JP2001143723A - Electrolyte for fuel cell and fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte for a fuel cell that can be used in the temperature range up to approximately 200 deg.C degree from a room temperature without requiring a troublesome operation such as a differential pressure control, etc. SOLUTION: An electrolyte for a fuel cell which consists of 5 oxidized phosphorus : silica = 2:98-50:50 in a mole ratio and consists of an amorphous silica molding body whose electric conductivity is equal to or more than 10 mS/cm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an electrolyte for a fuel cell and a fuel cell.

[0002]

2. Description of the Related Art In general, fuel cells are classified according to an electrolyte to be used. "Alkaline electrolyte type-AFC""solid polymer electrolyte type-PEFC""phosphoric acid type-PAFC""molten carbonate type-MCFC""solid electrolyte Type-SOFC ". Among these fuel cells, PEFC and PAFC have in common that the charge transfer material is a proton. Therefore, although not generally performed, classification as “proton fuel cell” is also possible. In such a proton-type fuel cell, in PEFC, a perfluoroalkylsulfonic acid membrane having fluorine in a polymer skeleton [produced by DuPont, trade name
Nafion] or the like. Further, there is a similar proposal of a perfluoroalkylsulfonic acid membrane having a different perfluorovinyl ether side chain. However, when these are used as fuel cell electrolytes, they have a problem that they cannot be used under high-temperature conditions because of their low heat resistance. Specifically, from the viewpoint of product quality, it could not be used when it could be used in an environment of 80 ° C. or higher. Further, PAFC can be used even at a temperature exceeding 100 ° C. However, since the viscosity of phosphoric acid used as an electrolyte decreases at a high temperature condition of about 200 ° C., strict differential pressure control is required. there were.

[0003]

There is a need for an electrolyte for a fuel cell that can be used in a temperature range from room temperature to about 200 ° C. and does not require complicated operations such as differential pressure control.

[0004]

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, a specific amorphous material containing phosphorus, silicon, oxygen, and hydrogen exhibits performance as an electrolyte for a fuel cell. And found that it can be used as a fuel electrolyte without operation such as differential pressure control even under high temperature conditions,
The present invention has been completed. That is, the gist of the present invention is to provide a fuel cell comprising an amorphous silica molded body having a molar ratio of phosphorus pentoxide: silica = 2: 98 to 50:50 and an electric conductivity at 50 ° C. of 10 mS / cm or more. Exists in the electrolyte.

[0005] In a preferred embodiment of the present invention, the content of phosphorus pentoxide is 100% of the total of phosphorus pentoxide and silica.
2 moles or more and less than 10 moles of the above electrolyte for a fuel cell; the content of phosphorus pentoxide is 10 moles or more and 50 moles or less when the total of phosphorus pentoxide and silica is 100 moles The fuel cell electrolyte having a specific surface area of 10 m ² / g or more according to the BET method; the fuel cell electrolyte containing 1 to 20% by weight of water. Further, another embodiment of the present invention includes a fuel cell using the above-described fuel cell electrolyte, and is preferably a fuel cell used in an environment of 80 to 200 ° C.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
In the present invention, a fuel cell is a power generation device that supplies fuel to a negative electrode and oxidant to a positive electrode, extracts a potential difference between the positive and negative electrodes as a voltage, and supplies the voltage to a load. Hydrazine has been studied for a long time as the negative electrode fuel, and technically completed. However, hydrazine is rarely used at present because of its low output voltage and expensive hydrazine. Currently,
Since the output voltage is high, hydrogen is most often used as the anode fuel. However, direct introduction of an organic substance containing a hydrogen atom, such as methanol, methane, and butane, into a fuel cell is also being studied. The oxidizing agent is generally oxygen in the air, and pure oxygen is used for special applications such as spacecraft.

[0007] In a fuel cell, an oxidation catalyst is arranged with one surface as a negative electrode around an electrolyte, and a reduction catalyst is arranged with the other surface as a positive electrode. The fuel in the anode is
An oxidant is supplied to the positive electrode to generate a voltage. further,
In order to extract the generated voltage, an electron conductor is brought into contact. However, since it is necessary to allow fuel to flow through the negative electrode and oxidant to flow through the positive electrode, a porous material made of an excellent electronic conductor and durable in use atmosphere Position the body. Further, a partition plate for preventing the fuel and the oxidant from being scattered behind is arranged to constitute a unit cell. In many cases, grooves are formed in the porous body or the partition plate for the purpose of uniformly supplying the fuel and the oxidizing agent.

[0008] As described above, the unit cell is from the partition plate to the partition plate. Since the voltage of one cell is as low as 1.0 V or less, several tens to several hundred cells are stacked until the voltage required by the load is reached. Usually used. In most cases, the electrolyte is used after forming a catalyst layer on its surface. When hydrogen gas as a fuel reaches the negative electrode catalyst layer, it is dissociated into electrons and protons by an electrochemical reaction process. When the electrolyte contains moisture, hydrogen of the sulfone group is ionized to generate protons. The protons sequentially move in the electrolyte by the protons generated at the negative electrode and reach the positive electrode. Electrons are sent to the positive electrode via the porous body, the partition plate, and an external load.
Oxygen in the air as an oxidizing agent and protons that have moved through the electrolyte are combined by an electrochemical reaction process to form water. Therefore, the speed of proton conduction = proton conductivity is an important property that determines the performance of a fuel cell. That is, the higher the proton conductivity, the better.

As understood from the above reaction process, the electrolyte functions as a partition so that hydrogen in the fuel and oxygen in the air as the oxidant do not mix, that is, the electrolyte needs to have a gas barrier property. The electrons need to be an insulator because they want to move to the positive electrode via an external load. The fuel cell electrolyte of the present invention has a molar ratio of phosphorus pentoxide: silica = 2: 98-5.
0:50, electric conductivity at 50 ° C. is 10 m
It is composed of an amorphous silica molded body having a S / cm or more.

The amorphous silica molded article of the present invention comprises phosphorus pentoxide: silica = 2: 98 to 50:50 in a molar ratio, and contains no other metal element or polymer material as a substantial constituent. preferable. If the content of phosphorus pentoxide is too small, the proton conductivity will be low, and if it is too large, the mechanical strength of the glass will decrease, and handling will be inconvenient. Similarly, when the amount of silica is too large, the proton conductivity tends to be low, and when the amount is too small, the chemical stability tends to decrease. Preferably, it is an amorphous molded body composed of phosphorus pentoxide: silica in a molar ratio of 5:95 to 20:80. In some cases, according to the present invention, 3
0 mol% or less, desirably 5 mol% or less of TiO ₂ and A
l ₂ O _3, etc. may be those obtained by introducing a. Further, from the viewpoint that phosphoric acid is deliquescent and must be stored in a dry atmosphere when its content increases, the content of phosphorus pentoxide is
When the total of phosphorus pentoxide and silica is 100 mol, 2
From the viewpoint of proton conductivity, the content of phosphorus pentoxide is preferably 10 mol or more and 50 mol or less when the total of phosphorus pentoxide and silica is 100 mol. It is preferably a ratio.

In the present invention, the term "amorphous" refers to a substance which does not contain a crystalline component, for example, a substance which does not have a clear diffraction line peak when measured by powder X-ray. The crystalline component may be contained as long as it does not affect the phosphoric acid content. However, since the crystalline component cannot contain phosphoric acid, the crystalline component is desirably as small as possible. Also,
In the present invention, the term "formed body" means a shaped body (usually a sheet shape) used as a fuel cell, and has a shape that can maintain a certain shape during assembly work and during operation after assembly. What is necessary is just to have the property.

The amorphous silica molded article of the present invention has porosity and has a specific surface area of 10 m
² / g or more, preferably 50 m ² / g or more. The larger the specific surface area, the higher the proton conductivity, and its maximum value is 900 m ² / g. Above 600 m ² / g, the proton conductivity does not change much. In the phosphate-silica glass obtained by the conventional melting method, the proton conductivity was very low because the specific surface area was almost zero.

In the present invention, the amorphous silica molded product 50
The electric conductivity at 0 ° C is 10 mS / cm or more, preferably 15 mS / cm. In the present invention, the electric conductivity is such that a silver paste or a gold electrode is vacuum-deposited on both surfaces after pretreating an amorphous silica molded article having a thickness of about 0.1 mm in an atmosphere having a relative humidity of 30% or more. It is measured by the AC impedance method in a constant humidity atmosphere. This measurement temperature is 50 ° C. If the electric conductivity at 50 ° C. is too low, the electric resistance increases and the output voltage decreases.

The amorphous silica molded body of the present invention is a material in which a hydrogen atom is bonded to oxygen, exists in a water state, or exists as a proton in a basic skeleton. The proton is 0.1 to 5 for the amorphous silica molded body.
% By weight, and preferably 1 to 20% by weight of water. The content of the proton can be determined as (phosphoric acid content x ionization degree), ignoring the amount of dissociation of hydrogen bonded to oxygen, since it is small in quantity.

Since the proton is a conductive carrier, it is desirable to increase the amount thereof. However, if the amount is too large, the proton enters the amorphous silica molded body in the form of water, and the chemical stability of the basic skeleton is reduced. Become. Water is an essential component because it promotes the transfer of protons in the amorphous silica molded article. However, if it is too much, the chemical stability of the glass is reduced. If the amount of protons or water is too small, sufficient proton conductivity cannot be obtained.

The method for producing an amorphous silica molded article according to the present invention may be carried out, for example, by the method of preparing a sol-
This is a production method by a gel method, in which a predetermined phosphoric acid raw material is added in the raw material at that time or in the process, and gelled in a container having a desired shape and size, or the obtained gel is formed into a desired shape and shape. It is formed into a size, dried and fired. By adjusting the firing temperature at this time to usually 100 to 1000 ° C., a silica molded body having high conductivity can be obtained. This silica molded article is amorphous, and the amount of protons and water can be adjusted within the above range.

The silica raw material is preferably a silicon alkoxide or a condensate of a silicon alkoxide obtained by adding water and a catalyst thereto and partially hydrolyzing it in advance. As the silicon alkoxide, a tetraalkoxysilane such as tetramethoxysilane or tetraethoxysilane is particularly preferable. As the phosphoric acid raw material, phosphoric acid can be used as it is, but an alkoxide derivative of phosphoric acid is preferable because it has a uniform composition with the reactivity with silicon alkoxide. Examples of alkoxide derivatives of phosphoric acid include trialkyl phosphoric acids such as trimethoxy phosphoric acid

In the above-mentioned method of mixing the silica raw material and the phosphoric acid raw material, there is no problem even if either is added first or both are added together. It is preferable that these are mixed and then heated in the state of a solution to cause the reaction between them to proceed. Thereafter, water and a catalyst necessary for gelation to proceed are added. The amount of water to be added is not less than the amount at which the alkoxy group in the raw material is replaced by the silanol group, including the amount of water brought in from the raw material, and is preferably such that substantial gelation proceeds even at room temperature. Therefore, the amount of water used when using tetraalkoxysilane and trialkylphosphoric acid is 4 times the amount of tetraalkoxysilane and 3 times the amount of trialkylphosphoric acid, although it depends on the raw materials used and various conditions. And, for example, 0.
Choose 5 to 5 times the amount of water.

The composition thus mixed is gelled in a predetermined container so as to obtain a sample of a desired shape. Heating can be performed at a temperature of up to about 100 ° C. in order to promote gelation, but depending on the composition, cracks may occur in the sample, making that part unusable. If you want to use the gelled shape almost as it is as the product,
It is preferable to leave at room temperature for a certain period after gelation and gelation.

The sample after gelation is usually 100 to 10
A heat treatment is performed at 00 ° C. to prepare an amorphous silica molded body. Heating temperature is higher than the temperature using high proton conductor,
For example, it is preferable to heat at 200 ° C. or higher. Also,
The upper limit temperature varies depending on the composition. As the phosphorus pentoxide content increases, the heating temperature decreases. If the heating temperature is too high, the pores disappear, the amount of protons extremely decreases, and the proton conductivity decreases. The upper limit is 1000 ° C. for a composition containing 10 mol% phosphorus pentoxide and 700 ° C. for a composition containing 50 mol% phosphorus pentoxide.

The heating time varies depending on the heating temperature and the composition, but is preferably in the range of 1 hour to 24 hours. The heating atmosphere is not particularly limited, but is preferably in air. The heating conditions are selected so that the specific surface area of the product after heating is at least 10 m ² / g, preferably at least 50 m ² / g. Most preferably, an amorphous material having a specific surface area of 600 m ² / g or more can be obtained by selecting, for example, the range of 600 ° C. to 800 ° C., and a high proton conductor can be obtained.
The mechanical and chemical stability of the material is also increased.

The amorphous silica molded article of the present invention is generally in the form of a film, a film, a sheet or the like, and has a thickness of, for example, 0.01 to 20 mm.
It is about. Further, a fibrous material, a crushed material, or a product formed into any other shape may be used.

The amorphous compact obtained by the above-mentioned production method can be used in almost any shape, but can be processed into a desired shape if necessary. In addition, by performing a pretreatment of introducing water before using as a proton conductor, stable performance can be obtained from the beginning. The amorphous silica molded body thus obtained is
It has higher proton conductivity from room temperature to 200 ° C. than conventional materials, and is stable even at temperatures of 80 ° C. or more. In addition, the proton transport number is close to 1 and is good. further,
According to this manufacturing method, processing into an arbitrary shape is easy,
The mechanical strength is also good.

The amorphous silica molded article of the present invention is used as an electrolyte instead of a perfluoroalkylsulfonic acid resin membrane in a solid polymer electrolyte type fuel cell. In other words, a substance capable of generating protons such as hydrogen gas is circulated on one side of the electrolyte, and oxygen or the like that consumes protons by a chemical reaction is flowed on the other side, so that protons are conducted in the electrolyte and the electrolyte is connected. Electrons flow through the external circuit to form a battery. In a hydrogen concentration cell, a low-concentration hydrogen gas flows on one side of the electrolyte and a high-concentration hydrogen gas flows on the other side, so that protons are conducted in the electrolyte and current flows to an external circuit connecting the membranes.

A method for manufacturing a fuel cell will be described. The most important component that determines the performance is an “amorphous silica molded body-gas diffusion electrode assembly” in which an amorphous silica molded body, a catalyst, and a carbon porous body are integrally molded. Currently,
It is the mainstream to use a catalyst called "platinum-supported carbon" in which fine particles of platinum are supported on the surface of carbon particles such as carbon black, acetylene black, and Ketjen black.

This catalyst is added to the phosphoric acid aqueous solution and mixed.
At this time, if there is a concern that the catalyst may aggregate, an operation may be performed in which the container is immersed in an ultrasonic cleaner to break the aggregation. As a binder, PTFE (P) is used to impart water repellency to the catalyst layer.
(OlyTetraFluoroEthyleneen)
The catalyst slurry thus obtained can be applied to one or both of the amorphous silica molded body and the carbon porous body by spraying or screen printing.

As the carbon porous body, carbon paper, carbon cloth or the like is used. The catalyst layer is laminated on both sides of the amorphous silica molded body so as to form a carbon porous body behind the catalyst layer, and the temperature is 120 to 160 ° C. (preferably 1 to 160 ° C.).
30 to 150 ° C.), press pressure 5 to 20 MPa (preferably 6 to 12 MPa), time 5 minutes or more (preferably 10
Min) to obtain an amorphous silica molded article-gas diffusion electrode assembly.

The required number of the amorphous silica molded body-gas diffusion electrode assembly and the partition plate having grooves formed so that hydrogen gas and air flow uniformly are laminated via a gasket. Bolts and nuts are tightened to the laminated body with a predetermined torque, and an appropriate pressing force is applied to complete the fuel cell. Since the fuel cell of the present invention has excellent heat resistance,
Suitable for use in an environment at 80 to 200 ° C., which is difficult to use with conventional fuel cells, and especially 100 to 200 ° C.
Suitable for use in an environment.

[0029]

The present invention will be described in more detail with reference to the following examples. [Method of Measuring Electric Conductivity] After pretreating a sample in an atmosphere with a relative humidity of 30%, a silver paste or a platinum electrode is attached to both surfaces of the sample by a vacuum deposition method or the like, and the relative humidity is set to 5%.
The sample was placed under an atmosphere of 4%, and the conductivity (mS / cm) of the sample was measured by an AC impedance method.

Example 1 To 15.44 g of tetraethoxysilane, a mixed solution of water-ethanol-hydrochloric acid adjusted to a molar ratio to tetraethoxysilane of 1: 1: 0.0027 was added and hydrolyzed at room temperature for 1 hour. Decomposition was performed. Trimethoxyphosphoric acid 1.0
9 g was added to the above-mentioned hydrolysis solution of tetraethoxysilane, and a mixed solution of water-ethanol-hydrochloric acid adjusted to a molar ratio of 4: 4: 0.011 with respect to silica glass as a final product was added thereto. For 1 hour. 3 ml of formamide was added to this solution, and the mixture was stirred for 30 minutes, poured out into a Teflon mold, allowed to gel in a room, and further allowed to solidify. After heating the obtained gel to 600 ° C. at 10 ° C./h,
For 5 hours. Then, it was left in an electric oven until it returned to room temperature. Thus, 5P ₂ O ₅ • 95S
A colorless and transparent amorphous silica molded body having an iO ₂ composition (mol%) was synthesized. This was a glass-like circular sample having a thickness of 1 mm and a diameter of about 5 cm. The specific surface area measured by the BET method is 650 m ² / g, and the average pore radius is about 1.5 nm.
In this case, it was glassy but had porosity. The electric conductivity at 50 ° C. was 0.02 mS / cm.

Example 2 A sealing test was performed with nitrogen gas before introducing hydrogen gas and air. At 200 ° C., a nitrogen gas at a pressure of 0.1 MPa was introduced into both the cathode chamber and the anode chamber at a gauge pressure, and an inlet was provided.
The outlet was closed and maintained for a certain period of time to confirm that there was no gas leakage. Subsequently, only the anode chamber was released and held for a certain period of time to confirm that there was no gas leakage. Thereby, it was confirmed that the hermetic seal can be maintained even when there is a differential pressure. next,
At 200 ° C., hydrogen gas was introduced into the anode chamber and air was introduced into the cathode chamber, and the voltage was measured.
Open circuit voltage was obtained. Subsequently, the voltage was measured while changing the current density. The result is shown in FIG.

Example 3 The summer was measured in the same manner as in Example 2 except that the measurement was performed at 50 ° C., and an open circuit voltage of 0.76 V was obtained. Subsequently, the voltage was measured while changing the current density. The result is shown in FIG.

Comparative Example 1 A perfluorosulfonic acid membrane (manufactured by DuPont, trade name: Nafion) having fluorine in the polymer skeleton was used in place of the amorphous silica molded article obtained in Example 1. The current and voltage at 200 ° C. were measured in the same manner as in Example 2. However, the perfluorosulfonic acid film having fluorine in the polymer skeleton was degraded to brown, and neither current nor voltage could be measured.

Discussion In the model test of the fuel cell of Example 2 using the electrolyte as an electrolyte, the amorphous silica molded article of Example 1 was sufficient even at a high temperature of 200 ° C. without operation such as differential pressure control. It shows excellent cell performance and is useful as an electrolyte for a fuel cell that can withstand use under high temperature conditions. Comparative Example 1 shows that the perfluorosulfonic acid film having fluorine in the polymer skeleton cannot function as an electrolyte under high temperature conditions.

[0035]

According to the present invention, it is possible to provide an electrolyte for a fuel cell which can be used in a temperature range from room temperature to about 200 ° C. and does not require complicated operations such as differential pressure control.

[Brief description of the drawings]

FIG. 1 is a diagram showing current-voltage characteristics in a hydrogen-air battery system.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tamotsu Muto 1000 Kamoshitacho, Aoba-ku, Yokohama-shi, Kanagawa Prefecture Mitsubishi Chemical Corporation Yokohama Research Laboratory F-term (reference) 5H026 AA04 EE11 EE12 HH02 HH05 HH06 HH08

Claims (7)

[Claims] 1. A molar ratio of phosphorus pentoxide: silica = 2: 98
~ 50: 50, and the electric conductivity at 50 ° C is 1
An electrolyte for a fuel cell comprising an amorphous silica molded body having a value of 0 mS / cm or more. 2. The fuel cell according to claim 1, wherein the content of phosphorus pentoxide is a ratio of 2 mol or more and less than 10 mol when the total of phosphorus pentoxide and silica is 100 mol. For electrolyte. 3. The content of phosphorus pentoxide is 10 moles or more and 50 moles when the total of phosphorus pentoxide and silica is 100 moles.
2. The electrolyte for a fuel cell according to claim 1, wherein the proportion is not more than mol. 4. A specific surface area of 10 m ² / g by a BET method.
The electrolyte for a fuel cell according to any one of claims 1 to 3, wherein: 5. The fuel cell electrolyte according to claim 1, wherein the electrolyte contains 1 to 20% by weight of water. 6. A fuel cell using the fuel cell electrolyte according to claim 1. 7. The fuel cell according to claim 6, wherein the fuel cell is used in an environment at 80 to 200 ° C.