Factors controlling surface oxygen exchange in oxides.

Reducing the working temperature of solid oxide fuel cells is critical to their increased commercialization but is inhibited by the slow oxygen exchange kinetics at the cathode, which limits the overall rate of the oxygen reduction reaction. We use ab initio methods to develop a quantitative elementary reaction model of oxygen exchange in a representative cathode material, La0.5Sr0.5CoO3-δ, and predict that under operating conditions the rate-limiting step for oxygen incorporation from O2 gas on the stable, (001)-SrO surface is lateral (surface) diffusion of O-adatoms and oxygen surface vacancies. We predict that a high vacancy concentration on the metastable CoO2 termination enables a vacancy-assisted O2 dissociation that is 102-103 times faster than the rate limiting step on the Sr-rich (La,Sr)O termination. This result implies that dramatically enhanced oxygen exchange performance could potentially be obtained by suppressing the (La,Sr)O termination and stabilizing highly active CoO2 termination.

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).

In this work, we performed density functional theory (DFT) calculations with inclusion of Hubbard U corrections for the transition metal d-electrons, to investigate stability and electrocatalytic activities of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for the ABO3 (A = La; B = Cr, Mn, Fe, Co, and Ni) (001) surfaces. We showed surface binding energies of relevant ORR/OER species are coupled strongly to surface polarity and local oxidation states, giving large (∼1 eV scale per adsorbate) differences in binding between (001) AO and BO2 surfaces, where the more oxidized BO2 bare surfaces in general exhibit weak coverage dependence, while the more reduced AO bare surfaces of the LaCrO3, LaMnO3, and LaFeO3 perovskites with lower d-electron filling show strong/moderate coverage dependences. We then predicted that surface coverage can play a key role in determining surface stability, and when coverage effects are included the AO and BO2(001) surfaces have either similar stability or the AO surface is more stable, as found for 1 monolayer HO* covered AO surfaces of LaCrO3 and LaFeO3 under ORR conditions and 1 monolayer O* covered LaNiO3 AO surface under OER conditions. For the (001) AO surfaces with strong coverage dependent surface adsorption, we predicted a decrease in ORR overpotential of 1-2 V with proper treatment of coverage effects as compared to those of the bare surface simulations. Our results indicated that the GGA+U method and proper treatment of coverage effects more accurately predict ORR and OER overpotentials relative to experimental values as compared to the GGA method and bare surfaces. The overall ORR activity trends vs. the LaBO3 series were predicted to be Co > Mn ≈ Ni > Fe > Cr.

Anomalous Interface and Surface Strontium Segregation in (La1-ySry)2CoO4±Î´/La1-xSrxCoO3-δ Heterostructured Thin Films.

Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden-Popper/perovskite (La1-ySry)2CoO4±Î´/La1-xSrxCoO3-δ heterostructure is remarkably enhanced relative to the Ruddlesden-Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La1-ySry)2CoO4±Î´/La1-xSrxCoO3-δ grown on SrTiO3. We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden-Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O2. Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.

Tuning the Spin State in LaCoO3 Thin Films for Enhanced High-Temperature Oxygen Electrocatalysis.

The slow kinetics of oxygen surface exchange hinders the efficiency of high-temperature oxygen electrocatalytic devices such as solid oxide fuel cells and oxygen separation membranes. Systematic investigations of material properties that link to catalytic activity can aid in the rational design of highly active cathode materials. Here, we explore LaCoO3 thin films as a model system for tuning catalytic activity through strain-induced changes in the Co spin state. We demonstrate that Raman spectroscopy can be used to probe the Co-O bond strength at different temperatures to determine the relative spin occupancies of LaCoO3. We find that strain can be used to reduce the spin transition temperature and promote the occupation of higher spin states that weaken the Co-O bond. The decrease in Co-O bond strength and increased spin moment of the thin films result in significant enhancements of the oxygen surface exchange kinetics by up to 2 orders of magnitude.

Cation interdiffusion model for enhanced oxygen kinetics at oxide heterostructure interfaces.

An interface between the perovskite La(0.8)Sr(0.2)CoO(3-δ) (LSC-113) and the K(2)NiF(4)-type (La(0.5)Sr(0.5))(2)CoO(4-δ) (LSC-214) heterostructure was recently shown to enhance oxygen surface exchange and the rate of the oxygen reduction reaction (ORR) by orders of magnitude compared to either the LSC-113 or LSC-214 phase alone. This result is of interest to develop better optimized materials for solid-state electrochemical devices, e.g. solid oxide fuel cells. The effect has been attributed to the interface itself, rather than changes in the bulk LSC-113 or LSC-214 phases. Using density functional theory (DFT)-based simulations, we demonstrate that there is a ∼0.9 eV (∼1.3 eV) energy gain for exchanging a Sr from LSC-113(25%Sr) (LSC-113(40%Sr)) with a La from LSC-214(50%Sr). These changes in energy create a large driving force for interdiffusion across the heterostructure interface from Sr into LSC-214 and La into LSC-113. We estimate that the Sr concentrations (in the LSC-214 phase) in a typical experimental temperature range of 500-600 °C and in equilibrium with LSC-113(25%Sr) and LSC-113(40%Sr), may be about 75% Sr and 90% Sr, respectively. Based on the bulk behavior of the LSC-214 phase (Vashook et al., Solid State Ionics, 2000, 138, 99-104), an Sr enrichment from x = 0.5 to x = 0.75 in (La(1-x)Sr(x))(2)CoO(4-δ) is expected to enhance the oxygen vacancy concentration by 2-2.5 orders of magnitude under typical experimental conditions. An increased vacancy concentration in LSC-214 near the interface can explain most of the enhanced oxygen kinetics observed up until now in these heterostructures.