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 (ϕ^M), which can be easily varied within 0.5 V in experiments, and the potential at the reaction plane in solution (ϕ^OHP), 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 ϕ^M and ϕ^OHP 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 ϕ^OHP, 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.