JP2009059585A - Power generation control method of direct methanol fuel cell, and fuel cell using its method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a control method wherein electric power can be generated stably for a long time in a direct methanol fuel cell in which a water permeation coefficient α is 1 or less. <P>SOLUTION: This is a first power generation control method in which concentration control of fuel concentration control mechanism is carried out according to an inequality a: 0≤((C1-C0)/C0)/((P0-P1)/P0)≤20. Here, C0: methanol concentration in the fuel at the initial starting time of the power generation, C1: methanol concentration after lapse of time t, P0: the initial power output, and P1: power output after lapse of time t. This is a second power generation control method when the output voltage V becomes a threshold voltage Vs or less, methanol concentration C of the fuel is raised by an increment ΔC, and power generation is carried out until the output voltage becomes equal to the lowest usable voltage VL or more in a stationary state. This is a third power generation control method in which the standard condition (temperature T0, cathode gas flow amount F0, time tn) and an especial condition (temperature T1, cathode gas flow amount F1, time ta) are carried out while alternately repeating them under ta/(tn+ta)≤30%. Here, 40°C≤T0<T1≤90°C, and F0≤F1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a power generation control method for a direct methanol fuel cell. More specifically, the present invention relates to a control method for driving a direct methanol fuel cell (DMFC) for a long time while maintaining a stable output, and a direct methanol fuel cell using this method.

  In recent years, electronic devices have been miniaturized, and many information terminals can be carried around, so that the so-called ubiquitous society in which necessary information can be obtained everywhere is changing.

  These information terminals are equipped with abundant functions such as high-speed arithmetic processing, wireless LAN, and multimedia playback, and power consumption tends to increase. At present, such information terminals mainly use a primary battery or a secondary battery as a driving power source. A battery with a large capacity is required to drive the information terminal for a long time, but a primary or secondary battery with sufficient capacity has not been developed due to environmental problems and safety problems. Expectations are rising.

  Among fuel cells, direct methanol fuel cells that generate electricity by extracting hydrogen ions (protons) directly from methanol are high in energy density of methanol, which is a fuel, and can be downsized without a reformer. In some respects, there is an increasing expectation for application in various fields as a power source for portable devices.

  A direct methanol fuel cell (DMFC) starts power generation by supplying methanol fuel and oxygen or an oxygen-containing gas (for example, air) to a power generation unit.

The basic reaction of DMFC is expressed by the following formula.
Anode electrode: CH ₃ OH + H ₂ O → 6H ⁺ + 6e ⁻ + CO ₂ (1)
^{Cathode: 6H + + 6e - + 3} / 2O 2 → 3H 2 O (2)
That is, as is clear from the equation (1), the DMFC ideally generates power by supplying a mixture of methanol and water at a molar ratio of 1: 1 as a fuel to the anode electrode. Methanol-type fuel cells have a methanol-to-water molar ratio of 1: 1 and adjusted to a predetermined concentration because of the material limitations of the solid polymer electrolyte membrane used in the power generation section. It is necessary to supply fuel to the anode.

  At the cathode, water molecules are generated from the protons that have been generated at the anode electrode by the reaction of formula (2) and moved through the electrolyte membrane, and are usually discharged to the atmosphere, but this water is recovered using a cooler or the like. If possible, it can be used for the reaction of the formula (1) by supplying it to the anode, and power generation can be performed by supplying only a water: methanol mixture or methanol having a higher methanol concentration.

  However, in an actual DMFC, more water is consumed from the methanol fuel than shown by the equation (1). Specifically, water that moves from the anode to the cathode accompanied by protons, water that moves from the anode to the cathode due to a concentration gradient, and the like. Depending on the material characteristics and operating conditions used for the electromotive section, more than 10 times the water molecules consumed in the basic reaction of formula (1) may move to the cathode side. Since the moved water is finally discharged together with the reaction product water on the cathode side, there is a problem that inconvenience occurs in the downsizing of the DMFC main body because the size of the cooler and the cooling fan becomes large when trying to collect with a cooler or the like. . Another problem was the need to use a more diluted fuel with a lower volumetric energy density.

  In order to discuss the amount of water transfer from the anode to the cathode, the following coefficient α can be used as an index.

The total amount of water discharged on the cathode side during power generation is mtotal (mol), the amount of water generated by equation (2) by power generation is me (mol), and methanol crossed over from the anode to the cathode is the cathode. Assuming that the amount of water produced by oxidation is mx (mol), the constant load current during power generation is I (A), and the power generation time is p (sec), the amount of proton transfer from the anode electrode to the cathode electrode x And the apparent water transfer amount y is x = I * p / 96500 (mol) (3)
y = total-me-mx (mol) (4)
Calculated by The amount of methanol crossover from the anode electrode to the cathode electrode can be calculated from the concentration of carbon dioxide (C CO ₂ ) (vol%) contained in the gas discharged on the cathode side. If the expected flow rate discharged on the cathode side is G (L / sec),
mx = G * t * C Co 2/100/22400 (mol) (5)
Therefore, the apparent water permeability coefficient α is α = y / x (6)
It becomes. It is considered that the above problem can be solved by reducing the coefficient α.

Japanese Patent Application Laid-Open No. 61-107666 discloses a technique relating to a method for controlling a liquid fuel cell. Here, the fuel cell output decrease is circulated by utilizing the fact that the fuel cell output decreases when the concentration of the circulating fuel decreases in the liquid fuel cell to which liquid fuel including water and water is circulated and supplied. It is used as a method for detecting a decrease in fuel concentration, and it is described that fuel or a fuel-rich mixture is replenished from the tank to the circulating fuel when the fuel cell output decreases. On the other hand, in the present invention, the circulating fuel concentration is kept constant by the fuel concentration control mechanism, and basically the fuel cell output does not decrease due to the decrease of the circulating fuel concentration. In a fuel cell having a water permeability coefficient α of less than 1 from the anode to the cathode, it has been found that even if the concentration of the circulating fuel is kept constant, the output decreases due to a change in the material. Provided is a fuel concentration control method for preventing a decrease in fuel cell output when a change in the material occurs, and a concentration control method from a viewpoint different from that disclosed in Japanese Patent Laid-Open No. 61-107666. It is.
JP-A-61-107666

  As described above, reducing the amount of water permeating from the anode to the cathode (ie, reducing the α value) reduces the amount of water generated from the cathode and simplifies the cathode water recovery mechanism. Or, it can be omitted, but if a methanol fuel cell with a small α value of 1 or less is generated for a long time with a constant load current, a rapid voltage drop is observed after a certain time, and the required output It became clear that there was a problem that could not be maintained. The cause of this phenomenon is not clear, but now water that has returned from the cathode side to the anode side accumulates in the anode catalyst layer, resulting in diluting the concentration of methanol fuel supplied. It is estimated that this is the cause.

  An object of the present invention is to provide a power generation control method aiming at preventing such a rapid decrease in output in advance when a direct methanol fuel cell having a small α value is generated for a long time. .

A first power generation control method for a fuel cell according to the present invention comprises an anode electrode, a cathode electrode, a membrane electrode assembly comprising a solid polymer electrolyte membrane sandwiched between both electrodes, a fuel supply mechanism to the anode, and a fuel concentration control mechanism A fuel cell having a gas supply mechanism for a fuel tank and a cathode, wherein the water moves along with the number x of hydrogen ions (protons) that move from the anode to the cathode in accordance with power generation in the membrane electrode assembly. The ratio α of the number y of molecules [where α = y / x. ] Is a direct methanol fuel cell in which the concentration control of the fuel concentration control mechanism is performed according to the following equation (a).
0 ≦ ((C1-C0) / C0) / ((P0−P1) / P0) ≦ 20 (a)
[Where C0 is the methanol concentration in the methanol fuel at the beginning of power generation, C1 is the methanol concentration in the methanol fuel after elapse of t time from the start of power generation, and P0 is the initial output (voltage x current), P1 is the output (voltage × current) after elapse of t time. ]

  A second fuel cell power generation control method according to the present invention comprises an anode electrode, a cathode electrode, a membrane electrode assembly comprising a solid polymer electrolyte membrane sandwiched between both electrodes, a fuel supply mechanism to the anode, a fuel concentration A fuel cell having a control mechanism, a fuel tank, and a gas supply mechanism to the cathode, which moves with the number x of hydrogen ions (protons) that move from the anode to the cathode in accordance with power generation in the membrane electrode assembly. The ratio α of the number y of water molecules to be performed [where α = y / x. ] Is 1 or less, the methanol concentration of the fuel supplied to the anode when it is detected that the voltage V output from the fuel cell is less than or equal to a preset threshold voltage Vs C is increased by a preset increment ΔC, and concentration control of the fuel concentration control mechanism is performed until the fuel cell reaches a minimum voltage VL that can be used in a steady state [here, The threshold voltage Vs is lower than the lowest voltage VL that the fuel cell can be used in a steady state.

  A third fuel cell power generation control method according to the present invention comprises an anode electrode, a cathode electrode, a membrane electrode assembly comprising a solid polymer electrolyte membrane sandwiched between both electrodes, a fuel supply mechanism to the anode, a fuel concentration A fuel cell having a control mechanism, a fuel tank, a gas supply mechanism to a cathode, and a temperature control mechanism, wherein the number of hydrogen ions (protons) that move from the anode to the cathode with power generation in the membrane electrode assembly is increased The ratio α of the number y of water molecules that move together [where α = y / x. ] In the direct methanol fuel cell in which the power generation of the direct methanol fuel cell is 1 or less, (b) standard operating conditions in which power generation is continuously performed for tn time at the temperature T0 and the cathode gas flow rate F0; ) Electric power is generated at the temperature T1 and the cathode gas flow rate F1 under the special operation condition for continuous ta time, and the ratio of the above-mentioned tn and ta is controlled so that ta / (tn + ta) ≦ 30%. , Wherein the standard operation condition and the special operation condition are alternately repeated [wherein, the temperature T0 and the temperature T1 are both 40 ° C. or more and 90 ° C. or less, and the temperature T1 under the special operation condition is used. And at least one of the cathode gas flow rate F1 is higher than the temperature T0 and the cathode gas flow rate F0 in the standard operating conditions. The cathode stoichiometric ratios ξ0 and ξ1 corresponding to the cathode flow rates F0 and F1 satisfy 1 ≦ ξ0 ≦ ξ1 ≦ 4].

  A direct methanol fuel cell according to the present invention is one in which any one of the above-described direct methanol fuel cell power generation control methods is implemented.

  According to the present invention, an anode electrode, a cathode electrode, and a membrane electrode assembly comprising a solid polymer electrolyte membrane sandwiched between both electrodes, a fuel supply mechanism to the anode, a fuel concentration control mechanism, a gas to the fuel tank and the cathode A fuel cell having a supply mechanism, wherein the ratio α of the number y of water molecules moving accompanying the number x of hydrogen ions (protons) moving from the anode to the cathode accompanying power generation in the membrane electrode assembly For a direct methanol fuel cell in which α = y / x] is 1 or less, a power generation control method capable of generating power stably for a long time is provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, the figure used for description is a representative example shown for understanding the content of the present invention, and does not limit the scope of the present invention.

FIG. 1 shows an example of an electromotive member (MEA) of a direct methanol fuel cell used in the present invention.
In FIG. 1, a power generation unit 1 of a direct methanol fuel cell includes an anode catalyst layer 2 and a cathode catalyst layer 3, and a proton conductive solid polymer electrolyte membrane 4 sandwiched between them. Usually, the anode catalyst layer 2 is formed for the purpose of taking out hydrogen ions (protons) from a methanol fuel supplied to the anode electrode by a chemical reaction according to the above formula (1). As the catalyst, a catalyst that has been used as an anode catalyst of a conventional direct methanol fuel cell can be used in the present invention. Preferably, for example, platinum ruthenium (PtRu) -based alloy metal particles with little poisoning are used alone or in a form supported on carbon fine powder, and mixed with a perfluorocarbon sulfonic acid solution to impart proton conductivity. The As a commercially available product, for example, catalyst powder (Pt / Ru Black Hisspec 6000) manufactured by Johnson & Matthey can be used. On the other hand, in the cathode catalyst layer 3, protons moving from the anode electrode side through the solid polymer electrolyte membrane 4 according to the above formula (2), electrons flowing through the external circuit, and oxygen supplied to the cathode electrode react. Water is produced. As the catalyst on the cathode side, those conventionally used can also be used in the present invention. Preferably, for example, platinum (Pt) fine particles are used in the form of a simple substance or a carbon fine powder, and are mixed with a perfluorocarbon sulfonic acid solution in order to impart proton conductivity in the same manner as the anode catalyst layer. The As a commercially available product, it is preferable to use a Pt / C catalyst (HP 40 wt% Pton Vulcan XC-72R) manufactured by E-TEK. As the solid polymer electrolyte membrane 4, a perfluorocarbon sulfonic acid electrolyte membrane having high proton conductivity (such as a Nafion membrane manufactured by Dupont) is suitable.

  Furthermore, in the direct methanol fuel cell of the present invention, in order to suppress the generation of water at the cathode as much as possible, an anode MPL (Micro Porous Layer) is provided on the anode electrode side opposite to the electrolyte membrane of the catalyst layer. 5 can be formed. By forming this anode MPL layer 5, the penetration of methanol fuel into the catalyst layer on the anode electrode side is suppressed to the minimum necessary for power generation, and as a result, water permeation to the cathode electrode side is suppressed. it can. Since the anode MPL layer 5 must have water repellency, it can be formed, for example, by mixing carbon powder and water repellant PTFE emulsion. Moreover, even if the anode MPL layer is formed of a single layer, it is allowed to be formed of several layers having different properties. The anode MPL layer 5 is usually formed on an anode GDL (Gas Diffusion Layer) layer 6. As the anode GDL layer 6, for example, carbon paper manufactured by Toray Industries, Inc., which has been subjected to water repellent treatment with an appropriate amount of PTFE can be used. It is desirable to form the cathode MPL layer 7 and the cathode GDL layer 8 on the cathode electrode side on the opposite side of the catalyst layer from the electrolyte membrane. As the cathode MPL layer 7 and the cathode GDL layer 8, Elat GDL LT-2500-W (a cathode MPL layer formed on the cathode GDL layer) manufactured by E-TEK can be used.

  The electromotive member (MEA) manufactured as described above generates electricity by flowing a fuel such as an aqueous methanol solution on the anode electrode side and a gas containing oxygen such as air on the cathode side. Generally, in a fuel cell, an MEA is installed between a conductive material called a separator formed with a flow path so that fuel and gas do not leak through a sealing material, and the fuel and gas are placed in the flow path. To generate electricity. The separator is not particularly limited as long as it is conductive, but it is preferable to use a carbon member because of ease of processing.

  An example of the separator is shown in FIG. Taking the anode-side separator 9 as an example, the concentration-adjusted methanol-containing fuel supplied from the fuel supply port 10 flows through the anode GDL layer surface of the MEA according to the flow path 11 and is discharged from the fuel discharge port 12. . In the separator on the cathode side, a gas containing oxygen such as air similarly flows along the surface of the cathode GDL along the flow path and is discharged from the discharge port.

  FIG. 3 shows an example of setting when generating power in a single cell. A membrane electrode assembly (MEA) 13 as an electromotive member is composed of an anode electrode 13a, a cathode electrode 13b, and a solid polymer electrolyte membrane 13c sandwiched between both electrodes 13a and 13b. The body 13 is sandwiched between two separators 16 and 17 in which flow paths are formed via sealing materials 14 and 15. Further, current extraction plates 18 and 19 for extracting the current obtained by the power generation to the outside are installed above and below and connected to a load (not shown). The voltage of the membrane electrode assembly 13 is obtained by measuring the voltage between the two current extraction plates. A temperature control mechanism for adjusting the temperature of the membrane electrode assembly 13, specifically, heaters 20 and 21, is installed on the outside thereof. The temperature of the membrane electrode assembly 13 during power generation can be measured, for example, by inserting a thermocouple or thermistor into an observation hole formed in the separator. By using this measured temperature and controlling the output to the heater with a temperature controller or the like, the membrane electrode assembly 13 can be easily maintained at a predetermined temperature.

  In order to maintain the confidentiality of the membrane electrode assembly 13 and the separator, the entire electrode electrode 13a, the cathode electrode 13b and the separators 16 and 17 are kept in electrical contact with each other, and the separators 16 and 17 and the current extraction plate 18 are further maintained. In order to hold the 19 contacts, the insulation plates 22 and 23 are sandwiched and clamped from the outside by the pressing plates 24 and 25. It is also possible to obtain a high voltage by stacking a plurality of MEAs sandwiched between two separators.

  A concentration-controlled aqueous methanol solution is supplied to the anode electrode from the fuel supply port 26 by, for example, a fuel pump. The surplus methanol aqueous solution remaining for power generation at the electromotive section is discharged from the fuel discharge port 27, but this is temporarily collected in a container and replenished with methanol and water consumed by the power generation, and the methanol concentration It is also possible to readjust and recirculate to the fuel supply port. In order to adjust the methanol concentration automatically, a concentration sensor is installed in one of the circulating methanol fuel piping or piping branched from the piping, the signal obtained from this sensor is processed, and the necessary The concentration can be adjusted by supplying high-concentration methanol fuel and / or water from each separately prepared tank using a pump or the like. As a methanol concentration sensor, one using an optical refractive index, one using an electrostatic capacity, one using an ultrasonic method, one using a density measuring method, a method for electrochemically detecting the oxidation current of methanol It is possible to use various systems such as the ones. A gas containing oxygen such as air is supplied from the gas supply port 28 to the cathode electrode by the action of a pump or the like, and is discharged from the gas discharge port 29.

  FIG. 4 shows a preferred configuration example of a direct methanol fuel cell to which the power generation control method according to the present invention is applied. In FIG. 4, A is an electromotive member. This electromotive member A includes one membrane voltage assembly (MEA) 13 composed of a solid polymer electrolyte membrane 13c sandwiched between an anode electrode 13a and a cathode electrode 13b and both electrodes 13a and 13b shown in FIG. It consists of a stack with multiple pieces. The current I of the voltage V generated between the anode electrode 13a and the cathode electrode 13b can be used by the load F. B is a fuel supply mechanism to the anode. The fuel supply mechanism B to the anode is provided with a mixing tank B1 and a circulation pump B2, and fuel (specifically, a mixture of methanol and water) in the mixing tank B1 is generated by an electromotive member by the circulation pump B2. After being supplied to A and used for power generation in the electromotive member A, it is discharged from the electromotive member A, introduced into the mixing tank B1, and circulated so that it can be supplied to the electromotive member A again. The fuel concentration control mechanism C includes a methanol concentration sensor C1 that measures the methanol concentration in the fuel supplied to the electromotive member A, and fuel pumps C2 and C3. Based on information from the methanol concentration sensor C1. The supply amount of methanol is controlled by the pump C2, and the supply amount of water (fuel) is controlled by the pump C3. D is a fuel tank, D1 is a fuel tank containing high-concentration methanol fuel, and D2 is a fuel tank containing water (fuel). E is a gas supply mechanism to the cathode. The gas supply mechanism E to the cathode includes a cathode gas supply pump E1, supplies an oxygen-containing gas (for example, air) to the electromotive member A, and is used for power generation by the electromotive member A. It is configured to be discharged to the outside of the member A. Since liquid or gaseous water contained in the discharge from the electromotive member A can be used as fuel, like the water stored in the fuel tank D2, the discharge from the electromotive member A All or part of the water contained in the product can be supplied to the fuel supply mechanism B to the anode.

In the first fuel cell power generation control method according to the present invention, the concentration control of the fuel concentration control mechanism is performed according to the following equation (a).
0 ≦ ((C1-C0) / C0) / ((P0−P1) / P0) ≦ 20 (a)
[Where C0 is the methanol concentration in the methanol fuel at the beginning of power generation, C1 is the methanol concentration in the methanol fuel after elapse of t hours from the start of power generation, and P0 is the initial output (voltage x current), P1 is the output (voltage × current) after elapse of t time. ]
The range of ((C1-C0) / C0) / ((P0-P1) / P0) needs to be 0 or more and 20 or less. By being within this range, it is possible to suppress the amount of methanol permeating from the anode side to the cathode side (crossover), to improve the fuel utilization efficiency and to suppress the decrease in output. Preferably they are 2 or more and 15 or less, Most preferably, they are 3 or more and 10 or less. The value of ((C1-C0) / C0) / ((P0-P1) / P0) does not have to be always constant until t time elapses from the start of power generation. It may have changed.

  In the first power generation control method for a fuel cell according to the present invention, the methanol concentration of the fuel to be supplied is periodically exchanged from the initial methanol concentration C0 to the fuel whose fuel concentration is adjusted to be higher sequentially with the elapsed time. It is possible to finally reach the methanol concentration C1 after the time t, or (b) when automatically adjusting the concentration by the circulation method, the relationship information between the elapsed time and the methanol concentration Based on the above, it is also possible to start power generation at an initial methanol concentration C0, measure the elapsed time while integrating the power generation time, and automatically control the concentration corresponding to the elapsed time to reach C1. Is possible.

  FIG. 5 schematically shows how the fuel concentration is controlled. In the first fuel cell power generation control method of the present invention, for example, the methanol concentration is increased stepwise as in (1), the methanol concentration is increased linearly as in (2), or It is also possible to change the methanol concentration according to a certain function as in (3). By doing so, even if water is accumulated in the anode catalyst layer, it is considered that dilution is suppressed by increasing the concentration of methanol fuel supplied so as to correspond to it.

  In the second power generation control method of the fuel cell according to the present invention, when it is detected that the voltage V output from the fuel cell is equal to or lower than a preset threshold voltage Vs, methanol of fuel supplied to the anode The concentration C is increased by a preset increment ΔC, and concentration control of the fuel concentration control mechanism is performed until the fuel cell reaches a minimum voltage VL that can be used in a steady state. Here, the threshold voltage Vs is lower than the lowest voltage VL that the fuel cell can use in a steady state.

  In this second power generation control method, the voltage V of the fuel cell (specifically, for example, the voltage V of the whole stack in which a single cell or a plurality of single cells made of the membrane electrode assembly (MEA) 13 is stacked) is used. It can be easily performed by continuously generating power while taking a load at a constant current value, periodically or continuously observing the voltage between the two current extraction plates, taking it in a personal computer or memory, and monitoring it. Triggered by detecting that the voltage V has fallen below a predetermined threshold voltage Vs, the methanol concentration supplied to the anode electrode is manually or automatically increased by a preset increment ΔC from the current supply concentration C. Change to methanol concentration. The voltage V is compared with the lowest voltage VL in the steady state of the fuel cell. If VL> V, the methanol fuel concentration is increased again by the increment ΔC. Thereafter, when it is detected again that the voltage V has become equal to or lower than the predetermined threshold voltage Vs as power generation continues, the concentration control of the fuel concentration control mechanism is repeatedly performed (FIG. 6). Here, the minimum voltage VL in the steady state of the fuel cell is a minimum voltage that can supply sufficient power to an electronic device or the like even if a normal fuel cell undergoes performance degradation due to long-time power generation. means. Vs can be appropriately determined in consideration of, for example, the value of the minimum voltage VL, ΔC, the degree of voltage recovery, and the like, but generally a voltage corresponding to 50 to 95% of VL, preferably 60 to 90 of VL. The voltage corresponding to%. By setting Vs within the above range, it becomes easy to obtain an effect of preventing a reversal phenomenon in which the voltages of some cells are reversed, particularly in a stack in which a plurality of single cells are stacked. The value of ΔC can also be determined as appropriate, but is generally 0.05 mol / L to 3.0 mol / L, preferably 0.1 mol / L to 1.5 mol / L. There is no particular limitation on the upper limit of the number of repetitions.

  In the third fuel cell power generation control method according to the present invention, power generation of the direct methanol fuel cell is as follows: (a) Standard operating conditions in which power generation is continued for tn time at a temperature T0 and a cathode gas flow rate F0; B) Control is performed so that the ratio of the above-mentioned tn and ta is ta / (tn + ta) ≦ 30% under special operating conditions in which power generation is continued at the temperature T1 and the cathode gas flow rate F1 for ta time. Meanwhile, the standard operation condition and the special operation condition are alternately repeated. [Temperature T0 and temperature T1 are both 40 ° C. or higher and 90 ° C. or lower, and at least one of temperature T1 and cathode gas flow rate F1 under special operating conditions is higher than temperature T0 and cathode gas flow rate F0 under standard operating conditions. high. The cathode stoichiometric ratios ξ0 and ξ1 corresponding to the cathode flow rates F0 and F1 satisfy 1 ≦ ξ0 ≦ ξ1 ≦ 4. ]

In the third power generation control method, power generation is performed at the flow rate F0 of the gas containing oxygen to the cathode electrode and the power generation temperature T0 under standard operating conditions, and the ratio of tn and ta is periodically ta / (tn + ta) ≦ Power generation is performed for a long time while power generation is performed under special operating conditions (temperature T1, cathode gas flow rate F1) while controlling to be 30%. Here, the cathode gas flow rate in the cathode stoichiometry ξc calculated for both F0 and F1 is in the range of 1 ≦ ξc ≦ 4. Here, the cathode stoichiometry ξc is represented by the ratio of the amount of oxygen contained in the gas flowing to the cathode to the theoretically required amount of oxygen calculated from the equation (2). For example, assuming that the current value flowing through power generation is I (A), the cathode gas flow rate is F (mL / min), and the oxygen content is Vo (%),
ξc = F / (I * 60/96500/6 * 1.5 * 22400 / (Vo / 100))
(7)
It is possible to ask for.

  The standard operation condition, the special operation condition, and the number of repetitions may be at least twice. There is no particular limitation on the upper limit of the number of repetitions.

  In special operation conditions, the temperature conditions need not be the same each time, and the temperature conditions may be different each time. Similarly, under special operating conditions, the cathode gas flow rates need not be the same for each time, and the cathode gas flow rates for each time may be different.

  When changing the operating conditions of the fuel cell (when changing from standard operating conditions to special operating conditions), if the temperature conditions are changed, the temperature conditions may be changed from 5 ° C to 30 ° C, preferably from about 10 ° C to 20 ° C. preferable. Further, when changing the operating condition of the fuel cell, when changing the cathode gas flow rate, it is preferable to change it to 1.05 to 3 times, preferably 1.2 to 2.5 times the standard operating conditions.

  The cathode stoichiometric ratio ξ0 corresponding to the cathode flow rate F0 is 1 ≦ ξ0, preferably 1.2 ≦ ξ0, and the cathode stoichiometric ratio ξ1 corresponding to the cathode flow rate F1 is ξ1 ≦ 4, preferably ξ1 ≦. . If ξ0 and ξ1 do not satisfy 1 ≦ ξ0 and 1 ≦ ξ1, the theoretically necessary amount of oxygen cannot be supplied, so power generation cannot be performed stably, and ξ0 ≦ 4 and ξ1 ≦ 4 are satisfied. If not, it is not preferable because not only the amount of water discharged on the cathode side increases but also the drying of the electrolyte membrane is accelerated and the life of the fuel cell is shortened. It should be noted that the cathode stoichiometric ratios in each round need not be the same and may be different.

FIG. 7 is a schematic diagram showing a state of temperature control and fuel flow rate control in the third power generation control method according to the present invention. 7A and 7B are schematic diagrams when FIG. 1A changes only the temperature, FIG. 7B shows a case where only the cathode flow rate is changed, and FIG. 7C shows a case where both the temperature and the cathode flow rate are changed. Taking Figure (a) as an example, standard operating conditions (temperature T0, cathode gas flow rate F0, time tn),
Special operating conditions (temperature T1, cathode gas flow rate F0, time ta),
A power generation control method comprising repeatedly performing the above is shown.

  The ratio between tn and ta at each time is within a range of ta / (tn + ta) ≦ 30%, preferably within a range of ta / (tn + ta) ≦ 25%, particularly preferably within a range of ta / (tn + ta) ≦ 20%. Controlled within. If it exceeds 30%, not only the amount of water discharged on the cathode side increases, but also the drying of the electrolyte membrane is accelerated and the life of the fuel cell is shortened. If it is 20% or less, the water recovery mechanism can be simplified or omitted. The lower limit of ta / (tn + ta) is preferably 15%, particularly preferably 10% or less.

  In a direct methanol fuel cell in which α is less than 1, when power generation is performed for a long time under a constant condition as in the conventional example, the voltage drops because water accumulates in the anode catalyst layer as described above. . However, even in such direct methanol fuel cells where α is less than 1, according to the first to third power generation control methods according to the present invention, water discharge on the cathode side is facilitated, and as a result It becomes possible to prevent the accumulation of water in the anode catalyst layer by reducing the return of water from the cathode electrode to the anode electrode.

  Hereinafter, in order to facilitate understanding of the present invention, a detailed description will be given using examples.

<Example 1>
A mixture of a Pt / Ru alloy catalyst (Pt / Ru “Black Hisspec 6000”) manufactured by Johnson & Matthey and a perfluorocarbon sulfonic acid solution (Nafion solution “Aldrich SE-20092” manufactured by Dupont) as an anode catalyst layer. As a cathode catalyst layer, a commercially available perfluorocarbon sulfonic acid membrane (Dupont) was prepared by mixing and dispersing a Pt / C catalyst (HP 40 wt% Pt on Vulcan XC-72R) manufactured by E-TEK and a perfluorocarbon sulfonic acid solution (same as above). CCM (Catalyst Coated Membrane) was produced by thermocompression bonding on Nafion 112). On the other hand, TGPH-090 manufactured by Toray Industries, Inc. was used as the anode GDL, and a slurry of which viscosity was adjusted by mixing and dispersing a carbon powder (VULCAN XC-72R) manufactured by CABOT and a water-repellent PTFE 60 wt% emulsion onto the GDL. It was applied by the method. After drying at 100 ° C. and firing at 350 ° C., an anode GDL layer with an anode MPL layer was produced. As the cathode GDL, commercially available Elat GDL LT-2500-W (MPL layer already formed) manufactured by E-TEK was used. The above-mentioned CCM, anode GDL with anode MPL, and cathode GDL with cathode MPL were overlapped and thermocompression bonded at, for example, 125 ° C. and 5 kg / cm ² for 1 minute to produce an MEA (3 cm × 4 cm) having the same configuration as FIG. .

  The obtained MEA was installed using a seal material made of Teflon between two separators in which the same serpentine-like flow path as in FIG. 2 was formed, and a power generation test was performed with the configuration shown in FIG. An aqueous methanol solution with an initial concentration of 1.0 mol / L is pumped as a fuel at a flow rate of 1.0 mL / min on the anode side, and air in the atmosphere is fed to the cathode side at a rate of 60 mL / min using a small pump. The air was supplied, and a current of 1.8 A was drawn using an electronic loader while heating to 60 ° C. with a heater, and electricity was generated while measuring and recording the voltage with a personal computer. The fuel concentration was changed to a higher concentration by 0.1 mol / L every time the power generation test time was 100 hours, and increased to 1.5 mol / L after 500 hours. Although the initial α value was 43 V and the initial α value was 0.1, the voltage after 500 hours was 0.39 V and the α value was slightly changed to 0.2, but stable power generation was possible. .

<Comparative Example 1>
When a power generation test was performed on the MEA produced under the same conditions as in Example 1 under the same conditions except that the fuel concentration was kept constant at 0.1 mol / L, the voltage rapidly decreased to 0.35 V or less after 145 hours. As a result, the current of 1.8 A could not be passed.

<Example 2>
A MEA produced under the same conditions as in Example 1 was set in a jig having the configuration shown in FIG. The power generation conditions were as follows: fuel was supplied to the anode electrode at a methanol fuel concentration of 1.0 mol / L and a feed rate of 1.2 mL / min, and dry air was supplied to the cathode electrode at 65 mL / min to set the heater temperature to 60 ° C. A power generation test was conducted while pulling a current of 1.8 A with an electronic load machine while controlling at a constant speed. The methanol fuel was supplied while being adjusted to a predetermined concentration using an automatic adjusting machine that mixed pure methanol and pure water with an ultrasonic methanol concentration meter. The voltage was measured and recorded using a personal computer. Although the initial voltage was as high as 0.45 V, the voltage suddenly dropped after 160 hours and became the threshold voltage Vs of 0.33 V or less, so the methanol fuel concentration was changed from 1.0 mol / L to 1.2 mol / L. As a result, the power generation test was continued after recovering to VL = 0.35V to 0.42V. Further, after 350 hours, the voltage suddenly decreased again and fell below the threshold voltage of 0.33 V. Therefore, when the methanol fuel concentration setting value was changed to 1.5 mol / L again, the voltage recovered to 0.40 V, and after 500 hours, the voltage recovered to 0. The power generation was stopped when it became 38V. The α value was 0.15 at the initial stage and 0.22 after 500 hours.

<Example 3>
Four 24 cm ² MEAs prepared by the same method as in Example 1 were prepared, and each was installed using a Teflon sealing material between two thin separators each having a serpentine-shaped flow path. Four MEAs sandwiched between two sheets were laminated. The methanol aqueous solution was fed with a small pump and sent all the way to the front of the fuel supply port of each cell, and then branched into four just before the supply port so that it could be uniformly supplied to each cell. Similarly, air was sent all the way to the front of the gas supply port of each cell and branched into four immediately before the supply port so that it could be uniformly supplied to each cell. The aqueous methanol solution had a concentration of 1.5 mol / L, and the amount of liquid fed was 6 mL / min for the entire four cells. The methanol fuel was supplied while being adjusted to a predetermined concentration using an automatic adjusting machine that mixed pure methanol and pure water with an ultrasonic methanol concentration meter. The cathode air supply rate was 600 mL / min for the entire four cells. The heater was controlled so that the power generation temperature was 60 ° C. While taking an electric current of 3.6 A with an electronic load machine, a power generation test was performed while measuring and recording all voltages of four cells with a personal computer. On the way, when the power generation test was continued for a long time while the cathode flow rate was increased to 700 mL / min at a rate of 1 hour every 6 hours during the power generation test and returned to 600 mL / min after 1 hour, the initial voltage was 1.85V. At the beginning, the voltage after 500 hours dropped to 1.60 V, but no sudden voltage drop phenomenon was observed, and a stable output could be obtained. The α value was 0.08 in the initial stage and 0.21 after 500 hours.

<Comparative example 2>
A 4-cell stack produced in the same manner as in Example 3 was subjected to power generation for a long time at a constant flow rate of 600 mL / min without controlling to increase the cathode air flow rate at a rate of 1 hour per 6 hours. After that, the voltage suddenly decreased to 1.2 V or less, and a current of 3.6 A could not be taken stably. The α value was 0.11 at the initial stage and 0.17 after 350 hours.

<Example 4>
A power generation test was performed on a 4-cell stack produced in the same manner as in Example 3 while measuring and recording the total voltage of 4 cells with a personal computer while taking a current of 3.6 A with an electronic load machine. On the way, when the power generation test was continued for a long time with the control of increasing the power generation temperature to 70 ° C. at a rate of 2 hours every 10 hours of the power generation test and returning to 60 ° C. after 2 hours, the initial voltage started at 1.80 V The voltage after 500 hours dropped to 1.50 V, but no sudden voltage drop phenomenon was observed, and the output could be taken stably. The α value was 0.12 at the initial stage and 0.19 after 500 hours.

As described above, according to the power generation control method of the present invention, it is possible to provide a direct methanol fuel cell capable of generating power stably for a long time even when using an electromotive member MEA having a water permeability coefficient α of 1 or less. Can do.

The figure which shows the principal part of the electromotive member (MEA) of a direct methanol type fuel cell. The figure which shows an example of the separator applied to a direct methanol type fuel cell. The figure which shows the example of a setting at the time of generating electric power with a direct methanol type fuel cell. The figure which shows the preferable structure of the direct methanol type fuel cell to which the electric power generation control method by this invention is applied. The schematic diagram which shows the mode of the fuel concentration control in the 1st electric power generation control method by this invention. The schematic diagram which shows the mode of the fuel concentration control in the 2nd electric power generation control method by this invention. The schematic diagram which shows the mode of the temperature control and fuel flow control in the 3rd electric power generation control method by this invention.

Claims (4)

A membrane electrode assembly comprising a solid polymer electrolyte membrane sandwiched between an anode electrode, a cathode electrode, and both electrodes;
Fuel supply mechanism to the anode,
Fuel concentration control mechanism,
A fuel cell having a gas supply mechanism to a fuel tank and a cathode,
The ratio α of the number y of water molecules moving accompanying the number x of hydrogen ions (protons) moving from the anode to the cathode with power generation in the membrane electrode assembly [where α = y / x. . In a direct methanol fuel cell in which is 1 or less,
A fuel cell power generation control method, wherein the concentration control of the fuel concentration control mechanism is performed according to the following equation (a).
0 ≦ ((C1-C0) / C0) / ((P0−P1) / P0) ≦ 20 (a)
[Where C0 is the methanol concentration in the methanol fuel at the beginning of power generation, C1 is the methanol concentration in the methanol fuel after elapse of t hours from the start of power generation, and P0 is the initial output (voltage x current), P1 is the output (voltage × current) after elapse of t time. ] A membrane electrode assembly comprising a solid polymer electrolyte membrane sandwiched between an anode electrode, a cathode electrode, and both electrodes;
Fuel supply mechanism to the anode,
Fuel concentration control mechanism,
A fuel cell having a gas supply mechanism to a fuel tank and a cathode,
The ratio α of the number y of water molecules moving accompanying the number x of hydrogen ions (protons) moving from the anode to the cathode with power generation in the membrane electrode assembly [where α = y / x. . In a direct methanol fuel cell in which is 1 or less,
When it is detected that the voltage V output from the fuel cell is equal to or lower than a preset threshold voltage Vs, the methanol concentration C of the fuel supplied to the anode is increased by a preset increment ΔC. A method for controlling the power generation of a direct methanol fuel cell, wherein concentration control of the fuel concentration control mechanism is performed until the fuel cell reaches a minimum voltage VL that can be used in a steady state.
[Here, the threshold voltage Vs is lower than the lowest voltage VL that the fuel cell can use in a steady state. ] A membrane electrode assembly comprising a solid polymer electrolyte membrane sandwiched between an anode electrode, a cathode electrode, and both electrodes;
Fuel supply mechanism to the anode,
Fuel concentration control mechanism,
Fuel tank,
A fuel cell having a gas supply mechanism to a cathode and a temperature control mechanism,
The ratio α of the number y of water molecules moving accompanying the number x of hydrogen ions (protons) moving from the anode to the cathode with power generation in the membrane electrode assembly [where α = y / x. . In a direct methanol fuel cell in which is 1 or less,
The power generation of the direct methanol fuel cell is as follows: (a) standard operating conditions in which power generation is continued for tn time at temperature T0 and cathode gas flow rate F0; and (b) power generation is performed at temperature T1 and cathode gas flow rate F1 for ta time. The standard operation condition and the special operation condition are alternately controlled while the ratio of tn and ta is controlled to be ta / (tn + ta) ≦ 30%. A method for controlling power generation of a direct methanol fuel cell, characterized in that the method is carried out repeatedly.
[Temperature T0 and temperature T1 are both 40 ° C. or higher and 90 ° C. or lower, and at least one of temperature T1 and cathode gas flow rate F1 under special operating conditions is higher than temperature T0 and cathode gas flow rate F0 under standard operating conditions. high. The cathode stoichiometric ratios ξ0 and ξ1 corresponding to the cathode flow rates F0 and F1 satisfy 1 ≦ ξ0 ≦ ξ1 ≦ 4. ]   A direct methanol fuel cell in which the power generation control method for a direct methanol fuel cell according to any one of claims 1 to 3 is implemented.

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