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Abstract

The chief aim of this work has been to contribute towards the development of a rechargeable metal-air battery. This part of the work is an investigation of factors affecting the rechargeability of the air electrode in particular, the mechanism of gas evolution from porous Teflon bonded electrodes in general. Platinum black and silver on graphite electrodes have been chosen as examples since they are the two most commonly used oxygen reduction electrocatalysts in primary metal-air cells.

The problems of using porous electrodes for gas evolution are (a) finding an electrode structure which is not damaged by and makes effective use of the available surface area during gas evolution, (b) finding an electrocatalyst which is stable under strongly anodic conditions, which has a surface oxide which is reduced at potentials > 1.2 V and which has a low oxygen evolution overvoltage.

Since Teflon bonded electrodes have proved to be the most effective for gas dissolution reactions it was decided to use this type of electrode structure. Oxygen and hydrogen evolution currents were drawn from several Teflon bonded electrodes using different electrocatalysts (eg. platinum black, silver on graphite, platinised graphite, platinised tungsten trioxide and tungsten carbide) and current collectors (eg. nickel screen, platinum gauze and graphite cloth) and it was found that only graphite based electrodes were damaged by gas evolution. Since these electrodes were damaged by both oxygen and hydrogen evolution and no carbon monoxide or dioxide was formed it was concluded that the damage was due to mechanical weakness rather than oxidation. By increasing the Teflon content of the electrode, graphite electrodes can be made to withstand gas evolution.

All common oxygen evolution electrocatalysts form surface oxides at high positive potentials. These surface oxides lower the oxygen evolution performance and if not reduced at potentials above 1.2 V they seriously lower the oxygen reduction performance. Only gold fulfills this condition; the oxide film on platinum remains more than a monolayer thick until the potential is < 0.85 V while the oxide layer on graphite cannot be completely removed even by hydrogen evolution, which rules out the use of graphite for rechargeable air electrodes.

Graphite and nickel are the materials with the lowest oxygen evolution overvoltage and a recently developed oxygen reduction electrocatalyst, cobalt nickel oxide, with a performance equivalent to silver on graphite, can be used for oxygen evolution taking advantage of the relatively low oxygen evolution overvoltage on nickel.

The oxygen reduction reaction on bare and oxidised platinum foil surfaces was compared with the oxide reduction reaction on oxidised platinum foil surfaces and it was shown that in order to obtain accurate kinetic data for oxygen reduction on an oxide surface it was important to ensure that the transient oxide reduction current had decayed to zero, and that a monolayer of oxide still remained on the electrode.

The kinetics of the oxide formation reaction on platinum foil were studied and shown to obey the Elovich law. However, it was also shown that the oxide film can grow to thicknesses greater than a monolayer and this together with the decay of current with time over long periods conflicts with the currently accepted mechanism for oxygen evolution and oxide formation. A method of separating oxygen evolution and oxide formation currents was developed from an analysis of current-time curves in the potential range 1.0–2.0 V.

A comparison between gas evolution on porous and planar electrodes showed that the percentage reduction in effectively employed surface due to blockage by gas bubbles is of the same order for both types of electrode. The potentiostatic pulse technique was applied to porous electrodes making a correction for double layer charging, the instantaneous currents thus determined enabled the effectiveness factors and intrinsic exchange current densities of porous electrodes to be determined.

The effect of Teflon content on gas evolution from a Teflon bonded platinum black electrode was minimal. This corresponds to a model postulated for Teflon bonded gas evolution electrodes where the factor limiting mass transfer of gas away from the electrode is the pressure needed before gas can escape from the micropores of the electrocatalyst.

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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