Density functional theory study of LaMnO3 and its competing oxides: an insight into a prospective alkaline fuel cell cathode

LaMnO3 is an inexpensive alternative to precious metals (e.g. platinum) as a catalyst for the oxygen reduction reaction (ORR) in alkaline fuel cells (AFCs). In fact, recent studies have shown that among a range of non-noble metal catalysts, LaMnO3 provides the highest catalytic activity. However, further development of this catalyst is limited by the fact that very little is known about LaMnO3 in the AFC environment. While it has been established that the bulk phase possesses an orthorhombic structure, it has not been possible to determine the structure of the surfaces or the sites active towards the ORR. In this work, therefore, periodic hybrid-exchange (B3LYP) density functional calculations are performed in order to understand the origins of the catalytic activity of LaMnO3. The long term goal is to suggest strategies for optimising the activity of LaMnO3 through control of its crystallite morphology. Initially, the phase stability of LaMnO3 with respect to its competing (La, Mn) oxides is determined by accurate calculation of the Gibbs formation energies of each compound (1.6% mean error). The accuracy achieved is higher than in previous literature, validating the methodology adopted and the reliability of the chemical potentials determined to limit the stability of the bulk and surfaces of LaMnO3. Having determined the ground state of each Mn oxide it was possible to simulate electron energy-loss spectroscopy (EELS) for Mn in different valence states and local environments. The simulated EELS demonstrated that it is possible to identify its oxidation state and local coordination (i.e. the surface structure) on LaMnO3 surface terminations, based on the shift and shape of predicted L3 edges, which correlate well with measured EEL spectra. Calculations of the low-index, stoichiometric and non-polar surfaces of LaMnO3 were then performed in order to predict the equilibrium crystal morphology. For each low energy surface the adsorption sites were also identified. The energetics of the surfaces are rationalised in terms of the cleavage of Jahn-Teller distorted Mn-O bonds, the compensation of undercoordination for ions in the terminating layer and relaxation effects. Finally the adsorption sites identified are investigated by adsorption of molecular O2. The binding energies, adsorbate structure and charge transfer are analysed to predict the reactivity of each site. Results indicate that Jahn-Teller distortion and the coordination of Mn sites modulate the binding strength of O2. The main results presented are the crystallite morphology, the identification of surface reaction sites and the chemical characterisation of those sites. This is a theoretical characterisation of the LaMnO3 catalyst providing detailed atomistic information that has not been possible to deduce from experiment.Open Acces