DE10053734C2 - Hydrogen storage alloy based on zirconium - Google Patents

Description

The invention relates to a hydrogen storage alloy Zirconium based and especially a non stoichiometric one Zr-Ni-based hydrogen storage alloy for a cathode material for a nickel / metal hydride secondary cell for stei capacity and performance.

Most ongoing research in the field of Ni / MH Secondary cells are concerned with achieving high levels Capacity and high performance including properties a high degree of discharge and a long life of the Ni / MH secondary cell. These properties of the cell will be mainly due to the properties of hydrogen spit cher alloy determines the negative electrode or Katho de forms. Achieving high capacity and high Ni / MH secondary cell performance is directly related to capacity and the performance of the negative electrode from the Hydrogen storage alloy connected. Therefore the For to a significant extent on increasing the Performance of the cathode material from the hydrogen storage alloy directed. Recently it was reported that Hydrogen storage alloys with a non-stoichiometry a high degree of discharge characteristics and have a longer life, so that such alloys now come to the fore as effective Material for negative electrodes or cathodes are moved.

A Ni / MH secondary cell which uses a hydrogen storage alloy as the active cathode material has the following electro-chemical reaction principle:
When the cell is discharged, the hydrogen storage alloy releases a stored hydrogen atom, which combines with a hydroxyl ion OH ^- in a KOH electrolyte to form a water molecule, while an electron is moved to a positive electrode or anode via an external circuit. On the other hand, when the cell is charged, the water is decomposed into a hydrogen ion H ⁺ and a hydroxyl ion OH ^- . The hydroxyl ion remains in the electrolyte, while the hydrogen ion is combined with the ion coming from outside to form a hydrogen atom, which in turn combines with and is stored in the hydrogen storage alloy. This principle of reaction uses inherent reversible properties of the hydrogen storage alloy, ie its stability in the aqueous alkali solution and its ability to absorb / release hydrogen quickly.

In order for the hydrogen storage alloy to be used as an effective cathode material of the Ni / MH secondary cell, it must essentially satisfy the following two conditions: (1) It must have good hydrogen attachment properties together with a suitable absorption / release pressure for hydrogen in a gas Solid reaction (generally 0.01 to 1 atm at room temperature), a large hydrogen storage capacity (the theoretical discharge capacity of the electrode is proportional to the hydrogen storage capacity (C _H wt .-%), a theoretical discharge capacity (mAh / g = 268 .C _H ) and have a high reaction rate. (2) A charge transfer reaction associated with the decomposition and synthesis of hydrogen at an interface between the alloy and the electrolyte when the alloy is brought into an electrochemical reaction with the electrolyte must be easy to carry out. That means that only one such hydrogen Storage alloy can be used as the cathode of the Ni / MH secondary cell, the surface of which acts as a catalyst for the charge transfer reaction.

Various hydrogen storage alloys are already known which meet these requirements, and typical alloys of this type can be defined as follows: (1) AB ₅ type with La-Nd-Ni-Co-Al with a hexagonal structure according to US Pat. No. 4,488,817 and Mixed metal (Mm) -Mn-Ni-Co-Al with hexagonal structure according to JPs 61-1,132,501 and 61-214,361, as well as (2) AB ₂ type with Ti-V-Ni-Cr with a C14, 15-hexagon len, cubic space-centered multi-phase structure according to US-A-4,551,400 and with Zr-V-Ni with C14 structure according to the "Journal of the Less-Common Metals, 172-174: 1219 (1991).

Of these alloys, the AB ₅ -type La-Ni-based hydrogen storage alloy electrode shows a substantial reduction in electrode capacity with charge / discharge cycles in the alkali electrolyte, as described in "Journal of the Less-Common Metals, 161: 193 ( 1990) and 155: 119 (1989), the overall phenomenon of which is referred to as degradation, For example, US-A-4,488,817 discloses that the cycle life of the La-Ni based alloy can be extended by Co and Al are replaced in small amounts for a Ni element and Nd in small amounts for a La element, the problem of capacity reduction still being present.

A Zr-based hydrogen storage alloy, at least Contains 30 atomic percent Zr and at least 40 atomic percent Ni, as disclosed in US-A-4,946,646 is also on a discharge capacity of only 300 to 270 mAh / g is limited.

US-A-4,849,205, 4,728,586 and 4,551,400 disclose Ti-Zr-  V-Ni-Cu-Mn-M (M is Al, Co, Fe, etc.) - hydrogen storage alloy with capacities of not more than 300 to 380 mAh / g.

The previously developed hydrogen storage alloys for Ni / MH secondary cells can be roughly divided into two types, namely (1) in the AB ₅ type with alloys made of mixed metal (Mm) -Ni base, where A is an element with a strong affinity for Hydrogen, for example a rare earth metal such as La, Ti, Zr, Ce, Pr, Nd etc. and B is a transition element such as Ni, Mn, Co, Fe, Al etc., and (2) in AB ₂ - Type with alloys based on Zr-Ni and Ti-Ni. However, the first alloy of the AB ₅ type has the disadvantage of a low energy storage density, while the second AB ₂ type is poor in almost all performance profiles. The further development of Ni / MH secondary cells with high capacity and high performance will be directed towards the production of high-performance hydrogen storage alloys of the AB ₂ type, which have a higher capacity than the AB ₅ hydrogen storage alloys.

The object underlying the invention is that to overcome the above problems and a hydrogen spit To provide zirconium-based alloy, the one has non-stoichiometric composition ratio, d. H. an ABZ high capacity hydrogen storage alloy and Develop performance.

This task is solved with a hydrogen storage alloy based on zirconium, which can be represented by the following general formula:

Zr _1-X Ti _X (Mn _U V _V Cr _Y Ni _1-UVY ) _Z

where X, U, V, Y and Z are atomic proportions of each alloy composition element  and is 0 <X ≦ 0.4; 0.3 ≦ U ≦ 0.4; 0.1 ≦ V ≦ 0.2; 0.0 ≦ Y ≦ 0.2; 0.45 ≦ U + V + Y ≦ 0.65 and 1.6 ≦ Z ≦ 1.9 applies.

In the hydrogen storage alloy according to the invention, Zr a base alloy composition element with an optima len composition ratio of at least 0.6 to less as 1 atomic part taking into account the composition ratio of Ti as a substitution metal element.

Ti is an element that increases discharge capacity rate of the hydrogen storage alloy according to the invention tion, whereby its optimal composition ratio nis on from more than 0 to 0.4 or less atomic content is bordered. If the composition ratio of Ti au would be outside its optimal range, the Discharge capacity reduced to about 300 mAh and the elec trode life can be shortened. The reason why Ti as Replacement for Zr in the alloy composition elements is used is that in the case of an alloy the alloy with a stoichiometric composition elements in the A position have a stronger affinity for water substance than those in the B-position and thus the Alloy tends to have a lower total hydrogen to have equilibrium pressure. Therefore Zr is replaced by Ti which has similar crystallographic properties Exception has a lower affinity for hydrogen to to provide a suitable hydrogen equilibrium pressure len and thus the reversible hydrogen storage capacity increase.

The optimal Mn composition ratio in the fiction According to hydrogen storage alloy is at least 0.3 limited to 0.4 or less atomic part because of the discharge efficiency  the alloy is greatly reduced if that Ratio is less than 0.3, and a reduction in Charging efficiency and deterioration of the catalytic Function occur when the ratio is more than 0.4.

An optimal composition ratio for V at the erfin hydrogen storage alloy according to the invention is at least Limited 0.1 to 0.2 or less atomic proportion. An optimal one Composition ratio of Cr, which is used to increase the Hydrogen storage capacity and to enlarge the elec electrode life is up to 0.2 or less atom limited share. Because Cr has a strong affinity for hydrogen has and counteracts a solution of V, it becomes ver applies the degradation of the alloy electrode by Ti sub is prevented and the discharge capacity increases improve. In the alloy according to the invention, Ni is one Element which is responsible for a catalytic function of the alloy in a KOH electrolyte. Its optimal composition ratio is preferably at least 0.35 and 0.55 or less atomic fraction.

The water according to the invention which meets the above conditions material storage alloy has a higher discharge density of 370 to 425 mAh / g and their discharge efficiency is 80% at 400 mAh / g based on a discharge capacity at 25 mAh / g.

Below are manufacturing methods for the Invention appropriate hydrogen storage alloy and for the process for Measurement of their hydrogen deposition characteristics in one Gas-solid reaction and its hydrogen storage described in the electrolyte.

First, appropriate amounts of the alloy are put together Settlement elements determined and weighed so that they with  their corresponding compositional relationship in the alloy tion and the total weight of the elements 5 g is. Then the alloy composition elements melted with an arc under an argon atmosphere and solidified into a sample. In the present case is the process of implementing and remelting the solidified sample repeated at least four times to obtain the To improve the homogeneity of the sample.

The sample obtained is then minced and the powder with a size of 100 to 200 mesh corresponding to a Lich sieve mesh size from 0.15 mm to 0.07 mm, is in one Reactor tube introduced with a high pressure hydrogen injection device in the design according to Sievert becomes. Then there is an activation process without any thermal treatment by the interior of the reactor 30 Minutes at a pressure of about 102 torr whereupon hydrogen is supplied at 20 atm. Usually becomes the hydrogen is completely absorbed in one hour.

After completion of the hydrogen absorption, the reactor tube again held under a vacuum to the hydrogen completely from the inside of the reactor tube. This process of hydrogen absorption / removal was completed within several minutes. The water substance injection device with the reactor tube is on a constant temperature using an automatic Thermostats during the whole activation process Hydrogen is obtained at a given temperature equilibrium pressure composition or concentration Curves, so-called PCT isotherms, around the thermodynamic Determine properties of the alloy from these curves. For the PCT isotherms, the abscissa gives the number of Hydrogen molecules related to the alloy sphere and the  Ordinates a hydrogen partial pressure.

In order to investigate the properties of the hydrogen storage alloy in the alkali electrolyte, alloys based on Zr-Ti-Mn-V-Cr-Ni-based (atomic content X = 0.0; 0.2; 0.4 and 0 , 6) produced by arc melting under an argon atmosphere. The alloys obtained are mechanically finely comminuted in air in order to measure the PCT isotherm and thus the thermodynamic properties of the alloys using an automatic PCt isotherm measuring device. X-ray spectroscopic analyzes (XRD analyzes) are also carried out in order to analyze the structure and to obtain a lattice constant of the alloy produced. Of the finely comminuted alloys, alloy powders with a particle size of up to 400 mesh, corresponding to a mesh size of up to 0.07 mm, were taken, mixed with 300% by weight of nickel powder and cold-pressed at a pressure of 10 t / cm ² to obtain tablet electrodes which are then subjected to a half-cell test. In the test according to the invention, the current densities during charging / discharging are 100 mA / g each. The cells are charged for 6 hours and then discharged until they are switched off at a final voltage of at least -0.75 V (compared to Hg / HgO). After sufficient activation of the measurement of the discharge capacities, the discharge efficiency and the electrode life, the electrodes are subjected.

A hydrogen storage alloy is thus according to the invention with high performance and high capacity for Ni / MH secondary cells len developed, which the conventional commercial Co containing hydrogen storage cells (hydrogen storage cells based on mixed metal (Mm) -Ni) can replace. By Increasing their performance and increasing their energy density  the market for the Ni / MH secondary cells and becomes the Development of electric vehicles accelerated their performance mainly through the performance and the capacity the secondary cell can be determined.

The invention is explained in more detail with reference to drawings.

Fig. 1 is a diagram showing atomic proportion dependent discharge potential of a Zr _1-X Ti _X (Mn _0.2 V ₀₂ Ni _{0.6) 1.8} (X = 0.0, 0.2, 04 or 0.6) alloy , which are plotted against the discharge capacity in mAh / g at a temperature of 30 ° C and a discharge current density of 50 mA / g.

Fig. 2 is a diagram showing atomic proportion depending Ände approximations of the discharge capacity plotted as behaves nis C / C _max in% of a Zr _1-X Ti (Mn. _0.2 V _0.2 Ni _{0.6) 1.8} (X = 0 , 0; 0.2; 0.4 or 0.6) alloy, which is plotted against the discharge current density in mA / g at a temperature of 30 ° C.

Fig. 3 is a diagram showing atomic proportion depending PCT isotherms of a Zr _1-X Ti _X (Mn _0.2 V _0.2 Ni _{0.6) 1.8} (X = 0, 0; 0, 2, 04 or 0 , 6) alloy, which are worn at a temperature of 30 ° C.

Fig. 4 is a diagram showing atomic proportion dependent changes of discharge capacity plotted as Kapazitätsver ratio C / C _max in% of a Zr _1-X Ti _X (Mn _0.2 V _0.2 Ni _{0.6) 1.8} (X = 0 , 0, 0.2, 0.4 or 0.6) alloy, which are plotted against the number of charge and discharge cycles at a temperature of 30 ° C. and a discharge current density of 100 mA / g.

Fig. 5 is a diagram showing atomic proportion dependent PCT isotherms of a Zr _0.7 Ti _0.3 (Mn _0.2 V _0.2 Cr _0.6 Ni _{Y-Y) 1.8} (Y = 0.0 0, 05 or 0.15) alloy with partial replacement by Cr, which are applied at a temperature of 30 ° C.

Fig. 6 is a diagram showing atomic proportion dependent changes in the discharging potentials of a Zr _0.7 Ti _0.3 (Mn _0.2 V _0.2 Cr _0.6 Ni _{Y-Y) 1.8} (Y = 0.0 0, 05 or 0.15) alloy with a partial replacement with Cr, which are applied at a temperature of 30 ° C and a current density of 50 mA / g.

FIG. 7 shows in a diagram atomic fraction-dependent PCT isotherms of a Zr _0.65 Ti _0.35 (Mn _0.3 V _0.14 Cr _0.11 Ni _0.45 ) _1.76 alloy which is at a temperature of 30 ° C are plotted.

Fig. 8 is a diagram showing atomic proportion dependent Ent charge potentials of a Zr _0.65 Ti _0.35 (Mn _0.3 V _0.14 Cr _0.11 Ni _{0.45) 1.76} - alloy above the discharge capacity in mAh / g are applied at a temperature of 30 ° C and a discharge current density of 50 mA / g.

Fig. 9 is a diagram showing atomic proportion dependent changes of discharge capacity plotted as a ratio C / C _max in% of a Zr _0.65 Ti _0.35 (Mn _0.3 V _0.14 Cr _0.11 Ni _{0.45) 1 76} alloy, which are plotted against the discharge current density in mA / g at a temperature of 30 ° C.

Fig. 10 is a diagram showing atomic proportion dependent changes of discharge capacity in mAh / g of Zr _0.65 Ti _0.35 (Mn _0.3 V _0.15 Cr _0.11 Ni _{0.45) 1.76} alloy and a known Alloy plotted against the number of charge and discharge cycles at a temperature of 30 ° C.

The Zr _X-1 Ti _X (Mn _0.2 V _0.2 Ni _0.6 ) _1.8 alloy is used as the base alloy. All changes in the properties of the alloy due to the Ti exchange are determined when an atomic proportion of Ti from 0.0 to 0.2; 04 is changed to 0.6, with the results in Fig. 1 to Fig. 4 are shown.

Furthermore, a Zr _0.7 Ti _0.3 (Mn _0.2 V _0.2 Cr _Y Ni _0.6-Y ) _1.8 alloy is used as the base alloy, and all changes in the properties of the alloy due to the Cr- Exchanges found when an atomic fraction of Cr is changed from 0.0 to 0.05 and 0.15, the results being shown in FIGS. 5 and 6.

The invention is further illustrated by the examples below explained.

example 1

The properties of a Zr _1-X Ti _X (Mn _0.2 V _0.2 Ni _0.6 ) _1.8 (X = 0.0, 0.2, 0.4 and 0.6) alloy are analyzed ,

A Zr (Mn _0.2 V _0.2 Ni _0.6 ) _1.8 hydrogen storage alloy is chosen as the base alloy, since it has a relatively large hydrogen storage capacity and a suitable equilibrium hydrogen pressure. Using this alloy, a hydrogen storage alloy with a non-stoichiometric composition ratio is obtained by partially substituting a Ti element for a Zr element of the base alloy. The thermodynamic and discharge properties of the resulting non-stoichiometric alloy are tested, while the amount of Ti in the non-stoichiometric alloy is increased step by step.

Fig. 1 shows a measurement result of the change in the discharge capacity of the alloy electrode with the increase in Ti. It can be seen from the drawing that there is a maximum for a certain titanium portion.

Fig. 3 shows the change in the PCT characteristic with the increase in Ti. It can be seen that the hydrogen equilibrium pressure increases and the total hydrogen storage capacity decreases as the atomic proportion of Ti increases. However, as can be seen from Fig. 2, the discharge capacity shows the above-mentioned tendency because the discharge efficiency of the alloy electrode increases as the amount of Ti is increased. In contrast, it can be seen from Fig. 4 that the shorter life of the alloy electrode grows the more Ti is exchanged. The reason for this is that the porosity of an oxide film formed on the surface of the alloy electrode increases with the increase of the exchanged Ti, and thus the penetration of oxygen and the solution of V in the electrolyte also increase, with the result that the oxide film formed becomes thicker ,

Example 2

The properties of a Zr _0.7 Ti _0.3 (Mn _0.2 V _0.2 CR _Y Ni _0.6-Y ) _1.8 (Y = 0.0, 0.05, 0.15) - Alloy analyzed.

Fig. 5 shows the change of a PCT isotherm shows a Zr _0.7 Ti _0.3 (Mn _0.2 V _0.2 Cr _0.6 Ni _{Y-Y) 1.8} alloy with the exchange of Cr. The PCT isotherm is plotted at a temperature of 30 ° C. From the isothermal curve it is confirmed that the hydrogen storage capacity of the alloy electrode increases with an increase in the exchanged Cr. In order to improve the degradation and the discharge capacity of the alloy electrode with exchanged Ti, the Ni of the alloy is partly replaced by Cr, which has a stronger affinity for hydrogen and can prevent the dissolution of V. The hydrogen storage capacity of the alloy electrode is thus increased when the exchanged Cr is increased. From Fig. 6 it can also be seen that the discharge capacity of the alloy electrode increases as the exchanged Cr increases.

Example 3

The properties of a Zr _0.65 Ti _0.35 (Mn _0.3 V _0.14 Cr _0.11 Ni _0.45 ) _1.76 alloy are analyzed.

FIGS. 7 and 8 show a PCT isotherm or a discharge curve of a Zr _0.65 Ti _0.35 (Mn _0.3 V _0.14 Cr _0.11 Ni _{0.45) 1.76} alloy, wherein the a temperature of 30 ° C are applied. The alloy is obtained by optimizing the amounts of exchanged Ti and Cr and the stoichiometric ratio based on the above alloy design measures. As can be seen from the diagrams, the reversible hydrogen storage capacity (0.01 to 1 atm) of the alloy is 1.6 percent by weight. The alloy has a very high discharge capacity of 425 mA / h, which is increased by 45% compared to the known alloys.

FIGS. 9 and 10 show changes of the discharge capacity with the discharge current density and the charging and discharging cycles. The drawings confirm that all other performances of the alloy, such as electrode life, discharge efficiency, etc., are as good as or better than that of the known alloys.

Claims (1)

Zirconium-based hydrogen storage alloy with the following general formula
Zr _1-X Ti _X (Mn _U V _V Cr _Y Ni _1-UVY ) _Z
where X, U, V, Y and Z are atomic proportions of each alloy composition element and
0 <X ≦ 0.4;
0.3 ≦ U ≦ 0.4;
0.1 ≦ V ≦ 0.2;
0.0 ≦ Y ≦ 0.2;
0.45 ≦ U + V + Y ≦ 0.65; and
1.6 ≦ Z ≦ 1.9 applies.