JP2003308869A - Fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell using methanol as a fuel, haying improved power efficiency by completely preventing the leakage of the methanol and achieving stable operation with no leakage of the fuel even in intermittent power generation. <P>SOLUTION: The fuel cell using the methanol as the fuel for generating electric power comprises an electrolytic unit for electrochemically decomposing the methanol to produce hydrogen and a PEM fuel cell unit using the hydrogen as the fuel, integrated therewith. The electrolytic unit can hold a methanol utilization factor higher and the fuel cell unit using the hydrogen as the fuel can hold energy efficiency extremely higher. In no consideration of the power consumption of the electrolytic unit, the fuel cell can be operated at higher efficiency than a direct type methanol fuel cell and it can be easily energized/ deenergized momentarily. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell of the methanol type, which uses methanol as a fuel to generate electricity by an electrochemical reaction, and which produces hydrogen by electrochemically decomposing methanol and hydrogen. PE to
The present invention relates to a fuel cell in which an M-type fuel cell is integrated to generate electricity using methanol as a fuel.

[0002]

2. Description of the Related Art With the development of so-called solid polymer electrolyte fuel cells, it is said that the practical application of fuel cells is approaching, and the range of application thereof has become extremely large. In particular, a fuel cell using a hydrogen fuel is said to be capable of energy efficiency of 50 to 60% with respect to hydrogen, and has a current density of from extremely low to a large range of 2 to 3 A / cm2. There is. Moreover, since the operating temperature may be about 100 ° C. at the maximum, it is attracting attention as a power source for next-generation automobiles for in-vehicle use, from a small combined heat and power supply system. However, although it is recognized that hydrogen as a fuel is important, it is certain that there are still handling problems such as means for obtaining hydrogen, and means for storing and transporting the obtained hydrogen. In other words, there is an idea of a conventional hydrogen cylinder for transporting hydrogen fuel, but with a normal cylinder, one cylinder has a capacity of 7 m3 and only 500
Can only carry about g. It has also been proposed to use a hydrogen storage metal, but up to now, the maximum practical storage capacity is about 2.5% by weight, which is disadvantageous in terms of weight and also causes hydrogen storage and release. There is a problem of proper conditions.
For this purpose, the pyrolysis method is modified. That is, when methanol is used as a fuel, steam reforming of CH3OH + H2O → CO2 + 3H2 is performed in the presence of a catalyst at a temperature of about 300 ° C., and in the case of natural gas or gasoline, the reforming temperature is increased from 700 to 800 ° C. Taking natural gas as an example, hydrogen is obtained by a reaction of CH4 + 2H2O → CO2 + 4H2 or CH4 + O2 → CO2 + 2H2. However, although hydrogen can be obtained from these, even if the stationary type is good, even in the presence of a catalyst, even if the stationary type is good, further temperature reduction is desired. Moreover, the energy loss due to the reforming is considerably large, and thus there is a problem that the total energy consumption for power generation becomes large. That is, the power generation efficiency of the fuel cell is about 30% when the reformer is inserted even if the fuel cell main body has 50% or more. In addition, the equipment becomes large. In particular, as a future problem, with respect to the increase in power consumption that accompanies the higher functionality of portable communication devices and computers, secondary batteries have problems such as excessive weight and insufficient capacity. Ultra-small fuel cells have come to the forefront as a means to deal with this, but pyrolysis and reforming make them large in size, and due to the high temperature problem, practical questions have arisen. ing.

For these purposes, so-called direct methanol fuel cell, which uses methanol as a fuel and sends it directly to the fuel cell as a fuel, has been considered and many studies have been made. In particular, it has been attempted to be used as a power source for a golf cart, a transportation device such as an electric forklift truck, and the above-mentioned portable electronic device that require small size and light weight. However, in a fuel cell that uses methanol as a direct fuel, the overvoltage of the electrode is large, it is difficult to obtain a large current, and the power efficiency is low. The actual generated voltage is about 0.4V against the theoretical voltage of 1.18V, which is about 30%. Is. Further, the ion exchange membrane used as the solid electrolyte has a problem that the methanol fuel easily passes because it is not in an ionic state. There was a problem of reducing efficiency. This problem of passage of methanol can hardly be solved with currently available materials, and even if special treatment is applied to the ion exchange membrane,
It is said that the loss will be about 0%. Therefore, there is a problem that the overall energy efficiency is about 20 to 25%. However, there are great expectations in terms of miniaturization,
In particular, small mobile devices are being studied as the next-generation power source that is expected most.

As described in the prior art, the direct methanol fuel cell has a merit that the mechanism is extremely simple, and on the contrary, has a problem that energy efficiency is deteriorated. However, the low power efficiency of direct power generation is equivalent to the efficiency of a fuel cell that uses a reformer, which includes a reformer. In that respect, there are few problems, but the leakage of methanol, which is the fuel, to the counter electrode is large. This is a problem, and in addition, in a portable device, etc., methanol comes out from the oxygen electrode side as it is, so that it has an additional problem of giving off an odor to the surroundings. In addition, if the fuel is left connected for intermittent use, which is inevitable with mobile devices, so-called flooding will occur and oxygen supply to the air electrode will be insufficient, and in some cases the fuel cell will not work. Had. Further, in the process using pure methanol, there is a problem that corrosion of metal parts and the like progresses because methanol itself is a corrosive agent.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and further improves the power efficiency of a fuel cell using methanol as a fuel and completely prevents the leakage of methanol. An object of the present invention is to provide a fuel cell which can be eliminated and improved in efficiency, and can be operated stably without fuel leakage even in intermittent power generation.

[0006]

The present invention relates to an electrolytic unit in which an anode material and a cathode material are in close contact with a polymer solid electrolyte, and a fuel cell unit in which a hydrogen gas diffusion electrode and an air / oxygen gas diffusion electrode are in close contact with the polymer solid electrolyte. A fuel cell using methanol as a fuel, in which the liquid is impermeable and the gas is permeable through a thin layer, methanol is supplied to the electrolysis unit, and an oxygen-containing gas is supplied to the air / oxygen gas diffusion electrode. Patent application by the present inventors, Japanese Patent Laid-Open No. 2000-
An electrolytic reformer for obtaining hydrogen from methanol water shown in 917470 and JP-A-2000-91740 and a hydrogen fuel cell are connected in series through a membrane through which hydrogen gas or gas is passed. This makes it possible to close the methanol part by generating electricity in the fuel cell using hydrogen generated in the electrolytic reformer as fuel, and since the fuel cell part is completely separated from methanol, the problem of methanol leakage is completely eliminated. It became possible to maintain high energy efficiency.
The details will be described below. The reaction of the fuel cell according to the present invention is shown below. That is, Therefore, the theoretical voltage is 1.24V-0.0 when the electrolysis unit and the fuel cell unit are connected in series.
6V = 1.18V, which theoretically provides almost the same electric power as 1.24V of the hydrogen fuel cell.

[0007] However, it is well known that in the case of electrolysis, there are overvoltages, liquid resistances, etc., and a voltage slightly higher than the theory is required. When an electrolyzed part in which electrode materials are formed on both sides of a polymer solid electrolyte is placed in a methanol solution and electricity is applied, it depends on the conditions, but at a temperature of about 40 to 60 ° C., a cell voltage at a current density of about 10 A / dm 2 is obtained. It was found that electrolysis can be performed at 0.2 to 0.3 V and hydrogen can be obtained from the cathode. It was also found that hydrogen generation occurred on the cathode side almost according to theory. No decomposition of methanol was observed even if the cathode side was methanol water. The present invention utilizes this phenomenon, and even if the electrolysis voltage is 0.3 V, the fuel cell operates with hydrogen, so that a power generation efficiency of 60% or more is possible. Therefore, the total power generation efficiency is 10 A / current density. Since the fuel cell unit has a charging voltage of 0.7 to 0.75 V at dm, it is possible to reduce the supply voltage to the electrolysis unit from 0.3 V to 0.4 to 0.45 V, at least directly. Type methanol fuel cells can be as good or good numbers.

In this way, the hydrogen generation part of the electrolysis unit and the hydrogen electrode part which is the negative electrode of the fuel cell unit are made common,
By electrically connecting in series, when the supply of hydrogen occurs, the fuel cell operates to start power generation, and the electric power causes electrolysis to continuously operate the hydrogen fuel cell. However, if liquid methanol enters the hydrogen electrode on the fuel cell side, it may interfere with the supply of hydrogen, and the use of methanol as fuel on the fuel cell side may lead to deactivation of the fuel cell electrode. Therefore, it is important that the partition wall between the electrolysis portion and the fuel cell is a thin film that prevents liquid permeation and allows only hydrogen to pass. Although foil-like palladium was used as the thin film here, any film may be used as long as it can achieve the purpose. In other words, it can be manufactured under the following conditions. First, the fuel cell can be manufactured under normal conditions. For example, a so-called MEA (Membrane E) in which an ion-exchange membrane is used as a solid electrolyte and graphite powder having platinum supported on both sides thereof is used as a catalyst.
Electrode assembly) is prepared, and a palladium thin film is formed on the hydrogen electrode surface. Although the forming method is not particularly specified, it is necessary that there is no through hole, and optimally, a thin film having a thickness of about 0.1 to 5 μm is formed by the PVD method. However, other methods may be used as long as the object can be achieved, and the thickness is not particularly specified. An electrolytic portion is formed on the palladium film thus formed. This is also acceptable under normal conditions, but the hydrogen electrode catalyst is desirable because platinum or ruthenium is active, but this supporting method is not specified in particular, and platinum or ruthenium black is directly or platinum similarly to the fuel cell electrode. Powder of ruthenium-loaded graphite or the like is formed as a thin bath on a palladium film using a suitable binder. The binder is not specified, but PTFE (Poly Tet) of about 5 to 10% of the catalyst amount is used.
Ra Fluoro Ethylene) can be used. As a result, it can be held extremely stable. However, it is not limited to this. A cation exchange membrane as a solid electrolyte and an ion exchange resin as a binder are attached thereto. It is needless to say that the opposite surface of the ion exchange membrane may be MEA in which a methanol decomposition catalyst having a methanol resistance characteristic such as platinum / ruthenium is formed in advance. In this way, a unit in which the fuel cell and the electrolysis portion are integrated can be formed.

Another manufacturing method is the same up to the step of forming the palladium thin film on the hydrogen electrode side of the MEA of the fuel cell unit, but the hydrogen generating electrode of the electrolysis section is provided on the opposing surface of the hydrogen electrode of the palladium film. Formed, and the product name Na on the upper surface
It is also possible to apply and fix a liquid substance of an ion exchange resin such as a fion liquid to form a solid electrolyte membrane. The membrane formed here must prevent hydrogen or CO2 on the anode side from migrating to the counter electrode side through the membrane. Methanol containing platinum / ruthenium is formed on the opposing surface of the solid electrolyte membrane thus formed. Form a cracking catalyst. In addition, with the palladium thin film as the center, a cathode of the electrolysis unit is formed on one side of the palladium thin film, and a negative electrode (hydrogen electrode) of the fuel cell unit is formed on the other side thereof in advance. You can also stick it. It goes without saying that any method may be used as long as the fuel cell portion and the electrolysis portion can be integrated in this way. The methanol electrode and the oxygen electrode on the fuel cell side of the integrated unit manufactured in this way are electrically connected by applying a load, while blowing a mixed liquid of methanol and water into the methanol electrode on the fuel cell side, First, when a small amount of voltage is applied to a cell or the like to cause hydrogen to be generated, a current will continuously flow due to the power generation action of the fuel cell. The first power source for hydrogen generation may be a secondary battery or a primary battery, but it can be effectively used as a portable device by connecting to a solar cell. FIG. 1 shows a schematic diagram of a cross-sectional structure of a fuel cell according to the present invention. The embodiments will be described below, but it goes without saying that the invention is not limited thereto.

[0010]

[Embodiment] "Embodiment 1" A diameter of 0.2 as a base material and an electric conductor
Using a net mesh made by knitting a nickel wire of mm,
MEA manufactured by E-TEK was attached to the surface as a fuel cell unit by hot pressing. Nickel mesh is on the oxygen electrode side, hot press conditions are 150 ° C, pressure 2 kg
It was / cm2. This MEA has an ion exchange membrane of N
The afion 115 was 40 g / m2 of platinum on the hydrogen electrode and 20 g / m2 of platinum on the air electrode side. The platinum was supported on carbon black Vulkan XC-72. A palladium metal film was applied to the surface facing the nickel mesh by vapor deposition to a thickness of about 1 μm. Pd
Naf made by DuPont as a solid polymer electrolyte on the surface
The same platinum / carbon as the fuel cell oxygen electrode was supported on the palladium side using the ion115, and an electrode material composed of platinum: ruthenium = 1: 1 (mol) was mounted on the opposite side of the methanol side on the carbon black Vulkan XC72. Then, an electrode material supported so as to be 20% with respect to carbon black was attached. The binder was Nafion liquid on both sides, and was 10% (weight) of the catalyst.
The amount of electrode material was 40 g / m 2 with platinum and ruthenium. The loading was carried out by hot pressing at 2 kg / cm2 and a temperature of 150 ° C. The platinum electrode of this electrolysis part MEA was bonded to the fuel cell Pd side. The same nickel knitted mesh as that on the oxygen electrode side was attached to the methanol electrode side as an electric conductor. A 1: 1 mixture of methanol and water was added dropwise to the electrolysis side of the structure thus prepared. Further, oxygen gas was supplied to the oxygen electrode side. A variable resistor was attached as a load, and the battery was first energized. Power generation started due to hydrogen generation.
When the generated voltage was measured while changing the current density, the results were as follows. The operating temperature was 50 ° C. [Example 2] Using the same fuel cell part as in Example 1, a 1 μm palladium foil was placed on the surface of the hydrogen electrode and platinum was carried on the carbon black, and a paint prepared using ethyl alcohol as a binder was applied and dried, followed by Nafion.
The solution was applied three times. A methanol electrolysis electrode material for electrolysis prepared in the same manner as in Example 1 was painted on the surface. This was integrated under the same hot press conditions as in Example 1. The power generation of this product was examined under the same conditions as in Example 1. For the first energization, a reverse current prevention device was attached to the solar cell and used. In this case, it is 0 A at 10 A / dm2.
It was from 5V to 0.52V.

[0010]

EFFECTS OF THE INVENTION According to the present invention, the portable fuel cell system is substantially the same as the direct methanol type in a small fuel cell system, but the power efficiency is improved, and the efficiency is not lowered due to the permeation of methanol. The prevention of gas supply to the oxygen electrode due to the elimination of the problem of reduced efficiency associated therewith has become extremely easy to handle. In other words, 1) By carrying out hydrogen generation by reforming methanol in the electrolysis portion, methanol permeation (cross over)
The problem of reduced efficiency due to r) has been completely resolved. 2) Since the fuel cell part is a hydrogen fuel PEMFC, the efficiency can be kept extremely high and stable operation is possible. 3) The reforming portion and the fuel cell portion can be formed with substantially the same size as the conventional direct methanol fuel cell system, and the structure is extremely simple. 4) Since the fuel cell part is a pure hydrogen fuel type, it has a feature that the electrode catalyst is simple and the amount thereof is small. 5) In a methanol fuel cell, if the fuel is not completely drained when the power is off, the cell will fill up and must be drained when the operation is restarted.
A mechanism for draining fuel at the time of ff is required, which makes the structure complicated. However, since the present invention may fill the electrolytic cell with fuel, the structure of the fuel system is simplified and the on- It is particularly excellent as a power source for a device that performs off. 6) Since no special operation of the fuel system is required, its operability is almost the same as that of an ordinary secondary battery and is extremely good.
Moreover, unlike secondary batteries, charging operation that requires a long time just by adding fuel is not necessary, and handling is easy.
And so on.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a cross-sectional structure of a fuel cell according to the present invention.

[Explanation of symbols]   Electrolysis part for producing hydrogen from methanol   Fuel cell part using generated hydrogen as fuel   Hydrogen permeable membrane   Current collector   Cation exchange membrane (polymer solid electrolyte)   Electrolytic partial anode (methanol electrode)   Electrolytic partial cathode (hydrogen generating electrode)   Fuel cell negative electrode (hydrogen electrode)   Fuel cell positive electrode (oxygen / air electrode)

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 4K021 AA01 BA06 DB31 DB53 DC15                 5H026 AA06 BB04 CX04 CX05 EE02                       EE18                 5H027 AA06 BA11

Claims (5)

[Claims] 1. A liquid electrolyte comprising an electrolytic unit in which an anode material and a cathode material are adhered to a solid polymer electrolyte, and a fuel cell unit in which a hydrogen gas diffusion electrode and an air / oxygen gas diffusion electrode are adhered to the solid polymer electrolyte. A fuel cell that uses methanol as a fuel, which is made to be in close contact with a gas-permeable thin layer to supply methanol to the electrolysis unit and to supply an oxygen-containing gas to the air / oxygen gas diffusion electrode. 2. The fuel cell according to claim 1, wherein the thin film which is permeable to gas and impermeable to liquid is a palladium thin film. 3. A thin film which is permeable to gas and impermeable to liquid is a palladium thin film, a cathode of an electrolysis unit is formed on one surface of the palladium thin film, and a hydrogen gas diffusion electrode of a fuel cell unit is formed on the opposite surface of the palladium film. The fuel cell according to claim 1, wherein the fuel cell comprises: 4. The fuel cell according to claim 1, wherein the thin film which is permeable to gas and impermeable to liquid is a thin film in which palladium is coated on the surface of the polymer film. 5. The fuel cell according to claim 1, wherein the methanol is a mixture of methanol and water.