Hydrogen gas production method and hydrogen gas production device

Abstract

To provide hydrogen gas with improved production efficiency.SOLUTION: A hydrogen gas production method involves salty water being irradiated with a pulsed laser 20 having a pulse width of 1 picosecond or less to produce hydrogen gas from the water. A hydrogen gas may be produced from water by storing water in a cylindrical container made of a material transparent to a pulsed laser 20 having a pulse width of 1 picosecond or less and applying the pulsed laser 20 from outside the cylindrical container toward water in the cylindrical container.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method and an apparatus for producing hydrogen gas. [Background technology]

[0002] In recent years, hydrogen has been attracting attention as an energy source that does not emit greenhouse gases such as carbon dioxide. In particular, hydrogen produced without emitting carbon dioxide during the manufacturing process is called "green hydrogen" and is attracting attention as an energy source with a low environmental impact. Green hydrogen is produced, for example, by electrolyzing water using electricity generated from natural energy sources such as solar and wind power. One proposed method for producing hydrogen is to irradiate water with a femtosecond laser and multiphoton ionize the water molecules to produce hydrogen gas. [Prior art documents] [Patent documents]

[0003] [Non-Patent Document 1] HK Pawlak et. al, "Hydrogen Production in Liquid Water by Femtosecond Laser-Induced Plasma", Appl. Energy 2019, 247, 24-31. Summary of the Invention [Problem to be solved by the invention]

[0004] There is a demand for increasing the efficiency of hydrogen gas production.

[0005] The present disclosure has been made in view of these problems, and one of its exemplary purposes is to provide a technique for increasing the efficiency of hydrogen gas production. [Means for solving the problem]

[0006] A method for producing hydrogen gas according to an embodiment of the present disclosure involves irradiating salty water with a pulsed laser having a pulse width of 1 picosecond or less to produce hydrogen gas from the water.

[0007] Another aspect of the hydrogen gas production method of the present disclosure involves placing water in a cylindrical container made of a material transparent to a pulsed laser having a pulse width of 1 picosecond or less, irradiating the water in the cylindrical container with a pulsed laser from outside the cylindrical container, and generating hydrogen gas from the water.

[0008] A hydrogen gas production apparatus according to yet another aspect of the present disclosure includes a laser light source that generates a pulsed laser having a pulse width of 1 picosecond or less, a cylindrical container made of a material that is transparent to the pulsed laser, and a lens that focuses the pulsed laser toward water contained in the cylindrical container.

[0009] Any combination of the above components, or mutual substitution of the components or expressions of the present disclosure between methods, systems, etc., are also valid aspects of the present disclosure. [Effects of the Invention]

[0010] According to the present disclosure, the efficiency of generating hydrogen gas can be improved. [Brief explanation of the drawings]

[0011] [Figure 1] 1(a) and 1(b) are diagrams schematically showing the configuration of a hydrogen gas production device according to the first embodiment. [Figure 2] 1 is a graph showing the relationship between the salinity concentration of raw water and the amount of hydrogen gas produced. [Figure 3] 3(a) to 3(e) are images of gas bubbles generated from raw water irradiated with a pulsed laser. [Figure 4] 10 is a graph showing the relationship between pulse laser irradiation time and the amount of hydrogen gas generated. [Figure 5] 10 is a graph showing the relationship between the lens numerical aperture and the amount of hydrogen gas produced. [Figure 6]FIG. 4 is a diagram schematically illustrating the configuration of a hydrogen gas production device according to a second embodiment. [Figure 7] FIG. 10 is a diagram schematically illustrating the configuration of a hydrogen gas production device according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Before describing the present disclosure in detail, a brief overview will be provided. -12 This technology involves irradiating water with a pulsed laser having a pulse width of 1 picosecond or less to generate hydrogen gas from water. Pulsed lasers with a pulse width of 1 picosecond or less are generally called femtosecond lasers. By irradiating water molecules with a femtosecond laser, hydrogen gas can be generated through multiphoton ionization of the water molecules.

[0013] In this disclosure, salty water is used as the raw material for generating hydrogen gas. One example of salty water is seawater. By irradiating seawater with a femtosecond laser, the amount of hydrogen gas generated can be improved compared to irradiating ultrapure water containing no impurities with a femtosecond laser. Here, "ultrapure water" refers to water with extremely high purity, with a resistivity of 18.2 MΩ·cm or higher.

[0014] In this disclosure, water is placed in a cylindrical container made of a material transparent to the femtosecond laser, and a femtosecond laser is irradiated from outside the cylindrical container toward the water inside the cylindrical container. The femtosecond laser is focused by a lens outside the cylindrical container, and is further focused by refraction on the surface of the cylindrical container and then focused onto the water inside the cylindrical container. By using a cylindrical container to focus the femtosecond laser, the amount of hydrogen gas produced can be improved compared to using a container with a flat laser incident surface.

[0015] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description, the same elements are designated by the same reference numerals, and redundant description will be omitted as appropriate. Furthermore, to facilitate understanding of the description, the dimensional ratios of the components in the drawings do not necessarily correspond to the actual dimensional ratios.

[0016] (First embodiment) 1(a) and 1(b) are diagrams schematically illustrating the configuration of a hydrogen gas production device 10 according to a first embodiment. Fig. 1(a) is a side view, and Fig. 1(b) is a top view. The hydrogen gas production device 10 includes a laser light source 12, a reaction vessel 14, and a lens 16.

[0017] 1(a) and 1(b), the irradiation direction of the pulse laser 20 is the x direction, the extension direction of the reaction vessel 14 is the z direction, and the direction perpendicular to the x direction and the z direction is the y direction.

[0018] The laser light source 12 generates a pulsed laser 20 having a pulse width of 1 picosecond or less. The pulsed laser 20 is, for example, a femtosecond laser having a pulse width of 10 femtoseconds or more and 500 femtoseconds or less. The center wavelength of the pulsed laser 20 is not particularly limited, but is, for example, 200 nm or more and 2000 nm or less. The repetition frequency of the pulsed laser 20 is, for example, 10 Hz or more and 1 MHz or less, for example, 100 Hz or more and 10 kHz or less. The pulse energy of the pulsed laser 20 is, for example, 0.1 mJ or more and 10 mJ or less.

[0019] The laser light source 12 is, for example, a femtosecond laser light source that uses titanium-doped sapphire crystal as a laser medium. The laser light source 12 includes an oscillator that generates a femtosecond laser beam and a regenerative amplifier that amplifies the femtosecond laser beam output from the oscillator. The pulsed laser 20 generated by the laser light source 12 has, for example, a center wavelength of 810 nm, a repetition frequency of 1 kHz, a pulse width of 130 femtoseconds, and pulse energy of 0.1 mJ or more and 1 mJ or less.

[0020] The reaction vessel 14 is a vessel that contains raw water 18. The reaction vessel 14 is made of a material that is transparent to the pulsed laser 20, for example, a glass material such as borosilicate glass or fused silica. The reaction vessel 14 may be made of a transparent material that transmits only a portion of the pulsed laser 20, or may be made of a transparent material entirely. The reaction vessel 14 is preferably a cylindrical vessel, and the incident surface of the pulsed laser 20 is preferably a cylindrical surface or a convex curved surface. The outer diameter of the cylindrical vessel is 1 mm or more and 100 mm or less, for example, 5 mm or more and 50 mm or less.

[0021] The lens 16 focuses the pulsed laser 20 output from the laser light source 12. The lens 16 is disposed outside the reaction vessel 14 so that the focal point of the lens 16 is located inside the reaction vessel 14. The lens 16 is, for example, an objective lens, and preferably has a numerical aperture (NA) of 0.28 or more and 0.42 or less.

[0022] Raw water 18 is water that serves as a raw material for generating hydrogen gas. Raw water 18 may be ultrapure water that does not contain impurities, but is preferably water that contains salt, such as seawater. The salt concentration of raw water 18 is, for example, 0.1% by weight or more and 8% by weight or less, and preferably 0.3% by weight or more and 4% by weight or less. The salt concentration of raw water 18 may be the same as the salt concentration of ordinary seawater (3.4% by weight).

[0023] Raw water 18 contains salts such as chlorides, bromides, iodides, sulfates, bicarbonates, and borates of metals such as sodium (Na), calcium (Ca), potassium (K), magnesium (Mg), strontium (Sr), lithium (Li), aluminum (Al), iron (Fe), cobalt (Co), and manganese (Mn). Raw water 18 contains salts such as sodium chloride (NaCl), magnesium chloride (MgCl), calcium chloride (CaCl), sodium sulfate (NaSO), potassium chloride (KCl), sodium bicarbonate (NaHCO), potassium bromide (KBr), sodium borate (NaBO), strontium chloride (SrCl), sodium fluoride (NaF), lithium chloride (LiCl), and potassium iodide (KI). The salts contained in raw water 18 may be any one of these or a combination of two or more of these. The raw water 18 may have the same salt composition as seawater, or may be seawater itself.

[0024] When a pulsed laser 20 is focused by a lens 16 and irradiated onto the raw water 18 in the reaction vessel 14, a filament 22 extending in the beam irradiation direction (x direction) is generated near the focal point. 13 W / cm 2 The beam self-converges due to the nonlinear optical effect (Kerr effect) caused by the above, and a filament 22 is generated that extends in the beam irradiation direction with a constant spot diameter without expanding. The length of the filament 22 in the beam irradiation direction is, for example, about 300 μm to 500 μm.

[0025] In the filament 22, the bonds of water molecules are broken by multiphoton ionization and tunnel ionization due to the extremely high laser light intensity, and hydrogen radicals (H * ) is generated, and hydrogen gas (H2) is generated by combining with hydrogen radicals. Also, hydroxyl radicals (OH * Oxygen gas (O2) is also produced due to the generation of hydrogen and oxygen gases. In the vicinity of the filament 22, bubbles 24 containing hydrogen and oxygen gases are generated.

[0026] If raw water 18 contains salt, the amount of hydrogen gas produced increases due to the nonlinear optical effect caused by the metal components of the salt. However, if the salt concentration is too high, the amount of hydrogen gas produced decreases due to the ion suppression effect caused by metals with low ionization potential (Na, Mg, Ca, etc.). If raw water 18 contains salt, trace amounts of hydrogen chloride gas (HCl) are detected, but harmful gases such as chlorine gas (Cl2) and hypochlorous acid (HClO) are not detected. The amount of hydrogen chloride gas produced is about five orders of magnitude smaller than the amounts of hydrogen gas and oxygen gas produced.

[0027] FIG. 2 is a graph showing the relationship between the salinity of the raw water 18 and the amount of hydrogen gas produced. (a) shows the case where ultrapure water containing no salt was used, and (b) to (e) show the cases where water containing salt was used. (d) is artificial seawater, which has a salinity of 3.4% by weight, the same as seawater, and has the same salt composition as seawater. (b) is the artificial seawater of (d) diluted to 1 / 10 its salinity, containing 0.34% by weight. (c) is the artificial seawater of (d) diluted to 1 / 5 its salinity, containing 0.68% by weight. (e) is the artificial seawater of (d) concentrated to 2.5 times its salinity, containing 8.5% by weight.

[0028] As shown in FIG. 2, using salty water (b) to (d) increases the amount of hydrogen gas produced compared to using ultrapure water (a). For the salt concentrations (b) to (d), the amount of hydrogen gas produced increases as the salt concentration increases. On the other hand, using concentrated seawater (e) decreases the amount of hydrogen gas produced compared to using ultrapure water (a). Therefore, when using salty water as raw water 18, it is preferable to use a salt concentration lower than that of (e), and the salt concentration of raw water 18 is preferably 0.1% by weight or more and 8% by weight or less. Furthermore, based on the results of (b) to (d), it is preferable to use a salt concentration of raw water 18 of 0.3% by weight or more and 4% by weight or less, and particularly a salt concentration of 3% by weight or more and 4% by weight or less, similar to that of seawater, is preferable.

[0029] 3(a) to 3(e) are images of gas bubbles generated from raw water 18 irradiated with pulsed laser 20, captured using a high-speed camera at 1,000 frames per second. Figures 3(a) to 3(e) show raw water 18 with different salt concentrations, corresponding to graphs (a) to (e) in FIG. 2, respectively. In other words, FIG. 3(a) shows ultrapure water containing no salt, FIG. 3(b) shows seawater diluted 1 / 10, FIG. 3(c) shows seawater diluted 1 / 5, FIG. 3(d) shows undiluted and unconcentrated seawater, and FIG. 3(e) shows seawater concentrated 2.5 times.

[0030] 3(b) to (d), in the case of water containing a low concentration of salt, relatively large bubbles are present in a densely packed state. The reason for this is thought to be that the pulsed laser 20 is more strongly focused due to the nonlinear optical effect caused by the metal components of the salt, causing bubbles to be generated in a densely packed state in the filament 22. The generation of many bubble nuclei in a densely packed state allows the many bubble nuclei to coalesce and grow into larger bubbles, which is thought to ultimately lead to an increase in the amount of extractable hydrogen gas produced.

[0031] In the case of ultrapure water in Figure 3(a), small bubbles are dispersed over a wide area. This is thought to be because, in the case of ultrapure water, there is no contribution from the nonlinear optical effect caused by the metal components of the salt, so bubble nuclei are less likely to form densely and grow into large bubbles.

[0032] In the case of water with a high salt content, as shown in Figure 3(e), small bubbles are densely packed together. This is thought to be because, when the water contains a high salt content, bubbles are generated more densely in the filament 22, while the ion suppression effect reduces the amount of bubble nuclei generated.

[0033] 4 is a graph showing the relationship between the irradiation time of the pulsed laser 20 and the amount of hydrogen gas produced. Line A shows the case where a cylindrical container with an outer diameter of 10 mm was used as the reaction container 14 and seawater was used as the raw water 18. Line B shows the case where a cylindrical container with an outer diameter of 10 mm was used as the reaction container 14 and ultrapure water was used as the raw water 18. Line C shows the case where a rectangular container with a flat incident surface for the pulsed laser 20 was used as the reaction container 14 and ultrapure water was used as the raw water 18.

[0034] As shown in the graph of FIG. 4, the amount of hydrogen gas produced in the example of line B is 2.5 times greater than that in the comparative example of line C. This shows that the amount of hydrogen gas produced can be improved by using a cylindrical container as the reaction container 14. Furthermore, the amount of hydrogen gas produced in the example of line A is 1.3 times greater than that in the example of line B. This shows that the amount of hydrogen gas produced can be improved by using water containing salt as the raw water 18. Furthermore, since the amount of hydrogen gas produced increases depending on the irradiation time of the pulsed laser 20, hydrogen gas can be produced continuously by continuously irradiating the pulsed laser 20.

[0035] Figure 5 is a graph showing the relationship between the numerical aperture of the lens 16 and the amount of hydrogen gas produced. Figure 5 shows a case where a cylindrical container with an outer diameter of 10 mm is used as the reaction container 14 and ultrapure water is used as the raw water 18. As shown in Figure 5, the amount of hydrogen gas produced can be improved by setting the numerical aperture of the lens 16 to be between 0.28 and 0.42. By appropriately setting the numerical aperture of the lens 16, the shape and size of the filament 22 can be optimized, and the amount of hydrogen gas produced can be improved.

[0036] According to this embodiment, the amount of hydrogen gas produced can be improved by using water containing salt as the raw water 18. Even if the raw water 18 contains salt, harmful gases such as Cl2 and HClO are not produced, so when seawater, which is inexhaustible on Earth, is used as the raw material, hydrogen gas can be produced without desalination. This is in contrast to the electrolysis of seawater, which produces Cl2.

[0037] According to this embodiment, by using a cylindrical container as the reaction container 14 that contains the raw water 18, the amount of hydrogen gas produced can be improved compared to when a rectangular container is used. In addition, by setting the numerical aperture of the lens 16 that focuses the pulsed laser 20 on the raw water 18 to be 0.28 or more and 0.42 or less, the amount of hydrogen gas produced can be improved.

[0038] 6 is a diagram schematically illustrating the configuration of a hydrogen gas production device 10A according to the second embodiment. The hydrogen gas production device 10A includes a laser light source 12, a reaction vessel 14, a lens 16, a tank 30, a return flow path 32, and a gas recovery device 34. The laser light source 12, the reaction vessel 14, and the lens 16 are configured in the same manner as in the first embodiment. The following description of the second embodiment will focus on the differences from the first embodiment, and descriptions of the commonalities with the first embodiment will be omitted as appropriate.

[0039] The tank 30 stores raw water 18 and supplies the raw water 18 to the reaction vessel 14. The reaction vessel 14 is provided above the tank 30 in the vertical direction (gravity direction, z direction) and extends vertically. The return flow path 32 extends from the top of the reaction vessel 14 toward the bottom of the tank 30, and is configured to return the raw water 18 that has passed through the reaction vessel 14 to the tank 30.

[0040] The gas recovery device 34 recovers the bubbles 24 generated in the reaction vessel 14. The gas recovery device 34 recovers the gas through a gas recovery port 36 that opens at the top of the reaction vessel 14. The gas recovery device 34 may be equipped with a gas separation device that separates hydrogen gas from the recovered gas.

[0041] In the reaction vessel 14, a vertically upward flow is generated in the raw water 18 due to the buoyancy of the bubbles 24. Due to the vertically upward flow generated in the reaction vessel 14, raw water 18 is supplied from the tank 30 to the reaction vessel 14, as shown by arrow F1. Furthermore, the raw water 18 located at the top of the reaction vessel 14 flows through the return flow path 32, as shown by arrow F2, and returns to the tank 30, as shown by arrow F3. The raw water 18 stored in the tank 30 is circulated due to the buoyancy of the bubbles 24, so that fresh raw water 18 can be continuously supplied to the reaction vessel 14. The pulsed laser 20 is irradiated toward the raw water 18, in which a flow is generated due to the buoyancy of the bubbles 24 generated from the raw water 18.

[0042] According to this embodiment, the raw water 18 can be circulated by the buoyancy of the bubbles 24 without using a power source such as a pump, thereby reducing the energy required to generate hydrogen gas. Furthermore, by continuously supplying fresh raw water 18 to the reaction vessel 14, hydrogen gas can be generated efficiently and continuously.

[0043] 7 is a diagram schematically illustrating the configuration of a hydrogen gas production apparatus 10B according to a third embodiment. The hydrogen gas production apparatus 10B includes a laser light source 12, a reaction vessel 14, a lens 16, and a spatial light modulator 40. The third embodiment differs from the first embodiment in that the third embodiment further includes the spatial light modulator 40. The following description of the third embodiment will focus on the differences from the first embodiment, and descriptions of the commonalities with the first embodiment will be omitted as appropriate.

[0044] The spatial light modulator 40 is disposed between the laser light source 12 and the lens 16. The spatial light modulator 40 is configured to modulate the beam profile of the pulsed laser 20 output from the laser light source 12. The pulsed laser 20, whose beam profile has been adjusted by the spatial light modulator 40, is focused by the lens 16 toward the raw water 18.

[0045] The spatial light modulator 40 modulates the beam profile of the pulsed laser 20 so that, for example, multiple beams 42 are focused from the lens 16 toward the raw water 18. In this case, filaments 22 are formed at multiple focusing positions within the reaction vessel 14. By forming the filaments 22 at multiple focusing positions, multiple sources of hydrogen gas can be generated, thereby improving the amount of hydrogen gas produced.

[0046] The spatial light modulator 40 may modulate the beam profile of the pulsed laser 20 so that the focal position in the reaction vessel 14 varies over time. In this case, it becomes possible to irradiate the next pulsed laser 20 at a location different from the location where the bubbles 24 have been generated by irradiation with the pulsed laser 20. This makes it possible to suppress a decrease in the amount of hydrogen gas produced due to irradiation of the bubbles 24 with the pulsed laser 20.

[0047] As a modification of the third embodiment, the pulsed laser 20 may be scanned using optical elements such as a mirror or a lens, so that the focusing position in the reaction vessel 14 varies over time. For example, a galvanometer mirror or a polygon mirror that scans the pulsed laser 20 and an fθ lens that focuses the scanned pulsed laser 20 toward the reaction vessel 14 may be used.

[0048] The present disclosure has been described above based on the embodiments. It will be understood by those skilled in the art that the present disclosure is not limited to the above embodiments, that various design changes are possible, and that various modifications are possible, and that such modifications are also within the scope of the present disclosure. [Explanation of symbols]

[0049] 10...hydrogen gas production device, 12...laser light source, 14...reaction vessel, 16...lens, 18...raw material water, 20...pulse laser, 22...filament, 24...gas bubble, 30...tank, 32...return flow path, 34...gas recovery device, 36...gas recovery port, 40...spatial light modulator.

Claims

1. Storing water in a cylindrical container made of a material transparent to a pulsed laser having a pulse width of 1 picosecond or less; irradiating the pulsed laser from outside the cylindrical container toward the water in the cylindrical container to generate hydrogen gas from the water, The diameter of the cylindrical container is 1 mm or more and 100 mm or less, A method for producing hydrogen gas, wherein the pulsed laser is focused onto the water by a lens having a numerical aperture of 0.28 or more and 0.42 or less and the cylindrical container.

2. A hydrogen gas production method as described in claim 1, wherein the diameter of the cylindrical container is 5 mm or more and 50 mm or less.

3. A method for producing hydrogen gas as described in claim 1 or 2, wherein the water contains salt.

4. The method for producing hydrogen gas according to claim 1 , wherein the water includes seawater.

5. 5. The method for producing hydrogen gas according to claim 3, wherein the salinity of the water is 0.1% by weight or more and 8% by weight or less.

6. 5. The method for producing hydrogen gas according to claim 3, wherein the salinity of the water is 0.3% by weight or more and 4% by weight or less.

7. The method further comprises generating a flow from the bottom to the top of the cylindrical container by the buoyancy of bubbles generated from the water, and circulating the water in the cylindrical container through a return flow path from the top to the bottom of the cylindrical container; The method for producing hydrogen gas according to claim 1 , wherein the pulsed laser is irradiated onto the water in which a flow is occurring due to the circulation.

8. The method for producing hydrogen gas according to claim 1 , wherein the pulsed laser is irradiated onto the water while varying a focal position over time.

9. a laser light source that generates a pulsed laser having a pulse width of 1 picosecond or less; a cylindrical container made of a material transparent to the pulsed laser; a lens that focuses the pulsed laser toward the water contained in the cylindrical container, The diameter of the cylindrical container is 1 mm or more and 100 mm or less, the numerical aperture of the lens is equal to or greater than 0.28 and equal to or less than 0.42; A hydrogen gas production device in which the pulsed laser is focused onto the water by the lens and the cylindrical container.

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