Enhancement of oxygen surface exchange on epitaxial La0.6Sr0.4Co0.2Fe0.8O3-δ thin films using advanced heterostructured oxide interface engineering

Abstract

Engineering of a novel heterostructured oxide interface was used to enhance the oxygen surface exchange kinetics of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF 113 ) thin films. A single-layer decoration of mixed (LaSr) 2 CoO 4±δ (LSC 214 ) and La 1−x Sr x CoO 3−δ (LSC 113 ) and a double-layer decoration of stacked LSC 214 and LSC 113 grown on the LSCF 113 markedly enhanced the surface exchange coefficients of the LSCF 113 by up to ∼1.5 orders of magnitude relative to the undecorated LSCF 113 . It is hypothesized that two different types of surface decorations can enable Sr segregation at the interface and surfaces of LSC 113 and LSC 214 , leading to enhancement of the oxygen surface exchange kinetics of decorated LSCF 113 .The development of highly active cathode materials is essential to lower the operating temperature of solid oxide fuel cells (SOFCs), where the slow kinetics of the oxygen surface exchange on the cathode surface limits the efficiency of SOFCs at intermediate temperatures (500-750°C). [1,2] Current cathode materials such as La 1−x Sr x MnO 3−δ (LSM 113 ) [3][4][5] with high electronic conductivity but low ionic conductivity [6] are inadequate for the usage in the intermediate temperature range due to insufficient surface activity.La 1−x Sr x Fe 1−y Co y O 3−δ (LSCF 113 ), which has beneficial materials properties such as high ionic and electronic conductivity, [7] and fast oxygen surface exchange, [8] therefore, has been developed as one of the most promising commercial cathode materials for intermediate temperature SOFCs. In particular, a solution infiltration process, in which a phase transition occurs from a liquid into a solid has been widely used to further enhance the surface activity of LSCF 113 . [9][10][11][12] Utilizing infiltrated LSM 113 coatings, it has been shown the enhanced electrocatalytic activity of LSCF 113 cathodes. [11,12] Infiltrated La 0.4875 Ca 0.0125 Ce 0.5 O 2−δ (LCC) [9] and Sm 0.5 Sr 0.5 CoO 3−δ (SSC) [10] coatings have also been used for better stability and activity of LSCF 113 electrodes. Although many studies have shown the enhanced cathodic performance of LSCF 113 by surface modification through a solution-based infiltration process, the origin responsible for the enhanced stability and activity of decorated LSCF 113 cathode is poorly understood.Ruddlesden-Popper (RP) phases (A 2 BO 4 ) have been utilized as a material for the La 1−x Sr x CoO 3−δ (LSC 113 ) surface modification, which results in the enhanced surface activity of LSC 113 significantly due to the formation of heterostructured oxide interfaces. [13][14][15][16][17][18] Using well-defined epitaxial thin film systems, remarkably enhanced oxygen surface exchange kinetics (up to ∼2 orders of magnitude) of LSC 113 has been reported by decorating (La 0.5 Sr 0.5 ) 2 CoO 4±δ (LSC 214 ) phase on the LSC 113 surface. [13,19] Coherent Bragg rod analysis (COBRA) and density functional theory (DFT) have suggested that the enhanced oxygen surface exchange kinetics may be attributed to the Sr segregation ...

Controlling Oxygen Mobility in Ruddlesden–Popper Oxides

“…So far, substantial efforts have been focused on developing ABO 3 perovskite oxides, which exhibit fascinating physicochemical properties—i.e., high electronic and ionic conductivities and high catalytic activities [1,2,3]—for use in energy applications. Mixed ionic and electronic conductors (MIECs), such as La 1−x Sr x CoO 3−δ (LSC 113 ) [4,5,6,7,8,9,10,11,12] and La 1−x Sr x Co 1−y Fe y O 3−δ (LSCF 113 ) [13,14,15,16,17,18,19,20,21,22,23,24,25,26], that show high degrees of oxygen deficiency are commonly used to promote oxygen diffusivity and surface exchange kinetics at intermediate temperatures. However, these materials suffer from some serious inherent limitations, including poor thermal and chemical stability [27,28,29,30] and long-term instability [22,31,32,33,34].…”

Controlling the Oxygen Electrocatalysis on Perovskite and Layered Oxide Thin Films for Solid Oxide Fuel Cell Cathodes

“…In addition, the viability of controlling the crystallographic orientation of epitaxial thin films has enabled investigation of the anisotropic nature of oxygen transport and kinetic properties in RP oxides [61,65]. Furthermore, oxide interface engineering has brought a new concept to design highly active and stable cathode materials for intermediate SOFCs [66][67][68]. The growth of high quality epitaxial thin films is made possible using several deposition methods such as pulsed laser deposition (PLD), molecular beam epitaxy (MBE), atomic layer deposition (ALD), and sputtering.…”

“…Therefore, high electrochemical performance at the cathode could be achieved relative to undoped ABO 3 perovskite oxides. In this sense, LSC 113 [17,21,27,28,46,64,73,75,76] and La 1−x Sr x Co 1−y F y O 3−δ (LSCF 113 ) [22,45,47,66,[77][78][79][80][81][82][83][84][85][86] perovskite oxides are commonly used as these oxides are known to show oxygen deficiency and the oxygen nonstoichiometry (δ) affects the electrochemical properties, conductivity and lattice expansion of the materials.…”

“…To achieve enhanced surface exchange kinetics and stability, recent efforts have been focused on developing advanced cathode materials based on MIECs with surface modification [27,33,66,68,76,82,86,125,143,[145][146][147][148]. Oxide heterostructure interfaces, the combination of a Ruddlesden-Popper (RP, LSC 214 ) layer on top of the perovskite LSC 113 , have shown remarkably high oxygen surface exchange kinetics [27,33,68,86,125,143,148,149].…”

“…Vast majority of research has been performed on porous LSCF 113 electrodes, which lead to ambiguous structure and geometry, and therefore the physical origin responsible for enhanced cathodic performance associated with surface decoration of perovskites is not yet completely understood. However, Lee et al [66] recently demonstrated using new forms of heterostructure oxide interfaces-(1) LSCF6428 thin films with a single layer decoration of mixed LSC 214 and LSC 113 and (2) LSCF6428 thin films with a double layer decoration of stacked LSC 214 and LSC 113 -significantly enhanced ORR activity of LSCF6428. More recently, Chen et al [155] showed the improved electrocatalytic activity and durability of LSCF6428 thin films by coating PrO 2 .…”