CN117089749A - A nano-K2MgF4 catalyzed high-capacity Mg-Cu-Sr-based hydrogen storage alloy and its preparation method - Google Patents

Abstract

The invention relates to the technical field of solid hydrogen storage alloy materials. It provides a nano-K ₂ MgF ₄ catalyzed high-capacity Mg-Cu-S-based hydrogen storage alloy powder for fuel cells containing a small amount of catalyst. The chemical formula of the alloy is: Mg ₉₅ Cu _5‑x Sr _x +y(wt)%K ₂ MgF ₄ , where x is the atomic ratio, 0<x≤2, y is the percentage of K ₂ MgF ₄ in the alloy, 2≤y≤8. The preparation method of the alloy powder is to use induction heating and smelting in a helium atmosphere, and inject the molten alloy into a copper casting mold to obtain an as-cast master alloy ingot. The ingot is loaded into a quartz tube, and after induction heating and melting, it is continuously sprayed on the surface of the rotating water-cooled copper roller through a slit nozzle at the bottom of the quartz tube to obtain a rapidly cooled alloy; the rapidly quenched alloy thin strip is mechanically crushed and mixed with a small amount of The catalyst K ₂ MgF ₄ is mixed and ball milled to obtain alloy powder with a nanocrystalline-amorphous structure. Alloy powder can significantly reduce the thermal stability of magnesium hydride, improve the hydrogen absorption and release thermodynamics and kinetics of the alloy, and meet the fuel cell's solid-state hydrogen storage performance requirements.

Description

A nano-K2MgF4 catalyzed high-capacity Mg-Cu-Sr-based hydrogen storage alloy and its preparation method

Technical field

The invention belongs to the technical field of solid hydrogen storage alloy materials, and in particular provides a high-capacity Mg-Cu-Sr-based hydrogen storage alloy powder for nano-K ₂ MgF ₄ catalyzed fuel cells and a preparation method thereof.

Background technique

The content described in the background art reflects the inventor's process of discovering and analyzing problems and should not necessarily be regarded as prior art.

The application of hydrogen fuel cells in the vehicle field has attracted great attention from all over the world. An efficient and safe hydrogen supply system is one of the key technical keys to fuel cells. At present, most fuel cell hydrogen supply systems use high-pressure gaseous hydrogen storage. Due to the excessive technical requirements for pressure vessels and users' concerns about the safety of high-pressure systems, researchers are forced to develop safer and more reliable hydrogen supply systems. Researchers have turned their attention to low-pressure solid-state hydrogen storage materials and have found that a variety of solid-state hydrogen storage materials have potential for fuel cell applications. Among them, the rare earth-based AB ₅ hydrogen storage material has good hydrogen absorption and desorption kinetics and can meet the requirements of fuel cells. However, this alloy system limits its use in fuel cells due to its low theoretical hydrogen storage capacity (1.4%). Applications. Although the theoretical capacity (1.86%) of Ti Fe-based hydrogen storage materials is higher than that of AB ₅ alloy, it still cannot meet the requirements of vehicle fuel cells.

Magnesium and magnesium-based alloys have attracted much attention from researchers due to their extremely high hydrogen storage capacity (7.6%). At the same time, magnesium has abundant reserves and low cost. In terms of hydrogen storage capacity, it fully meets the requirements of vehicle-mounted fuel cells. . However, the thermal stability of magnesium hydride is extremely high, and its decomposition temperature is >300°C. Moreover, the formation and decomposition kinetics of hydride are extremely poor, making the hydrogen absorption and desorption kinetics unsatisfactory.

Contents of the invention

The object of the present invention is to provide a solid-state hydrogen storage alloy for Mg-Cu-Sr-based fuel cells with high capacity and excellent hydrogen absorption and release kinetic properties and a preparation method thereof. Through the present invention, the hydrogen storage performance of the magnesium-based alloy is improved. been greatly improved. This provides a nanocrystalline-amorphous Mg-Cu-Sr-based hydrogen storage alloy with high hydrogen storage capacity and excellent dynamic properties and a corresponding preparation process. The present invention achieves the above objectives through the following technical route.

A first aspect of the invention provides a multi-component Mg-Cu-Sr-based hydrogen storage alloy for fuel cells. The alloy contains elements Cu and Sr, a small amount of K ₂ MgF ₄ catalyst, and its composition is Mg ₉₅ Cu _5-x Sr. _x +y wt.%K ₂ MgF ₄ , where x is the atomic ratio, 0<x≤2, y is the percentage of K ₂ MgF ₄ in the alloy, 2≤y≤8, preferably x=1.5, y= 5.

Another aspect of the present invention provides a method for preparing a hydrogen storage alloy for fuel cells. The preparation steps include:

A Batching: Batching according to the chemical composition formula Mg ₉₅ Cu _5-x Sr _x (x=0,0.5,1,1.5,2), where x is the atomic ratio. Among them, when the ingredients are formulated according to the chemical composition formula, the Mg element increases the burning loss by 8%, and the purity of the raw materials is greater than 99.6%.

B Smelting: Place the prepared raw materials in order in a magnesium oxide crucible, evacuate to 1×10 ^-2 -5×10 ^{- 4} Pa, and introduce 0.01-0.1MPa inert gas as a protective gas. The protective gas is Pure helium or helium+argon mixed gas, the volume ratio of the mixed gas is about 1:1. Use conventional heating methods, such as arc melting, induction heating melting or other heating methods. The heating conditions are: heating temperature 1400-1550°C to obtain molten liquid master alloy, keep it in the molten state for 3-5 minutes; then melt it The alloy is poured into a water-cooled copper mold to obtain an as-cast master alloy ingot.

C quick quenching: Place the alloy ingot prepared in step B above into a quartz tube with a slit at the bottom, use induction heating under helium protection until the alloy ingot is completely melted, and use the pressure of the protective gas to remove it from the bottom of the quartz tube. The slit is ejected and falls on the surface of a water-cooled copper roller rotating at a linear speed of 20m/s, and a fast-quenching alloy sheet with a thickness of about 50-100μm is obtained.

D mechanical ball milling: After mechanically crushing _the quick- _quenching Mg ₉₅ Cu _{5-x Sr} , ball milling in an all-round planetary high-energy ball mill for 2-5 hours (excluding downtime), the ball-to-material ratio is 1:20, and the rotation speed is 350 rpm. During the ball milling process, the ball mill should be shut down for 0.5 hours every hour of operation to prevent the temperature of the ball mill tank and abrasive from being too high.

E. Use XRD to test the phase structure of the ball-milled powder, use high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) to observe the morphology and microstructure of the ball-milled alloy particles, and use selected area electron diffraction (SAED) to determine the crystalline state of the alloy. Semi-automatic Si everts equipment was used to test the solid-state hydrogen storage capacity and hydrogen absorption and release kinetics of alloy powders. The hydrogen absorption and release temperature is 280°C, the initial hydrogen pressure of hydrogen absorption is 3MPa, and the hydrogen release is carried out at a pressure of 1×10 ^-4 MPa.

The beneficial effects of the present invention are:

In terms of alloy materials, the present invention adds Cu and Sr for alloying. Mg and Cu form the Mg ₂ Cu phase, and Sr and Cu form the CuSr phase and Cu ₅ Sr phase. The intermetallic compounds formed can significantly improve the hydrogen absorption and release of the magnesium alloy. Thermodynamic and kinetic properties. A special microstructure with nanocrystalline + amorphous is obtained through rapid quenching treatment. This structure has obvious advantages in improving the hydrogen absorption and release kinetic properties of Mg-based alloys. Adding a trace amount of nanometer K ₂ MgF ₄ catalyst, the catalyst is evenly distributed on the surface of the alloy particles through mechanical ball milling. Because K ₂ MgF ₄ has high activity, it has a very beneficial effect on improving the hydrogen absorption and release thermodynamics of magnesium-based alloys. The hydrogen storage alloy powder prepared in this way not only has high hydrogen absorption and release capacity and excellent hydrogen absorption and release kinetics, but also has good hydrogen absorption and release cycle stability, and is very promising to become a hydrogen supply carrier for hydrogen fuel cells.

Description of the drawings

Figure 1 is a diagram of the as-cast alloy ingot of Example 1;

Figure 2 is a diagram of the rapidly quenched alloy thin strip of Example 1;

Figure 3 XRD pattern of the as-cast alloy of Examples 1-5;

Figure 4 is the SEM morphology of the ball-milled powder of Examples 1-4;

Figure 5 is the XRD pattern of the ball-milled powder of Example 1-8;

Figure 6 is the HRTEM morphology of the ball-milled alloy of Examples 1-4.

Detailed ways

The embodiments of the present application will be described in detail below with reference to the drawings and examples, so that the implementation process of how the present application applies technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly.

The present invention finds through research that the multiphase structure can increase the density of grain boundaries and phase boundaries, improve the diffusion ability of hydrogen atoms, and shorten the diffusion distance of hydrogen atoms, thereby improving the kinetics of hydride formation and decomposition. At the same time, alloys containing multiple components can significantly reduce the thermal stability of hydrides and improve the hydrogen release thermodynamic properties of the alloy. In terms of alloy composition design, adding a small amount of Cu and Sr can form Mg ₂ Cu, CuSr and Cu ₅ Sr phases, which can significantly improve the hydrogen absorption and release kinetic properties of the magnesium-based alloy, and improve the alloy's absorption and release properties to a certain extent. Hydrogen release thermodynamic properties. At the same time, adding highly active catalyst K ₂ MgF ₄ can form a highly active coating layer on the surface of magnesium alloy particles through mechanical ball milling, reducing the thermal stability of magnesium hydride and improving the hydrogen absorption and release kinetic properties of the alloy. Research shows that improving the microstructure of the alloy and increasing crystal defects can significantly improve the hydrogen absorption and release kinetic properties of the alloy. In particular, the formation of nanocrystals and a small amount of amorphous structures in the alloy is extremely beneficial to improving the hydrogen absorption and release kinetics of the alloy. Increasing the density of crystal defects, especially dislocations and stacking faults, is extremely beneficial to the nucleation and growth of hydrides, thus improving the hydrogen absorption and release kinetics of the alloy.

In terms of preparation process, the master alloy is first rapidly quenched to obtain a nanocrystalline + amorphous structure and to form quick-quenching crystal defects in the alloy. Studies have shown that crystal defects formed by quick quenching are more stable than ball milling defects. properties, which is beneficial to improving the hydrogen absorption and release cycle stability of the alloy. Adding a small amount of catalyst to the rapidly quenched alloy and performing ball milling can improve the surface properties of the alloy, increase defects on the alloy surface, and further improve the hydrogen absorption and release thermodynamics and kinetic properties of the alloy.

The present invention further explains the composition and preparation method of the magnesium-based hydrogen storage alloy for fuel cells involved in the present invention through the following examples.

The chemical formula of the magnesium-based hydrogen storage alloy for fuel cells of the present invention is: Mg ₉₅ Cu _5-x Sr _x +ywt.%K ₂ MgF ₄ , where x is the atomic ratio, 0<x≤2, and preferably x= 1.5. y is the percentage of K ₂ MgF ₄ in the alloy, 2≤y≤8, preferably y=5. wt.% means weight percentage.

The preparation method of the high-capacity hydrogen storage alloy for fuel cells of the present invention includes the following steps:

A. Make ingredients according to the chemical formula of Mg ₉₅ Cu _5-x Sr _x . Since magnesium is easy to volatilize, increase the burning loss by 8% when mixing;

B. Place the prepared raw materials in the magnesium oxide crucible, place the lump metal magnesium on the top of the crucible, and add other materials to the crucible in no particular order. Use induction heating for melting, evacuate to 1×10 ^-2 -5×10 ^-4 Pa, and then fill in 0.01-0.1MPa high-purity helium as a protective gas. In order to reduce costs, argon + helium mixed gas can also be used As a protective gas, the volume ratio of the mixed gas is about 1:1, and the melting temperature is 1400-1550°C. The temperature is adjusted depending on the composition of the alloy to ensure complete melting of the metal raw materials. After maintaining the temperature for 3-5 minutes, the molten alloy is poured into a water-cooled copper mold to obtain an as-cast master alloy ingot.

C. Put the ingot prepared in step B above into a quartz tube with a slit at the bottom, use medium frequency induction heating to completely melt the ingot, and use the pressure of the protective gas to eject the liquid alloy from the slit at the bottom, falling at a linear speed of On the surface of a water-cooled copper roller rotating at 20m/s, a quick-quenching alloy flake is obtained, with a thickness of approximately 50-100μm.

D. After _mechanically crushing _the quick-quenching Mg ₉₅ Cu _{5-x Sr} Ball milling in an all-round planetary high-energy ball mill takes 2-5 hours (excluding downtime); the ball-to-material ratio is 1:20, and the speed is 350 rpm. During the ball milling process, the machine should be shut down for 0.5 hours for every 1 hour of ball milling to prevent the temperature of the ball mill tank and abrasive from being too high. The optimal ball milling time is 3 hours, and the alloy powder described in the patent is obtained.

F. Use XRD to test the structure of the ball-milled powder, use high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) to observe the morphology and microstructure of the alloy particles before and after hydrogen absorption, and use selected area electron diffraction (SAED) to determine the crystal state of the alloy. Fully automatic Si everts equipment was used to test the solid-state hydrogen storage capacity and hydrogen absorption and release kinetics of alloy powders. The hydrogen absorption temperature is 280°C, the initial hydrogen pressure of hydrogen absorption is 3MPa, and the hydrogen release is performed at 280°C and 1×10 ^-4 MPa pressure.

The chemical components and proportions of specific embodiments of the present invention are selected as follows:

Example 1: Mg ₉₅ Cu _3.5 Sr _1.5 +5wt.%K ₂ MgF ₄

Example 2: Mg ₉₅ Cu ₃ Sr ₂ +2wt.%K ₂ MgF ₄

Example 3: Mg ₉₅ Cu ₄ Sr ₁ +4wt.%K ₂ MgF ₄

Example 4: Mg95Cu5+3wt.%K2MgF4

Example 5: Mg ₉₅ Cu _4.5 Sr _0.5 +7wt.%K ₂ MgF ₄

Example 6: Mg ₉₅ Cu _3.5 Sr _1.5 +8wt.%K ₂ MgF ₄

Example 7: Mg ₉₅ Cu _3.5 Sr _1.5 +3wt.%K ₂ MgF ₄

Example 8: Mg ₉₅ Cu _3.5 Sr _1.5 +6wt.%K ₂ MgF ₄

Bulk metal magnesium, metal copper and metal strontium are selected according to the chemical formula composition of each embodiment. The metal purity requirement is ≥99.5%. After the selected block metal is polished to remove the surface oxide layer, it is weighed according to the chemical dosage ratio. Among them, the proportion of metallic magnesium is increased by 8wt.% when mixing to compensate for the burning loss during melting; during the preparation process, the technical parameters of each stage are as follows: during induction heating, the vacuum is evacuated to 1×10 ^-2 -5×10 ^-4 Pa, Fill with high-purity helium gas with a pressure of 0.01 to 0.1MPa or helium + argon gas mixture with a volume ratio of 1:1; melting temperature is 1400-1550°C; vacuum to 10 ^-2 -10 ^- during rapid quenching and heating ⁴ Pa, the surface linear speed of the water-cooled copper roller is 20m/s. The quick-quenching flakes are mechanically crushed and passed through a 200-mesh sieve. They are mixed with the nano-K ₂ MgF ₄ catalyst and put into a stainless steel ball mill tank. They are ball milled with an all-round planetary ball mill for 2-5 hours. The ball mill is shut down for 0.5 hours every hour of operation. All process parameters can be appropriately selected within the above range to prepare the hydrogen storage alloy powder described in the patent. Therefore, although the present invention only cites a typical embodiment, this embodiment is applicable to preparation methods with different parameters.

Process technical parameters of Example 1: According to the chemical formula Mg ₉₅ Cu _3.5 Sr _1.5 , bulk metal magnesium, metal copper and metal strontium are selected. The purity of these metals is 99.8%, and they are weighed according to the chemical dosage ratio. Among them, 959.5 grams of metal magnesium, 85.6 grams of metal copper, and 28.0 grams of strontium metal are placed. The weighed bulk metal is placed in the magnesium oxide crucible of the medium frequency induction furnace, and then Cover the furnace cover, evacuate for about 30 minutes until the vacuum degree is above 5×10 ^-2 Pa, then fill in high-purity helium protective gas until the pressure reaches -0.04MPa, adjust the power to 5kW, and control the temperature at 650°C to make the metal Mg is melted, then the power is adjusted to 25kW and the temperature is controlled at 1550°C to melt metal copper and strontium. After maintaining the temperature in the molten state for 5 minutes, pour the molten liquid into the copper casting mold. When pouring into the ingot mold, adjust the power to 8kW. After cooling for 30 minutes under a helium protective atmosphere, it was released from the furnace to obtain a cylindrical ingot with a diameter of 30 mm.

Place about 100 grams of as-cast Mg ₉₅ Cu _3.5 Sr _1.5 alloy rod into a quartz tube with a diameter of 35mm and a slit of 0.05mm×20mm at the bottom; heat it to melt with a radio frequency of 245 kilohertz, and protect it under a helium atmosphere , the heating power is 15kW; the molten alloy is sprayed onto the surface of a water-cooled copper roller with a surface linear speed of 20m/s under a helium pressure of 1.05atm to obtain a fast-quenching alloy thin strip with a thickness of approximately 85μm;

Mechanically crush the quick-quenching Mg ₉₅ Cu _3.5 Sr _1.5 alloy flakes and pass through a 200-mesh sieve. Weigh 20 grams of sieved alloy powder, 1 gram of nano K ₂ MgF ₄ catalyst and 400 grams of stainless steel grinding balls and put them into a stainless steel with a volume of 250 ml. The ball mill jar is evacuated and filled with high-purity argon gas and then sealed. Ball milling in an all-round planetary high-energy ball mill for 3 hours. The ball mill will be shut down for 0.5 hours every hour.

Figure 1 shows the ingot of Example 1. Macroscopic observation of the fracture surface of the ingot shows that the cross-section composition of the ingot alloy is uniform and there is no macrosegregation.

Figure 2 shows a fast-quenched alloy thin strip. The thickness of the test thin strip is about 85 μm. The structure of the fast-quenched alloy thin strip is nanocrystalline + a small amount of amorphous structure;

Figure 3 is the XRD pattern of the as-cast alloy of Examples 1-8. The results show that adding Cu and Sr forms a variety of intermetallic compounds, including Mg ₂ Cu, CuSr, and Cu ₅ Sr;

Figure 4 shows the SEM morphology of the ball-milled powder of Examples 1-4. It is observed that the particles of the alloy after ball milling are well dispersed and no obvious agglomeration occurs, which is obviously related to the role of the catalyst. No presence of the catalyst K ₂ MgF ₄ was found in the observation. It can be inferred that it is evenly distributed on the surface of the ball-milled alloy particles. Since K ₂ MgF ₄ is very soft in nature, it is impossible to cut into the interior of the alloy particles during the ball milling process.

Figure 5 shows the XRD pattern of the ball-milled alloy in Examples 1-8. It is found that the ball-milled material has nanocrystalline and amorphous structural characteristics. XRD does not show the existence of free K ₂ MgF ₄ , indicating that through ball milling, the catalyst and the matrix alloy formed Consistent nanocrystalline structure.

Figure 6 is the HRTEM morphology of the ball-milled alloy in Examples 1-4, showing that the alloy has a nanocrystalline and amorphous structure.

The gaseous hydrogen absorption and release capacity and kinetics of the alloy powder were tested using fully automatic Sieverts equipment. The results are shown in Table 1.

Table 1 Hydrogen absorption and desorption kinetics and cycle stability of alloy powders with different compositions

—Hydrogen absorption amount (wt.%) within 10 minutes when the initial hydrogen pressure is 3MPa and 280℃,/> —The amount of hydrogen released within 30 minutes (wt.%) at an initial pressure of 1×10 ^-4 MPa and 280°C. S ₁₀₀ =C ₁₀₀ /C _max ×100%, where C _max is the saturated hydrogen absorption capacity of the alloy and the hydrogen absorption capacity of C ₁₀₀ after the 100th cycle.

The above results show that the ball-milled alloy powder has high hydrogen absorption and desorption capacity and excellent kinetic properties. Obviously, the hydrogen absorption and release capacity of the alloy of the present invention can fully meet the hydrogen capacity requirements of the fuel cell for the hydrogen supply system. Compared with existing similar alloys, the hydrogen storage performance of the alloy of the present invention has been significantly improved.

After analysis, the thermodynamic and kinetic properties of hydrogen absorption and release of magnesium-based alloys can be significantly improved through alloying. The improvement of microstructure and the formation of crystal defects further improve the hydrogen absorption and release kinetic properties of magnesium-based alloys. Mechanical ball milling is considered the most effective preparation technology for the formation of nanocrystalline and amorphous structures. Adding a small amount of active catalyst can significantly improve the thermodynamic and kinetic properties of hydrogen absorption and release of magnesium-based alloys.

The present invention adopts multi-element alloying in component design to form special intermetallic compounds. During the ball milling process, a small amount of nanometer K ₂ MgF ₄ is added as a catalyst to obtain powder with a nanocrystalline-amorphous structure and greatly improve the magnesium base. Thermodynamic and kinetic properties of hydrogen absorption and release of alloys.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that it can still be used Modifications are made to the technical solutions described in the foregoing embodiments, or equivalent substitutions are made to some of the technical features; however, these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims (5)

1. A high-capacity Mg-Cu-Sr-based hydrogen storage alloy catalyzed by nano-K ₂ MgF ₄ for fuel cells, which is characterized in that it contains Cu, Sr and nano-catalyst K ₂ MgF ₄ , and its chemical formula is: Mg ₉₅ Cu _5-x Sr _x +y wt.%K ₂ MgF ₄ , where x is the atomic ratio, 0<x≤2, y is the percentage of K ₂ MgF ₄ in the alloy, 2≤y≤8. 2. The alloy according to claim 1, characterized in that the atomic ratio of the chemical formula is: x=1.5; the content of _the catalyst _K2MgF4 is y=5. 3. A method for preparing a hydrogen storage alloy for fuel cells, characterized in that the steps of the method are: Step 1: Make ingredients according to the chemical formula Mg ₉₅ Cu _5-x Sr _x , where x is the atomic ratio, 0<x≤2, x=1.5; Step 2: Use induction heating to melt all elements to obtain molten Mg ₉₅ Cu _{5-x Sr} , pour the molten alloy into a copper mold to obtain an as-cast master alloy ingot; Step 3: Place the ingot prepared in step 2 above into a quartz tube with a slit at the bottom, heat it again with an induction coil to a molten state, use the pressure of the protective gas to eject it from the slit at the bottom of the quartz tube, and spray it continuously on A fast quenching alloy thin strip is obtained on the smooth surface of a copper roller rotating at a linear speed of 20m/s; Step 4. _Mechanically crush _the quick-quenching Mg ₉₅ Cu _{5-x Sr} , ball milling is carried out in an all-round planetary high-energy ball mill. 4. In the fourth step, _the quick-quenching _{Mg 95} Cu _{5-x Sr} Pour in high-purity argon and ball-mill in an all-round planetary high-energy ball mill for 2-5 hours, with a ball-to-material ratio of 1:20. 5. The preparation method according to claim 3 or 4, characterized in that the fast-quenching alloy sheet has a nanocrystalline-amorphous structure, and the thickness of the alloy sheet is 50-100 μm.