Results And Discussion

The role of macrocycles or N4 chelates in oxygen reduction in alkaline electrolytes has been extensively studied. The activity and stability of the macrocycles impregnated on carbon supports are improved by thermal treatment at higher temperatures. Fig. 1 and Fig. 2 show the potential versus current density characteristics of the gas diffusion electrodes, prepared from CoTPP with loadings of approximately 1.5 mg/cm2. As shown in the figures, 6M KOH and 80 "C gives a higher performance improvement than the other concentrations and temperatures studied. This is due to the influence of temperature on the electrode kinetics and the characteristics of the electrolyte where higher ionic conductivity and lower diffusion coefficient depend on the strength of the bulk concentration (2). However, the high performance of the CoTPP-based electrode is not sustainable due to factors mainly such as dissolution of cobalt (3), wetting-in properties of the electrode (4) or corrosion of the electrode materials (5). Atomic absorption spectrophotometer (AAS) analysis of the electrolyte after 200 hours of electrochemical operation of the electrode showed an almost 60% loss of cobalt, which together with the charred residue of the macrocycle constitute the active component in the reduction of oxygen.

Fig. 3 shows polarization curves for electrodes catalyzed by CoTPP, platinum-cobalt alloy and platinum. CoTPP shows superior activity among the electrocatalysts considered. Surface, structural, chemical and electrochemical characterizations of the Pt-Co electrocatalysts have been reported elsewhere (6). The Pt-Co alloy catalyzed electrode was superior by approximately 20 mV than the pure platinum-based electrode. Jalan et al (7) suggested that the smallest nearest-neighbor distance of platinum atoms with the transitional metals would be ideal for the dual site adsorption of oxygen or peroxide. Paffett et al (8) proposed that surface roughness effects due to dissolution of the base metal gives surface area effect and hence improvement of the oxygen reduction activity.

Fig. 4 shows comparison of the long-term operation of the Pt-Co and CoTPP-based electrodes. At higher temperatures, the decay rates of the CoTPP-based electrode are higher than lower temperature operation, where small decay rates after 7000 hours of operation of the CoTPP-based electrodes were reported (3). The operation time of the Pt-Co electrocatalyst increased approximately four-fold compared to the CoTPP-based electrode. TEM analysis of a fresh and used electrode after 670 hours of electrochemical operation has shown agglomeration of the catalyst particles, which is the main factor for the deterioration of the electrode performance (9).

Acknowledgement-The authors wish to thank Professor Olle Lindstrom for discussions. Financial support by the Swedish Agency for Research Cooperation with Developing Countries (SAREC) is gratefully acknowledged.

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