is shown, there is no description about specific means for suppressing hydrogen generation.
Electrochemical and Solid-State Letters, 8 (1) A52-A54 (2005) Electrochemical and Solid-State Letters, 8 (4) A211-A214 (2005)

  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 capable of suppressing unintended hydrogen generation and a control method thereof. This is the issue.

The present invention employs the following means in order to solve the above problems.
According to the first aspect of the present invention, when the supply of the oxidant to the cathode is started at start-up, or when the supply of the oxidant to the cathode is stopped at the time of stoppage, the fuel is not substantially in contact with the anode. The direct fuel cell system is characterized in that it is set to be controlled as described above.
According to a second aspect of the present invention, in the direct fuel cell system according to the first aspect, when the supply of the oxidant to the cathode is started at start-up, or the supply of the oxidant to the cathode is stopped at the time of stoppage. The fuel is in contact with the anode so that the concentration of the fuel is controlled to be 0.005 mol / l or less.
According to a third aspect of the present invention, there is provided a control method for a direct fuel cell system, wherein supply of an oxidant to a cathode is started at the start of the direct fuel cell system, or the direct fuel cell system When stopping the supply of the oxidant to the cathode at the time of stopping, control is performed so that the fuel is not substantially in contact with the anode.

  According to the first, second, or third invention of the present invention, hydrogen generation from a direct fuel cell system can be reduced or prevented.

  In the direct fuel cell system of the present invention, it is preferable to use a direct fuel cell having a solid polymer electrolyte membrane as an electrolyte. 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 supplied to the anode of the direct type fuel cell may be any liquid or gas fuel that permeates an electrolyte membrane such as a solid polymer electrolyte membrane having proton conductivity and is oxidized electrochemically to generate protons. Liquid fuels containing alcohols such as methanol, ethanol, 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, among which an aqueous solution containing alcohol, particularly an aqueous solution containing 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 a gas containing oxygen (for example, air) as an oxidizing agent, the reaction (A) on the anode side and the reaction (B) on the cathode side are usually performed as follows. Waking up and generating electricity.

(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 a direct fuel cell system, since power is normally generated at a sufficient air flow rate, oxygen is sufficiently supplied to the entire cathode, so that hydrogen is not generated. However, when the direct fuel cell operation is stopped, the air flow rate gradually decreases, or in a state where sufficient oxygen has not yet reached the entire cathode immediately after the operation is started, etc. Local oxygen deficiency occurs. In such a state, if fuel (methanol) remains in the anode, the reaction (D) occurs on the cathode side and the reaction (C) occurs on the anode side in the region where oxygen shortage occurs at the cathode. At the same time, in the region where oxygen shortage does not occur at the cathode, the reaction (B) occurs on the cathode side and the reaction (A) occurs on the anode side. At this time, even when the load is not connected between the cathode and the anode and there is no electrical conduction, the reaction between (B) and (D) at the cathode and ( Since electrons are transferred between the reactions A) and (C), the reactions (A), (B), (C), and (D) proceed. As a result, there arises a problem that hydrogen is generated.

Therefore, in the present invention, control is performed so that the fuel is not substantially in contact with the anode when the supply of the oxidant to the cathode is started at the start or when the supply of the oxidant to the cathode is stopped at the time of stop. To do. By doing so, even if oxygen deficiency is partially generated at the cathode at the time of starting and stopping, since the fuel is not substantially in contact with the anode at that time, the above reaction (A) is suppressed or Is prevented. As a result, the above reactions (A), (B), (C) and (D) are suppressed or prevented as a whole, so that generation of hydrogen is suppressed or prevented.
In the present invention, when the supply of the oxidant to the cathode is started at the time of start-up, or when the supply of the oxidant to the cathode is stopped at the time of stop, the fuel is substantially not in contact with the anode. A specific approach is to flow a substantially fuel-free fluid to the anode before starting the supply of oxidant to the cathode at start-up or before stopping the supply of oxidant to the cathode at stop. Thus, it is preferable to remove the fuel in contact with the anode.

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 fuel as an example.
(Comparative Examples 1-4)
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 are joined to both surfaces of the electrolyte membrane (2) made of Nafion 117 (registered trademark) by hot pressing. Ten membrane electrode assemblies 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 from the power terminals (64) and (65) to the load. 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 (66) 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 (97) 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 (97). If the methanol concentration in the fuel is estimated to be 2 weight percent or less, the controller (60) sends a signal (96) 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 (97) is sent from the liquid level sensor to the control device (60). ) Is sent. Upon receiving the signal (97), the control device (60) sends a signal (94) 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. When the control device (60) estimates that the methanol concentration in the fuel exceeds 4 weight percent, the control device (60) sends a signal (94) 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 8 liters of air per minute and 150 milliliters of fuel per minute to the fuel cell stack (11), 60 W of power was generated. After performing this power generation for 1 hour, the load was removed, the supply of fuel and oxidant was stopped, and the fuel cell system was stopped. 30 minutes after the stop, the fuel cell system was started by the method of Comparative Examples 1 to 4 described in Table 1. The air supply during startup was 8 liters per minute and the fuel supply was 150 milliliters per minute.

During the start-up, the gas discharged from the vent (67) attached to the fuel tank (56) was collected and analyzed for components by gas chromatography. Hydrogen was detected in all of Comparative Examples 1 to 4. From this result, it is understood that hydrogen is generated from the anode when the supply of the oxidant to the cathode is started with fuel in the anode.

Example 1 according to the present invention was carried out in the same manner as Comparative Examples 1 to 4 except that the starting method of Example 1 described in Table 1 was used instead of the starting method described in Table 1 in Comparative Examples 1 to 4. In this way, the fuel cell system was started. The air supply during startup was 8 liters per minute and the fuel supply was 150 milliliters per minute. Hydrogen was measured by the same method as in Comparative Examples 1 to 4, but no hydrogen was detected. From the above results, it is understood that the generation of hydrogen from the anode is suppressed or prevented when the supply of the oxidant to the cathode is started in the absence of fuel at the anode.
(Comparative Examples 5 and 6)
The following experiment was performed using the fuel cell system used in Comparative Examples 1 to 4. When 8 liters of air per minute and 150 milliliters of fuel per minute were supplied to the fuel cell stack (11), 60 W of power was generated. After this power generation was performed for 1 hour, when the load was removed and stopped, the fuel cell system was stopped by the methods of Comparative Examples 5 and 6 shown in Table 2.

When the gas was discharged, the gas discharged from the vent (67) attached to the fuel tank (56) was collected and analyzed for components by gas chromatography. Hydrogen was detected in both Comparative Examples 5 and 6. From this result, it is understood that hydrogen is generated from the anode when the supply of the oxidant to the cathode is stopped in a state where there is fuel in the anode.

Example 2 according to the present invention was carried out in the same manner as Comparative Examples 5 and 6, except that the stopping method of Example 2 described in Table 2 was used instead of the stopping method described in Table 2 in Comparative Examples 5 and 6. The fuel cell system was stopped by this method. Hydrogen was measured by the same method as in Comparative Examples 5 and 6, but no hydrogen was detected. From the above results, it is understood that hydrogen generation from the anode is suppressed or prevented when the supply of the oxidant to the cathode is stopped in the absence of fuel at the anode.
(Confirmation test)
Next, check the level of fuel concentration allowed to contact the anode when starting the supply of oxidant to the cathode during start-up or when stopping the supply of oxidant to the cathode during stoppage. A test was conducted. In this confirmation test, the same fuel cell system as in Example 1 was used except that the number of laminated membrane electrode assemblies was one. First, the temperature of the fuel cell stack (11) was raised to 50 ° C. Then, the following test was done in each methanol aqueous solution of each fuel concentration shown in Table 3.

An aqueous methanol solution was supplied to the anode side at 15 ml / min for 5 minutes. Thereafter, air was supplied to the cathode side at a rate of 800 milliliters per minute, and the amount of gas generated at that time was measured in the same manner as in Example 1 (during startup). Thereafter, the air was stopped, and the amount of gas generated at that time was measured by the same method as in Example 1 (when stopped). Table 3 shows measured values of the amount of hydrogen generated at the time of starting and stopping. Further, the results of Table 3 are shown in FIGS. FIG. 6 is an enlarged view of FIG. 5 in the horizontal axis direction. From Table 3 and FIGS. 5 and 6, it is understood that if the fuel concentration is 0.005 mol / l or less, the hydrogen generation amount is below the detection limit, and the hydrogen generation amount increases rapidly from 0.01 mol / l. . Therefore, in the present invention, when the supply of the oxidant to the cathode is started at start-up or when the supply of the oxidant to the cathode is stopped at the time of stop, the concentration of the fuel in contact with the anode is 0.005 mol / l or less. It is understood that In general, when generating power with a direct fuel cell, an aqueous solution of about 3 wt% of fuel is used, so that a fuel of 0.005 mol / l or less, which is significantly lower than 3 wt%, contacts the anode. It can be said that the fuel is not substantially in contact with the anode.

  The results of the above confirmation test slightly differ depending on what is used for the fuel. Therefore, the allowable concentration of fuel that can be determined as “substantially no fuel is in contact with the anode” described in the present specification is numerically strict. It is difficult to specify. However, the hydrogen concentration is less than 1 / 30th of the fuel concentration used for normal power generation, and the hydrogen generation amount at startup and stop changes the fuel concentration from 0 to the concentration used for normal power generation. If the fuel concentration can be reduced to 1/10 or less of the maximum value of the generated amount, it can be determined that “the fuel is not substantially in contact with the anode”. The permissible concentration of fuel that can be determined as “substantially no fuel is in contact with the anode” in each fuel can be easily measured by performing a test similar to the above confirmation test while changing the fuel concentration.

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

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 an Example and a comparative example. It is a figure which shows the relationship between the density | concentration of the fuel which an anode contacts, and hydrogen generation amount. It is a figure which shows the relationship between the density | concentration of the fuel which an anode contacts, and hydrogen generation amount.

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 64, 65 Power terminal 66 Exhaust Port 67 Vent 90-97 Signal 98 Operation stop command

Claims (3)

  It was set to control the fuel so that it was not substantially in contact with the anode when starting the supply of oxidant to the cathode at start-up or when stopping the supply of oxidant to the cathode at stop. A direct fuel cell system characterized by the above.   Control is performed so that the concentration of the fuel in contact with the anode is 0.005 mol / l or less when the supply of the oxidant to the cathode is started at the start or when the supply of the oxidant to the cathode is stopped at the time of stop. The direct fuel cell system according to claim 1, wherein the direct fuel cell system is set to A control method for a direct fuel cell system,
When starting supply of oxidant to the cathode at the start of the direct fuel cell system, or when stopping supply of oxidant to the cathode when the direct fuel cell system is stopped,
A control method for a direct fuel cell system, wherein control is performed so that fuel does not substantially contact the anode.