Ab initiodefect energetics of perovskite (001) surfaces for solid oxide fuel cells: A comparative study ofLaMnO3versusSrTiO3and

Abstract

In this paper, we perform a comparative study based on ab initio modeling for perovskite ABO 3 (001) surfaces and surface defect energetics in order to understand the influence of polarity and redox active Mn in the LaMnO 3 system. We consider LaMnO 3 , LaAlO 3 , SrTiO 3 , and briefly LaFeO 3 systems for comparison, which illustrate the interplay between properties of polar surfaces and the varying d-electron shell of transition metals. We are motivated by the need to understand the surfaces of mixed electronic and ionic conductors typically used in solid oxide fuel cell cathodes and other ion conducting technologies, which are represented here by the LaMnO 3 system. We focus on the influence of the metal character and surface polarity on the surface and surface defect chemistry in these selected systems. We demonstrate that the facile redox of the TM (3d 4 ) in LaMnO 3 with partial e g orbital occupation (or specifically e g occupancy close to 1) allows the polar surfaces to be compensated by changes in charge density over relatively short length scales (3 to 4 unit cells or ∼1.5 nm) near the surface as compared to LaAlO 3 . In contrast to LaAlO 3 , this low-energy and short-range screening mechanism leads to low surface energies without any additional reconstruction, rapidly converging surface properties with film thickness (by ∼8 unit cells), bulklike defect chemistry more than ∼1.5 nm from the surface, and surface defect energetics that are primarily governed by the local charge doping or the created electric field near the polar surfaces. We show that LaMnO 3 exhibits very different surface properties from LaAlO 3 and SrTiO 3 , thereby demonstrating that these properties are due to the presence of the redox active transition metal with partial e g orbital occupation and a polar surface, respectively. These understandings can help guide qualitative analysis, computational study, and design of surfaces of mixed electronic and ionic conductors.

Ab initio GGA+U study of oxygen evolution and oxygen reduction electrocatalysis on the (001) surfaces of lanthanum transition metal perovskites LaBO3(B = Cr, Mn, Fe, Co and Ni)

“…Internal relaxations of the (001) slab coordinates were first performed without adsorbates. We note that the adopted vacuum thickness and slab thickness were previously shown to provide converged surface energy as well as defect formation energies with an accuracy of ~0.1 eV/defect for the LaMnO 3 (001) surfaces 27,34 . In addition, for the perovskite systems with band gaps such as LaFeO 3 , the nonstoichiometry of the symmetric slab forces surface charge equal to half of the bulk layer charge, which is equivalent to the surface compensating charge to the bulk polarity that would come from the large bulk-like region of a thick film, resulting in thickness independent surface energy 27 .…”

“…Nonetheless, the relative adsorption energy difference between the two (001) AO and BO 2 surfaces does not depend on the electrochemical conditions and is intrinsically set by surface polarity of the surface terminations, due to opposite surface compensating charge originating from bulk polarity 26,27 and different surface metal-oxygen bonding environments (metaloxygen coordination). In general we found stronger binding energies on the (001) AO termination and weaker binding energies on the bare (001) BO 2 termination for HO*, O* and HOO*, with 0.6~4 eV differences between the two ideal (001) surfaces, as shown in Figure S1, Supporting Information.…”

“…More detailed discussions on surface compensating charge occurring at the surfaces of polar transition metal perovskite systems are given in Ref. 27. Ref.…”

“…Ref. 27 shows that extra surface charge doping can cause surface band bending and alter the O 2p-band center position relative to the Fermi level, which results in distinct surface binding energy between the (001) AO and BO 2 surfaces. Qualitatively, the difference in binding energy can be understood from simple formal charge observations.…”

“…Furthermore, detailed experimental characterizations 24 has also observed a correlation between the surface HO* coverage and the ORR activities of LaBO 3 (001) films (B=Cr, Mn, Fe, Co, and Ni). Overall, the effects of surface stability 6,12,25 , termination and polarity 26,27 and coverage on the OER and ORR activities still remain largely unexplored.…”