The oxygen reduction reaction (ORR) is the reduction of oxygen to water, which releases energy that can be used to generate electricity. In terms of the efficiency of electricity generation, electrochemical fuel cells containing an appropriate catalyst are more efficient than burning H2 and O2 to generate heat to run a turbine. The best catalysts for ORR contain platinum, and while alternative materials have been considered, it has proven quite challenging to optimize the efficiency of ORR electrocatalysts. In terms only of the efficiency of ORR, articulating the effect of this ideal electrocatalyst on the catalytic cycle is easy, but rather difficult to actualize. The ideal electrocatalyst would harvest the same amount of energy per electron transferred in each step of the overall electrochemical reaction 4H ! + 4e " + O # → 2H # O. The associative reaction mechanism, the mechanism generally understood to occur for many successful ORR catalysts, proceeds through the steps,