City Research Online

Abstract

All aqueous secondary battery cells evolve hydrogen and oxygen towards the end of the charging cycle and when being overcharged. This leads to a progressively diminishing volume of electrolyte being present in the battery cell, and a maintenance function involving topping up the electrolyte with distilled water is, therefore, necessary to restore the electrolyte to its original volume. In order to avoid such function, catalyst such as Pd/AL2O3 has been used commercially to recombine the H2/O2 mixture so that the reformed water is allowed to return to the cell. Unfortunately, as the catalyst ages, it tends to suffer from ”start-up ” problems and eventually fails to function completely. Although many investigations have been carried out in the past 15 years or so, the problems remain unsolved. Therefore, the life and reliability of the catalysts in a catalytic reaction would be of paramount importance with secondary cell systems.

In the course of these investigations, the concepts of free-space and water condensation on the catalyst surface were discovered. These have led to an understanding of the problems which affect the life and reliability of catalysts in catalytic devices or hydrocaps. Generally, the ”start-up” problems were due to flooding by a thin film of water on the catalyst surface. As a result, the catalyst loading and specific surface area of the catalyst were increased significantly, and more active catalysts (e.g. Pt or Pd dispersed on NiCo2O4) were used. However, all these methods failed to solve the ”start-up” problem. On the other hand, the incorporation of a valve system to isolate the catalyst from the free-space during shut-down period, so that the water arising out of the slow reaction between residual H2/O2 on the catalyst surface is kept to minimum. Furthermore, the water droplets lying on the sides of the hydrocap containing the H2/O2 recombination catalysts tend to vaporise on standing and the water molecules are finally adsorbed on the packing material and/or catalyst. Such problems were successfully solved using a packing material (NiO) capable of adsorbing water vapour from its surrounding so as to reduce condensation of water on the catalyst, and itself capable of catalysing H2/O2 recombination under the temperature conditions attained in use.

The modification of commercial hydrocaps for the ingress of air (and thus oxygen) to the catalyst bed, and the provision of such an external source of oxygen enables substantially complete reaction of all hydrogen evolved in cases where there is a shortage of evolved oxygen, and in situations where substantially only hydrogen is evolved.

The catalytic activities of platinized NiCo2O4, Co3O4 and La0.5Sr0.5CoO3 have been found to be greater than that of platinum dispersed on alumina or glass powder. The mechanisms for the promotional effect of these metal oxides as active support for the reaction of hydrogen with oxygen to form water were studied by means of a differential system. Electrochemical studies and gas chromatography results show that the metal oxides did not absorb hydrogen nor oxide reduction as confirmed by x-ray powder diffraction analysis. However, the kinetic characteristics and specific platinum activity measurements show that the interaction of hydrogen and oxygen over the metal oxides is due to a temperature effect. The mechanism of catalytic oxidation of hydrogen by the oxide catalysts proceeds via alternate reduction and oxidation of the oxide surface, the reduction of the cations takes place on the oxide surface, which is reoxidised by oxygen.

The information on Co3O4 agreed reasonably well with the only published data, whilst the information on NiCo2O4, La0.5Sr0.5CoO3 and lithiated Co3O4 is new.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science Q Science > QD Chemistry T Technology > TP Chemical technology
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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