JP2005021860A - Hydrogen storage material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive hydrogen storage material which does not lose the hydrogen occlusion capability on exposure to the air, and occludes hydrogen under a low hydrogen pressure. <P>SOLUTION: Part of the surface of a metal or an alloy which has a hydride phase whose heat of formation is negatively larger than -45kJ/mol.H<SB>2</SB>, is covered with palladium, thereby forming the hydrogen storage material. Even the exposure to the air causes a less degradation of hydrogen occlusion capability because the material is covered with palladium. The material can occlude hydrogen under a hydrogen pressure lower than the equilibrium hydrogen pressure of palladium. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hydrogen storage material capable of reversibly storing and releasing hydrogen.

  In recent years, hydrogen energy has attracted attention as a clean alternative energy due to environmental problems such as global warming caused by carbon dioxide emissions and energy problems such as exhaustion of petroleum resources. For the practical application of hydrogen energy, it is important to develop technology for safely storing and transporting hydrogen. As hydrogen storage materials capable of storing hydrogen, development of carbon materials such as activated carbon, fullerene, and nanotubes, and hydrogen storage alloys are underway. Among these, hydrogen storage alloys are expected as a new transportable storage medium because hydrogen can be stored in large quantities in the form of a safe solid called a metal hydride.

  However, hydrogen storage alloys are oxidized when exposed to the atmosphere. As a result, a so-called barrier layer such as an oxide is formed on the surface of the hydrogen storage alloy. In the portion where the barrier layer is formed, hydrogen molecules cannot be dissociated. For this reason, when the hydrogen storage alloy is exposed to the atmosphere, its hydrogen storage capacity decreases.

As a method for preventing oxidation of the hydrogen storage alloy and imparting a hydrogen dissociation function to the hydrogen storage alloy, for example, a method of forming a palladium thin film on the entire surface of the magnesium thin film is disclosed (for example, non-patent document). 1 and 2).
A. Krozer et al., “Hydrogen uptake by Pd-coated Mg: Absorption-decomposition isotherms and uptake kinetics”, “Journal of the Less-Common Metals”, 1990, 160, p.323-342. A. Krozer et al., “Unusual kinetics of Hydrogen formation in Mg-Pd sandwiches, studied by Hydrogen profiling and quartz crystal microbalance measurements”, “Journal of the Less-Common Metals”, 1989, 152, p.295-309.

  Non-Patent Documents 1 and 2 disclose a material composed of two layers in which a magnesium thin film and a palladium thin film are laminated. In this material, hydrogen molecules are first adsorbed on the surface of the palladium thin film and dissociated into hydrogen atoms. Next, hydrogen atoms diffuse inside the palladium thin film and then diffuse into the magnesium thin film. That is, in order for magnesium to be hydrogenated, it is necessary to hydrogenate palladium. Therefore, in order for magnesium to be hydrogenated, a hydrogen pressure higher than the equilibrium hydrogen pressure of palladium is required. Moreover, in the said material, since the whole surface of a magnesium thin film is coat | covered with palladium, the usage-amount of palladium is large and cost is high.

  The present invention has been made in view of such a situation, and provides a hydrogen storage material that is inexpensive, does not lose hydrogen storage capacity even when exposed to the atmosphere, and can store hydrogen even under a low hydrogen pressure. The task is to do.

The hydrogen storage material of the present invention is characterized in that a part of the surface of a metal or alloy having a hydride phase whose hydride heat of generation is more negative than −45 kJ / mol H ₂ is covered with palladium.

  In the hydrogen storage material of the present invention, a predetermined metal or alloy (hereinafter referred to as “metal etc.” as appropriate) plays a role of storing hydrogen. When a metal or the like reacts with hydrogen to generate a metal hydride, heat of hydride generation (ΔH) is generated. The stability of the hydride produced is determined by the value of the heat of hydride production (hereinafter referred to as “ΔH” as appropriate). If the value of ΔH is negative, the hydride is stable, and the greater the negative absolute value, the more stable the hydride. That is, a large negative absolute value of ΔH means that the equilibrium hydrogen pressure at the same temperature is smaller.

In the hydrogen storage material of the present invention, the metal or the like serving as a base material for storing hydrogen has a hydride phase whose ΔH is negatively larger than −45 kJ / mol H ₂ . Here, ΔH of hydride of palladium (PdH _0.56 ) is −43.1 kJ / molH ₂ at a temperature of 298K. Therefore, a metal having a hydride phase whose ΔH at a temperature of 298 K is negatively larger than −45 kJ / mol H ₂ can generate a hydride at a hydrogen pressure lower than that of palladium at the same temperature. When the pressure-composition isotherm of a metal or the like has a multistage plateau and there are a plurality of hydride phases, it is only necessary that ΔH of any of the hydride phases is negatively larger than the above value.

  In the hydrogen storage material of the present invention, a part of the surface of the metal or the like having the hydride phase is coated with palladium. That is, the remainder of the surface of the metal or the like that is not covered with palladium is exposed. Therefore, a boundary between the metal or the like and palladium exists on the surface of the metal or the like. Hydrogen molecules adsorb at the boundary and dissociate into hydrogen atoms. Thereafter, it is considered that hydrogen atoms diffuse into the metal or the like. In addition, it is considered that hydrogen atoms adsorbed and dissociated on the surface of palladium diffuse on the palladium surface and diffuse into the metal or the like from the boundary. Thus, since it is not necessary for hydrogen atoms to diffuse inside the palladium, the metal or the like is hydrogenated, so that it is not a condition that the palladium is hydrogenated. Therefore, the hydrogen storage material of the present invention can store hydrogen even under a hydrogen pressure lower than the equilibrium hydrogen pressure of palladium.

  In addition, since a part of the surface of the metal or the like is covered with palladium, the metal or the like is hardly oxidized even when exposed to the atmosphere. Therefore, in the hydrogen storage material of the present invention, even when exposed to the atmosphere, the decrease in hydrogen storage capacity is small. Furthermore, in the hydrogen storage material of the present invention, since the coating with palladium is a part of the surface of metal or the like, the amount of palladium used is smaller than that with which the entire surface is coated. Therefore, the hydrogen storage material of the present invention is a relatively inexpensive material.

  In the hydrogen storage material of the present invention, a part of the surface of the metal or alloy that stores hydrogen is covered with palladium, so that even when exposed to the atmosphere, the decrease in hydrogen storage capacity is small. Moreover, hydrogen can be occluded under a hydrogen pressure lower than the equilibrium hydrogen pressure of palladium. Therefore, according to the hydrogen storage material of the present invention, hydrogen can be sufficiently recovered even in an atmosphere in which hydrogen is lean.

  Hereinafter, the hydrogen storage material of the present invention will be described in detail. In addition, the hydrogen storage material of this invention is not limited to the following embodiment. The hydrogen storage material of the present invention can be implemented in various forms with modifications and improvements that can be made by those skilled in the art without departing from the scope of the present invention.

The hydrogen storage material of the present invention is formed by coating a part of the surface of a metal or alloy having a hydride phase whose hydride generation heat is negatively greater than −45 kJ / mol H ₂ . The metal or alloy serving as a base material for storing hydrogen is not particularly limited as long as ΔH has a hydride phase negatively larger than −45 kJ / mol H ₂ . In this specification, “metal” means a single metal. However, “metal” may contain impurities that are inevitably mixed in production. Examples of the metal include alkali metals such as magnesium, titanium, vanadium, calcium, strontium, barium, zirconium, niobium, and lithium. Among these, it is preferable to use magnesium, titanium, or vanadium because the hydrogen storage amount per unit weight is large and hydrogen can be released at a relatively low temperature. Examples of the alloy include Mg-Ni alloy, Mg-Zn alloy, Mg-Ca alloy, Mg-Cu alloy, Mg-Li alloy, Mg-Al alloy and other magnesium alloys, Ti-Co alloy, Ti-Fe, and the like. -Titanium alloys such as Ni-V alloy and the like. Among them, it is preferable to use an alloy containing at least one of magnesium, titanium, and vanadium because hydrogen storage amount per unit weight is large and hydrogen can be released at a relatively low temperature.

  The form of the metal or alloy is not particularly limited, and can be implemented in various forms such as powder, plate, thin film, lump, and line. In any form, it is sufficient that a part of the surface of metal or the like is covered with palladium. As a coating mode, a mode in which palladium is dispersed and coated on the surface of the metal or the like is desirable from the viewpoint of increasing the boundary between the metal or the like and palladium and promoting the reaction between the metal or the like and hydrogen. The covering ratio is preferably 10% or more and 90% or less of the entire surface area of the metal or alloy. This is because when the coating ratio is less than 10% of the entire surface area of the metal or the like, the oxidation of the metal or the like in the atmosphere cannot be sufficiently suppressed. It is more preferable that the coating ratio is 20% or more. Further, when the covering ratio exceeds 90% of the entire surface area of metal or the like, hydrogen cannot be sufficiently occluded under a hydrogen pressure lower than the equilibrium hydrogen pressure of palladium. It is more preferable that the coating ratio is 80% or less. In particular, it is preferable that the covering ratio is about 50%.

  As a preferred embodiment of the hydrogen storage material of the present invention, a metal or alloy is in the form of a thin film, and a thin film of palladium is formed on the surface of the metal or alloy thin film to form an island shape. In FIG. 1, the cross-sectional schematic diagram of the hydrogen storage material which is an example of this aspect is shown. As shown in FIG. 1, the hydrogen storage material 1 includes a vanadium thin film 2 and a palladium thin film 3. The vanadium thin film 2 is formed on the upper surface of the glass substrate 4. The film thickness of the vanadium thin film 2 is 50 nm. The palladium thin film 3 is formed on the upper surface of the vanadium thin film 2 by being dispersed in an island shape. The average thickness of the palladium thin film 3 is 2 nm over the entire surface of the vanadium thin film 2.

  In this embodiment, hydrogen molecules are adsorbed on the boundary 5 between the vanadium thin film 2 and the palladium thin film 3 and dissociated into hydrogen atoms. Thereafter, hydrogen atoms diffuse into the vanadium thin film 2. Further, hydrogen atoms adsorbed and dissociated on the surface of the palladium thin film 3 diffuse on the surface of the palladium thin film 3 and diffuse into the vanadium thin film 2 from the boundary 5. Thus, the hydrogen storage material of this aspect can store hydrogen at a hydrogen pressure lower than the equilibrium hydrogen pressure of the palladium thin film 3. Moreover, even when exposed to the atmosphere, the hydrogen storage capacity is not lost.

  In the case of the above aspect, it is desirable that the average film thickness of the palladium thin film on the surface of the metal or alloy thin film is 5 nm or less. This is because if the average film thickness exceeds 5 nm, the coverage ratio of palladium may exceed 90% of the entire surface area of the metal or the like. The average film thickness is more preferably 3 nm or less. Here, the “average film thickness” is a thin film thickness on the assumption that a palladium thin film formed in an island shape on the surface of a thin film of metal or the like is uniformly formed on the entire surface of the thin film of metal or the like. . In the present specification, a value measured using a crystal oscillator thickness meter is employed.

  What is necessary is just to employ | adopt the optimal method for the manufacturing method of the hydrogen storage material of this invention according to the form of the metal etc. which become a base material. For example, when the metal or the like is in a powder form, palladium particles may be attached to a part of the particle surface of the metal or the like by a method such as mechanical grinding. In addition, when the metal or the like is plate-like, thin-film-like, lump-like, or linear, palladium is applied to a part of the surface of the metal or the like by various methods such as thermal spraying, plating, vacuum deposition, sputtering, or ion plating. What is necessary is just to make it adhere. For example, in order to form a thin film of palladium in an island shape on the surface of a thin film of metal or the like, first, a thin film of metal or the like is formed on the surface of a substrate such as glass by sputtering or the like. Next, the palladium thin film may be dispersed in the form of islands by sputtering or the like on the surface of the metal thin film.

  Based on the above embodiment, a hydrogen storage material was produced in which a thin palladium film was dispersed in an island shape on the surface of a thin vanadium film. For comparison, a hydrogen storage material in which a palladium thin film was uniformly formed on the entire surface of the vanadium thin film was manufactured. Then, both hydrogen storage materials were once exposed to the atmosphere, then hydrogenated, and their hydrogen storage capacity was evaluated. Hereinafter, it demonstrates in order.

(1) Manufacture of hydrogen storage material (a) Hydrogen storage material of Example As shown in FIG. 1 above, a hydrogen storage material formed by dispersing a thin palladium film in an island shape on the surface of a thin film of vanadium. Manufactured. First, a thin film (film thickness 50 nm) of vanadium was formed on the surface of a glass substrate (length 5 mm × width 35 m × thickness 1 mm) by an RF-assisted magnetron sputtering method in an argon atmosphere. Next, a palladium thin film was dispersed in the form of islands on the surface of the vanadium thin film by the same method. The average film thickness of the formed palladium thin film was measured with a quartz oscillator film thickness meter and found to be 2 nm. The produced hydrogen storage material was used as the hydrogen storage material of the example. The hydrogen storage material including the glass substrate was used as Sample 1.

(B) Hydrogen Storage Material of Comparative Example A hydrogen storage material in which a palladium thin film was uniformly formed on the surface of a vanadium thin film was manufactured. First, a thin film of vanadium (thickness: 50 nm) was formed on the surface of a glass substrate similar to the above by RF-assisted magnetron sputtering in an argon atmosphere. Next, a palladium thin film was uniformly formed on the surface of the vanadium thin film by the same method. When the average film thickness of the formed palladium thin film was measured with a quartz oscillator film thickness meter, it was 20 nm. The produced hydrogen storage material was used as a hydrogen storage material of a comparative example. The hydrogen storage material including the glass substrate was used as Sample 2.

(C) Reference Example As a reference example, a vanadium thin film (film thickness 50 nm) and a palladium thin film (film thickness 20 nm) were formed on the surface of a glass substrate similar to the above. Both thin films were formed by RF-assisted magnetron sputtering in an argon atmosphere. The manufactured vanadium thin film was used as a sample 3 including a glass substrate. Similarly, a thin film of palladium was used as sample 4 including the glass substrate.

(2) Evaluation of hydrogen storage capacity (a) Electrical resistance measurement The said samples 1-4 were once taken out in air | atmosphere. And the electrode for the electrical resistance measurement by a 4-terminal method was attached to the surface of the hydrogen storage material or the thin film in each sample by dotite. The distance between the voltage measurement terminals was about 15 mm. Each sample to which the electrode was attached was attached to the chamber of the electrical resistance measuring apparatus, and then the inside of the chamber was evacuated to about 1.33 × 10 ⁻⁴ Pa by a rotary pump and a turbo molecular pump. Subsequently, hydrogen gas (purity 99.99999%) was introduced into the chamber so as to have a predetermined pressure. A constant current of 1 mA was applied to the sample in the chamber using a constant current power source. The voltage between the voltage measuring terminals was measured with a multimeter, and the electrical resistance was determined. The electrical resistance was determined after holding for 10 minutes under a predetermined hydrogen gas pressure. The pressure of hydrogen gas was increased in order from 2.66 × 10 ⁻⁴ Pa to 1.01 × 10 ⁵ Pa.

(B) Results Table 1 shows the values of electrical resistance at each hydrogen gas pressure. FIG. 2 shows the rate of change in electrical resistance at each hydrogen gas pressure. The rate of change in electrical resistance is a value obtained by dividing the electrical resistance value at each hydrogen gas pressure by the initial electrical resistance value before hydrogenation.

表１および図２に示すように、実施例の水素吸蔵材料（試料１）では、１．３３×１０〜１．３３×１０²Ｐａの水素ガス圧力で、電気抵抗値が急激に上昇した。電気抵抗値の上昇は、水素吸蔵材料が水素化されたことを示すものである。これより、実施例の水素吸蔵材料は、大気に曝されたにも関わらず、１．３３×１０〜１．３３×１０²Ｐａという極めて低い水素ガス圧力で、水素を吸蔵できることがわかる。一方、比較例の水素吸蔵材料（試料２）では、１．３３×１０³〜１．３３×１０⁴Ｐａの水素ガス圧力で、電気抵抗値が急激に上昇した。つまり、比較例の水素吸蔵材料は、実施例の水素吸蔵材料の約１００倍の水素ガス圧力でしか、水素を吸蔵できないことがわかる。なお、この水素ガス圧力は、後述するパラジウム薄膜の水素化圧力と、ほぼ同じである。

As shown in Table 1 and FIG. 2, in the hydrogen storage material (sample 1) of the example, the electrical resistance value rapidly increased at a hydrogen gas pressure of 1.33 × 10 to 1.33 × 10 ² Pa. An increase in the electrical resistance value indicates that the hydrogen storage material has been hydrogenated. From this, it can be seen that the hydrogen storage materials of the examples can store hydrogen at a very low hydrogen gas pressure of 1.33 × 10 to 1.33 × 10 ² Pa, despite being exposed to the atmosphere. On the other hand, in the hydrogen storage material of the comparative example (sample 2), the electrical resistance value increased rapidly at a hydrogen gas pressure of 1.33 × 10 ^{3 to} 1.33 × 10 ⁴ Pa. That is, it can be seen that the hydrogen storage material of the comparative example can only store hydrogen at a hydrogen gas pressure of about 100 times that of the hydrogen storage material of the example. This hydrogen gas pressure is substantially the same as the hydrogenation pressure of the palladium thin film described later.

In the palladium thin film (sample 4) which is a reference example, the electrical resistance value rapidly increased at a hydrogen gas pressure of 1.33 × 10 ^{3 to} 6.55 × 10 ³ Pa. That is, it can be seen that the palladium thin film is hydrogenated at the hydrogen gas pressure. Further, in the vanadium thin film (sample 3) as a reference example, the value of electric resistance hardly changed even when the hydrogen gas pressure was increased. It is considered that the vanadium thin film was oxidized by exposure to the atmosphere and became inactive against hydrogen.

  From the above, it was confirmed that the hydrogen storage material of the present invention can store hydrogen at a hydrogen pressure lower than the hydrogen pressure at which palladium is hydrogenated. Further, it was confirmed that the hydrogen storage material of the present invention does not lose its hydrogen storage ability even when exposed to the atmosphere.

The cross-sectional schematic diagram of the hydrogen storage material which is one Embodiment of this invention is shown. The rate of change in electrical resistance at each hydrogen gas pressure is shown.

Explanation of symbols

1: Hydrogen storage material 2: Vanadium thin film 3: Palladium thin film 4: Glass substrate 5: Boundary

Claims (6)

A hydrogen storage material in which a part of the surface of a metal or alloy having a hydride phase having a hydride generation heat negatively larger than −45 kJ / mol H ₂ is coated with palladium.   2. The hydrogen storage material according to claim 1, wherein a coverage ratio of the palladium is 10% or more and 90% or less of the entire surface area of the metal or alloy. The metal or alloy is in the form of a thin film,
The hydrogen storage material according to claim 1, wherein the palladium thin film is formed in an island shape on the surface of the metal or alloy thin film.   The hydrogen storage material according to claim 3, wherein an average film thickness of the palladium thin film on the surface of the metal or alloy thin film is 5 nm or less.   The hydrogen storage material according to claim 1, wherein the metal is any one of magnesium, titanium, and vanadium.   The hydrogen storage material according to claim 1, wherein the alloy is an alloy containing at least one of magnesium, titanium, and vanadium.

Images