Halogen substitutions leading to enhanced oxygen evolution and oxygen reduction reactions in metalloporphyrin frameworks†
The oxygen evolution and oxygen reduction reactions (OER and ORR, respectively) are important in the field of renewable and clean energy, particularly for hydrogen production and fuel cells. These applications have so far been limited because of the high price of the catalysts and the energy loss due to overpotentials. Hence, non-precious metal catalysts with high efficiency toward the OER/ORR are desirable. In this work, we employ density functional theory (DFT) calculations to study the OER/ORR on metalloporphyrin and halogenated metalloporphyrin frameworks. The free energies of the reaction intermediates, including OH, O and OOH, were measured on 14 metal sites (Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Ir, Pt and Au) of the metalloporphyrin frameworks. Adsorption free energy relations were found and used to establish the reaction trend. The group 9 metals, namely Co, Rh and Ir, turn out to be potential candidates for both the OER and ORR because they provide intermediate free energies close to those of an ideal catalyst. The substitution of halogen atoms at the beta-positions of the metalloporphyrins of group 9 metals modifies the adsorption free energies of the intermediates so that they become closer to the ideal values and in turn reduce the OER and ORR overpotentials. After functionalization, Co-Por-F provides the lowest ORR overpotential and reduces the OER overpotential, approaching the value for an expensive Ir catalyst. Analysis of the electronic structure shows that controlling the d-band splitting by chemical manipulation of the single active site catalyst can be the key to enhancing the efficiency of these reactions.