JP2004319437A - Elution prevention method, quality control method, operation method of fuel electrode for direct methanol fuel cell, and direct methanol fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent degradation of characteristics of a direct methanol fuel cell and obtain operating conditions and quality control for stably generating power for a ling period of time. <P>SOLUTION: Elution of a fuel electrode material to fuel is monitored in the direct methanol fuel cell 2. The elution is caused by an elution of perfluorosulfonic acid polymer in a fuel electrode into fuel in the case of high-density fuel of 2 M or more at an operating temperature of 80°C or more, which makes electrode catalyst elute into the fuel and deteriorate the characteristics. The elution is prevented by limiting fuel density to less than 2 M, and the operating temperature to 80°C or less, and elution characteristics are evaluated at manufacturing of the fuel cell to effect a quality control. Further, presence of the elution is detected from a color of the fuel or the like, and if detected, an upper limit of the operating temperature and the fuel density is lowered to prevent further elution. With this, elution of the fuel electrode material is prevented to improve durability of the direct methanol fuel cell 2. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a direct methanol fuel cell capable of directly generating power by supplying methanol and water as fuel and air as oxidizing gas. More specifically, the present invention relates to operating conditions, quality control, and the like for preventing deterioration in characteristics of a direct methanol fuel cell and stably generating power for a long period of time.

  In recent years, countermeasures against environmental problems and resource problems have become important, and as one of the countermeasures, the development of direct methanol fuel cells has been actively conducted. In particular, a direct methanol fuel cell that can be used for direct power generation without performing reforming and gasification using methanol as a fuel has a simple structure, and can be easily reduced in size and weight.

  In a direct methanol fuel cell, when an aqueous methanol solution is supplied to the fuel electrode 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 oxidizing agent on the air electrode side, water is generated by a battery reaction and discharged from the air outlet.

In such a direct methanol fuel cell, a proton conductive polymer solid electrolyte mainly composed of perfluorosulfonic acid represented by Nafion (registered trademark of DuPont), which is used for an electrolyte membrane, is added with methanol as a fuel. It is known that methanol having permeated through the electrolyte membrane increases polarization of the air electrode. For this reason, it is known that methanol in a fuel has an optimum concentration at which characteristics are maximized, and it has been considered that the concentration of a methanol aqueous solution of a fuel is approximately 1 M (mol / dm ³ ). .
However, the above methanol concentration is a discussion based on the characteristic loss due to the permeation of methanol through the electrolyte membrane, and it is considered that a fuel with a higher concentration may be used as long as the characteristics can be slightly lowered. (See Non-Patent Document 1).

  Also, regarding the battery operating temperature, since the direct methanol fuel cell originally has lower output characteristics than the polymer electrolyte fuel cell, it is operated at a high temperature of about 90 ° C. in order to obtain as large an output characteristic as possible. Things have been done. In addition, as described in Non-Patent Document 2, in recent years, operation at a higher temperature of about 130 ° C. has been studied in consideration of application as a fuel cell for an automobile.

Investigations have been made to develop a proton conductive electrolyte membrane other than perfluorosulfonic acid, which has low methanol permeation, and sulfonated aromatic polymers and the like are promising.
Preprints of the Society of Automotive Engineers of Japan No.46-00,20005062 Journal of the Electrochemical Society (J. Electrochem. Soc.) Vol 143, No. 1, (1996), L12

  In order to put the above-mentioned direct methanol fuel cell into practical use, it is necessary that the cell does not deteriorate and can be operated stably for the required life of the fuel cell of several thousands to tens of thousands of hours. In order to prevent such battery characteristics deterioration and improve the battery life, it is essential to clarify the battery deterioration modes and take effective measures for each of the deterioration modes. .

  In contrast to the conventionally considered deterioration modes, in the present invention, perfluorosulfonic acid used in the solid electrolyte membrane of the direct methanol fuel cell, and the fuel electrode catalyst and PTFE fine particles in the fuel electrode are added. A mode was found in which particulate perfluorosulfonic acid was eluted into an aqueous methanol solution as a fuel. When this deterioration mode occurs, the particulate perfluorosulfonic acid functioning as a kind of binder in the fuel electrode loses its function, so that the electrode material such as the fuel electrode catalyst and the PTFE fine particles is replaced by the aqueous methanol solution of the fuel. It melts into the fuel and rapidly blackens the fuel, and the cell characteristics are significantly reduced. It is considered that this blackening of the fuel occurs because the black fuel electrode catalyst is mixed into the fuel. In this deterioration mode, the battery characteristics are rapidly lowered, and the deterioration phenomenon is irreversible, and it is considered that there is no recovery means once it occurs.

The present invention provides, on both sides of a proton conductive polymer solid electrolyte membrane, an electrode catalyst comprising at least a noble metal or a carbon supporting a noble metal, and a fuel electrode containing a proton conductive polymer solid electrolyte such as perfluorosulfonic acid. An air electrode is provided, and methanol and water as fuel are supplied to the fuel electrode side, and oxygen in the air is supplied to the air electrode side to generate electricity. By making the methanol concentration in the supplied fuel less than 2M (mol / dm ³ ), elution of the proton conductive polymer solid electrolyte and the electrode catalyst from the fuel electrode into the fuel is prevented. To prevent elution of the fuel electrode of a direct methanol fuel cell. The proton conductive polymer solid electrolyte to be added to the fuel electrode or the air electrode may be a perfluorosulfonic acid compound or the like, or an aromatic polymer sulfone oxide or the like.
Preferably, the methanol concentration is 1.5M or less and the operating temperature is 90 ° C or less, and particularly preferably the operating temperature is 80 ° C or less.

Further, the present invention provides a fuel electrode comprising, on both sides of a proton conductive polymer solid electrolyte membrane, an electrode catalyst comprising at least a noble metal or carbon carrying a noble metal, and a proton conductive polymer solid electrolyte such as perfluorosulfonic acid. And an air electrode, methanol and water as fuel are supplied to the fuel electrode side, and oxygen in the air is supplied to the air electrode side to generate power. A direct methanol fuel cell quality control method characterized by evaluating the elution characteristics of the fuel electrode material into the fuel.
Preferably, elution of the anode material into the fuel when the anode is brought into contact with a fuel having a concentration of more than 2M or a fuel having a concentration of more than 80 ° C, particularly preferably a fuel having a concentration of more than 2M and having a temperature of more than 80 ° C. The elution characteristics are evaluated by detecting the accompanying change in the characteristics of the fuel electrode.

Further, the present invention provides a fuel electrode comprising, on both sides of a proton conductive polymer solid electrolyte membrane, an electrode catalyst comprising at least a noble metal or carbon carrying a noble metal, and a proton conductive polymer solid electrolyte such as perfluorosulfonic acid. A direct methanol fuel cell comprising a fuel electrode and a fuel electrode, wherein methanol and water as fuel are supplied to the fuel electrode side, and oxygen in the air is supplied to the air electrode side to generate power. When the elution of the fuel electrode material into the fuel is detected, feedback is provided to the side for lowering the fuel concentration, the side for lowering the operating temperature, or the side for limiting the output of the fuel cell.
Preferably, a window for observing the color of the fuel or a sensor for detecting the color of the fuel is provided to detect the elution of the fuel electrode material into the fuel by the change in the color of the fuel.

Further, the present invention provides a fuel electrode comprising, on both sides of a proton conductive polymer solid electrolyte membrane, an electrode catalyst comprising at least a noble metal or carbon carrying a noble metal, and a proton conductive polymer solid electrolyte such as perfluorosulfonic acid. A direct methanol fuel cell in which methanol and water are supplied as fuel to the fuel electrode side and oxygen in the air is supplied to the air electrode side to generate power. Means for detecting or inputting elution of the fuel electrode material to the side, when the detection or input is performed, on the side for lowering the fuel concentration, or the side for lowering the operating temperature, or the side for limiting the output of the fuel cell And control means for feedback.
Preferably, a window for viewing the color of the fuel or a sensor for detecting the color of the fuel is provided.

  According to the present invention, a phenomenon in which a solid electrolyte such as perfluorosulfonic acid in a fuel electrode, an air electrode, and an electrolyte membrane, particularly, a fuel electrode is eluted into an aqueous methanol solution as a fuel is caused by a methanol concentration in the fuel and an operating temperature of the fuel cell. It is based on the finding that it is closely related to The phenomenon that the solid electrolyte in the fuel electrode elutes into the aqueous methanol solution, which is the fuel, occurs more remarkably as the methanol concentration in the fuel becomes higher and the operating temperature of the battery becomes higher. did.

  There is also a correlation between the methanol concentration in the fuel and the cell temperature, and solid electrolyte membranes such as perfluorosulfonic acid, which are used for the electrolyte membrane of direct methanol fuel cells, originally have the property of easily permeating methanol. Therefore, even when a part of methanol supplied to the fuel electrode does not take out current from the battery, a part of the methanol permeates to the air electrode side. Then, the permeated methanol is rapidly oxidized by the catalyst of the air electrode and the air supplied to the air electrode, and the entire battery generates heat. Therefore, when a rich fuel is supplied to the cell, the permeation of methanol further increases the cell temperature. Further, since the methanol permeation increases as the cell temperature increases, once a rich fuel is supplied to the cell, the temperature rises rapidly and the deterioration of the cell proceeds rapidly.

  In order to prevent the elution of the fuel electrode material from the fuel cell into the fuel, in the present invention, the concentration of methanol in the fuel is specified to be less than 2M, preferably 1.5M or less. The operating temperature of the battery is preferably set at 90 ° C. or lower, more preferably at 80 ° C. or lower. In order to completely prevent the elution of the fuel electrode material, it is effective to operate at a methanol concentration as low as less than 2.0M, preferably from 0.5 to 1.5M, particularly preferably from 0.5 to 1.0M. Operation within the range is desirable.

  In order to directly control the methanol fuel cell system at the operating concentration range and operating temperature of the present invention, a dummy cell for detecting the methanol concentration in the fuel is inserted into the fuel cell stack, and the concentration can be detected. Alternatively, the detection may be performed by installing a methanol sensor in the fuel tank or the fuel pipe. Regarding the battery temperature, a thermocouple or thermistor may be directly installed on the fuel cell stack to detect the temperature, or the fuel temperature in the fuel outlet or fuel pipe of the stack may be detected.

  The detected methanol concentration in the fuel and the battery temperature are taken into the control circuit so that the fuel having a methanol concentration of 2.0 M or more is not supplied to the stack, and the temperature is preferably kept at 90 ° C. or less at a concentration of 1.5 M or less. Control so that However, when the system is started, it is desirable to supply methanol having a high concentration within a range of less than 2.0 M to the stack.

  Regarding the methanol concentration and the operating temperature, it is desirable to operate the battery at the lowest possible methanol concentration and the lowest possible operating temperature as long as the required output of the battery can be maintained. In order to perform such control, such a logic circuit may be incorporated in the above control circuit.

  Next, when the elution characteristics of the fuel electrode material into the fuel are evaluated, the quality of the direct methanol fuel cell can be controlled. The quality control may be performed by, for example, using a high-concentration methanol or at a high operating temperature to accelerate the elution of the fuel electrode material, or may be performed under normal operating conditions.

  During fuel cell operation, if the elution of the anode material is detected and the fuel concentration is reduced, the operating temperature is reduced, or feedback is provided in the direction of limiting the output, the deterioration of the fuel cell is prevented from progressing. As a result, the durability of the fuel cell can be increased. For this purpose, a viewing window may be provided in a fuel pipe, a circulation tank, or the like, so that blackening of the fuel or the like may be visually observed, and the occurrence of deterioration may be input to a control circuit. Alternatively, blackening of the fuel or the like may be detected by a sensor such as a colorimetric sensor and fed back.

    Examples will be described below, but the present invention is not limited to these examples.

  For the electrolyte membrane, Nafion (Nafion is a registered trademark) of DuPont, which is general as a perfluorosulfonic acid-based electrolyte membrane, was used. The air electrode is formed by mixing an air electrode catalyst in which fine particles of platinum as an air electrode material are supported on carbon powder and a solution of a perfluorosulfonic acid-based electrolyte (Nafion) with PTFE fine particles to form a paste. A paste was applied to carbon paper which had been impregnated with a PTFE (polytetrafluoroethylene) solution and subjected to a water-repellent treatment, and dried at 100 ° C. The fuel electrode is a fuel electrode catalyst in which fine particles of platinum-ruthenium as a fuel electrode material are supported on carbon powder using carbon paper which has been subjected to water-repellent treatment in the same manner as the air electrode, and PTFE fine particles. , And a paste obtained by mixing a Nafion solution was applied, and dried at 100 ° C. to produce a paste. After drying these air electrode and fuel electrode, they were laminated on an electrolyte membrane and joined by hot pressing at 130 ° C. to obtain an MEA (electrolyte / electrode assembly). In the MEA, the Nafion solution in the electrode is dried to be in a resin state, imparts proton conductivity to the electrode portion, and serves as a kind of binder to bind a catalyst and PTFE particles. Further, the perfluorosulfonic acid is present in the electrode, for example, in the form of fine particles, but may be present in the form of a continuous film. Also, fine powder of platinum called platinum black may be used for the air electrode catalyst, and fine powder of platinum-ruthenium called platinum-ruthenium black may be used for the fuel electrode catalyst.

  Further, the MEA manufactured in this manner is impregnated with a phenolic resin to prevent gas leakage. The graphite air electrode separator plate (groove depth: 3 mm, groove width: 3 mm), and the fuel electrode separator plate (groove depth) (1 mm, groove width: 3 mm) to form a unit cell.

Next, in order to confirm the effect of the present invention, the characteristic deterioration due to the above-described deterioration mode was verified for a unit cell. The initial characteristics of the cell were evaluated under standard conditions of a fuel flow rate of 4 ml / min and an air flow rate of 1 l / min using a 1 M aqueous methanol solution as a fuel at 80 ° C. Next, the cells whose initial characteristics were evaluated were continuously operated at 200 mA / cm ² at various operating temperatures and methanol concentrations for 8 hours. After the continuous operation, the test was performed again under the same standard conditions as those used for measuring the initial characteristics, the output density at a current density of 200 mA / cm ² before and after the continuous test was calculated, and the deterioration of the characteristics was evaluated from the change. Further, the drop of the fuel electrode catalyst was performed by visually checking the color of the fuel waste liquid after the test. The blackened fuel waste liquid was analyzed, and the presence or absence of platinum and ruthenium was confirmed by the atomic absorption method.

The test results are shown in Tables 1 to 5. At a fuel density of 0.5 M and below 80 ° C., continuous operation for 8 hours was not possible at a current density of 200 mA / cm ² . It was found that continuous operation was not possible when the fuel concentration was too low. However, at 1.0 M and 1.5 M, continuous operation was possible in a temperature range of 50 to 90 ° C., blackening of fuel and deterioration of characteristics were not observed, and a phenomenon that characteristics were somewhat improved was observed.

Table 1
Table 1 Effect of the present invention Continuous test condition set temperature (° C) 50 60 70 80 85 90
Fuel methanol concentration 0.5M 0.5M 0.5M 0.5M 0.5M 0.5M
Before continuous operation of power density 72 75 75 74 71 73
(mW / cm ² ) After continuous operation --- 75 73 74
Whether there is blackening of the fuel waste liquid---None None None Remarks Set temperature is 50 to 70 ° C and fuel methanol concentration is
At 0.5 M, continuous operation at 200 mA / cm ² cannot be performed.

Table 2
Table 2 Effect of the present invention Test condition setting temperature (° C) 50 60 70 80 85 90
Fuel methanol concentration 1.0M 1.0M 1.0M 1.0M 1.0M 1.0M
Before power density continuous operation 75 74 71 74 73 72
(mW / cm ² ) After continuous operation 76 75 75 76 73 76
Whether there is blackening of the fuel waste liquid None None None None None None

Table 3
Table 3 Effect of the present invention Test condition set temperature (° C) 50 60 70 80 85 90
Fuel methanol concentration 1.5M 1.5M 1.5M 1.5M 1.5M 1.5M
Before power density continuous operation 72 74 74 73 73 75
(mW / cm ² ) After continuous operation 74 76 75 74 73 74
Whether there is blackening of the fuel waste liquid None None None None None None

Table 4
Table 4 Effect of the present invention Test condition set temperature (° C) 50 60 70 80 85 90
Fuel methanol concentration 2.0M 2.0M 2.0M 2.0M 2.0M 2.0M
Before continuous power density operation 70 73 73 73 74 74
(mW / cm ² ) After continuous operation 74 75 74 72 70 68
Whether there is blackening of the fuel waste liquid No No No Yes Yes Yes Yes
Remarks Pt, Ru Pt, Ru Pt, Ru
Detection Detection Detection

Table 5
Table 5 Effect of the present invention Test condition set temperature (° C) 50 60 70 80 85 90
Fuel methanol concentration 2.5M 2.5M 2.5M 2.5M 2.5M 2.5M
Before continuous power density operation 72 73 71 72 74 73
(mW / cm ² ) After continuous operation 71 65 60 62 51 48
Whether there is blackening of the fuel waste liquid Yes Yes Yes Yes Yes Yes Remarks Pt, Ru Pt, Ru Pt, Ru Pt, Ru Pt, Ru Pt, Ru
Detection Detection Detection Detection Detection Detection Detection

  In the continuous operation at 2.0 M or higher at 80 ° C., the fuel turned black, and Pt and Ru were confirmed in the fuel. Under these conditions, it was found that although the potential of the fuel electrode did not exceed the elution potential of Ru, Nafion of the fuel electrode was eluted by the above-mentioned deterioration mode, and a part of the electrode catalyst flowed out. Further, the phenomenon became remarkable in the case of 2.5 M fuel, and a sharp deterioration of the characteristics was observed at any temperature examined in this study. Also, the higher the operating temperature, the more remarkable this phenomenon. Here, a phenomenon in which the fuel electrode catalyst is eluted into the fuel when the potential of the fuel electrode exceeds the elution potential of Ru is referred to as “transversion”.

  Next, an example of an embodiment of the present invention is shown in FIG. In the figure, 2 is a direct methanol fuel cell, 4 is its fuel cell stack, 6 is a high-concentration methanol tank, which stores high-concentration methanol such as pure methanol or 60% by weight methanol. Reference numeral 8 denotes a circulation tank for storing methanol-water fuel having a concentration of 0.5 to 2M, reference numeral 10 denotes a waste liquid tank, which also functions as a gas-liquid separation tank, and reference numeral 12 denotes a control circuit. 14 is a radiator for cooling, 16 is its fan, and 18 is a bypass valve for bypassing the radiator. P1 is a fuel supply pump, P2 is a fuel adjustment pump, and supplies high-concentration methanol to the circulation tank. P3 is an air pump, P4 is a waste liquid pump, and injects waste liquid into the circulation tank 8.

  The fuel cell 2 is provided with various sensors, CS is a methanol concentration sensor, TS is a temperature sensor, and detects a waste liquid temperature. This temperature is substantially equal to the temperature in the stack 4, and the temperature of the temperature sensor TS is set as the operating temperature. LS1 to LS3 are level sensors, the level sensor LS1 detects the liquid level of the circulation tank 8, the level sensor LS2 detects the liquid level of the high-concentration methanol tank 6, and the level sensor LS3 is the level of the waste liquid tank 10. Detect the liquid level.

  DS is a deterioration sensor that detects elution and reversal of the electrode, and detects, for example, a blackening or a change in color of the fuel due to elution of the electrode catalyst using an optical sensor or a colorimetric sensor. It also detects changes in electrical properties of the fuel, such as conductivity, dielectric constant, and dielectric loss. Alternatively, fluorine ions based on Nafion eluted in the fuel are detected by an ion sensor. Further, a PH sensor detects a change in PH due to elution of Nafion which is a strongly acidic substance. When detecting a change in color or a change in electrical characteristics, in addition to elution of the fuel electrode material, it is also possible to detect inversion, and when detecting fluorine ions or monitoring PH, mainly detecting elution of the fuel electrode material. Become. The change in the color of the fuel and the change in the electrical characteristics are mainly due to the catalyst eluted in the fuel (carbon powder, Pt, Ru, etc.), and the deterioration sensor DS is located at the lower part of the circulation tank where these tend to accumulate, especially. It is preferable to provide them. In the case of detecting fluorine ions or detecting PH, the installation position of the deterioration sensor DS is arbitrary. The blackening of the fuel can be detected visually, and for example, a viewing window W is provided at a lower portion of the circulation tank 8 so that the color of the fuel tank can be confirmed from the outside. When the blackening of the fuel is visually found, the fuel can be input to the control circuit 12 from the manual switch SW.

  The opening and closing of these pumps, sensors, valves and the like must be managed by the control circuit 12 and controlled according to the state of the fuel cell system. Tables 6 and 7 show examples of the control method. When the fuel concentration falls to 0.5M or less, the fuel adjusting pump is operated to supply a constant amount of high-concentration methanol to the fuel circulation tank, and the methanol concentration of the fuel in the fuel circulation tank is increased within a range not exceeding 2M. . In addition, when the fuel level in the fuel circulation tank is full, high-concentration methanol cannot be added.Therefore, it is necessary to raise the operating temperature within a range not exceeding 80 ° C, increase the evaporation of water, and lower the fuel level. It becomes. Therefore, for example, the radiator mechanism is bypassed to increase the operating temperature. In this case, it is desirable that the fuel flow rate is not too high. As another method, a method in which the fuel adjustment pump is rotated in the reverse direction, a part of the fuel in the fuel circulation tank is pumped into the high-concentration methanol tank, and the fuel level is lowered may be considered.

Table 6
Table 6 Control method of fuel cell system (when circulation tank is not full)

Fuel concentration cell temperature
Normal operation at startup
Set during normal operation Set temperature
Less than 0.5M (0.5M to 2M) 2M or more Less than temperature Within range 80 ° C or more
(1) Air
Pump------
(2) Fuel supply
Pump High flow rate-High flow rate Low flow rate-High flow rate
(3) Fuel concentration Fuel concentration Fuel Fuel adjustment Large concentration Low concentration Large concentration
Pump (less than 2M)-(0.5M or more) (less than 2M)--
Fuel concentration
To small
(4) Waste liquid (0.5M or more)
Pump High flow rate-High flow rate Low flow rate-High flow rate
(5) Radiator bypass Radiator
Bypass valve---side
(6) Radiator
Fan---Stop Run Run

Table 7
Table 7 Control method of fuel cell system (when circulation tank is full)

Fuel concentration cell temperature
Normal operation at startup
Set during normal operation Set temperature
Less than 0.5M (0.5M to 2M) 2M or more Less than temperature Within range 80 ° C or more
(1) Air
Pump------
(2) Fuel supply
Pump Low flow rate-High flow rate Low flow rate-High flow rate
Reverse rotation Reverse rotation
(3) Fuel Fuel Circulation tank Fuel Circulation tank Concentration adjustment Increase concentration Increase concentration Increase concentration
Pump (less than 2M)-lower (less than 2M)-lower
Fuel fuel
To low concentration To low concentration
(4) Waste liquid (0.5M or more) (0.5M or more)
Pump Low flow rate-High flow rate Low flow rate-High flow rate
(5) Radiator by bypass
Pass valve side-Tar side Tar side Tar side Tar side
(6) Radiator
Tarfan Stop-Run Stop Run Run

  When the fuel concentration becomes 2M or more, the drain pump is operated to supply the low-concentration methanol waste liquid collected in the waste liquid tank into the fuel circulation tank. Also, when the fuel level in the fuel circulation tank is full, it is not possible to add low-concentration methanol waste liquid, so it is possible to bypass the radiator mechanism and raise the operating temperature as before, If the temperature becomes high, the deterioration of the MEA in the deterioration mode of the present invention proceeds, and it is desirable not to raise the operating temperature at this time. Therefore, it is considered effective to reverse the fuel adjustment pump to pump up a part of the fuel in the fuel circulation tank into the high-concentration methanol tank to lower the fuel level.

  Next, when the battery is started or when the operating temperature becomes lower than the set temperature, in order to raise the battery temperature, the radiator bypass valve is set to the bypass side and cooling is not performed by the radiator, and the fuel concentration is set within a range of less than 2M. It is possible to increase the battery temperature by using methanol cross leak.

  Further, when the battery temperature exceeds 80 ° C., in order to lower the battery temperature, the radiator bypass valve is used as the radiator side to cool the fuel, and the fuel concentration can be reduced within a range that does not become less than 0.5 M. .

  FIG. 2 shows the configuration of the control circuit 12. An input interface 20 receives signals from the sensors CS, TS, LS1 to LS3, etc., and grasps the state of the fuel cell 2. The CPU (control processing device) 22 uses the sensor signals from the input interface 20, the start signal, and the signal from the deterioration detection unit 26 to output the pumps P1 to P4, the bypass valve 18, The fan 16 and the like are driven. The start signal is turned on when the fuel cell 2 is started, and the target temperature is slightly increased as shown in Tables 6 and 7 for a predetermined time from the start or until the fuel cell temperature reaches the predetermined temperature (Table 6). At 80 ° C. or lower), the control target is changed to a slightly higher fuel concentration (less than 2M).

  The deterioration detection unit 26 detects deterioration based on signals from the manual switch SW and the deterioration sensor DS, and changes the control target. For example, it is assumed that the state of the fuel cell can be determined into four levels, that is, no degradation, degradation level 1, degradation level 2, and degradation level 3 based on the signal of the degradation sensor DS. In the case where there is no deterioration, control is performed under the conditions described in Tables 6 and 7, etc., the upper limit of the fuel concentration is changed to 1.5 M at the deterioration level 1 and the upper limit of the battery temperature is changed to 70 ° C. The upper limit of the concentration is changed to 1 M and the upper limit of the cell temperature is changed to 60 ° C., and the operation of the fuel cell is ended at the deterioration level 3. The deterioration detecting unit 26 displays the degree of deterioration and the like via a display unit such as an LCD 28. The processing when the deterioration is detected is to lower the target value of the fuel concentration in the fuel cell and to lower the target value of the operating temperature to prevent the deterioration from progressing. Alternatively, instead of or in addition to lowering the target values of the fuel concentration and the operating temperature, an upper limit is imposed on the output (output current, output power, etc.) of the fuel cell, or if an upper limit already exists, the upper limit is set. For example, the output may be limited by lowering the output. As a result, the life and durability of the fuel cell can be greatly extended.

  Next, elution of the fuel electrode will be described from the aspect of quality control of the fuel cell. For example, the quality control of the fuel cell can be performed by operating the manufactured fuel cell as a unit cell or a fuel cell stack under a predetermined condition to check whether or not the fuel electrode material is eluted. As the operating conditions, for example, for a representative sample, conditions more severe than normal operating conditions, a methanol concentration of more than 2M, an operating temperature of more than 80 ° C. such as 4M to 60% methanol, for example, 90 to 110% The fuel cell is operated at ℃ or the like, and the elution characteristics such as the presence or absence of the elution of the fuel electrode material in the fuel and the elution amount are measured. Alternatively, a change in voltage / current characteristics before and after such endurance conditions is measured.

  Instead of the acceleration test as described above, the presence or absence of elution of the fuel electrode material and the change in voltage / current characteristics when the fuel cell is operated under normal use conditions may be measured. In addition, instead of using a cell such as a unit cell or a fuel cell stack as a sample to evaluate the elution characteristics, the elution characteristics may be evaluated for the fuel electrode alone. For example, as described as a method of manufacturing the MEA, a paste of the fuel electrode material is applied to the water-repellent carbon paper, and dried at a temperature corresponding to a hot press temperature. The drying temperature is preferably the highest temperature experienced when manufacturing the MEA, but may be in a range that can be correlated with the elution characteristics of the fuel electrode material in the MEA. Then, for a single dried fuel electrode, the elution of the fuel electrode material into the fuel may be evaluated.

  Whether the fuel electrode material is eluted or not and whether or not it is evaluated using single cells or fuel cell stacks, or when evaluating the characteristics of the fuel electrode alone, whether the fuel blacks, changes in electrical characteristics, or detects fluorine ions , PH change, trace element analysis such as atomic absorption analysis, or the like. Further, the elution of the fuel electrode material can be indirectly evaluated from the change in the voltage / current characteristics before and after the durability test. The solvent used for the evaluation is not limited to methanol-water and the like, but a polar solvent such as isopropanol-water can be used.

  FIG. 3 shows the voltage / current characteristics before and after the endurance test in which the fuel cell was operated at 80 ° C. (lower limit temperature) to 90 ° C. (upper limit temperature) for 30 minutes using 6M methanol-water mixed fuel. Indicates a change. The test conditions of 1M MeOH and 70 ° C. in the figure are measurement conditions of the voltage / current characteristics before and after the durability test. In this example, the hot pressing temperature of the solid electrolyte membrane (Nafion membrane) of the fuel electrode or the air electrode was set to 190 ° C. to suppress the elution of Nafion in the fuel electrode. When 6 M methanol is used, elution of the anode catalyst into the fuel is detected, and the elution amount increases as the hot press temperature decreases, and the effect of operation in 6 M methanol for 30 minutes is that the hot press temperature is low. It was about big.

In the supplementary examples, Nafion (registered trademark of DuPont) was shown as the proton conductive polymer solid electrolyte, but other perfluorosulfonic acid polymers or aromatic polymer sulfonic acid compounds may be used.

Block diagram of a direct methanol fuel cell according to an embodiment Block diagram of a control unit in the embodiment Characteristic diagram showing changes in voltage-current characteristics due to elution of fuel electrode

Explanation of reference numerals

2 Direct methanol fuel cell 4 Fuel cell stack 6 High concentration methanol tank 8 Circulation tank 10 Waste liquid tank 12 Control circuit 14 Radiator 16 Fan 18 Bypass valve 20 Input interface 22 Control processing unit (CPU)
24 Output interface 26 Deterioration detector 28 LCD
P1 Fuel supply pump P2 Fuel adjustment pump P3 Air pump P4 Waste liquid pump CS Methanol concentration sensor TS Temperature sensors LS1 to LS3 Level sensor DS Deterioration sensor W Viewing window SW Manual switch

Claims (8)

On both sides of the proton conductive polymer solid electrolyte membrane, an electrode catalyst made of at least a noble metal or carbon carrying a noble metal, and a fuel electrode and an air electrode containing a proton conductive polymer solid electrolyte are provided, and the fuel electrode side Direct methanol fuel cell, which supplies methanol and water as fuel to the air and supplies oxygen in the air to the air electrode side to generate power,
The concentration of methanol in the fuel supplied to the fuel cell is less than 2M to prevent the elution of the proton conductive polymer solid electrolyte and the electrode catalyst from the fuel electrode into the fuel. A method for preventing elution of a fuel electrode of a methanol fuel cell. The method for preventing elution of a fuel electrode of a direct methanol fuel cell according to claim 1, wherein the methanol concentration is 1.5M or less and the operating temperature is 90C or less. On both sides of the proton conductive polymer solid electrolyte membrane, an electrode catalyst made of at least a noble metal or carbon carrying a noble metal, and a fuel electrode and an air electrode containing a proton conductive polymer solid electrolyte are provided, and the fuel electrode side A method for controlling the quality of a direct methanol fuel cell in which methanol and water are supplied as fuel to the air electrode and oxygen in the air is supplied to the air electrode side to generate power,
A quality control method for a direct methanol fuel cell, comprising evaluating the elution characteristics of a fuel electrode material into fuel. The elution characteristics may be evaluated by detecting a change in the characteristics of the anode accompanying the elution of the anode material into the fuel when the anode is brought into contact with a fuel having a concentration of more than 2M or a fuel having a temperature of more than 80 ° C. 4. The quality control method for a direct methanol fuel cell according to claim 3, wherein: On both sides of the proton conductive polymer solid electrolyte membrane, an electrode catalyst made of at least a noble metal or carbon carrying a noble metal, and a fuel electrode and an air electrode containing a proton conductive polymer solid electrolyte are provided, and the fuel electrode side A method for operating a direct methanol fuel cell in which methanol and water are supplied as fuel to the air electrode, and oxygen in the air is supplied to the air electrode side to generate power.
Direct methanol type, characterized in that when the elution of the fuel electrode material into the fuel is detected, feedback is made to the side that lowers the fuel concentration, the side that lowers the operating temperature, or the side that limits the output of the fuel cell. How to operate the fuel cell. A window for viewing the color of the fuel, or provided with a sensor for detecting the color of the fuel, characterized by detecting the elution of the fuel electrode material into the fuel by the change in the color of the fuel, The method for operating a direct methanol fuel cell according to claim 5. On both sides of the proton conductive polymer solid electrolyte membrane, an electrode catalyst made of at least a noble metal or carbon carrying a noble metal, and a fuel electrode and an air electrode containing a proton conductive polymer solid electrolyte are provided, and the fuel electrode side A direct methanol fuel cell that supplies methanol and water as fuel to the fuel cell and supplies oxygen in the air to the air electrode side to generate power,
Means for detecting or inputting elution of the anode material into the fuel;
When the detection or input is performed, a control unit for feedback is provided on the side for lowering the fuel concentration, the side for lowering the operating temperature, or the side for limiting the output of the fuel cell, Direct methanol fuel cell. 8. The direct methanol fuel cell according to claim 7, further comprising a window for checking the color of the fuel, or a sensor for detecting the color of the fuel.

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