JP2009076272A - Fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell capable of supplying liquid fuel of a required flow down to terminal ends of branched channels of a fuel distribution mechanism without bringing forth air bubble plugging, and alleviating a load of a liquid-sending pump. <P>SOLUTION: The fuel battery is provided with a membrane electrode assembly 2 comprising a fuel electrode 13, an air electrode 16, and an electrolyte membrane 17, a fuel distribution mechanism 3, a fuel housing part 4, and a supply flow channel 5 connecting the fuel housing part with the fuel distribution mechanism. The fuel distribution mechanism 3 is provided with a fuel injection port 31 communicated with the supply flow channel, a main channel 20 with a given flow channel cross section uniformly continued from the fuel injection port, a plurality of fuel exhaust ports opened in opposition to the fuel electrode, and a plurality of branched channels 21 to 27 with a given flow channel resistance fitted midway between the main channel and the fuel exhaust channel and with a flow channel cross section shape and a branching structure adjusted in movement from an upstream side toward a downstream side between the main channel and the fuel exhaust ports. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fuel cell having a planar arrangement effective for the operation of a portable device, and more particularly to an internal vaporization type direct methanol fuel cell (DMFC).

  In recent years, various electronic devices such as personal computers and mobile phones have been miniaturized with the development of semiconductor technology, and attempts have been made to use fuel cells as power sources for these small devices. Fuel cells have the advantage that they can generate electricity simply by supplying fuel and oxidant, and can continuously generate electricity if only the fuel is replenished / replaced. For this reason, if the size can be reduced, it can be said that the system is extremely advantageous for the operation of the portable electronic device. Direct methanol fuel cells (DMFCs), in particular, use methanol with a high energy density as the fuel, and can extract current directly from the methanol on the electrocatalyst, enabling downsizing and handling of fuel compared to hydrogen gas fuel. Since it is easy, it is promising as a power source for small devices, and its practical application is expected as an optimal power source for cordless portable devices such as mobile phones, portable audio devices, portable game machines, and notebook computers.

  The DMFC fuel supply method includes gas supply type DMFC that vaporizes liquid fuel and then feeds it into the fuel cell with a blower, etc., liquid supply type DMFC that feeds liquid fuel directly into the fuel cell with a pump and the like, and liquid fuel An internal vaporization type DMFC that vaporizes in a cell is known.

  Among these, the internal vaporization type DMFC is disclosed in, for example, Patent Document 1, in which a membrane electrode assembly (MEA) having a fuel electrode, an electrolyte membrane, and an air electrode is placed on a fuel containing portion made of a resin box-like container. Arranged structures have been proposed. When supplying fuel vaporized directly from the fuel storage part to the MEA, it is important to improve the controllability of the fuel cell output, but the conventional internal vaporization DMFC does not always provide sufficient output controllability. Is the actual situation.

  On the other hand, in Patent Documents 2 to 4, it is proposed that the MEA of the DMFC and the fuel storage portion are connected via a flow path. By supplying the liquid fuel supplied from the fuel storage unit to the MEA via the flow path, the supply amount of the liquid fuel can be adjusted based on the shape, diameter, and the like of the flow path. However, depending on the supply structure of the liquid fuel from the flow path, the fuel supply state to the MEA may become uneven, and the output of the fuel cell may be reduced. For example, when liquid fuel is allowed to flow along the flow path in the groove, the fuel concentration is reduced on the outlet side of the flow path because the fuel is consumed sequentially as the liquid fuel flows in the flow path. For this reason, the power generation reaction is reduced near the MEA flow path outlet, and as a result, the output is reduced.

  Patent Document 3 proposes a fuel cell system that supplies liquid fuel from a fuel storage unit to an MEA through a flow path by a pump. Patent Document 3 also describes using an electric field forming means (electroosmotic flow pump) for forming an electroosmotic flow in the flow path instead of the general-purpose pump.

Patent Document 4 proposes a fuel cell system that supplies liquid fuel using an electroosmotic pump. Although the pump is effective in the fuel cell to which the fuel circulation structure is applied, if the fuel is not circulated like the internal vaporization type DMFC, even if the pump is applied, the fuel consumption will increase and the entire MEA will be increased. It is difficult to cause a uniform power generation reaction in
International Publication No. 2005/112172 Pamphlet JP 2005-518646 A JP 2006-089552 A US Patent Application Publication No. 2006/0029851

  However, in the conventional internal vaporization type DMFC, in order to supply the liquid fuel evenly over the entire surface of the MEA fuel electrode, a distribution plate is disposed immediately before the gas-liquid separation membrane in the vaporization chamber and is formed inside the distribution plate. Although liquid fuel is allowed to flow through a number of branch passages, the pressure loss in the branch passages is large, so that it is necessary to increase the pump back pressure, and the load on the pump is large. Further, if the pump back pressure is excessively large, bubbles are likely to be generated in the flow path, which may cause so-called bubble clogging that hinders the smooth flow of liquid fuel. When bubble clogging occurs, power generation output decreases and varies.

  The present invention has been made to solve the above problems, and can supply a desired flow rate of liquid fuel to the end of the branch passage of the fuel distribution mechanism without causing clogging of bubbles, and can reduce the load of the liquid feed pump. An object is to provide a fuel cell that can be reduced.

  The present inventors have proposed the basic structure of the fuel distribution mechanism based on the application specification of the previous Japanese Patent Application No. 2006-353947, etc., but as a result of earnest research and development thereafter, further improvements have been made to this. In addition, a technology for supplying liquid fuel uniformly and efficiently to the fuel electrode without the risk of bubble clogging was established as follows.

  A fuel cell according to the present invention includes a membrane electrode assembly having a fuel electrode, an air electrode, and an electrolyte membrane sandwiched between the fuel electrode and the air electrode, and a fuel electrode side of the membrane electrode assembly A fuel distribution mechanism that distributes and supplies fuel to a plurality of locations of the fuel electrode, a fuel storage section that stores liquid fuel, and a supply flow path that connects the fuel storage section to the fuel distribution mechanism, The fuel distribution mechanism includes a fuel injection port that communicates with the supply flow channel, a main passage having a predetermined flow path cross section that is uniformly continuous from the fuel injection port, and the fuel. A plurality of fuel discharge ports that are opened to face the poles, and are provided between the main passage and the fuel discharge port, and transition from the upstream side to the downstream side between the main passage and the fuel discharge port. The cross-sectional shape of the channel and the branch structure are It is adjusted, and having a plurality of branch passages having a desired flow resistance.

  In the above case, the branch passage branches from the main passage or the upstream branch passage so that the cross-section of the passage gradually decreases as it moves from the upstream side to the downstream side, and its end communicates with the fuel discharge port. It is preferable. In this way, an appropriate amount of liquid fuel can be distributed to a plurality of branch passages using an appropriate pump back pressure without increasing the pump back pressure. Further, bubbles are not generated in the liquid fuel in the passage, and bubbles are not clogged.

  The main passage and the branch passage are preferably formed by a single or a plurality of thin tubes. In particular, the main passage is preferably a single capillary having a uniform diameter. This is because the main passage has a header function for distributing the liquid fuel to the plurality of branch passages, and thus it is necessary to distribute and supply the liquid fuel evenly to the respective branch passages.

  It is preferable that the downstream branch passage has a smaller equivalent diameter than the upstream branch passage. The equivalent diameter is defined as follows.

  The “equivalent diameter” is an index obtained by converting a shape other than a circle (for example, a rectangle) into a circle (perfect circle) diameter, and the cross-sectional area (a × b) of the flow path is the circumference of the flow path cross section (2a + 2b). It is given by multiplying by 4 divided by. That is, the equivalent diameter de can be calculated by substituting the cross-sectional sizes a and b of the flow path into the following equation (1). For example, in a flow path having a height a of 50 μm and a width b of 25 μm, the equivalent diameter de is 1.33.

de = 4ab / (2a + 2b) (1)
The main passage and the branch passage are preferably formed so that the aspect ratio of the cross section of the flow path is close to 1. In particular, when the aspect ratio of the main passage is approximated to 1, the pressure loss in the main passage itself is suppressed, and the header function of the main passage is sufficiently exhibited.

  It is preferable that the flow path cross-sectional area of the branch passage is reduced in the vicinity of the fuel discharge port so that the transport amount of the liquid fuel is controlled by a driving force mainly composed of a capillary force. In a so-called semi-passive type fuel distribution mechanism that supplies and distributes liquid fuel to the membrane electrode assembly by combining capillary force and pump driving force, the load applied to the pump increases exponentially as the number of fuel outlets increases. This is because the role of capillary force is relatively increased. When the end of the branch passage is set to, for example, the length a = 50 μm and the width b = 25 μm (equivalent diameter 1.33) as described above, a sufficient capillary force is generated, and the load on the pump is greatly reduced.

  The main passage and the branch passage are preferably formed so that the liquid fuel flows in a laminar flow state having a Reynolds number of 2000 or less in the branch passage. This is because the critical Reynolds number Rec at which the fluid flow changes from laminar flow to turbulent flow is in the range of about 2000 to 3000.

  The Reynolds number Re (dimensionalless) is an index representing the magnitude of inertia with respect to the state of the flow in the flow path, that is, the viscosity of the fluid, and is given by the following equation (2).

Re = (u × de × ρ) / μ (2)
However, u: flow velocity, de: equivalent diameter, ρ: fluid density, μ: fluid viscosity The branch passage has the same sum of the channel cross-sectional area before branching and the channel cross-sectional area after branching, and plural after branching It is preferable to branch so that the cross-sectional areas of the channels are substantially equal. In the fuel distribution mechanism having such a branch structure, the flow path resistance is minimized and the occurrence of bubble clogging is effectively prevented.

  There is preferably only one fuel inlet. This is because when liquid fuel is introduced into the fuel distribution mechanism from a single fuel inlet, variations in the supply pressure and concentration of the fuel are minimized, and liquid fuel is easily distributed evenly over the entire fuel electrode. Of course, the fuel injection ports may be arranged at a plurality of locations, and liquid fuel may be introduced into the fuel distribution mechanism from the plurality of fuel injection ports.

  The liquid fuel is desirably a methanol aqueous solution or a pure methanol solution having a methanol concentration of 80 mol% or more. This is because when the fuel concentration is 80 mol% or less, the output tends to decrease and the supply frequency of the liquid fuel increases.

  According to the present invention, liquid fuel at a desired flow rate can be supplied to the end of the branch passage of the fuel distribution mechanism without causing bubble clogging, and the load on the liquid feed pump can be reduced. A fuel cell is provided as an excellent power source for cordless portable devices such as portable computers and notebook computers.

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

  First, an overall outline of the fuel cell will be described with reference to FIG.

  The outer side of the fuel cell 1 is covered with an outer case 18 and a distribution plate 30 of the fuel distribution mechanism 3, and a membrane electrode assembly (MEA) 2 is housed inside. The outer case 18 and the distribution plate 30 are integrated with the MEA 2 sandwiched between them and screwed, and the end of the outer case 18 is crimped to the distribution plate 30. A pair of O-rings 19 are provided on the outer periphery of the MEA 2 to seal between the outer case 18 and the MEA 2 and between the distribution plate 30 and the MEA 2 so that the internal fuel does not leak to the outside. .

  The MEA 2 is a power generation element having a multipolar structure having a plurality of strip-shaped single electrodes (unit cells). The plurality of single electrodes are arranged side by side on substantially the same plane, and are electrically connected in series. In the present embodiment, an example of a fuel cell having a four-series arrangement in which four single electrodes are connected in series will be described. Each of the single electrodes includes MEA 2, a positive current collector (cathode conductive layer) and a negative current collector (anode conductive layer) (not shown).

  A moisturizing plate (not shown) is provided on the side of the positive electrode current collector so as not to block the passage of outside air, and to prevent the entry of smiling dust and foreign matters from the outside, and further contact. ing. As the moisturizing plate, a porous film having a porosity of, for example, 20 to 60% is preferably used. A plurality of ventilation holes (not shown) are opened on the main surface of the outer case 18 for air introduction. Air enters the inside through these ventilation holes, passes through the moisture retaining plate, and is supplied to the air electrode (cathode) 16 of the MEA 2.

  Examples of the catalyst contained in the fuel electrode 13 and the air electrode 16 include platinum group elemental metals (Pt, Ru, Rh, Ir, Os, Pd, etc.), alloys containing platinum group elements, and the like. it can. Although it is desirable to use Pt-Ru which has strong resistance to methanol and carbon monoxide as the anode catalyst and platinum as the cathode catalyst, it is not limited to this. Also, a supported catalyst using a conductive support such as a carbon material may be used, or an unsupported catalyst may be used.

  The electrolyte membrane 17 is for transporting protons generated in the fuel electrode 13 to the air electrode 16 and is made of a material that does not have electron conductivity and can transport protons. For example, fluorinated resins having a sulfonic acid group (for example, perfluorosulfonic acid polymer), hydrocarbon-based resins having a sulfonic acid group, tungstic acid, phosphotungstic acid, and the like can be mentioned. Nafion membrane (registered trademark) manufactured by Asahi Glass Co., Ltd., Flemion membrane (registered trademark) manufactured by Asahi Glass Co., Ltd., Aciplex membrane (registered trademark) manufactured by Asahi Kasei Kogyo Co., Ltd., or the like. In addition to polyperfluorosulfonic acid-based resin films, copolymer films of trifluorostyrene derivatives, polybenzimidazole films impregnated with phosphoric acid, aromatic polyether ketone sulfonic acid films, or aliphatic hydrocarbon-based films The electrolyte membrane 17 capable of transporting protons such as a resin soot may be configured.

  A fuel distribution mechanism 3 is arranged on the fuel electrode (anode) 13 side of the MEA 2 as shown in FIG. The fuel distribution mechanism 3 is connected to the fuel storage unit 4 via the supply flow path 5. Liquid fuel 41 is introduced into the fuel distribution mechanism 3 from the fuel storage portion 4 through the supply flow path 5 by a predetermined fuel supply method. A pure passive system or a semi-passive system can be adopted as the fuel supply system. In the fuel cell 1 of the present embodiment shown in FIG. 1, a pure passive method using only capillary force is used, but a semi-passive method combining capillary force and pump driving force may be used. The semi-passive method is described in detail in the application specification of Japanese Patent Application No. 2006-353947, which is an earlier application of the present inventors. The supply flow path 5 is not limited to a pipe independent from the fuel distribution mechanism 3 and the fuel storage unit 4. For example, when the fuel distribution mechanism 3 and the fuel storage unit 4 are stacked and integrated, a liquid fuel flow path connecting them may be used.

  As shown in FIG. 2, the fuel distribution mechanism 3 includes a distribution plate 30. The distribution plate 30 includes a single fuel inlet 31, an introduction pipe 20 that communicates with the fuel inlet 31, a main passage 21 that communicates with the introduction pipe 20, and first to first branches that sequentially branch from the main passage 21. The sixth branch passages 22 to 27 and a fuel discharge port 27 a that opens at the end of the last sixth branch passage 27. The fuel injection port 31 is continuous with one end (starting end) side of the introduction pipe 20. The introduction pipe 20 is formed of a thin tube having a rectangular cross section having a uniform diameter (for example, an equivalent inner diameter of 0.05 to 5 mm), and functions as a header that distributes liquid fuel to the passages 21 to 27 that follow the pipe. is there.

  Four main passages 21 branch from the introduction pipe 20, two first branch passages 22 branch from each of the main passages 21, and two first passages 22 from each of the first branch passages 22. Each of the second branch passages 23, two third branch passages 24 branch from each of the second branch passages 23, and two fourth branches from each of the third branch passages 24. The branch passages 25 respectively branch, two fifth branch passages 26 branch from each of the fourth branch passages 25, and two sixth branch passages from each of the fifth branch passages 26. 27 is branched. The total number of sixth branch passages 27 located at the tail is 128, and fuel discharge ports 27a are opened at the respective ends. These fuel discharge ports 27a all face the fuel electrode 13 of the MEA 2.

  Next, the branch passage in the fuel distribution mechanism will be described in detail with reference to FIG. 3 and Table 1.

  In this embodiment, a rectangular cross-section tube (nonmetal such as resin or ceramic) having an equivalent inner diameter of 1.2 mm is used as the introduction tube 20. For the main passage 21, a square section square tube (nonmetal such as resin or ceramic) having an inner height of 400 μm × width of 400 μm × length of 3 mm was used. The first branch passage 22 has a rectangular cross section with a height a of 50 μm and a width b of 800 μm, a length of 2 mm, and the second branch passage 23 has a rectangular cross section with a height a of 50 μm and a width b of 400 μm. The third branch passage 24 has a rectangular cross section with a height a of 50 μm and a width b of 200 μm, a length of 5 mm, and the fourth branch passage 25 has a height a of 50 μm and a width b. Is a rectangular cross section with a length of 14 mm, the fifth branch passage 26 is a square cross section with a height a of 50 μm and a width b of 50 μm, a length of 25 mm, and the sixth branch passage 27 has a height. The rectangular cross section with a being 50 μm and width b being 25 μm, the length was 45 mm. As described above, the reason why the first to sixth branch passages 22 to 27 are all set to the same height a (= 50 μm) except for the main passage 21 is due to manufacturing circumstances described later. The total length of the flow path from the fuel injection port 31 to the fuel discharge port 27a was about 100 mm.

  Next, a method for manufacturing the distribution plate 30 of the fuel distribution mechanism 3 will be briefly described.

  The distribution plate 30 is made of a resin that can be subjected to pattern etching, such as polyethylene (PE). Two resin plates are prepared, and a space for the first to sixth branch passages 22 to 27, the main passage 21 and the like and a fuel discharge port 27a are formed on one surface of one resin plate by pattern etching using a photolithography process. Form. A ceramic square tube is sandwiched between the pattern-etched resin plate and the other resin plate (flat plate), and these are bonded together with an adhesive. The ceramic square tube should serve as the introduction tube 20 and the main passage 21. The two resin plates and the ceramic square tube are integrated by bonding. The periphery of the preform is trimmed and burrs are removed from the fuel outlet 27a. As a result, a desired distribution plate 30 is obtained. When the flow path 5 from the fuel storage portion 41 is connected to the fuel inlet 31 of the distribution plate 30 manufactured as described above and further combined with the outer case 18 and the MEA 2, the desired fuel cell 1 is obtained. A manufacturing method of such a microchannel flow path is described in detail in Japanese Patent Application Laid-Open No. 2006-181740.

  The liquid fuel flows through the fuel cell 1 as follows.

  The liquid fuel introduced into the distribution plate 30 from the fuel injection port 31 passes through the main passage 21 from the introduction pipe 20 and passes through the first to sixth branch passages 22 to 27 branched into a plurality of fuel discharge ports. 27a, respectively. For example, a gas-liquid separation membrane (not shown) that transmits only the vaporized component of the liquid fuel and does not transmit the liquid component is disposed in the plurality of fuel discharge ports 27a, so that only the vaporized component of the liquid fuel is transmitted. The fuel electrode (anode) 13 of the MEA 2 is supplied. Accordingly, the vaporized component of the liquid fuel is discharged from the plurality of fuel discharge ports 27a toward a plurality of locations on the fuel electrode 13. The separator may be installed as a gas-liquid separation membrane or the like between the fuel distribution mechanism 3 and the fuel electrode 13.

  A gas-liquid separation membrane (not shown) is provided between the fuel distribution mechanism 3 and the MEA 2, and the liquid fuel discharged from the plurality of fuel discharge ports 27 a or the vaporized components thereof is transmitted to the gas diffusion layer 12 of the fuel electrode 13. Let The gas-liquid separation membrane has a property of allowing only the vaporized component of the liquid fuel (for example, methanol solution) to permeate but not the liquid fuel itself. As the gas-liquid separation membrane, for example, a porous membrane such as a silicon sheet or a PTFE membrane is used. Here, the vaporization component of liquid fuel means vaporized methanol when liquid methanol is used as the liquid fuel, and from the vaporization component of methanol and the vaporization component of water when an aqueous methanol solution is used as the liquid fuel. Is a mixed gas.

A plurality of fuel discharge ports 27a are provided on the surface of the distribution plate 30 in contact with the fuel electrode 13 so that fuel can be supplied to the entire MEA 2. The number of the fuel outlets 27a may be four or more in the case of four series connection, but in order to equalize the fuel supply amount in the surface of the MEA ² , the fuel outlets 27a of 1 to 16 pieces / cm ² are used. It is preferable to form so that there exists. If the number of fuel discharge ports 27a is less than 1 / cm ² , the amount of fuel supplied to the MEA ² cannot be made sufficiently uniform. Even if the number of the fuel discharge ports 27a exceeds 16 / cm ² , no further effect can be obtained.

  The fuel released from the fuel distribution mechanism 3 is supplied to the fuel electrode 13 of the MEA 2 as described above. In the MEA 2, the fuel diffuses through the anode gas diffusion layer 12 and is supplied to the anode catalyst layer 11. When methanol fuel is used as the liquid fuel, the internal reforming reaction of methanol shown in the following formula (1) occurs in the anode catalyst layer 11. When pure methanol is used as the methanol fuel, the water generated in the cathode catalyst layer 14 or the water in the electrolyte membrane 17 is reacted with methanol to cause the internal reforming reaction of the formula (1). Alternatively, the internal reforming reaction is caused by another reaction mechanism that does not require water.

CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (1)
Electrons (e ⁻ ) generated by this reaction are guided to the outside via a current collector, and are operated to a cathode (air electrode) 16 after operating a portable electronic device or the like as so-called electricity. Further, protons (H ⁺ ) generated by the internal reforming reaction of the formula (1) are guided to the cathode 16 through the electrolyte membrane 17. Air is supplied to the cathode 16 as an oxidant. Electrons (e ⁻ ) and protons (H ⁺ ) that have reached the cathode 16 react with oxygen in the air in the cathode catalyst layer 14 according to the following equation (2), and water is generated with this reaction.

6e ⁻ + 6H ⁺ + (3/2) O ₂ → 3H ₂ O (2)
Since the plurality of fuel discharge ports 27a are arranged so that fuel is supplied to the entire surface of the MEA 2, the amount of fuel supplied to the MEA 2 can be made uniform. That is, the fuel distribution in the plane of the anode (fuel electrode) 13 is leveled, and the fuel required for the power generation reaction in the MEA 2 can be supplied as a whole without excess or deficiency. Therefore, the MEA 2 can efficiently generate a power generation reaction without causing an increase in size or complexity of the fuel cell 1. As a result, the output of the fuel cell 1 can be improved. In other words, the output and its stability can be improved without impairing the advantages of the passive fuel cell 1 that does not circulate the fuel.

  By using the fuel distribution mechanism 3 having such a structure, the liquid fuel injected from the fuel injection port 31 into the fuel distribution mechanism 3 is evenly distributed to the plurality of fuel discharge ports 27a regardless of the direction and position. be able to. Therefore, the uniformity of the power generation reaction in the plane of MEA 2 can be further enhanced.

  In the fuel cell 1 of the present embodiment, the fuel distribution mechanism 3 having the plurality of fuel discharge ports 27a is applied as described above. The liquid fuel 41 is introduced into the fuel distribution mechanism 3 from the fuel inlet 31 through the supply flow path 5. In the fuel distribution mechanism 3, the liquid fuel 41 flows into the introduction pipe 20 made of a straight thin tube, and the four main passages 21 and the first to sixth branch passages 22 to branch sequentially from the introduction pipe 20. 27, and finally ejects all at once from the total 128 fuel discharge ports 27a communicating with the end of the sixth branch passage 27 toward the fuel electrode 13 of the MEA 2.

  Since the introduction pipe 20 and the main passage 21 function as headers, the liquid fuel 41 having a specified concentration is discharged from each of the plurality of fuel discharge ports 27a. Further, since the plurality of fuel discharge ports 27a are arranged so that fuel is supplied to the entire surface of the MEA 2, the amount of fuel supplied to the MEA 2 can be made uniform. That is, the fuel distribution in the plane of the fuel electrode 13 is leveled, and the fuel required for the power generation reaction in the MEA 2 can be supplied as a whole without excess or deficiency. Therefore, the MEA 2 can efficiently generate a power generation reaction without causing an increase in size or complexity of the fuel cell 1. As a result, the output of the fuel cell 1 is improved.

  The fuel distribution mechanism 3 used in the above-described embodiment distributes liquid fuel to a plurality of fuel discharge ports 27a from an introduction pipe 20 provided therein. For this reason, strictly speaking, a phenomenon is observed in which the temperature on the side close to the fuel injection port 31 is slightly high and the temperature decreases toward the back.

  The liquid fuel 41 introduced into the fuel distribution mechanism 3 from the fuel injection port 31 is led to the plurality of fuel discharge ports 27a through the main passage 21 and the branch passages 22 to 27 branched into a plurality of portions. By using the fuel distribution mechanism 3 having such a structure, the liquid fuel 41 injected from the fuel injection port 31 into the fuel distribution mechanism 3 is evenly distributed to the plurality of fuel discharge ports 27a regardless of the direction and position. can do. Therefore, the uniformity of the power generation reaction in the plane of MEA 2 can be further enhanced.

  Further, the fuel injection port 31 and the plurality of fuel discharge ports 27 a are connected by the introduction pipe 20, the main passage 21, and the branch passages 22 to 27, so that more fuel is supplied to a specific portion of the fuel cell 1. Is possible. For example, in the case where the heat radiation of the half part of the fuel cell 1 is improved due to the convenience of mounting the device, the temperature distribution is conventionally generated, and the average output is inevitably lowered. On the other hand, by adjusting the formation pattern of the branch pipes 22 to 27 and arranging the fuel discharge ports 27a densely in a portion with good heat dissipation in advance, the heat generated by the power generation in that portion can be increased. Thereby, the in-plane power generation degree can be made uniform, and it is possible to suppress a decrease in output.

  FIG. 4 shows the fuel cell of Example 1 and the fuel cell of the comparative example having the branch flow path shown in Table 1, with the tube length L (mm) on the horizontal axis and the pressure P (relative value) on the vertical axis. It is a characteristic diagram which shows the result of having investigated pressure loss by comparing. The characteristic line A in the figure shows the result of Example 1, and the characteristic line B shows the result of the comparative example. The pressure P on the vertical axis is expressed as a relative pressure with the pressure at the fuel injection port 31 immediately after being pumped as a reference value (= 1).

  FIG. 5 (a) shows a non-branching channel concept 1, and FIG. 5 (b) shows a branching channel concept 2. FIG. The results shown in FIG. 4 were obtained by simulation based on the channel concepts 1 and 2 as shown in FIG. As preconditions, the inflow flow rate Qin was set to 0.5 μl / min (flow rate per fuel discharge port), the outlet pressure Pout was set to zero relative pressure, and the tube length L was set to 100 mm in total length.

  As is clear from FIG. 4, the pressure loss is significantly reduced in Example 1 as compared with the comparative example, and in Example 1, the pump back pressure is reduced by two orders of magnitude (less than 1/100) as compared with the comparative example. I found it possible. From such knowledge, for example, a semi-passive type fuel cell using an electroosmotic flow pump (EO pump) described in US 2006/0029851 A1 is sufficiently practical. The semi-passive type fuel cell will be described later. Further, there is an advantage that bubble clogging does not occur by improving the flow of the liquid fuel in the fuel distribution mechanism.

  Next, a semi-passive type fuel cell will be described.

  A pump is attached to the flow path 5 between the fuel storage part 4 and the fuel distribution mechanism 3, and the liquid fuel can be transported more efficiently with the assistance of the pump driving force as well as the capillary force. The type of pump is not particularly limited, but from the viewpoint that a small amount of liquid fuel can be fed with good controllability and can be reduced in size and weight, an electroosmotic flow pump (EO pump), rotary pump ( It is preferable to use a rotary vane pump), a diaphragm pump, a squeezing pump, or the like. The electroosmotic flow pump uses a sintered porous material such as silica that causes an electroosmotic flow phenomenon. The electroosmotic flow pump is described in the above-mentioned Patent Document 2. A rotary pump rotates a wing with a motor and feeds liquid. The diaphragm pump is a pump that feeds liquid by driving the diaphragm with an electromagnet or piezoelectric ceramics. The squeezing pump presses a part of the flexible fuel flow path and squeezes the fuel. Among these, it is more preferable to use an electroosmotic pump or a diaphragm pump having piezoelectric ceramics from the viewpoint of driving power, size, and the like.

  Since the main object of the fuel cell 1 is a small electronic device, the pumping amount of the pump is preferably in the range of 10 μL / min to 1 mL / min. When the amount of liquid delivery exceeds 1 mL / min, the amount of liquid fuel delivered at a time becomes too large. For this reason, the fluctuation in the amount of fuel supplied to the MEA 2 increases, and as a result, the fluctuation in output increases. A reservoir for preventing this may be provided between the pump and the fuel distribution mechanism 3, but even if such a configuration is applied, fluctuations in the fuel supply amount cannot be sufficiently suppressed, and the device Incurs an increase in size.

  On the other hand, if the pumping amount is less than 10 μL / min, there may be a shortage of supply capacity when the amount of fuel consumption increases as the apparatus is started up. As a result, the starting characteristics of the fuel cell 1 are deteriorated. From such a point, it is preferable to use a pump having a liquid feeding capacity in the range of 10 μL / min to 1 mL / min. Furthermore, it is more preferable that the pumping amount be in the range of 10 to 200 μL / min. In order to stably realize such a liquid feeding amount, it is preferable to apply an electroosmotic flow pump or a diaphragm pump to the pump.

  Further, a liquid fuel impregnated layer laminated inside the fuel distribution mechanism 3 may be provided. As the liquid fuel-impregnated layer, for example, multi-rigid fibers such as porous polyester fiber and porous olefin resin, and open-cell porous resin are preferable. The liquid fuel-impregnated layer allows liquid fuel to be evenly supplied to a gas-liquid separation membrane (not shown) even when the liquid fuel in the fuel storage portion is reduced or when the fuel cell body is inclined and placed and the fuel supply is biased. As a result, it is possible to supply the vaporized liquid fuel evenly to the fuel electrode catalyst layer 11. In addition to the polyester fiber, it may be composed of various water-absorbing polymers such as acrylic resin, and is composed of a material that can hold the liquid by utilizing the permeability of the liquid, such as a sponge or an aggregate of fibers. . This liquid fuel impregnation part is effective for supplying an appropriate amount of fuel regardless of the posture of the main body.

  The liquid fuel is not necessarily limited to methanol fuel, for example, ethanol fuel such as ethanol aqueous solution or pure ethanol, propanol fuel such as propanol aqueous solution or pure propanol, glycol fuel such as glycol aqueous solution or pure glycol, dimethyl ether, It may be formic acid or other liquid fuel. In any case, liquid fuel corresponding to the fuel cell is used. In particular, a methanol aqueous solution or a pure methanol solution having a fuel concentration exceeding 80 mol% is preferred.

  In addition, the mechanism which sends liquid fuel from the fuel accommodating part 4 to the fuel distribution mechanism 3 is not specifically limited. For example, when the installation location at the time of use is fixed, the liquid fuel can be dropped from the fuel storage unit 4 to the fuel distribution mechanism 3 and fed using gravity. Further, by using the supply flow path 5 filled with a porous body or the like, the liquid can be fed from the fuel storage portion 4 to the fuel distribution mechanism 3 by capillary force. In addition, if the fuel is supplied from the fuel distribution mechanism 3 to the MEA 2, a fuel cutoff valve may be arranged instead of the pump. In this case, the fuel cutoff valve is provided to control the supply of liquid fuel through the flow path. Furthermore, in order to improve the stability and reliability of the fuel cell, a fuel cutoff valve can be arranged in series with the pump.

  However, when the shut-off valve is installed in the flow path 5 between the pump and the fuel storage unit 4, for example, if the fuel of the pump is depleted (evaporated) during long-term storage, the function of sucking out liquid fuel from the fuel storage unit 4 is hindered. May occur. For this reason, it is preferable to install a shutoff valve in the supply flow path 5 between the pump and the fuel distribution mechanism 3 to prevent evaporation of liquid fuel from the pump 31 during long-term storage.

  In this way, by inserting the shutoff valve between the fuel storage unit 4 and the fuel distribution mechanism 3, a small amount of fuel inevitably generated even when the fuel cell 1 is not used or the above-described pump re-operation is performed. Can be avoided. These greatly contribute to the improvement of practical convenience of the fuel cell 1.

Although various embodiments have been described above, the present invention is not limited only to the above-described embodiments, and the constituent elements can be modified and embodied without departing from the spirit of the invention in the implementation stage. . In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

1 is an internal perspective sectional view showing a fuel cell according to an embodiment of the present invention. The top view which shows typically the outline | summary of the fuel flow path in a fuel distribution mechanism. The cross-sectional schematic diagram which shows the fuel flow path of the Example in which a flow-path cross section reduces in steps whenever it branches. The characteristic diagram which shows the relationship between the flow path length L and the pressure P in an Example and a comparative example, respectively. (A) is a channel conceptual diagram showing a straight pipe channel, (b) is a channel conceptual diagram showing a branch pipe channel.

Explanation of symbols

1 ... Fuel cell,
2 ... Membrane electrode assembly (MEA),
3 ... Fuel distribution mechanism,
4 ... Fuel container, 41 ... Liquid fuel,
5 ... Supply channel,
11 ... anode catalyst layer, 12 ... negative electrode current collector, 13 ... fuel electrode,
14 ... Cathode catalyst layer, 15 ... Positive electrode current collector, 16 ... Air electrode,
17 ... electrolyte membrane (proton conductive membrane),
19 ... O-ring (seal member),
20 ... Fuel passage (introducing pipe),
21 ... Fuel passage (main passage),
22 ... Fuel passage (first branch passage),
23 ... Fuel passage (second branch passage),
24 ... Fuel passage (third branch passage),
25 ... Fuel passage (fourth branch passage),
26 ... Fuel passage (fifth branch passage),
27 ... Fuel passage (sixth branch passage),
27a ... Fuel outlet,
30 ... distribution plate, 31 ... fuel inlet.

Claims (10)

A membrane electrode assembly having a fuel electrode, an air electrode, and an electrolyte membrane sandwiched between the fuel electrode and the air electrode; and disposed on the fuel electrode side of the membrane electrode assembly; A fuel cell comprising: a fuel distribution mechanism that distributes and supplies fuel to a plurality of locations; a fuel storage portion that stores liquid fuel; and a supply passage that connects the fuel storage portion to the fuel distribution mechanism. ,
The fuel distribution mechanism includes:
A fuel inlet communicating with the supply channel;
A main passage having a predetermined flow path section that is uniformly continuous from the fuel inlet;
A plurality of fuel outlets that open to face the fuel electrode;
Provided between the main passage and the fuel discharge port, the flow path cross-sectional shape and the branch structure are respectively adjusted as the transition from the upstream side to the downstream side from the main passage to the fuel discharge port, A plurality of branch passages having flow path resistance;
A fuel cell comprising:   The branch passage branches off from the main passage or the upstream branch passage so that the cross section of the flow path gradually decreases as it moves from the upstream side to the downstream side, and its end communicates with the fuel discharge port. The fuel cell according to claim 1, wherein:   The fuel cell according to claim 1, wherein the main passage and the branch passage are formed by a single or a plurality of thin tubes.   The fuel cell according to any one of claims 1 to 3, wherein the downstream branch passage has a smaller equivalent diameter than the upstream branch passage.   The fuel cell according to any one of claims 1 to 4, wherein the main passage and the branch passage are formed so that an aspect ratio of a cross section of the flow passage is close to 1.   The flow path cross-sectional area of the branch passage is reduced in the vicinity of the fuel discharge port so that the transport amount of liquid fuel is controlled by a driving force mainly composed of capillary force. Fuel cell.   7. The main passage and the branch passage are formed so that liquid fuel flows in a laminar flow state having a Reynolds number of 2000 or less in the branch passage. Fuel cell.   The branch passage is branched such that the sum of the cross-sectional area of the flow path before branching and the cross-sectional area of the flow path after branching are equal, and the cross-sectional areas of the plurality of flow paths after branching are substantially equal. The fuel cell according to any one of claims 1 to 7, wherein:   The fuel cell according to claim 1, wherein the number of the fuel inlets is only one.   10. The fuel cell according to claim 1, wherein the liquid fuel is a methanol aqueous solution or a pure methanol solution having a methanol concentration of 80 mol% or more.

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