Hydrogen occluding alloy, process for its preparation and electrode

Abstract: A Ni-based hydrogen occluding alloy having a composition comprising, by wt%, 32 to 38% of rare earth elements essentially consisting of La and/or Ce, 0.5 to 3.5% of Al, 0.5 to 10% of Mn, 0.005 to 0.5% of hydrogen, optionally 0.1 to 17% of Co, and the balance being Ni and unavoidable impurities; wherein the alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu 5 -type crystal structure in a ratio of 0.5 to 20% by area. The alloy exhibits high hydrogen absorption and desorption rates, and excellent initial activation in practical use.

Description: Industrial Field of the Invention The present invention relates to a hydrogen occluding alloy exhibiting significantly high hydrogen absorption and desorption rates, and excellent initial activation characteristics in practical use for electrodes of batteries, for example. Description of the Related Art Hitherto, a variety of hydrogen occluding alloys have been proposed, and a hydrogen occluding alloy which is disclosed on page 369 of the abstract of "The 35th Battery Symposium in Japan" held in November, 1994, in Nagoya-shi, has particularly attracted attention to battery electrodes. The hydrogen occluding alloy is a Ni-based alloy having a reduced composition comprising, by wt% (hereinafter "%" indicates "wt%"), 33.2% of rare earth elements essentially consisting of La and/or Ce, 9.8% of Co, 1.9 % of Al, 5.2% of Mn, and the balance being Ni and unavoidable impurities; and having a single phase CaCu 5 -type crystal structure. A hydrogen occluding alloy is typically made by preparing a molten alloy having a given composition and casting it into an ingot. When putting it to practical use as a battery electrode, for example, the ingot is subjected to temper annealing in a vacuum or nonoxidizing inert gas atmosphere at a given temperature between 900 and 1,050 °C for a given time period, if necessary, and the as-cast or temper-annealed ingot is mechanically pulverized to a predetermined particle size or pulverized by a hydrogenation process under a pressurized hydrogen atmosphere which includes hydrogen absorption at a given hot temperature between 10 and 200 °C and hydrogen desorption by vacuum evacuation. In addition, when the hydrogen occluding alloy is applied to, for example, a battery electrode, the battery can be put to practical use after an initial activation treatment in a pressurized hydrogen atmosphere for a given time period until the electrode including the hydrogen occluding alloy has a sufficient discharge capacity at an initial stage of use. Problems to be Solved by the Invention On the other hand, there recently have been large requirements, such as larger output, higher performance, and energy saving, for batteries and heat pumps in which the hydrogen occluding alloy is widely applied. Therefore, the hydrogen occluding alloy has been required to have higher hydrogen absorption and desorption rates and a shorter initial activation time than those of the foregoing conventional hydrogen occluding alloy. Means for Solving the Problems In view of the above, the present inventors have studied to improve the hydrogen absorption and desorption rates and initial activation of the hydrogen occluding alloy comprising the Ni-based alloy set forth above. As a result, the following conclusion was obtained: When the conventional hydrogen occluding alloy of the as-cast or temper-annealed ingot set forth above is maintained in a hydrogen atmosphere e.g. at a pressure within the range from 1 to 2 bar (1 to 2 atms), preferable 1 to 1.2 bar (1 to 1.2 alms) at a temperature within the range of 0°C to 100°C and is subjected to hydrogenation heat treatment in which the alloy is held at a given temperature between 600 and 950 °C, and preferably 700 and 900 °C, for a given time period in a hydrogen atmosphere and cooled, the alloy has a microstructure in which fine rare earth element hydride is dispersively distributed in a CaCu 5 -type crystal matrix. When the ratio of the rare earth element hydride is 0.5 to 20% by area (for example the upper limit is 10% by area), the alloy exhibits catalytic effects to remarkably promote hydrogen absorption and desorption without discharge capacity deterioration. Therefore, the alloy can absorb and desorb hydrogen atoms at rates higher than those of the conventional hydrogen occluding alloy, and initial activation is significantly promoted. 1. The present invention was achieved based on the results set forth above, and characterized by a Ni-based hydrogen occluding alloy having a composition comprising, by wt%, 32 to 38% of rare earth elements essentially consisting of La and/or Ce, 0 to 17% of Co, 0.5 to 3.5% of Al, 0.5 to 10% of Mn, 0.005 to 0.5% of hydrogen, and the balance being Ni and unavoidable impurities; wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu 5 -type crystal structure in a ratio of 0.5 to 20% by area. 2. According to an embodiment of the invention the Ni-based hydrogen occluding alloy has a composition comprising, by wt%, 32 to 38% of rare earth elements essentially consisting of La and/or Ce, 0.1 to 17% of Co, 0.5 to 3.5% of Al, 0.5 to 10% of Mn, 0.005 to 0.5% of hydrogen, and the balance being Ni and unavoidable impurities; wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu 5 -type crystal structure in a ratio of 0.5 to 20% by area. 3. According to a further embodiment of the invention the Ni-based hydrogen occluding alloy has a composition comprising, by wt%, 32 to 35% of rare earth elements essentially consisting of La and/or Ce, 0.5 to 3.5% of Al, 0.5 to 10% of Mn, 0.005 to 0.2% of hydrogen, and the balance being Ni and unavoidable impurities; wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu 5 -type crystal structure in a ratio of 0.5 to 10% by area. 4. According to another embodiment of the invention the Ni-based hydrogen occluding alloy having a composition comprising, by wt%, 32 to 35% of rare earth elements essentially consisting of La and/or Ce, 4 to 17% of Co, 0.5 to 3.5% of Al, 0.5 to 10% of Mn, 0.005 to 0.2% of hydrogen, and the balance being Ni and unavoidable impurities; wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu 5 -type crystal structure in a ratio of 0.5 to 10% by area. The grounds for the choice of the composition of the Ni-based hydrogen occluding alloy and the rate of the rare earth element hydride according to the present invention will now be described. (a) Rare earth elements essentially consisting of La and Ce These rare earth elements form a matrix of a CaCu 5 -type crystal structure exhibiting hydrogen occlusion effects together with Ni, and form rare earth element hydride which contributes to increased hydrogen charge and discharge rates and improved initial activation. Since discharge capacity decreases when the content is less than 32% or over 38%, the content is determined to 32 to 38%, preferably 32 to 35% and more preferably 33 to 34%. (b) Co The Co component is dissolved into a matrix and has effects which reduce volume expansion/shrinkage during hydrogen absorption/desorption, prevent pulverization of the alloy and prolong its usable life. When the Co content is less than 0.1%, these desired effect cannot be achieved. Whereas, when the content exceeds 17%, the discharge capacity and initial activation effects tend to decrease. Accordingly, the content is preferably determined to 0.1 to 17%, preferably 4 to 17% and more preferably 6 to 12%. (c) Al The Al component is dissolved into the matrix and improves the corrosion resistance of the alloy. When the content is less than 0.5%, the desired corrosion resistance effects cannot be achieved. On the other hand, when the content exceeds 3.5%, the discharge capacity decreases. Therefore, the content is determined to 0.5 to 3.5%, and preferably 1 to 2%. (d) Mn The Mn component is dissolved into the matrix, decreases the equilibrium pressure for dissociating hydrogen, and contributes to increasing discharge capacity. When the content is less than 0.5%, a desired increase in discharge capacity cannot be achieved, whereas a content over 10% tends to decrease discharge capacity. Thus, the content is determined to 0.5 to 10%, and preferably 1 to 4.5%. (e) Hydrogen and Rare Earth Element Hydride Hydrogen predominantly bonds to rare earth elements by thermal hydrogenation at a high temperature to form rare earth element hydride which contributes to hydrogen absorption and desorption rates and improved initial activation. When the content is less than 0.005%, the ratio of the rare earth element hydride is less than 0.5% by area, and thus the desired effects cannot be achieved. Whereas a content exceeding 0.5 % forms rare earth element hydride at a ratio over 20% by area and thus drastically decreases discharge capacity. Therefore, the content is determined to 0.005 to 0.5%, preferably 0.005 to 0.2%, more preferably 0.01 to 0.2% and most preferably 0.01 to 0.08 so that the resulting ratio of rare earth element hydride finely distributed in the matrix is 0.5 to 20% by area, preferably 0.5 to 10% by area, more preferably 0.7 to 9% by area and most preferably 0.7 to 4% by area. The Ni-based hydrogen occluding alloy according to the invention can for example be prepared by melting Ni, La, Ce, Al, Mn and if desired Co, in the desired amounts and/or a corresponding misch metal in a vacuum, casting the melt into a cooled casting mold, maintaining the ingot at a temperature within the range from 0°C to 100°C, in a hydrogen atmosphere, heating and holding the ingot at a temperature within a range from 600°C to 950°C and cooling it to a temperature of 300°C or less. In accordance with a preferred embodiment the hydrogen occluding alloy of the present invention can be treated with a temper-annealing after casting. Temper-annealing at a temperature of e.g. 850°C and more furnishes an improved homogenization of the alloy. It is preferred that the temper-annealing temperature does not exceed 1050°C in order to avoid the possibility that the contents of the alloy change because of vaporization of elements (e.g. Mn) in the alloy. Therefore, the temper-annealing temperature is preferably 850°C to 1050°C. The hydrogen occluding alloy of the present invention is treated in a hydrogen atmosphere after said casting or after the possible temper-annealing. Though it is preferable for a said hydrogen treating to the hydrogen occluding alloy to be at room temperature, the same effect can be achieved as much as at 0°C. If the hydrogen temperature exceeds 100°C, the alloy does not occlude enough hydrogen so that a reaction of hydrogen and the alloy is not achieved to be even and/or uniform. Therefore, the hydrogen temperature is determined to 0°C to 100°C, preferable 20°C to 60°C. The hydrogen occluding alloy of the present invention is heated and held after a said hydrogen treating. If the heating and holding temperature is less than 600°C, a reaction for generating rare earth element hydride does not occur. On the other hand, if the heating and holding temperature exceeds 950°C the desired microstructure cannot be achieved because of decomposition of rare earth element hydride. Therefore, the heating and holding temperature is determined to 600°C to 950°C, preferably 700°C to 900°C. Embodiment The hydrogen occluding alloy in accordance with the present invention will now be described in further detail with reference to an exemplifying embodiment. Ni, La, Ce, Co, Al and Mn, as well as a misch metal, as raw materials, each preferably having a purity not less than 99.9 % were melted in a vacuum (e.g. in an ordinary high-frequency induction melting furnace) to prepare a Ni-based molten alloy having a given composition and casted into a cooled (e.g. water-cooled) casting mold (e.g. a copper casting mold) to form an ingot. The ingot was temper-annealed at a predetermined temperature within the range from 850°C to 1050°C (e.g. for about for 10 hours). After the ingot was maintained at room temperature (about 20°C) (e.g. for about one hour) in a hydrogen atmosphere of a given pressure within a range from 1 to 2 bar (1 to 2 atms), preferable 1 to 1.2 bar( 1 to 1.2 atms) it was heated to a predetermined temperature within a range from 600 to 950°C, preferably 700°C to 900°C, held at the predetermined temperature (e.g. for about 1 hour), and cooled to a temperature of 300°C or less so that hydrogenation heat treatment was achieved. Hydrogen occluding alloys 1 through 47 in accordance with the present invention (hereinafter referred to as alloy(s) of the present invention) each having a composition set forth in Table 1 through 4 and comprising powder of a particle size of 0.074 mm (200 mesh) or less were prepared in such a way. For comparison, a conventional hydrogen occluding alloy (hereinafter called "a conventional alloy") having a composition shown in Table 4 was prepared under the same conditions as those for the alloys of the present invention, except that the hydrogenation treatment after temper-annealing was omitted, and alternatively, the alloy was subjected to hydrogenation pulverization involving hydrogen absorption under conditions of a heating temperature of 200°C and a holding time of 1 hour and a hydrogen atmospheric pressure of 8 bar (8 atms) in a pressure vessel and hydrogen desorption by vacuum evacuation, so that the alloy had a particle size of 0.074 mm (200 mesh) or less. Microstructures of the resulting hydrogen occluding alloys were observed by scanning electron microscopy at a magnification of 50,000, transmission electron microscopy at a magnification of 50,000 and powder X-ray diffractometry. The alloys 1 through 47 of the present invention have a structure in which fine rare earth element hydride is dispersively distributed in a matrix of a CaCu 5 -type crystal structure. The observed ratios (percent by area) of the rare earth element hydride are shown in Tables 1 through 4. Also, X-ray diffraction patterns confirmed that the matrix had a CaCu 5 -type crystal structure and the compound dispersively distributed in the matrix comprises rare earth element hydride. The conventional alloy had a single phase CaCu 5 -type crystal structure. Then, the hydrogen absorption rate and the hydrogen desorption rate of each of alloys 1 through 47 of the present invention and the conventional alloy were measured according to JIS H7202 "Method for Measuring Hydrogenation Rate of Hydrogen Occluding Alloy" as follows: Regarding the hydrogen absorption rate, as set forth in an schematic view in Figure 1; (a) Alloy powder was enclosed in a container which was immersed in a oil or water bath, then a valve Vb was closed and valves Va and Vc were opened while maintaining the bath temperature at 200 °C to introduce pressurized hydrogen into the system from a hydrogen cylinder. When the pressure in the system reached 30 atms, the valve Va was closed, followed by allowing to stand until the pressure in the system decreased to a predetermined level (until the absorption of hydrogen by means of the alloy powder was completed). Thus, the powder was initially activated; (b) When the pressure in the system decreased to a predetermined level of around 20 atms, the valve Vb was opened, followed by lowering the pressure in the system to 10 -2 Torr by a vacuum pump. Then, the bath temperature was lowered to 20°C, and the valves Vb and Vc were closed and the valve Va was opened to introduce hydrogen into the system, except for the container. When the pressure was raised to 30 atms, the valve Va was closed and the valve Vc was opened. In such a state, the pressure drop in the system with respect to time was measured. The amount of occluded hydrogen at the point when the amount of hydrogen occluded by powder reached 80%, and the time taken to the foregoing moment are obtained from the pressure-drop curve so that the equation: (amount of occluded hydrogen when occlusion of 80% is realized)/(time taken to realize occlusion of 80%) was calculated. The thus-obtained value was defined as hydrogen absorption rate. The hydrogen desorption rate was determined by the following procedure: The bath was maintained at a temperature of, for example, 120°C, suitable for hydrogen desorption within a range of from 100 to 300°C, in the state in which measurement of the hydrogen absorption rate had been completed, that is, in the state in which the valves Va and Vb were closed, the valve Vc was opened and the pressure in the system reached a predetermined level of around 20 atms. After, the valve Vb was opened and the valve Vc was closed to evacuate the system, except for the container, to 10 -2 Torr, the valve Vb was closed and the valve Vc was opened. In such a state, the rise in pressure of the system with respect to time was measured. The amount of desorbed hydrogen at the point when the amount of hydrogen desorbed from powder reached 80%, and time den to the foregoing moment are obtained from the pressure-rise curve so that the equation: (amount of desorbed hydrogen when desorption of 80% is realized)/(time taken to realize desorption of 80%) was calculated. The thus-obtained value was defined as hydrogen desorption rate. The results are shown in Tables 5 and 6. For the purpose of evaluating the initial activation of alloys 1 through 47 of the present invention and the conventional alloy, each alloy was used as an active material for a negative electrode of a battery, and the battery was subjected to repeated charge/discharge cycles until the battery showed a maximum discharge capacity as shown below in detail. The initial activation was taken as the number of charge/discharge cycles at which the discharge capacity corresponds to 97% of the maximum discharge capacity. Cuprous oxide (Cu 2 O) as a conductive agent, polytetrafluoroethylene (PTFE) as a binder and carboxymethyl cellulose (CMC) as a thickener were added to each of alloys 1 through 47 of the present invention and the conventional alloy, and the resulting paste was loaded on a commercially available foamed nickel plate having a porosity of 95%. The foamed nickel plate was dried, pressed, and shaped into a cut plate of 30 mm by 40 mm having a thickness of 0.40 to 0.43 mm. The amount of loaded active material was approximately 1.8 g. A nickel thin plate as a lead was welded to a side of the cut plate to form a negative electrode. On the other hand, a positive electrode was formed by preparing a paste from Ni(OH) 2 as an active material, cobalt monoxide (CoO) as a conductive agent, polytetrafluoroethylene (PTFE) as a binder and carboxymethyl cellulose (CMC) as a thickener; loading the paste on the foamed nickel plate; drying, pressing and shaping the foamed nickel plate into a cut plate of 30 mm by 40 mm having a thickness of 0.71 to 0.73 mm; and welding the nickel thin plate to a side of the cut plate. The positive electrodes were provided on both sides of the negative electrode through separators made of a polypropylene/polyethylene copolymer, and protection plates made of polyvinyl chloride were integrated therewith at both sides of the positive electrodes so as to prevent omission of the active material from the outside of the positive electrodes. A battery was fabricated by inserting the integrated electrodes into a cell made of polyvinyl chloride and pouring a 28% aqueous KOH solution as an electrolyte solution into the cell. The resulting battery was subjected to charge/discharge cycles under conditions of a charging rate of 0.25 C, discharging rate of 0.25 C, and an amount of charged electric variable corresponding to 135% of the negative electrode capacity. The charge/discharge cycles were repeated until the battery showed a maximum discharge capacity, where one charge and discharge cycle is counted as one charge/discharge. Tables 5 and 6 show the maximum discharge capacity obtained by the procedure set forth above, as well as the number of charge/discharge cycles as a measure evaluating the initial activation, at which the discharge capacity is 97% of the maximum discharge capacity. Advantages Results set forth in Tables 1 through 6 evidently demonstrate that each of alloys 1 through 47 of the present invention exhibits a similar discharge capacity to the conventional alloy and a microstructure in which fine rare earth element hydride is dispersively distributed in a matrix having a CaCu 5 -type crystal structure, the hydrogen absorption and desorption rates are extremely high and initial activation is significantly promoted by the effect of the rare earth element hydride, compared to the conventional alloy comprising a single phase CaCu 5 -type crystal structure which exhibits relatively low hydrogen absorption and desorption rates and delayed initial activation, due to non-existence of rare earth element hydride. As described above, since the hydrogen occluding alloy in accordance with the present invention exhibits significantly high hydrogen absorption and desorption rates, and excellent initial activity in practical use, it significantly contributes to the achievement of high output, high performance, and energy saving in various mechanical apparatuses using the hydrogen occluding alloy. Brief Description of the Drawing Fig. 1 Fig. 1 is a schematic view illustrating the apparatus used for measuring hydrogen absorption and desorption rates of the hydrogen occluding alloy. Kind Hydrogen absorption rate (wt%/sec.) Hydrogen desorption rate (wt%/sec.) Maximum discharge capacity (mAh/g) Charge/disch arge cycles (Number) Alloys of the present invention 1 0.28 0.25 357 5 2 0.31 0.27 362 3 3 0.33 0.30 355 2 4 0.28 0.26 366 2 5 0.30 0.27 361 3 6 0.29 0.27 358 4 7 0.29 0.26 353 5 8 0.30 0.27 349 5 9 0.31 0.27 366 2 10 0.30 0.26 360 3 11 0.28 0.25 351 4 12 0.29 0.27 354 4 13 0.30 0.27 358 3 14 0.30 0.27 362 2 15 0.29 0.26 361 2 16 0.30 0.26 357 2 17 0.29 0.26 349 3 18 0.29 0.27 359 3 19 0.30 0.27 356 3 20 0.30 0.27 353 4 21 0.35 0.37 351 2 22 0.33 0.30 359 2 Kind Hydrogen absorption rate (wt%/sec.) Hydrogen desorption rate (wt%/sec.) Maximum discharge capacity (mAh/g) Charge/discharge cycles (Number) Alloys of the present invention 23 0.25 0.23 361 4 24 0.28 0.25 365 3 25 0.32 0.28 359 2 26 0.33 0.31 357 2 27 0.36 0.35 355 2 28 0.29 0.25 367 2 29 0.27 0.24 362 3 30 0.29 0.26 355 3 31 0.29 0.26 357 3 32 0.28 0.24 362 3 33 0.29 0.25 365 2 34 0.27 0.25 363 2 35 0.30 0.27 360 2 36 0.29 0.25 355 3 37 0.31 0.28 353 3 Conventional Alloy 0.18 0.16 345 11 38 0.32 0.30 364 3 39 0.32 0.30 362 3 40 0.31 0.27 363 3 41 0.31 0.28 364 3 42 0.31 0.27 364 3 43 0.31 0.27 360 3 44 0.32 0.29 361 3 45 0.31 0.29 362 3 46 0.31 0.28 361 3 47 0.31 0.27 361 3

Claims: A Ni-based hydrogen occluding alloy having a composition comprising, by wt%, 32 to 38% of rare earth elements essentially consisting of La and/or Ce, 0 to 17% Co, 0.5 to 3.5% of Al, 0.5 to 10% of Mn, 0.005 to 0.5% of hydrogen, and the balance being Ni and unavoidable impurities; wherein said alloy has a microstructure characterized in that fine rare earth element hydride is dispersively distributed in a matrix having a CaCu 5 -type crystal structure in a ratio of 0.5 to 20% by area. A Ni-based hydrogen occluding alloy according to claim 1 comprising, by wt%, 0.1 to 17% of Co. A Ni-based hydrogen occluding alloy according to claim 1 comprising, by wt%, 32 to 35% of rare earth elements essentially consisting of La and/or Ce, and 0.005 to 0.2% of hydrogen, and wherein the fine rare earth element hydride is dispersively distributed in the matrix having a CaCu 5 -type crystal structure in a ratio of 0.5 to 10% by area. A Ni-based hydrogen occluding alloy according to claims 1 and 2 comprising, by wt%, 32 to 35% of the rare earth elements essentially consisting of La and/or Ce, and 4 to 17% of Co and wherein the fine rare earth element hydride is dispersively distributed in the matrix having a CaCu 5 -type crystal structure in a ratio of 0.5 to 10% by area. A process for preparing the Ni-based hydrogen occluding alloy according to any of claims 1 to 4, characterized by melting Ni, La, Ce, Al, Mn and optionally Co, in the desired amounts and/or a corresponding misch metal, in a vacuum, casting the melt into a cooled casting mold, optionally temper-annealing the obtained ingot at a temperature within the range from 850°C to 1050°C, maintaing the ingot at a temperature within the range from 0°C to 100 °C, in a hydrogen atmosphere, heating and holding the ingot at a temperature within a range from 600°C to 950°C and cooling it to a temperature of 300°C or less. The process according to claim 5, wherein the hydrogen atmosphere is at a pressure between 1 and 2 bar, preferably between 1 and 1,2 bar. Use of the Ni-based hydrogen occluding alloy of any of claims 1 to 4 for the preparation of electrodes. An electrode containing or consisting of an alloy according to any of claims 1 to 4.

Inventors: TAKIZAWA YOSHIO, WADA MASAHIRO, TAKIZAWA, YOSHIO, WADA, MASAHIRO, Takizawa, Yoshio, c/o Sohgou-kenkyusho, Wada, Masahiro, c/o Sohgou-kenkyusho
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1997
Main classification: H01M4
IPC codes: C22F1, H01M4