City Research Online

Abstract

The oxygen electrode is highly irreversible and is the main cause of efficiency losses in many electrochemical systems. In recent years the use of oxides as oxygen electrodes have been suggested and many oxide electrodes were studied in this respect. Of such oxides, the spinel structured NiCO₂O₄ was shown to be an active oxygen elect-rode in alkaline medium.

This study reports the work that was carried out to investigate the reasons for the good activity of NiCO₂O₄ as an oxygen electrode. Studies on the oxygen evolution aspects were carried out by investigating the oxidation state transitions of the surface compounds formed before oxygen is evolved and ascertaining the Tafel and reaction order parameters for oxygen evolution. The results of such studies and a knowledge of the transition metal cationic sites on NiCO₂O₄ enabled the postulation of a mechanism, suggesting that trivalent cations of Ni and Co are more active for oxygen evolution rather than the divalent ones. This postulation was confirmed by carrying out oxygen evolution studies on similar lines, on 4 oxides of the Li doped CO₃O₄ system. This work enabled the development of a better oxygen evolution electrocatalyst than NiCO₂O₄. The performance of a Teflon bonded 10 at % Li doped CO₃O₄ electrode was found to be 1.52V vs the dynamic hydrogen electrode at a current density of 1A cm-2 in 5 mol dm–3 KOH at 70°C. This performance is significantly better than that obtained on NiCO₂O₄.

This thesis also discusses some factors that govern the selection of oxides for oxygen evolution, such as the role of the lower metal oxide/higher metal oxide couple and the corrosion stability of oxides.

Since the efficiency of an oxygen evolving elect-rode is determined by the electrocatalyst characteristics and the electrode characteristics, some effort was also directed in comparing the efficiency of porous non Teflon bonded and Teflon bonded electrodes. The effectiveness factor of Teflon bonded electrodes for oxygen evolution was found to be higher than the non Teflon bonded porous electrodes.

On oxygen reduction aspects, the potentiostatic pulse technique was used to study the mechanism of oxygen reduction on NiCO₂O₄. A mechanism has been proposed by considering the experimentally observed values of the kinetic parameters and the nature of the cation sites on NiCO₂O₄

Oxygen reduction studies were also carried out on composite NiCO₂O₄/graphite electrodes. In these electrodes the HO₂ formed by oxygen reduction on graphite is broken down by NiCO₂O₄, thus, bringing about a synergetic enhancement in performance. This study establishes by means of simple models, the factors that govern the attainment of the maximum oxygen reduction performance in composite electrodes. It also establishes that the peroxide decomposition activity of oxides such as NiCO₂O₄ is sufficient to bring about the maximum possible oxygen reduction performance, if the two electrocatalysts are in a state of interparticulate mix.

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

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