Towards a unified way of comparing the electrocatalytic activity MN4 macrocyclic metal catalysts for O2 reduction on the basis of the reversible potential of the reaction

We have revisited correlations between catalytic activity and formal potentials of MN4 catalysts for the reduction of O-2 (ORR). When comparing the activities of Cr, Mn, Co and Fe MN4 complexes for ORR as a plot of (log i/n)(E) versus the M(III)/(II) formal potential of the catalyst (n = number of electrons involved in the ORR), two linear correlations are obtained: one for Cr, Mn and Fe complexes and another for Co complexes. Mn and Fe are 4-electron reduction catalysts and Cr and Co are 2-electron reduction catalysts for ORR in alkaline media. We show that instead of plotting the formal potential E degrees' of the catalyst but the difference between this parameter and the reversible potential E-rev of the corresponding catalytic reaction (O-2/OH-) or (O-2/HO2-), as (E degrees' - E-rev) and using the Co(II)/(I) formal potential for cobalt macrocyclics, instead of the Co(III)/(II) redox couple, all data points merge into one single linear correlation of slope + 0.170 V/decade. It is possible that when (E degrees' - E-rev) approaches zero, the maximum activity could be achieved and this is especially important for 4-electron catalysts and the linear correlation might be part of an incomplete volcano correlation. (C) 2014 Elsevier B.V. All rights reserved.