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Abstract

The use of fuel cells as an energy supply in the space programme during the 1960’s in the U.S.A, stimulated enormous interest in the investigation of electrocatalysts for oxygen reduction reaction, due to its vital importance in fuel cell research and development. However, despite the large research effort and extensive literature, the efficiency achieved in fuel cells is well below the theoretical value, and the kinetics and mechanism of oxygen reduction, particularly on gas diffusion electrodes, remain mostly a matter of speculation.

In this thesis extensive studies of the role of Co3o4 catalysts in oxygen reduction on Teflon-bonded Co3o4/graphite electrodes in alkaline solution were carried out both chemically and electrochemically. It was proposed that during oxygen reduction, the intermediate HO2- formed was homogeneously decomposed by dissolved Co2+ ions: Co3O4 is capable of being dissolved in concentrated alkaline solution. Impedance responses for 02 reduction on Teflon-bonded graphite electrodes, Co3o4/graphite electrodes and semiconducting cobalt oxide electrodes, were, both experimentally and theoretically, studied. The accuracy and limitation of using a.c. impedance technique in gas diffusion electrodes were also discussed. In the final part of this thesis (Chapter 9) a novel deposition method to produce highly porous cobalt electrodes at room temperatures was reported. Preliminary experiments
for the regeneration of air electrodes in situ were also carried out.

A high surface area Co3O4 (HSAC) was prepared by oxidizing and 2+ precipitating Co ions in alkaline solution. The crystallographic structure of the prepared oxide was needle-shaped. The HSAC catalysts were characterized by high surface area (89.88 m2/g) and low electrical resistivity (1121.6 ohm’cm). It was shown that HSAC catalysts are highly active for both peroxide decomposition and oxygen reduction. When HSAC was coprecipitated with graphite to form a composite catalyst, a polarization potential of about 825 mV vs. DHE at 200 mA/cm2 (25°C, 5N KOH and under air) was obtained, and the electrode performance was quite stable.

The kinetics of the decomposition of hydrogen peroxide in the presence of 1 ppm Co2+ ions in 5N KOH solution were examined in Chapter 4. It was demonstrated that Co2+ ions in alkaline solutions are very active towards H2O2 decomposition. The decomposition reaction was found to be a 1.2 -order reaction.

A homogeneous decomposition mechanism involving the chain reactions was proposed. The peroxide decomposition was considered to be initiated by the formation of free radicals. The heterogeneous behaviour of cobalt oxide catalysts in alkaline solutions was explained by the termination processes of the free radical on Co2+ ions in the spinel lattice. The equivalent circuit coupled with an adsorption process was employed to study the impedance responses in oxygen reduction on Teflon-bonded graphite electrodes. The characteristic exchange current density io' calculated from the charge transfer resistance Rct was about 2 mA/cm , which is in good agreement with the data reported on pyrolytic graphite electrodes. The adsorption resistance R evaluated was found to be much higher that RQt value, indicating again that the surface adsorption process of oxygen molecule is the rate determining step during 0^ reduction. The stability of graphite electrodes was examined by measuring the apparent double layer capacitance. It was concluded that the instability of a graphite electrode is, mainly, caused by the attack of peroxide, formed during oxygen reduction, on the active sites of the graphite electrode.

In order to correlate the electrochemical activity of the electrodes and catalytic activity of the oxides incorporated, the impedance responses on Co3O4/graphite electrodes were analysed, using an analytical method for an electrode reaction coupled with a catalytic process. Results obtained showed that there is a close correlation between the catalytic parameters kc and electroactive parameter iQ' values, both evaluated from the impedance measurements. The increasing iQ' values with the catalytic activities of Co^O^ were mainly due to the increase of recycling efficiency inside the pores of the electrodes. The dependence of kc values on the measured potentials demonstrated, electrochemically, that HO2 formed during 02 reduction on Co3O4/graphite electrodes is homogeneously decomposed by the dissolved Co2+ ions.

In Chapter 8, 0? reduction reactions on HSAC and 3.1 at% Li-doped Co3O4 (LDC) electrodes were studied, using steady state polarization and impedance methods. An empirical impedance model, incorporating a surface oxide layer model and a modified Randles circuit, was developed. Though the theoretical background of this approach has not been established unanimously, it was shown clearly that the kinetics and mechanism of oxygen reduction are strongly dependent on the surface state of the semiconducting cobalt oxides, and the porous profile of the electrode has a more profound influence on the diffusion processes. A novel reactive deposition method to prepare a highly porous cobalt electrode at room temperatures was described in Chapter 9. In this method, the cobalt was deposited electrochemically from cobalt salt solutions in the presence of bubbling oxygen. The porous cobalt electrodes prepared by the deposition method showed very fine grains (about 1 micron) and the pores are uniformly distributed. Significantly, the deposited cobalt electrodes showed very high electrical performance in alkaline solutions. The preliminary results of the regeneration of air electrodes in situ, based on the deposition of cobalt catalysts on the porous electrodes, was also reported. The potential application of such a reactive deposition method in the manufacture of storage batteries such as nickel-cobalt and silver-cobalt, and regeneration of air electrodes were discussed in Chapter 10.

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

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