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Abstract

(THE MATHMETICAL EQUATION HAS NOT BEEN FORMATTED PROPERLY, PLEASE READ THE ABRACT THROUGH THE PDF TO GET THE CORRCT FORMATT)

The electrochemical reduction of oxygen in alkaline solution:-

2H20 + 02 + 4e + 40H-

presents one of the challenging problems of electrochemistry. An improvement in the kinetics and efficiency of the process would have a profound effect on power generation and on other aspects of every day life.

The reduction is generally regarded as a two stage process :-

H20 + O02 + 2e + 20H + HOZ (1)

H20 + HO> + 2e > 30H- (2)

This results in a mixed potential and most oxygen electrocatalysts have open circuit voltages in the region of 1.0 - 1.1 volts (vs RHE), significantly lower than the theoretical value of 1.229 V vs RHE at 25°C. Stage (2) is normally the rate-determining step and significant improvement in performance and open circuit voltage can be expected if the peroxide step is eliminated. The key to the solution of the oxygen electrode problem may lie in the electro-catalyst's ability to dissociatively chemisorb oxygen. If this condition is fulfilled, no HO2- intermediate is formed and the oxygen will go directly to hydroxyl ion via a 4-electron process. According to the joint pseudosplitting mechanism, a necessary condition for dissociative chemisorption of oxygen is for the oxygen molecule to to be chemisorbed 'side-on' on the electrocatalyst surface. Theoretical considerations suggest that the requirement for side-on chemisorption of oxygen can be satisfied using paramagnetic or ferromagnetic oxides. According to this consideration, perovskite oxide catalysts (La1 -x,Srx,Co03) enable the paramagnetic oxygen molecule to be chemisorbed side-on, since Sr doped LaCo03 is paramagnetic at room temperature. One such compound La0. 5Sr0-5Co03 exhibits complete reversible behaviour towards oxygen reduction in 45% KOH solution at room temperature. Also, the electrochemical performance of this catalyst increases linearly with temperature, surpassing that of platinum black at 170°C. However, its performance below 100°C was too low to be of practical interest. The purpose of this investigation is to study the kinetics of oxygen reduction on semiconducting oxides with particular reference to its electronic structure and magnetic properties and to optimize the performance of a practical oxygen electrode for low temperature fuel cell application. The electrochemical reduction of oxygen on teflon-bonded Nd0.5Sr0-5CoO3 electrode in 453 KOH was studied as a function of temperature and oxygen partial pressure. The activation energy was 10.75 kcal/mol, similar to that for dissociative oxygen chemisorption. The relationship between i and P02 is of the form (mathematical equation, cannot be formatted. Please read the pdf abstract) Suggesting that the rate of oxygen chemisorption is the rate-controlling step. Galvanostatic oxygen stripping showed that the surface coverage of oxygen was only 1% for both Sr doped and LaCo03 and NdCo03 and that the coverage was independent of temperature in the range 250G 80°C. This Suggests that the improvement in performance with temperature is not due to the increase in the number of active sites.

Further investigation on the series La1x Srx C003 showed that both the maximum performance and the maximum oxygen coverage were obtained at x = 0.5. This indicates that the improvement in steady state performance with x is due to the increase in the number of the active sites. The results also suggest that the double exchange couple (mathematical equation, cannot be formatted. Please read the pdf abstract) satisfies an important condition for the splitting of the 0-0 bond and is acting as an active site on the perovskite oxides. Mathematical calculation shows that only a small portion of Co ions are available for the oxygen chemisorption process. Crystallographic considerations suggest that the number of active sites on La1-x,SrxCo03 is governed by geometric factors.

In view of the higher proportion of available transition metal ions, NiCo204, is expected to be a more suitable oxygen electrocatalyst than the perovskite oxides. Unfortunately, its electrochemical instability below 750 mV rules out the possibility of using NiCo204, as a practical oxygen electrode. However, by mixing graphite with the NiCo204, it is possible to achieve significantly higher performance at 750 mV.

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

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