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 O₂ (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 ′ of the catalyst but the difference between this parameter and the reversible potential E_rev of the corresponding catalytic reaction (O₂/OH^-) or (O₂/HO₂ ^-), as (E ′ - 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 ′ - 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.