Aromatic hydrocarbon based proton exchange membrane and direct methanol fuel cell using same

Abstract

A proton exchange membrane is obtained which can give an excellent power generation characteristic when the membrane is applied to, in particular, a fuel cell wherein high-concentration methanol is used as a fuel. In the aromatic hydrocarbon based proton exchange membrane of the invention, the ion exchange capacity is set into the range of 0.6 to 1.3 meq / g. Moreover, the area swelling rate for a 30% by mass methanol aqueous solution at 40° C. is set into the range of 2 to 30%. Preferably, a sulfonic acid group is bonded to an aromatic ring of the aromatic hydrocarbon based polymer contained in the aromatic hydrocarbon based proton exchange film. Preferably, the aromatic hydrocarbon based polymer is a polyarylene ether based polymer.

Description

TECHNICAL FIELD [0001] The present invention relates to an aromatic hydrocarbon based proton exchange membrane useful as a polymeric electrolyte membrane for a direct methanol fuel cell wherein a high-concentration methanol aqueous solution is used as a fuel, and a direct methanol fuel cell using the same. BACKGROUND ART [0002] Direct methanol fuel cells are solid polymer fuel cells wherein methanol is used as a fuel to generate electric power, and are expected to be used as power sources for notebook size personal computers, PDAs, cellular phones, and so on. Direct methanol fuel cells have, as their center, a structure called a membrane electrode assembly (MEA), wherein a pair of electrodes are jointed to both faces of a proton exchange membrane. A methanol aqueous solution is supplied to one of the electrodes, and an oxidizing gas such as air is supplied to the other, whereby the structure can be operated as a cell. As the concentration of the methanol aqueous solution is higher, ...

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Benefits of technology

[0027] The aromatic hydrocarbon based proton exchange membrane of the invention (i.e., any one of the aromatic hydrocarbon based proton exchange membranes of the invention) contributes in particular to an increase in the energy density of fuel cells and a decrease in the size thereof since the membrane gives an excellent power generation characteristic in the case that the membrane is used in a direct methanol fuel cell wherein high-concentration methanol is used as a fuel. BEST MODES FOR CARRYING OUT THE INVENTION

[0028] The inventors have made eager investigations about the selection and optimization of an aromatic hydrocarbon based proton exchange membrane used particularly in a direct methanol fuel cell wherein a high concentration methanol aqueous solution is used as a fuel. As a result, the invention has been made.

[0029] In a proton exchange membrane obtained by copolymerizing a component having, in its polymer skeleton, an anionic functional group contributing to the expression of proton conductivity, for example, a sulfonic acid group with a hydrophobic component not contributing to proton conductivity, which is made of, for example, an aromatic skeleton, the proton conductivity increases when the ratio of the amount of the proton conductive component is increased. However, in correlation with this matter, a methanol crossover also increases. On the other hand, if the ratio of the amount of the proton conductive component is decreased, a methanol crossover can also be restrained; however, the proton conductivity also falls. In other words, the channel wherein protons can be shifted is basically a hydrophilic moiety as well as the channel wherein methanol can be sifted; therefore, a positive correlation is present between the two.

[0030] Thus, when the proton exchange membrane which contains a large amount of a proton conductive component is compared with the proton exchange membrane which does not contain a very large amount of a proton conductive component, basic membrane properties therebetween, such as proton conductivity and methanol permeability, are different; however, factors producing effects on the generation of electric power are film (electric) resistance and the permeation rate of methanol, which depend on film thickness in addition to proton conductivity and the permeation coefficient of methanol. Therefore, when the film thickness of the proton exchange membrane wherein both of the proton conductivity and the methanol permeation coefficient are small is made small, the film can be rendered a film having properties close to those of a film wherein both of the proton conductivity and the methanol permeation rate are relatively high since the film resistance decreases and further the methanol permeation rate increases. In short, it can be generally said that it is important to select a proton exchange membrane wherein the methanol permeation coefficient, the proton conductivity and the film thickness are optimized.

[0031] In the case of using, in particular, a high-concentration methanol aqueous solution as a fuel, proton exchange membranes wherein various factors are combined can be supposed; the inventors have found out that only the following direct methanol fuel cell can endure long-term power generation: a direct methanol fuel cell formed by use of a proton exchange membrane wherein the area swelling rate for a methanol aqueous solution is restrained into a specific range. Thus, the invention has been made.

[0032] Accordingly, the invention is characterized by selecting and using, as a proton exchange membrane used in a direct methanol fuel cell wherein a high-concentration methanol aqueous solution is used as a fuel, a proton exchange membrane in which an aromatic hydrocarbon based polymer is contained and, in particular, the area swelling property for a methanol aqueous solution is small.

Problems solved by technology

However, when the Nafion (registered trade name) membrane is used in a fuel cell wherein methanol is used as a fuel, there is remarkably caused a problem called a methanol crossover, which is a problem that methanol permeates into the Nafion (registered trade name) membrane to flow into the side of its air electrode.

Thus, there arises a problem that the performance thereof as a cell falls.

Furthermore, it is pointed out that an excessively high cost for the membrane hinders the establishment of fuel cell technique thereof.

Consequently, the energy density becomes low and further the fuel tank becomes large-sized.

Thus, this hinders the practical use.

In membranes having high methanol permeability, such as a perfluorocarbon sulfonic acid membrane, power generating performance is not easily exhibited unless a diluted methanol aqueous solution is used.

However, proton conductivity and methanol blocking property are in general properties incompatible with each other; thus, if the proton conductivity is made preferential, the methanol permeability becomes high so as to cause a fall in the power generation characteristic easily, and if the methanol blocking property is made preferential, the resistance of the membrane becomes high, thereby causing a fall in the power generation characteristic easily.

As a result, the power generation characteristic does not become sufficient.

However, in this case also, the methanol concentration used in the direct methanol fuel cell is small, so that the above-mentioned problem is not solved.

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