Ab initioenergetics ofLaBO3(001)(B=Mn, Fe, Co, and Ni) for solid oxide fuel cell cathodes

Abstract

$\text{La}B{\text{O}}_{3}$ ($B=\text{Mn}$, Fe, Co, and Ni) perovskites form a family of materials of significant interest for cathodes of solid oxide fuel cells (SOFCs). In this paper ab initio methods are used to study both bulk and surface properties of relevance for SOFCs, including vacancy formation and oxygen binding energies. A thermodynamic approach and the density functional theory plus $U$ method are combined to obtain energies relevant for SOFC conditions ($T\ensuremath{\approx}800\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, $P{\text{O}}_{2}\ensuremath{\approx}0.2\text{ }\text{atm}$). The impact of varying ${U}_{\text{eff}}$ $({U}_{\text{eff}}=U\ensuremath{-}J)$ on energy and electronic structure is explored in detail and it is shown that optimal ${U}_{\text{eff}}$ values yield significantly better agreement with experimental energies than ${U}_{\text{eff}}=0$ (which corresponds to the standard generalized gradient approximation). $\text{La}B{\text{O}}_{3}$ oxygen vacancy formation energies are predicted to be in the order $\text{Fe}g\text{Mn}g\text{Co}g\text{Ni}$ (where the largest implies most difficult to form a vacancy). It is shown that (001) $B{\text{O}}_{2}$ terminated surfaces have 1--2 eV lower vacancy formation energies and therefore far higher vacancy concentrations than the bulk. The stable surface species at low temperature are predicted to be the superoxide $\text{O}_{2}{}^{\ensuremath{-}}$ for $B=\text{Mn}$, Fe, Co and a peroxide $\text{O}_{2}{}^{2\ensuremath{-}}$ with a surface oxygen for $B=\text{Ni}$. Entropy effects are predicted to stabilize the monomer oxygen surface state for all $B$ cations at higher temperatures. Overall oxygen coverage of the (001) $B{\text{O}}_{2}$ surface is predicted to be quite low at SOFC operating conditions. These results will aid in understanding the oxygen reduction reaction on perovskite SOFC cathodes.