AB5 hydrogen storage alloys have been intensively studied due to its superior ability to store hydrogen and release at ambient conditions. It is also a major component in the negative electrode of Ni-MH batteries. However, it has poor high rate capability and cycle life stability. Carbon nanotubes (CNTs) were found to store a tremendous amount of hydrogen, owing to the fact that they possess very large surface areas. It is because the hydrogen storage capacity is in general highly dependent on the surface area of the storing materials. The aim of this project has been to investigate the effect on electrochemical behaviours of Ab5 negative electrode in Ni-MH batteries by adding carbon nanotubes. The research also studied the influence of the ball milling treatments applied to both the Ab5 and CNTs. La0.59Ce0.27Nd0.08Pr0.06 (Ni0.76Mn0.08Al0.01Co0.15)5 AB5 alloy powder was used as active material in the negative electrode in the Ni-MH batteries, CNTs were used as additive, nickel powers as conductor in a three-electrode cell. Electrodes with compositions of AB5 + x wt.% CNTs (x=0, 5, 10) were studied. Activation, high rate capability and cycle life stability were investigated. The three-electrode cell in an open container with 6 M of KOH as electrolyte was connected to charge/discharge machine where galvanostatically charging and discharging took place. Hydrogenation of ball milled and as-received AB5 alloy powders were examined by conventional volumetric method. Morphology of AB5 and CNTs was examined by scanning electron microscopy (SEM) and transition electron microscopy (TEM), respectively. The phase identification and crystal lattice parameters were analysed by multi-purpose X-ray diffraction before and after ball milling treatments for both materials. The chemical composition of Ab5 alloy powders was tested using ICP chemical method. The results show the addition of CNTs in negative electrode in a Ni-MH battery enhanced the specific discharge capacity remarkably. A maximum discharge capacity of 252 mAh/g was observed for electrode with low energy ball-milled (LEBM) Ab5 with 5 wt.% of CNTs. This was due to the superior properties and great surface area of CNTs which allow more hydrogen to be stored and diffused onto the surface. Not only CNTs could act as a hydrogen reservoir in the negative electrode, it also acted as a conductor by building a conductive network between active material and nickel powders, and hence an increase in discharge capacity. However, the milling on CNTs alone will not improve the electrochemical properties of the electrode. In contrary, the activation profiles, high rate capability and cycle stability have been enhanced significantly when Ab5 alloy powders were ball-milled. The possible explanation is the smaller particle size and rough surface (and hence large surface area) obtained after ball milling induces a better hydrogen diffusion between the particles, as a result of shorter distance between particles after ball milling. Ball milling treatments on AB5 alloy powders did not improve the hydrogen absorption capacity. A highest value of 1.27 wt.% was observed for LEBM alloy powders. Ball milled samples have a slightly lower plateau pressure as compared with that of as-received alloy powders. In addition, only 4% of the maximum absorption capacity was lost after 10 repeated absorption and desorption cycles due to pulverisation of the particle over cycling. It can be concluded that LEBM Ab5 with addition of 5 wt.% CNTs, can significantly improve the electrochemical properties of negative electrode in Ni-MH batteries.
| Identifer | oai:union.ndltd.org:ADTP/258306 |
| Date | January 2007 |
| Creators | Tsai, Ping-Ju (Ben), Materials Science & Engineering, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. Materials Science & Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Tsai Ping-Ju (Ben)., http://unsworks.unsw.edu.au/copyright |
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