EFFECT OF LIGAND, SUPPORTAND SOLVENT ON THE O_2 BINDING OF NON- PRECIOUS METAL CATALYSTS: AN AB INITIO STUDY

摘要

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 ML_2 catalysts where M = Cu(Ⅰ), Cu(Ⅱ), Fe(Ⅱ), Fe(Ⅲ), Ni(Ⅱ), and Co(Ⅱ), and L = diaminetriazole (M-N_2). O_2 binding was determinedto involve the d-based anti-bonding d_(xz) and d_(x2) orbitals of thecatalysts and two singly occupied π~* orbitals of O_2. When the catalysts were considered in an aqueous media (i.e., unsupported), Fe(Ⅱ) exhibited the strongest O_2 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 O_2, with the smaller the gap resulting in stronger O_2 binding. When a support and an explicit solvent moleculewere considered, the catalysts showed comparable O_2 and H_2O bindingmaking the O_2 binding through H_2O displacement much less favorable. Hence, the difference in the binding of H_2O and O_2 is seen to be critical in evaluating the activity of the catalyst.