JP2008210548A - Fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell excellent in output characteristics, suppressed in excess heat generation, and easy in handling. <P>SOLUTION: The fuel cell includes: a membrane electrode assembly having a fuel electrode, an air electrode, and an electrolyte membrane interposed between the fuel electrode and the air electrode; and a fuel supply mechanism arranged on the fuel electrode side of the membrane electrode assembly and supplying fuel to the fuel electrode of the membrane electrode assembly. Heat resistance from the outermost surface on the air electrode side to the air electrode is made lower than that from the outermost surface on the fuel electrode side to the fuel electrode. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel cell using liquid fuel.

  In recent years, attempts have been made to use a fuel cell as a power source for portable electronic devices such as notebook computers and mobile phones so that they can be used for a long time without being charged. A fuel cell is characterized in that it can generate electric power simply by supplying fuel and air, and can generate electric power continuously for a long time if fuel is replenished. For this reason, if the fuel cell can be reduced in size, it can be said that the system is extremely advantageous as a power source for portable electronic devices.

  A direct methanol fuel cell (DMFC) is promising as a power source for portable electronic devices because it can be miniaturized and the fuel can be easily handled. As the liquid fuel supply method in the DMFC, there are known an active method such as a gas supply type and a liquid supply type, and a passive method such as an internal vaporization type in which the liquid fuel in the fuel container is vaporized inside the cell and supplied to the fuel electrode. It has been.

Among these, a passive system such as an internal vaporization type is particularly advantageous for downsizing of the DMFC. In the passive DMFC, for example, a structure is proposed in which a membrane electrode assembly (fuel cell) having a fuel electrode, an electrolyte membrane, and an air electrode is disposed on a fuel containing portion made of a resin box-like container ( For example, see Patent Document 1). In addition, it has been studied to connect a fuel cell of DMFC and a fuel storage part via a flow path (see Patent Documents 2 to 4). By supplying the liquid fuel supplied from the fuel storage unit to the fuel cell via the flow path, the supply amount of the liquid fuel can be adjusted based on the shape and diameter of the flow path.
International Publication No. 2005/112172 Pamphlet JP 2005-518646 A JP 2006-089552 A US Patent Publication No. 2006/0029851

  By the way, the voltage generated by the power generation reaction in the DMFC is 1.2 V in the theoretical calculation, but decreases to about 0.3 to 0.5 V under practical operating conditions. In order to increase the power generated by the power generation reaction of the fuel cell, it is important to suppress the voltage drop by smoothly performing the catalytic reaction and to effectively generate power over the entire fuel cell.

  Regarding the improvement of the catalyst, many researchers and engineers have been energetically researching since the discovery of the function of the fuel cell, but no promising catalyst other than platinum-based catalysts has been found. However, even when a platinum-based catalyst is used, the reaction activity is improved by raising the reaction temperature, and the mass transfer in the vicinity of the catalyst becomes smooth, so that an improvement in voltage is recognized.

  Methods for increasing the reaction temperature of the catalyst include, for example, covering the entire fuel cell with a heat insulator, or the so-called methanol crossover phenomenon in which fuel is supplied excessively and the fuel flows directly to the cathode without undergoing an anode reaction. It is conceivable to use the heat generated by However, simply applying such a method increases the generated voltage, and the amount of heat generated continues to increase as the power density increases, making it extremely difficult to design the heat. Further, in the method using the methanol crossover phenomenon, the cathode temperature becomes too high, and the generated water transpirations increases, so that the adverse effect of performance degradation due to drying of the electrolyte membrane becomes obvious.

  In an active fuel cell that forcibly supplies fuel and air, the reaction temperature is raised by heating and supplying the fuel and air with a heat exchanger or a heater. However, although such a method can increase the generated voltage, the apparatus becomes complicated and large-scale, and is mainly limited to application to a large-sized fuel cell for stationary use.

  The present invention has been made in order to solve the above-described problems, and an object of the present invention is to provide a fuel cell that is excellent in output characteristics, suppressed excessive heat generation, and easy to handle.

  The fuel cell of 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 is disposed on the fuel electrode side of the membrane electrode assembly. And a fuel supply mechanism for supplying fuel to the fuel electrode of the membrane electrode assembly, wherein the thermal resistance from the outermost surface on the air electrode side to the air electrode is from the outermost surface on the fuel electrode side to the fuel electrode. It is characterized by being smaller than the thermal resistance up to.

  In the fuel cell of the present invention, the thermal resistance from the outermost surface on the air electrode side to the air electrode is preferably 70% or less of the thermal resistance from the outermost surface on the fuel electrode side to the fuel electrode. Such a thermal resistance is preferably a thermal resistance against a heat flow in a direction perpendicular to the main surface of the membrane electrode assembly.

  As the fuel used in the fuel cell of the present invention, methanol fuel is preferably used, and in particular, an aqueous methanol solution having a methanol concentration of 80% or more or pure methanol is preferably used.

  According to the present invention, the thermal resistance from the outermost surface on the air electrode side to the air electrode is smaller than the thermal resistance from the outermost surface on the fuel electrode side to the fuel electrode. Heat generation is suppressed and the fuel cell can be easily handled.

  Hereinafter, the present invention will be described in detail. FIG. 1 is a cross-sectional view showing a first embodiment of a fuel cell of the present invention. A fuel cell 1 shown in FIG. 1 covers a fuel cell 2 constituting an electromotive unit, a fuel supply mechanism 3 for supplying fuel to the fuel cell 2, and the entire anode 13 side of the fuel cell 2. It is mainly comprised from the heat insulating body 4 provided in this way.

  The fuel cell 2 includes an anode (fuel electrode) 13 having an anode catalyst layer 11 and an anode gas diffusion layer 12, and a cathode (air electrode / oxidant electrode) 16 having a cathode catalyst layer 14 and a cathode gas diffusion layer 15. And a membrane electrode assembly (MEA) composed of a proton (hydrogen ion) conductive electrolyte membrane 17 sandwiched between the anode catalyst layer 11 and the cathode catalyst layer 14.

  Examples of the catalyst contained in the anode catalyst layer 11 and the cathode catalyst layer 14 include a simple substance of a platinum group element such as Pt, Ru, Rh, Ir, Os, and Pd, and an alloy containing the platinum group element. For the anode catalyst layer 11, it is preferable to use Pt—Ru, Pt—Mo, or the like having strong resistance to methanol, carbon monoxide, or the like. Pt, Pt—Ni or the like is preferably used for the cathode catalyst layer 14. However, the catalyst is not limited to these, and various substances having catalytic activity can be used. The catalyst may be either a supported catalyst using a conductive support such as a carbon material or an unsupported catalyst.

  Examples of the proton conductive material constituting the electrolyte membrane 17 include a fluorine-based resin (Nafion (trade name, manufactured by DuPont) or Flemion (trade name, manufactured by Asahi Glass Co., Ltd.) such as a perfluorosulfonic acid polymer having a sulfonic acid group. Etc.), organic materials such as hydrocarbon resins having sulfonic acid groups, or inorganic materials such as tungstic acid and phosphotungstic acid. However, the proton conductive electrolyte membrane 17 is not limited to these.

  The anode gas diffusion layer 12 laminated on the anode catalyst layer 11 serves to uniformly supply fuel to the anode catalyst layer 11 and also serves as a current collector for the anode catalyst layer 11. The cathode gas diffusion layer 15 laminated on the cathode catalyst layer 14 serves to uniformly supply the oxidant to the cathode catalyst layer 14 and also serves as a current collector for the cathode catalyst layer 14. The anode gas diffusion layer 12 and the cathode gas diffusion layer 15 are made of a porous substrate.

  A conductive layer is laminated on the anode gas diffusion layer 12 and the cathode gas diffusion layer 15 as necessary. As these conductive layers, for example, a mesh made of a conductive metal material such as Au, a porous film, a thin film, or the like is used. Rubber O-rings 19 are interposed between the electrolyte membrane 17, the fuel distribution mechanism 3 (here, the fuel storage unit 6), and the cover plate 18, thereby preventing fuel leakage from the fuel cell 2. Prevents oxidant leakage.

  The cover plate 18 has an opening 18a for taking in air as an oxidant. A moisture retaining layer and a surface layer are disposed between the cover plate 18 and the cathode 16 as necessary. The moisturizing layer is impregnated with a part of the water generated in the cathode catalyst layer 14 to suppress the transpiration of water and promote uniform diffusion of air to the cathode catalyst layer 14. The surface layer adjusts the amount of air taken in, and has a plurality of air inlets whose number, size, etc. are adjusted according to the amount of air taken in.

  The fuel supply mechanism 3 includes a fuel storage unit 6 that stores liquid fuel. The fuel storage unit 6 is disposed on the anode 13 side of the fuel battery cell 2 and has a substantially box shape with an opening provided on the anode 13 side. The opening allows only the vaporized component of the liquid fuel to pass therethrough, A gas-liquid separation membrane 9 in which liquid components are difficult to permeate is disposed.

  Liquid fuel corresponding to the fuel cells 2 is stored in the fuel storage unit 6. As the liquid fuel, methanol fuel is preferable, and an aqueous methanol solution or pure methanol having a methanol concentration of 80% or more is particularly preferably used. The liquid fuel may be, for example, an ethanol fuel such as an ethanol aqueous solution or pure ethanol, a propanol fuel such as a propanol aqueous solution or pure propanol, a glycol fuel such as a glycol aqueous solution or pure glycol, dimethyl ether, formic acid, or other liquid fuel. Good. In any case, liquid fuel corresponding to the fuel cell 2 is stored in the fuel storage unit 6.

  The gas-liquid separation membrane 9 is in the form of a sheet made of a material that is inert to liquid fuel and does not dissolve, and specifically includes a silicone rubber thin film, a low density polyethylene (LDPE) thin film, and polyvinyl chloride (PVC). A thin film, a polyethylene terephthalate (PET) thin film, a fluororesin (for example, polytetrafluoroethylene (PTFE), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), etc.), a microporous film, or the like is used.

  The heat insulator 4 is provided, for example, so as to cover the entire anode 13 side of the fuel cell 2, and specifically, provided so as to cover the electrolyte membrane 17, the anode 13, and the fuel storage unit 6 as the fuel supply mechanism 3. It has been. In the fuel cell 1 of the first embodiment, since the heat insulator 4 is provided on the entire anode 13 side of the fuel cell 2, the thermal resistance (hereinafter simply referred to as the cathode 16) from the outermost surface on the cathode 16 side. The cathode side thermal resistance is set to be smaller than the thermal resistance from the outermost surface on the anode 13 side to the anode 13 (hereinafter simply referred to as the anode side thermal resistance).

  Here, in the fuel cell 1 shown in FIG. 1, the surface of the cover plate 18 is the outermost surface on the cathode 16 side. Therefore, the cathode-side thermal resistance is a thermal resistance from the cover plate 18 to the cathode 16 (the cathode catalyst layer 14 and the cathode gas diffusion layer 15). Further, the surface of the bottom 4a of the heat insulator 4 is the outermost surface on the anode 13 side. Therefore, the anode-side thermal resistance is a thermal resistance from the surface of the bottom 4a of the heat insulator 4 to the anode 13 (the anode catalyst layer 11 and the anode gas diffusion layer 12). The cathode side thermal resistance includes the thermal resistance of the cathode 16 (cathode catalyst layer 14, cathode gas diffusion layer 15) itself. Similarly, the anode side thermal resistance also includes the anode 13 (anode catalyst layer 11, anode gas diffusion). Layer 12) includes its own thermal resistance.

  As described above, when the cathode-side thermal resistance is made smaller than the anode-side thermal resistance, when heat is generated along with power generation, the generated heat is retained on the anode 13 side, and the generated heat is generated on the cathode 16 side. Is dissipated. As a result, the anode 13 side quickly rises to a temperature suitable for the catalytic reaction, the fuel cell output increases and stabilizes in a short time, and the output voltage is also maintained at a high temperature in the vicinity of the catalyst. Higher than the ones. Furthermore, it not only increases output power but also improves fuel utilization efficiency, which is effective from the viewpoint of energy saving.

  Further, since the cathode 16 side is maintained at a relatively low temperature with respect to the power generation output by heat radiation, excessive water transpiration due to the generated heat, and performance degradation due to drying of the electrolyte membrane 17 due to the water evaporation are greatly suppressed. The

  In addition, it is preferable that the cathode side thermal resistance and the anode side thermal resistance are specifically thermal resistances to heat flow in a direction perpendicular to the main surface of the membrane electrode assembly 2. Regarding the cathode side thermal resistance and the anode side thermal resistance, the cathode side thermal resistance is made smaller than the anode side thermal resistance, thereby making it easier to obtain the effects as described above.

  In particular, the cathode side thermal resistance is preferably 70% or less of the anode side thermal resistance. If at least the cathode-side thermal resistance is smaller than the anode-side thermal resistance, the output power can be improved. However, by setting the cathode-side thermal resistance to 70% or less of the anode-side thermal resistance, for example, a high output of 200 mW or more. Can also be obtained stably.

  As a heat insulating material which comprises the heat insulating body 4, various heat insulating materials used as heat insulating materials for buildings, such as a foamed polystyrene, a urethane foam, glass wool, are applicable, for example. The thickness of the heat insulator 4 is suitably adjusted so that the cathode side thermal resistance is smaller than the anode side thermal resistance, and preferably the cathode side thermal resistance is 70% or less of the anode side thermal resistance.

  The anode side thermal resistance and the cathode side thermal resistance can be measured as follows. 2 and 3 schematically show a method for measuring the thermal resistance of the fuel cell 1 shown in FIG. 1, FIG. 2 shows a method for measuring the anode-side thermal resistance, and FIG. 3 shows a cathode. A method for measuring the side thermal resistance is shown.

  First, as shown in FIG. 2, when measuring the anode-side thermal resistance, the anode composed only of the constituent members on the anode 13 side of the electrolyte membrane 17 in the fuel cell 1 to be measured shown in FIG. 1. A side measurement member 1a is prepared. A heating element 21 connected to the power source 20 is installed on the cathode side of the anode side measuring member 1a, and the periphery of the heating element 21 is covered with a heat insulator 22 having a sufficiently high thermal resistance.

Thereafter, a constant electric power is applied to the heating element 21 from the power source 20 to generate heat, and a thermocouple (not shown) installed on the anode 13 and a thermocouple installed on the surface of the bottom 4a of the heat insulator 4 which is the outermost surface. (Not shown) and the temperature change is measured. The temperature when the temperature change is eliminated and the temperature becomes constant is read, and the temperature of the anode 13 at that time is T _A , and the temperature of the surface of the bottom portion 4a which is the outermost surface is T _AA . In addition, the power applied to the heating element 21 is P (W). From these, the anode side thermal resistance (R _A ) is calculated by the following formula.
_{R A = (T A -T AA} ) / P [℃ / W]

  The cathode side thermal resistance can be obtained in the same manner. That is, as shown in FIG. 3, a cathode-side measurement member 1b consisting of only the constituent members on the cathode 16 side of the electrolyte membrane 17 in the fuel cell 1 as the measurement object shown in FIG. A heating element 21 connected to the power source 20 is installed on the anode side of the cathode side measuring member 1b, and the periphery of the heating element 21 is covered with a heat insulator 22 having a sufficiently high thermal resistance.

Thereafter, heat is generated by energizing the heating element 21 from the power source 20 to generate heat, and a thermocouple (not shown) installed on the cathode 16 and a thermocouple (not shown) installed on the outermost surface of the cover plate 18. )) And measure the temperature change. The temperature when the temperature change is eliminated and the temperature becomes constant is read, and the temperature of the cathode 16 at that time is T _C , and the temperature of the outermost cover plate 18 is T _CC . In addition, the power applied to the heating element 21 is P (W). From these, the cathode side thermal resistance (R _C ) is calculated by the following formula.
_{R C = (T C -T CC} ) / P [℃ / W]

In the present invention, the anode-side thermal resistance (R _A ) and the cathode-side thermal resistance (R _C ) thus obtained are only required to satisfy R _C <R _A as described above, and R _C ≦ R _It is more preferable that _A × 0.7 is satisfied.

The power generation reaction in the fuel cell 1 shown in FIG. 1 is performed as follows. First, the vaporized component of the liquid fuel stored in the fuel storage unit 6 passes through the gas-liquid separation membrane 9 and is supplied to the anode 13 of the fuel cell 2. In the fuel cell 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, an 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)

The electrons (e ⁻ ) generated by this reaction are guided to the outside via a current collector, and are guided to the cathode 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 formula (2), and water is generated along with this reaction.
6e ⁻ + 6H ⁺ + (3/2) O ₂ → 3H ₂ O (2)

  At this time, since the cathode side thermal resistance is made smaller than the anode side thermal resistance, the anode 13 quickly rises to a temperature suitable for the catalytic reaction, and the fuel cell output is increased and stabilized in a short time. The output voltage at that time is also higher than in the prior art. In addition, since the cathode 16 is prevented from being excessively heated, the generated water is less likely to evaporate, and the electrolyte membrane 17 is also less likely to be dried.

  Next, another embodiment of the fuel cell of the present invention will be described. FIG. 4 shows a second embodiment of the fuel cell of the present invention. The fuel cell 1 shown in FIG. 4 is configured such that the fuel accommodating portion 6 of the fuel cell 1 shown in FIG. 1 is also integrally formed from the heat insulator 4 and the cathode side thermal resistance is smaller than the anode side thermal resistance. is there. As described above, the fuel storage unit 6 may be integrally formed by the heat insulator 4, and by doing so, the configuration of the fuel cell 1 can be simplified, and the productivity and reliability are improved. Can be improved. For example, it is possible to make the cathode-side thermal resistance smaller than the anode-side thermal resistance by making the constituent member itself such as the fuel storage portion 6 from a material having a large thermal resistance, for example, a resin containing foam beads inside. it can.

  FIG. 5 shows a third embodiment of the fuel cell of the present invention. The fuel cell 1 shown in FIG. 5 extends the heat insulator 4 of the fuel cell 1 shown in FIG. 4 to the side surface portion of the cover plate 18 that is the outermost surface on the cathode 16 side, and the cathode side thermal resistance is increased to the anode side thermal resistance. Is smaller than that. Thus, as long as the cathode-side thermal resistance can be made smaller than the anode-side thermal resistance, the heat insulator 4 may be provided up to the outermost surface on the cathode 16 side. The mechanical strength can be improved, and the reliability can be improved and the productivity can be greatly improved.

  For example, in the fuel cell 1 as shown in FIG. 5, the heat insulator 4 can also be provided on the surface of the cover plate 18 that is the outermost surface on the cathode 16 side. However, in such a case, it is necessary to appropriately adjust the thickness of the heat insulator 4 so that the cathode-side thermal resistance is smaller than the anode-side thermal resistance.

  FIG. 6 shows a fourth embodiment of the fuel cell of the present invention. In the fuel cell 1 of the fourth embodiment shown in FIG. 6, a fuel distribution mechanism 5 is disposed on the anode 13 side of the fuel cell 2, and a flow path 7 is supplied to the fuel distribution mechanism 5 from the fuel storage portion 6 by a pump 8. The liquid fuel is fed through. Here, a portion including the fuel distribution mechanism 5, the fuel storage unit 6, the flow path 7 and the pump 8 is the fuel supply mechanism 3. The fuel cell 1 is configured such that the cathode side thermal resistance is smaller than the anode side thermal resistance by covering the anode 13 of the fuel cell 2 and the fuel distribution mechanism 5 with the heat insulator 4. Has been.

  In the case where the fuel distribution mechanism 5 and the fuel storage portion 6 are arranged separately as described above, the anode-side thermal resistance is that of a portion that directly forms a laminated structure with the fuel cell 2. That is, in the fuel cell 1 of the fourth embodiment shown in FIG. 6, the outermost surface on the anode 13 side is the surface of the bottom portion 4 a of the heat insulator 4, and the anode-side thermal resistance is the surface of the bottom portion 4 a of the heat insulator 4. To the anode 13 (the anode catalyst layer 11 and the anode gas diffusion layer 12). In the fuel cell 1 of the fourth embodiment shown in FIG. 6, for example, when the fuel storage portion 6 is disposed in contact with the bottom 4 a of the heat insulator 4, the fuel storage capacity is included in the anode side thermal resistance. Part 6 is also included.

  For example, as shown in FIG. 7, the fuel distribution mechanism 5 includes at least one fuel inlet 5a through which liquid fuel flows through the flow path 7, and a plurality of fuel outlets 5b through which liquid fuel and its vaporized components are discharged. The fuel distribution plate 5c having As shown in FIG. 6, a gap 5e serving as a liquid fuel passage led from the fuel injection port 5a is provided inside the fuel distribution plate 5c. The plurality of fuel discharge ports 5b are directly connected to the gaps 5e that function as fuel passages.

  The liquid fuel introduced into the fuel distribution plate 5c from the fuel inlet 5a enters the gap 5e, and is guided to the plurality of fuel discharge ports 5b through the gap 5e functioning as the fuel passage. For example, a gas-liquid separator (not shown) that transmits only the vaporized component of the liquid fuel and does not transmit the liquid component may be disposed in the plurality of fuel discharge ports 5b. As a result, the vaporized component of the liquid fuel is supplied to the anode 13 of the fuel cell 2. The gas-liquid separator may be installed as a gas-liquid separation membrane or the like between the fuel distribution mechanism 5 and the anode 13. The vaporized component of the liquid fuel is discharged from a plurality of fuel discharge ports 5b toward a plurality of locations on the anode 13.

The number of the fuel discharge ports 5b may be one or more. However, in order to equalize the fuel supply amount in the surface of the fuel cell 2, there are 0.1 to 10 / cm ² fuel discharge ports 5b. It is preferable to form so as to. If the number of fuel discharge ports 5b is less than 0.1 / cm ² , the amount of fuel supplied to the fuel cells 2 cannot be made sufficiently uniform. Even if the number of the fuel discharge ports 5b exceeds 10 / cm ² , no further effect can be obtained.

  The pump 8 is not a circulation pump that circulates fuel, but is a fuel supply pump that sends liquid fuel from the fuel storage unit 6 to the fuel distribution mechanism 5 to the last. By supplying liquid fuel with such a pump 8 when necessary, the controllability of the fuel supply amount can be improved. Moreover, by using such a pump 8, fuel can be discharged uniformly from each fuel discharge port 5b, and high output can be stably obtained.

  The fuel supplied from the fuel distribution mechanism 5 to the fuel cells 2 is used for the power generation reaction, and is not circulated thereafter and returned to the fuel storage unit 6. As described above, since the fuel cell 1 shown in FIG. 6 does not circulate the fuel, unlike the conventional active method, it does not impair the downsizing of the apparatus. In addition, the fuel cell 1 shown in FIG. 6 uses a pump 8 for supplying liquid fuel, and is pure as in the fuel cells 1 of the first to third embodiments shown in FIGS. Since it is different from the passive method, for example, it is called a semi-passive method.

  The type of the pump 8 is not particularly limited, but a rotary vane pump, an electroosmotic flow pump, a diaphragm pump, from the viewpoint that a small amount of liquid fuel can be sent with good controllability and can be reduced in size and weight. It is preferable to use an ironing pump or the like. A rotary vane pump feeds liquid by rotating a wing with a motor. The electroosmotic flow pump uses a sintered porous material such as silica that causes an electroosmotic flow phenomenon. 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 liquid feeding capability of the pump 8 is preferably in the range of 10 μL / min to 1 mL / min. When the liquid feeding capacity exceeds 1 mL / min, the amount of liquid fuel fed at one time becomes too large, and the stop time of the pump 8 occupying the entire operation period becomes long. For this reason, fluctuations in the amount of fuel supplied to the fuel cells 2 increase, and as a result, fluctuations in output increase. A reservoir for preventing this may be provided between the pump 8 and the fuel distribution mechanism 5, but even if such a configuration is applied, fluctuations in the fuel supply amount cannot be sufficiently suppressed, and This will increase the size of the device.

  On the other hand, when the liquid feeding capacity of the pump 8 is less than 10 μL / min, there is a risk of insufficient supply capacity when the amount of fuel consumption increases as when 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 the pump 8 having a liquid feeding capacity in the range of 10 μL / min to 1 mL / min. The pumping capacity of the pump 8 is more preferably 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 8.

  Note that a fuel cutoff valve (not shown) may be arranged in place of the pump 8 as long as fuel is supplied from the fuel distribution mechanism 3 to the fuel cells 2. This fuel cutoff valve is provided to control the supply of liquid fuel through the flow path.

  FIG. 8 shows a fifth embodiment of the fuel cell of the present invention. In the fuel cell 1 shown in FIG. 8, the fuel distribution mechanism 5 of the fuel cell 1 shown in FIG. 6 is also configured integrally with the heat insulator 4, and the cathode side thermal resistance is smaller than the anode side thermal resistance. is there. As described above, the fuel distribution mechanism 5 may be integrally formed from the heat insulator 4, and by doing so, the configuration of the fuel cell 1 can be simplified, and the productivity and reliability are improved. Can be improved.

  FIG. 9 shows a sixth embodiment of the fuel cell of the present invention. The fuel cell 1 shown in FIG. 9 is provided by extending the heat insulator 4 of the fuel cell 1 shown in FIG. 8 to the side surface portion of the cover plate 18 which is the outermost surface on the cathode 16 side, and providing the cathode side thermal resistance on the anode side. It is smaller than the thermal resistance. Thus, as long as the cathode-side thermal resistance can be made smaller than the anode-side thermal resistance, the heat insulator 4 may be provided up to the outermost surface on the cathode 16 side. The mechanical strength can be improved, and the reliability can be improved and the productivity can be greatly improved.

  For example, in the fuel cell 1 as shown in FIG. 9, the heat insulator 4 can be further provided on the surface of the cover plate 18 which is the outermost surface on the cathode 16 side. However, in such a case, it is necessary to adjust the thickness and the like of the heat insulator 4 so that the cathode side thermal resistance is smaller than the anode side thermal resistance.

  Although the present invention has been described above, the present invention is not limited to the above-described embodiment itself, and can be embodied by modifying constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements 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.

  For example, a description has been given by taking a passive fuel cell as an example of the configuration of the fuel cell body, but the present invention can also be applied to an active fuel cell. Even in such a case, the same effect as described above can be obtained. In addition to supplying all the liquid fuel as vapor, the fuel supply to the membrane electrode assembly may be partly supplied in a liquid state, and the present invention is partially in a liquid state as described above. It can also be applied to what is supplied.

  Next, the present invention will be described in detail with reference to examples.

(Example 1, Comparative Examples 1 and 2)
As the fuel cell of Example 1, the anode side of the fuel cell as shown in FIG. 1 and the fuel storage portion which is the fuel supply mechanism are covered with a heat insulator, and the cathode side heat resistance is set to 100 when the anode side thermal resistance is 100. A resistor having a resistance of 30 was produced. On the other hand, as shown in FIG. 10, the fuel cell of Comparative Example 1 has the same configuration as that of the fuel cell of Example 1 except that no heat insulator is provided, and the anode side thermal resistance and the cathode side thermal resistance are the same. Was produced. In addition, as shown in FIG. 11, the fuel cell of Comparative Example 2 is provided with a heat insulator on the entire surface excluding the opening of the cover plate, and when the anode side thermal resistance is 100, the cathode side thermal resistance is 130. Produced.

  In the fuel cells of Example 1 and Comparative Examples 1 and 2, the electrolyte membrane was Nafion 112 (manufactured by DuPont), and the anode was carbon particles carrying platinum ruthenium alloy fine particles on carbon paper as an anode gas diffusion layer. Assume that an anode catalyst layer made of DE2020 (manufactured by DuPont) was formed, and a cathode formed a cathode catalyst layer made of platinum-supported carbon particles and DE2020 (manufactured by DuPont) on carbon paper as a cathode gas diffusion layer. It was supposed to be. The cover plate was made of stainless steel (SUS304), the fuel storage part was made of polyethylene terephthalate, and the gas-liquid separation membrane was a silicone sheet having a thickness of 0.2 mm. Furthermore, the heat insulator was made of expanded polystyrene having a thickness of 10 mm. The anode side thermal resistance and the cathode side thermal resistance were measured by the method shown in FIGS.

  Next, the fuel cells of Example 1 and Comparative Examples 1 and 2 were subjected to power generation using pure methanol as a liquid fuel in an environment of a temperature of 25 ° C. and a relative humidity of 50%, and attached to the outermost surface on the cathode side. The temperature was measured with a thermocouple and the output was measured. The results are shown in FIGS.

  As is clear from FIG. 12, the temperature of the fuel cell of Example 1 rapidly rises after the start of power generation, reaches about 50 ° C. after 1 hour, and then stabilizes although there is a slight fluctuation. . In addition, when this data was closely observed with the original data, it was confirmed that the temperature was actually stabilized in about 10 minutes from the start of power generation.

  On the other hand, in the fuel cell of Comparative Example 1, it takes about 2 hours for the temperature to stabilize, and the temperature after stabilization is about 43 ° C., which is lower than that of the fuel cell of Example 1. Further, in the fuel cell 1 of Comparative Example 2, it is found that the temperature rapidly rises after the start of power generation and reaches about 60 ° C. after 1 hour, but then rapidly decreases and stabilizes at around 35 ° C.

  Corresponding to such a temperature rise, as shown in FIG. 13, the output characteristics of the fuel cell of Example 1 increase rapidly after the start of power generation and reach 250 mW in the first hour. When this data was closely observed with the original data, it was actually confirmed that it reached 250 mW in about 10 minutes from the start of power generation. On the other hand, in the fuel cell of Comparative Example 1, it takes time until the temperature stabilizes, and the temperature at that time is low, and it takes about 2 hours from the start of power generation to reach about 160 mW, and then about 160 mW. You can see that Further, in the fuel cell of Comparative Example 2, the output increases rapidly after power generation starts and reaches about 240 mW, but then decreases rapidly, and after 10 hours, it decreases to about 90 mW.

  As a cause of this, since no heat insulator is provided in the fuel cell of Comparative Example 1, in particular, the thermal resistance on the anode side is not sufficient, and the heat generated with power generation is quickly dissipated and the temperature does not rise, Moreover, since the temperature is low, it is presumed that the activity of the catalyst is low and the output remains low.

  On the other hand, in the fuel cell of Comparative Example 2, although the heat insulator was applied to almost the entire surface, the cathode side thermal resistance was larger than the anode side thermal resistance, and the temperature rapidly increased due to the heat generated by the initial power generation. Because the temperature rises too much because the fuel is not balanced, the methanol crossover phenomenon that the methanol of fuel flows directly to the cathode without contributing to power generation becomes obvious, and as a result, the voltage of the fuel cell decreases and the output decreases. Inferred.

  In addition, after these evaluations, the fuel was filled again after 24 hours of rest, and power generation was performed to evaluate the output characteristics. The output of the fuel cells of Example 1 and Comparative Example 1 was reproduced. In the fuel cell of Example 2, the output remained low. For this cause investigation, when the AC impedance at 1 kHz was measured, both the fuel cell of Example 1 and Comparative Example 1 showed an impedance change within ± 10%, whereas the fuel cell of Comparative Example 2 showed an initial change. It was observed that the impedance increased to more than 5 times.

  In general, a large increase in impedance is often caused by drying of the electrolyte membrane or peeling of the catalyst layer. Therefore, the fuel cell of Comparative Example 2 was disassembled to investigate drying of the electrolyte membrane or peeling of the catalyst layer. It was. As a result, for the fuel cell of Comparative Example 2, it was confirmed that the moisture content of the electrolyte membrane was reduced to about half of the initial value, and the decrease in proton conductivity due to drying of the electrolyte membrane caused an increase in impedance. It was confirmed that the voltage during power generation decreased.

(Examples 2 and 3, Comparative Examples 3, 4, and 5)
As shown in FIG. 5, as the fuel cell of Example 2, the fuel storage portion is integrally formed from a heat insulator, and the heat insulator is provided up to the side surface portion of the cover plate, which is the outermost surface on the cathode side. When the thermal resistance was 100, a cathode side thermal resistance of 30 was produced. Further, as the fuel cells of Example 3 and Comparative Examples 3 to 5, when a heat insulator is further arranged on the cover plate of the fuel cell of Example 2 and the anode side thermal resistance is 100, the cathode side thermal resistance is respectively 70, 100, 130, and 200 were produced. The adjustment of the cathode-side thermal resistance with respect to the anode-side thermal resistance was performed by changing the thickness of the heat insulator disposed on the cover plate. The configuration other than the heat insulator (including the fuel storage portion) was the same as that of the fuel cell of Example 1.

  Next, the fuel cells of Examples 2 and 3 and Comparative Examples 3, 4, and 5 were subjected to power generation using pure methanol as a liquid fuel in an environment of a temperature of 25 ° C. and a relative humidity of 50%. The output after 10 hours was measured. The results are shown in Table 1. In addition, the anode side thermal resistance and the cathode side thermal resistance in Table 1 are shown as relative values when the anode side thermal resistance is 100.

  As is clear from Table 1, the fuel cells of Examples 2 and 3 in which the cathode side thermal resistance is 70% or less of the anode side thermal resistance can maintain a high output of 200 mW or more. It turns out that the output of the fuel cells of Comparative Examples 3 to 5 exceeding 70% of the thermal resistance is lowered. From this result, it is understood that the cathode side thermal resistance is preferably 70% or less of the anode side thermal resistance.

Sectional drawing which shows 1st Embodiment of the fuel cell of this invention. Sectional drawing which shows the measuring method of anode side thermal resistance. Sectional drawing which shows the measuring method of cathode side thermal resistance. Sectional drawing which shows 2nd Embodiment of the fuel cell of this invention. Sectional drawing which shows 3rd Embodiment of the fuel cell of this invention. Sectional drawing which shows 4th Embodiment of the fuel cell of this invention. The external view which shows an example of a fuel distribution mechanism. Sectional drawing which shows 5th Embodiment of the fuel cell of this invention. Sectional drawing which shows 6th Embodiment of the fuel cell of this invention. Sectional drawing which shows the fuel cell of the comparative example 1. FIG. Sectional drawing which shows the fuel cell of the comparative example 2. FIG. The figure which showed the relationship between the elapsed time after the electric power generation start about the fuel cell of an Example and a comparative example, and the temperature of the outermost surface by the side of a cathode. The figure which showed the relationship between the elapsed time after the start of electric power generation, and an output about the fuel cell of an Example and a comparative example.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Fuel cell, 2 ... Fuel cell, 3 ... Fuel supply mechanism, 4 ... Heat insulator, 5 ... Fuel distribution mechanism, 6 ... Fuel accommodating part, 7 ... Flow path, 8 ... Pump, 11 ... Anode catalyst layer, 12 DESCRIPTION OF SYMBOLS ... Anode gas diffusion layer, 13 ... Anode (fuel electrode), 14 ... Cathode catalyst layer, 15 ... Cathode gas diffusion layer, 16 ... Cathode (air electrode / oxidant electrode), 17 ... Proton (hydrogen ion) conductive electrolyte film

Claims (5)

A membrane electrode assembly including a fuel electrode, an air electrode, and an electrolyte membrane sandwiched between the fuel electrode and the air electrode; and the membrane electrode assembly disposed on the fuel electrode side of the membrane electrode assembly. A fuel cell having a fuel supply mechanism for supplying fuel to the fuel electrode;
A fuel cell, wherein a thermal resistance from the outermost surface on the air electrode side to the air electrode is smaller than a thermal resistance from the outermost surface on the fuel electrode side to the fuel electrode.   2. The fuel cell according to claim 1, wherein a thermal resistance from the outermost surface on the air electrode side to the air electrode is 70% or less of a thermal resistance from the outermost surface on the fuel electrode side to the fuel electrode.   3. The fuel cell according to claim 1, wherein the thermal resistance is a thermal resistance against a heat flow in a direction perpendicular to a main surface of the membrane electrode assembly.   The fuel cell according to any one of claims 1 to 3, wherein the fuel is methanol fuel.   5. The fuel cell according to claim 4, wherein the methanol fuel is a methanol aqueous solution or pure methanol having a methanol concentration of 80% or more.

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