Effect of RE Composition on Microstructure and Electrochemical Performance of RE (NiCoMnAl)_5 Electrode

Chinese Library Classification No.: TG113.12: A Article ID: 1002 - Received the first draft date: 2001-03-09; Received revised version date: 2001 AB5-type rare earth hydrogen storage electrode alloy has easy activation, electrochemical capacity High (the theoretical capacity of 372mAhg) and good dynamic properties, etc., are currently widely used in the NiMH battery 111. As the mother alloy of AB5-type hydrogen storage electrode alloy, LaNi5 alloy due to the large volume in the process of hydrogen absorption The change (volume expansion rate when hydrogen is adsorbed is 23.5) causes serious powdering of the alloy, and the poor corrosion resistance in the alkali solution makes the electrochemical capacity of the alloy decrease rapidly with the increase in the number of charge and discharge cycles. Studies have shown that LaNi5 is 13. The capacity retention rate of the alloy after only 150 charge-discharge cycles is only about 30%, and the cost is too high to meet the practical requirements. In 1984, Willemes adopted the method of alloying to effectively improve the electrochemical cycle stability of LaNi5 alloy and make AB5 type electrode alloy practical. 14. Multi-alloying of LaNi5 alloy mainly includes A side (La) substitution and B side (Ni ) Alternatives. After systematically studying the effects of Co, Mn, Al, Si, Ti and other elements in place of Ni on the electrochemical performance of the alloy, people basically grasped the effect of substitution of each element, and currently used AB5 type rare earth hydrogen storage in production and application. The B side component of the electrode alloy has been basically shaped. 151. The A side element of the AB5 type alloy is the hydrogen absorbing element of the alloy and has an important influence on the hydrogen storage performance of the alloy. M, the substitution on the A side is mainly the use of inexpensive mixing. Rare earth replaces pure La, but due to the different contents of various elements (mainly La, CePr, Nd4 rare earth elements) in mixed rare earths produced in different regions, the influence of rare earth elements on the electrochemical performance of alloys is relatively delayed. This work is to study the changes of rare earth elements on the A side through the combination of LaCe and miscellaneous rare earths from different origins. In particular, the change in the content of Lat:e affects the activation performance, electrochemical capacity, cycle stability, and stability of AB5-type rare earth hydrogen storage electrode alloys. Discharge performance and other electrochemical properties.

1. During the solidification process, due to the inconsistent segregation tendency of the constituent elements of the alloy, especially the segregation of the Mn element is quite serious 181, resulting in uneven alloy composition after solidification, plus the alloy has a certain degree of solidification in the water-cooled copper crucible The direction of heat dissipation (dissipation of heat from the core of the sample to the direction of the copper crucible) thus forms this directional dendritic microstructure. SEM4DS study showed that 19, the as-cast alloy segregation phase (black area) in the Mn content is higher, Co content is lower, Al is in the main segregation phase and the matrix phase boundary.

Table 2 Cell parameters of RE (NiCoMnAl) electrode alloys with different rare earth components Table2Thece! The XRD patterns of the unitary electrodes of RE (NiCoMnAl) electrode alloys containing different rare earth elements are shown. XRD pattern analysis showed that each sample consisted of a single hexagonal CaCu5-type hexagonal structure, and no second phase was found. According to the XRD data, the cell parameters of the alloy were calculated using the least squares method. The results are shown in Table 2. It can be seen that with the decrease of the A side:La content, the lattice parameter a of the alloy decreases, resulting in a decrease in the alloy cell volume V. Small, this is related to La's atomic radius is significantly greater than that of Ce, Pr, Nd's atomic radius. 110. RE (NiCoMnAl) Electrode Electrode's Electrochemical Performance Four electrode alloy's activation performance curves are shown. It is easy to see from the figure that when the A side of the alloy is pure La, its electrochemical capacity is much higher than that of the alloy when the A side is mixed rare earth; and when the A side is mixed rare earth, the electrochemical capacity of the alloy is The decrease in La content slightly decreases. Explain the anodicity of A-side rare earth element composition on the alloy. Table 3 The electrochemical performance of RE (NiCoMnA) electrode alloy with different rare earth components. From the above. As can be seen from the figure, the composition of rare earths on the side of AB5 type electrode alloy has a certain influence on the rate discharge performance of the alloy, especially when the discharge current density is greater than 600mA/h, the effect is more obvious: when the A side is pure La rare earth The alloy's large current discharge performance is poor (such as A - 1 sample) and using a small amount of Ce instead of La can significantly improve the alloy's large current discharge capacity (such as A - 2 samples) with the increase of Ce content and a small amount With the addition of Pr and Nd elements, the rate discharge performance of the alloy is reduced (eg A-3 and A-4 specimens). The electrochemical capacities of the alloys at discharge rates of 1.0C and 3.0C are listed in Table 3. Previous studies have shown 1121 as one of the kinetic parameters, the high-current discharge capacity of the hydrogen storage electrode alloys mainly depends on 2 processes, ie The electrochemical reaction between the electrode alloy and the electrolyte and the diffusion of protons in the alloy. When the discharge current is small, the rate discharge performance of the electrode alloy is mainly determined by the electrochemical reaction speed, ie, the exchange current density/. The discharge rate of the electrode alloy is mainly determined by the diffusion of protons in the alloy during high-current discharge. The experimental results of the electrochemical charge-discharge cycle life tests of the alloys on the four alloys show that when the A side is pure La, the cycle life of the alloy is the worst. With the increase of the Ce content on the A side, the cycle life of the alloy is gradually improved. .

This is consistent with the experimental results of Adzic et al. After 300 times of 1.0 C charge and discharge, the electrochemical capacity retention rate of the alloy is shown in Table 3. The influence of the variation of the rare earth composition on the side of the A side of the alloy to the cycle life of the AB5 type electrode alloy can be attributed to the following two aspects: On the one hand, From the previous XRD test results, it can be seen that the increase of Ce content on the A side causes a decrease in the cell volume of the alloy, which in turn leads to a decrease in the electrochemical capacity of the alloy. We know that the volume change of the AB5-type hydrogen storage electrode alloy during charge and discharge is proportional to the electrochemical capacity. Therefore, the higher Ce content on the side of alloy A, the smaller the volume change during charging and discharging, and the less tendency of the alloy to pulverize, which is beneficial to the improvement of charge and discharge cycle life of the electrode alloy. On the other hand, because the electrode alloy is weakly immersed in a strong alkali (6mol/KOH solution), it is also an important factor that affects the cycle life. Studies have shown that 114, in the alkaline electrolyte, +3 Ce ions can be reduced by oxidation. The reaction was converted to +4 Ce ions and an oxide film of Ce2 was formed on the surface of the alloy. This layer of dense oxide film is distributed on the surface of the alloy, which prevents further segregation and oxidation of the alloy surface components, reduces the corrosion rate of the alloy, and improves the cycle stability of the alloy.

3 Conclusions The microstructures of the four as-cast RE (NiCoMnAl) electrode alloys with different A-side rare earth compositions are dendritic morphology and the structure is a CaCu5 type hexagonal structure, but the unit cell volume of the alloy is The rare earth elements such as Ce, PrNd and other rare earth elements decrease to increase the content of La; the rare earth component of the A side of the AB5 hydrogen storage electrode alloy has a certain influence on its electrochemical performance: when the A side is pure La, RE (NiCdMnAl) The maximum electrochemical capacity of the alloy is 329.3mAhg. The rate discharge performance and the charge-discharge cycle life are poor. After replacing La with a small amount of Ce, PrNd, etc., the electrochemical capacity of the alloy is reduced, and the rate discharge performance and cycle life are improved. Since La on the A side is replaced by a small amount of other rare earth elements, the cell structure of the alloy changes.