JP5219121B2 - Direct fuel cell - Google Patents

Description

  The present invention relates to a direct fuel cell that supplies a sugar solution as a fuel.

A polymer electrolyte fuel cell (PEFC) is an all-solid-state fuel cell that uses an ion-conducting polymer thin film as its electrolyte, and is a simple and highly efficient power generation system that can operate in the range from room temperature to 100 ° C. From the viewpoint of the global environment, energy problems, etc., it is expected to be put into practical use and spread as a fuel cell vehicle, a stationary home generator, a power source for mobile devices, and the like.

  Among these applications, as a power source for mobile devices, methanol can be used as a fuel on the electrode because of its advantages such as excellent fuel transportability and responsiveness to start / stop, and a compact design that does not require a reformer. Direct methanol fuel cells (DMFC) that generate electricity by direct oxidation are mainly used.

  The DMFC has a structure in which electrodes are attached to both sides of a solid polymer electrolyte membrane, like a PEFC that uses pure hydrogen or reformed hydrogen as a fuel. Usually, a platinum-ruthenium catalyst is used for the anode (fuel electrode). A platinum catalyst is used for the cathode (air electrode). In the fuel cell having such a structure, when an aqueous methanol solution is supplied to the anode side, the methanol reacts with water and is oxidized to generate carbon dioxide, protons and electrons. The generated proton moves 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.

  In recent years, high performance of information terminal devices and medical electronic devices has progressed, and there is an urgent need to develop portable, wearable, and implantable power sources that can be used continuously without the need for battery replacement or charging. Direct fuel cells such as DMFC are of interest because they can be used continuously just by supplying fuel, and are currently surpassing the R & D competition.

  In PEFC, many fuel compounds can be used in addition to methanol, and alcohols such as ethanol (Non-Patent Document 1), 2-propanol (Non-Patent Document 2), and ethylene glycol (Non-Patent Document 3) can be used. Examples of direct fuel cells that use direct oxidation of formic acid (Non-patent Document 4), dimethyl ether (Patent Document 1), borohydride (Patent Document 2), hydrazine (Patent Document 3), etc. There is. Utilization of these various fuels is thought to lead to the development of a power source according to the output scale and various applications.

  Among direct fuel cells using these various fuel compounds, the direct methanol fuel cell (DMFC), where research on practical application is the most advanced, is harmful to formaldehyde and formic acid that may occur as fuel methanol and by-products. There is a concern that it will be a factor that hinders practical application and spread. Furthermore, in DMFC, 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 the methanol crossover phenomenon, when a perfluorosulfonic acid solid polymer electrolyte membrane developed for conventional PEFC using hydrogen gas as a fuel is used, sufficient performance as DMFC is demonstrated. There are many things that cannot be done. Therefore, the development of electrolyte membranes with low methanol permeability is an urgent task for the practical application of DMFC.

Furthermore, as a problem of DMFC, it is necessary to use a large amount of expensive noble metal-based electrode catalyst such as platinum-ruthenium alloy for methanol electrooxidation reaction. In order to solve this problem, a great deal of research has been done on methanol oxidation electrode catalysts, and various alloy catalysts and metal complexes have been proposed, but the activity of platinum-ruthenium alloys known since the 1960s has been greatly increased. No more electrocatalyst has been found. Since the cost of precious metal catalysts accounts for a large percentage of the total cost of the fuel cell system, in order to realize the practical use and cost reduction of PEFC, the use of electrode catalysts with reduced precious metal usage or no precious metal Development has become a major issue. However, since a sulfonic acid proton conductive membrane is usually used for the electrolyte membrane, the electrode catalyst material must have excellent acid resistance that does not corrode or deteriorate for a long time in a strongly acidic atmosphere. Is done. In this respect, the catalyst material is limited to some materials such as precious metals, which hinders the search for a highly active catalyst and cost reduction.

  In addition, PEFCs that use conventional proton-conducting cation exchange membranes typically have a large cathode overvoltage, and platinum catalysts are usually used in consideration of the acid resistance as described above. The necessity of a noble metal catalyst for both the anode and the cathode is a serious problem from the viewpoint of the cost of the fuel cell and the amount of resources in the diffusion stage.

  Furthermore, in PEFC using a proton-conducting cation exchange membrane, water generation accompanying power generation occurs on the cathode side. For this reason, a phenomenon called flooding in which the wetness of the electrode by the generated water proceeds and the reactivity of the oxygen reduction reaction decreases is often observed on the cathode side. In order to solve this problem, it is necessary to devise water repellent treatment on the cathode side to improve the diffusibility of oxygen gas.

  As described above, PEFCs that use ion-conductive polymer thin films as electrolytes are expected to be put to practical use and spread in various applications, but there are currently many issues that need to be improved. .

On the other hand, in order to realize a sustainable society, interest in biomass energy has increased rapidly in recent years. The use of biomass fuels as an energy source, since the CO ₂ is discharged at the time of use in which originally CO ₂ in air is fixed by the plant, it is not necessary recorded as emissions (carbon neutral). Attempts have already been made to reduce CO ₂ emissions by mixing ethanol (bioethanol) obtained by fermentation of crops such as corn and sugarcane with gasoline. The use of biomass-derived fuel is significant from the viewpoint of using renewable petroleum alternative energy, and it can be expected that CO ₂ emission reduction will be more effective when combined with a fuel cell, which is a highly efficient power generation system.

When sugars such as glucose are used as fuel, the overvoltage of the electrochemical oxidation reaction is high, so that only extremely low power generation performance can be achieved by applying conventional PEFC technology. For this reason, research in the field of biofuel cells based on the enzyme reaction by glucose dehydrogenase has become the mainstream (Patent Document 4), and application as a power source for portable electronic devices and living body embedded chips is expected. . However, the maximum output density is only about 1 mW / cm ² , and it is far from the applicable output as a power source for portable devices.
Electrochimica Acta, 49, 3901 (2004) J. Power Sources, 124, 12 (2003) J. Power Sources, 150, 27 (2005) J. Power Sources, 144, 28 (2005) Japanese Patent Laid-Open No. 11-144751 Japanese Patent Laid-Open No. 2002-50375 JP 2006-244961 JP 2005-310613 A

  The present invention has been made in view of the current state of the prior art described above, and its main purpose is to reduce or eliminate the various problems described above in the conventional direct fuel cell using methanol as a fuel. It is an object of the present invention to provide a novel direct fuel cell that can be used.

  As a result of intensive studies to achieve the above-described object, the present inventor uses an alkaline anion exchange membrane instead of an acidic proton conductive cation exchange membrane as an electrolyte membrane, and uses saccharides as a fuel compound. In some cases, it has been found that a novel direct fuel cell capable of achieving the above-described object can be obtained, and the present invention has been completed here.

That is, the present invention provides the following direct fuel cell.
1. A polymer electrolyte fuel cell comprising a cathode, an anode, and an electrolyte membrane disposed between the anode and the cathode,
The electrolyte membrane is an anion exchange membrane;
A direct fuel cell in which a saccharide solution is supplied to the anode as a fuel.
2. Item 2. The above item 1, wherein the anion exchange membrane is a solid polymer membrane having at least one anion exchange group selected from the group consisting of a quaternary ammonium group, a pyridinium group, an imidazolium group, a phosphonium group, and a sulfonium group. Direct fuel cell.
3. Item 3. The direct fuel cell according to Item 1 or 2, wherein the saccharide supplied as fuel is at least one selected from the group consisting of monosaccharides, disaccharides and polysaccharides.
4). Item 3. The direct fuel cell according to Item 1 or 2, wherein the saccharide supplied as fuel is a monosaccharide.

  The fuel cell of the present invention includes a cathode, an anode, and an electrolyte membrane disposed between the anode and the cathode as constituent elements, and an anion exchange membrane is used as the electrolyte membrane, and saccharides are used as fuel. It is what is used.

FIG. 1 is a conceptual diagram schematically showing the structure of a direct fuel cell of the present invention and its power generation mechanism. As shown in FIG. 1, the fuel cell of the present invention includes an anion conductive electrolyte membrane, an anode layer and a cathode layer bonded on both sides as constituent elements, and a saccharide serving as a fuel on the anode side. The solution is supplied and oxygen or air is supplied to the cathode. By reduction of oxygen at the cathode side, hydroxide ion (OH ^-) generated reaches the anode move through the film, the anode sugar and OH ^- react oxide and water sugar Arise. At this time, electrons can flow through the external circuit to extract current.

The fuel cell having the structure described above is particularly characterized in that an anion exchange membrane is used as the electrolyte membrane. By using the anion exchange membrane as the electrolyte membrane, hydroxide ions move in the membrane, and the electrolyte The membrane is in an alkaline atmosphere. The electrochemical oxidation reaction of saccharides such as glucose used as fuel in the fuel cell of the present invention has an overvoltage reduced in an alkaline atmosphere as compared to an acidic atmosphere. For example, FIG. 2 is a cyclic voltammogram measured in 0.1 M potassium hydroxide containing 0.1 M d-glucose using a platinum electrode as a working electrode, and in comparison, in 0.1 M sulfuric acid containing 0.1 M d-glucose. The results are also shown. As is clear from the results shown in FIG. 2, a higher current density is obtained at the same potential when measured in potassium hydroxide than in sulfuric acid.

  Similarly, it is known that the overvoltage of the oxygen reduction reaction is also reduced in an alkaline atmosphere. Therefore, the fuel cell of the present invention using an anion exchange membrane as an electrolyte membrane and using an aqueous saccharide solution as fuel can be expected to be a high-power fuel cell.

  Hereafter, each component of the fuel cell of this invention is demonstrated concretely.

(1) Electrolyte Membrane In the present invention, an anion exchange membrane capable of transferring OH ⁻ produced at the cathode to the anode is used as the electrolyte membrane. The type of anion exchange membrane is not particularly limited. For example, a hydrocarbon resin having an anion exchange group such as a quaternary ammonium group, a pyridinium group, an imidazolium group, a phosphonium group, or a sulfonium group (for example, polystyrene, polysulfone, Anion exchange membranes made of solid polymers such as polyethersulfone, polyetheretherketone, polyphenylene, polybenzimidazole, polyimide, polyarylene ether, etc.) and fluorine resins can be used. The ion exchange capacity of the anion exchange membrane is preferably about 0.1 to 10 meq / g, and more preferably about 0.5 to 5 meq / g. The thickness of the anion exchange membrane is preferably about 5 to 300 μm, preferably 10 to 100 μm.
More preferably, it is about.

(2) Catalyst component In the fuel cell of the present invention, various catalysts such as metals, metal alloys, metal complexes and the like conventionally known as electrode catalysts can be used as the electrode catalyst used for the anode and the cathode.

  In a conventional fuel cell using a sulfonic acid-based cation exchange membrane, high acid resistance is required for an anode and a cathode catalyst, and therefore, a material centering on an expensive noble metal is used. On the other hand, since the fuel cell of the present invention uses an alkaline anion exchange membrane, even a base metal can be used without being corroded. Therefore, in the catalyst used in the fuel cell of the present invention, the metal species include noble metals such as platinum, palladium, iridium, rhodium, ruthenium and gold used in conventional PEFC, as well as nickel, silver, cobalt, iron and copper. Base metals such as zinc can be used. A single metal catalyst or metal complex selected from these metals, or an alloy or metal complex complex composed of any combination of two or more metals can be used. It is also possible to use a composite catalyst of a metal catalyst selected from the above and another metal oxide, or a supported catalyst in which catalyst fine particles are dispersed on a carrier such as carbon or metal oxide.

  Therefore, in the fuel cell of the present invention, both the anode and the cathode are not limited to expensive noble metal catalysts, and can be selected from a wide range. Therefore, various highly active catalysts can be developed, and the cost can be reduced.

(3) Fuel In the direct fuel cell of the present invention, saccharides are used as fuel. As described above, the fuel cell of the present invention uses an anion exchange membrane as an electrolyte membrane, thereby reducing the overvoltage of the electrochemical oxidation reaction of saccharides, and even when saccharides are used as fuel. High battery output can be achieved.

As the saccharide, monosaccharide, disaccharide, polysaccharide and the like can be used. Among these, monosaccharides include erythrose, threose, erythrulose having 4 carbon atoms, ribose having 5 carbon atoms, arabinose, xylose, lyxose, ribulose, xylulose, glucose having 6 carbon atoms, mannose, galactose, fructose. Sorbose, allose, altrose, gulose, idose, talose, psicose, tagatose, inositol and the like. As the disaccharide, maltose, lactose, cellobiose, saccharose and the like can be used. Examples of polysaccharides include starch, glycogen, inulin, cellulose, pectin and the like. The fuels illustrated above can be used alone or in combination of two or more.

  Among the above-mentioned saccharides, ribose, arabinose, xylose, lyxose, glucose, mannose, galactose, allose, altrose, gulose, idose, talose and the like, which are monosaccharides having an aldehyde group, are more preferable.

  The sugars described above are supplied to the anode of the fuel cell as a solution containing sugars. Although the kind of solvent for dissolving saccharides is not specifically limited, For example, water, alcohol (for example, methanol, ethanol, propanol etc.) etc. can be used.

The concentration of the saccharide in the solution is not particularly limited, but is usually preferably about 10 ⁻⁶ to 10 M (mol / L), more preferably about 10 ^{−3 to} 5 M.

Further, potassium hydroxide, sodium hydroxide, ammonia, or the like can be added to the solution containing saccharides. The concentration of the alkali component at this time is usually preferably about 10 ⁻³ to 10 M (mol / L), more preferably about 0.1 to 3 M.

(4) Configuration of the Fuel Cell of the Present Invention The fuel cell of the present invention includes a cathode, an anode, and an anion exchange membrane disposed between the anode and the cathode as constituent elements.

In the fuel cell having such a structure, the electrode layer and the electrolyte membrane, which are the cathode and the anode, are usually used as a joined body as in the conventional fuel cell. The joined body of the electrode layer and the electrolyte membrane can be produced by a known method. For example, after thinning a catalyst ink prepared by mixing catalyst powder and a resin solution, a method of hot pressing on an electrolyte membrane or a method of directly applying and drying on a polymer membrane can be applied. As the resin solution, an electrolyte solution having an anion exchange capacity of an ion exchange capacity of about 0.1 to 10 meq / g (more preferably 0.5 to 5 meq / g) is preferable as in the case of the electrolyte membrane. Polymer resins such as polyvinylidene fluoride and polyvinyl butyral that do not have may be used. In addition, the catalyst can be directly attached to the polymer electrolyte membrane by an adsorption reduction method, electroless plating, electroplating, sputtering, CVD, or the like. Alternatively, the electrode may be produced by a method in which a catalyst ink is directly applied to a gas diffusion layer or a current collector and dried, or a method in which a metal complex as a precursor is impregnated or reduced.

  A fuel battery cell can be produced by sandwiching both surfaces of the obtained membrane-electrode assembly with a current collector such as carbon paper or carbon cloth and incorporating it into the cell. A solution containing the above saccharides as a fuel may be supplied to the anode side, and oxygen or air may be supplied or naturally diffused to the cathode side.

  The direct fuel cell of the present invention having the above-described configuration has the following excellent characteristics.

  (I) By using an anion exchange membrane as an electrolyte membrane, the overvoltage of the anode reaction and the cathode reaction is reduced. For this reason, it is possible to increase the output of the fuel cell.

(Ii) By using saccharides as fuel, the use of harmful fuels and the risk of by-products generated during fuel cell operation can be avoided. Saccharides are biofuels that can be produced from biomass including corn, sugar cane, and wood. If these biofuels are used in place of fossil fuels or fuel compounds produced from fossil fuels, it is possible to reduce the burden on the global environment and contribute to the realization of a sustainable society. Furthermore, by generating electricity using glucose present in the body, it can be used as a power source for pacemakers, artificial organs, biosensors, and micromachines embedded in the body.

(Iii) Saccharides have a higher molecular weight than methanol and a greater resistance to movement in the electrolyte membrane. For this reason, it is possible to prevent the crossover phenomenon that causes the methanol to permeate the cathode side, which is a problem in the conventional DMFC, and causes a decrease in performance.

  (Iv) The anion exchange membrane used as the electrolyte membrane is alkaline, and high acid resistance is not required for the catalyst used in the anode and cathode. For this reason, it is not limited to a noble metal catalyst, Base metals, such as nickel, iron, silver, cobalt, can also be used as a catalyst, without being corroded. Therefore, it is possible to search for a highly active catalyst from a wide range of options, and it is possible to develop a highly active catalyst and reduce the cost.

  (V) Flooding, which is a problem with PEFCs using conventional proton-conducting cation exchange membranes, can also be improved by using an anion exchange membrane fuel cell. The power generation mechanism of the fuel cell of the present invention can be expressed by the following equation.

Anode: sugar + n OH ^- → oxide of sugar + n H ₂ O + ne ^-
_{Cathode: n / 4 O 2 + n} / 2 H 2 O + ne - → n OH -
Total reaction: Sugar + n / 4 O ₂ → Sugar oxide + n / 2 H ₂ O
Thus, since water is generated not on the cathode side but on the anode side, there is no possibility of adversely affecting the oxygen reduction reaction at the cathode. The anode is originally an electrode to which a fuel solution is supplied, and the progress of electrode wetting by generated water does not matter.

  As described above, the direct fuel cell of the present invention is characterized by using an anion exchange membrane as an electrolyte membrane and using saccharides as a fuel. As a result, a high-performance fuel cell using saccharides with high safety as a fuel can be obtained, the problem of flooding can be improved, and the crossover phenomenon that has been a problem in conventional DMFCs can be prevented. Moreover, since not only precious metals but also base metals can be used as catalysts, it is possible to develop highly active catalysts at the same time as cost reduction.

  As described above, the direct fuel cell of the present invention can solve or alleviate various problems in the conventional polymer electrolyte fuel cell (PEFC). For example, a portable small power source (mobile device, IT device) , A power source for medical devices and robots), and a power cell that is very useful in various applications such as an in-vivo power source (artificial organ, in-vivo sensor, micro-machine power source).

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
Platinum-ruthenium black was used as the anode catalyst, and platinum black was used as the cathode catalyst. Each catalyst was mixed with an anion exchange resin solution or ethanol to form a catalyst ink, which was thinned to produce an electrode sheet. As the anion exchange membrane, a hydrocarbon membrane with an ion exchange group of quaternary ammonium base of 27 μm and ion exchange capacity of 1.7 mmol / g was used, and anode and cathode electrode sheets were hot pressed on both sides respectively. Thus, a membrane-electrode assembly was obtained. The amount of catalyst metal supported in the obtained membrane-electrode assembly was 3 mg / cm ² on the anode side and the cathode side, the anion exchange resin content in the electrode layer was 5% by weight, and the electrode layer thickness was about 1 μm. It was.

A fuel cell was assembled by sandwiching both sides of the obtained membrane-electrode assembly with carbon cloth, 0.5 M potassium hydroxide containing 0.5 M d-glucose was supplied to the anode at 4 ml / min, and the cathode was humidified Oxygen was supplied at 100 ml / min, and the power generation performance of the fuel cell was evaluated at room temperature.

  FIG. 3 shows a graph showing the current density-voltage characteristics and current density-output density characteristics measured for the fuel cells obtained in Example 1 and Comparative Example 1 below.

  From the graph of FIG. 3, the power generation performance of the fuel cell of Example 1 using the anion exchange membrane was significantly improved as compared with the fuel cell using the conventional cation exchange membrane shown in Comparative Example 1 ( It is clear that the maximum power density is 13 times.

Example 2
In the fuel cell of Example 1, in place of d-glucose, d-mannose, d-galactose, d-xylose, d-maltose, d-lactose, d-cellobiose, d-fructose, d-sorbose and saccharose were each used. Using as a fuel, the power generation performance was evaluated in the same manner as in Example 1.

  FIG. 4 is a graph showing current density-voltage characteristics when these saccharides are used as fuels. From FIG. 4, it is clear that the above-described monosaccharide and disaccharide can be used as fuel in the fuel cell of the present invention.

Example 3
In the fuel cell of Example 1, a 0.5 M aqueous potassium hydroxide solution containing 0.5 M d-glucose was supplied to the anode at 4 ml / min, and humidified nitrogen was supplied to the cathode at 100 ml / min to form a membrane. The oxidation current (crossover current) of d-glucose that permeated and reached the cathode side was measured.

  FIG. 5 shows the measurement results of the crossover current in Example 3 together with the results obtained in Comparative Example 2 and Comparative Example 3. The fuel cell of Example 3 that uses an anion exchange membrane as an electrolyte membrane and supplies glucose as a fuel uses a fuel cell that uses an anion exchange membrane as an electrolyte membrane and supplies methanol as a fuel (Comparative Example 2), and an electrolyte Compared with a fuel cell using a cation exchange membrane as a membrane and supplying methanol as a fuel (Comparative Example 3), the crossover current is a very small value of 1/7 or less, and the influence of the crossover is substantially reduced. Is clearly negligible.

Comparative Example 1
Platinum-ruthenium black was used as the anode catalyst, and platinum black water-repellently treated with polytetrafluoroethylene was used as the cathode catalyst. Each catalyst was mixed with a Nafion (trade name, manufactured by DuPont) solution, which is a perfluorosulfonic acid cation exchange resin, to form a catalyst ink. After forming a thin electrode sheet, the perfluorosulfonic acid A membrane-electrode assembly was obtained by hot pressing on both surfaces of a Nafion-117 membrane (trade name, manufactured by DuPont) of a cation exchange membrane.

  The fuel cell was assembled by sandwiching both sides of the obtained membrane-electrode assembly with carbon cloth, 0.5 M d-glucose aqueous solution was supplied to the anode at 4 ml / min, and humidified oxygen was supplied to the cathode at 100 ml / min. The power generation performance of the fuel cell was evaluated at room temperature.

Comparative Example 2
In the fuel cell of Example 1, a 0.5 M aqueous solution of potassium hydroxide containing 0.5 M methanol was supplied to the anode at 4 ml / min, and humidified nitrogen was supplied to the cathode at 100 ml / min to permeate the membrane. Then, the oxidation current (crossover current) of methanol reaching the cathode side was evaluated.

Comparative Example 3
In the fuel cell of Comparative Example 1, 0.5 M methanol aqueous solution was supplied to the anode at 4 ml / min, humidified nitrogen was supplied to the cathode at 100 ml / min, and the methanol passed through the membrane and reached the cathode side. The oxidation current (crossover current) of was evaluated.

1 is a schematic diagram showing one embodiment of a direct fuel cell of the present invention. It is a cyclic voltammogram measured in 0.1 M potassium hydroxide or 0.1 M sulfuric acid containing 0.1 M glucose using a platinum electrode as a working electrode. 4 is a graph showing current density-voltage characteristics and current density-output density characteristics of fuel cells in Example 1 and Comparative Example 1. FIG. 6 is a graph showing current density-voltage characteristics of a fuel cell in Example 2. 6 is a graph showing crossover currents of fuel cells in Example 3, Comparative Example 2, and Comparative Example 3.

Claims (2)

A polymer electrolyte fuel cell comprising a cathode, an anode, and an electrolyte membrane disposed between the anode and the cathode,
The electrolyte membrane is an anion exchange membrane;
The anode is supplied with a solution containing a saccharide and an alkali component as fuel ,
The anion exchange membrane is a solid polymer membrane having at least one anion exchange group selected from the group consisting of a quaternary ammonium group, a pyridinium group, an imidazolium group, a phosphonium group and a sulfonium group;
Saccharide supplied as fuel, monosaccharides, disaccharides, and at least one Der Ru direct methanol fuel cell selected from the group consisting of polysaccharides. The direct fuel cell according to claim 1, wherein the saccharide supplied as fuel is a monosaccharide.

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