Segregation and phase separation of aliovalent dopants on perovskite oxide (ABO 3 ) surfaces is detrimental to the performance of energy conversion systems such as solid oxide fuel/electrolysis cells and catalysts for thermochemical H 2 O and CO 2 splitting. One key reason behind the instability of perovskite oxide surfaces is the electrostatic attraction of the negatively We take La 0.8 Sr 0.2 CoO 3 (LSC) as a model perovskite oxide, and modify its surface with additive cations that are more and less reducible than Co on the B-site of LSC. By using ambient pressure X-ray absorption and photoelectron spectroscopy, we proved that the dominant role of the less reducible cations is to suppress the enrichment and phase separation of Sr while reducing the concentration of ! •• and making the LSC more oxidized at its surface. Consequently, we found that these less reducible cations significantly improve stability, with up to 30x acceleration of the oxygen exchange kinetics, after 54 hours in air at 550 o C achieved by Hf addition onto LSC.Finally, the results revealed a "volcano" relation between the oxygen exchange kinetics and the oxygen vacancy formation enthalpy of the binary oxides of the additive cations. This volcano relation highlights the existence of an optimum surface oxygen vacancy concentration that balances the gain in oxygen exchange kinetics and the chemical stability loss. However, significant degradation of the ORR kinetics because of dopant segregation and phase separation is also associated with surface oxygen vacancies 11 . Therefore, here we propose to decrease the surface oxygen vacancy concentration for suppressing the electrostatic driver to this detrimental process.In this paper we hypothesized that the perovskite oxide surface stability can be tuned as a function of the reducibility of the surface. We took La 0. and stability on LSC, while the addition of V and excess Co lead to stronger degradation.Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and X-ray absorption spectroscopy (AP-XAS) 33,34 measurements up to 550 o C revealed that these less reducible cations make the LSC surface more oxidized and decrease the surface oxygen vacancy concentration, leading to a smaller electrostatic driving force for Sr segregation. Electrochemical performance of LSC with surface chemical modificationsWe compared the evolution of the surface oxygen exchange coefficients, k q , which represents the oxygen reduction reactivity of LSC cathodes as a function of time at 530 °C in air.The The morphology of the electrochemically tested cathode surfaces, shown in Fig. 1b, indicates the correlation of the electrochemical stability to the surface chemical stability. On the films with fast degradation of k q , i.e., LSC and LSC-V12, a large surface roughness and particle coverage is evident. Electrochemically stable films such as LSC-Ti15, LSC-Al15 and LSC-Hf16 have more stable surface morphology with significantly lower roughness. Our previous investigation on the nature of these segregated pa...

Attaining fast oxygen exchange kinetics on perovskite and related mixed ionic and electronic conducting oxides is critical for enabling their applications in electrochemical energy conversion systems. This study focuses on understanding the relationship between surface chemistry and the surface oxygen exchange kinetics on epitaxial films made of (La 1-x Sr x ) 2 CoO 4 , a prototypical Ruddlesden-Popper structure that is considered as a promising cathode material for fuel cells. The effects of crystal orientation on the surface composition, morphology, oxygen diffusion and surface exchange kinetics were assessed by combining complementary surface-sensitive analytical techniques, specifically low energy ion scattering, x-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning transmission electron microscopy, atomic force microscopy and secondary ion mass spectroscopy. The films were grown in two different crystallographic orientations, (001) and (100), and with two different Sr compositions, at x=0.25 (LSC25) and 0.50 (LSC50), by using pulsed laser deposition. In the as-prepared state, a Sr enriched layer at the top surface and a Co enriched subsurface layer were found on films with both orientations. After annealing at elevated temperatures in oxygen, the Sr enrichment increased, followed by clustering into Sr-rich secondary phase particles. Both the LSC25 and LSC50 films showed anisotropic oxygen diffusion kinetics, with up to 20 times higher oxygen diffusion coefficient along the (ab) plane compared that along the c-axis at 400-500 o C. However, no dependence of surface oxygen exchange coefficient was found on the crystal orientation. This result indicates that the strong Sr segregation at the surface overrides the effect of the structural anisotropy that was also expected for the surface exchange kinetics. The larger presence of Co cations exposed at the LSC25 surface compared to that at the LSC50 surface is likely the reason for the faster oxygen surface exchange kinetics on LSC25 compared to LSC50. This work demonstrated the critical role of surface chemistry on the oxygen exchange kinetics on perovskite related oxides, which are thus far under-explored at elevated temperatures, and provides a generalizable approach to probe the surface chemistry on other catalytic complex oxides.

No abstract

Chemical-looping water splitting is a novel and 8 promising technology for hydrogen production with CO 2 separation. that the reduction is the rate-limiting step, and it determines the total amount of hydrogen produced in the following oxidation 22 step. The redox kinetics is modeled using a two-step surface chemistry (an H 2 O adsorption/dissociation step and a charge-23 transfer step), coupled with the bulk-to-surface transport equilibrium. Kinetics and equilibrium parameters are extracted with 24 excellent agreement with measurements. The model reveals that the surface defects are abundant during redox conditions, and 25 charge transfer is the rate-determining step for H 2 production. The results establish a baseline for developing new materials and 26 provide guidance for the design and the practical application of water splitting technology (e.g., the design of OC characteristics, 27 the choice of the operating temperatures, and periods for redox steps, etc.). The method, combining well-controlled experiment 28 and detailed kinetics modeling, enables a new and thorough approach for examining the defect thermodynamics in the bulk and 29 at the surface, as well as redox reaction kinetics for alternative materials for water splitting.

The hetero-interfaces between the perovskite (La 1Àx Sr x )CoO 3 (LSC 113 ) and the Ruddlesden-Popper (La 1Àx Sr x ) 2 CoO 4 (LSC 214 ) phases have recently been reported to exhibit fast oxygen exchange kinetics.Vertically aligned nanocomposite (VAN) structures offer the potential for embedding a high density of such special interfaces in the cathode of a solid oxide fuel cell in a controllable and optimized manner.In this work, VAN thin films with hetero-epitaxial interfaces between LSC 113 and LSC 214 were prepared by pulsed laser deposition. In situ scanning tunneling spectroscopy established that the LSC 214 domains in the VAN structure became electronically activated, by charge transfer across interfaces with adjacent LSC 113 domains above 250 C in 10 À3 mbar of oxygen gas. Atomic force microscopy and X-ray photoelectron spectroscopy analysis revealed that interfacing LSC 214 with LSC 113 also provides for a more stable cation chemistry at the surface of LSC 214 within the VAN structure, as compared to single phase LSC 214 films. Oxygen reduction kinetics on the VAN cathode was found to exhibit approximately a 10-fold enhancement compared to either single phase LSC 113 and LSC 214 in the temperature range of 320-400 C. The higher reactivity of the VAN surface to the oxygen reduction reaction is attributed to enhanced electron availability for charge transfer and the suppression of detrimental cation segregation.The instability of the LSC 113/214 hetero-structure surface chemistry at temperatures above 400 C, however, was found to lead to degraded ORR kinetics. Thus, while VAN structures hold great promise for offering highly ORR reactive electrodes, efforts towards the identification of more stable heterostructure compositions for high temperature functionality are warranted.