POLYMER ELECTROYTE MEMBRANE, MEMBRANE/ELECTRODE ASSEMBLY AND FUEL CELL USING THE ASSEMBLY

Abstract

Disclosed is an electrolyte membrane which is slowly degraded by a peroxide occurring at an air electrode catalyst layer, is low in cost, and is long in life, and a membrane electrode assembly. The electrolyte membrane has a first electrolyte layer which has an ion conductivity, a second electrolyte layer which has an ion conductivity, and, upon surface contact with methanol, is thicker than the first electrolyte layer, has a larger ion exchange equivalent, or a larger number average molecular weight, and a porous layer which has an ion conductive electrolyte impregnated therein, formed between the first electrode layer and the second electrode layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a solid polymer electrolyte membrane in accordance with the present invention;

FIG. 2 is a view of a direct methanol type fuel cell electricity production apparatus in accordance with the present invention; and

FIG. 3 is a solid polymer type hydrogen-oxygen type fuel cell electricity production apparatus in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Below, the present invention will be described in more details by way of examples, which should not be construed as limiting the scope of the present invention.

EXAMPLE 1

(Manufacturing of Electrolyte Composite Membrane)

Sulfonated polyether sulfone (S-PES) with a number average molecular weight of 4×10⁴ and an ion exchange equivalent weight of 8×10² g/equivalent was dissolved in N,N-dimethylacetamide to manufacture a 30 wt % electrolyte solution. The electrolyte solution was cast and coated on a glass substrate. A polyolefin porous membrane was placed thereon, and impregnated therewith. Further, an electrolyte solution was cast and coated from thereabove. At this step, by controlling the amount of the electrolyte to be cast, the thicknesses of the electrolyte layers on the opposite sides of the polymer layer were changed. Thereafter, heating and drying were carried out at 80° C. for 20 minutes, and then at 120° C. for 20 minutes, thereby to remove the solvent in the solution. As a result, there was manufactured a solid polymer electrolyte composite membrane having a porous layer on the inner sides of a pair of electrolyte layers in which the thickness of one electrolyte layer is larger than the thickness of the other electrolyte layer. FIG. 1 shows a cross sectional view of a configuration of the solid polymer electrolyte composite membrane. A reference numeral 1 represents an electrolyte composite membrane; 2, a porous layer; 3, an electrolyte layer; and 4, an electrolyte layer having a larger thickness than that of the electrolyte layer 3. The observation of the cross-section of the resulting electrolyte composite membrane indicates as follows: the total thickness of the electrolyte composite membrane is 40 μm, the thickness of the electrolyte layer 4 is 20 μm, and the thickness of the electrolyte layer 3 is 5 μm.

(Manufacturing of Membrane Electrode Assembly)

For a fuel electrode, an electrode catalyst including Pt and Ru carried in each amount of 25 wt % on carbon black was used. Whereas, for an air electrode, an electrode catalyst having Pt in an amount of 50 wt % carried thereon was used. To each electrode catalyst, a Nafion solution was weighed and mixed in a ratio such that the weight ratio of the electrode catalyst and Nafion was 1 to 9, thereby to manufacture an electrode catalyst paste. The electrode catalyst paste was spray coated on the electrolyte membrane to form an electrode catalyst layer. At this step, an air electrode catalyst layer was formed on the electrolyte layer having a larger thickness, and a fuel electrode catalyst layer was formed on the other.

EXAMPLE 2

(Manufacturing of Electrolyte Composite Membrane)

Sulfonated polyether sulfone with a number average molecular weight of 4×10⁴, and an ion exchange equivalent weight of 11×10² g/equivalent was dissolved in N,N-dimethylacetamide to manufacture a 30 wt % electrolyte solution. The electrolyte solution was cast and coated on a glass substrate. A polyolefin porous membrane was placed thereon, and impregnated therewith. Further, the electrolyte solution formed in Example 1 was cast and coated from thereabove. Thereafter, heating and drying were carried out at 80° C. for 20 minutes, and then at 120° C. for 20 minutes, thereby to remove the solvent in the solution. As a result, there was manufactured a solid polymer electrolyte composite membrane having a porous layer on the inner sides of a pair of electrolyte layers in which the ion exchange equivalent weight of one electrolyte layer is larger than the ion exchange equivalent weight of the other electrolyte layer. The observation of the cross section of the resulting electrolyte composite membrane indicates as follows: the total thickness of the electrolyte composite membrane is 40 μm, and each thickness of the electrolyte layers is 12 μm.

(Manufacturing of Membrane Electrode Assembly)

Manufacturing was carried out in the same manner as in Example 1. In the process, an air electrode catalyst layer was formed at the electrolyte layer having a larger ion exchange equivalent weight, and a fuel electrode catalyst layer was formed at the other.

EXAMPLE 3

(Manufacturing of Electrolyte Composite Membrane)

Sulfonated polyether sulfone with a number average molecular weight of 7×10⁴ and an ion exchange equivalent weight of 8×10² g/equivalent was dissolved in N,N-dimethylacetamide to manufacture a 30 wt % electrolyte solution. The electrolyte solution was cast and coated on a glass substrate. A polyolefin porous membrane was placed thereon, and impregnated therewith. Further, the electrolyte solution formed in Example 1 was cast and coated from thereabove. Thereafter, heating and drying were carried out at 80° C. for 20 minutes, and then at 120° C. for 20 minutes, thereby to remove the solvent in the solution. As a result, there was manufactured a solid polymer electrolyte composite membrane having a porous layer on the inner sides of a pair of electrolyte layers in which the average molecular weight of one electrolyte layer is larger than the average molecular weight of the other electrolyte layer. The observation of the cross section of the resulting electrolyte composite membrane indicates as follows: the total thickness of the electrolyte composite membrane is 40 μm, and each thickness of the electrolyte layers is 12 μm.

(Manufacturing of Membrane Electrode Assembly)

Manufacturing was carried out in the same manner as in Example 1. In the process, an air electrode catalyst layer was formed at the electrolyte layer having a larger average molecular weight, and a fuel electrode catalyst layer was formed at the other.

EXAMPLE 4

(Manufacturing of Electrolyte Composite Membrane)

Sulfonated polyether sulfone (S-PES) with a number average molecular weight of 9×10⁴ and an ion exchange equivalent weight of 7×10² g/equivalent was dissolved in N,N-dimethylacetamide to manufacture a 23 wt % electrolyte solution. The electrolyte solution was cast and coated on a glass substrate. A polyolefin porous membrane was placed thereon, and impregnated therewith. Further, the electrolyte solution formed in Example 1 was cast and coated from thereabove. Thereafter, heating and drying were carried out at 80° C. for 20 minutes, and then at 120° C. for 20 minutes, thereby to remove the solvent in the solution. As a result, there was manufactured a solid polymer electrolyte composite membrane having a porous layer on the inner sides of a pair of electrolyte layers in which the average molecular weight of one electrolyte layer is larger than the average molecular weight of the other electrolyte layer. The observation of the cross section of the resulting electrolyte composite membrane indicates as follows: the total thickness of the electrolyte composite membrane is 40 μm, and each thickness of the electrolyte layers is 12 μm.

(Manufacturing of Membrane Electrode Assembly)

Manufacturing was carried out in the same manner as in Example 1. In the process, an air electrode catalyst layer was formed at the SM-PES electrolyte layer, and a fuel electrode catalyst layer was formed at the other.

COMPARATIVE EXAMPLE 1

The electrolyte solution manufactured in Example 1 was cast and coated on a glass substrate. A polyolefin porous membrane was placed thereon, and impregnated therewith. Further, the electrolyte solution was cast and coated from thereabove. Thereafter, heating and drying were carried out at 80° C. for 20 minutes, and then at 120° C. for 20 minutes, thereby to remove the solvent in the solution. As a result, there was manufactured a solid polymer electrolyte composite membrane having a porous layer on the inner sides of a pair of electrolyte layers in which the thickness of one electrolyte layer is equal to the thickness of the other electrolyte layer. The observation of the cross section of the resulting electrolyte composite membrane indicates as follows: the total thickness of the electrolyte composite membrane is 40 μm, and each thickness of the electrolyte layers is 12 Mm.

(DMFC Cell Performance Evaluation)

The DMFC electricity production apparatus single cell shown in FIG. 2 was used, and each membrane electrode assembly manufactured in Examples 1 to 3, and Comparative Example 1 was mounted therein. Thus, the cell performances were measured. In FIG. 1, a reference numeral 1 represents a polymer electrolyte membrane; 5, an anode; 6, a cathode; 7, an anode diffusion layer; 8, a cathode diffusion layer; 9, an anode collector; 10, a cathode collector; 11, fuel; 12, air; 13, an anode terminal; 14, a cathode terminal; 15, an anode end plate; 16, a cathode end plate; 17, a gasket; 18, an O-ring; and 19, bolt/nut. As a fuel, a 10 wt % methanol solution was circulated on the fuel electrode side, and air was fed to the air electrode side in a natural exhalation form. Continuous operation was carried out at 35° C. under a load of 50 mA/cm².

Table 1 shows the results when the DMFC continuous electricity production test has been carried out by the use of each membrane electrode assembly manufactured in Examples 1 to 3, and Comparative Example 1. As apparent from Table 1, the DMFC using the solid polymer electrolyte composite membrane according to the present invention has a longer life as compared with the electrolyte membrane of Comparative Example. The same results can also be expected for a PEFC continuous test.

TABLE 1
					Comparative
Item	Example 1	Example 2	Example 3	Example 4	Example 1
Total thickness of electrolyte composite membrane (μm)	40	40	40	40	40
Type of electrolyte adjacent to air electrode catalyst	S-PES	S-PES	S-PES	SM-PES	S-PES
layer
Thickness of electrolyte layer adjacent to air	20	12	12	12	12
electrode catalyst layer (μm)
Ion exchange equivalent weight of electrolyte layer	8 × 102	11 × 102	8 × 102	7 × 102	8 × 102
adjacent to air electrode catalyst layer
(g/equivalent)
Number average molecular weight of electrolyte layer	4 × 104	4 × 104	7 × 104	9 × 104	4 × 104
adjacent to air electrode catalyst layer
Type of electrolyte adjacent to air electrode catalyst	S-PES	S-PES	S-PES	S-PES	S-PES
layer
Thickness of electrolyte layer adjacent to fuel	4	12	12	12	12
electrode catalyst layer (μm)
Ion exchange equivalent weight of electrolyte layer	8 × 102	8 × 102	8 × 102	8 × 102	8 × 102
adjacent to fuel electrode catalyst layer
(g/equivalent)
Number average molecular weight of electrolyte layer	4 × 104	4 × 104	4 × 104	4 × 104	4 × 104
adjacent to fuel electrode catalyst layer
Initial voltage (V)	0.35	0.33	0.37	0.38	0.34
Ratio of electricity production time until voltage	>1.5	>1.2	>1.2	>1.2	1
is reduced to 0.3 V (Example/Comparative Example)

(PEFC Cell Performance Evaluation)

The compact single cell using hydrogen as a fuel shown in FIG. 3 was used, and each membrane electrode assembly manufactured in Example 1 and Comparative Example 1 was mounted therein. Thus, the cell performances were measured. In FIG. 3, a reference numeral 1 represents a polymer electrolyte membrane; 5, an anode; 6, a cathode; 7, an anode diffusion layer; 8, a cathode diffusion layer; 20, a fuel passage of an electrically conductive separator (bipolar plate) for pole chamber separation, and also serving as a gas feed channel to the electrode; 21, a passage for air of an electrically conductive separator (bipolar plate) for pole chamber separation, and also serving as a gas feed channel to the electrode; 22, hydrogen of the fuel and water; 23, hydrogen; 24, water; 25, air; and 26, air and water. The compact single cell was set in a thermostat, and the temperature of the thermostat was controlled such that the temperature by a thermocouple (not shown) inserted into the separator was 70° C. For humidification of the anode and the cathode, an external humidifier was used. Thus, the temperature of the humidifier was controlled within the range between 70 to 73° C. such that the dew point at the vicinity of the outlet of the humidifier was 70° C. For the dew point, other than the measurement by a dew-point hygrometer, the amount of wetting water consumed was continuously measured. Thus, the dew point determined from the flow rate, temperature, and pressure of a reaction gas was confirmed to be a prescribed value. By setting the load current density at 250 mA/cm², the hydrogen utilization rate at 70%, the air utilization rate at 40%, electricity was produced at about 8 hours/day, and hot keep operation was carried out during the residual time.

Table 2 shows the results when the PEFC continuous electricity production test has been carried out by the use of each membrane electrode assembly manufactured in Example 1 and Comparative Example 1. As apparent from Table 2, the PEFC using the solid polymer electrolyte composite membrane according to the present invention has a longer life as compared with the electrolyte membrane of Comparative Example.

TABLE 2
		Comparative
Item	Example 1	Example 1
Total thickness of electrolyte composite	40	40
membrane (μm)
Type of electrolyte adjacent to air electrode	S-PES	S-PES
catalyst layer
Thickness of electrolyte layer adjacent to air	20	12
electrode catalyst layer (μm)
Ion exchange equivalent weight of electrolyte	8 × 102	8 × 102
layer adjacent to air electrode catalyst layer
(g/equivalent)
Number average molecular weight of	4 × 104	4 × 104
electrolyte layer adjacent to air electrode
catalyst layer
Type of electrolyte adjacent to air electrode	S-PES	S-PES
catalyst layer
Thickness of electrolyte layer adjacent to fuel	4	12
electrode catalyst layer (μm)
Ion exchange equivalent weight of electrolyte	8 × 102	8 × 102
layer adjacent to fuel electrode catalyst
layer (g/equivalent)
Number average molecular weight of	4 × 104	4 × 104
electrolyte layer adjacent to fuel electrode
catalyst layer
Ratio of electricity production time until	>1.1	1
voltage is reduced by 10%
(Example/Comparative Example)

The polymer electrolyte composite membrane of the present invention can be used not only for a hydrogen-oxygen fuel cell but also for a DMFC of the type using alcohol for a fuel, and directly feeding it to the fuel cell.

Claims

1. An electrolyte membrane for a methanol fuel cell, comprising a first electrolyte layer having an ion conductivity, a second electrolyte layer having an ion conductivity and being thicker than the first electrolyte layer, and a porous layer having an ion conductive electrolyte impregnated therein, formed between the first electrode layer and the second electrode layer.

2. An electrolyte membrane for a methanol fuel cell, comprising a first electrolyte layer having an ion conductivity, a second electrolyte layer having an ion conductivity and having a larger ion exchange equivalent than that of the first electrolyte layer, and a porous layer having an ion conductive electrolyte impregnated therein, formed between the first electrode layer and the second electrode layer.

3. An electrolyte membrane for a methanol fuel cell, comprising a first electrolyte layer having an ion conductivity, a second electrolyte layer having an ion conductivity and having a larger number average molecular weight than that of the first electrolyte layer, and a porous layer having an ion conductive electrolyte impregnated therein, formed between the first electrode layer and the second electrode layer.

4. The solid polymer electrolyte membrane according to claim 2, wherein the first electrolyte is a hydrocarbon electrolyte layer, and the second electrolyte layer is a hydrocarbon electrolyte layer having a different chemical formula.

5. The solid polymer electrolyte membrane according to claim 1, wherein the solid polymer electrolyte membrane is an aromatic hydrocarbon polymer electrolyte having an ion exchange group.

6. The solid polymer electrolyte membrane according to claim 1, wherein the solid polymer electrolyte membrane is polyether sulfone having an ion exchange group.

7. The solid polymer electrolyte membrane according to claim 6, wherein the ion exchange group is a sulfonic acid group.

8. The solid polymer electrolyte membrane according to claim 1, wherein the ratio of the thickness of the first electrolyte layer and the thickness of the second electrolyte layer is 1:10 to 4:5.

9. The solid polymer electrolyte membrane according to claim 1, wherein the thickness of the first electrolyte layer is 5 to 40 μm, and the thickness of the second electrolyte layer is 10 to 50 μm.

10. A membrane electrode assembly according to claim 1, comprising the solid polymer electrolyte membrane, wherein an air electrode catalyst layer is formed adjacent to the second electrolyte layer, and a fuel electrode catalyst layer is formed adjacent to the first electrolyte layer.

11. A membrane electrode assembly according to claim 2, comprising the solid polymer electrolyte membrane, wherein an air electrode catalyst layer is formed adjacent to the second electrolyte layer, and a fuel electrode catalyst layer is formed adjacent to the first electrolyte layer.

12. A membrane electrode assembly according to claim 3, comprising the solid polymer electrolyte membrane, wherein an air electrode catalyst layer is formed adjacent to the second electrolyte layer, and a fuel electrode catalyst layer is formed adjacent to the first electrolyte layer.

13. A fuel cell having the membrane electrode assembly according to claim 10.