The effects of the ligand, central metal, and solvent on the O2 binding of non-precious metal catalyst model systems: An ab initio study

Non-precious metal (NPM) catalysts are currently under development to replace the expensive platinum-based materials currently utilized for oxygen reduction in PEM fuel cells. In this work, systematic studies were carried out to examine the effect of central metal, chelating ligand, and solvent on the O2 binding activity of a series of MN2 and MN4 NPM catalysts (Im)MLn where M = Cu2+, Fe2+, Fe3+, Ni2+ and Co2+, L = diaminotriazole or porphyrin, and a support ligand imidazole (Im). O2 and H2O binding energies were calculated for all the catalysts. Cu2+-based catalysts exhibit no activity toward O2 regardless of the ligand, Fe2+- and Co2+-based catalysts show the strongest O2 binding, and the rest fall in between. This is in alignment with the energy gap between the metal 3dz23dz2 and the in-plane anti-bonding π* orbital on the O2 with the larger the energy gap, the weaker the interaction. Porphyrin-based catalysts bind weakly with H2O compared to their diaminotriazole counterparts, which is attributed to the larger energy gap between the HOMO of H2O and the higher lying LUMO of Porphyrin-based catalysts resulted from a stronger anti-bonding interaction between their metal d orbitals and the σN orbitals of porphyrin. We propose that the initial O2 absorption activity of MN4 or MN2 catalysts in the oxygen reduction reaction be considered on the basis of the relative binding of O2 to H2O.