Mechanism for enhanced oxygen reduction kinetics at the (La,Sr)CoO3−δ/(La,Sr)2CoO4+δ hetero-interface

Abstract

The recently reported fast oxygen reduction kinetics at the interface of (La,Sr)CoO 3-δ (LSC 113 ) and (La,Sr) 2 CoO 4+δ (LSC 214 ) phases opened up new questions for the potential role of dissimilar interfaces in advanced cathodes for solid oxide fuel cells (SOFCs). Using first-principles based calculations in the framework of density functional theory, we quantitatively probed the possible mechanisms that govern the oxygen reduction activity enhancement at this hetero-interface as a model system. Our findings show that both the strongly anisotropic oxygen incorporation kinetics on the LSC 214 and the lattice strain in the vicinity of the interface are important contributors to such enhancement. The LSC 214 (100) surface exposed to the ambient at the LSC 113 /LSC 214 interface facilitates oxygen incorporation because the oxygen molecules very favorably adsorb on it compared to the LSC 214 (001) and LSC 113 (001) surfaces, providing a large source term for oxygen incorporation. Lattice strain field present near the hetero-interface accelerates oxygen incorporation kinetics especially on LSC 113 (001). At 500 °C 4×10 2 times faster oxygen incorporation kinetics is predicted in the vicinity of the LSC 113 /LSC 214 heterointerface with 50% Sr-doped LSC 214 compared to that on the single phase LSC 113 (001) surface.Contributions from both the anisotropy and local strain effects are of comparable magnitude. The insights obtained in this work suggest that hetero-structures which have a large area of (100) surfaces and smaller thickness in [001] direction of the Ruddlesden-Popper phases, and larger tensile strain near the interface would be promising for high-performance cathodes.Keywords: solid oxide fuel cell, oxygen reduction reaction, cathode, anisotropy, strain, perovskite, Ruddlesden-Popper, (La,Sr)CoO 3 , (La,Sr) 2 CoO 4+δ , density functional theory * Corresponding author. Email: byildiz@mit.edu 2 Graphical AbstractAnisotropic oxygen incorporation on (La,Sr) 2 CoO 4+δ and the lattice strain near the (La,Sr)CoO 3-δ / (La,Sr) 2 CoO 4+δ interface serve as two possible sources to accelerate the oxygen reduction kinetics on the hetero-structure by 4×10 2 times at 500 °C. Broader contextHigh efficiency and fuel flexibility of Solid Oxide Fuel Cells (SOFCs) render them attractive as a sustainable energy conversion technology. There is growing interest in the design of dissimilar interfaces at the nano-scale to enable high-performance SOFC cathodes with fast oxygen reduction reaction (ORR) kinetics at lower temperatures than their traditional operation temperature of 800 °C. A motivating example for this purpose has been the hetero-structure made of the (La,Sr)CoO 3-δ and (La,Sr) 2 CoO 4+δ phases with highly active interfaces. Two previous experimental reports have observed 10 3 -10 4 times enhancement in ORR kinetics at 500-550 °C arising from the interface of these two phases. However, these empirical observationshave not yet reached a fundamental understanding of why these interfaces are so highly active...

Segregated Chemistry and Structure on (001) and (100) Surfaces of (La1–xSrx)2CoO4 Override the Crystal Anisotropy in Oxygen Exchange Kinetics

“…Anisotropic oxygen diffusion along the 2 main crystallographic directions (ab plane and c-axis) were measured for LSC25 between 250 and 500ºC in both the transversal and longitudinal configurations (Figure 9(a) ). The 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 large difference in oxygen diffusivity between the two orientations in this composition is consistent with the previous computational predictions which showed an interstitial oxygen transport via an interstitialcy mechanism within the ab plane 34,44,47,48 . …”

“…In ref. 34 An anisotropy (temperature dependent) of circa 2 orders of magnitude is apparent in the tracer diffusion coefficients, D*, shown in Figure 1 (a). The anistropy for the surface exchange coefficients, k*, shown in Figure 1 (b) is less strong than that for diffusion.…”

“…34 calculations suggest that doping of Sr into LSC hinders the oxygen adsorption and incorporation process relative to the undoped LSC due to structural reasons at the surface 34 . Furthermore, similarly to our argument for the Sr composition dependence of D*, the ability of the material to accommodate oxygen interstitials is also important for k*.…”

Double Columnar Structure with a Nanogradient Composite for Increased Oxygen Diffusivity and Reduction Activity

“…Sase et al confi rmed that used epitaxial (La,Sr) CoO 3− δ (LSC113) on (La,Sr) 2 CoO 4− δ (LSC214) gave ORRs more than three orders of magnitude faster, at 500 °C, than were observed by using only the single-phase LSC113 surface. [33][34][35][36][37] Several research groups have investigated this enhancement of ORR kinetics caused by the heterostructured LSC113/LSC214 system to attempt to explain the mechanisms involved and the wileyonlinelibrary.com causes of the increased reaction rate. There have been some disagreements between the hypotheses and mechanisms that have been proposed; however, the theory that interfacial regions contribute to the enhanced ORR kinetics is now regarded as being established.…”

B-Site Metal (Pd, Pt, Ag, Cu, Zn, Ni) Promoted La1−xSrxCo1−yFeyO3–δ Perovskite Oxides as Cathodes for IT-SOFCs

“…One study interprets the experimentally observed increased oxygen exchange rate at such interfaces [68] as caused mainly by a redistribution of Sr from the perovskite to the interface region coupled with a respective strong local increase of the oxygen vacancy concentration. Another investigation focuses more on the strain caused at the hetero interface leading to increased oxygen vacancy mobility [69]. There is also an idea that the transition state structure of the perovskite may play an important role in the course of the vacancy diffusion [70][71][72][73].…”