JPS63237363A - Methanol fuel cell - Google Patents

Abstract

PURPOSE:To reduce the cell weight by using vapor phase water and methanol as the fuel for power generation. CONSTITUTION:Methanol and water in the vapor phase state are fed to a fuel electrode. The short circuit due to the liquid junction is thereby eliminated, and the cell voltage is increased. The number of laminated unit cells can be decreased accordingly, thus a methanol fuel cell with the reduced cell weight can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an acidic electrolyte methanol fuel cell that uses methanol as direct fuel.

[Conventional technology]

Fuel cells that use methanol as fuel are easy to handle and can be made compact, so they are being put to practical use in applications ranging from small household power sources to portable power sources of several tens of kilowatts. is expected.

This fuel cell uses a porous electrode using a catalyst containing a platinum metal element as an oxidizer electrode and a fuel electrode.

During this time, a plurality of unit cells each consisting of an electrolyte containing an acidic electrolyte are stacked and used, and oxidizing gas (air and/or oxygen) is supplied to the oxidizing electrode, and fuel is supplied to the fuel electrode. This is the power generation device used.

Broadly speaking, two types of batteries have been developed for this power supply system.

One such method is, for example, the International Fuel Cell Seminar held in 1981.
As reported in ellS eminar), 2
A device that produces hydrogen-containing gas by reforming methanol in a reformer at 50 to 450°C, and supplies this gas to the fuel electrode of a battery (operating temperature approximately 200°C) that uses phosphoric acid as the electrolyte to generate electricity. There is. Since this system operates at a higher temperature than the above-mentioned systems and is more complicated, it has been researched and developed for applications ranging from several kilowatts to several kilowatts. Additionally, in order to reduce the complexity of the system, a new proposal (Japanese Patent Application Laid-open No. 60-224166) has been proposed in which methanol reforming activity is imparted to the fuel electrode within the battery body, thereby omitting the reformer in the front stage of the battery. ing.

However, a power source that requires such a methanol reforming step requires a temperature of 200° C. or higher, and is limited to high-output batteries due to system and operability considerations.

In addition to the methanol reforming reaction, side reactions also proceed in an equilibrium manner, and the production of carbon monoxide, which is a poisonous component of the catalyst, is unavoidable, necessitating a shift converter in front of the battery, and disproportionation. Due to the reaction (2co-〇+C0z), clogging due to carbon precipitation must be taken into consideration, which further complicates the system. Therefore, another method, a methanol fuel cell, in which methanol is directly supplied to the fuel electrode, has been selected.

This battery generates electricity by supplying methanol directly in liquid form to the fuel electrode of the battery, causing it to electrochemically react with air at temperatures as low as room temperature to about 60°C using sulfuric acid as an electrolyte. . It operates at low temperatures, is highly efficient,
In particular, it is attracting attention as a portable power source that is easy to handle due to the miniaturization of the device, especially for power supplies of several kilowatts or less.

Generally, electrodes are produced by applying a catalyst layer containing a mixture of catalyst powder, a binder, and polytetrafluoroethylene as a water repellent onto a substrate such as carbon paper, and firing the mixture. A unit cell, which is a basic structure of a battery using, for example, an ion exchange membrane (containing an aqueous sulfuric acid electrolyte) as an electrode and an electrolyte, is as follows. An oxidizer electrode and a fuel electrode are located in the center via a cation exchange membrane substituted with acid type,
It consists of these and an oxidizer electrode and a separator (for example, a press molded product of expanded graphite) that has a function of supplying and discharging fuel and also collects current. Air (or oxygen) is supplied to the oxidizer electrode chamber, and an anolite (methanol + sulfuric acid aqueous solution) is supplied to the fuel electrode chamber.

The ion exchange membrane has the function of transporting hydrogen ions and preventing methanol from diffusing to the oxidizer electrode and being directly burned.

The reaction of a fuel cell using methanol as a direct fuel and using an acidic electrolyte consists of a fuel electrode reaction (1) and an oxidant electrode reaction (2), so that the CH3OH+ H that is supplied to the fuel electrode is
2O-')C02+ 68++ 6 e--(1)6
H++3/20z+6e--+3H20 mountain (2) is methanol and water, and carbon dioxide gas is generated by power generation.

In order to discharge this carbon dioxide gas to the outside of the battery, an acid-type electrolyte is used, and in particular, an aqueous solution of sulfuric acid or sulfonic acid, which has high ionic conductivity, is used.

In the case where an aqueous sulfuric acid solution with the lowest ionic conductive resistance is used as the electrolyte, since the fuel is an aqueous methanol solution, methanol is directly oxidized at the oxidizer electrode. Therefore, the battery performance deteriorates, and since the fuel and electrolyte mix in an equilibrium state, there are technical problems such as short-circuiting of each unit cell due to the electrolyte in a stacked battery.

Therefore, in order to prevent direct oxidation of fuel, there was a proposal to use an ion exchange membrane containing polymeric sulfonic acid as an electrolyte and as a resistor for liquid movement (Japanese Patent Application Laid-Open No. 154048/1983), which is an important element of methanol fuel cells. It is a material.

In addition, as a method to prevent liquid short circuits, batteries using polymeric sulfonic acid and ion exchange membranes as electrolytes instead of sulfuric acid (
JP-A-60-62064), or optimization of the number of stacked layers and improvement of the fuel liquid supply structure (JP-A-60-220570)
There are other suggestions.

[Problem that the invention seeks to solve]

However, regarding the relationship between the temperature of the ion exchange membrane and the amount of methanol permeated through the ion exchange membrane, the higher the temperature, the greater the amount of methanol permeated. Therefore, in view of battery performance and methanol utilization efficiency, an operating temperature of about 60° C. or lower is desirable. This reduces the reaction rate of the fuel electrode and the power density (20 to 24 mW/a+T
).

Here, the relationship between reaction temperature, monopolar performance of the oxidizer electrode and fuel electrode, and performance of the unit cell will be shown. The oxidizer electrode has a small potential change due to reaction temperature. The fuel electrode is highly dependent on temperature, which means that the higher the temperature, the greater the rate of one reaction.

The relationship between the temperature and voltage of a unit cell consisting of an oxidizer electrode, electrolyte (ion exchange membrane) and fuel electrode is shown, as well as the relationship between the potential difference of a single electrode. The unit battery voltage is from room temperature to about 60℃,
The potential difference is almost the same as that of a single electrode, but as the temperature rises further, the battery voltage decreases. This is because the function of the ion exchange membrane as a resistor for liquid movement decreases at high temperatures.
This is thought to be due to the fact that more methanol permeates and the methanol is directly oxidized at the oxidizer electrode.

As a result, there is a problem in that the long-term stability of battery performance, which is an important characteristic for putting batteries into practical use, deteriorates. In addition,
The use of polymeric sulfonic acid as an electrolyte cannot avoid a decrease in battery pressure due to ionic conduction resistance.

Also, considering the durability of the battery components, (1) a liquid fuel cell in which the supplied fuel contains an electrolyte, (2) a separator,
In cases where the electrodes are made of conductive carbon material, a short circuit is formed at the liquid junction between each unit cell that makes up the stacked battery. The reaction causes electrolytic corrosion and damage.

Furthermore, when using liquid as fuel, it is necessary to circulate and supply more liquid (including fI & decomposition) than the methanol and water required for the reaction, and when using a fuel with a high methanol concentration, it is necessary to circulate and supply more liquid than the methanol and water required for the reaction. In addition, the fuel must be dilute due to the influence of methanol, which has a large output weight, and the output density of the unit cell is low due to the large output weight, which requires a large number of stacked layers. growing.

In order to solve the above-mentioned problems, the present invention provides a methanol fuel cell in which the cell voltage is increased as a result of no methanol short-circuit, and the cell weight can be reduced because the number of stacked unit cells can be reduced. With the goal.

[Means for solving problems]

In order to achieve the above object, the present invention consists of a unit cell in which an oxidizer electrode and a fuel electrode are arranged opposite to each other with an electrolyte interposed therebetween. Fuel for power generation is supplied to the fuel electrode, and oxidant gas is supplied to the oxidizer electrode. The methanol fuel cell is characterized in that the power generation fuel is water and methanol in a gas phase.

[Effect]

(1)? ! ! The solute acts as a movement resistor for gas consisting of methanol and water, making it possible to raise the reaction temperature to 60° C. or higher, thereby increasing the reaction rate of the fuel electrode.

Accordingly, the output density of the unit battery can be increased.

For example, at a temperature of 75℃, compared to a temperature of 60℃, the
b (2) Is there no liquid junction between each unit battery, forming a short circuit? It can prevent galvanic corrosion.

(3) There is no circulating liquid or auxiliary equipment, reducing weight.

〔Example〕

Next, examples of the present invention will be described.

The present invention will be further illustrated in detail with reference to Examples below.
The present invention is not limited to the following examples.

Example 1 In this example, an electrode for use in a fuel cell that uses methanol as direct fuel was prepared, and the unipolar characteristics of the electrode were measured.

(1) Take 0.77 g of a catalyst powder in which 20 wt% of platinum is supported on an oxidizing agent polar carbon carrier (Furnace Black Cabot), add water and knead it, and mix it with Polyflon dispersion (D-1:
Daikin Corporation) liquid as polytetrafluoroethylene 5
0 wt%, and then carbon paper (E
-715: Kenbetsu Kagakusha) 100 x 128, and after air drying, it was burned in an air electrode at 300°C for 30 minutes to obtain oxidizer electrode A.

The monopolar performance of this oxidizer electrode A was evaluated. Measured at 25℃,
In 1,5 mol/Q sulfuric acid electrolyte at 60°C and 80°C, air was supplied and the potential was determined at a current density of 60 mA/aJ. The results are o, 79V vs NHE, respectively.
, 0.80 V vs NHE and 0.81 V vs NHE. The results are shown in FIG. 29.

(2) A catalyst with 50 wt% of platinum and ruthenium supported on a fuel electrode carbon carrier (furnace black: Cabot)
g, add water and knead. Next, as PTFE, 2
．． Polyflon dispersion was added and mixed in an amount of 8 g, and this was coated on a 250 x 300 mm carbon paper. After drying, it was fired at 285° C. for 30 minutes, and this was used as fuel electrode B.

The monopolar performance of this fuel electrode B was evaluated. Measurement is carried out at fuel electrode B
20m of 4,5 mol/Q sulfuric acid electrolyte was added to the catalyst layer.
After impregnation equivalent to g/aJ, the catalyst layer was impregnated with 1.5 mol/Q
Immerse in sulfuric acid electrolyte. Next, a methanol:water=1=1 aqueous solution was prepared on the substrate (carbon paper) side at 60° C. by blowing nitrogen at iQ/min using a 3G filter to supply gaseous fuel (methanol+water).

The temperature of this monopolar cell is 40°C, 60°C and 80°C,
The fuel electrode potential at 60 mA/aJ was determined for each. The results are 0.44V vs NHE at 40℃, 60
0.38 V vs NHE at °C and 0.32 V at 80 °C
It was vsNHE.

Comparative Example 1 Using fuel electrode B, the potential was determined at 60 mA/cd in an anolite (1.0 mol/12 methanol-1, 5 mol/Q sulfuric acid aqueous solution) while changing the temperature from 17°C to 80°C.

The results are shown in 10 of FIG.

Example-2 A unit cell as shown in FIG. 1 was produced by using oxidizer electrode A and fuel electrode B of 100×128n+m and interposing an ion exchange membrane (CMV=Asahi Glass Co., Ltd.) containing 1.5 mol/Q sulfuric acid between the two electrodes. was configured. Let this unit battery be I. In Fig. 1, an oxidizer electrode 1 and a fuel electrode 2 are located in the center via a cation exchange membrane 3 substituted with an acid type, and are equipped with an oxidizer electrode and a fuel supply/exhaust function, as well as current collection. It is composed of a separator 6 (for example, a press-molded product of expanded graphite) that also has a large diameter. Air 7 (or oxygen) is supplied to the oxidant electrode chamber 4.

For the electrodes used in batteries, the oxidizer electrode catalyst is one in which platinum is supported on a carbon carrier, and the fuel electrode catalyst is made of noble metals such as ruthenium, tin, rhenium, iridium, etc., which have a high cocatalyst effect, as platinum alone has a slow reaction rate. Those containing semiconductor elements are preferable. In this unit battery (methanol: water = 1:1)
The aqueous solution was kept at 60°C and filtered with nitrogen (IQ) using a 3G filter.
/min), and air was added as an oxidizing agent.
Q/min, and the operating temperature of the unit battery is 40 to 75.
The performance of the unit battery was determined at a current density of 60 mA/ffl when the temperature was changed to .degree. The results are shown in FIG.

A simple method for converting methanol and water into a vapor phase is to provide a fuel vaporization chamber. One example is to add nitrogen and/or carbon dioxide produced by a battery to a mixed aqueous solution of methanol:water = 1:1. Gas (CO2) is bubbled through a ball filter or the like to turn methanol into gaseous fuel. By recycling and using battery exhaust gas,
The fuel utilization rate can be increased, and the temperature of the aqueous solution (methanol + water) can also be increased, making it easier to vaporize the fuel.

Comparative Example-2 The performance of the unit cell (2) was determined in the same manner as in Example-2, except that the fuel electrode hairnolite was supplied in circulation at a rate of 200 m Q /min and the operating temperature was 30 to 75°C.

The results are shown in 11 of FIG. In Figure 4, 1
2 shows the unipolar potential difference.

Example-3 A powdered matrix (average particle size 3 μm) consisting of silicon carbide and zirconium phosphate as an electrolyte and 3mo
A unit battery was constructed in the same manner as in Example 2, except that a paste prepared by mixing l/Q sulfuric acid electrolyte was used. This is called a unit battery ■.

Using this unit battery, the performance of the unit battery was measured in the same manner as in Example 2 while changing the operating temperature to 40°C, 60°C, and 75°C.As a result, the battery voltage at 60mA/a# was , 0.30V at 40℃, 0.39 at 60℃
V and 75°C were 0.45V.

Example-4 Unit cell I was used, fuel and oxidizer were supplied in the same manner as in Example-2, operating temperature was 60°C ± 5°C, and current density was 60mA/
ad, and its characteristics were measured.

The results are shown in 13 of FIG. In this experiment, part of the gaseous fuel (methanol + water) was recycled.

Comparative Example-3 Using unit battery I, anolite (
1,5+mol/Qmethanol-L,5+mo
l/Q sulfuric acid aqueous solution) and an oxidizing agent, and the operating temperature was 6.
Continuous operation was performed at 0°C ± 2°C and a current density of 60 mA/aJ.

The results are shown in 14 of FIG.

Example 5 Example using unit batteries r and ■ at an operating temperature of 75°C
An experiment similar to 4 was conducted. The results are 15 and 1 in Figure 6.
6.

Comparative Example 4 An experiment similar to Comparative Example 3 was conducted using unit battery 1 at an operating temperature of 75°C. The results are shown in 17 of FIG.

Example-6 A stacked battery (
A) was constructed.

The weight of this battery (A) was 12.5 kg.

Example-7 A stacked battery (
B) was constructed.

The weight of this battery (B) was 11.25 kg. 1 Comparative Example-5 The anolyte was stored in the fuel electrode chamber to simulate the circulating state of the anolyte in the stacked battery (A) of Example-6, and the weight of the battery at that time was measured. The result was 13.88 seagulls.

Example 8 As shown in FIG. 7, a gas phase methanol fuel cell using methanol as direct fuel was operated.

Next-layer battery (A) 1B is used, and the fuel vaporizer 19 is 60°C.
It has a structure that maintains the temperature and bubbles nitrogen. This allows gas phase methanol and water to be supplied to the fuel electrode. The vaporizer 19 is connected to a control device 22 that includes a methanol concentration sensor 23 and a liquid level sensor 24 and performs feedback control so that the ratio of methanol and water in the vaporizer 19 is 1:1. Methanol and water are replenished from the methanol tank 20 and the water tank 21 through the blower 2S. Continuous operation was performed for 100 hours at a current density of 60 mA/cof. The amount of nitrogen to be supplied was manually adjusted and supplied so that the temperature of the battery was 55 to 60 °C (
The gas (methanol + water) was used in circulation.

The results are shown in 27 of FIG. 8 as a relationship between operating time and output power.

Comparative Example-6 A conventional liquid methanol fuel cell shown in FIG. 9 was operated. In this example, the stacked battery (A) 18 is used, and the fuel tank 28 contains 1 mol/Q methanol+1, 5 mol/
Pour the Q sulfuric acid anolyte solution into the stacked battery 18.
It was circulated and supplied to Further, the fuel tank 27 is equipped with a methanol concentration sensor 23 and a liquid level sensor 24, so that the methanol concentration in the fuel tank is 0.8 to 1.2 mol/
Q. The control device 22 controls the anolyte liquid level to ±5%, and the methanol tank 20 and water tank 2
Supplied from 1. Air was supplied to the oxidizer electrode at 35 Q/win. The results of continuous operation for 100 hours at 60 mA/aJ are shown at 30 in FIG.

Example-9 Using a laminated battery (B), the operating temperature of the battery is 70-75
Continuous operation was carried out in the same manner as in Example-8 except that the temperature was set at ℃.

The results are shown at 31 in FIG.

Example 10 The stacked batteries used in Examples 8 and 9 were disassembled, and the outer awt and weight of the separators were measured, but there was no change from the initial value.

Comparative Example-7 The laminated battery used in Comparative Example-6 was disassembled, and the weight and weight were measured in the same manner as in Example-10. As a result, damage due to electrolytic corrosion was observed at the anolite inlet and outlet of the separator near both ends of the battery. Furthermore, the weight of the entire separator was reduced by 2.5% compared to the initial weight.

〔Effect of the invention〕

As explained above, according to the methanol fuel cell according to the present invention, since methanol water is supplied to the heating electrode in a gaseous state, there is no formation of a short circuit due to a liquid junction, and the battery voltage can be increased. Moreover, electrolytic corrosion of battery constituent materials can be prevented. Moreover, since there is no liquid junction, the battery temperature can be kept at a suitably high temperature, making it possible to further increase the battery voltage. As a result, the battery output is stabilized for a long period of time, and the output density is increased.

Furthermore, the methanol fuel cell according to the present invention is lightweight and compact, and its usefulness as a portable power source is further improved.

Furthermore, circulation of the liquid (anolyte) is no longer necessary, and problems with the durability of the liquid pump and poor sealing of the battery liquid can be solved.

In future commercialization, it would be even more effective if (1) automatic control of heat balance, electrolyte balance, and fuel balance and a high-performance solid electrolyte were created.

[Brief explanation of drawings]

Fig. 1 is a unit cell configuration diagram in an embodiment of the methanol fuel cell according to the present invention, Fig. 2 is a graph showing the temperature-potential of a single electrode, and Figs. 3 and 4 are graphs showing the temperature-voltage of the unit cell. 5 and 6 are graphs showing changes in the performance of unit cells over time. FIG. 7 is a configuration diagram of an embodiment of a methanol fuel cell according to the present invention. FIG. Graph showing the results of continuous operation of a fuel cell. FIG. 9 is a block diagram of a conventional methanol fuel cell in which an anolite is supplied to a fuel electrode. 1... Oxidizer electrode, 2... Fuel electrode, 3... Electrolyte (
ion exchange membrane), 4... Oxidizer electrode chamber, 5... Fuel electrode chamber, 6... Separator, 7... Air (oxygen), 8...
...Anorite. 9.10... Unipolar potential, 11, 13, 14, 15°1
6.17... Unit battery voltage, 12... Unipolar potential difference,
18... Stacked battery, 19... Fuel vaporizer, 20...
・Methanol tank, 21...Water tank, 22...
Control unit, 23... Methanol concentration sensor, 24...
Liquid level sensor, 25...Blower, 26...Nitrogen, 27,30°31...Battery output, 29...Liquid pump.

Claims (1)

[Claims] 1. Consisting of a unit cell in which an oxidizer electrode and a fuel electrode are arranged opposite to each other with an electrolyte interposed between them, power generation fuel is supplied to the fuel electrode, and oxidized gas is supplied to the oxidizer electrode. A methanol fuel cell characterized in that the power generation fuel is water and methanol in a gas phase. 2. The oxidizer electrode and the fuel electrode include a conductive carbon carrier and 1
Methanol according to claim 1, characterized in that it is composed of a catalyst layer made of a mixture of a catalyst containing more than one type of platinum metal element and polytetrafluoroethylene, and a conductive carbon substrate. Fuel cell. 3. The methanol fuel cell according to claim 1, wherein the electrolyte is composed of an ion exchange membrane and/or a matrix of fine particles of silicon carbide and zirconium phosphate, and contains an electrolyte. 4. The methanol fuel cell according to any one of claims 1 to 3, wherein the electrolyte comprises an aqueous sulfuric acid solution and/or an aqueous solution containing a polymeric sulfonic acid.