(Invited) Oxygen Reduction Reaction at MN4 and MN5 Catalysts - Calculated and Experimental O2-Fe Binding Energies, Activity Indexes, Volcano Correlations
Authors: Tasca Federico , Zagal Jose H , Orellana Walter , Marco Jose 
Published: February 2020

The oxygen reduction reaction (ORR) is a fundamental chemical reaction in industrial processes as well as in living systems, being O2, the final electron (e-) acceptor of many reactions. Since then its importance in the fuel cell technology where energy would be stored in the form of H2 to be oxidized in conjunction with the reduction of the cheap and abundant O2. However, the ORR is a complicated reaction which involves the transfer of 4 electrons and 4 protons and for its nature, it proceeds through the consequent formation of more than one intermediate accounting for the so-called “scaling correlations” [1]. The complexity of the reaction determines the slow kinetics and the difficulties into finding an adequate catalyst for it to proceed without energy losses. The “industrial standard” catalysts for this reaction are based on the very expensive and rare Pt metal. An uncountable number of non-precious metal catalysts (NPMC) have been studied with the purpose to replace Pt. In our research group, we are adopting various strategies to increase the activity and the stability of metal phthalocyanines like the substitution of planar neutral residues with more electron-negative ones, and the addition of axial back ligands [2-5]. In this research work we studied the electrocatalytic activity towards the ORR of a new compound i.e. the iron 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadeca(fluoro) phthalocyanine (16(F)FePc) in the absence and in the presence of a fifth pyridine axial ligand (FeN5). Interesting the 16(F)FePc appears to be the most active among all the FeN4 studied to the moment. The very high redox potential of the active Fe(III)/Fe(II) redox couple (-0.075 V and 0.44 V vs. SCE at pH 13 and at pH 1 respectively) confirms the correlation between the activity of the catalyst vs. redox potential of the active redox couple of the metal in the catalyst. 16(F)FePc was so active towards the ORR that the production of H2O2 measured at rotating ring electrodes was nil at pH 13 and very low at pH 1. In the presence of pyridine axial ligand, the redox potential increased of almost 60 mV and catalytic currents and stability of the catalyst also drastically increased. The catalysts were characterized with electrochemical techniques and by EPR and XPS spectroscopy in the presence and in the absence of O2. We could therefore experimentally evaluate the binding energy of O2 to the Fe metal centre. Ab initio calculations, confirm the experimental data and the importance of the pyridine axial ligand to lower the binding energy of O2 to the Fe metal centre because of decoupling of the Fe from the carbon support and changes in the geometrical and electronic structure.
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Acknowledgments
F.T thanks Fondeyct project 1181840. J.H.Z thanks the Millennium Project RC120001, Fondecyt 1181037. W.O. thanks Fondecyt 1170480.