JPS6286662A - Fuel cell - Google Patents

Abstract

PURPOSE:To increase the electrolyte absorption quantity of an air pole, and check a potential drop with a variation in electrolyte volume in an air pole catalytic layer in continuous discharge of a cell as well as to aim at the promotion of long service life in the cell, by adding a hydrophilic agent to the air pole catalytic layer. CONSTITUTION:A methanol fuel cell is provided with air poles (an air pole substrate 10 and an air pole catalytic layer 11), and methanol poles (a methanol pole substrate 14 and a methanol pole catalytic layer 13 via an ion-exchange film 12, and the atmosphere is led into an air chamber 9, while H2SO4-CH3H- water are fed to a methanol pole chamber 15. A mixture of an electrode catalyz er bearing Pt on a furnace black having carbon powder and polytetrafluoroethylene or a water-repellent agent is applied onto a conductive porous substrate and burned, thereby forming the catalytic layer 11 to which SiC is added as a hydrophilic member.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to fuel cells, and more particularly to methanol fuel cells.

[Background of the invention]

Fuel cells that use liquid fuels, particularly methanol, have not yet been put into practical use in the world, and their initial characteristics are still being evaluated at various research institutions.

Gas diffusion electrodes generally used in fuel cells are made of a mixture of an electrode catalyst with improved specific activity by highly dispersing fine PT particles on carbon powder, and polytetrafluoroethylene (hereinafter abbreviated as PTFE), which is a water repellent. This type of electrode is prepared by coating and firing PTFH on a conductive porous substrate (for example, Japanese Patent Application Laid-Open No. 60-86767).

The amount of electrolyte absorbed into the electrode catalyst layer varies depending on the type of carbon carrier and the type of electrolyte.

Regarding the details of gas diffusion electrodes used in fuel cells, most of them are related to phosphoric acid fuel cells.If the gas diffusion electrodes used in phosphoric acid fuel cells are applied to methanol fuel cells, their performance will not be exhibited at all. . This is due to differences in the type of electrolyte and the operating temperature of the cell, and because sulfuric acid, which is the electrolyte used in methanol fuel cells, is not absorbed into the electrode catalyst layer, a three-phase interface of liquid and gas, which is the site of reaction, is formed. This is due to the fact that it is difficult to do so.

As described above, the configuration of the gas diffusion electrode needs to be optimized depending on the type of fuel cell.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel cell that improves the problem that conventionally used gas diffusion electrodes can absorb only a small amount of electrolyte.

[Summary of the invention]

The present invention was developed based on the results of detailed studies on the initial performance of gas diffusion electrodes used in methanol fuel cells and the life span of cells combined with fuel electrodes.

As mentioned above, a typical gas diffusion electrode consists of an electrode catalyst that has improved specific activity by highly dispersing fine PT particles on carbon powder, and PTFI, a water repellent. It is produced by coating and baking a mixture of the following on a porous conductive substrate. In this type of electrode, it is necessary to have appropriate water repellency and to increase and stabilize the surface quality of the three-phase interface where liquid, gas, and solid contact, which is the reaction site.

The type of carbon powder carrier is a factor that affects the water repellency of the electrode, that is, its wettability with the electrolyte.

Amount of PTFE added. The electrode firing temperature or the concentration of sulfuric acid, which is an electrolyte, is increased.

It is necessary to understand how the wettability of the electrode changes when the above-mentioned parameters are changed, or the relationship between the degree of wetting of the electrode and the electrode property.

As for the evaluation method, for the former, the electrode catalyst layer is suspended on the liquid side on a sulfuric acid electrolyte of a predetermined concentration, and the value when the weight of the electrode that has absorbed sulfuric acid reaches a constant value is used to calculate the equilibrium absorption amount of the electrolyte. did. The magnitude of the electrolyte equilibrium absorption amount was used as an index of the wettability of the electrode. On the other hand, regarding the relationship between electrode wettability and electrode performance, we defined the amount of electrolyte occupying the pore volume of the electrode catalyst layer as pore occupancy, and organized the relationship between this pore occupancy and electrode performance.

The relationship between pore occupancy and air electrode potential is shown in FIG. 1 as a model. As shown in Figure 1, the air electrode potential is.

It exhibits high performance because the three-phase interface is ideally formed within a certain range of pore occupancy. On the other hand, in a region where the pore occupancy is small, the catalyst layer is not sufficiently wetted by the electrolytic solution, so the H+ migration resistance increases and the potential decreases. On the other hand, in areas where the pore occupancy is large, the catalyst layer is sufficiently covered with the electrolyte and gas diffusion is inhibited, resulting in a decrease in potential.

Therefore, in order to obtain an air electrode with stable performance.

It is necessary to create a catalyst layer structure that does not deviate from the pore occupancy region where the potential is stable even when the operating conditions of the battery change.

First, we determined the equilibrium absorption amount of electrolyte for air electrodes with different types of carbon carriers. The results are shown in FIG. FIG. 2 shows an electrode catalyst in which 15 wt% of PT is supported on furnace black (hereinafter referred to as the first carrier) having 250 rrF/g carbon powder as a carbon carrier, and PTFE! was prepared at a concentration of 20 wt%, and incubated in air for 320'
This figure shows the change in electrolyte absorption of the air electrode fired with C. The electrolyte was 3mou/Q sulfuric acid and the temperature was 60°C. In addition, 2 is an electrolytic solution of an air electrode prepared in the same manner as above using an electrode catalyst in which 50 wt% of PT was supported on furnace black having 1400 ni'/g carbon powder as a carrier (hereinafter referred to as the second carrier). It showed an absorption change.

Although the amount of absorption is constant at any air electrode, 14
Although a time of 0 hours or more is used, a value that can be considered as the equilibrium absorption amount of electrolyte can be obtained.6 Comparing both air electrodes, it can be seen that the equilibrium absorption amount differs depending on the type of carbon carrier.

Next, the equilibrium absorption amount of electrolyte of the air electrode was determined by changing the amount of PTFE in the catalyst layer. The electrode catalyst has 15 wt% of PT supported on the first carrier, and PTFE is added to this by 3Q, 4
0.50wt% mixed in air at 300'C-0.5h
An air electrode was produced by firing.

Absorption test was conducted at 60°C, 1.5 moQ/Q Hi
I went inside s O&. The results are shown in FIG. As seen in FIG. 3, under these experimental conditions, the equilibrium absorption amount does not change significantly depending on the amount of PTFE in the catalyst layer.

FIG. 4 shows the absorption test results for air electrodes with different firing temperatures. The air electrodes used in the test shown in FIG. 4 were manufactured in the following procedure. PTFE was mixed with an electrode catalyst in which 15 wt% of PT was supported on the first carrier so that the amount was 30 wt%, and this was applied to a conductive porous substrate and heated at 300°C, 320°C and 340°C in air. Firing was performed for 0.5 hours each.

The absorption test was conducted at 60°C-3moQ/QHas04. In Figure 1, curve 4 shows the change in electrolyte absorption amount of the air electrode fired at 300°C, curve 5 at 320°C, and curve 6 at 340°C fired. The electrolyte equilibrium absorption amount takes an extremely small value as the air electrode firing temperature increases, and for those fired at 340°C, equilibrium is not reached even after 200 hours of N immersion.

One of the reasons why the absorption amount of the air electrode fired at 340° C. is small due to the high firing temperature is considered to be as follows.
It is generally known that PTFE melts and undergoes a state change at around 320 to 330°C.

Therefore, in the case of the one fired at 340° C., as a result of undergoing a history of a semi-molten state, it is thought that the water repellency as an air electrode was strengthened as the state of PTFE changed within the catalyst layer. However, no clear proof has been obtained.

Next, an absorption test was conducted on air electrodes with varying amounts of electrode catalyst applied. Electrocatalyst transfers 10 pt to the second support
5 mg per 12 electrodes.
and applied in an amount of 10 mg. PTF at this time
The amount of E is 30 wt%.

The above electrode was baked in air at 300° C. for 5 hours to obtain an air electrode. For these air electrodes, 60℃-3n
The results of an absorption test in on/Q Has Oa are shown in FIG. Curve 7 in the figure shows the amount of electrode catalyst applied is 5 m.
g/aA, curve 8 shows the change in absorption of the air electrode at 10 mg/cd.

The figure shows that when the amount of PTFE and the firing temperature are constant, the equilibrium absorption amount of electrolyte in the air electrode is proportional to the amount of electrode catalyst applied, that is, the thickness of the air electrode catalyst layer.

As described above, the equilibrium absorption amount of the electrolyte was determined by changing the type of carbon carrier used for preparing the electrode catalyst, the amount of PTFE added to the air electrode catalyst layer, the firing temperature of the air electrode, and the thickness of the air electrode catalyst layer. In order to make effective use of the value of equilibrium absorption amount, it is necessary to check the performance of each air electrode when it reaches equilibrium absorption with the electrolyte. To do this, as mentioned above, it is sufficient to find an index of how much of the pore volume of the air electrode catalyst layer is occupied by the amount of electrolyte that has reached equilibrium absorption, that is, the relationship between the pore occupancy and the air electrode potential. become.

First, the pore volumes of the air electrode catalyst layers prepared under various preparation conditions were measured by mercury intrusion method. The results are shown in FIG. The values in the figure are based on an electrode catalyst in which 15 wt% of PT is supported on the first carrier, and PTFE is added at 30 wt% on the first carrier.
．． A mixture of 40 and 50 wt% was applied onto a conductive porous substrate, and 30 wt% of each PTFE content was applied onto a conductive porous substrate.
The pore volume per gram of catalyst N is shown for air electrodes fired at 0°C, 320°C, and 340°C. From the results shown in the figure, it can be seen that the pore volume of the air electrode catalyst layer does not change much depending on the firing temperature, but is greatly influenced by the amount of PTFE added.

The pore occupancy was calculated from the value of the equilibrium absorption amount obtained in FIGS. 2 to 5 and the value of the pore volume of the air electrode catalyst layer obtained in FIG. The relationship between the pore occupancy and the air electrode potential is summarized and shown in FIG. The value indicated by the white circle in the figure is 1.5 mo
Q/QHxSO<The value at which equilibrium absorption was reached with the electrolytic solution, and the black circle was determined using the value at which equilibrium absorption was reached with the 3moQ/ΩHx No. 80 electrolyte. Further, the air electrode potential is relative to the hydrogen standard electrode potential obtained at a current density of 60 mA/cd.

According to the results obtained under the present experimental conditions, a high and stable air electrode potential was obtained when the pore occupancy was in the range of 15 to 33%.

Next, let's consider the configuration and lifespan of a methanol fuel cell. FIG. 8(A) shows a battery configuration model diagram; Bl shows an enlarged view of the area around the electrodes. Diagram (A) shows ion exchange 1
An air electrode (an air electrode substrate 10, an air electrode catalyst layer 11) and a methanol electrode (a methanol electrode substrate 14, a methanol electrode catalyst layer 13) are arranged via the air electrode 1g12, and the atmosphere is introduced into the air chamber 9.

1.5 mon/Q in the methanol electrode chamber 15
Has 04-1. Onon/QCHsH-Power is generated by supplying water. Enlarged view (Bl shows the liquid-gas-isomer interface of the catalyst layer of the air electrode, that is, the gas diffusion electrode.
This is a dimensional illustration. Therefore, 1 in Figure (B)
The portion 6 corresponds to the pore occupancy rate described above.

Based on the basic study results of air electrodes obtained so far, 1.5 l
A life test was conducted using an air electrode with l1oQ/QHz SO4 electrolyte tenopore occupancy of about 20%, and the results are summarized in the 9th section.
As shown in the figure. The battery voltage is the value when discharged at a current density of 60 mA/a#. The initial voltage of 0.41V is during operation 1.
After 0 hours, it decreased by about 50 mV, and then 50 mV/1. O
decreases at a rate of h.

The cause of this decrease in battery voltage is thought to be changes on the air electrode side, and in particular, a decrease in the air electrode potential due to a decrease in the volume of electrolyte in the catalyst layer is predicted. In other words, even when operating with stacked air electrodes that have reached equilibrium absorption with the electrolyte,
Dry air is supplied to the air chamber and moisture evaporates, so the
It is thought that the volume of the electrolytic solution 16 occupying the pores in Figure mB1 decreases, the pore occupancy rate decreases, and shifts to the lower left portion of Figure 7, resulting in a tendency for the battery voltage to decrease.

From the above, in order to stabilize the battery voltage, it is necessary to have a catalyst layer structure that has a large equilibrium absorption amount of electrolyte and a large pore occupancy while maintaining the water repellency of the air electrode.

For this purpose, we came up with the idea that it would be sufficient to add a hydrophilic agent to the air electrode catalyst layer.

Examples based on this idea will be described below.

[Embodiments of the invention]

Examples of the present invention will be described below, but the present invention is not limited to the following examples.

Example 1 In this example, an electrode catalyst in which 15 wt% of PT was supported on a first carrier and a 30 wt% ['TFE kneaded product were applied to a conductive porous substrate to form a catalyst layer. The effect of adding SiC will be described. The procedure for producing the air electrode is described below.

, 5iC1 with 4.5 g of electrocatalyst and average particle size of 0.3 μm
, 8g, add water and knead thoroughly. Polyflon dispersion (PTFE) was added to this in an amount of 2.7 g and kneaded. This paste was applied onto porous carbon paper (25o x 3oO!rn), air-dried, and then fired at temperatures of 300'C and 320C in an air atmosphere.

About these air electrodes 3t@oQ/Q Has
An absorption test was conducted in 04 electrolyte. The result is the 10th
As shown in the figure. Curve 17 in the figure is the one fired at 300℃, curve 1
8 is an air electrode at 320°C. The equilibrium absorption amount obtained is
The value was approximately twice that of conventional air electrodes. Furthermore, 30
In the air electrode fired at 0°C, the occupancy rate of the electrolyte is 50%.
It also reached Next, we evaluated the performance of the monopolar air electrode that reached equilibrium absorption. Measurement was carried out at 60℃-3moQ/QHxSO4
The results are shown in Figure 11.6 Figure 11 shows
The current density-potential characteristics of each air electrode were evaluated, meaning that the higher the potential at a constant current density, the better the performance. The air electrode obtained in the present invention has a current density of 0.8Vv at a practical current density of 60mA/a#.
It showed a potential higher than sNHE.

Comparative Example 1 In this comparative example, the procedure for producing an air electrode without adding a hydrophilic agent to the catalyst layer, the amount of electrolyte absorbed by the produced air electrode, and its performance were evaluated.

Water is added to 4.5 g of the first carrier-15% PT electrode catalyst and thoroughly kneaded. Polytetrafluoroethylene dispersion (PTFE) was added to this in an amount of 2.7 g and kneaded. This paste was applied onto porous carbon paper (250×300 m), air-dried, and then fired at temperatures of 300° C. and 320° C. in an air atmosphere.

Absorption tests were conducted on these air electrodes in a 3IIIOQ/aH2S04 electrolyte. The results are shown in Figure 12. In Figure 1, curve 19 is for firing at 300℃, curve 20 is for firing at 300℃.
It was fired at 0°C. The equilibrium absorption amount obtained is +30
Approximately 6 mg/cd when fired at 0°C.

The value of the air electrode fired at 320°C was approximately 4 mg/d, which was approximately 1/2 of the value of the improved air electrode according to the present invention.Next, the performance of the single electrode of the air electrode that reached equilibrium absorption was evaluated. did. The measurements were carried out in the same manner as in Example 1.6 As a result, the air electrode produced in this comparative example was as follows.

At any firing temperature, a potential of 0.80 V was exhibited at a current density of 60 mA/cxl.

Example 2 In this example, changes in battery performance during continuous discharge of a single cell in which the 300"C fired air electrode and methanol electrode prepared in Example 1 were combined were evaluated.

The effective electrode area of the battery was 140 d, and the air electrode used had reached equilibrium absorption with the am liquid. The operation was carried out at 60℃, with air on the air electrode side and an anolite (on the methanol electrode side).
1,5-0Q/QHzSOa-1,0+++oQ/QC
HaOH) was circulated. Current density 60 mA/a
The discharge characteristics of this battery at 1 are shown in FIG.

As seen in the figure, the voltage which was initially 0.41 V was 0.40 V after about 10 hours, and the voltage drop was 5 mV per 10 hours thereafter.

Comparative Example 2 Changes in battery performance during continuous discharge of the unit cell prepared in Comparative Example 1, which was a combination of an air electrode and a methanol electrode fired at 300° C., were evaluated.

The battery configuration and operating conditions were exactly the same as in Example 2. As a result, the battery voltage, which was 0.41V, was approximately 1
o, asv after 0 hours, and the voltage drop per 10 hours thereafter was 10 mV.

Example 3 In this example, Z was added as a hydrophilic member to a catalyst layer formed by coating an electrode catalyst in which 15 wt% of PT was supported on a first carrier and a 30 wt% PTFE kneaded product onto a conductive porous substrate.
The effect of adding rOx will be described.

The air electrode was prepared using the following procedure. Electrocatalyst 4.5
g and 1.8 g of Zr0z having an average particle size of 1 μm are mixed, water is added, and the mixture is sufficiently kneaded. Polyflon dispersion (PTFE) was added to this in an amount of 2.7 g and kneaded. This paste was applied onto porous carbon paper (250x3001!m), air-dried, and then baked in air at a temperature of 300°C. This air electrode is 60℃
In the absorption test of -3ago j2/n HzSOt, it absorbed 10 mg/ci of electrolyte. This value becomes 45% when converted to pore occupancy. For the air electrode that has reached this equilibrium absorption.

The current density-potential characteristics as a single electrode and the characteristics of a single cell in combination with a methanol electrode were evaluated. The evaluation method was the same as that used in Examples 1 and 2.

As a result, the unipolar performance showed a high potential of 0.80 V at a current density of 60 mA/a#. Also, the battery voltage is initially 0.
．． The voltage was 40 V, and after about 10 hours it was 0.39 V, and the voltage drop was 5 mV per 10 hours.

Example 4 In this example, phosphoric acid was added as a hydrophilic member to a catalyst layer formed by coating an electrode catalyst in which 15 wt% of PT was supported on a first carrier and a 30 wt% PTFE kneaded product onto a conductive porous substrate. The effect of adding zirconium (Zr(HPOa)z) will be described.

The air electrode was prepared using the following procedure. Xc-72R-
15% Pt electrode contact g4.5 g and average particle size 200 m
Mix 8 g of Zr (HPOth) below esh, add water, and thoroughly knead.Add polyromendis version as PTFE to a total weight of 2.7 g and knead.This paste is mixed with porous carbon. paper(
250x300m) and air dried for 30 minutes in the air.
It was fired at 0°C. This air electrode is 60'C-3moQ/
In the absorption test of jlHzsOa.

Absorbed 10.5 mg/aJ of electrolyte. This value is.

When converted to pore occupancy rate, it is 48%. For the air electrode that had reached this equilibrium absorption, current density-potential characteristics as a single electrode and unit cell characteristics in combination with a methanol electrode were evaluated. The evaluation was performed in the same manner as in Examples 1 and 2.

As a result, the monopolar performance showed a potential of 0.78 V at a current density of 60 mA/a#. Also, the battery voltage is initially 0.3
It showed a voltage of 8 V, 0.37 V after about 10 hours, and then a voltage drop of 5 mV per 10 hours.

In addition, when producing a gas diffusion electrode, the carrier supporting the active metal must have a hydrophobic part and a hydrophilic part that have conductivity to a degree that does not impair electronic conductivity and can maintain water repellency as a gas diffusion electrode. Furthermore, the active metal may be present only in the parent part of the carrier to form the catalyst layer.

〔Effect of the invention〕

According to the present invention, it is possible to increase the amount of electrolyte absorbed by the gas diffusion electrode, that is, the air electrode, and to expand the pore occupancy region that exhibits a high potential. Since it is possible to suppress the drop in potential due to changes, a longer battery life can be achieved.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the ratio of the amount of electrolyte occupying the pore volume of the air electrode catalyst layer, that is, the relationship between pore occupancy and air electrode performance;
Figure 2 shows the change in the amount of electrolyte absorbed by the air electrode over time;
Figure 3 shows the equilibrium absorption amount of electrolyte in the air electrode with varying amounts of PTFE in the catalyst layer, Figure 4 shows the amount of electrolyte absorbed versus time in the air electrode with varying firing temperature, and Figure 5 shows: Figure 6 shows the amount of electrolyte absorbed by air electrodes with different catalyst layer thicknesses. Figure 6 shows the changes in pore volume of air electrode catalyst layers prepared by various methods. Figure 7 shows the amount of electrolyte absorbed by air electrodes with different pore occupancy rates. Potential change, Figure 8 (A
) (B) is an enlarged model diagram of the unit cell configuration of a methanol fuel cell and the vicinity of the air electrode - ion exchange membrane - methanol pole,
Figure 9 shows the characteristics of a conventional battery. Figure 10 shows the amount of electrolyte absorbed by the air electrode according to the present invention over time, and Figure 11 shows the current density of the air electrode according to the present invention.
12 shows the electrolyte absorption amount versus time for the conventional air electrode, and FIG. 13 shows the continuous discharge characteristics of a fuel cell using the air electrode according to the present invention. 1... Amount of electrolyte absorbed by the air electrode using the first carrier - 15% pt electrode catalyst, 2... Amount of electrolyte absorbed by the air electrode using the second carrier - 50% pt electrode catalyst, 3...Equilibrium electrolyte absorption amount of air electrodes with varying amounts of PTFE in the catalyst layer, 4...Air electrodes using first carrier-15% pt electrode catalyst fired at 300°C Absorption amount of electrolyte, 5... Also fired at 320°C, 6... 340
7. Electrolyte absorption amount of the air electrode coated with 5 mg of second carrier-50% PT electrode catalyst per 12 electrodes, 8...
Same <10mg/ffl'! ! I clothed, 9...
air chamber. 10... Air electrode substrate, 11... Air electrode catalyst layer, 12
...Ion exchange membrane, 13...methanol electrode catalyst layer,
14... Methanol electrode substrate, 15... Anolyte chamber, 16... Electrolyte layer occupying the air electrode catalyst layer, 17...
・Amount of electrolyte absorbed by the air electrode fired at 300°C in the present invention, 18...Those also fired at 320°C, 19.
...17 air electrode current density-potential characteristics, 20...1
8 Characteristics of the air electrode 221...Amount of electrolyte absorbed by a conventional air electrode fired at 300°C, 22...Amount of electrolyte absorbed versus time of an air electrode also fired at 320°C.

Claims (1)

[Claims] 1. A fuel cell comprising a pair of opposing electrodes consisting of an oxidizer electrode and a fuel electrode, and an ion exchange membrane containing an electrolyte,
A fuel cell characterized in that a material more hydrophilic than the substance constituting the oxidant electrode catalyst layer is added to the catalyst layer to increase the holding volume of the electrolyte. 2. The fuel cell according to claim 1, wherein the electrolyte is made of a proton dissociation type strong acid. 3. The fuel cell according to claim 2, wherein the strong acid comprises at least one of sulfuric acid, phosphoric acid, and an organic acid having a sulfonic acid group. 4. The fuel cell according to claim 1, wherein the ion exchange membrane is a cation exchange membrane that transports protons. 5. In claim 1, the oxidizing agent electrode catalyst layer constituent material is composed of a mixture of an electrode catalyst in which platinum is supported on a carbon-based carrier and polytetrafluoroethylene having water repellency and binding properties. A fuel cell featuring: 6. In claim 1, the hydrophilic member is made of a material more hydrophilic than the catalyst layer constituent material, and includes titanium oxide, zirconium oxide, tin oxide, zircon,
boron nitride, silicon nitride, tantalum carbide, silicon carbide,
A fuel cell comprising at least one of zirconium phosphate, titanium phosphate, etc. 7. The fuel cell according to claim 1, wherein the oxidizing agent is air and the fuel is methanol.