The stability of LaMnO3 surfaces: a hybrid exchange density functional theory study of an alkaline fuel cell catalyst

Abstract

LaMnO₃ is an inexpensive alternative to precious metals (e.g. platinum) as a catalyst for the oxygen reduction reaction in alkaline fuel cells. In fact, recent studies have shown that among a range of non-noble metal catalysts, LaMnO₃ provides the highest catalytic activity. Despite this, very little is known about LaMnO₃ in the alkaline fuel cells environment, where the orthorhombic structure is most stable. In order to understand the reactivity of orthorhombic LaMnO₃ we must first understand the surface structure. Hence, we have carried out calculations on its electrostatically stable low index surfaces using hybrid-exchange density functional theory, as implemented in CRYSTAL09. For each surface studied the calculated structure and formation energy is discussed. Among the surfaces studied the (100) surface was found to be the most stable with a formation energy of 0.98 J m⁻². The surface energies 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 equilibrium morphology of orthorhombic LaMnO₃ crystals is predicted, allowing us to speculate about likely surface reaction sites.