Kinetic Aspects of Electrochemical Oxygen Reduction

Oxygen reduction in an acid medium may occur according to the following reactions  

The apparent number of electrons transferred during ORR, , has been measured for several Fe-based catalysts obtained by the pyrolysis of various Fe precursors, including Fe-N4 chelates, iron phenanthroline or salts such as iron acetate. Values of n and the associated yields of hydrogen peroxide, %H202, are reported in Table 3.2 for Fe precursors and in Table 3.3 for Co precursors. %H202 is obtained from the following equation %H202 = 100 (4 - n)tl. 

According to these tables, reduction of O2 on Fe-based catalysts occurs mainly to water (n = 4), except for cationic and anionic FeTMPP, while the lower value of n obtained for Co-based precursors indicates a larger release of peroxide (n = 2) by these catalysts. The reduction of O2 to water may either occur directly with the transfer of 4 electrons (4e ) or O2 may be first reduced to H2O2 with the transfer of two electrons, followed by the transfer of another two electrons during a subsequent reduction of H2O2 (2 x 2e ). The rate constant for reaction (3.1) in the 4e transfer mechanism is A i in the following scheme, while two rate constants k2 and ks for the consecutive reactions (3.2) and (3.3) are involved in the 2 x 2e transfer mechanism for ORR to water. 

Wroblowa proposed an analysis to discriminate between k and k2 using the ring and disk currents, 7r and 7d, respectively measured at various angular rates, CO, and at a fixed potential value with a rotating-ring disk electrode (RRDE) The following relation has been demonstrated  

Precursor Carbon support Heat-treatment temperature (°C) n %H202 References 

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