Sr Segregation Research Articles

Improving the Electrocatalytic Activity and Durability of the La0.6Sr0.4Co0.2Fe0.8O3-δ Cathode by Surface Modification.

Electrode materials with high activity and good stability are essential for commercialization of energy conversion systems such as solid oxide fuel cells or electrolysis cells at the intermediate temperature. Modifying the existing perovskite-based electrode surface to form a heterostructure has been widely applied for the rational design of novel electrodes with high performance. Despite many successful developments in enhancing electrode performance by surface modification, some controversial results are also reported in the literature and the mechanisms are still not well understood. In this work, the mechanism of how surface modification impacts the oxygen reduction reaction (ORR) activity and stability of perovskite-based oxides was investigated. We took La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) as the thin-film model system and modified its surface with additive Pr xCe1- xO2 layers of different thicknesses. We found a strong correlation between surface oxygen defects and the ORR activity of the heterostructure. By inducing higher oxygen vacancy concentration compared to bare LSCF, PrO2 coating is proved to greatly facilitate the rate of oxygen dissociation, thus significantly enhancing the ORR activity. Because of low oxygen vacancy density introduced by Pr0.2Ce0.8O2 and CeO2 coating, on the one hand, it does not boost the rate of ORR but successfully suppresses surface Sr segregation, leading to an enhanced durability. Our findings demonstrate the vital role of surface oxygen defects and provide important insights for the rational design of high-performance electrode materials through surface defect engineering.

Cyclic polarization enhances the operating stability of La0.57Sr0.38Co0.18Fe0.72Nb0.1O3-δ oxygen electrode of reversible solid oxide cells

Reversing the direction of polarization current is essential for reversible solid oxide cells technologies, but its effect on cobaltite based perovskite oxygen electrodes is largely unknown. Herein, we report the operating stability and microstructure at the electrode/electrolyte interface of La0.57Sr0.38Co0.18Fe0.72Nb0.1O3-δ (LSCFN) oxygen electrodes assembled on barrier-layer-free Y2O3ZrO2 electrolyte under cyclic anodic/cathodic polarization mode at 0.5 A cm−2 and 750 °C. During the cyclic polarization, the electrocatalytic activity of LSCFN electrode is drastically deteriorated in cathodic mode, but the performance loss is largely recoverable in anodic mode. This is due to the fact that the surface segregation of Sr and accumulation at the electrode/electrolyte interface by cathodic polarization can be remarkably mitigated by anodic polarization. The time period in each cycle plays a key role in determining the accumulation of Sr species at the electrode/electrolyte interface. A full cell operating in a time period of 12 h fuel-cell/12 h electrolysis is reversible for a duration of 240 h, in contrast to the performance degradation in a shorter time period of 4 h fuel cell/4 h electrolysis. The present study sheds lights on applying cobaltite based perovskite oxygen electrodes on barrier-layer-free YSZ electrolyte for reliable solid oxide cells.

The Effect of Sr Doping on La2NiO4+Δ Doped with 10% Mn

Cathodes based on K2NiF4 phases have been shown as high performance cathodes for solid oxide fuel cells (SOFC). The main candidate in this family is La2NiO4+δ (LNO) which is known to have high electronic conductivity, high ionic conductivity and good surface exchange at low temperatures (500-600°C). However, performance of these cathodes needs further improvement to compete with the high temperature cathodes such as LaxSr1-xMnO3 (LSM). An increase in the surface exchange is expected by replacing Ni with a low amount of Mn, however such a replacement is not stable and Sr needs to replace the La in the A site to compensate for the small Mn ion. Samples with varying Sr content have been produced using a citric acid sol gel route and their electrical properties have been studied. Changing the Mn:Sr ratio induced a change in the lattice parameters and as such affected properties of the material. The biggest change was an increase in the electronic conductivity across the solid solution range of 10-50% Sr. However, while the 10% sample behaves very similarly to LNO despite having much lower electronic conductivity, Sr segregation in the high Sr content samples, shown using XPS, has some detrimental effects on the cathode behaviour. The correlation between the structure, electronic conductivity, Sr segregation and impedance results will be presented.

Measurement of oxygen reduction/evolution kinetics enhanced (La,Sr)CoO3/(La,Sr)2CoO4 hetero-structure oxygen electrode in operating temperature for SOCs

To make full use of the advantages of solid oxide cells (SOCs) under actual operating conditions, hetero-structured La0.6Sr0.4CoO3-δ/LaSrCoO4±δ (LSC113/214) thin film electrodes are prepared and investigated by a novel high temperature micro-probe electrochemical test platform for SOCs. The results show that the surface exchange coefficient (kq) of LSC113/214 thin films is 3–10 times higher than that of single phase LSC113 in 773–1123 K. ToF-SIMS and XPS characterizations show that LSC113/214 hetero-interface leads to Sr enrichment at interfacial region and stabilizes it against detrimental Sr segregation. This hetero-interface further induces increased number of active oxygen vacancies and leads to accelerated oxygen exchange kinetics by raising O 2p center closer to Fermi level. This work provides significantly enhanced ORR/OER activity of hetero-structured LSC113/214 oxygen electrode at operation conditions and brings substantial technical benefits for the SOC systems.

Thermochemical stability of LaxSr1-xCoyFe1-yO3-δ and NiFe2O4-Ce0.8Tb0.2O2-δ under real conditions for its application in oxygen transport membranes for oxyfuel combustion

This work addresses the thermochemical stability of ceramic materials –typically used in oxygen transport membranes– under the harsh gas environments found in oxyfuel combustion processes. Specifically, a dual-phase NiFe2O4-Ce0.8Tb0.2O2-δ (NFO-CTO) composite and a single-phase La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) were studied. The effect of the main contaminants present in this kind of processes (CO2, SO2 and H2O) has been tested. NFO-CTO composite remains stable under all the conditions studied whereas LSCF presents a poor stability in the presence of CO2 and SO2. Regardless of the treatment, NFO-CTO conserves its crystalline structure, without giving rise to new species due to segregation or incorporation of sulphur and/or carbon. On the contrary, LSCF is prone to degradation in contact with CO2 and SO2, segregating Sr in the form of SrCO3 and SrSO4 and Co and Fe in the form of CoO and Fe3O4. It is also shown that SO2 interaction with LSCF is stronger than in the case of CO2. A concentration of just 2000 ppm of SO2 in CO2 is enough to subdue the formation of SrCO3, promoting the segregation of Sr only in the form of SrSO4. With the results presented in this work, it is possible to conclude that the NFO-CTO is a suitable candidate from the thermochemical viewpoint to be used as membrane material in 4-end modules for oxygen generation integrated into oxyfuel combustion processes whereas the use of LSCF should be dismissed.

The Impact of in Situ Crystallization on Oxygen Surface Exchange Kinetics of Mixed Conducting Thin Film Oxygen Electrodes

The high temperature oxygen surface exchange kinetics of mixed conducting oxides play a critical role in the efficiency of solid oxide fuel/electrolysis cells (SOCs). Recently, an in situ Optical Transmission Relaxation (OTR) approach has been applied to quantify the oxygen surface exchange coefficients (kchem ) of thin films, with the advantage of providing contact-free, in situ and continuous measurements of native surfaces. The technique relies upon the application of the Beer-Lambert law, where optical absorption is proportional to the concentration of absorbing species, e.g. oxidized Pr (~Pr4+) in PrxCe1-xO2- δ (PCO) [1 ,2 ] or oxidized Fe (~Fe4+) in SrTi1-xFexO3- δ (STF), and to the oxygen stoichiometry via electroneutrality. For example in STF, during reduction (oxygen evolution), absorbing Fe4+ is replaced by Fe3+, resulting in an increase in measured optical transmission through the STF with time, which can be described by the surface exchange-limited kinetics equation to determine kchem . [ 3 ]. As we know, the grain size and degree of crystallinity could affect the thin films’ electrical, optical and catalytic properties. In our previous work, we examined the impact of crystallinity, grain boundaries, orientation, and surface chemistry on kchem for STF (x=0.35, STF35) by preparing films of different structures by pulsed laser deposition under different conditions. We found that fast oxygen surface exchange needs both crystallinity (typically obtained at high growth temperatures) and low Sr surface concentration (typically obtained at low growth temperatures). Amorphous STF thin films were not able to exhibit optically measurable oxygen exchange, while excellent crystallinity, obtained at high growth temperatures, coexisted with high Sr segregation and therefore non-optimal kchem . However, crystallization at intermediate temperatures was used to obtain much faster surface exchange kinetics [ 4 ]. In this work, we further studied the effect of crystallization on oxygen surface exchange kinetics, by post-annealing as-grown amorphous STF35 thin films and observing the evolution of their oxygen exchange behavior in situ during crystallization. The impact of annealing temperature (500-800 °C) and time (2-100 h) on the crystallinity and microstructure was observed by scanning probe microscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and in situ optical transmission changes. The consistent appearance of a significant decrease in optical transmission during heating suggested that crystallization began around 545-550 ºC. The best kchem was found for annealing at 600 ºC for 2 h. The thin film with lower (or perhaps no) crystallinity annealed at 500 °C was not able to exhibit detectable oxygen surface exchange, while higher annealing temperatures and/or times led to non-optimal kchem , which is consistent with our previous research. In order to assess whether the benefits of crystallization under mild conditions for rapid surface exchange also applied to other compositions and structures, we investigated the surface exchange kinetics during in situ crystallization of other materials, such as perovskite SrTi0.65Co0.35O3- δ (STC35), fluorite Pr0.1Ce0.9O2- δ (PCO), and Ruddlesden-Popper Sr2Ti0.65Fe0.35O4± δ (RP-STF35). Optical relaxations were observed in these materials, enabling the quantification of their kchem . For the perovskite STC35 and RP-STF35 thin films, in-situ crystallization was observed to benefit the oxygen exchange kinetics. On the other hand, for a fluorite PCO thin film, we found that it was already crystalline after deposition at 25 ºC. Therefore, the annealing process for PCO thin film did not show a positive effect on kchem . Implications for use of thin film electrodes in intermediate temperature devices will be addressed. Acknowledgements Support from WPI-I2CNER and a JSPS Kakenhi Grant-in-aid for Young Scientists (B) project number JP15K18213 (to N. H. Perry) and JSPS Fellowship (201702103) are gratefully acknowledged. Reference [1] J. J. Kim, S. R. Bishop, N. J. Thompson, D. Chen and H. L. Tuller, “Investigation of nonstoichiometry in oxide thin films by simultaneous in s itu optical absorption and chemical capacitance measurements: Pr-doped ceria, a case study”, Chemistry of Materials , 2014, 26, 1374-1379. [2] J. J. Kim, S. R. Bishop, N. J. Thompson and H. L. Tuller, “Investigation of redox kinetics by simultaneous i n s itu optical absorption relaxation and electrode impedance measurements: pr doped ceria thin films”, ECS Transactions, 57 (1) 1979-1984 (2013) [3] I. Denk, F. Noll and J. Maier, “In situ profiles of oxygen diffusion in SrTiO3: bulk behavior and boundary effects”, Journal of the American Ceramic Society , 1997, 80, 279-285. [4] T. Chen, G.F. Harrington, K. Sasaki, and N.H. Perry, “Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach”, Journal of Materials Chemistry A , 2017, 5, 23006-23019

Suppressed Sr segregation and performance of directly assembled La0.6Sr0.4Co0.2Fe0.8O3-δ oxygen electrode on Y2O3-ZrO2 electrolyte of solid oxide electrolysis cells

Active and stable oxygen electrode is probably the most important in the development of solid oxide electrolysis cells (SOECs) technologies. Herein, we report the successful development of mixed ionic and electronic conducting (MIEC) La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) perovskite oxides directly assembled on barrier-layer-free yttria-stabilized zirconia (YSZ) electrolyte as highly active and stable oxygen electrodes of SOECs. Electrolysis polarization effectively induces the formation of electrode/electrolyte interface, similar to that observed under solid oxide fuel cell (SOFC) operation conditions. However, in contrast to the significant performance decay under SOFC operation conditions, the cell with directly assembled LSCF oxygen electrodes shows excellent stability, tested for 300 h at 0.5 A cm−2 and 750 °C under SOEC operation conditions. Detailed microstructure and phase analysis reveal that Sr segregation is inevitable for LSCF electrode, but anodic polarization substantially suppresses Sr segregation and migration to the electrode/electrolyte interface, leading to the formation of stable and efficient electrode/electrolyte interface for water and CO2 electrolysis under SOECs operation conditions. The present study demonstrates the feasibility of using directly assembled MIEC cobaltite based oxygen electrodes on barrier-layer-free YSZ electrolyte of SOECs.

Microstructure optimization for high-performance PrBa0.5Sr0.5Co1.5Fe0.5O5+δ-La2NiO4+δ core-shell cathode of solid oxide fuel cells

Four PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF)-La2NiO4+δ (LN) core-shell cathodes, designated as PL-0, PL-1, PL-3 and PL-5, are prepared by infiltrating LN solution into PBSCF scaffold, and they are investigated in terms of the effect of LN thickness on their electrochemical performance. PL-3 with a continuous LN coating of a moderate average thickness (∼9 nm) demonstrates the lowest initial polarization resistance (0.51 Ω cm2) and highest power density (0.71 W cm−2) among all the cathodes. Polarized at 400 mA cm−2 and 700 °C for up to 40 h, the polarization resistance of all the prepared cathodes increases to approach a stable level after early stage decrease due to current activation, and PL-3 exhibits a slower average rate of performance degradation (25%). The electrochemical performance improvement is mainly attributed to that LN has a relatively high oxygen surface exchange coefficient and continuous LN coating depresses Sr segregation at PBSCF/LN interface.

A FIB-STEM Study of Strontium Segregation and Interface Formation of Directly Assembled La0.6Sr0.4Co0.2Fe0.8O3-δ Cathode on Y2O3-ZrO2 Electrolyte of Solid Oxide Fuel Cells

Electrode/electrolyte interface plays a critical role in the performance and stability of solid oxide fuel cells (SOFCs). Here, interface formation, Sr segregation and reaction of directly assembled La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode on barrier-layer-free Y2O3-ZrO2 (YSZ) electrolyte are studied at 1000 mAcm−2 and 750°C using focused ion beam and scanning transmission electron microscopy (FIB-STEM) techniques. The results indicate that polarization promotes the formation of LSCF/YSZ interface with a high level of periodicity and symmetry but no amorphous phases or solid solutions. Further polarization induces the Sr segregation and diffusion toward the LSCF/YSZ interface, forming Sr rich layer (SRL, primarily SrO) at LSCF and YSZ interface. Segregated Sr species are highly mobile and deposition of SrO occurs at the LSCF/YSZ interface as well as on the YSZ electrolyte surface. The reaction between SRL and YSZ is fast, forming SrZrO3 secondary phase. The growth of SrZrO3 phase at the interface are affected by the crystallography lattice plane orientation of the YSZ electrolyte, forming a coherence interface between SrZrO3 and YSZ electrolyte. Further polarization accelerates Sr segregation and formation of the SrZrO3 reaction layer, leading to the disintegration of LSCF structure at the interface. The results indicate that chemical reaction between LSCF and YSZ occurs at 750°C under polarization conditions, kinetically induced by the segregated SrO at the interface and on the YSZ electrolyte surface.

Temporal and thermal evolutions of surface Sr-segregation in pristine and atomic layer deposition modified La0.6Sr0.4CoO3−δ epitaxial films

The bulk-to-surface Sr segregation vs. temperature and time in La0.6Sr0.4CoO3−δ have been studied. The atomic fraction of surface-Sr was effectively reduced by atomic layer deposition of ZrO2 overcoat.

Structural and surface changes in glassy carbon due to strontium implantation and heat treatment

There are still questions around the microstructure of glassy carbon (GC), like the observation of the micropores. These were proposed to explain the low density of GC. This paper explains the effect of ion bombardment (200 keV Sr+, 1 × 1016 Sr+/cm2 at RT) on the microstructure of GC. TEM and AFM show that micropores in pristine GC are destroyed leading to densification of GC from 1.42 g/cm3 to 2.03 g/cm3. The amorphisation of glassy carbon was also not complete with graphitic strands embedded within the GC. These were relatively few, as Raman analysis showed that the Sr implantation resulted in a typical amorphous Raman spectrum. Annealing of the sample at 900 °C only resulted in a slight recovery of the GC structure. AFM and SEM analysis showed that the surface of the sample became rougher after Sr implantation. The roughness increased after the sample was annealed at 600 °C due to segregation of Sr towards the surface of the GC. SEM measurements of a sample with both implanted and un-implanted edges after annealing at 900 °C, showed that the high temperature heat treatment did not affect the surface topography of un-irradiated GC.

Insights into the Surface Reactivity of Cermet and Perovskite Electrodes in Oxidizing, Reducing, and Humid Environments.

Understanding the surface chemistry of electrode materials under gas environments is important in order to control their performance during electrochemical and catalytic applications. This work compares the surface reactivity of Ni/YSZ and La0.75Sr0.25Cr0.9Fe0.1O3, which are commonly used types of electrodes in solid oxide electrochemical devices. In situ synchrotron-based near-ambient pressure photoemission and absorption spectroscopy experiments, assisted by theoretical spectral simulations and combined with microscopy and electrochemical measurements, are used to monitor the effect of the gas atmosphere on the chemical state, the morphology, and the electrical conductivity of the electrodes. It is shown that the surface of both electrode types readjusts fast to the reactive gas atmosphere and their surface composition is notably modified. In the case of Ni/YSZ, this is followed by evident changes in the oxidation state of nickel, while for La0.75Sr0.25Cr0.9Fe0.1O3, a fine adjustment of the Cr valence and strong Sr segregation is observed. An important difference between the two electrodes is their capacity to maintain adsorbed hydroxyl groups on their surface, which is expected to be critical for the electrocatalytic properties of the materials. The insight gained from the surface analysis may serve as a paradigm for understanding the effect of the gas environment on the electrochemical performance and the electrical conductivity of the electrodes.

Mechanism of Chromium Poisoning in SOFC Cathode Investigated by Using Pattern Thin Film Model Electrode

Chromium poisoning is one of main causes of the degradation of SOFC cathodes. Cr-poisoning is caused by the deposition of Cr-containing oxides through reactions between the cathode material and the Cr vapor, such as CrO3 and CrO2(OH)2, coming from constitutional metals of the SOFC system. SrCrO4 and Cr2O3 are known as such oxides. Since Cr ions in the vapors and Cr2O3 are hexa- and tri-valent, respectively, the formation of Cr2O3 requires the reduction of Cr ions. On the other hand, in this case of the SrCrO4 deposition, the reduction of Cr ions is not needed. So far, it was pointed out that the type of the Cr deposition and their amounts significantly depends on the electrode material and SOFC operation conditions. But yet the mechanism of Cr-poisoning is not well-clarified. In this study, in order to investigate the mechanism of Cr-poisoning systematically and quantitatively, the patterned thin film electrode proposed by our group was applied [1]. This model electrode was a kind of a columnar electrode which simplified the complicated microstructure of the practical porous electrode. The electrode contacted with the electrolyte only at the limited and defined area, and the reaction current was distributed simply as a function of the distance from the interface. Because of such a simple structure of the model electrode, it was expected that the influence of the polarization on the Cr-poisoning can be separately evaluated. La0.6Sr0.4Co3-δ (LSC), which is a typical mixed ionic and electronic conducting oxide, was chosen as a model SOFC cathode. The LSC patterned thin film electrode was fabricated on a Ce0.9Gd0.1O1.95 (GDC) electrolyte by means of photolithography and pulsed laser deposition. One-week degradation tests were carried out at 973 K under p(O2) = 1 bar and p(H2O) = 0.019 bar. The wet O2 gas with or without Cr-vapor was flowed to the patterned electrode under cathodic overpotential of -190 mV. When Cr-vapor was introduced, the gas was preliminarily passed through a column of Cr2O3 powder kept at 973 K. After the degradation tests, the patterned electrode was analyzed by using SEM/EDS, SIMS, EPMA , and STEM/EDS/SAED. From SEM observation, many deposits were observed on the electrode surface after the degradation test with Cr-vapor. Elementary mapping by EPMA revealed that Cr preferentially deposited near the electrode/electrolyte interface together with Sr. The area where Cr deposits were observed corresponded to the effective electrode-reaction area evaluated by our previous works using operando micro X-ray absorption spectroscopy [1]. On the other hand, in the positions far from the electrode/electrolyte interface, where the electrode reaction did not take place, Cr deposits were rarely found. These results clearly demonstrated that the cathodic polarization accelerated the Cr-poisoning. From the selected area electron diffraction, the deposits were identified as SrCrO4. After the degradation test without Cr-vapor, Sr segregation was observed near the electrode/electrolyte interface. These indicated that the acceleration of the Cr-poisoning by the cathodic polarization was originally due to the Sr segregation followed by the SrCrO4 formation through the chemical reaction with Cr-vapor. References (1) K. Amezawa, et al., Electrochem. Soc. Trans., 66 (2), 129-135 (2015).

A Fundamental Study on the Chemical Stability of La1−xSrxCo0.2Fe0.8O3−δ Cathodes for Intermediate Temperature Solid Oxide Fuel Cells

The chemical stability of La1−xSrxCo0.2Fe0.8O3−δ (x = 0, 0.4, 0.6, and 1) oxides before and after annealing at 750 °C in air is investigated by field emission scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and environmental transmission electron microscopy (TEM). Results indicate that Sr surface segregation has initially occurred at the sintering stage, and then, the secondary-phase particles are formed with increasing the heat-treatment time at 750 °C in air. Increasing Sr content accelerates Sr segregation on the surface, because of two driving forces including interaction forces in the crystal lattice and thermal activation. AES and XPS results reveal that Sr and Co segregations toward the surface have great contributions to the chemical instability of La1−xSrxCo1−yFeyO3−δ (LSCF) during annealing.

Effect of a (La,Sr)2CoO4 Phase on the Oxygen Exchange Reaction of Dense and Porous (La,Sr)CoO3 Electrodes

Promotion of oxygen exchange rate on La0.6Sr0.4CoO3-δ (LSC) by the coexistence of the Ruddlesden-Popper phase was investigated for dense film and porous electrodes. Application of a porous (La,Sr)2CoO4 (LSC214) layer on a dense film electrode increased the reaction rate and decreased the activation energy. The oxygen reaction rate on a bare LSC varied from sample to sample, suggesting strong effect of the surface chemistry such as Sr segregation. On the other hand, the electrode modified with a LSC214 layer showed less dependence. The presence of an LSC214 phase appeared to suppress the Sr segregation, which accounts for the improvement of the reaction rate. In contrast to dense electrodes, porous electrodes showed no significant improvement with addition of LSC214. From the estimation of the effective utilization length, the surface reaction rate of a porous electrode was much higher than that on a dense electrode, which may influence the different effect observed by addition of LSC214 in dense and porous electrodes.

Effect of a (La,Sr)2CoO4 Phase on the Oxygen Exchange Reaction of Dense and Porous (La,Sr)CoO3 Electrodes

Oxygen reduction reactions on a mixed conducting lanthanum cobaltite, (La,Sr)CoO3: LSC, have been widely investigated both from scientific interest and from the technological importance as a cathode of a solid oxide fuel cell. Since the reaction pathway is not simple in a practical porous electrode, model electrode approaches have been adopted by many researchers. However, the reaction mechanism and rate determining processes may not be always identical between model and porous electrodes, which can mislead the research and development. The authors have been making efforts to improve oxygen reduction activity of LSC by utilizing the promotion effect of a co-existing Ruddlesden-Popper phase, (La,Sr)2CoO4 : LSC214, which was first observed at the surface of a dense ceramic LSC(1). The concept worked for a dense film electrode but not for a practical porous electrode except when the initial performance of LSC was somehow poor(2). The difference between dense and porous LSC may account for the results. The followings are what we found in the course of the research: 1. The oxygen exchange rate around on LSC was faster around the thermally precipitated LSC214 particles. (1) 2.Application of a porous LSC214 layer on a dense film LSC electrode showed increased reaction rate and decreased activation energy. (3) 3. Application of a porous LSC214 layer on a porous LSC electrode appeared to improve the performance when the base LSC layer had somehow large electrochemical resistance. (2) 4. Well-prepared LSC electrode showed the electrochemical resistance of about 0.5 ohm cm2 in air at 600˚C, and no further improvement was achieved by application of LSC214 layer on the electrode ore mixing LSC214 particles in the electrode. 5. Effective utilization length of a porous LSC electrode evaluated by the electrochemical impedance was thinner than expected from the surface reaction and bulk diffusion constants of dense ceramic LSC. 6. Surface reaction rate of a dense LSC decreased significantly with Sr segregation in the surface/sub-surface layers. 7. Strontium on the surface of LSC film appeared to be removed when it was in contact with porous LSC214 layer. Based on the above observations, the difference of dense and porous LSC electrodes will be discussed in terms of Sr segregation and in the role of LSC214. References (1) M. Sase, K. Yashiro, K. Sato, J. Mizusaki, T. Kawada, N. Sakai, K. Yamaji, T. Horita, H. Yokokawa, Solid State Ionics 178(35), 1843 (2008). (2) K. Yashiro, T. Nakamura, M. Sase, F. Hermes, K. Sato, T. Kawada, J. Mizusaki, Electrochem. Solid State Lett. 12(9), B135-B137 (2009). (3) T. Kawada, T. Tairako, K. Nagao, A. Unemoto, M. Sase, K. Yashiro, S. Hashimoto, and K. Amezawa, Proc. 16th Intern. Conf. Nanotechnology, Sendai, Japan, August 22-25, 2016

(Invited) Towards an Improved Understanding of Electrochemical Oxygen Exchange Reactions on Mixed Conducting Oxides

It is generally agreed that the oxygen reduction reaction on oxide surfaces is strongly affected by defects, and the same is true for the counter reaction of oxygen evolution from an oxide. Relevant defects include the “standard” point defects also present in the bulk, i.e. oxygen vacancies and electrons or holes, but also defects within the ideal termination of an oxide, e.g. cation segregates, and higher dimensional defects such as dislocations or grain boundaries. However, it is far from trivial to clearly identify the role of specific defects on the oxygen reduction or release reaction. Often, one neither knows exact defect concentrations nor can one vary those in a defined manner without affecting the entity of all defects. This also hampers development of theoretical models describing the current voltage characteristics of oxygen reduction or evolution, i.e. oxygen exchange, at gas/solid interfaces. In this contribution, two types of experiments are employed to shed further light on the role of defects in oxygen exchange of mixed conducting oxides such as Sr doped LaCoO3 (LSC) and LaFeO3 (LSF). i) The first approach makes use of the fact that both oxygen partial pressure p(O2) and overpotential affect the oxygen chemical potential and thus defect concentrations in oxides. However, the oxygen reduction or evolution kinetics may still be affected very differently by overpotential and p(O2), since only the latter also changes the concentration of reacting molecules. By properly combining oxygen partial pressure changes and overpotential, we could tune the chemical potentials of LSF and gas phase independently and thus we extracted “true” reaction orders of O2 and defects in the oxygen reduction and evolution reaction. Additional bias dependent three point impedance measurements revealed the chemical capacitance of LSF and this was used for calculating the absolute bulk defect concentration in dependence of the oxygen chemical potential. This analysis also revealed that defect interaction strongly affects the thermodynamic data of LSF and a new defect chemical model including non-dilute defects is suggested. ii) The second type of experiments deals with the effect of certain cations in the topmost layer of LSC. It is already known that Sr segregation to the surface of mixed conducting oxides may reduce their activity towards oxygen reduction. A kind of SrO termination layer seems to be unavoidable on LSC at higher temperatures but still additional degradation may take place and reasons are not fully understood. A novel type of experiment was thus developed, with impedance spectroscopic analysis of samples inside a PLD (pulsed laser deposition) chamber (in-situ impedance PLD). In this manner, the surface of an LSC electrode in an YSZ based electrochemical cell could be decorated by very tiny amounts of other oxides (SrO, Co3O4, La2O3) and the effect on oxygen exchange kinetics could be detected immediately by impedance measurements. Already small fractions of monolayers of Sr strongly modified the polarization resistance, thus indicating that only a small number of very active sites was present on LSC and those became deactivated by Sr. Co, on the other hand, can significantly activate the LSC electrode surface.

Highly Stable Sr-Free Cobaltite-Based Perovskite Cathodes Directly Assembled on a Barrier-Layer-Free Y2 O3 -ZrO2 Electrolyte of Solid Oxide Fuel Cells.

Direct assembly is a newly developed technique in which a cobaltite-based perovskite (CBP) cathode can be directly applied to a barrier-layer-free Y2 O3 -ZrO2 (YSZ) electrolyte with no high-temperature pre-sintering steps. Solid oxide fuel cells (SOFCs) based on directly assembled CBPs such as La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ show high performance initially but degrade rapidly under SOFC operation conditions at 750 °C owing to Sr segregation and accumulation at the electrode/electrolyte interface. Herein, the performance and interface of Sr-free CBPs such as LaCoO3-δ (LC) and Sm0.95 CoO3-δ (SmC) and their composite cathodes directly assembled on YSZ electrolyte was studied systematically. The LC electrode underwent performance degradation, most likely owing to cation demixing and accumulation of La on the YSZ electrolyte under polarization at 500 mA cm-2 and 750 °C. However, the performance and stability of LC electrodes could be substantially enhanced by the formation of LC-gadolinium-doped ceria (GDC) composite cathodes. Replacement of La by Sm increased the cell stability, and doping of 5 % Pd to form Sm0.95 Co0.95 Pd0.05 O3-δ (SmCPd) significantly improved the electrode activity. An anode-supported YSZ-electrolyte cell with a directly assembled SmCPd-GDC composite electrode exhibited a peak power density of 1.4 W cm-2 at 750 °C, and an excellent stability at 750 °C for over 240 h. The higher stability of SmC as compared to that of LC is most likely a result of the lower reactivity of SmC with YSZ. This study demonstrates the new opportunities in the design and development of intermediate-temperature SOFCs based on the directly assembled high-performance and durable Sr-free CBP cathodes.

Degradation of La0.6Sr0.4Fe0.8Co0.2O3-δ Oxygen Electrodes on Ce0.9Gd0.1O2-δ Electrolytes during Reversing Current Operation

In order to assess the durability of La0.6Sr0.4Co0.2Fe0.8O3 oxygen electrodes in reversible solid oxide cells, current switched ∼1000 h galvanostatic (0.7, 1.0, and 1.5 A/cm2) life tests were performed on symmetrical Ce0.9Gd0.1O2–electrolyte cells at 700°C. Cell operating voltage and resistance, the latter measured by impedance spectroscopy, were monitored throughout. Degradation was minimal for the 0.7 A/cm2 case. For the higher current densities, the cell voltage and resistance increased with time, although the cell appeared to stabilize after ∼500 h in the 1.5 A/cm2 case. Post-test analyses showed no evidence of electrolyte cracking or delamination for any current. However, 3D imaging revealed measureable microstructural coarsening after 1.5 A/cm2 operation that was not present after 0.7 A/cm2 operation. Furthermore, the amount of Sr segregated onto LSCF surfaces was higher for the cells operated with current switching versus the as-prepared and zero-current cells. Analysis of the results suggest that much of the degradation was due to decreased oxygen surface exchange rate due to current-enhanced Sr segregation, with a smaller contribution due to microstructural coarsening. The possibility of extrapolating these accelerated tests to longer times is discussed.

A robust high performance cobalt-free oxygen electrode La 0.5 Sr 0.5 Fe 0.8 Cu 0.15 Nb 0.05 O 3−δ for reversible solid oxide electrochemical cell

Abstract A novel cobalt-free perovskite oxide La0.5Sr0.5Fe0.8Cu0.15Nb0.05O3−δ (LSFCN) has been synthesized and evaluated as oxygen electrode for reversible solid oxide electrochemical cells (RSOCs). The performance and stability of the LSFCN based RSOCs have been characterized in fuel cell and electrolysis modes, and the reversibility of the cells has been proven. In FC mode, the cell exhibits the maximum power density of 1.10 Wcm−2 at 800 °C, and a stable output under 0.7 V at 700 °C during 108 h. The performance and stability of the cell in electrolysis mode are also remarkable. An electrolysis current of 0.85 A cm−2 is achieved at 750 °C with an applied voltage of 1.3 V, and no degradation as well as delamination are observed for the cell after 50 h electrolysis under voltage of 1.60 V (∼1.27 A cm−2) at 800 °C. The high performance of the LSFCN at both cathodic and anodic conditions may be attributed to the inherent high electrochemical activity of copper-iron based perovskites; and the incorporation of Nb5+ cations into perovskite lattice is responsible for the stability of LSFCN, which leads to the more stable crystal structure, lower thermal expansion coefficient and the reduced Sr segregation at surface.