JPH113724A - Direct type methanol fuel cell having solid polymer electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a crossover of methanol and to improve a discharge characteristic by using a three-layer electrolyte layer, arranging an acidic aqueous solution layer between two slid polymer electrolyte layers, and fluidizing the acidic aqueous solution layer. SOLUTION: An electrolyte layer consists of three layers formed, by arranging an acidic aqueous solution layer between two solid polymer electrolyte layers, and the acidic aqueous solution layer is fluidized. As the solid polymer electrolyte, ion exchange film resin, such as perfluorocarbon sulfonate-based resin or styrene-divinyl benzene copolymer-based resin is used, and as the acidic aqueous solution, an aqueous solution of an acid, such as sulfuric acid or hydrochloric acid is used. Nafion (R) 112 films 1, 2 which serve as the solid polymer electrolyte are interposed between a positive electrode and a negative electrode, so that the surface of a catalyst on the electrode is faced to the Nafion film, and they are pressed with a hot press to form an electrode/electrolyte film joined body. Air is supplied to the positive electrode, and a methanol - containing aqueous solution is supplied to the negative electrode, catalytic activity of the positive electrode is maintained, and deterioration in a battery characteristic is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for supplying methanol to a negative electrode and performing an electrochemical reaction directly at the negative electrode to obtain electric power.
The present invention relates to a direct methanol fuel cell.

[0002]

2. Description of the Related Art A fuel cell has two electrodes on both sides of an electrolyte which is an ion conductor, supplies an oxidizing gas (oxidizing agent) such as oxygen or air to one electrode, and supplies hydrogen or carbonized gas to the other electrode. A battery that supplies fuel (reducing agent) such as hydrogen and causes an electrochemical reaction to generate electricity.

There are many types of fuel cells. A direct methanol fuel cell (abbreviated as DMFC) directly supplies methanol, which is a fuel, to a negative electrode. Many fuel cells use hydrogen or hydrogen as a fuel. Compared to using hydrocarbon reformed hydrogen, the equipment is not only simpler, but also easier to transport and store the fuel itself, and may be able to operate at temperatures below 100 ° C Therefore, it is considered to be most suitable for small size and portable use,
It is regarded as a promising power source for future vehicles.

[0004] As an electrolyte of a direct methanol fuel cell, an alkaline type is changed from an initial type to an acid type, and in recent years, a solid polymer electrolyte is often used. By using solid polymer electrolytes, the operating temperature could be higher than with liquid electrolytes, and the performance of direct methanol fuel cells was significantly improved over the earlier ones.

A direct methanol fuel cell (PEM-DMFC) using a solid polymer electrolyte is a Du Pont
It has a structure in which both sides of a proton-conductive solid polymer electrolyte such as Nafion manufactured by the company are sandwiched between two porous electrodes with a catalyst attached, methanol is directly supplied to the negative electrode, and oxygen or air is supplied to the positive electrode. It is. At the negative electrode,
Methanol and water react to generate carbon dioxide, protons, and electrons, which work through an external circuit before reaching the positive electrode. Further, protons reach the positive electrode through the solid polymer electrolyte. At the positive electrode, oxygen, protons, and electrons react to generate water. Therefore, the entire reaction of the direct methanol fuel cell is a reaction in which water and carbon dioxide are generated from methanol and oxygen. These reactions proceed with the aid of a catalyst in the electrode. The theoretical voltage for this reaction is 1.1
The voltage is 8 V, but in an actual battery, the voltage is lower than this value due to IR drop and the like.

[0006] Although the characteristics of direct methanol fuel cells have been considerably improved, they have the disadvantage that the output and efficiency of the cells are lower than those of other fuel cells. This is due to the low activity of the catalyst that oxidizes methanol, and the short-circuit phenomenon in which methanol diffuses through the electrolyte to reach the anode, where it reacts directly with the oxidant on the catalyst of the cathode (this phenomenon is called “crossover” 2)
[M. P. Hoga
rth and H.R. A. Hards Platinu
m Metals Rev. 40 (4) 150
(1996)].

[0007] In the direct methanol fuel cell, a catalyst is required for both the positive electrode and the negative electrode, and the catalyst of the negative electrode is particularly problematic. That is, when methanol is oxidized on a platinum catalyst, carbon monoxide adsorbed on the platinum is generated, which poisons the platinum and reduces the catalytic activity [R. Parsons
and T. Vandernote J .; Elect
roanal. Chem. , 257 9 (1988)]
It is believed that. In order to quickly remove carbon monoxide from the surface of platinum, the addition of a secondary metal has been studied. At present, it is known that a platinum-ruthenium system is the most highly active catalyst.

The ion exchange resin membrane as a solid polymer electrolyte does not show any conductivity in a dry state, but usually shows high conductivity by swelling with water.

The structure of the Nafion membrane of Du Pont, which is best known as a solid polymer electrolyte, has a water-repellent polyfluoroethylene [-(CF ₂ ) main chain.
_n -] and skeletal portion, consisting of the portion of the sulfonic acid group is a hydrophilic ion exchange groups attached to side chains (-SO ₃ H). When this membrane absorbs water, the model in which hydrophilic ion exchange groups are aggregated to form spherical clusters, and the clusters are dispersed in a polyfluoroethylene matrix, is a powerful model. In the model,
Water is contained in the cluster portion, these clusters are connected by a narrow passage, believed to [Takenaka carpenter試季Report 36 81 (1985)]. It is presumed that other solid polymer electrolytes have a similar structure.

[0010]

When methanol comes into contact with a solid polymer electrolyte that has absorbed water, methanol easily dissolves in water. Therefore, the methanol dissolves in water in clusters in the solid polymer electrolyte and passes through it. The positive electrode is reached and oxidized on the positive electrode catalyst.

On the other hand, in the positive electrode, the noble metal as a catalyst electrochemically oxidizes methanol passing through the electrolyte from the negative electrode, so that the characteristics of the positive electrode are significantly deteriorated. The phenomenon that methanol reaches the positive electrode through the electrolyte,
As a method of reducing the so-called crossover even slightly, a method of increasing the pressure of oxygen or air and a method of increasing the operating temperature of the battery to 100 ° C. or higher have been studied, and the characteristics have been considerably improved. Properties have not been obtained. In addition, in order to increase the pressure of oxygen or air and to raise the operating temperature of the battery, a device for this is required, and the whole battery becomes complicated.

In order to improve the characteristics of a direct methanol fuel cell, it is necessary that the fuel, methanol, reaches the cathode through the solid polymer electrolyte, that is, the crossover of methanol must be minimized or eliminated. There was a need for specific means for that.

[0013]

According to the present invention, there is provided a direct methanol fuel cell comprising three electrolyte layers each having an acidic aqueous solution layer between two solid polymer electrolyte layers. , To make the acidic electrolyte layer flow.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a direct methanol fuel cell using a solid polymer electrolyte according to the present invention, a porous electrode having a catalyst layer attached to both sides of a proton conductive solid polymer electrolyte is joined to a negative electrode. A mixture of methanol and water is supplied to the positive electrode with oxygen or air to extract electricity.

As the substrate of the porous electrode, for both the positive and negative electrodes, a porous substrate such as carbon paper, a molded carbon article, a sintered carbon article, a sintered metal, or a foamed metal is used after being subjected to a water-repellent treatment. Polytetrafluoroethylene or the like can be used as the water repellent.

As the noble metal catalyst, platinum, platinum alloy, gold, gold alloy, palladium, palladium alloy and the like can be used for the positive electrode, and platinum or an alloy of platinum and ruthenium, gold and rhenium can be used for the negative electrode. Fine powder or a carbon powder supporting a noble metal can be used.

The porous electrode according to the present invention is produced by applying a catalyst dispersion solution to the surface of a water-repellent electrode.
The catalyst dispersion solution is obtained by uniformly mixing fine particles of a catalyst such as platinum black or a carbon powder carrying the catalyst, a water repellent such as polytetrafluoroethylene, and a solid polymer electrolyte dissolved in an alcohol or the like in an appropriate solvent. It is prepared by mixing.

The electrolyte layer of the direct methanol fuel cell according to the present invention is composed of three layers having an acidic aqueous solution layer between two solid polymer electrolyte layers, and allows the acidic aqueous solution to flow. The following two methods can be considered for flowing the acidic aqueous solution.

First, when the direct methanol fuel cell is small and is to be mounted on a mobile body such as an automobile, the acidic aqueous solution is stored in a tank in advance, and is supplied to the fuel cell to supply two fuel cells. After passing between the solid polymer electrolyte layers,
Take it out of the battery. The acidic aqueous solution that has passed between the two solid polymer electrolyte layers contains a small amount of methanol contained in the negative electrode side solid polymer electrolyte layer. When this acidic aqueous solution containing methanol is circulated between the solid polymer electrolyte layers, the concentration of methanol in the acidic aqueous solution increases, and when passing between the solid polymer electrolyte layers, methanol is removed from the positive-electrode side solid polymer electrolyte. It moves to the layer, further reaches the positive electrode, and causes crossover, so it is stored in another tank, and after the moving body is stopped, it is taken out, and the acidic aqueous solution and methanol may be separated by another device. .

The second method is that when a direct methanol fuel cell is large and used as a stationary type, an acidic aqueous solution containing a small amount of methanol that has passed between two solid polymer electrolyte layers of the fuel cell is used. The treatment may be carried out by a device provided in the fuel cell for separating the acidic aqueous solution and methanol to remove the methanol, and then the acidic aqueous solution may be circulated.

As the solid polymer electrolyte used in the direct methanol fuel cell, various ion exchange membrane resins such as a perfluorocarbon sulfonic acid resin and a styrene-divinylbenzene copolymer resin can be used.

As the acidic aqueous solution, an aqueous solution of any acid such as sulfuric acid or hydrochloric acid can be used.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure and characteristics of a direct methanol fuel cell according to the present invention will be described in detail using preferred embodiments.

Example 1 A direct methanol fuel cell was prepared in which three electrolyte layers having a dilute sulfuric acid layer were provided between two solid polymer electrolyte layers, and the dilute sulfuric acid layer was flowed.

First, a carbon paper having a porosity of 75% and a thickness of 0.40 mm was cut into a size of 50 mm × 50 mm, washed with 2-propanol, and dried to obtain a dispersion containing 20% by weight of polytetrafluoroethylene. It is immersed in a polytetrafluoroethylene aqueous solution for several seconds, taken out and air-dried, and then fired at 300 ° C. for 10 minutes in an argon gas atmosphere. About 3 mg / cm ² of polytetrafluoroethylene is attached to the obtained water-repellent carbon paper.

Next, a catalyst dispersion solution was prepared. First,
5 g of platinum-supported carbon containing 10% by weight of platinum is put into a stainless steel beaker, 80 ml of water is added and stirred, and 80 ml of 2-propanol is further added and stirred for 1 hour. Next, 2 ml of an aqueous dispersion of polytetrafluoroethylene containing 20% by weight of polytetrafluoroethylene was added, stirred, and 10 ml of a commercially available Nafion solution (containing 5% by weight of Nafion, manufactured by Aldrich Chemical) was added. 1 with a stirrer
After stirring for an hour, a catalyst dispersion solution for a positive electrode was prepared.

Separately, a catalyst-dispersed solution for a negative electrode was prepared in the same procedure as for the positive electrode except that 10 g of platinum-ruthenium-supported carbon containing 10% by weight of platinum and 10% by weight of ruthenium was used.

For both the positive electrode and the negative electrode, a catalyst dispersion solution was applied to the surface of the water-repellent carbon paper, respectively.
Naturally dried. Further, after applying again and drying naturally, 11
After drying at 0 ° C. for 1 hour, an electrode for a direct methanol fuel cell having a catalyst layer attached to one surface was obtained. The thickness of the catalyst layer of the positive electrode was about 0.05 mm, the amount of platinum on the electrode surface was about 1.0 mg / cm ² , the thickness of the catalyst layer of the negative electrode was about 0.08 mm, and the amount of platinum on the electrode surface was about 0.08 mm / cm ^2. Is about 2.
It was 0 mg / cm ² .

The positive electrode thus obtained and the Nafion 112 membrane as a solid polymer electrolyte are sandwiched such that the surface of the electrode on which the catalyst is attached is on the Nafion side, and hot pressed at 140 ° C. for 3 minutes. To form a positive electrode / electrolyte membrane assembly. Similarly, the negative electrode and the Nafion 112 film as the solid polymer electrolyte were hot-pressed and joined to produce a negative electrode / electrolyte membrane assembly.

Next, the positive electrode / electrolyte membrane assembly and the negative electrode / electrolyte membrane assembly were placed with the solid polymer electrolyte membranes facing each other, and a dilute sulfuric acid (1 mol / l) having a thickness of about 0.5 mm was interposed therebetween. Attach the (sulfuric acid aqueous solution) layer. The dilute sulfuric acid is supplied from outside the fuel cell, passes between the two solid polymer electrolyte layers of the fuel cell, and flows out of the fuel cell. Dilute sulfuric acid may flow naturally from the upper part to the lower part of the fuel cell, or may be forced to flow using a pump.

FIG. 1 shows a cross-sectional structure of a direct methanol fuel cell according to the present invention. In FIG. 1, reference numeral 1 denotes a Nafion 112 membrane as a negative-side solid polymer electrolyte;
2 is Nafion 112 as a solid polymer electrolyte on the positive electrode side
The membrane 3, 3 is a diluted sulfuric acid layer, 4 is a diluted sulfuric acid inlet, 5 is a diluted sulfuric acid outlet, and the diluted sulfuric acid 3 is supplied to the battery from the diluted sulfuric acid inlet 4.
It flows out of the dilute sulfuric acid outlet 5 to the outside of the battery. Reference numeral 6 denotes a negative electrode catalyst layer, 7 denotes carbon paper as a negative electrode porous current collector, 8 denotes a supply port of an aqueous methanol solution as a fuel, 9 denotes
Denotes an outlet for methanol which has not reacted with carbon dioxide as a reaction product of the negative electrode and water as a solvent. Reference numeral 10 denotes a positive electrode catalyst layer, 11 denotes carbon paper as a porous current collector for the positive electrode, 12 denotes a supply port for air or oxygen, and 13 denotes a discharge port for excess air or oxygen and water for reaction products. Reference numeral 14 denotes a negative terminal, 15 denotes a positive terminal, and 16 denotes a fuel cell frame.

In the direct methanol fuel cell (cell A) according to the present invention, dilute sulfuric acid is flowed at a rate of 50 ml / min between two Nafion membranes in advance.
On the other hand, in a direct methanol fuel cell for comparison (referred to as battery B), a dilute sulfuric acid layer was provided between the Nafion film on the negative electrode side and the Nafion film on the positive electrode side, and was kept inside the cell, and was not allowed to flow.

Next, air humidified with steam at 60 ° C. was supplied to the positive electrode at a rate of 2 l / min, and an aqueous solution at 60 ° C. containing 1 mol / l of methanol was supplied to the negative electrode to obtain characteristics of the direct methanol fuel cell. Was measured. FIG. 2 shows an iV curve, and the characteristics of the battery A according to the present invention were considerably superior to those of the comparative battery B in which the sulfuric acid layer did not flow.

Example 2 A direct-type methanol fuel cell (Cell C) similar to that of Example 1 was used except that a 1 mol / l aqueous hydrochloric acid solution was used as an acidic aqueous solution flowing between two solid polymer electrolytes. ). As a result of measuring the characteristics of Battery C under the same conditions as in Example 1, the iV curve was almost the same as the characteristics of Battery A.

[0035]

In the conventional direct methanol fuel cell, an aqueous solution in which methanol as a fuel is dissolved is supplied to the negative electrode. However, when the methanol comes into contact with the solid polymer electrolyte that has absorbed water, the methanol is rapidly increased. It mixes with the water in the solid polymer electrolyte, diffuses in the water contained in the solid polymer electrolyte, reaches the positive electrode, and reacts on the catalyst of the positive electrode. As a result, the catalytic activity of the positive electrode decreases, and the characteristics of the battery deteriorate.

However, in the direct methanol fuel cell of the present invention, the electrolyte layer is a three-layer structure provided with an acidic aqueous solution between two solid polymer electrolyte layers, and the acidic aqueous solution is flown. When methanol is diffused from the negative electrode through the negative electrode side solid polymer electrolyte layer into the acidic aqueous solution,
The acidic aqueous solution containing methanol is taken out of the fuel cell, and methanol hardly moves to the solid polymer electrolyte layer on the positive electrode side. As a result, the catalytic activity of the positive electrode is maintained in the original state, and deterioration of battery characteristics can be prevented. Further, in the present invention, methanol not used in the reaction of the fuel cell can be recovered and reused.

Although the acidic aqueous solution depends on its concentration, the ionic conductivity is usually higher than that of the solid polymer electrolyte. Therefore, even when the acidic aqueous solution layer is provided, the discharge characteristics of the fuel cell may deteriorate. There is no. In the examples, sulfuric acid and hydrochloric acid were used as the acidic aqueous solution. However, the same effect can be obtained when another acidic aqueous solution is used as the acidic aqueous solution used in the present invention.

[Brief description of the drawings]

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

FIG. 2 is a diagram comparing the characteristics of a direct methanol fuel cell A according to the present invention and a comparative cell B.

Claims (5)

[Claims] 1. A direct methanol fuel cell provided with a solid polymer electrolyte, wherein an acidic aqueous solution is provided between two solid polymer electrolyte layers, and the acidic aqueous solution is caused to flow. 2. An acidic aqueous solution is supplied from outside the battery,
2. The direct methanol fuel cell provided with a solid polymer electrolyte according to claim 1, wherein the fuel cell is discharged outside the cell after passing between two solid polymer electrolyte layers. 3. A configuration in which an acidic aqueous solution circulates between two solid polymer electrolyte layers and a device for reducing the content of methanol contained in the acidic aqueous solution provided outside the battery. A direct methanol fuel cell provided with the solid polymer electrolyte according to claim 1. 4. The direct methanol fuel cell provided with the solid polymer electrolyte according to claim 1, wherein the acidic aqueous solution is a sulfuric acid aqueous solution. 5. The direct methanol fuel cell provided with a solid polymer electrolyte according to claim 1, wherein the acidic aqueous solution is an aqueous hydrochloric acid solution.