Effect of Ligand, Support and Solvent on the O 2 Binding of Nonprecious Metal Catalysts: An Ab Initio Study

Abstract

Non-precious metal (NPM) catalysts have been under extensive studyto replace expensive platinum based catalysts utilized in the Oxygen Reduction Reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells. In this work, density functional theory (DFT) calculations were undertakento understand the effect of structure, choice of the central metal, and support on a series of ML2 catalysts where M = Cu(I), Cu(II), Fe(II), Fe(III), Ni(II), and Co(II), and L = diaminetriazole (M-N2). O2 binding was determinedto involve the d-based anti-bonding dxz and dz2 orbitals of thecatalysts and two singly occupied π* orbitals of O2. When the catalysts were considered in an aqueous media (i.e., unsupported), Fe(II) exhibited the strongest O2 binding,followed by Co(II), Fe(III), Ni(II) and Cu(I); whereas the binding was observed in Cu(II).This order in the binding seems to correlate with the energy gap between the d orbitals of the metal cation and the π* of the triplet O2, with the smaller the gap resulting in stronger O2 binding. When a support and an explicit solvent moleculewere considered, the catalysts showed comparable O2 and H2O bindingmaking the O2 binding through H2O displacement much less favorable. Hence, the difference in the binding of H2O and O2 is seen to be critical in evaluating the activity of the catalyst.