Reaction Pathways for the Reduction of Molecular Oxygen

In spite of the considerable effort expended in trying to unravel the fundamental aspects of the O2 electroreduction reaction, many details about the mechanism are not fully understood. The electrochemical reduction of oxygen is a multielectron reaction that occurs via two main pathways one involving the transfer of two electrons to give peroxide, and the so-called direct four-electron pathway to give water. The latter involves the rupture of the 0-0 bond. The nature of the electrode strongly influences the preferred pathway. Most electrode materials catalyze the reaction via two electrons to give peroxide Peroxide pathway in acid 

Peroxide formation during O2 reduction can be followed by its reduction 

In strongly alkaline solutions or in organic solvents, 02is reduced via the transfer of a single electron to give superoxide ion 

Noble metals (e.g., platinum, gold, and silver), metal oxides, mixed oxides such as spinels, perovskites, and pyrochlores have been investigated, but by far the best catalytic material is highly dispersed platinum. The high cost of this metal stimulated research to look for less expensive alternative materials 

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