Year

1996

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

This study was motivated by a desire to understand more about the performance of Ti2Ni hydrogen storage alloy as the negative electrode for rechargeable nickel-metal hydride batteries.

A starting point for the study was a survey of the literature pertaining to the characteristics and applications of hydrogen storage alloys. A review of the application of hydrogen storage alloys in nickel-metal hydride batteries is also presented.

Based on the literature review, studies aimed at understanding the hydrogen storage performance of Ti2Ni alloy electrode have been carried out.

Room temperature microencapsulation technique has been applied to the surface modification of the Ti2Ni alloy powder. The electrodes fabricated from the Ti2Ni alloy powder microencapsulated at room temperature exhibit a superior performance to that ofthe Ti2Ni alloy powders that are either uncoated or coated at higher temperature in respect of specific capacity and especially the cycle life of the electrode.

The formation of Ti2NiHo.5 hydride phase has been found responsible for the capacity decay of the alloy during early cycles. Once formed, the compound cannot be reversibly charged and discharged. Therefore, it provides no contribution to the discharge capacity of Ti2Ni electrodes.

Elemental substitutions of the alloy have also been carried out. Cobalt and aluminium have been characterised as having some beneficial effects to aspects of the performance of Ti2Ni hydrogen storage alloy electrodes. Cobalt addition is beneficial to the specific capacity and the cycle life of the Ti2Ni electrode. Aluminum addition into Ti2Ni alloy is beneficial to the cycle life but is detrimental to the specific capacity of the electrode because of the passivation of the electrode owing to the existence of a new Ti2Al phase.

The electrochemical impedance spectroscopy measurements provide us with some preliminary understanding of the discharge process of the Ti2Ni electrode. Discharge temperature and the discharge state of the electrode have been shown to influence the discharge kinetics of the electrode.

Charging and discharging of the Ti2Ni hydrogen storage alloy electrode in heavy water electrolyte exhibit sluggish kinetic processes which give rise to extreme difficulty in in-situ neutron diffraction analysis. Electrochemical ac impedance analysis shows that the reaction resistance of the electrode charged/discharged in D20 solution is much greaterthan in H20 solution. This reveals the reason for the difficulty encountered in our neutron diffraction studies and provides a useful indication for further work with neutron diffraction measurements, especially the in-situ measurements during the charge/discharge process of the hydrogen storage alloy electrode.

In summary, this study has characterised the surface modification and elemental substitution effects on the performance of the Ti2Ni hydrogen storage alloy electrode,and has also presented analysis of the electrode early capacity decay as well as preliminary understanding of the discharge kinetics of the electrode.

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