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Abstract

Oxygen Evolution

The oxygen evolution reaction is of particular interest to secondary metal-air batteries, water electrolysis plants and oxygen extractors. Criteria have been suggested for the selection of possible active materials for the oxygen evolution reaction. A survey of various metal oxides based on these criteria suggested that NiCo₂O₄ is of particular interest. Teflon-bonded electrodes based on this material gave 3 A/cm² at 1.72 V vs D.H.E., 70°C, 5N KOH. The anodic performance of the other compositions in the NiₓCoᵧO₄ series, where 0 < x < 3 and 0 ≤ y < 2, were evaluated at different temperatures.

Two types (Teflon-bonded and non-Teflon-bonded) of NiCo₂O₄ porous electrodes were investigated. The Teflon-bonded electrodes displayed higher performance and greater mechanical strength during the evolution of oxygen.

The effect of modifying the hydrophobic nature of Teflon-bonded NiCo₂O₄ electrode surfaces by depositing a thin layer of NiCo₂O₄ (by thermal decomposition of Co/Ni nitrates) on the surface was investigated, and the modified surface electrode showed a significant improvement in anodic performance.

Steady-state measurements for oxygen evolution on porous electrodes are inaccurate for the determination of the kinetic parameters necessary for kinetic studies due to concentration polarization and the continuous change of the available surface area with current densities due to the formation of bubbles. Therefore, it is suggested that the potentiostatic pulse technique could solve the above problems. Reliable pulse measurements could be obtained, provided that the electrodes were initially anodized sufficiently to form stable higher oxides and that the potentials of these electrodes were maintained above that for reduction of the higher oxides.

Oxygen Extraction

A novel, integrated electrochemical-chemical method is described in this study. The system consists of a Teflon-bonded graphite cathode, which reduces oxygen via a two-electron process with the formation of HO₂⁻ ions in alkaline solution (5N KOH). The HO₂⁻ ions are chemically decomposed using a suitable catalyst to yield oxygen, and a Teflon-bonded NiCo₂O₄ anode is used to evolve oxygen electrolytically. At 40°C, 5N KOH, the power consumption of this device is only 2.69 kWhr/1000 litres of oxygen, significantly lower than conventional electrochemical oxygen extractors (4.4 kWhr/1000 litres oxygen).

Oxygen reduction on high surface area graphite in 5N KOH was investigated and the concentration of the peroxide ions produced was measured at different currents.

For the decomposition of hydrogen peroxide, two catalysts (CoFe₂O₄ and NiCo₂O₄) were evaluated, and the kinetics of H₂O₂ decomposition on these catalysts was investigated.

Publication Type:	Thesis (Doctoral)
Subjects:	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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