Roles of radical characters of pristine and nitrogen-substituted hydrographene in dioxygen bindings

Abstract

We investigate by means of density functional theory (DFT) calculations how hydrogen-terminated graphenes (hydrographenes) with and without nitrogen impurities interact with dioxygen. The current study aims at searching whether hydrographenes can be utilized as cathode catalysts in fuel cell with a focus on dioxygen binding, the first step in oxygen reduction reaction (ORR). If hydrographenes have a nanometer-size rhombic structure with zigzag edges, unpaired electrons are localized at their edges with or without the nitrogen impurities. Spin localization comes from frontier orbitals of the nanometer-size hydrographenes whose amplitudes appear only at their edges. Due to their radical characters, dioxygen can bind to an edge carbon atom of the hydrographenes under the condition where fuel cell is usually operated. There are two types of dioxygen binding into a hydrographene: one is a Pauling fashion where one C-O bond is formed and the other is a bridging fashion with two formed C-O bonds. In the bridging fashion, the formation of the two C-O bonds activates dioxygen, and then radical characters of the oxygen atoms completely disappear. In contrast, the Pauling fashions retain an unpaired electron on the oxygen atom that does not participate to the C-O bond formation. The existence of radical oxygen atoms would facilitate the next step in ORR (the initial proton transfer to an adsorbed dioxygen), whereas such facilitative effects cannot be seen in its absence. According to DFT calculations, the Pauling-type bindings are always energetically preferred over the bridging-type bindings. In particular, the C→N substitution enhances the preferences of the Pauling-type binding over the bridging-type binding compared with the pristine case. Accordingly DFT calculations demonstrate that radical characters of edge carbons of a nanometer-sized rhombic hydrographene play a crucial role in dioxygen bindings in a Pauling fashion that would be responsible for enhancing the catalytic activity in fuel cell.