JP5144183B2 - Electrolyte / electrode assembly and method for producing the same - Google Patents

Description

  The present invention relates to an electrolyte / electrode assembly in which an electrolyte membrane is interposed between an anode side electrode and a cathode side electrode, and a method for manufacturing the same.

  The fuel cell includes a stack configured by stacking a predetermined number of unit cells each having an electrolyte / electrode assembly sandwiched between a pair of separators. Here, the electrolyte / electrode assembly has a form in which an electrolyte film made of a solid polymer film or an oxide film is interposed between the anode side electrode and the cathode side electrode. In this case, it functions as a proton conductor that conducts protons from the anode side electrode toward the cathode side electrode. Examples of the material of the solid polymer film include fluorine resins such as perfluorocarbon sulfonic acid resin and vinylidene fluoride resin, or hydrocarbon resins such as polystyrene resin and polyethylene resin.

  This type of solid polymer membrane has a problem that proton conductivity decreases in a dry environment. Therefore, during operation of the fuel cell, it is generally performed to keep the wet state by controlling the humidity of the reaction gas supplied to each electrode.

  However, there is a problem that a slight time delay occurs until the actual humidity of the reaction gas reaches the control value, and a complicated control device is required to quickly control the actual temperature. .

Therefore, in Patent Document 1, from the viewpoint of ensuring proton conductivity regardless of a decrease in humidity, a conductive component that can occlude hydrogen and can ionize hydrogen into protons is dispersed in a matrix to form an electrolyte membrane. Has been proposed. As the conductive component, ZrO ₂ , SiO ₂ , TiO ₂ , Al ₂ O ₃ , SiO ₂ .Al ₂ O ₃ , Sb ₂ O ₅ , zeolite, activated carbon, carbon nanotube, titania nanotube, silica nanotube, fullerene, etc. In this porous particle, a metal particle such as Pd, Pt, Ru, Ag, Ni, Cu, Sn, Zn or the like is supported.

  Patent Document 2 discloses that when at least one of a solid polymer film, a catalyst layer of an anode side electrode, and a catalyst layer of a cathode side electrode contains silica fine particles or silica fibers, the water retention amount of the solid polymer film There is a description that the solid polymer membrane is prevented from drying for this reason.

JP 2003-282097 A JP-A-6-1111827

  The technique described in Patent Document 1 is not intended to improve the water retention ability of the solid polymer membrane but to improve proton conductivity. For this purpose, metal fine particles are placed in the pores of the porous particles. Is to be carried. However, in this case, there is a concern that impurities are mixed in the process of supporting the metal fine particles. When such a situation occurs, the proton conductivity of the solid polymer membrane decreases.

In addition, when silica is used as described in Patent Document 2, if the reaction gas is in a wet state, the voltage is 1 V at 80 ° C., which is a general operating condition of the polymer electrolyte fuel cell. There is also a concern that it may be changed to H ₄ SiO ₄ or H ₂ SiO ₃ . This is because H ₄ SiO ₄ and H ₂ SiO ₃ are thermodynamically more stable than SiO ₂ under these operating conditions. Of course, in this case, alteration occurs in the electrolyte / electrode assembly.

  As described above, the electrolyte / electrode assembly according to the prior art has an inconvenience that the proton conductivity of the solid polymer membrane is lowered or the concern that the alteration occurs cannot be eliminated.

  The present invention has been made in order to solve the above-described problems, and maintains an excellent water retention ability over a long period of time, does not cause alteration, and has an electrolyte membrane having an electrolyte membrane excellent in strength, and It aims at providing the manufacturing method.

In order to achieve the above object, the present invention provides an electrolyte / electrode assembly in which an electrolyte membrane is interposed between an anode side electrode and a cathode side electrode,
The electrolyte membrane is characterized in that unsupported titania nanotubes having an aspect ratio of 10 to 10 ⁵ are dispersed in a polymer membrane at a ratio of 0.01 to 50% by mass. The unsupported titania nanotube means a titania nanotube in which metal fine particles or the like are not supported on both the outer wall and the inner wall.

  In the present invention, moisture is retained on the outer and inner walls of the unsupported titania nanotubes dispersed in the electrolyte membrane. Since this moisture becomes a path for proton conduction, the electrolyte membrane exhibits excellent proton conductivity.

  Further, in the dry environment, the electrolyte membrane is kept in a wet state by the retained moisture, so that a decrease in proton conductivity is suppressed even in the dry environment.

  Moreover, titania is thermodynamically stable even in a humid environment at a temperature of 100 ° C. or less, which is a general operating temperature of a polymer electrolyte fuel cell. In other words, since it does not change into another substance by reacting with moisture, the concern that the electrolyte membrane is altered is eliminated.

  In addition, since the titania nanotube has a fiber shape, the titania nanotube functions as a filler like the fiber in the fiber-reinforced plastic, thereby improving the tensile strength of the electrolyte membrane. For this reason, the electrolyte membrane excellent in intensity | strength can also be comprised. That is, an electrolyte / electrode assembly having excellent durability can be configured.

The effect of retaining moisture becomes even more pronounced when the outer diameter of the unsupported titania nanotube is 3 to 10 ⁵ nm. That is, as the unsupported titania nanotubes dispersed in the electrolyte membrane, those having an outer diameter of 3 to 10 ⁵ nm are preferable.

Further, the present invention is a method for producing an electrolyte / electrode assembly in which an electrolyte membrane is prepared by interposing an electrolyte membrane between an anode side electrode and a cathode side electrode,
A step of dispersing unsupported titania nanotubes having an aspect ratio of 10 to 10 ⁵ in the solution to be the electrolyte membrane at a ratio of 0.01 to 50% by mass after film formation;
Forming a film using the solution in which the unsupported titania nanotubes are dispersed, and providing an electrolyte membrane;
Providing each of the end surfaces of the electrolyte membrane with an anode side electrode or a cathode side electrode;
It is characterized by having.

  In general, simply adding titania nanotubes to the solution makes it difficult to aggregate or precipitate the titania nanotubes and disperse them. Therefore, for example, ultrasonic dispersion, bead mill, or dispersion using centrifugal force is performed. As a result, titania nanotubes are dispersed in the solution, and an electrolyte membrane in which titania nanotubes are dispersed can be easily obtained by forming a film from the solution.

  Moreover, in the present invention, it is not particularly necessary to hold the metal fine particles or the like on the titania nanotube or to perform surface treatment on the titania nanotube. For this reason, it can avoid that an impurity mixes in an electrolyte membrane, Therefore, it can avoid that a proton conductivity falls by mixing of an impurity.

As described above, it is preferable to use unsupported titania nanotubes having an outer diameter of 3 to 10 ⁵ nm.

  When performing the bead mill, it is preferable to use beads having a diameter of 0.0001 to 1 mm and a peripheral speed of 1 to 50 m / sec. Thereby, the titania nanotube can be dispersed in the solution without bending or breaking.

  According to the present invention, an electrolyte membrane is configured by dispersing titania nanotubes having a predetermined aspect ratio in a solid polymer film at a predetermined ratio. For this reason, while maintaining the outstanding water retention ability over a long period of time, the electrolyte-electrode assembly which shows the outstanding proton conductivity and intensity | strength can be comprised.

  Moreover, titania nanotubes are thermodynamically stable even in a humid environment at a temperature of 100 ° C. or lower, which is a general operating temperature of a polymer electrolyte fuel cell. Therefore, the concern that the electrolyte membrane may change during operation can be eliminated.

  Hereinafter, preferred embodiments of the electrolyte / electrode assembly and the manufacturing method thereof according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is an enlarged cross-sectional view of a main part of a stack 12 in which an electrolyte / electrode assembly 10 according to the present embodiment is incorporated. As shown in FIG. 1, the stack 12 includes a plurality of unit cells 14 each including the electrolyte / electrode assembly 10 and constitutes a fuel cell.

  The unit cell 14 sandwiches an electrolyte / electrode assembly 10 having an electrolyte membrane 20 interposed between a pair of separators 22 and 22 between an anode side electrode 16 and a cathode side electrode 18 each having a noble metal catalyst layer and a gas diffusion layer. Is provided. As a material of the separator 22, carbon, stainless steel, titanium alloy, or the like is generally selected.

  The separators 22 project in opposite directions to supply a fuel gas containing hydrogen to the anode side electrode 16 or to supply an oxidant gas containing oxygen to the cathode side electrode 18. The corrugated portion 28 in which the first convex portion 24 and the second convex portion 26 are alternately continued is provided. And the horizontal convex surfaces 24a and 26a are provided in the 1st convex part 24 and the 2nd convex part 26, respectively.

  When the stack 12 is configured, for example, the horizontal top surface 24 a of the first convex portion 24 contacts the anode side electrode 16, and the horizontal top surface 26 a of the second convex portion 26 contacts the cathode side electrode 18. Then, the oxidant gas is circulated through the oxidant gas flow path 30 between the first convex part 24 and the cathode side electrode 18, and the fuel gas flow path between the second convex part 26 and the anode side electrode 16. The fuel gas is circulated to 32. That is, the wavy portion 28 functions as a supply passage for supplying the reaction gas to the electrodes 16 and 18.

  On the other hand, the electrolyte membrane 20 is configured by dispersing titania nanotubes in a solid polymer membrane.

  The solid polymer membrane is not particularly limited as long as it is a substance capable of conducting protons, but is not limited to fluoropolymers such as perfluorocarbon sulfonic acid resin and vinylidene fluoride resin, or polystyrene resin or polyethylene resin. A hydrocarbon resin is a preferred example.

  A titania nanotube is a cylindrical or hollow substance, and has an inner wall and an outer wall. The titania nanotube employed in the present embodiment is a non-supported one in which metal fine particles are not supported on both the inner wall and the outer wall. As will be described later, moisture is retained on the inner wall and the outer wall.

The titania nanotubes preferably have an outer diameter as small as possible and as long as possible, in other words, a large aspect ratio. Specifically, the length / outer diameter of the titania nanotube, that is, the aspect ratio is set to 10 to 10 ⁵ . In a titania nanotube having an aspect ratio of less than 10, the proton conductivity of the electrolyte membrane 20 does not increase. Further, the effect of ensuring the strength of the electrolyte membrane 20 is poor.

The aspect ratio of a general titania nanotube is 10 ⁵ or less. Accordingly, the preferred aspect ratio of the titania nanotube is 10 to 10 ^5, more preferably from 50 to 10 ^5, more preferably from 100 to 10 ^5.

Moreover, it is preferable that the outer diameter of a titania nanotube exists in the range of 3-10 < ⁵ > nm. If it is less than 3 nm, it is not easy to maintain a tubular or hollow shape, and therefore, the effect of retaining moisture on the inner wall is poor. On the other hand, when it exceeds 10 ⁵ nm, proton conductivity tends to decrease. The outer diameter is more preferably 3 to 100 nm, and further preferably 5 to 20 nm.

  The ratio of the titania nanotubes configured as described above to the electrolyte membrane 20 is 0.01 to 50 when the mass of the electrolyte membrane 20 is 100. That is, titania nanotubes are dispersed in the electrolyte membrane 20 at a ratio of 0.01 to 50% by mass. When the content is less than 0.01% by mass, sufficient retention ability is not exhibited in the electrolyte membrane 20. On the other hand, if it exceeds 50% by mass, the solid polymer membrane that contributes to proton conduction is relatively reduced, making it difficult to ensure sufficient proton conductivity. The ratio of the titania nanotube is more preferably 0.1 to 30% by mass, and further preferably 0.5 to 10% by mass.

  As the anode side electrode 16 and the cathode side electrode 18 that interpose the electrolyte membrane 20, known ones may be provided.

  When the fuel cell including the electrolyte / electrode assembly 10 thus configured is raised to a predetermined temperature and the reaction gas is supplied, power generation occurs. That is, oxidant gas and fuel gas are respectively supplied through the oxidant gas flow path 30 and the fuel gas flow path 32, and hydrogen in the fuel gas is ionized at the anode side electrode 16 to generate protons. The membrane 20 moves to the cathode side electrode 18 to generate water together with oxygen in the oxidant gas and electrons that have reached the cathode side electrode 18 via an external load. The produced water is discharged to the outside together with the reaction gas in the form of water vapor or water droplets.

  Here, titania nanotubes are dispersed in the electrolyte membrane 20 as described above. The water vapor entrained in the reaction gas to control the humidity of the reaction gas and the water vapor generated by the above reaction are not supported by metal fine particles on the outer wall or inner wall of the titania nanotube. The inner wall is promptly held. As a result, moisture is retained in the electrolyte membrane 20.

  Therefore, according to the present embodiment, the wet state of the electrolyte membrane 20 can be maintained for a long time without depending on the humidity of the reaction gas. In addition, because of this, it is not necessary to strictly and rapidly control the humidity of the reaction gas, so that the operation management of the fuel cell becomes easy.

  Moreover, the water vapor retained on the outer and inner walls of the titania nanotube serves as a proton conduction path. As is well known, titania nanotubes are linear elongated objects, and for this reason, protons move linearly along the extending direction of titania nanotubes. As can be seen from this, in the present embodiment in which titania nanotubes are dispersed, proton transfer is less likely to be inhibited compared to the case where titania having a bent portion such as a cluster structure is added. Will improve.

Furthermore, titania (TiO ₂ ) nanotubes are thermodynamically stable at 100 ° C. or lower, which is a general operating condition of a fuel cell using a solid polymer membrane. That is, there is no concern of changing from TiO ₂ to other substances. In other words, it is possible to eliminate the concern that the titania nanotube is altered and the proton conductivity is changed accordingly.

  In addition, since the titania nanotube has a fiber shape, swelling of the electrolyte membrane 20 is suppressed. That is, since the electrolyte membrane 20 is prevented from swelling / shrinking due to a change in the water content accompanying the operation / stop of the fuel cell, the stress generated with the volume change is significantly reduced. For this reason, it can avoid that the electrolyte membrane 20 deteriorates. Further, the fiber-shaped titania nanotube functions as a filler in the same manner as the fiber in the fiber-reinforced plastic, thereby improving the tensile strength of the electrolyte membrane. Therefore, the strength of the electrolyte membrane 20 is also improved.

  As described above, according to the present embodiment, the electrolyte / electrode assembly 10 that maintains excellent water retention ability over a long period of time, exhibits excellent proton conductivity and strength, and has no fear of deterioration during operation. Can be configured.

  Next, a manufacturing method of the above-described electrolyte / electrode assembly 10 will be described. This manufacturing method includes a first step in which titania nanotubes are dispersed in a solution to be the electrolyte membrane 20, a second step in which the electrolyte membrane 20 is formed by forming a film from the solution, and an anode side electrode on each end face of the electrolyte membrane 20 16 or the third step of providing the cathode side electrode 18 respectively.

  First, in a 1st process, the solution used as the electrolyte membrane 20, for example, the predetermined concentration solution of perfluorocarbon sulfonic acid resin is prepared. In this solution, titania nanotubes are dispersed.

As described above, as the titania nanotube, those having an aspect ratio in the range of 10 to 10 ⁵ and having no metal fine particles supported on the outer wall and the inner wall are used, and are provided by the second step. The electrolyte film 20 is added to the solution at a ratio of 0.01 to 50% by mass. Of course, the titania nanotube preferably has an outer diameter in the range of 3 to 10 ⁵ nm.

  Here, simply adding the titania nanotubes to the solution causes the titania nanotubes to aggregate or precipitate, making it difficult to disperse them in the solution. For this reason, in this embodiment, for example, ultrasonic dispersion or bead milling is performed to forcibly disperse. Or you may make it disperse | distribute using a centrifugal force.

When adopting a bead mill, for example, titania nanotubes are added to a mixture of an appropriate solvent such as n-propyl alcohol or water and the above solution, and this mixture is further contained in a vessel containing ZrO ₂ beads. Then, the rotating body (pin rotor) provided at the center of the vessel is rotated.

  In this case, it is preferable to use beads having a diameter of 0.0001 to 1 mm. In beads having a diameter of less than 0.0001 mm, it is not easy to disperse titania nanotubes. In addition, when beads having a diameter exceeding 1 mm are used, excessive stress is applied to the titania nanotube, and there is a concern that the titania nanotube may be bent or damaged.

  Moreover, it is preferable to set the peripheral speed of the outermost periphery of a pin rotor within the range of 1-50 m / sec. If it is less than 1 m / sec, it is not easy to disperse titania nanotubes. On the other hand, when rotating at a peripheral speed exceeding 50 m / sec, the titania nanotube may be bent or broken.

  The duration of the bead mill may be, for example, 1 to 360 hours, depending on the amount of titania nanotubes.

  Using the solution in which the titania nanotubes are dispersed in this manner, a film is formed in the second step. That is, fluidity is lost by drying the solution. In drying, heating may be performed at a temperature lower than the glass transition point. Thereby, the electrolyte membrane 20 is obtained.

  Next, in the third step, the anode side electrode 16 or the cathode side electrode 18 is provided on each end face of the electrolyte membrane 20. For example, both the noble metal catalyst layer and the gas diffusion layer constituting each electrode may be pressure-bonded to each end face of the electrolyte membrane 20 by hot pressing.

  As described above, the electrolyte / electrode assembly 10 including the electrolyte membrane 20 in which titania nanotubes are dispersed in the solid polymer membrane is obtained.

  Thus, in the manufacturing method according to the present embodiment, the titania nanotubes do not hold metal fine particles or the like, and the surface treatment is not performed on the titania nanotubes. For this reason, it is possible to avoid a decrease in proton conductivity due to impurities mixed into the electrolyte membrane 20.

  In the above-described embodiment, the perfluorocarbon sulfonic acid resin is exemplified as the solid polymer film. However, it goes without saying that other solid polymer films may be used.

  A Nafion (trade name, perfluorocarbon sulfonic acid resin manufactured by DuPont) with a concentration of 20%, n-propyl alcohol, and water were mixed in a vessel to obtain a mixed solution. Titania nanotubes were added to the mixture so that the ratio after film formation was 1% by mass.

Further, ZrO ₂ beads having a diameter of 1 mm or less were put in a vessel, the vessel was sealed, and the pin rotor was rotated at a peripheral speed of 50 m / sec for about 300 minutes. Thereby, a Nafion solution in which titania nanotubes were dispersed was obtained. The bead mill may be performed while circulating the Nafion solution outside the pot.

  Next, a part of this Nafion solution was weighed, transferred to a container, dried, and then heat-treated. The heating temperature was less than the glass transition point. As a result, the fluidity of the Nafion solution was lost, and an electrolyte membrane made of Nafion in which titania nanotubes were dispersed was formed.

After that, carbon carrying Pt (Pt carrying amount: about 0.5 mg / cm ² ) and carbon paper are sequentially laminated on each end face of the electrolyte membrane, and both are pressed against each end face by hot pressing. Then, an anode side electrode and a cathode side electrode each having a noble metal catalyst layer and a gas diffusion layer were provided to form an electrolyte / electrode assembly. Further, the unit cell was provided by sandwiching the electrolyte / electrode assembly with a pair of separators. This is an example.

  For this unit cell, a power generation test was performed by supplying a reaction gas having a humidity of 100% to both the anode side electrode and the cathode side electrode. In addition, while supplying hydrogen gas to an anode side electrode, air was supplied to the cathode side electrode, and the ratio of hydrogen gas and air was 1.5: 2 by volume ratio. Then, after the power generation test was completed, both hydrogen gas and air were switched to dry gas, and the current-voltage characteristics were examined with the same volume ratio as above. FIG. 2 is a graph showing the relationship between current density and potential after 2 hours have elapsed after switching to dry gas and after 3.5 hours have elapsed.

  For comparison, an electrolyte membrane made of Nafion was produced according to the example except that titania nanotubes were not added to the mixed solution, and a unit cell including this electrolyte membrane was produced. This is a comparative example.

  For this unit cell, a power generation test was performed using a wet gas, and the current-voltage characteristics were examined by switching hydrogen gas and air to a dry gas, as in the example. FIG. 2 also shows the relationship between the current density and the potential after 3.5 hours have elapsed after switching to dry gas for 2 hours.

  From FIG. 2, it is clear that a high potential is exhibited over a long period of time as the titania nanotube is dispersed in the electrolyte membrane. This means that the titania nanotubes exhibited an excellent water retention capability in the electrolyte membrane.

  That is, by dispersing titania nanotubes in the electrolyte membrane, an electrolyte / electrode assembly that exhibits sufficient power generation characteristics even in a dry environment can be configured.

It is a principal part expanded sectional view of the stack in which the electrolyte-electrode assembly which concerns on this Embodiment was integrated. It is a graph which shows the relationship between the current density of each unit cell of Example and a comparative example at the time of changing to dry gas after the power generation test by wet gas, and a predetermined time, and potential.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Electrolyte electrode assembly 12 ... Stack 14 ... Unit cell 16 ... Anode side electrode 18 ... Cathode side electrode 20 ... Electrolyte membrane 22 ... Separator 28 ... Wave-shaped part 30 ... Oxidant gas flow path 32 ... Fuel gas flow path

Claims (5)

An electrolyte / electrode assembly in which an electrolyte membrane is interposed between an anode side electrode and a cathode side electrode,
The electrolyte membrane is unsupported titania nanotube aspect ratio of 10 to 10 ⁵ is dispersed in a polymer film in a proportion of 0.01 to 50 wt%,
An electrolyte / electrode assembly characterized by retaining moisture on the inner wall and outer wall of the unsupported titania nanotube . 2. The electrolyte / electrode assembly according to claim 1, wherein an outer diameter of the unsupported titania nanotube is 3 to 10 ⁵ nm. An electrolyte / electrode assembly manufacturing method for producing an electrolyte / electrode assembly by interposing an electrolyte membrane between an anode side electrode and a cathode side electrode,
To a solution serving as the electrolyte membrane, the ratio the aspect ratio Ri 10 to 10 ⁵ der, the unsupported titania nanotubes that holds moisture on the inner wall and the outer wall proportion after film is 0.01 to 50 wt% And the step of dispersing in
Forming a film using the solution in which the unsupported titania nanotubes are dispersed, and providing an electrolyte membrane;
Providing each of the end surfaces of the electrolyte membrane with an anode side electrode or a cathode side electrode;
A method for producing an electrolyte / electrode assembly, comprising: The manufacturing method of claim 3, wherein the unsupported titania nanotube manufacturing method of the electrolyte electrode assembly having an outer diameter and wherein the use of what is 3 to 10 5 ^nm.   5. The manufacturing method according to claim 3, wherein beads are milled at a dispersion speed of 1 to 50 m / sec using beads having a diameter of 0.0001 to 1 mm in the dispersion. Manufacturing method of joined body.

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