Superoxide Production by Electrocatalysis

In aqueous systems, superoxide ion was only observed on eleetrodes modified in alkaline solutions by surfaetant (triphenylphosphine oxide) [91], organie groups (methyl phenyl) [88], or metal maeroeyelie eompounds [88, 89, 92]. The surfaetant and organie groups ean eover the eleetrode surfaee, preventing water from reaehing the eleetrode surfaee and stabilizing the produeed O2. The reversibility of the O2/O2 eleetroehemieal response depends on the alkalinity [88, 90]. In less alkaline solutions, the redox behavior beeomes less reversible, due to the kineties of the reaetion. 

The formal potential of the O2/O2 redox couple is very close to flie redox potential of the catalyst. For both Co(lll)2-DSP and Co(ll)HFPC systems, the formal potential of O2/O2 is 0.0 V vs. NHE, normalizing the air pressure to O2 pressure. This value is close to that reported in [78] for the initial reversible 1-electron transfer process of O2 in aqueous solution at inert electrodes  

Electrocatalytic ORR carries out in three pathways the 1-electron transfer pathway, producing superoxide ion the 2-electron transfer pathway, producing hydrogen peroxide and the 4-electron transfer pathway, producing water. In a non-aqueous aprotic solvent system, a room-temperature ionic liquid system, and on specific transition-metal, macrocyclic-compounds-coated graphite electrodes in alkaline solutions, 1-electron reduction can be observed. Carbon materials, quinone and derivatives, mono-nuclear cobalt macrocyclic compounds, and some chalcogenides can only catalyze 2-electron ORR. Noble metal, noble metal alloy materials, iron-macrocyclic complexes, di-nuclear cobalt macrocyclic complexes, some chalcogenides, and transition-metal carbide-promoted Pt catalysts can catalyze 4-electron reduction. 

Zhang L, Zhang J, Wilkinson DP, Wang H. Progress in preparation of non-noble electrocatalysts for PEM fuel cell reactions. J Power Sources 2006 156.2 171—82. 

Yeager E. Dioxygen electrocatalysis mechanism in relation to catalyst structure. J Mol Catal 1986 38 5-25. 

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