Reinterpreting volcano correlations in bio-inspired Electrocatalysts: The role of active site availability in oxygen reduction

The binding energy E M − O 2 of dioxygen to active sites is the most common reactivity descriptor for the electroreduction of O 2 promoted by metals and for macrocyclic MN4 molecular catalysts. When activity is plotted as log j E versus the M − O 2 binding energy, a volcano-shaped correlation is obtained with perfluorinated iron phthalocyanine (16(F)FePc), showing the highest activity. However, we report here that, surprisingly, in spite of its high activity, 16(F)FePc exhibits the largest activation energy of a series of MN4 complexes examined. In contrast, biomimetic Fe porphyrins appear on the low activity section of the descending region of the volcano plot and exhibit very low and even negative activation energies. The apparent low activity of Fe porphyrins as log j E is attributed to the extremely low concentration of Fe(II) active sites at the potential E chosen for comparing the activity in a volcano plot. In alkaline media, under those conditions Fe porphyrins predominate in the inactive state Fe(III)-OH state. Fe porphyrins exhibit strong O 2 binding energies that probably facilitate fast electron transfer to the bound O 2 , as in aerobic life. Thus, the descending region of the volcano can be understood in terms of a new Principle, stated as follows “ For an electrocatalytic process to proceed at an optimal rate, the applied overpotential must not be high enough to drive sufficient current, but low enough to avoid converting the metal center to an inactive oxidation state ”. • New interpretation of activity volcano correlations on the basis of activation energies. • New interpretation of the falling region of the volcano. • Experimental energies of activation do not match data calculated in the abnsece of electrolyte and electrode potential. • We find a possibility that volcano correlations are potential dependent which has never mentioned in the literature. • Our finding possibly applies to metal electrodes which in the descending portion of the volcano they exist as oxides in real situations (applied potential).