JP2006278173A - Direct methanol type fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct methanol type fuel cell of which a factor in lowering power generation efficiency and stability is reduced. <P>SOLUTION: The direct methanol type fuel cell is composed of a film-electrode assembly having an anode, a cathode, and a proton transmitting film pinched between them; a container containing at least dimethylether and methanol, maintaining inside pressure imparted by vapor pressure of the fuel; and a fuel supply passage supplying the fuel to the anode by the above pressure. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a direct methanol fuel cell.

  A direct methanol fuel cell (hereinafter referred to as DMFC) is a type of fuel cell that uses methanol directly as fuel. A typical DMFC includes a membrane electrode assembly including an anode, a cathode, and a proton permeable membrane sandwiched between them. Methanol or a mixture of methanol and water is supplied as fuel to the anode, and air is supplied as oxidant to the cathode, thereby generating electric power. As a result of power generation, carbon dioxide is produced at the anode and water is produced at the cathode. Such DMFC technology is disclosed in Patent Document 1, for example.

DMFC has the potential advantage of being lightweight and compact because it does not require a reformer to extract hydrogen. However, since the fuel is a liquid, a liquid delivery means such as a pump is necessary, and the potential advantage is greatly reduced. In addition, since the liquid feeding means such as a pump includes a drive portion, a part of the power generated by the membrane electrode assembly must be used for the drive, which causes a decrease in the power generation efficiency of the entire DMFC. Patent Documents 2 and 3 disclose a technique of sending liquid using pressurized gas such as inert gas instead of a pump.
JP 2004-127659 A JP-A-6-310166 JP 2003-317755 A

  According to the techniques disclosed in Patent Documents 2 and 3, a DMFC can be configured by reducing the number of liquid feeding devices such as pumps. However, a cylinder for storing the pressurized gas is required, and it is still difficult to configure the DMFC in a lightweight and compact manner. Furthermore, the inert gas may be mixed into the fuel to form a gas-liquid two-phase flow, causing a problem that the inflow of fuel to the anode becomes uneven and power generation becomes unstable.

  A direct methanol fuel cell according to one aspect of the present invention contains a membrane electrode assembly including an anode, a cathode, and a proton permeable membrane sandwiched between the anode, a fuel containing at least dimethyl ether and methanol, and the fuel A container for maintaining the internal pressure by the vapor pressure of the fuel, a fuel supply path for allowing the fuel to flow out from the container toward the anode by the pressure, and a vapor of dimethyl ether provided in the fuel supply path. Separating means.

  A direct methanol fuel cell according to one aspect of the present invention contains a membrane electrode assembly including an anode, a cathode, and a proton permeable membrane sandwiched therebetween, and a fuel containing at least dimethyl ether and methanol. A container that holds an internal pressurization due to the vapor pressure of the fuel, a fuel supply passage that supplies the fuel to the anode by the pressurization, and a separation unit that is provided in the fuel supply passage and separates dimethyl ether vapor. Is provided.

  A DMFC having high power generation efficiency and stability is realized.

  In order to solve the above-mentioned problems, the present inventors diligently studied a pressurizing means not using an inert gas or the like, and focused on dimethyl ether. Dimethyl ether has a boiling point of −24.8 ° C. and a saturated vapor pressure that greatly exceeds 1 atm near room temperature. It is soluble in both methanol and water, and can be adjusted to an appropriate vapor pressure using the vapor pressure drop by mixing with these in an appropriate ratio. Moreover, since the vapor | steam of dimethyl ether can be easily isolate | separated from a liquid phase using a gas-liquid separation membrane, it can prevent flowing in into an anode with a gas-liquid two-phase flow. Further, even when dimethyl ether reaches the anode while being dissolved in the fuel, since dimethyl ether does not contain a C—C bond in the molecule, carbon does not deposit on the anode, and deterioration thereof can be avoided. The present inventors have conceived that the adjusted vapor pressure is used as a pressurizing pressure for discharging fuel from a fuel tank, and that a gas-liquid separation membrane prevents the generation of a gas-liquid two-phase flow. The present invention has been made by constructing a suitable fuel cell system.

  A first embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a schematic diagram of a direct methanol fuel cell according to the present embodiment.

  The fuel cell system 1 includes a membrane electrode assembly 3 including an anode 5 (fuel electrode) and a cathode 7 (air electrode), a fuel tank 9, and a fuel supply path that connects the fuel tank 9 and the anode 5. The fuel supply path includes a connection path 13, a discharge path 17, and a fuel supply path 15, and is a flow path partially looped by connecting the outlet of the anode 5 to the connection path 13. The fuel cell system 1 further includes a mixing tank 11, a gas-liquid separator 28, an anode-side cooler 29, and a cathode-side cooler 33, and the gas-liquid separator 28, the anode-side cooler 29, and the like. The mixing tank 11 is a part of a looped portion of the fuel supply path.

  The fuel tank 9 is a container for storing a mixed liquid containing at least methanol and dimethyl ether as fuel of the fuel cell system 1 and further containing water as necessary, as will be described later. The fuel tank 9 may be configured to be detachable from other parts of the direct methanol fuel cell. As an example of such a configuration, a detachable joint is provided between the fuel tank 9 of the connection path 13 and an on-off valve V1 described later so as to be kept airtight with the outside. The mixing tank 11 is a container for receiving the fuel and mixing and storing it with a fluid containing water discharged from the membrane electrode assembly 3.

  The membrane electrode assembly 3 includes a proton permeable membrane, an anode 5 (fuel electrode) and a cathode 7 (air electrode) laminated on both surfaces thereof. In FIG. 1, for convenience of explanation, the anode 5 and the cathode 7 are drawn separately, and the proton permeable membrane interposed therebetween is omitted. Although omitted in FIG. 1, each of the anode 5 and the cathode 7 is provided with a current collector, and the generated electric power is extracted to an external electric wire (not shown). The proton permeable membrane is made of a resin having proton conductivity. As an example of such a resin, a copolymer of tetrafluoroethylene and perfluorovinyl ether sulfonic acid can be exemplified, but in place of this, other appropriate resins having proton conductivity can be applied. . For convenience of explanation, a case where a set of membrane electrode assemblies is included will be described. Of course, the present invention can be applied to a fuel cell stack in which a plurality of sets of membrane electrode assemblies are stacked.

  The fuel tank 9 is a substantially sealed container so as to maintain an internal pressure due to the vapor pressure of the liquid inside. A connection path 13 including an on-off valve V1 connects the fuel tank 9 and the anode side cooler 29. A gas-liquid separation unit 28 including a gas-liquid separation film 27 is provided between the connection path 13 and the anode side cooler 29. The gas-liquid separation unit 28 has a liquid pool with an appropriate depth, and a gas-liquid separation wall 28A is separated between the gas-liquid separation unit 28 and the anode-side cooler 29. By ensuring communication only in the lower part, only the liquid phase can flow to the anode-side cooler 29. The upper part of the gas-liquid separation unit 28 is provided with a gas-liquid separation film 27, and is further connected to an exhaust path 27A for guiding the gas phase separated from the liquid phase to a discharge path 25 described later.

  The mixing tank 11 is also provided with a gas-liquid separation membrane 11A, and an exhaust passage 21 is similarly connected thereto. The exhaust passage 21 merges with the exhaust passage 27A, and further includes an opening / closing valve V2 on the downstream side thereof. The merged exhaust passage 21 joins a later-described exhaust passage 25 further downstream of the opening / closing valve V2.

  The mixing tank 11 is connected to the anode 5 by a fuel supply path 15 in which a pump P1 is interposed. A discharge port of the anode 5 is in communication with the connection path 13, and an exhaust (fluid containing a gas phase and a liquid phase) containing methanol, water, and carbon dioxide from the anode 5 is supplied from the fuel tank 9. It is mixed and introduced into the anode side cooler 29.

  The anode side cooler 29 includes a plurality of heat radiating fins 29A, and the internal fluid is cooled by aeration. Although not shown, a blower can be provided as appropriate. The cooling efficiency can be adjusted by adjusting the amount of ventilation by the blower.

  The fuel cell system 1 further includes an air supply path 23 for supplying outside air to the cathode 7. The air supply path 23 includes a filter 31, a pump P2, and an on-off valve V3 in order from the upstream side.

  The discharge port of the cathode 7 is connected to the discharge path 25, and the discharge path 25 joins the exhaust path 21 and is connected to the cathode side cooler 33. Similarly to the anode side cooler 29, the cathode side cooler 33 includes a plurality of heat radiation fins 33A for cooling the internal fluid. The cathode side cooler 33 further includes a water recovery tank 35 so that the liquid condensed by cooling is collected. The water recovery tank 35 is connected to a connection path 41 having a pump P3 and a check valve CV, and communicates with the mixing tank 11. The cathode side cooler 33 includes an exhaust passage 37 for discharging the gas after being cooled and the liquid component being removed from the fluid to the outside air. The exhaust passage 37 includes a VOC removal means 39 for removing volatile organic substances (VOC) and an open / close valve V4 downstream thereof, and communicates with the outside air. The VOC removal means 39 has a built-in oxidation catalyst and oxidizes dimethyl ether or methanol.

  The fuel tank 9 stores a mixed liquid containing methanol and further dimethyl ether as fuel. The mixing ratio can be determined in consideration of a pressure drop in the connection path 13 or the like, and it can be exemplified that 5% dimethyl ether is mixed with methanol in a weight ratio. As described above, since the fuel tank 9 is configured to maintain the internal pressure, the fuel flows out of the fuel tank 9 by opening the opening / closing valve V1 with the adjusted vapor pressure as a pressurization. The vapor pressure is uniquely determined by the mixing ratio and temperature, and can be stably supplied without being affected by conditions such as the remaining amount of fuel.

  The fuel outflow rate can be adjusted by the opening degree of the on-off valve V1. The vapor pressure of the mixed liquid varies depending on the shape of the connection path 13 and the power generation capacity, but for example, 1 kPa or more is sufficient. In addition, even when the environmental temperature changes, the amount of dimethyl ether is set in advance so that a sufficient vapor pressure can be obtained at the assumed operating temperature so as not to hinder the fuel supply. Since methanol is used for power generation, the fuel tank 9 can be made more compact when the value of dimethyl ether in the mixed solution is lower in the range where the required vapor pressure can be obtained.

  Since the fuel discharged from the fuel tank 9 passes through the connection path 13, a pressure loss occurs and the pressure decreases, so that a part of dimethyl ether is vaporized and a gas-liquid two-phase flow state including a gas phase of dimethyl ether. It becomes. The vapor phase of dimethyl ether is separated from the liquid phase of the fuel by the gas-liquid separation membrane 27 and exhausted to the outside air via the exhaust path 27A. When the vapor of dimethyl ether passes through the gas-liquid separation membrane 27, a pressure loss is generated according to the type and area of the gas-liquid separation membrane 27 and the flow rate of dimethyl ether. If a path from the exhaust path 27A to the outside is set so that the pressure loss becomes, for example, 50 Pa or less, dimethyl ether can be effectively exhausted from the anode circulation path.

  As described above, since the outlet of the anode 5 is connected to the connection path 13 of the fuel supply path, carbon dioxide and unreacted methanol as reaction products are also introduced into the anode-side cooler 29. Carbon dioxide generated at the anode during power generation also becomes a gas phase, is separated from the liquid phase by the gas-liquid separation membrane 27, and is exhausted to the outside air via the exhaust path 27A. Carbon dioxide also causes a pressure loss when passing through the gas-liquid separation membrane 27. For example, if a path leading to the discharge from the exhaust path 27A is set so that the pressure loss of the carbon dioxide is 50 Pa or less, the carbon dioxide can be effectively exhausted from the anode circulation path.

  The pressure loss in the gas-phase gas-liquid separation film 27 pressurizes the gas-liquid separation unit 28 and the anode side cooler 29. The pressurization in the gas-liquid separator 28 and the anode side cooler 29 is appropriately adjusted by the resistance of the gas-liquid separation film 27 independently of the pressurization in the fuel tank 9. When the pressure loss of the gas-liquid separation film 27 becomes larger than the liquid height h of the liquid pool of the gas-liquid separation unit 28, the gas-liquid separation unit 28 is passed in a gas-liquid two-layer flow state. Therefore, the pressure loss of the gas-liquid separation membrane 27 is made smaller than the liquid hydrostatic pressure ρgh. Here, ρ: liquid density, g: gravitational acceleration.

  The fuel that has become a liquid phase is cooled in the anode side cooler 29, then flows out into the discharge path 17, and is introduced into the mixing tank 11. Here, as will be described later, it is mixed with water recovered from the cathode 7, and further the gas phase is removed by the gas-liquid separation membrane 11 A, receiving the discharge force from the pump P 1, and going to the anode 5 via the fuel feed path 15. Supplied. The pressure loss when dimethyl ether is separated by the gas-liquid separation membrane 27 is supplied by the pump P1. Considering the pressure loss of the anode side cooler 29 and the anode 5, the discharge pressure of the pump P1 requires, for example, several hundred Pa to several kPa. Therefore, the pressure loss at the time of gas-liquid separation has little influence on the power consumption of the pump P1. Further, since the pressure loss when separating dimethyl ether by the gas-liquid separation membrane 27 also affects the pressure inside the mixing tank 11, the pressure on the inlet side of the pump P1 is also increased and the power consumption of the pump P1 for separating dimethyl ether is increased. There is almost no increase.

  At the same time as described above, the open / close valve V3 is opened to drive the pump P2, so that the outside air that has passed through the filter 31 is supplied to the cathode 7.

  An oxidation reaction occurs at the anode 5 and a reduction reaction occurs at the cathode 7, whereby electrons are transferred. Electric power is generated by taking it out by the current collector.

  When a reduction reaction occurs at the cathode 7, water is generated. The water flows out to the discharge path 25 together with the unreacted air, and flows into the cathode side cooler 33 together with the gas containing dimethyl ether and carbon dioxide flowing from the exhaust path 21. In the cathode side cooler 33, water is mainly condensed by cooling and is collected in the water collection tank 35. The recovered water is introduced into the mixing tank 11 via the connection path 41 and mixed with the fuel as described above to maintain the methanol aqueous solution concentration at the anode at an appropriate value with high power generation efficiency of, for example, about 1 to 3M. The gas from which the water has been collected is discharged into the outside air through the exhaust passage 37 and the open / close valve V4 after the volatile organic material such as dimethyl ether is burned and removed by the VOC removal means 39.

  As can be understood from the above description, an autonomously controlled pressurization occurs in the fuel tank, and fuel is discharged by the pressurization, so that the fuel supply from the fuel tank to the anode side cooler and the mixing tank There is no need for a pump on the road. Therefore, the direct methanol fuel cell system can have a simpler configuration. Further, by separating the vapor of dimethyl ether by the gas-liquid separation membrane, the fuel supplied to the anode is prevented from being destabilized as a gas-liquid two-phase flow.

  In this embodiment, a gas-liquid separation membrane was used for vapor separation of dimethyl ether, but regardless of the gas-liquid separation membrane, for example, a fuel containing dimethyl ether is supplied to a mixing tank or the like whose top is open to the outside air, It is also possible to release dimethyl ether to the outside air using gravity.

  A second embodiment of the present invention will be described below with reference to FIG. FIG. 2 is a schematic diagram of the direct methanol fuel cell according to the present embodiment. Unlike the first embodiment, the flow path is provided only on the anode side, and the cathode 7 is open to the outside air.

  Similar to the first embodiment described above, the fuel tank 9 stores a mixed liquid containing at least methanol and dimethyl ether, and further containing water as necessary. The mixing ratio of methanol and dimethyl ether can be determined as appropriate, and may be determined so as to obtain an appropriate vapor pressure (pressurization in the fuel tank 9) in consideration of a pressure drop in the connection path 13 or the like. As an example, mixing 5% dimethyl ether by weight with respect to methanol can be exemplified.

  As described above, since the fuel tank 9 is configured to maintain the internal pressure, the fuel flows out of the fuel tank 9 by opening the opening / closing valve V1 with the adjusted vapor pressure as a pressurization. The vapor pressure is uniquely determined by the mixing ratio and temperature, and can be stably supplied without being affected by conditions such as the remaining amount of fuel. The fuel outflow rate can be adjusted by the opening degree of the on-off valve V1. The vapor pressure of the liquid may be, for example, 1 kPa or more, but the vapor pressure at room temperature is set so as not to hinder the fuel supply even when the environmental temperature changes. Since methanol is used for power generation in the fuel cell, the amount of dimethyl ether in the fuel is preferably lower in the concentration range where the necessary vapor pressure can be obtained.

  The fuel tank 9 is connected to the anode 5 by a connection path 13 provided with an on-off valve V1, a gas-liquid separator 28 in which liquid is accumulated, and a fuel feed path 15 provided with an on-off valve V5. The upper part of the gas-liquid separation unit 28 is provided with a gas-liquid separation film 27, and is further connected to an exhaust path 27A for guiding the gas phase separated from the liquid phase to the outside.

  Since the fuel discharged from the fuel tank 9 passes through the connection path 13, a pressure loss occurs and the pressure decreases, so that a part of dimethyl ether is vaporized and a gas-liquid two-phase flow state including a gas phase of dimethyl ether. It becomes. The vapor phase of dimethyl ether is separated from the liquid phase of the fuel by the gas-liquid separation membrane 27 and exhausted to the outside air via the exhaust path 27A. Only the liquid phase is supplied to the fuel supply path 15.

  The pressure of the fuel supplied to the fuel supply path 15 is adjusted by the opening degree of the opening / closing valve V5. For example, the pressure of the fuel tank 9 is set to 1 kPa, and the pressure of the fuel supply path 15 is set to 100 Pa. The fuel is supplied to the anode 5 in a liquid phase of methanol or a mixture of methanol and water under a regulated pressure.

  On the other hand, the cathode 7 is open to the outside air, and air is supplied from the outside air by diffusion. Then, an oxidation reaction occurs at the anode 5 and a reduction reaction occurs at the cathode 7, whereby electrons are transferred. Electric power is generated by taking it out by the current collector.

  The oxidation-reduction reaction produces water and carbon dioxide, which are released to the outside air by diffusion.

  With the above configuration, after effectively removing dimethyl ether used for fuel preload from the fuel, it is possible to efficiently deliver methanol or an aqueous methanol solution to the anode without using a pump. .

  Although the present invention has been described with reference to preferred embodiments thereof, the present invention is not limited to the above-described embodiments. Based on the above disclosure, a person having ordinary skill in the art can implement the present invention by modifying or modifying the embodiment.

1 is a schematic diagram of a direct methanol fuel cell according to a first embodiment of the present invention. It is a schematic diagram of the direct methanol type fuel cell by the 2nd Embodiment of this invention.

Explanation of symbols

1: Fuel cell system 3: Membrane electrode assembly 5: Anode (fuel electrode)
7: Cathode (air electrode)
9: Fuel tank 11: Mixing tank 11A: Gas-liquid separation membrane 13: Connection path 15: Fuel supply path 17: Discharge path 21: Exhaust path 23: Air supply path 25: Discharge path 29: Anode-side cooler 29A: Fin 27: Gas-liquid separation membrane 27A: Exhaust passage 31: Filter 33: Cathode side cooler 33A: Fin 35: Water recovery tank 37: Exhaust passage 39: VOC removal means 41: Connection passage CV: Check valves P1 to P3: Pump V1 to V5: Open / close valve

Claims (5)

A membrane electrode assembly comprising an anode, a cathode, and a proton permeable membrane sandwiched therebetween,
A container containing a fuel containing at least dimethyl ether and methanol, and holding an internal pressurization due to a vapor pressure of the fuel;
A fuel supply path for causing the fuel to flow out of the container toward the anode by the pressurization;
Separating means for separating the vapor of dimethyl ether provided in the fuel supply path;
A direct methanol fuel cell.   2. The direct methanol fuel cell according to claim 1, wherein the separation means is a gas-liquid separation membrane.   3. The direct methanol fuel cell according to claim 1, further comprising combustion means for combusting the vapor of the separated dimethyl ether.   4. The direct methanol fuel cell according to claim 1, wherein the fuel supply path is further connected to a discharge port of the anode, and the separation means also separates carbon dioxide generated at the anode. A direct methanol fuel cell. A membrane electrode assembly comprising an anode, a cathode, and a proton permeable membrane sandwiched therebetween,
A container containing a fuel containing at least dimethyl ether and methanol, and holding an internal pressure by the vapor pressure of the fuel;
A fuel supply path for supplying the fuel to the anode by the pressurization;
Separating means for separating the vapor of dimethyl ether provided in the fuel supply path;
A direct methanol fuel cell.

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