JP2001026414A - Hydrogen storage material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a lightweight compact material capable of storing hydrogen at high density. SOLUTION: The hydrogen storage material comprises expanded graphite obtained by extending the distance between layers of graphite having superposed 6-membered ring faces by acid treatment and heat treatment in such a way that hydrogen is intercalated and concentrated between the layers. Such expanded graphite having hydrogen occluding ability has low bulk density. The bulk density of expanded graphite having particularly superior hydrogen occluding ability is 1.0-2.0 g/cc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage material capable of absorbing and storing hydrogen and releasing the stored hydrogen appropriately.

[0002]

2. Description of the Related Art In recent years, in the automobile industry and the like, natural gas and hydrogen gas, which emit a small amount of carbon dioxide per unit combustion energy, in order to suppress the emission of carbon dioxide, which is a main cause of global warming, The concept of using as a main fuel is being promoted. However, as a technology capable of storing fuel gas at high density, at present, a high pressure storage technology (a technology of storing under high pressure compression up to about 200 atm) is used.
Other than that, it has not been put to practical use.

However, in the case of high-pressure storage, the storage container must be rugged and the weight of the container must be increased. There were many circumstances where the application had to be restricted.

[0004] In particular, in the case of "hydrogen" which is attracting attention as a clean energy source, since the combustion energy of hydrogen is small (about one-third of that of natural gas), the stored energy is high even when stored at high pressure. Because of its small size, when it is applied to automotive fuel, the mileage per charge is less than half that of gasoline and natural gas vehicles, and it is significantly inferior to other fuels in terms of usability.

[0005] Therefore, a technique capable of storing fuel gas at a high density under a low pressure has been desired. With respect to hydrogen, a technology has been developed that can store hydrogen at a relatively high density on a volume basis at a low pressure of about 10 atm using a hydrogen storage alloy. However, when a hydrogen storage alloy is used, only about 1 to 2% by weight of the alloy weight can be stored on a weight basis, and this is used as a fuel source for automobiles, and is equivalent to that of a gasoline-powered vehicle. Mileage ''
To achieve the required alloy weight of 200 to 40
It weighs 0 kg (weight of 3 to 6 adults), which causes a reduction in fuel efficiency. Therefore, this is not at all acceptable from an energy conservation point of view.

Therefore, at least about twice the current hydrogen storage capacity (about 3 to 4% by weight of the alloy weight on a weight basis)
Research and development of hydrogen storage alloys that can store hydrogen have been progressing, but the prospects are difficult, and the hydrogen storage capacity of the developed alloys is becoming saturated.

[0007] In view of the above, an object of the present invention is to provide an energy source that does not emit carbon dioxide.
The object was to provide a lightweight and compact material capable of storing "hydrogen" expected as a "source" at a high density.

[0008]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and in the process, molecules such as methane, nitrogen, oxygen and carbon dioxide have the same size as these molecules. Focusing on the so-called “concentration effect” in which the presence of pores and slits of a certain degree causes the concentration and retention of such pores and pores, the concentration effect cannot be expected when appropriate pores and slits also exist in the hydrogen molecule. I thought. From this point of view, we will continue to study the existence of materials having pores and slits that are expected to have the effect of enriching hydrogen molecules, and seek the potential of hydrogen storage materials in carbon materials, especially "graphite". It became.

This is because graphite is a material that is extremely light and chemically extremely stable even in a high-density state, and has an affinity for hydrogen, and thus is suitable as a material for adsorbing hydrogen. In addition to the above, graphite is composed of hexagonal crystals in which flat six-membered ring polymer layers are overlapped, and the layer surface forming a hexagonal network is weak in van der Waals.
This is because hydrogen molecules are likely to enter between the layers of the six-membered ring polymerized surface of graphite because they are stacked by the Ruth bond.

That is, as described above, graphite has a hexagonal crystal structure in which layers of flat six-membered ring polymer surfaces are overlapped with each other. Thus, although graphite shows a remarkable anisotropy based on this structure, Rich in heat resistance, thermal shock resistance, corrosion resistance,
Although it is a material with very good electrical and thermal conductivity, the graphite intercalation compound can be formed by chemically inserting various heteroatoms between the layers of the "six-membered polymerized surface of graphite". It is known that can be generated. Therefore, it was examined whether or not this graphite intercalation compound exhibited a phenomenon of taking in hydrogen between the layers and concentrating it.

However, the interlayer distance between the layers of the six-membered ring polymerized surface of graphite is 3.34 °, which is an extremely difficult width for hydrogen molecules to enter, and the graphite intercalation compound is already different between the layers. Since the atoms have entered, the space into which hydrogen molecules enter is small, and thus it was judged that the material was not suitable as a material for storing a large amount of hydrogen.

However, it is known that graphite takes the form of "thermally expanded graphite" in which the layers are expanded by expansion. That is, in the graphite intercalation compound, sulfuric acid, nitric acid,
In the case where hydrogen peroxide and hydrochloric acid are intercalated (intercalated) between the graphite layers, when heated, the interlayer material is activated and gasified to generate fluid pressure, and the expansion action of the fluid pressure causes the graphite to expand. The interlayer is stretched as shown in FIG. 1 to form so-called "thermally expanded graphite". This thermally expanded graphite has heat resistance,
In addition to being excellent in chemical resistance, it is also excellent in electric conductivity and heat conductivity, and when formed in a sheet shape, elasticity and gas impermeability are imparted. Therefore, the thermal expansion graphite is used in the cylinder head gaskets of automobile engines,
It is used for rocker cover gaskets, manifold gaskets, etc., as well as for seal materials, heat insulating materials, and oil absorbing materials.

[0013] Then, this time, the hydrogen enrichment effect of the above-mentioned "thermally expanded graphite", in which the interlayer distance of the layer formed by the surface of the six-membered ring polymer is greatly increased, and the foreign atoms between the layers are gasified and dissipated. investigated. However, from this investigation, it was found that the above-described conventionally-expanded thermally expanded graphite does not show a significant hydrogen storage capacity.

[0014] The reason that the conventionally known thermally expanded graphite does not show a significant hydrogen storage capacity is that the degree of expansion of the thermally expanded graphite is very large, and the interlayer distance of the layer formed by the six-membered ring polymer surface is large. It was concluded that this was due to the widespread spread. That is, the conventional thermally expanded graphite applied to the gasket, the seal material, the heat insulating material, or the oil absorbing material expands its volume by 10 to 100 times or more as compared with the original graphite, and has a bulk density of 100% or more. G / cc to several thousandths of a g / cc (specifically
(0.03 g / cc or less, usually about 0.004 to 0.006 g / cc), so the interlayer distance of the layer formed by the 6-membered ring polymer surface is too large compared to the size of the hydrogen molecule We thought that this was the cause of not exhibiting the "hydrogen enrichment effect".

[0015] Therefore, the present inventors have further studied to find a means for adjusting the interlayer distance of graphite, and found that the time of acid treatment, the temperature of acid treatment, the heating temperature upon expansion, and the like are appropriately adjusted. As a result, it was confirmed that expanded graphite having various degrees of expansion was obtained. Then, powdery graphite is used as a sample, and an acid treatment and a heat treatment are performed on the graphite under the adjusted conditions, so that the interlayer distance is increased to an extent that hydrogen molecules can enter (that is, a bulk density of 2.0 to 1.0).
g / cc and the degree of expansion was extremely small compared to conventional thermal expansion graphite) .Expansion graphite with no foreign molecules between layers was manufactured and hydrogen storage was attempted. This expanded graphite was used as a hydrogen storage material. It was found to be very effective.

The present invention has been made based on the above findings and the like, and provides the following hydrogen storage material. (1) A hydrogen storage material made of “expanded graphite” that has been subjected to an acid treatment and a heat treatment to increase the distance between layers of the six-membered ring polymerized surface of graphite. (2) A hydrogen storage material made of “expanded graphite” that has been subjected to an acid treatment and a heat treatment to reduce the bulk density of graphite. (3) The bulk density is 1.0 to 2.0 g / cc, the above (1) or
The hydrogen storage material according to (2).

As described above, according to the present invention, graphite is used as a raw material in which the layers formed by the surface of the six-membered ring polymer are narrow, and hydrogen molecules cannot enter, and the layers are slightly expanded.
It provides a hydrogen storage material made of expanded graphite that secures a space for hydrogen molecules to enter and allows hydrogen molecules to enter and concentrate, but as a material before expansion, natural graphite is used. Any material may be used as long as it is "expandable and forms expanded graphite having a low bulk density after expansion", such as pyrolytic graphite and quiche graphite.

The hydrogen storage material according to the present invention is produced by immersing (acid-treating) graphite such as the above-mentioned natural graphite, pyrolytic graphite, and quiche graphite in an acid, washing with water, and then heating. Examples of the acid used in the acid treatment include sulfuric acid, nitric acid, hydrochloric acid, perchloric acid, hydrogen peroxide, phosphoric acid, and a mixed solution thereof. Further, a gas obtained by blowing chlorine gas into water can also be used. By this acid treatment, an acid which is activated and gasified when heated to generate a fluid pressure is intercalated (intercalated) between the graphite layers.

In the acid treatment, the graphite may be immersed in the acid as it is, but the graphite is evacuated at room temperature as a pretreatment, or immersed in the acid after heating and evacuating the graphite. It is preferable for obtaining a hydrogen storage material. The treatment temperature of the acid treatment can be selected from the range of -196 ° C to 300 ° C, and the treatment time is 0.05 to 200 ° C.
You can choose a time range. The acid treatment may be repeated any number of times, and the performance of the obtained hydrogen storage material is stabilized as the number of repetitions increases.

The graphite after the acid treatment is washed with water,
Next, a heat treatment is performed to expand the structure. The heating temperature is selected in the range of 0.1 to 100 hours. Here, the heating atmosphere may be any gas atmosphere such as nitrogen, oxygen, air, carbon dioxide, argon, and chlorine, and is not particularly limited. This heat treatment activates and gasifies the intercalated substance such as acid intercalated between the graphite layers by the acid treatment to generate a fluid pressure, thereby expanding the graphite and dispersing it from the interlayer. Therefore, expanded graphite having no foreign molecules between layers can be obtained.

[0021] The process of further expanding the "expanded graphite" obtained by expanding graphite, washing with water, and heating may be repeated many times. By selecting these conditions, it is possible to obtain expanded graphite having various degrees of expansion (expanded graphite in which the layers formed by the six-membered ring polymer surface have various interlayer distances). It is preferable to adjust the degree of expansion so that the interlayer distance is about twice (in terms of bulk density, 1.0 to 2.0 g / cc). As a result, the effect of infiltration and concentration of hydrogen between the expanded graphite layers is significantly improved. By the way, the layers of the six-membered ring polymerized surface of the expanded graphite used as the hydrogen storage material are preferably extended while keeping parallel to each other, but may be formed to have an interlayer gap which is inclined and expands in a wedge shape. No problem.

[0022]

The hydrogen storage material according to the present invention is made of expanded graphite. However, as can be seen from the presence of many hydrocarbon compounds, the carbon material has an affinity for hydrogen. Also from the viewpoint, it is suitable as a material for absorbing and storing hydrogen. Moreover, since the expanded graphite has a structure having slits and pores of an appropriate size in which no foreign molecules are present between the layers of the six-membered ring polymerized surface of graphite, hydrogen molecules are formed in the slits and fines. It exerts the effect of infiltrating into the pores and concentrating it, and also exerts an excellent hydrogen storage effect by synergistically exerting the effect of good affinity with the hydrogen. Note that, as described above, the hydrogen storage effect is such that the bulk density of the expanded graphite is 1.0 to 1.0.
When it is in the range of 2.0 g / cc (corresponding to approximately 6 to 9 ° in the interlayer distance between the layers of the six-membered ring-polymerized surface), it becomes significantly remarkable.

In addition, graphite has a high density in addition to being a lightweight material, so even if expanded graphite, it becomes a lightweight and compact member, and expanded graphite is also excellent in heat resistance and chemical resistance. Therefore, it has an advantage of being a very stable material, so that it can be said that its suitability as a hydrogen storage material is very high.

In addition, since the hydrogen storage material according to the present invention is made of expanded graphite, it can be used as a solid fuel even when the hydrogen storage function is reduced due to long-term use and the stage of disposal is reached. There is no problem because it is harmless even if it is disposed of by burial without incineration, and it is excellent as a material with a low environmental load that is particularly desired at present.

In the hydrogen storage material according to the present invention, when the hydrogen partial pressure in the atmosphere increases, the amount of hydrogen storage increases in accordance with the increase. However, when the hydrogen partial pressure in the atmosphere decreases, the stored hydrogen is reduced. Since it is released smoothly, it can be said that this is also a very simple hydrogen storage material.

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

EXAMPLE 5 g of commercially available graphite powder was prepared, and 200 g of the powder was prepared.
C. and a pre-treatment of drying by evacuating and drying with a vacuum pump at the same time.
6 ml of concentrated sulfuric acid and 10 ml of nitric acid were added, and an acid immersion treatment was performed for 6 hours. Next, the acid-treated graphite powder was washed with distilled water, and then subjected to a heat treatment at 550 ° C. for 3 hours in the air to obtain expanded graphite in which the interlayer distance between the layers of the six-membered ring polymerized surface was increased.

The expanded graphite thus obtained was heated again to 200 ° C. and evacuated by a vacuum pump and dried. Then, 30 ml of concentrated sulfuric acid and 10 ml of nitric acid were placed in a container containing the expanded graphite. In addition, an acid immersion treatment for 6 hours is performed, and a washing treatment with distilled water and 550 ° C. in the atmosphere are performed.
And a heat treatment for 3 hours were repeated three times. By the above treatment, the interlayer distance between the layers of the six-membered ring polymerized surface is increased (the interlayer distance is almost twice that of the original graphite) and the bulk density is 1.
Expanded graphite reduced to 0 g / cc was obtained.

Next, the hydrogen storage capacity of the obtained expanded graphite was investigated. Investigating hydrogen storage capacity, expanded graphite at room temperature (25 ° C)
By contacting with a hydrogen gas at a hydrogen pressure and examining the hydrogen storage weight per unit weight of graphite at various hydrogen pressures. The result is shown in FIG. In addition, for comparison, FIG. 2 also shows the results of an investigation using a raw graphite powder that was not subjected to expansion treatment.

As is evident from the results shown in FIG. 2, almost no hydrogen storage effect was observed in the raw graphite powder, whereas the expanded graphite according to the hydrogen storage material of the present invention showed a remarkable hydrogen storage capacity. It can be confirmed that it has an effect.

[0030]

As described above, according to the present invention, it is possible to provide a lightweight and inexpensive material capable of easily storing and releasing hydrogen at a high density, and to provide clean energy as a hydrogen fuel storage member. Industrially useful effects such as a great contribution to the promotion of fuel conversion are expected.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic diagram illustrating a situation in which the acid-treated graphite is heated to expand the six-membered ring polymer layer of the graphite into thermally expanded graphite.

FIG. 2 is a graph showing a comparison between the results of an investigation on the hydrogen storage capacity of graphite and expanded graphite according to the material of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yasuhiko Urabe 3-27-30 Toyo Park, Koto-ku, Tokyo 801 Central Bank Toyo Park Mansion 801 (72) Inventor Toshinari Araki 1-20-10 Momoi, Suginami-ku, Tokyo F-term (Reference) 4G040 AA02 AA36 AA42 4G046 EA05 EB02 EB04 EB06 EC02 EC05 EC07 EC08

Claims (3)

[Claims] 1. A hydrogen storage material comprising "expanded graphite" which has been subjected to an acid treatment and a heat treatment to increase the distance between layers of a six-membered polymerized surface of graphite. 2. A hydrogen storage material comprising “expanded graphite” in which the bulk density of graphite is reduced by performing an acid treatment and a heat treatment. 3. The hydrogen storage material according to claim 1, wherein the bulk density is 1.0 to 2.0 g / cc.