JP4210184B2 - Fuel cell and liquid fuel for the fuel cell - Google Patents

Description

  The present invention relates to a fuel cell that supplies liquid fuel directly to an anode, and a liquid fuel used in the fuel cell.

  In recent years, countermeasures to environmental problems and resource problems have been emphasized, and fuel cells have been actively developed as one of the countermeasures. In particular, a direct methanol fuel cell (DMFC) that directly uses electricity for power generation without reforming or gasifying the fuel is simple in structure, small in size, and easy to reduce in weight. It is attracting attention as a distributed power source.

  As shown in FIG. 7, the basic structure of the direct methanol fuel cell includes an electrolyte 22 made of a proton conductive solid polymer membrane, and a pair of cathode 23 and anode 24 provided with the electrolyte 22 interposed therebetween. Is. When an aqueous methanol solution or the like as a fuel is supplied to the anode 24 and air or the like is supplied as an oxidant gas to the cathode 23, carbon dioxide is generated at the anode 24 by an electrochemical reaction in which methanol and water react, and hydrogen ions ( Proton) and electrons are emitted, and at the cathode 23, water is generated by protons that have passed through the electrolyte 22, oxygen in the air, and electrons, and electric energy can be obtained in an external circuit. For the anode 24 and the cathode 23, a platinum or platinum alloy-based catalyst is used in order to improve output characteristics.

  However, direct methanol fuel cells (1) have a slow electrode reaction rate for methanol oxidation, and therefore an expensive platinum or platinum alloy catalyst is indispensable. (2) The electrode catalyst is a reaction intermediate for methanol oxidation. It is poisoned by carbon oxide, (3) Membrane permeability (crossover) of methanol in the solid polymer electrolyte membrane reduces output, (4) Methanol and its oxidative metabolites are extremely dangerous and severe for the human body It has problems such as requiring packaging.

In order to solve such problems, various proposals have been made to use dimethyl ether, aliphatic alcohols such as ethanol and isopropyl alcohol in place of methanol. (For example, see Patent Documents 1 and 2)
However, in a fuel cell using dimethyl ether, it is difficult to obtain output characteristics more than using methanol.
Also, when using aliphatic alcohols with more carbon atoms than methanol, the electrode reaction mechanism is similar to that of methanol, so all the fundamental problems of direct methanol fuel cells must be eliminated. It is difficult.

  In order to overcome such difficulties of the prior art, the present inventors have focused on benzyl alcohols and studied the electrode oxidation reaction (see, for example, Non-Patent Document 1). And this time, it discovered that the output characteristics etc. were remarkably improved by using a specific benzyl alcohol derivative and cyclodextrin in combination as a liquid fuel of a fuel cell that supplies liquid fuel directly to the anode, and completed the present invention. did.

JP 2002-231265 A JP 2003-217642 A Proceedings of the 2002 Electrochemical Fall Conference, p. 93, 1D25

  The present invention provides a fuel cell that does not necessarily require a safe and expensive platinum or platinum alloy catalyst, has excellent output characteristics, and can be reduced in size, weight, and cost, and a liquid fuel for the fuel cell. For the purpose.

In the present invention, the following configuration is adopted in order to solve the above problems.
1. Benzyl alcohol derivative and cyclodextrin represented by the following general formula (1) as a liquid fuel in a direct fuel cell having a pair of anode and cathode provided with an electrolyte in between and supplying liquid fuel directly to the anode A fuel cell characterized by being used in combination:

(In the formula, R represents an alkyl group having 1 to 5 carbon atoms, m and n are each an integer of 0 to 3,
≦ m + n ≦ 5. )
2. 2. The fuel cell according to 1, wherein a solution in which the benzyl alcohol derivative represented by the general formula (1) and cyclodextrin are dissolved in an inorganic acid aqueous solution or an acidic ethanol aqueous solution is used as the liquid fuel.
3. 3. The fuel cell according to 2, wherein the inorganic acid aqueous solution is a sulfuric acid aqueous solution.
4). 4. The fuel cell according to any one of 1 to 3, wherein the benzyl alcohol derivative is 4-hydroxy-3-methoxybenzyl alcohol.
5. 5. The fuel cell according to any one of 1 to 4, wherein the cyclodextrin is γ-cyclodextrin.
6). 6. The fuel cell according to any one of 1 to 5, wherein the blending ratio of the benzyl alcohol derivative and cyclodextrin is 1: 0.5 to 1:20 by weight.
7). The fuel cell according to any one of 2 to 6, wherein the pH of a liquid fuel obtained by dissolving a benzyl alcohol derivative and cyclodextrin in an aqueous inorganic acid solution is 1 to 4.
8). 8. The fuel cell according to any one of 2 to 7, wherein the liquid fuel obtained by dissolving a benzyl alcohol derivative and cyclodextrin in an inorganic acid aqueous solution further contains an electrolyte.
9. The fuel cell according to any one of 1 to 8, wherein the concentration of the benzyl alcohol derivative in the liquid fuel is 0.001 to 5 mol / liter.
10. 10. The fuel cell according to any one of 1 to 9, wherein the electrolyte is a proton conductive solid polymer electrolyte membrane.
11. A liquid fuel for a fuel cell comprising a solution of the benzyl alcohol derivative represented by the general formula (1) and cyclodextrin dissolved in an inorganic acid aqueous solution.
12 12. The liquid fuel for a fuel cell according to 11, wherein the inorganic acid aqueous solution is a sulfuric acid aqueous solution.
13. The liquid fuel for a fuel cell according to claim 11 or 12, wherein the benzyl alcohol derivative is 4-hydroxy-3-methoxybenzyl alcohol.
14 The liquid fuel for a fuel cell according to any one of 11 to 13, wherein the cyclodextrin is γ-cyclodextrin.
15. The liquid fuel for a fuel cell according to any one of 11 to 14, wherein the blending ratio of the benzyl alcohol derivative and cyclodextrin is 1: 0.5 to 1:20 by weight.
16. The liquid fuel for fuel cells according to any one of 11 to 15, wherein the pH of the liquid fuel obtained by dissolving a benzyl alcohol derivative and cyclodextrin in an aqueous inorganic acid solution is 1 to 4.
17. The liquid fuel for fuel cells according to any one of 11 to 16, wherein the liquid fuel obtained by dissolving a benzyl alcohol derivative and cyclodextrin in an inorganic acid aqueous solution further contains an electrolyte.
18. The liquid fuel for a fuel cell according to any one of 11 to 17, wherein the concentration of the benzyl alcohol derivative in the liquid fuel is 0.001 to 5 mol / liter.
19. The liquid fuel for a fuel cell according to any one of 11 to 18, which is stored in a replaceable cartridge.

  According to the present invention, unlike the conventional direct methanol fuel cell, the electrode catalyst is not subject to poisoning by carbon monoxide which is a reaction intermediate of methanol oxidation, and an expensive platinum or platinum alloy catalyst is used. Even if not, a fuel cell excellent in output characteristics and stability can be obtained. In the fuel cell of the present invention, unlike the conventional direct methanol fuel cell, there is no possibility that a reaction product harmful to the human body is generated, and the fuel cell can be reduced in size, weight and cost.

Next, a preferred embodiment of the fuel cell of the present invention will be described with reference to the drawings.
FIG. 1 is a conceptual diagram showing a single cell structure of a fuel cell according to the present invention. In this fuel cell, a container 1 has an ion exchange membrane 2 which is a solid polymer electrolyte membrane, a pair of cathodes 3 and an anode 4 which are sandwiched between them, and an oxidant flow path 5 on the outside thereof. A liquid fuel storage unit 6 is provided.

  The ion exchange membrane 2 may be either an anion or cation ion conduction type, but a proton conduction type is preferably used. As the ion exchange membrane 2, all known materials including a polymer membrane represented by perfluoroalkylsulfonic acid polymer can be used.

  As the cathode 3 and the anode 4, for example, porous carbon paper coated with a predetermined catalyst can be used. An ion exchange membrane 2 is disposed between the cathode 3 and the anode 4 and sandwiched, or the three members are joined by hot pressing, cast film formation or the like to form a membrane-one electrode structure. If necessary, a water repellent such as polytetrafluoroethylene may be added to or laminated on the porous carbon paper.

  The anode 4 can be configured by, for example, mixing carbon carrying platinum or a platinum alloy together with an ion conductive material and then bringing the carbon into contact with the ion exchange membrane 2. The platinum alloy may be an alloy of platinum and one or more elements selected from the group consisting of ruthenium, iridium, osmium, iron, nickel, gold, cobalt, palladium, tungsten, molybdenum and tin. it can.

  A preferable result is obtained when the ion conductive material is the same material as the ion exchange membrane 2. As a method for bringing the anode 4 into contact with the ion exchange membrane 2, known methods such as hot pressing and cast film formation can be used. In addition to carbon carrying platinum or a platinum alloy, known anodes 4 such as noble metals or those carrying them (electrode catalyst), organometallic complexes or those obtained by firing them can be used. A noble metal or a noble metal thin film or a laminate thereof as disclosed in US Pat. No. 5,759,712 can also be used as the anode 4.

In the fuel cell of the present invention, a conventional aliphatic alcohol such as methanol is used as a liquid fuel supplied directly to the anode by using a combination of the benzyl alcohol derivative represented by the general formula (1) and cyclodextrin. Oxidation electrode reaction is completely different from that of a fuel cell.
In other words, the oxidation reaction from methanol to carbon dioxide has a standard oxidation-reduction potential that is thermodynamically almost equal to that of hydrogen, but in reality, it is difficult to extract electrons from the hydroxyl group of methanol. A large overvoltage is required because the moving speed is very slow. In order to reduce this overvoltage, a noble metal catalyst such as platinum is used, but as an electrode reaction mechanism at this time, methanol was once adsorbed on the electrode catalyst and oxidized to carbon monoxide which is an adsorbed species of the reaction intermediate. Later, it has become clear from spectroscopic studies that it is further oxidized to carbon dioxide. This adsorbed carbon monoxide causes poisoning of the electrocatalyst.

In contrast, since benzyl alcohols have an aromatic ring with a relatively high electron density at the carbon atom to which the hydroxyl group is bonded, the electrode reaction is initiated by the generation of a cation radical by one-electron oxidation of the aromatic ring. . For this reason, the oxidation reaction depends on the electron donating property of the functional group bonded to the aromatic ring and is not significantly affected by the properties of the electrode surface.
Therefore, the catalyst dependence of the electrode reaction is extremely smaller than that of a conventional fuel cell, and excellent output characteristics can be obtained without using expensive platinum or a platinum-platinum-based catalyst as an electrode.

  The cathode 3 can be made of the same material as the anode 4. On the cathode 3 side, an oxidant introduction hole (not shown) for introducing an oxidant (in many cases air) into the oxidant flow path 5 is provided on the upper side, while unreacted air and products ( In many cases, an oxidant discharge hole (not shown) for discharging water) is provided. In this case, forced intake and / or forced exhaust means may be provided. The container 1 may be provided with a natural convection hole for air.

  A liquid fuel storage unit 6 is provided outside the anode 4. The liquid fuel storage unit 6 may be a flow path with a fuel storage unit (not shown) in which the liquid fuel of the present invention is stored in a replaceable cartridge, in addition to the fixed storage unit. The fuel is preferably stirrable by natural and / or forced convection. When the liquid fuel is stored in a replaceable cartridge, the size and shape of the cartridge can be selected as appropriate.

  Although the conceptual diagram of the fuel cell of FIG. 1 represents a single cell structure, this single cell can be used as it is as the fuel cell of the present invention. A plurality of cells may be connected in series and / or in parallel to form a mounted fuel cell. As a method for connecting cells, any known method such as a connection method using a bipolar plate can be used.

Next, the liquid fuel used in the fuel cell of the present invention will be described in detail.
In the present invention, as a liquid fuel supplied to the anode of the fuel cell, a benzyl alcohol derivative represented by the following general formula (1) and a cyclodextrin are used in combination.

In the general formula (1), R represents a lower alkyl group having 1 to 5 carbon atoms, m and n each represents an integer of 0 to 3, and 0 ≦ m + n ≦ 5.
Specific examples of R include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl and the like, and methyl and ethyl are preferable.
Preferred examples of the benzyl alcohol derivative represented by the general formula (1) include benzyl alcohol, mono- or di-hydroxybenzyl alcohol, mono- or di-methoxybenzyl alcohol, hydroxymethoxybenzyl alcohol, hydroxyethoxybenzyl alcohol and the like. Can be mentioned. In these benzyl alcohol derivatives, the substitution position of the hydroxy group or alkoxy group is not particularly limited. For example, in monohydroxybenzyl alcohol, any of 2-hydroxy, 3-hydroxy, and 4-hydroxy can be used. it can. These benzyl alcohol derivatives can be used alone or in combination of two or more.

  Cyclodextrin is a macrocyclic compound in which D-glucose forms a cyclic structure by α-1,4 bonds. It is α-cyclodextrin having a polymerization degree of 6, β-cyclodextrin having a polymerization degree of 7, and γ-cyclohexane having a polymerization degree of 8. Dextrins are common, but δ- and ε-cyclodextrins with a higher degree of polymerization are also known. Any of these known cyclodextrins can be used as the cyclodextrin used in the present invention, and these can be used alone or in combination of two or more. As the cyclodextrin, γ-cyclodextrin is preferably used.

Benzyl alcohol derivatives have properties as aromatic hydrocarbons, have extremely low affinity with proton conductive solid polymer electrolyte membranes, and are not oxidized with aliphatic alcohols such as methanol used in conventional fuel cells. The electrode reactions are completely different, and it is possible to prevent the decrease in output due to crossover and the poisoning of the catalyst by carbon monoxide when methanol is used.
However, the benzyl alcohol derivative has a relatively low solubility in an aqueous solution, and the oxidation product tends to undergo polymerization and cause a decrease in output as the reaction time elapses. In the present invention, the coexistence of a benzyl alcohol derivative and a cyclodextrin, which is a macrocyclic compound, improves the solubility in aqueous solution and the stability of the solution, and succeeds in remarkably improving the output characteristics as a fuel cell. It is.
The blending ratio of the benzyl alcohol derivative and the cyclodextrin is preferably 1: 0.5 to 1:20, particularly 1: 1 to 1:10 by weight.

In the present invention, a solution obtained by dissolving the benzyl alcohol derivative and cyclodextrin in an inorganic aqueous solution such as sulfuric acid or hydrochloric acid or an aqueous acidic ethanol solution is used as a liquid fuel for a fuel cell.
As the inorganic acid aqueous solution, a sulfuric acid aqueous solution is preferably used, and the concentration of sulfuric acid is preferably adjusted so that the pH of the liquid fuel is 1 to 4, particularly 1 to 2.

In the liquid fuel, it is preferable to add an electrolyte such as sodium sulfate, potassium sulfate, calcium sulfate, magnesium sulfate, and barium sulfate, particularly a neutral electrolyte, as a supporting electrolyte. By adding such an electrolyte, by adjusting the pH of the solution to 1 or more while suppressing the sulfuric acid concentration, it becomes possible to prevent white turbidity of the solution due to the oxidation reaction and maintain a high oxidation peak current.
The concentration of the benzyl alcohol derivative in the liquid fuel is preferably 0.001 to 5 mol / liter, particularly 0.01 to 1 mol / liter.

Next, the liquid fuel used in the fuel cell of the present invention will be further described with reference to examples. However, the following specific examples do not limit the present invention.
In the following example, cyclic voltammetry was performed using the three-electrode electrochemical cell shown in FIG. 2 to test the redox characteristics of the liquid fuel. In FIG. 2, 11 is a sample solution (liquid fuel), 12 is a working electrode, 13 is a reference electrode, 14 is an auxiliary electrode, 15 is a potentiostat, 16 is a function generator, and 17 is a recorder.

Example 1
Dissolve 0.01M vanillyl alcohol (4-hydroxy-3-methoxybenzyl alcohol) and 0.02M γ-cyclodextrin in 0.1M sodium sulfate aqueous solution, and adjust the pH with sulfuric acid to obtain a pH1 solution. It was. This solution was used as the sample solution 11, and cyclic voltammetry was performed with an electrochemical cell using a platinum electrode as the working electrode 12 in FIG. 2, a platinum wire as the auxiliary electrode 14, and a silver-silver chloride electrode as the reference electrode 13. Sweep speed 50 mV / s. A cyclic voltammogram is shown in FIG.
In FIG. 3, the dotted line shows a decrease in current value due to repeated potential sweeping, but it can be seen that the current value hardly decreases even with repeated potential sweeping in this sample solution.

(Comparative Example 1)
In Example 1, cyclic voltammetry was performed in the same manner as in Example 1 except that the solution was prepared without using γ-cyclodextrin. The result is shown in FIG.
In FIG. 4, the dotted line indicates a decrease in the current value due to repeated potential sweep, but in this sample solution, a decrease in the current value is observed as compared with Example 1 due to repeated potential sweep.

(Example 2)
Sample solution A was prepared by dissolving 0.01 M vanillyl alcohol and 0.02 M γ-cyclodextrin in 0.5 M sulfuric acid aqueous solution. Using this sample solution A, the result of cyclic voltammetry performed in the same manner as in Example 1 is shown by the solid line A in FIG. For comparison, sample solution B in which only 0.01 M vanillyl alcohol is dissolved in 0.5 M sulfuric acid aqueous solution and sample solution C in which 0.5 M methanol is dissolved in 0.5 M sulfuric acid aqueous solution are similarly processed. The results of cyclic voltammetry are shown by a broken line B and a dotted line C in FIG.

According to FIG. 5, it can be seen that vanillyl alcohol is oxidized with a larger oxidation peak current at substantially the same potential as methanol oxidation, although the concentration is 1/50 compared with methanol. Further, when γ-cyclodextrin is used in combination, a higher peak current value can be obtained.
In the case of the sample solution B in which vanillyl alcohol is dissolved alone, when the potential sweep is repeated, the solution becomes clouded over time, and the peak current value is reduced to about 10% after 3 hours. This is considered to be due to the polymerization of the oxidation product of benzyl alcohol.
On the other hand, in the case of the sample solution A combined with γ-cyclodextrin, the solution does not become clouded over time, and a high oxidation peak current value can be maintained for a long time.

(Reference Example 1)
In the electrochemical cell shown in FIG. 2, a glassy carbon disk electrode was used as the working electrode 12 instead of the platinum electrode. FIG. 6 shows the results of cyclic voltammetry by preparing a sample solution by dissolving 0.01M vanillyl alcohol in a 0.05M sulfuric acid aqueous solution containing 0.2M sodium sulfate.
For comparison, cyclic voltammetry was performed on a sample solution obtained by dissolving 0.5 M of methanol in a 0.5 M sulfuric acid aqueous solution using the electrochemical cell of FIG. 2 using a platinum electrode as the working electrode 12. The results are shown by dotted lines in FIG.
According to FIG. 6, in the case of a liquid fuel containing a benzyl alcohol derivative as a component, even when a relatively inexpensive electrode material such as glassy carbon that cannot be expected to have an electrode catalytic action is used, the same oxidation-reduction as a platinum electrode is achieved. Characteristics were observed. Therefore, it is considered that the cost of the fuel cell can be greatly reduced.

It is a conceptual diagram which shows the single cell structure of the fuel cell of this invention. It is the schematic diagram of the electrochemical cell which tested the oxidation reduction characteristic of the liquid fuel. 2 is a cyclic voltammogram obtained in Example 1. FIG. 2 is a cyclic voltammogram obtained in Comparative Example 1. 3 is a cyclic voltammogram obtained in Example 2. FIG. 2 is a cyclic voltammogram obtained in Reference Example 1. It is a schematic diagram which shows the basic structure of the conventional direct methanol fuel cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Container 2, 22 Solid polymer electrolyte membrane 3, 23 Cathode 4, 24 Anode 5 Oxidant flow path 6 Liquid fuel storage part 11 Sample solution 12 Working electrode 13 Reference electrode 14 Auxiliary electrode 15 Potentiostat 16 Function generator 17 Recorder

Claims (19)

Benzyl alcohol derivative and cyclodextrin represented by the following general formula (1) as a liquid fuel in a direct fuel cell having a pair of anode and cathode provided with an electrolyte in between and supplying liquid fuel directly to the anode A fuel cell characterized by being used in combination:

(In the formula, R represents an alkyl group having 1 to 5 carbon atoms, m and n are each an integer of 0 to 3,
≦ m + n ≦ 5. ) 2. The fuel cell according to claim 1, wherein a solution obtained by dissolving the benzyl alcohol derivative represented by the general formula (1) and cyclodextrin in an inorganic acid aqueous solution or an acidic ethanol aqueous solution is used as the liquid fuel. The fuel cell according to claim 2, wherein the inorganic acid aqueous solution is a sulfuric acid aqueous solution. The fuel cell according to any one of claims 1 to 3, wherein the benzyl alcohol derivative is 4-hydroxy-3-methoxybenzyl alcohol. The fuel cell according to claim 1, wherein the cyclodextrin is γ-cyclodextrin. The fuel cell according to any one of claims 1 to 5, wherein the blending ratio of the benzyl alcohol derivative and the cyclodextrin is 1: 0.5 to 1:20 by weight. The fuel cell according to any one of claims 2 to 6, wherein the pH of the liquid fuel in which the benzyl alcohol derivative and cyclodextrin are dissolved in an inorganic acid aqueous solution is 1 to 4. The fuel cell according to any one of claims 2 to 7, wherein the liquid fuel obtained by dissolving benzyl alcohol derivative and cyclodextrin in an aqueous inorganic acid solution further contains an electrolyte. The fuel cell according to any one of claims 1 to 8, wherein the concentration of the benzyl alcohol derivative in the liquid fuel is 0.001 to 5 mol / liter. The fuel cell according to any one of claims 1 to 9, wherein the electrolyte is a proton conductive solid polymer electrolyte membrane. A liquid fuel for a fuel cell comprising a solution of the benzyl alcohol derivative represented by the general formula (1) and cyclodextrin dissolved in an inorganic acid aqueous solution. The liquid fuel for a fuel cell according to claim 11, wherein the inorganic acid aqueous solution is a sulfuric acid aqueous solution. The liquid fuel for a fuel cell according to claim 11 or 12, wherein the benzyl alcohol derivative is 4-hydroxy-3-methoxybenzyl alcohol. The liquid fuel for a fuel cell according to claim 11, wherein the cyclodextrin is γ-cyclodextrin. The liquid fuel for a fuel cell according to any one of claims 11 to 14, wherein a blending ratio of the benzyl alcohol derivative and the cyclodextrin is 1: 0.5 to 1:20 by weight. The liquid fuel for a fuel cell according to any one of claims 11 to 15, wherein the pH of the liquid fuel obtained by dissolving a benzyl alcohol derivative and cyclodextrin in an aqueous inorganic acid solution is 1 to 4. The liquid fuel for a fuel cell according to any one of claims 11 to 16, wherein the liquid fuel obtained by dissolving a benzyl alcohol derivative and cyclodextrin in an inorganic acid aqueous solution further contains an electrolyte. The liquid fuel for a fuel cell according to any one of claims 11 to 17, wherein the concentration of the benzyl alcohol derivative in the liquid fuel is 0.001 to 5 mol / liter. The liquid fuel for a fuel cell according to any one of claims 11 to 18, which is stored in a replaceable cartridge.

Images