Thermal activation, transition metal macrocycles

This chapter provides a critical review of transition metal macrocycles, both in intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. Fundamental aspects of electrocatalysis, oxygen reduction and transition metal macrocycles will be highlighted in this brief introduction, which should serve as background material for the subsequent more specialized sections. 

In 1964, Jasinski discovered that certain macrocyclic compounds of transition metals (Fe, Co, etc.), N4 compounds such as phthalocyanins and tetra-azaannulenes, are very active catalysts for oxygen reduction in acidic solutions. However, their stability during prolonged work in acidic media was found to be very low. Sometime later it could be shown (Bagotsky et al., 1977-1978 that after thermal treatment at 700 to 800°C these compounds, despite their partial decomposition, not only retain their catalytic activity but acquire the chemical stability required for long-term operation. Sometimes their activity even increases. Heat-treated compounds of this type then became the subject of numerous studies. 

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