JP4238364B2 - Polymer electrolyte fuel cell - Google Patents

Description

The present invention relates to a polymer electrolyte fuel cell.

The polymer electrolyte fuel cell is a power source for transportation equipment such as electric vehicles and aerospace equipment, and a small portable power source because of its high current density at low temperature operation and its miniaturization. Research and development are underway. As fuel supplied to the polymer electrolyte fuel cell, hydrogen gas produced by reforming natural gas, methanol, gasoline or the like is generally used.
On the other hand, a polymer electrolyte fuel cell (direct methanol fuel cell) that directly supplies methanol as a fuel has been attracting attention in recent years because of its convenience as a liquid fuel for storage and transportation. In particular, direct methanol fuel cells are the mainstream for small power supplies and chargers for portable devices. At present, wearable information terminals, implantable biosensors, medical devices, and the like are also being developed. As a power source for these microelectronic devices, the fuel can be supplied for 24 hours without charging. Application of a working direct fuel cell is under consideration.
The direct methanol fuel cell has a structure in which electrodes are attached to both sides of a solid polymer electrolyte membrane, similarly to a solid polymer fuel cell using pure hydrogen or reformed hydrogen as a fuel. Usually, a platinum-ruthenium catalyst is used for the anode (fuel electrode), and a platinum catalyst is used for the cathode (air electrode). When an aqueous methanol solution is supplied to the anode side, methanol reacts with water and is oxidized to generate carbon dioxide, protons and electrons. The generated protons move to the cathode side in the electrolyte membrane, and oxygen is reduced on the cathode side to generate water. In this process, electrons flow through the external circuit and current can be taken out.
However, since the oxidation reaction rate of methanol is extremely slow and the overvoltage is large, the performance of the fuel cell is significantly reduced when methanol is used as the fuel, compared to when hydrogen gas is used as the fuel. In order to solve this problem, a great number of studies have been made on the electrode catalyst for methanol oxidation, and various alloy catalysts, organometallic complexes, etc. have been proposed, but the activity of platinum-ruthenium alloys known since the 1960s has been improved. Significantly more electrocatalysts have not yet been found.
In addition to methanol, ethanol (C. Lamy, E.M. Bergsir, and J.-ML.e.ger, J. Appl. Electrochem., 31, 799 (2001)), glycol (Japanese Patent Laid-Open No. 2002-2001). Various alcohols such as dimethyl ether (Japanese Patent Laid-Open No. 11-144751), borohydride solution (Japanese Patent Laid-Open No. 2002-50375), acetal (O. Savadogo and X. Yang, J. New Mat. Electrochem. Systems, 5, 9 (2002)) and the like have been studied.
The biggest problem with direct methanol fuel cells is the high overvoltage of the methanol electrode oxidation reaction. Therefore, it is necessary to use a large amount of an expensive noble metal-based electrode catalyst such as a platinum-ruthenium alloy. However, since the ratio of the cost of the noble metal catalyst to the total cost of the entire fuel cell system is large, in order to realize the practical use and cost reduction of the polymer electrolyte fuel cell, the amount of noble metal used is reduced or The development of electrocatalysts that do not use precious metals has become a major issue.
In addition to being a hazardous substance designated as a deleterious substance, methanol, when used as a fuel in a polymer electrolyte fuel cell, contains trace amounts of harmful substances such as formaldehyde and formic acid along with carbon dioxide. Since it may be generated as an oxidation product, safety of the direct methanol fuel cell is regarded as a problem when it is used for consumer equipment. In particular, when using a fuel cell in a portable device, wearable information terminal, implantable device, etc. that is in close contact with or attached to the human body or embedded, the safety and biocompatibility of the fuel are increasingly important. Become. In addition, methanol not only has an adverse effect on the human body, but is corrosive to some rubbers and plastics. Therefore, in the direct methanol fuel cell, there is a problem that the constituent material and the material that can be selected as the sealing member are limited, and the cost is increased.
Furthermore, in the direct methanol fuel cell, a phenomenon called “crossover” is known, in which the supplied methanol permeates the cathode side and causes performance degradation. That is, methanol that permeates the electrolyte membrane and reaches the cathode side reacts with oxygen on the cathode catalyst to generate carbon dioxide and water, thereby reducing the fuel utilization rate. In addition, since the cathode potential decreases as a hybrid potential, the output of the fuel cell also decreases. Due to this methanol crossover, the perfluorosulfonic acid solid polymer electrolyte membrane developed for conventional solid polymer fuel cells using hydrogen gas as fuel is sufficient as a direct methanol fuel cell. In many cases, the performance cannot be demonstrated. Therefore, the development of an electrolyte membrane with low methanol permeability is an urgent need for the practical use of direct methanol fuel cells.

The present inventor has conducted research on solid polymer direct fuel cells that supply other compounds as fuel instead of methanol while paying attention to the problems of the prior art. As a result, when a group of compounds such as ascorbic acid used as a reducing agent in reaction processes such as catalyst preparation, synthesis reaction, and electroless plating are used as fuel, solid polymers that can alleviate the problems of the prior art It has been found that a direct fuel cell can be obtained.
That is, the present invention provides the following polymer electrolyte direct fuel cell.
1. Direct operation using at least one selected from the group consisting of ascorbic acid, isoascorbic acid, citric acid, tartaric acid, sulfurous acid, thiosulfuric acid, hydrogen sulfite, dithionic acid, phosphorous acid, hypophosphorous acid and their salts as fuel A polymer electrolyte fuel cell characterized by the above.
2. 2. The solid polymer fuel cell as described in 1 above, wherein at least one selected from the group consisting of ascorbic acid, isoascorbic acid, citric acid, tartaric acid and salts thereof is used as the fuel.
3. 2. The polymer electrolyte fuel cell according to item 1, wherein at least one selected from the group consisting of sulfurous acid, thiosulfuric acid, hydrogen sulfite, dithionic acid, phosphorous acid, hypophosphorous acid and salts thereof is used as fuel.
4). 2. The polymer electrolyte fuel cell as described in 1 above, wherein a metal is used as the anode catalyst.
5. 2. The solid polymer fuel according to item 1, wherein at least one selected from the group consisting of ascorbic acid, isoascorbic acid, sulfurous acid, hydrogen sulfite, dithionic acid and salts thereof is used as a fuel, and a carbon material is used as an anode catalyst. battery.
6). 6. The polymer electrolyte fuel cell as described in 5 above, wherein carbon black is used as the anode catalyst.
7). At least one selected from the group consisting of ascorbic acid, isoascorbic acid, sulfurous acid, hydrogen sulfite, dithionic acid and their salts is used as a fuel, and a current collector made of carbon paper or carbon cloth is brought into contact with the solid polymer film. The solid polymer fuel cell as described in 1 above, which is directly operated in the absence of an electrode catalyst.
The direct fuel cell of the present invention is a solution of a compound selected from ascorbic acid, isoascorbic acid, citric acid, tartaric acid, sulfurous acid, thiosulfuric acid, hydrogen sulfite, dithionic acid, phosphorous acid, hypophosphorous acid and their salts. As a fuel to generate electricity.
Compounds such as ascorbic acid, isoascorbic acid, citric acid, tartaric acid, sulfurous acid, thiosulfuric acid, hydrogen sulfite, dithionic acid, phosphorous acid, hypophosphorous acid and their salts have a reducing action. The reaction rate is fast. Therefore, when these compounds are supplied as fuel to the anode, the overvoltage is lowered, and it is considered useful for improving the performance of the polymer electrolyte fuel cell.
For example, FIG. 1 is a cyclic voltammogram measured in a 0.5 M sulfuric acid solution on a platinum-ruthenium electrode using ascorbic acid, sodium sulfite, sodium hydrogen sulfite, sodium dithionite and hypophosphorous acid, respectively. is there. FIG. 1 also shows the results for methanol as a comparison.
As is apparent from the results shown in FIG. 1, a higher current density is obtained at the same potential when the other five compounds are used compared to when methanol is used. That is, when these compounds are used as fuel in the polymer electrolyte fuel cell, it is presumed that the anode potential is lowered and excellent fuel cell characteristics can be obtained. Thus, when a compound that can easily undergo an electrode oxidation reaction is used as a fuel, not only can a significant performance improvement of the fuel cell be expected, but also the possibility that an inexpensive catalyst other than platinum can be selected as the electrode catalyst. Further, there is a possibility that it can contribute to the reduction of the catalyst amount and the realization of a fuel cell that does not use an anode catalyst.
Among the above compounds, ascorbic acid, isoascorbic acid, citric acid, tartaric acid and their salts (sodium salt, potassium salt, magnesium salt, calcium salt, ammonium salt, etc.) are non-toxic and harmless which are also used as food additives Even when it is used in consumer equipment as a fuel directly supplied to the fuel cell, there is no problem in handling. Further, when ascorbic acid, isoascorbic acid, citric acid, tartaric acid and their salts are used as fuel, C-OH is only oxidized to a ketone group, and no dangerous compound is produced.
On the other hand, sulfurous acid, thiosulfuric acid, hydrogen sulfite, dithionic acid, phosphorous acid, hypophosphorous acid and their salts (sodium salt, potassium salt, magnesium salt, calcium salt, ammonium salt, etc.) are corrosive and irritating. However, it is not a compound designated as a deleterious substance such as methanol. Sulfurous acid, hydrogen sulfite, thiosulfuric acid, dithionic acid and their salts can produce sulfuric acid, and phosphorous acid, hypophosphorous acid and their salts can produce phosphoric acid. By collecting in the reservoir, release to the outside of the apparatus can be prevented, so that practical problems can be avoided.
Therefore, according to the present invention, it is possible to reduce the risk of diffusion of highly volatile harmful substances such as formaldehyde into the air.
In direct methanol fuel cells, it is considered that methanol supplied to the anode permeates the electrolyte membrane together with water and reaches the cathode side. On the other hand, compounds such as ascorbic acid used in the present invention are ionized in a solution, of which anion acts as a fuel. Currently, most of the electrolyte membranes commercialized for polymer electrolyte fuel cells are cation exchange membranes in which an anionic group such as sulfonic acid is fixed. Therefore, in the present invention, an anionic compound is used. Phenomenon such as crossover that is taken into the electrolyte membrane by ion exchange or permeates to the cathode side is unlikely to occur.
Basically, known polymer electrolyte membranes, electrode catalysts, membrane-electrode assemblies and cell structures used in ordinary solid polymer fuel cells can be applied to the fuel cell according to the present invention.
For example, as the electrode catalyst, conventionally known various metals and metal alloys can be used. Examples of anode and cathode catalysts include platinum, palladium, iridium, rhodium, ruthenium, platinum-ruthenium and other metal catalysts, or supported catalysts in which these catalyst fine particles are dispersed on a carrier such as carbon. .
Further, in the fuel cell according to the present invention, when ascorbic acid, isoascorbic acid, sulfurous acid, hydrogen sulfite, dithionic acid and salts thereof are used as fuel, a carbon material such as carbon black is used without using a noble metal. Can be used as an anode electrode as it is. As the carbon material, carbon black having a high specific surface area (50 m ² / g or more) is more preferable.
Furthermore, in the fuel cell of the present invention, when ascorbic acid, isoascorbic acid, sulfurous acid, hydrogen sulfite, dithionic acid and salts thereof are used as fuel, a collector such as carbon paper or carbon cloth is used as a solid high It is also possible to drive the battery without using a catalyst on the anode side by pressing or closely contacting the molecular film.
As the polymer electrolyte membrane, various ion exchange resin membranes including perfluorocarbon, styrene-divinylbenzene copolymer, and polybenzimidazole can be used.
The joined body of the solid polymer electrolyte membrane and the electrode catalyst is generally produced in the same manner as a known joined body. For example, after a catalyst ink produced by mixing a catalyst powder and an electrolyte solution is thinned, a method such as hot pressing on an electrolyte membrane or direct application / drying on a polymer membrane is applied. In addition, the catalyst can be directly attached to the solid polymer film by a method such as adsorption reduction, electroless plating, sputtering, or CVD. Alternatively, the electrode may be produced by a method such as applying and drying the catalyst ink directly on a gas diffusion layer or current collector such as carbon paper or carbon cloth, or impregnating / reducing a metal complex as a precursor.
In particular, on the anode side, as described above, when using a specific fuel such as ascorbic acid, a carbon material such as carbon black can be used as the anode catalyst. Further, when a specific fuel such as ascorbic acid is used, the current collector itself made of carbon paper, carbon cloth, or the like can also function as an anode catalyst.
Both sides of the obtained membrane-electrode assembly are sandwiched between current collectors such as carbon paper or carbon cloth and incorporated into a cell to produce a fuel cell. On the anode side, at least one of the aforementioned compounds is supplied as a fuel in the form of a solution of about 10 ^{−4 to} 5M (more preferably about 10 ^{−3 to} 2M), and air or oxygen is supplied to the cathode side. Naturally diffuse.
The operating temperature of the fuel cell of the present invention varies depending on the electrolyte membrane to be used, but is usually about 0 to 150 ° C, more preferably about 10 to 100 ° C.
According to the present invention, a direct fuel cell that uses a compound in place of methanol, which is designated as a deleterious substance, as a fuel for a polymer electrolyte fuel cell can be obtained.
In addition, each compound used in the present invention does not generate harmful substances such as formaldehyde and formic acid as oxidation products.
Furthermore, some of the compounds used in the present invention show no or little corrosiveness to rubbers and plastics which are constituent materials of fuel cells.
Furthermore, the direct fuel cell according to the present invention can reduce the crossover that reduces the performance of the direct methanol fuel cell.
Furthermore, some of the compounds used in the present invention can realize a high-performance solid polymer fuel cell even when a carbon material is used instead of an expensive noble metal as a catalyst on the anode side. . In particular, the effect is remarkable when carbon black having a large specific surface area is used as the anode catalyst.

FIG. 1 is a cyclic voltammogram of each fuel used in the present invention in a 0.5 M sulfuric acid solution.
FIG. 2 is a graph showing current-voltage characteristics of the fuel cell according to Example 1.
FIG. 3 is a graph showing current-voltage characteristics of the fuel cell according to Example 2.
FIG. 4 is a graph showing the cathode potential of the fuel cells according to Example 3 and Comparative Example 2.
FIG. 5 is a graph showing current-voltage characteristics of the fuel cell according to Example 4.
FIG. 6 is a graph showing current-voltage characteristics of the fuel cell according to Example 5.
FIG. 7 is a graph showing current-voltage characteristics of the fuel cell according to Example 6.
FIG. 8 is a graph showing current-voltage characteristics of the fuel cell according to Example 7.
FIG. 9 is a graph showing current-voltage characteristics of the fuel cell according to Example 8.
FIG. 10 is a graph showing current-voltage characteristics of the fuel cell according to Example 9.
FIG. 11 is a graph showing current-voltage characteristics of the fuel cell according to Example 10.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

また、第２図に、水溶液濃度０．５Ｍのアスコルビン酸、亜ジチオン酸ナトリウム、亜リン酸および次亜リン酸をそれぞれ燃料として用いた場合の電流−電圧特性を示す。Platinum-ruthenium black was used for the anode catalyst, and platinum black water-repellently treated with polytetrafluoroethylene was used for the cathode catalyst. Each catalyst is mixed with a polymer electrolyte solution to form a catalyst ink, which is made into a thin film to produce an electrode sheet, and then hot pressed on both sides of the polymer electrolyte membrane ("Nafion-117" brand name, manufactured by DuPont). Thus, a membrane-electrode assembly was obtained.
A fuel cell is assembled by sandwiching both surfaces of the obtained membrane-electrode assembly with carbon cloth, an aqueous solution of a predetermined fuel is supplied to the anode, and air is naturally diffused to the cathode. The power generation performance was evaluated.
Table 1 shows the values of the open circuit voltage when various fuels are used.

FIG. 2 shows current-voltage characteristics when ascorbic acid, sodium dithionite, phosphorous acid and hypophosphorous acid having an aqueous solution concentration of 0.5 M are used as fuels, respectively.

In place of platinum-ruthenium black in Example 1, platinum, ruthenium, palladium, iridium, and rhodium blacks were used as anode catalysts, and fuel cells were produced in the same manner as in Example 1.
In addition, a fuel cell was produced according to Example 1 by directly contacting the carbon cloth with the polymer electrolyte membrane without using a catalyst on the anode side.
Subsequently, 1 M ascorbic acid aqueous solution was supplied to the anode, and air was naturally diffused to the cathode, thereby evaluating the power generation performance of each fuel cell at room temperature.
The current-voltage characteristics of these fuel cells are shown in FIG. FIG. 3 also shows the current-voltage characteristics of the battery using ascorbic acid as a fuel according to Example 1.
FIG. 3 shows that the oxidation reaction of ascorbic acid progressed on all the anodes, including the case where no catalyst was used for the anodes, and the fuel cell was activated.
From the results of the present example, according to the present invention, the number of types of anode catalysts that can be used is increased, and even if no platinum-ruthenium catalyst is used on the anode side, fuel cell power generation is possible without using any catalyst. It is clear that this is possible.
Comparative Example 1
A 1M methanol solution was supplied to the anode side of the fuel cell produced in Example 2, and power generation performance was evaluated.
The performance of the fuel cell when methanol is used as the fuel is in the order of anode catalyst: platinum-ruthenium>platinum> iridium. When palladium, rhodium, ruthenium and no catalyst are used, the fuel cell is operated at room temperature. I could not.

In the fuel cell produced in the same manner as in Example 1, a triode cell was produced by taking a liquid junction with the hydrogen electrode (RHE) from the electrolyte membrane using 0.5 M sulfuric acid.
A 1M ascorbic acid aqueous solution was supplied to the anode side, and the potential on the cathode side was measured at room temperature using RHE as a reference electrode. FIG. 4 shows the cathode potential according to this example, together with the results of Comparative Example 2 (fuel = methanol) shown below.
Comparative Example 2
A 1M methanol solution was supplied to the anode side of the fuel cell produced in the same manner as in Example 3, and the cathode potential was measured at room temperature.
As is apparent from the results shown in FIG. 4, Example 3 using ascorbic acid as a fuel according to the present invention shows a higher cathode potential than Comparative Example 2 using methanol as a fuel. It is known that methanol has a large membrane permeability from the anode to the cathode, and the cathode potential becomes a mixed potential and decreases. On the other hand, the fuel used in the present invention has a high cathode potential, so it is considered that the crossover is small.

Fuel cells were prepared in the same manner as in Example 1, using platinum, ruthenium, palladium, iridium, rhodium, and various platinum-ruthenium blacks as the anode catalyst.
Further, a fuel cell was produced according to Example 1 by directly contacting a carbon cloth as a current collector with a polymer electrolyte membrane without using a catalyst on the anode side.
Next, 0.5 M isoascorbic acid aqueous solution was supplied to the anode and oxygen was supplied to the cathode at 100 ml / min, and the power generation characteristics of each fuel cell at room temperature were evaluated. The current-voltage characteristics of these fuel cells are shown in FIG.

Fuel cells were prepared in the same manner as in Example 1, using platinum, ruthenium, palladium, iridium, rhodium, and various platinum-ruthenium blacks as the anode catalyst.
Further, in the case where the carbon cloth was pressed against the polymer electrolyte membrane without using a catalyst on the anode side, a fuel cell was produced according to Example 1.
Next, 0.05 M aqueous sulfite solution was supplied to the anode, and air was naturally diffused to the cathode, thereby evaluating the power generation characteristics of each fuel cell at room temperature. The current-voltage characteristics of these fuel cells are shown in FIG.

  The power generation characteristics of each fuel cell were evaluated in the same manner as in Example 5 except that a 0.005M sodium hydrogen sulfite aqueous solution was used as the fuel. FIG. 7 shows the current-voltage characteristics of these fuel cells.

  The power generation characteristics of each fuel cell were evaluated in the same manner as in Example 5 except that 0.5 M sodium dithionite aqueous solution was used as the fuel. FIG. 8 shows the current-voltage characteristics of these fuel cells.

  The power generation characteristics of each fuel cell were evaluated in the same manner as in Example 5 except that 0.5M phosphorous acid aqueous solution was used as the fuel. FIG. 9 shows the current-voltage characteristics of these fuel cells.

  The power generation characteristics of each fuel cell were evaluated in the same manner as in Example 5 except that 0.5M hypophosphorous acid aqueous solution was used as the fuel. The current-voltage characteristics of these fuel cells are shown in FIG.

Various commercially available carbon blacks “Vulcan XC-72” (trade name, manufactured by Cabot), “Ketjen Black EC” (trade name, manufactured by Ketjen Black International), “Ketjen Black EC-600JD” (trade name, Ketjen Black International) or “Black Pearls 2000” (trade name, manufactured by Cabot) is dispersed in a mixed solvent of ethanol and water, impregnated in a carbon cloth, and dried to dry the carbon on the carbon cloth. Black was fixed.
In Example 1, without using the platinum-ruthenium black used as the anode catalyst, only the platinum black of the cathode was hot pressed on one side of the polymer electrolyte membrane to obtain a membrane-cathode assembly. Next, a fuel cell is placed by pressing a carbon cloth on which carbon black is attached by the above-described method on the opposite membrane surface of the obtained membrane-cathode assembly, and a normal carbon cloth on the cathode membrane surface. Assembled.
Using the obtained fuel cell, 0.5 M ascorbic acid aqueous solution was supplied to the anode side, and air was naturally diffused to the cathode to evaluate the power generation characteristics of the fuel cell at room temperature. The current-voltage characteristics of these fuel cells are shown in FIG.
FIG. 11 also shows current-voltage characteristics when only carbon cloth (without carbon black) is directly pressed against the anode side of the polymer electrolyte membrane surface in Example 2 (lowermost part). Curve).
FIG. 11 shows that the performance of the ascorbic acid fuel cell is improved by attaching various carbon blacks having a large specific surface area on the carbon cloth disposed on the anode side.
From the results of this example, it is clear that the performance of ascorbic acid fuel cells can be achieved by using a carbon material having a large specific surface area such as carbon black as the catalyst on the anode side. That is, it is clear that the carbon material having a large specific surface area is a high-performance electrode catalyst that can be substituted for a noble metal catalyst such as platinum or palladium.

Claims (7)

Direct operation using at least one selected from the group consisting of ascorbic acid, isoascorbic acid, citric acid, tartaric acid, sulfurous acid, thiosulfuric acid, hydrogen sulfite, dithionic acid, phosphorous acid, hypophosphorous acid and their salts as fuel A polymer electrolyte fuel cell characterized by the above. 2. The polymer electrolyte fuel cell according to claim 1, wherein at least one selected from the group consisting of ascorbic acid, isoascorbic acid, citric acid, tartaric acid and salts thereof is used as fuel. 2. The polymer electrolyte fuel cell according to claim 1, wherein at least one selected from the group consisting of sulfurous acid, thiosulfuric acid, hydrogen sulfite, dithionic acid, phosphorous acid, hypophosphorous acid and salts thereof is used as fuel. 2. The polymer electrolyte fuel cell according to claim 1, wherein a metal is used as the anode catalyst. 2. The solid polymer fuel according to claim 1, wherein at least one selected from the group consisting of ascorbic acid, isoascorbic acid, sulfurous acid, hydrogen sulfite, dithionic acid and salts thereof is used as a fuel, and a carbon material is used as an anode catalyst. battery. 6. The polymer electrolyte fuel cell according to claim 5, wherein carbon black is used as an anode catalyst. At least one selected from the group consisting of ascorbic acid, isoascorbic acid, sulfurous acid, hydrogen sulfite, dithionic acid and their salts is used as a fuel, and a current collector made of carbon paper or carbon cloth is brought into contact with the solid polymer film. 2. The polymer electrolyte fuel cell according to claim 1, which is directly operated in the absence of an electrode catalyst.

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