Does the breaking of adsorption-energy scaling relations guarantee enhanced electrocatalysis?

The adsorption energies of numerous species on homogeneous and heterogeneous catalysts scale linearly with each other. Such linear dependence lowers the degrees of freedom in multistep reactions, greatly simplifying computational electrocatalysis models. The downside of scaling relations is that they limit the efficiency of electrocatalytic reactions. For instance, the scaling relation between *OOH vs *OH supposedly limits the oxygen evolution and reduction reactions (OER, ORR): while the energetic separation of these intermediates should ideally be 2.46 eV, on most catalysts it is ∼3.20 eV. Thus, it is currently assumed that breaking such scaling relation might lead to significant enhancement of OER/ORR electrocatalysis. In this review, we evaluate this hypothesis using literature data. The analysis suggests that breaking the *OOH vs *OH scaling relation is a necessary yet insufficient condition to optimize OER/ORR electrocatalysts. Alternatively, we define a new descriptor: the electrochemical-step symmetry index (ESSI), the optimization of which effectively corresponds to low calculated overpotentials.