JP4977911B2 - Electrode catalyst powder for air electrode of hydrogen-air / solid polymer electrolyte type reversible cell, electrode-electrolyte membrane assembly (MEA) having air electrode and reversible cell using the same - Google Patents

Description

The present invention relates to an improvement of an electrode catalyst for a hydrogen-air / solid polymer electrolyte membrane type water electrolysis / fuel cell reversible cell. The present invention also relates to an air electrode using the electrode catalyst, and an electrode-electrolyte membrane assembly (MEA) having the air electrode as a constituent part, and further relates to a reversible cell in which a hydrogen electrode is combined with the MEA.

In recent years, fuel cells have attracted attention as on-site power generation systems. Solid polymer fuel cells originated from global energy problems and environmental problems, and for the past decade or so, rapid technological development has been promoted in order to use them as household cogeneration systems and automobile power supplies. Some are starting to be used as automobiles and household power generators. On the other hand, a power storage device that systematically combines a fuel cell that can receive power from hydrogen and a water electrolysis cell that can convert energy from power to hydrogen is a fuel cell type that uses hydrogen-oxygen as a medium. There is also a proposal to use a storage battery as a large-scale industrial facility or a consumer facility. By applying such a device, it becomes possible to level the power load by storing midnight power and taking out the power in the daytime, and to stably supply power by natural energy such as wind power and solar power generation. Further, it is possible to configure a secondary battery that can be charged by two methods such as electricity supply or fuel replenishment.

The basic structure of a fuel cell and a water electrolysis cell that can be configured as the same solid polymer electrolyte cell is almost the same. As shown in FIG. 1, one MEA (Membrane Electrode Assembly) Electrode-electrolyte membrane assembly) A basic cell unit is composed of two separator / bipolar plates and two reaction gas diffusers, which are stacked, and finally two power supply plates A solid polymer electrolyte reversible cell that integrates and integrates the functions of a water electrolysis cell and a fuel cell into a single cell by placing two insulating plates and two end plates into a stack. It is possible to configure. This is also referred to as an “integrated renewable fuel cell (URFC)”. If such a reversible cell is applied to a system that uses both the above fuel cell and water electrolysis, it can be a device with excellent practicality and versatility such as cost reduction, compactness, and high operating rate by integration. Expected.

The biggest difference between a fuel cell and a water electrolysis cell is that the electrode reaction is the opposite. This is shown in FIG. In FIG. 2, the name of each electrode chamber is an anode chamber (cathode at the time of fuel cell) that generates oxygen during water electrolysis as an oxygen electrode chamber, and a cathode that generates hydrogen at the time of water electrolysis (fuel cell). The anode chamber of the hour) was marked as the hydrogen electrode chamber.

A specific method of operating the reversible cell is performed as follows. During operation in the fuel cell mode, hydrogen gas supplied with a predetermined temperature and relative humidity by a humidifier is supplied from the outside of the cell to the hydrogen electrode chamber, and the predetermined temperature and relative humidity are also supplied into the oxygen electrode chamber by the humidifier. Is supplied with oxygen to generate electricity. The electrode reaction on the hydrogen electrode and oxygen electrode at this time is as follows:
Reaction on the hydrogen electrode: H ₂ → 2H ⁺ + 2e
Reaction at the top of oxygen: (1/2) O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O
During the reaction, hydrogen ions move in the electrolyte membrane from the hydrogen electrode side toward the oxygen electrode side. On the other hand, the electrode reaction when operated in the water electrolysis mode is as follows,
Reaction on the hydrogen electrode: 2H ⁺ + 2e → H ₂
Reaction at the top of oxygen: H ₂ O → (1/2) O ₂ + 2H ⁺ + 2e ⁻
Hydrogen and oxygen are generated by applying a direct current while supplying water into the oxygen chamber from the outside of the cell. Along with the reaction, hydrogen ions move in the electrolyte membrane from the oxygen electrode chamber side toward the hydrogen electrode chamber side.

For this reason, each member constituting the reversible cell is required to have a versatile function and material / material characteristics that are optimal for reactions of both the fuel cell and water electrolysis. The technical point is the oxygen electrode side. The reason is that the electrode reaction at the oxygen electrode has a high reaction overvoltage on the oxygen side in both the fuel cell and water electrolysis, and the electrode reaction is slower than that on the hydrogen side, and the oxygen reduction reaction in the fuel cell mode. Pt, which is a catalyst material with high reaction activity, is an activity for oxygen generation reaction during water electrolysis among platinum group elements such as Ru, Rh, Pd, Ir, and Pt, which are highly applicable to anode catalysts for industrial electrolysis. Is the lowest metal.

Along with this, the problem is that it is necessary to supply the electrode surface with water which is liquid in the water electrolysis mode, while actively supplying oxygen which is gas in the fuel cell mode. In general, as discussed in the following Patent Documents 1 to 3 and the like, the electrode for a fuel cell and the gas diffuser include moisture in a humidified gas and reaction water generated on the oxygen electrode according to the above reaction formula. In order to prevent this, the oxygen supply path in the diffusion layer or the electrode catalyst layer may be blocked, so in order to prevent this, a graphite-based material that is a water-repellent material is used as the electrode catalyst carrier or diffusion layer. In addition, a water repellent is added or impregnated to the base material to improve water repellency so that condensed water is easily removed. For example, Patent Document 4 discloses that the amount of the water repellent added is 5 to 30 wt.%.

However, in the reversible cell, if the amount of the water repellent added is too large, in the water electrolysis mode, the supply of water from the outside to the electrode surface is hindered and the water electrolysis performance deteriorates. In addition, graphite-based materials that themselves have water repellency, such as those used in fuel cells, are used as oxygen electrode components for reversible cells that require an anode resistance of 1.4 V vs. NHE or higher in the water electrolysis mode. This is practically impossible from the viewpoint of durability.

As described above, in order to improve the performance of a solid polymer electrolyte type reversible cell and put it to practical use, it is active against both water electrolysis reaction and fuel cell reaction, and balanced oxygen. A polar configuration is required. From this point of view, the inventors have so far attempted to develop a constituent member that enables an electrode reaction in the opposite direction to proceed with high efficiency at the same place. And the technology of Non-Patent Document 1 has been developed and disclosed.
JP2002-25575 JP 2001-332269 A JP 2004-262675 A JP 2006-107752 A JP 2000-342965 A JP 2004-134134 A JP 2004-134135 A Japanese Patent Application No. 2005-188583 T.Ioroi et al., Journal of Power Sources 112 (2002) 583-587

In the reversible cell manufactured by the technology developed by the inventors, for example, as described in Patent Document 8, when a reciprocating operation between water electrolysis and a fuel cell is performed, the current density is 0.2 to 0.6 A / cm. When operated at ² , the reciprocating operation efficiency of 57.1-44.2% is obtained. As the output voltage of the fuel cell, 0.786 to 0.730 V is obtained at a current density of 0.4 to 0.6 A / cm ² , which is highly efficient. As for water electrolysis performance, water electrolysis can be performed at an electrolysis voltage of 1.600 to 1.715 V at a current density of 1 A / cm ² , and the cell has performance comparable to that of a cell that exclusively performs water electrolysis. Is realized. The URFC disclosed in Non-Patent Document 1 is an electrode manufactured by a paste method, and in an oxygen electrode electrocatalyst composed of platinum black, PTFE and Nafion, 5 to 7 wt% PTFE and 7 to 9 wt%. % Nafion plus about 10 atomic% iridium showed high performance in both water electrolysis and fuel cell modes.

By the way, the above-mentioned reciprocal efficiency and fuel cell performance are obtained when pure oxygen is used as a reaction gas (which is an oxidant and sometimes referred to as such) for supplying a fuel cell operation of a reversible cell to an oxygen electrode chamber. Is the result of In the reversible cell, hydrogen and oxygen are generated simultaneously during water electrolysis, so both gases can be stored in a high-pressure cylinder or stored in a hydrogen storage alloy and used during power generation. On the other hand, in order to use a reversible cell in a place where space is limited, only hydrogen gas generated by water electrolysis is stored, oxygen gas is abandoned, and the fuel cell has air present in the atmosphere. It is advantageous to use oxygen in the inside. If oxygen in the air can be used, the versatility of the reversible cell will increase and it is expected to be widely used. By omitting the storage device, the device can be miniaturized and a mobile hydrogen / oxygen secondary battery can be realized.

However, as shown in FIG. 3, in the reversible cell according to the prior art, when oxygen in the air is used as the oxidant instead of pure oxygen, there is a problem that the characteristics are greatly deteriorated and the operation becomes unstable. It was. This is because most of the overvoltage of the fuel cell reaction is originally in the oxygen reduction reaction at the oxygen electrode, and the concentration of oxygen in the air is as thin as about 20%, and the supply of the reaction gas to the electrode / electrolyte interface is It can be slow.

When pure oxygen is used as the oxidant, oxygen gas is consumed on the electrode, and the reaction product is only water. Therefore, gas supply in a direction perpendicular to the electrode surface is naturally promoted by concentration diffusion. However, when oxygen in the air is used as the oxidizing agent, oxygen is consumed on the electrode surface, but a large amount of N ₂ gas that does not participate in the reaction remains, so that the gas supply effect due to concentration diffusion is reduced.

When using oxygen in the air, there is a difference in phenomenon as described above, and therefore, cell components that were usable when using pure oxygen are not always usable as they are. . In that case, it is necessary to develop cell components suitable for air operation.

An object of the present invention is to provide a solid polymer type reversible cell suitable for air operation and its components in response to the above-described needs. Specifically, a first object is to provide an electrode catalyst powder suitable for a solid polymer electrolyte type reversible cell using oxygen in the air as an oxidizing agent. The second object is to provide an air electrode mixed catalyst using the electrode catalyst powder, and the third object is an electrode for a solid polymer type reversible cell using the air electrode mixed catalyst. An electrolyte membrane assembly (hereinafter abbreviated as “MEA”) is provided. A fourth object is to provide a hydrogen-air / solid polymer electrolyte type reversible cell in which this MEA is assembled as a component. It is in.

An electrode catalyst powder for achieving the first object of the present invention is an electrode catalyst for a hydrogen-air / solid polymer electrolyte type reversible cell composed of platinum black and polytetrafluoroethylene (PTFE), and a mixture thereof. The PTFE content therein is in the range of 6.9 to 8.0% by weight.

A mixed catalyst for an air electrode that achieves the second object of the present invention is the above-mentioned electrode catalyst powder and iridium or its oxide powder in a metal weight ratio, Pt: Ir = 80-90: 20- 10 is a mixed catalyst for an air electrode of a hydrogen-air solid polymer type reversible cell obtained by mixing so as to be 10.

An MEA that achieves the third object of the present invention is an MEA for a solid polymer electrolyte type reversible cell having an air electrode using the above mixed catalyst.

A water electrolysis / fuel cell reversible cell that achieves the fourth object of the present invention is a reversible cell comprising the MEA as a constituent part.

The solid polymer electrolyte type water electrolysis / fuel cell reversible cell according to the present invention can operate a fuel cell using oxygen in the air instead of pure oxygen as an oxidant in the oxygen electrode. The power generation performance is improved by 7.0 to 9.3% compared to the same type of reversible cell using. This performance can be obtained stably. On the other hand, the water electrolysis performance is not substantially lowered. As a result, the reciprocal efficiency of the water electrolysis-fuel cell is improved by 5.7 to 7.2%. Needless to say, the fuel cell can be operated using oxygen gas as the oxidant.

With such an effect, it is not necessary to provide an oxygen gas storage facility as a reversible cell, so that the entire apparatus can be configured to be small and light. This effect makes it possible to construct a hydrogen-air / solid polymer electrolyte water electrolysis / fuel cell reversible cell in a smaller space than in the past and, in some cases, to be portable. Make reversible cells versatile.

The manufacturing process from producing a catalyst powder according to the present invention, using it as an electrode, constructing the MEA, and finally assembling the reversible cell is as follows.

The first step is a step of producing a mixture of platinum black and PTFE, and the second step is a step of forming an air electrode catalyst layer comprising the obtained mixture and solid polymer electrolyte particles. It is. The third step is a step of manufacturing a joined body, that is, MEA, in which the formed air electrode catalyst layer and the electrolyte membrane are joined together with the hydrogen electrode and the electrolyte membrane as counter electrodes by hot pressing, and the fourth step is This is the final process of assembling the reversible cell by laminating the manufactured MEA with a diffuser (current collector / feeder), a separator and an end plate, and fastening them with bolts.

In the present invention, the reason why the amount of PTFE, which is a water repellent added to platinum black, is in the range of 6.9 to 8.0 wt.% Is the typical amount added to the conventional catalyst. When the amount of PTFE is increased from 7 wt.% To the above range, the fuel cell performance when oxygen in the air is used as the oxidant is remarkably improved. On the other hand, the reason for setting the upper limit to 8.0 wt.% Is that when PTFE is added beyond this, the fuel cell performance is higher than that of the conventional catalyst, but the electrolysis voltage during water electrolysis tends to increase. This is because the round-trip efficiency, which is important as the performance of the reversible cell, decreases. If the amount of PTFE added is in the above-mentioned range of 6.9 to 8.0 wt.%, A reversible cell capable of performing fuel cell power generation using water electrolysis and air with high efficiency is realized. The highest efficiency within this range is obtained when the amount of PTFE added is around 7.0 wt.

PTFE containing a commercially available high molecular weight perfluoro compound capable of metal impregnation and metal coating is used. Specifically, a PTFE dispersion (manufactured by Daikin Industries) can be suitably used. As the platinum black, the same gold black as that of the platinum catalyst conventionally used in the solid polymer electrolyte fuel cell may be used. The specific surface area is preferably wide, but if the surface area measured by the BET method is 10 m ² / g or more, it can be used.

In order to perform the water repellent treatment with PTFE dispersion on platinum black, the method disclosed in the above-mentioned Patent Document 1 can be applied. Specifically, commercially available platinum black is mixed with a solution in which a polyether surfactant is dispersed in pure water, and a predetermined amount of PTFE dispersion is blended therein. By adjusting the blending amount, the PTFE content in the mixture of platinum black and PTFE is set to a desired value within the above range. Then, if it heats to 45-100 degreeC, stirring and hold | maintains until a supernatant liquid becomes transparent, the aggregation precipitation which consists of a mixture of platinum black-PTFE will be obtained.

The obtained agglomerated precipitate is dehydrated, dried, and fired. Firing may be performed at a temperature of 327 ° C. or higher, which is the melting point of PTFE, but usually about 360 ° C. is appropriate. If this temperature is maintained for 1 hour or longer, the platinum black-PTFE mixture exhibits water repellency, and a catalyst material that can be used for the air electrode of the reversible cell is obtained. Since this baking treatment also serves to remove organic substances such as surfactants adhered in the water repellency treatment by thermal decomposition, the atmosphere is preferably under vacuum or under inert gas flow. The obtained fired product is pulverized according to a known pulverization technique to obtain the electrode catalyst powder of the present invention.

In order to form the air electrode catalyst layer of the present invention, first, iridium black or iridium oxide particles are added to the electrode catalyst powder obtained in the above step as a metal weight ratio, and Pt: Ir = 80 to 90: A mixed catalyst is prepared by mixing so as to be 20 to 10. As iridium black and iridium oxide, those available on the market may be used. The highest efficiency is obtained when the mixing ratio of Pt: Ir is in the vicinity of 85:15. Next, an air electrode catalyst layer is formed by mixing the mixed catalyst and solid polymer electrolyte particles. Mixing with the solid polymer electrolyte can be performed using an ionomer-containing solution of a perfluorosulfonic acid-based solid polymer electrolyte having an ion exchange density of 900 to 1200. As such a solution, for example, a 5 wt.% Nafion solution that is easily available in the market can be suitably used. ("Nafion" is a trademark of DuPont)

For formation of the air electrode catalyst layer, a catalyst slurry is prepared by adding a mixed catalyst to a Nafion solution, and this slurry is applied to a fluororesin substrate such as doctor blade method, screen printing, brush coating, spray spraying method, etc. It can carry out by apply | coating by the method of and drying. The solid polymer electrolyte membrane is preferably a perfluorosulfonic acid-based proton type electrolyte membrane having an ion exchange capacity of 900 to 1200, and the above-mentioned “Nafion” is most preferable, but “Flemion” or “Aciplex” Can be substituted. In addition, any cation conductive ion exchange membrane having the above ion exchange density can be used. In recent years, membranes other than fluorine (perfluorosulfonic acid) membranes have also been developed. The electrolyte membrane on the catalyst layer side is Nafion and the electrolyte membrane is a non-perfluorosulfonic acid membrane (for example, an inexpensive hydrocarbon membrane). However, if bonding by hot pressing is possible, it was confirmed that it can be used for water electrolysis.

The amount of Nafion solution used in forming the air electrode catalyst layer is the content of the electrolyte particles in the catalyst layer as it is. The amount may be Pt: electrolyte = 90 to 95:10 to 5 by weight ratio of platinum black and electrolyte particles. The most preferable weight ratio is around Pt: electrolyte = 92.7: 7.3.

Since the solid polymer electrolyte type reversible cell of the present invention is a device that performs water electrolysis and fuel cell reaction in the same cell, the electrodes are provided on both sides of the solid polymer electrolyte membrane with an oxygen electrode (air electrode) and a hydrogen electrode. Each of the poles has a structure in which gas diffusion electrodes are arranged. This reversible cell has a gas diffusion electrode having the above-mentioned catalyst as an air electrode of a fuel cell, and a gas diffusion electrode having a known structure having platinum black as an electrode catalyst as a hydrogen electrode. Just do it. The production of the hydrogen electrode may be performed according to the above-described method for producing the air electrode. The catalyst is platinum that is highly active for both the reaction of oxidizing water to generate water and the reaction of reducing water to generate hydrogen, and on the hydrogen electrode side, it is a fuel cell or water electrolysis The carbon electrode can be used regardless of whether it is a hydrogen electrode that can be used for a normal hydrogen-oxygen type fuel cell, and is a counter electrode in the reversible cell of the present invention. Can be used as

The mixing of the mixed catalyst with the solid polymer electrolyte and the subsequent formation of the catalyst layer do not directly affect the water repellency of the electrode catalyst itself, so that the electrode catalyst, iridium or iridium oxide, and the electrolyte are finally predetermined. If the catalyst layer adjusted to the quantitative ratio is formed, the effect of the present invention can be obtained. For example, a mixed catalyst and an electrolyte are mixed in a predetermined amount, prepared in advance by drying and heat treatment at a temperature of 100 to 230 ° C., and pulverized into powder when necessary, The same effect can be obtained by a manufacturing process in which this powder is slurried with an organic solvent and the like, and the resulting slurry is applied onto a substrate to obtain a catalyst layer.

As the base material to which the catalyst slurry is applied, the bonding environment in the hot pressing process in the next step, that is, the resistance and thermal conductivity under the conditions of a temperature of 50 to 130 kg / cm ² and a pressure of 100 to 240 ° C., and formation of the slurry Any material can be used as long as it does not exhibit water repellency to the organic solvent used. This is because when the base material is damaged during hot press treatment, a portion where the catalyst layer is lost on the electrode is generated. If the heat insulation is high, the temperature rise of the joint surface is slow and the treatment time is long. The reason for this is that it is impossible to form a uniform catalyst layer. As described above, when the Nafion solution is directly used as the solvent for the slurry, a PTFE sheet having a thickness of 50 to 200 μm (manufactured by Nitto Denko, etc.) and other fluororesin sheets are readily available.

Like a known gas diffusion electrode, a porous material such as carbon paper, metal nonwoven fabric, foam metal, or micromesh is used as a gas diffuser (current collector / feeder), and a catalyst layer is formed on the gas diffuser. Even when the electrode is obtained by forming the catalyst layer, the catalyst layer can be formed by the above-described method, and the performance of the formed catalyst layer is not different, so that the same effect as the present invention can be obtained. In that case, as the base material of the air electrode, that is, the diffuser, the use of the diffuser disclosed in the above-mentioned Patent Documents 6 and 7 gives a higher performance electrode.

In the MEA of the present invention, the air electrode catalyst layer and the electrode obtained as described above are arranged on both sides of the hydrogen electrode catalyst layer and electrode and the proton type solid polymer membrane described above, It can be obtained by joining by the method. There is no particular limitation on the hot press joining conditions, and the heat press treatment may be performed for 5 minutes under the general hot press conditions of a temperature of 120 to 220 ° C. and a pressure of 5 to 130 kg / cm ² .

Since the MEA manufactured by the above-described method is liable to be contaminated by dust from the outside air, it is desirable to wash it with dilute sulfuric acid before use. After the washing operation, it is desirable to store it in sufficiently fresh, for example, pure water having a conductivity of 2 μS / cm, and store it in a sealed state so as not to be exposed to the outside air.

Add 1.6 g of ether surfactant polyoxyethylene octylphenyl ether to 200 mL of pure water, stir for 5 minutes, add 20 g of platinum black, stir for 10 minutes, and then use an ultrasonic generator to remove platinum black. Was dispersed.

To this platinum black dispersion, PTFE dispersion (manufactured by Daikin Industries) was added to 6.7, 7.0, 7.5, 8.0 wt.% (2.4, 2.5, 2.7) with respect to Pt. 3.3 g), and the mixture was kept at 45 to 100 ° C. and stirred to obtain an aggregated precipitate of platinum black and PTFE. This was dehydrated and placed in an electric furnace and baked under vacuum at 360 ° C. for 1 hour. The fired product was pulverized with a cutter mill until the particle diameter passed through a 220 mesh (aperture 75 μm) sieve to obtain an electrode catalyst powder.

The electrode catalyst powder prepared by the above-mentioned method and iridium black are mixed so that the metal weight ratio is Pt: Ir = 85: 15, and a Nafion 5% solution (ion exchange density 1100) is mixed with platinum ( The mixture was mixed in a nitrogen atmosphere so as to be 7.3 wt. The mixture was stirred for 5 minutes and sonicated to produce a catalyst slurry in which catalyst powder and Nafion were mixed finely and uniformly. This slurry was spread on a fluororesin sheet having a thickness of 50 μm with a doctor blade to form an air electrode catalyst layer.

The hydrogen electrode catalyst layer was formed on the fluororesin sheet by the same method as described above. Of the above electrode catalysts, only those containing 6.7 wt.% Of PTFE were used so that the Nafion content in the platinum and Nafion mixture was 7.3 wt.%. (The iridium black mixture is not necessary for the hydrogen electrode catalyst.)

Place the air electrode catalyst layer formed on the fluororesin on one side of the sheet of Nafion 115, which is a cationic ion exchange resin, and the hydrogen electrode catalyst layer on the other side. An MEA for a reversible cell was manufactured by heating and pressing at a temperature of 140 ° C. and a pressure of 90 kg / cm ² for 5 minutes. The effective electrode area of this MEA is 10 cm ² .

The manufactured MEA was hydrothermally treated in 100 mL of pure water at 85 ° C. for 1 hour, and then immersed in a 1M-H ₂ SO ₄ solution at 80 ° C. to remove impurity ions. Thereafter, the MEA is hydrothermally treated with pure water at 80 ° C. to remove the remaining sulfuric acid, and further washed with pure water at room temperature until the conductivity of the washing water becomes 2 μS / m or less. After confirming that it was removed, it was incorporated into the cell.

Using several types of MEAs produced as described above, a reversible cell was assembled together with a hydrogen electrode diffuser and an oxygen electrode diffuser of the same specification, and operated as a fuel cell and a water electrolysis device. The fuel cell performance of those reversible cells
Hydrogen flow rate: 100 ml / min Oxygen flow rate: 100 ml / min (500 ml / min for air)
Relative humidity: 99-100%
Cell temperature 80 ° C
The vehicle was operated and evaluated under the following conditions. The water electrolysis performance was similarly evaluated by operating at 80 ° C. and measuring the current-voltage characteristics under the operating conditions.

The efficiency was calculated by the following formula.
Fuel cell power generation efficiency: εFC = ΔG / ΔH = nFE _FC / ΔH ⁰ ₃₅₃
Water electrolysis _{^{efficiency: εWE = △ H 0 353 /}} △ G = △ H 0 353 / nFE WE
Round-trip conversion efficiency: εTOTAL = εFC × εWE = E _FC / E _WE
Here, F: Faraday constant (C), ΔH ⁰ ₃₅₃ : 284.038 kJ / mol, n = 2

Table 1 shows fuel cell voltage characteristic values when oxygen is used and Table 2 shows fuel cell voltage characteristic values when air is used for reversible cells equipped with air (oxygen) electrodes using electrode catalysts with various amounts of PTFE added. Show. Table 3 shows the water electrolysis operation characteristics. Table 4 shows the reciprocal conversion efficiency of the water electrolysis-fuel cell when the fuel cell is operated with air. FIG. 4 is a graph showing the data in Table 4 in comparison with the case of operating with pure oxygen.

From the results shown in Tables 1 to 4 and the comparison between FIG. 2 and FIG. 4, the following can be understood regarding the power generation performance of the assembled fuel cell.
(1) When pure oxygen is used, there is no significant difference in performance over the range of PTFE content in the electrode catalyst powder of 6.7 to 8.0 wt.%.
(2) When air is used, the oxygen electrode using an electrode catalyst containing 6.7 wt.% PTFE has a power generation efficiency of 40% or less at a current density of 400 mA / cm ² , and a higher current. Operation in the density range was not possible.
(3) Even when air is used, if the PTFE content is increased, the characteristics are greatly improved, and operation is possible even at a high current density of 700 mA / cm ² . The power generation characteristics have a strong dependence on the PTFE content, with a maximum around 7.0 wt.%.
(4) Eventually, the relationship between the PTFE content in the electrode catalyst powder and the power generation characteristics differs greatly depending on whether oxygen is supplied to the oxygen electrode or air, and fluctuations in the PTFE amount of about 1 wt. It was found that there was a difference in characteristics. This difference could not be predicted from the characteristics of the conventional oxygen electrode.

If air is used as the oxidant on the oxygen electrode side where the overvoltage of the fuel cell reaction is originally high, factors such as difficulty in supplying the gas to the electrode surface will be severely reflected in the performance. If the diffusion of water is hindered by agglomeration of steam that has humidified the reaction water or gas, the power generation performance will be greatly affected. In addition, when using air, the gas supply effect to the electrode surface due to the concentration gradient is less than when using oxygen, and in the vicinity of the electrode, the oxygen partial pressure is reduced due to the N ₂ gas remaining after the reaction. There is a problem of a significant drop. Therefore, for example, if the gas supply path is blocked by wetting with condensed water, even if the blockage is not a problem when operating with oxygen, it will have a significant effect when operating with air. Therefore, an air / oxygen electrode capable of high-performance operation with both air and pure oxygen is obtained by a difference in PTFE content of about 1%, or an electrode that exhibits high performance only in the case of oxygen. It is thought that there will be a difference.

On the other hand, when the amount of PTFE added is varied within the range of 6.7 to 8.0 wt.% Of the catalyst powder, the water electrolysis characteristics gradually decrease with the increase of PTFE as a whole. Until it reaches 5 wt.%, A gradual voltage increase is observed. Between 7.5 wt.% And 8.0 wt.%, When the current density is 1 A / cm ² , for example, there is a tendency that the electrolytic voltage increases by about 40 mV with the increase of PTFE, and the amount of PTFE is 8%. Setting it to 0.0 wt.% Or more leads to a decrease in water electrolysis performance. Even when the PTFE amount is increased to 8.0 wt.% Or more, the fuel cell characteristics do not tend to improve in both the air operation and the oxygen operation. As a result, the round-trip efficiency obtained by multiplying the efficiency of both the water electrolysis and the fuel cell also tends to decrease toward 8.0 wt.%, With the PTFE being at the maximum of 7.0 wt.%.

To summarize the above results, in a reversible cell using the electrode catalyst powder of the present invention, in order to supply air instead of oxygen and to generate power with a fuel cell, the PTFE content is 6.7 wt.% Or more, The PTFE content should be about 7.0 wt.% To be 8.0 wt.% Or less, and in order to obtain the highest reciprocating operation efficiency within this range.

Table 4 Water electrolysis / fuel cell reciprocation efficiency when using air (%)

The conceptual diagram which shows the basic structural member and structure of a solid polymer electrolyte type reversible cell. A conceptual diagram showing the principle of fuel cell power generation and water electrolysis of a solid polymer electrolyte reversible cell. The graph which shows the electric power generation characteristic when driving | running in the fuel cell mode using pure oxygen and air of the reversible cell assembled according to the prior art. The graph which shows the electric power generation characteristic when operating in the fuel cell mode using pure oxygen and air of the reversible cell according to the present invention.

Claims (4)

An electrode catalyst for a hydrogen-air / solid polymer electrolyte type reversible cell comprising platinum black and polytetrafluoroethylene (PTFE), wherein the PTFE content in the mixture is 6.9 to 8.0% by weight Electrocatalyst powder characterized by being in the range of A hydrogen-air solid obtained by mixing the electrode catalyst powder according to claim 1 and the powder of iridium or an oxide thereof in a metal weight ratio such that Pt: Ir = 80 to 90:20 to 10. Mixed catalyst for air electrode of polymer type reversible cell. An electrode-electrolyte membrane assembly (MEA) for a polymer electrolyte reversible cell having an air electrode using the mixed catalyst according to claim 2. A solid polymer electrolyte type water electrolysis / fuel cell reversible cell comprising the MEA according to claim 3 as a constituent part.

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