Potential-Dependent Volcano Plot for Oxygen Reduction: Mathematical Origin and Implications for Catalyst Design.

Climbing up the volcano peak stands as a challenging problem for oxygen reduction. Repeated efforts have been made to fine-tune the binding energy of oxygen reaction intermediates within a narrow region of 0.2 eV by adjusting the catalyst electronic structure. Herein, we address ourselves to two different, oft-neglected but nontrivial questions: (a) Does a superior oxygen reduction reaction catalyst in rotating disk electrode experiments still work well in practical fuel cells (usually at a different potential)? (b) For a given catalyst, can we place it on the volcano peak by adjusting the electrode potential (ϕM), which can be easily varied within 0.5 V in experiments, and the potential at the reaction plane in solution (ϕOHP), which is modulated by double-layer electrostatic effects? To answer these two questions, we articulate the mathematical origin of the volcano plot and reveal its dependence on ϕM and ϕOHP by combining a microkinetic model for the oxygen reduction reaction and a mean-field model for the double layer. Furthermore, we explore possible approaches of adjusting ϕOHP, for instance, by varying electrolyte concentration and particularly by tuning the electrostatic properties of the support material in a supported catalyst system. The investigation of how electrostatic properties of the support material affect the volcano plot of a supported catalyst opens an additional channel of catalyst-support interactions.