Novel Multinuclear Catalysts for the Electroreduction of Dioxygen Directly to Water

Abstract

The electroreduction of O_2 to H_2O in aqueous acid at potentials close to the thermodynamically permitted value remains a daunting challenge for designers of superior fuel cells and batteries that utilize dioxygen as the reducible reactant. The four-electron reduction of O_2, which involves the rupture of the O-O bond and the formation of four O-H bonds, requires the use of catalysts to obtain useful rates at cathode potentials of interest in practical applications. The standard potential of the O_2/H_2O couple in solutions containing 1 M H^+ and saturated with O_2 at 1 atm is ca. 1.0 V (vs the saturated calomel electrode, SCE), but the highest cathode potentials achievable with currently available catalysts are closer to 0.55 V. (Molecules, functional groups, or metallic deposits that accelerate the rates of electrode reactions when they are confined to the surfaces of electrodes are often called electrocatalysts, a terminology that will be adopted in this Account.) Finely divided platinum supported on high area carbon is the electrocatalyst employed most frequently to achieve the electroreduction of O_2 to H_2O in presently available fuel cells. However, this type of electrocatalyst suffers from the disadvantages of high cost and gradual loss in catalytic activity as the surface area of the active platinum particles decreases because of sintering, dissolution, physical dislodgment, and/or adsorption of impurities. Searches for superior electrocatalysts for the reduction of O2 have often focused on cobalt porphyrins which are well-known to exhibit electrocatalytic activity toward the reduction of O_2, although H_2O_2 instead of H_2O is the usual product. However, it was discovered in recent years that a variety of molecular catalysts consisting of dimeric cofacial cobalt porphyrins adsorbed on the surface of graphite electrodes are able to catalyze the direct four-electron electroreduction of O_2 without passing through H_2O_2 as an intermediate. Both dimeric and monomeric iridium por phyrins have also been found to accomplish the electroreduction of O_2 to H_2O at unusually positive potentials. The mechanisms through which dimeric electrocatalysts are believed to operate involve the simultaneous interaction of both metal centers with the two oxygen atoms of the O_2 molecule as the O-O bond is severed. The ideas and strategies that underlay the development of these electrocatalysts have been described.