JP3788491B2 - Direct methanol fuel cell with solid polymer electrolyte and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a direct methanol fuel cell in which methanol is supplied to a negative electrode, and electric power is obtained by an electrochemical reaction directly at the negative electrode.
[0002]
[Prior art]
A fuel cell has two electrodes on both sides of an electrolyte that is an ionic conductor, supplies an oxidizing gas (oxidant) such as oxygen or air to one electrode, and a fuel (such as hydrogen or hydrocarbon) to the other electrode ( This is a battery that generates electricity by supplying a reducing agent) and causing an electrochemical reaction.
[0003]
There are several types of fuel cells, but direct methanol fuel cells (abbreviated as DMFC) supply methanol as a fuel directly to the negative electrode, and many fuel cells use hydrogen or hydrocarbons as fuel. Compared to using reformed hydrogen, not only the equipment is simple, but also the transport and storage of the fuel itself is easy, and there is a possibility that it can operate at a temperature of 100 ° C. or less. It is considered to be most suitable for small size and portable use, and is regarded as a promising future power source for automobiles.
[0004]
As the electrolyte of the direct methanol fuel cell, the initial alkaline type is changed to the acid type, and recently, a solid polymer electrolyte is used in many cases. By using the solid polymer electrolyte, the operating temperature can be made higher than that of the liquid electrolyte, and the electrolyte layer can be a thin layer of 1 mm or less. It was much better than the initial one.
[0005]
A direct methanol fuel cell (PEM-DMFC) using a solid polymer electrolyte is sandwiched between two porous electrodes with a catalyst on both sides of a proton conductive solid polymer electrolyte such as Du Pont Nafion. It has a structure in which methanol is directly supplied to the negative electrode and oxygen or air is supplied to the positive electrode. In the negative electrode, methanol and water react to generate carbon dioxide, protons, and electrons. The electrons work through an external circuit and then reach the positive electrode. Protons reach the positive electrode through the solid polymer electrolyte. At the positive electrode, oxygen, protons, and electrons react to produce water. Therefore, the total reaction of the direct methanol fuel cell is a reaction in which water and carbon dioxide are generated from methanol and oxygen. These reactions proceed with the help of a catalyst in the electrode. The theoretical voltage of this reaction is 1.18 V, but in an actual battery, the voltage is lower than this value due to IR drop or the like.
[0006]
Although the direct methanol fuel cell has considerably improved characteristics, it has a drawback that the output and efficiency of the cell are lower than those of other fuel cells. This is due to the fact that the activity of the catalyst that oxidizes methanol is low and that the methanol diffuses through the electrolyte and reaches the anode, where it reacts directly with the oxidant on the catalyst of the positive electrode (this phenomenon is called “crossover It has become clear that it is called [M. P. Hogarth and H.M. A. Hards Platinum Metals Rev. , 40 (4) 150 (1996)].
[0007]
In a direct methanol fuel cell, a catalyst is required for both the positive electrode and the negative electrode, but the negative electrode catalyst is particularly problematic. That is, when methanol is oxidized on a platinum catalyst, carbon monoxide adsorbed on the platinum is produced, which poisons platinum and lowers the catalytic activity [R. Parsons and T. Vanderroot J. et al. Electroanal. Chem. , 257 9 (1988)]. In order to quickly remove carbon monoxide from the surface of platinum, the addition of a secondary metal has been studied, and at present, it is known that the platinum-ruthenium system is the most active catalyst.
[0008]
As a method for mounting the catalyst layer of the negative electrode, polytetrafluoroethylene or a solid polymer electrolyte that serves as a binder and a water repellent by supporting a mixture of platinum and ruthenium metal fine powder as it is or on a carbon having a large surface area A method of mixing with a solid polymer electrolyte by hot pressing or the like (USP 5,599,638), or a fine powder of platinum and ruthenium or an oxide thereof. Is mixed with an alcohol solution containing a solid polymer electrolyte, the catalyst mixture solution is applied onto a polytetrafluoroethylene plate, dried and then peeled off from the polytetrafluoroethylene plate, and then a porous electrode such as carbon paper. Transfer to top and join with solid polymer electrolyte by hot press etc. That way [X. Ren et. al. J. et al. Electrochem. Soc. , 143 L12 (1996)] and the like have been proposed.
[0009]
[Problems to be solved by the invention]
However, these conventional negative electrode catalyst layer mounting methods are extremely complicated in processes such as preparation, spraying, coating, drying, and hot pressing of a catalyst mixed solution, and the prepared catalyst layer becomes thick, and the reaction point is electrolyte- Since it is at or near the interface of the catalyst layer, the catalyst amount involved in the reaction is small out of the total catalyst amount, that is, the catalyst utilization rate is low. Therefore, there has been a demand for a negative electrode catalyst layer that has a simpler production method and a high utilization factor of the catalyst.
[0010]
[Means for Solving the Problems]
The present invention relates to a direct methanol fuel cell having a solid polymer electrolyte, the negative electrode catalyst layer is composed of two layers, the first layer is a platinum layer attached to the inside from the surface of the solid polymer electrolyte membrane, the second layer and wherein it is a layer containing a solid polymer electrolyte membrane of platinum and ruthenium and the solid polymer electrolyte attached from the surface to the outside of, the first and second layers are in contact with the solid polymer electrolyte surface To do. Further, it is characterized in that platinum and ruthenium contained in the negative electrode catalyst layer are attached by electroless plating, and further, a structure in which platinum and ruthenium contained in the second layer are alternately laminated can be used. is there.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
A direct methanol fuel cell having a solid polymer electrolyte according to the present invention has a porous electrode with a catalyst layer attached to both sides of a proton conductive solid polymer electrolyte, and a mixture of methanol and water on the negative electrode. Oxygen or air is supplied to the positive electrode to extract electricity.
[0012]
As the base of the porous electrode, both positive and negative electrodes can be used by subjecting a porous substrate such as carbon paper, carbon molded body, carbon sintered body, sintered metal, foam metal, etc. to water-repellent treatment, As the water repellent, polytetrafluoroethylene or the like can be used.
[0013]
As the noble metal catalyst, fine powder of noble metal such as platinum, platinum alloy, gold, gold alloy, palladium, palladium alloy or carbon powder supporting noble metal can be used for the positive electrode, and platinum and ruthenium for the negative electrode. Can be used.
[0014]
The porous electrode for the positive electrode is produced by applying a catalyst dispersion solution to the surface of the electrode subjected to water repellent treatment. The catalyst dispersion solution is obtained by uniformly mixing fine particles of catalyst such as platinum black or carbon powder supporting the catalyst, a water repellent such as polytetrafluoroethylene, and a solid polymer electrolyte dissolved in alcohol in an appropriate solvent. It is made by mixing.
[0015]
Methods for attaching the metal layer to the solid polymer electrolyte membrane include a physical method such as sputtering and an electroless plating method, but the electroless plating method is the simplest.
[0016]
Examples of methods for attaching a metal layer to a solid polymer electrolyte membrane by electroless plating include the following. When a Nafion 117 membrane manufactured by Du Pont is used as the solid polymer electrolyte membrane and a platinum electrode is attached thereto, the Nafion membrane is immersed in an aqueous solution of [Pt (NH ₃ ) ₅ Cl] Cl _{3 and} the sulfonic acid of the Nafion membrane is used. The proton of the group is replaced with [Pt (NH ₃ ) ₅ Cl] ³⁺ , which is reduced with NaBH ₄ to precipitate platinum particles near the surface of the Nafion membrane, and further, H ₂ PtCl ₆ in the aqueous solution is replaced with hydrazine. There is a method in which platinum particles are further grown on platinum particles by reduction with [Y. Fujiita et. al. , J .; Appl. Electrochem. , 16 935 (1986)].
[0017]
Also in the direct methanol fuel cell according to the present invention, the catalyst layer of the negative electrode is attached by the same method as this example. First, the platinum layer, which is the first layer, is attached to the solid polymer electrolyte membrane by electroless plating. That is, when the proton of the sulfonic acid group of the Nafion membrane is replaced with [Pt (NH ₃ ) ₅ Cl] ³⁺ and this is reduced with NaBH ₄ , [Pt (NH ₃ ) ₅ Cl] ³⁺ is converted into the interior of the Nafion membrane. Since NaBH ₄ which is a reducing agent is supplied from the aqueous solution side outside the Nafion membrane, [Pt (NH ₃ ) ₅ Cl] ³⁺ is reduced from the surface of the Nafion membrane toward the inside, As a result, the platinum particles are precipitated from the surface of the Nafion membrane toward the inside, and the platinum layer as the first layer is attached to the inside from the surface of the solid polymer electrolyte membrane.
[0018]
【Example】
Next, the second layer of the negative electrode catalyst layer is attached. First, when an aqueous solution such as RuCl ₃ or Na ₄ Ru (SO ₃ ) ₃ is reduced with N ₂ H ₄ or NaBH ₄ to deposit ruthenium on the platinum layer of the first layer, ruthenium nucleates platinum particles. And is deposited in the outward direction from the surface of the solid polymer electrolyte membrane. Next, H ₂ PtCl ₆ in the aqueous solution is reduced with NaBH ₄ to further deposit platinum on the ruthenium. Note that ruthenium deposition and platinum deposition may be alternately repeated to form a layer in which ruthenium and platinum are alternately stacked. Since the layer made of platinum and ruthenium obtained by electroless plating is porous, a commercially available Nafion solution (alcohol solution containing 5% by weight of Nafion) is uniformly applied on this layer and dried to form platinum. second layer comprising ruthenium and a solid polymer electrolyte is obtained as. The thus obtained second layer of the negative electrode catalyst layer is attached to the outside from the surface of the solid polymer electrolyte, and is in contact with the first layer on the surface of the solid polymer electrolyte.
[0019]
The obtained solid polymer electrolyte membrane to which the negative electrode catalyst layer is attached and a porous electrode such as carbon paper are joined by a method such as hot pressing. The positive electrode may be bonded to the solid polymer electrolyte membrane after the catalyst layer is attached to the surface of a water repellent carbon paper or other porous electrode, or the solid polymer electrolyte membrane may be electrolessly plated. After attaching a catalyst layer beforehand, you may join with a porous electrode.
[0020]
【Example】
The production method, structure and characteristics of a direct methanol fuel cell provided with the solid polymer electrolyte according to the present invention will be described in detail with reference to preferred embodiments.
[0021]
Example 1 A Nafion 117 membrane manufactured by Du Pont was used as the solid polymer electrolyte. First, the Nafion 117 membrane is cut out to a diameter of 70 mm, only the portion having a diameter of 36 mm at the center is exposed, and the remaining peripheral portion is covered with a polypropylene plate via a silicone rubber packing. Next, a polypropylene plate with a Nafion membrane sandwiched in the center of a container with an inner dimension of 70 mm in width, 20 mm in depth, and 80 mm in height is attached, and the interior of the container is divided into two rooms with a depth of about 9 mm. The liquid is not moved, and one is designated as the A chamber and the other as the B chamber.
[0022]
Room A is a plating room. This room contains 38 ml of an aqueous solution of [Pt (NH ₃ ) ₅ Cl] Cl ₃ containing 2 mg / ml of Pt, and room B contains 38 ml of purified water, keeping the whole at 40 ° C.・ Bubble air in both room A and room B for 30 minutes. By this operation, H ⁺ of the sulfonic acid group of the Nafion membrane is replaced with [Pt (NH ₃ ) ₅ Cl] ³⁺ .
[0023]
Next, after washing both room A and room B with purified water, 38 ml of an aqueous solution containing 0.5% NaBH ₄ is placed in room A, 38 ml of purified water is placed in room B, and the whole is kept at 40 ° C. Both room A and room B are bubbled with air and kept for 1 hour. By this operation, [Pt (NH ₃ ) ₅ Cl] ³⁺ in the Nafion film is reduced with NaBH ₄ to deposit platinum having a thickness of about 10 μm inward from the surface of the Nafion film. Get a layer.
[0024]
Further, after washing both room A and room B with purified water, place 20 ml of an aqueous solution containing 0.2 mol / l of Na ₄ Ru (SO ₃ ) ₃ and 5 ml of 0.5% NaBH ₄ aqueous solution into the room A. The pH is adjusted to 1.1 using the solution, 38 ml of purified water is put into the B chamber, the whole is kept at 40 ° C., and both the A and B chambers are bubbled with air and kept for 5 hours, the surface of the Nafion membrane Ruthenium with a thickness of about 1 μm is attached. Furthermore, after washing both chamber A and chamber B with purified water, 30 ml of a standard buffer solution of pH 1.1 and 3 ml of 5% H ₂ PtCl ₆ aqueous solution are added to chamber A, and 5 ml of 5% NaBH ₄ aqueous solution is further added. Place 38 ml of purified water in the chamber, keep the whole at 40 ° C., and bubbling with air in both the A and B chambers for 5 hours, and platinum of about 5 μm thickness is deposited on the ruthenium layer, and the solid electrolyte surface A porous layer made of ruthenium and platinum is formed. When a commercially available Nafion solution (alcohol solution containing 5% by weight of Nafion) is uniformly applied on this layer, the Nafion solution penetrates into the porous layer composed of ruthenium and platinum, and when this is dried, platinum and ruthenium are obtained. second layer comprising a solid polymer electrolyte is obtained as. The thus obtained second layer of the negative electrode catalyst layer is attached to the outside from the surface of the solid polymer electrolyte, and is in contact with the first layer on the surface of the solid polymer electrolyte.
[0025]
FIG. 1 shows a cross-sectional structure of the obtained negative electrode catalyst layer. In FIG. 1, 1 is a Nafion film, 2 is a platinum layer as a first layer of the negative electrode catalyst layer, and the inner direction from the surface of the Nafion film. It is attached to. 3 is ruthenium, which is attached in contact with the platinum layer 2 on the surface of the Nafion film, 4 is a layer containing Nafion and platinum , and the layer 4 containing ruthenium 3, Nafion and platinum is a Nafion film. Attached in the external direction, the second layer of the negative electrode catalyst layer is formed. Reference numeral 5 denotes a portion where no Nafion membrane catalyst exists.
[0026]
Carbon paper with a porosity of 75% and a thickness of 0.40 mm as a porous electrode is immersed in a dispersion polytetrafluoroethylene solution, and 0.5 mg / cm ² of polytetrafluoroethylene is attached to the surface for water repellent treatment. Then, a commercially available Nafion solution (alcohol solution containing 5% by weight of Nafion) was uniformly applied thereon, and after drying, the Nafion membrane with the negative electrode catalyst layer attached thereto was joined by hot pressing.
[0027]
Further, on the other surface of the Nafion film, the surface of the carbon paper treated with the same dispersion polytetrafluoroethylene as that used for the negative electrode is made of platinum-supported carbon, Nafion solution, and dispersion polytetrafluoroethylene. The catalyst solution is applied, dried, and the positive electrode with the catalyst layer attached is joined with a hot press to produce a direct methanol fuel cell electrode / electrolyte membrane assembly, which is used to produce the direct methanol fuel according to the present invention. A battery (referred to as battery A) was assembled.
[0028]
A direct methanol fuel cell for comparison uses a mixture of platinum-ruthenium-supporting carbon containing 10 wt% platinum and 10 wt% ruthenium, a Nafion solution, and a dispersion polytetrafluoroethylene as a negative electrode catalyst layer. Was applied to the surface of the water-repellent carbon paper, and a battery (battery B) equipped with a conventional negative electrode bonded to a Nafion film and a positive electrode similar to battery A was used.
[0029]
Next, air humidified with water vapor at 60 ° C. was supplied to the positive electrode at a rate of 2 l / min, and an aqueous solution at 60 ° C. containing 1 mol / l of methanol was supplied to the negative electrode, and the characteristics of the direct methanol fuel cell were measured. . FIG. 2 shows the i-V characteristics, and the characteristics of the battery A according to the present invention are superior to those of the comparative battery B using the conventional negative electrode.
[0030]
[Example 2] After using the same Nafion 117 membrane as in Example 1 and attaching the first and second layers of the negative electrode catalyst layer in the same procedure as in Example 1, the same Na ₄ Ru as in Example 1 was used. The procedure of attaching ruthenium using an aqueous solution of (SO ₃ ) _{3 and an} aqueous solution of NaBH ₄ and the procedure of attaching platinum using an aqueous solution of H ₂ PtCl _{6 and an} aqueous solution of NaBH ₄ were repeated twice, so that platinum and ruthenium were deposited on the surface of the solid electrolyte. A porous layer in which three layers were laminated was formed. When a commercially available Nafion solution (alcohol solution containing 5% by weight of Nafion) is uniformly applied on this layer, the Nafion solution penetrates into the porous layer composed of ruthenium and platinum, and when this is dried, platinum and ruthenium are obtained. Are stacked to obtain a second layer containing a solid polymer electrolyte.
[0031]
Using this negative electrode catalyst layer, a direct methanol fuel cell (cell C) similar to that in Example 1 was prepared under other conditions, and the characteristics were measured under the same conditions as in Example 1. The -V characteristic was almost the same as that of battery A.
[0032]
【The invention's effect】
In a direct methanol fuel cell having a solid polymer electrolyte according to the present invention, the anode catalyst layer is made from a second layer comprising a first layer comprising a platinum layer, the platinum and ruthenium and the solid polymer electrolyte, also The negative electrode catalyst layer is attached by electroless plating, and includes a conventional negative electrode catalyst layer using a catalyst-supporting carbon containing platinum and ruthenium, a Nafion solution, and a dispersion polytetrafluoroethylene. In comparison, the manufacturing process is very simple.
[0033]
In the catalyst layer according to the present invention, the thickness of the catalyst layer can be made extremely thin, and waste catalysts not involved in the reaction can be reduced.
[0034]
Further second layer of anode catalyst layer according to the present invention comprises platinum and ruthenium and the solid polymer electrolyte, for platinum and ruthenium and the solid polymer electrolyte it is mounted in contact with each other, the negative electrode active material When methanol is adsorbed on platinum as a catalyst to perform an electrochemical reaction, platinum as an electron conductor and a solid polymer electrolyte membrane as a proton conductor coexist, so that electrons are transferred and protons are transferred. Is easily done. In addition, carbon monoxide adsorbed on platinum produced during the methanol reaction is immediately removed from the platinum surface by ruthenium present on the platinum surface, and the catalytic ability of platinum is always kept constant. Moreover, the second layer containing platinum and ruthenium and the solid polymer electrolyte is spread three-dimensional directions, the number of reaction points becomes very large, in which electrochemical reaction progresses easily. When a larger amount of catalyst is required, electroless plating of ruthenium and platinum is performed alternately, and a structure in which ruthenium and platinum are alternately stacked from the surface of the solid polymer electrolyte membrane to the outside is formed. It is possible to further increase the number of reaction points by allowing the solid polymer electrolyte to be present in the catalyst.
[0035]
Further, in the catalyst layer mounting method according to the present invention, the amount of platinum and ruthenium in the catalyst layer, and the mixing ratio of platinum and ruthenium depend on the conditions such as the concentration of the reagent, the reaction time, and the temperature during electroless plating. Can be easily changed to any value.
[0036]
As the solid polymer electrolyte membrane, other ion exchange resin membranes such as other perfluorocarbon sulfonic acid resins and styrene-divinylbenzene copolymer resins are used in addition to the Du Pont Nafion described in the examples. be able to. In addition, as a salt containing platinum or ruthenium at the time of electroless plating, other than [Pt (NH ₃ ) ₅ Cl] Cl ₃ , H ₂ PtCl ₆ or Na ₄ Ru (SO ₃ ) ₃ described in the examples. Any platinum or ruthenium-containing salt can be used, and as the reducing agent, other reducing agents such as N ₂ H ₄ can be used in addition to NaBH ₄ described in the examples.
[Brief description of the drawings]
FIG. 1 is a diagram showing a cross-sectional structure of a negative electrode catalyst layer and a solid polymer electrolyte layer of a direct methanol fuel cell according to the present invention. FIG. 2 shows characteristics of a direct methanol fuel cell A and a comparative battery B according to the present invention. [Comparison of symbols]
1 Nafion membrane 2 First layer of negative electrode catalyst layer 3 Ruthenium 4 Layer containing Nafion and platinum

Claims (3)

The negative electrode catalyst layer consists of two layers, the first layer is a platinum layer attached from the surface of the solid polymer electrolyte membrane to the inside, and the second layer is platinum and ruthenium attached from the surface of the solid polymer electrolyte membrane to the outside. solid is a layer containing a polymer electrolyte, the said first layer to the second layer, characterized in that in contact with the solid polymer electrolyte surface, direct methanol fuel cell including a solid polymer electrolyte. 2. A direct methanol fuel cell comprising the solid polymer electrolyte according to claim 1, wherein platinum and ruthenium contained in the negative electrode catalyst layer are attached by electroless plating. Manufacturing method of fuel cell. 2. The direct methanol fuel cell with a solid polymer electrolyte according to claim 1, wherein platinum and ruthenium contained in the second layer of the negative electrode catalyst layer are alternately laminated.

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