In Search of the Most Active FeN4 and FeN5 Based Catalyst for Theoxygen Evolution and Reduction Reactions

Abstract

The oxygen evolution and reduction (OER and ORR) are fundamental chemical reactions 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 OER and the ORR are complicated reactions because of the high Gibbs free energy (ΔG) associated to the O double bond and because of the transfer of up to 4 electrons and 4 protons. For its nature, those reactions proceeds through the consequent formation of more than one intermediate accounting for the so-called “scaling correlations”. The complexity of the reactions determines the slow kinetics and the difficulties into finding an adequate catalyst for it to proceed without energy losses. In this study we tested various Fe phthalocyanines (FeN4) adsorbed on carbon nanotubes in the absence and in the presence of pyridine axial ligands to form penta-coordinated catalysts (FeN5). The composites were analyzed spectroscopically, and electrochemically characterized at pH 13 and pH 1. Computational analysis simulate the reactions on the Fe metal center. Based on the activity of the catalyst for the reactions and on the calculated binding energies volcano-correlations and activity indexes are drawn.