Sr Diffusion Research Articles

Performance and stability analysis of SOFC containing thin and dense gadolinium-doped ceria interlayer sintered at low temperature

Gadolinium-doped ceria (GDC) interlayers are required to prevent the interfacial reaction between La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode and Y2O3-stabilized ZrO2 (YSZ) electrolyte in solid oxide fuel cells (SOFCs). However, it's difficult to prepare a thin and dense GDC interlayer on the sintered half-cell at a low temperature. In this study, the physical vapor deposition (PVD) method was employed to successfully manufacture dense GDC interlayers with the thickness of 1 μm. The influences of GDC sintering temperature (900 °C, 1000 °C and 1100 °C) on cell performance characteristics and degradation behavior were investigated. The cell with GDC interlayer sintered at 1100 °C showed the lowest degradation rate during the 216-h operation. The best stability was attributed to the most effective inhibition of Sr diffusion by the GDC interlayer, which was demonstrated by the almost unchanged Ohmic and polarization resistances during the aging stage and the negligible Sr enrichment at YSZ/GDC interface. Compared to the conventional screen-printed GDC interlayers (sintered above 1250 °C), the GDC interlayer prepared by the PVD method and sintered at 1100 °C was significantly denser and thinner, showing a promising application prospect due to its benefits for cell stability.

Enhanced Long-Term Durability of Solid Oxide Electrolysis Cells by Microstructure Tailoring of the Air Electrode

High-temperature solid oxide electrolysis cells (SOECs) provide greater efficiency and may potentially provide future technology to meet the enormous energy storage needs generated by the surge in the availability of intermittent solar and wind electricity. However, in implementing this SOEC technology, the high polarization losses and long-term degradation of the SOEC electrodes are the bottlenecks. In this research, we tailored the microstructure of SOEC air electrode to improve electrochemical efficiency and increase the stability of the air electrode. The optimization of the microstructure culminated in a 30% improvement in SOEC electrochemical performance at 800 °C for H2O conversion and considerably improved long-term stability for >1000 hours. The Post-test SEM and TEM investigations suggested that delamination of the air electrode was prevented due to effective microstructure tailoring, and resistive interfaces forming Sr diffusion were suppressed inside the barrier layer and electrolyte. Due to the increased triple phase boundary density, porosity at the interface, and greater active surface area of the electrode, the buildup of the oxygen partial pressure over the electrolyte/barrier layer/air-electrode was decreased.

Microstructure tailoring of solid oxide electrolysis cell air electrode to boost performance and long-term durability

High-temperature solid oxide electrolysis cells (SOECs) offer higher efficiency compared to other electrochemical water splitting technologies and potentially could provide future technology to tackle the huge energy storage requirements created by the surge of intermittent solar and wind electricity availability. However, the lower electrochemical performance and long-term degradation of the SOEC electrodes are bottlenecks in the implementation of this technology. Herein, we report microstructure tailoring of a solid oxide cell air electrode via a simple method to significantly enhance the electrochemical performance and boost the air electrode stability. The air electrode microstructure was tailored by employing a graphite pore former and the cells were tested for SOEC performance and long-term durability under fuel cell (FC)-electrolysis cell (EC) cycles and a 1000 h chronopotentiometry test. The microstructural optimization resulted in a 30% increase in the SOEC performance for H2O conversion at 800 °C and significantly improved the long-term durability. Post-test SEM and TEM analyses indicated that, due to the microstructure tailoring, delamination of the air electrode was avoided and resistive interfaces forming Sr diffusion was suppressed within the barrier layer and electrolyte. Due to air electrode microstructure optimization, the buildup of the oxygen partial pressure across the electrolyte/barrier layer/air-electrode was reduced owing to increased triple phase boundary density, porosity at the interface, and larger active surface area of the electrode.

A comparative study of surface segregation and interface of La0·6Sr0·4Co0·2Fe0·8O3-δ electrode on GDC and YSZ electrolytes of solid oxide fuel cells

Electrode/electrolyte interface plays a critical role in the performance and stability of solid oxide fuel cells (SOFCs). In the case of La0·6Sr0·4Co0·2Fe0·8O3-δ (LSCF) cathode, it is well known that cathodic polarization promotes the Sr segregation and diffusion towards the LSCF electrode and Y2O3–ZrO2 (YSZ) electrolyte interface, leading to the formation of SrZrO3 secondary phase and the disintegration of LSCF structure at the interface. On the other hand, LSCF is chemically stable with doped ceria electrolytes such as Gd-doped CeO2 (GDC). However, there appears no comparative studies on the intrinsic relationship between the surface segregation, interface reaction and stability of LSCF in YSZ and GDC electrolytes. Here, a comparative study has been carried out on the segregation and interface formation of LSCF on GDC and YSZ electrolyte under identical cathodic polarization conditions at 750 °C and 1000 mAcm−2 using focused ion beam and scanning transmission electron microscopy (FIB-STEM) techniques. Segregation of Sr occurs in the LSCF-GDC system, however, the inertness of GDC electrolyte suppresses the segregation process of Sr species. Instead, surface segregation of B-site Co cation becomes dominant under the cathodic polarization, forming isolated CoOx particles. The results indicate that the existence of chemical catchers such as Zr in the case of YSZ electrolyte for the segregated Sr species is kinetically the driving force for the Sr segregation and stability of LSCF electrodes under SOFC operation conditions.

Effect of heat treatment on the migration behaviour of Sr and Ag CO-implanted in glassy carbon

The effect of annealing on the diffusion of silver, silver and strontium co-implanted in glassy carbon was investigated. Glassy carbon samples were implanted with 360 keV Ag ions at room temperature. The RBS profile showed that Fickian diffusion of Ag in glassy carbon is only observed at temperatures ranging from 500 °C–600 °C. At higher annealing temperatures, there was a significant loss of Ag and no Ag was retained in glassy carbon at 700 °C. Glassy carbon samples were also co-implanted with Ag and Sr. The diffusion behaviour of Ag when co-implanted with Sr was similar to that of the singly implanted Ag sample. However, the introduction of Sr into the glassy carbon matrix assisted in the retainment of the Ag ions. The co-implantation of Ag and Sr resulted in a change in the diffusion behaviour of Sr in glassy carbon. The implantation of Ag with Sr prevented the movement of Sr deeper into the bulk of the glassy carbon. The non-movement of Sr into the bulk of the glassy carbon was attributed to the increase of radiation damage near the surface of the glassy carbon making diffusion of Sr towards the surface of glassy carbon an easier choice.

Cesium and Strontium Contamination of Nuclear Plant Stainless Steel: Implications for Decommissioning and Waste Minimization.

Stainless steels can become contaminated with radionuclides at nuclear sites. Their disposal as radioactive waste would be costly. If the nature of steel contamination could be understood, effective decontamination strategies could be designed and implemented during nuclear site decommissioning in an effort to release the steels from regulatory control. Here, batch uptake experiments have been used to understand Sr and Cs (fission product radionuclides) uptake onto AISI Type 304 stainless steel under conditions representative of spent nuclear fuel storage (alkaline ponds) and PUREX nuclear fuel reprocessing (HNO3). Solution (ICP-MS) and surface measurements (GD-OES depth profiling, TOF-SIMS, and XPS) and kinetic modeling of Sr and Cs removal from solution were used to characterize their uptake onto the steel and define the chemical composition and structure of the passive layer formed on the steel surfaces. Under passivating conditions (when the steel was exposed to solutions representative of alkaline ponds and 3 and 6 M HNO3), Sr and Cs were maintained at the steel surface by sorption/selective incorporation into the Cr-rich passive film. In 12 M HNO3, corrosion and severe intergranular attack led to Sr diffusion into the passive layer and steel bulk. In HNO3, Sr and Cs accumulation was also commensurate with corrosion product (Fe and Cr) readsorption, and in the 12 M HNO3 system, XPS documented the presence of Sr and Cs chromates.

Influence of Current Load on the Growth of SrZrO3 at the GDC/YSZ Interface

The influence of current load on the growth of SrZrO3 formed at the interface of a gadolinia-doped ceria (GDC) interlayer and yttria-stabilized zirconia (YSZ) electrolyte was analyzed using high-resolution electron microscopy. A cell with LNF (LaNi0.6Fe0.4O3) cathode was prepared after removing the LSCF cathode using HCl. The LNF cathode was used to eliminate the effect of Sr diffusion during cell operation. These cells were operated under 0.2 A cm−2 at 800°C. For the cell with the LNF cathode, no significant change was observed in the amount of SrZrO3. At the SrZrO3/GDC interface, crystal orientation was the same from the GDC side to the SrZrO3 side. Before cell operation, the GDC grain had some defects and no clear boundary was distinguished between GDC and SrZrO3. After cell operation, the SrZrO3/GDC interface was clearer and crystallization of SrZrO3 proceeded.

Sr-diffusion-induced inhibition of (100)-oriented growth Ca1−xSrxVO3 thin film on a LaAlO3 substrate in pulsed laser deposition

Ca0.5 Sr0.5VO3 (CSVO) thin films were fabricated on LaAlO3 (LAO) and SrTiO3 (STO) substrates using pulsed laser deposition. The lattice mismatch between the CSVO and the substrates is +2.6% for STO and −0.4% for LAO, suggesting that LAO should be more appropriate for crystallization. However, the obtained results were not what was expected. While the CSVO on the STO substrate was highly oriented in (100) direction, the one on the LAO substrate was amorphous and a metal-insulator transition was absent from the electrical characteristics. Although the results clearly showed that STO is more suitable for CSVO deposition, this behavior is not explained merely by the lattice mismatch. Energy dispersive X-ray spectroscopy measurements revealed that Sr diffused from the CSVO thin film into the LAO substrate, which induced a large deviation in the chemical composition of the CSVO thin film. The results display evidence that the low quality of CSVO thin films on LAO is caused by Sr diffusion.

Numerical Simulation of the SrZrO3 Formation in Solid Oxide Fuel Cells

The Ce1−xGdxO2−δ (CGO) interlayer is a common reaction barrier layer employed in solid oxide fuel cells (SOFCs), to prevent chemical reactions between the (La1−xSrx)(Co1−yFey)O3−δ (LSCF) cathode and the Y2O3-stabilized ZrO2 (YSZ) electrolyte. However, even with the existence of the CGO layer, formation of SrZrO3 (SZO) insulating phase can still take place, causing cell degradation. Considering there have already been a large amount of experimental investigations conducted on the above degradation phenomenon, the current work is attempting to numerically model the process. A simplified numerical model of SZO formation at the YSZ–CGO interface is constructed. Based on the thermodynamic information and diffusion kinetics of the YSZ–CGO–LSCF system from the literature, the interdiffusion between YSZ and CGO, the Sr diffusion through the CGO layer, and the formation of SZO at the YSZ–CGO interface are modelled, reproducing the experimental data well. Limitations of the current modeling work are further discussed.

X-ray fluorescence nano-imaging of long-term operated solid oxide electrolysis cells

In this work, we investigate using nanoscale X-ray fluorescence spectroscopy the degradation processes of a cathode and an electrolyte supported cell, produced by two different suppliers, that have been long-term operated in electrolysis mode. High-resolution compositional analysis in combination with high lateral resolution are presented in the format of a 2D map with 50 nm resolution for Ni, Y, Ce, Gd, La, Sr, Co, Fe. We report on the different elements that are diffusing already during the sintering process as well as their evolution after long-term operation. In both type of cell, Co is the element that is highly segregating from the LSCF electrode during the electrochemical reaction. The CGO barrier does not prevent the diffusion of Sr in the electrolyte of the CSC cell while it does in the ESC cell. In addition, the contact layer between the hydrogen electrode and the electrolyte in the ESC cell seems to prevent the Ni depletion. A high concentration of gadolinium from the CGO layer is visible at the CGO/electrolyte interface on the oxygen electrode side as well as on the hydrogen electrode side in the ESC cell. The gadolinium concentration significantly increases during the cell operation to form a layer of a few micrometers.

Performance of La0.5Sr1.5MnO4±δ Ruddlesden-Popper manganite as electrode material for symmetrical solid oxide fuel cells. Part A. The oxygen reduction reaction

The electrochemical performance of La0.5Sr1.5MnO4±δ (L5S15 M) as SOFC cathode was studied using Electrochemical Impedance Spectroscopy (EIS) measurements. The influence of the sintering temperature on the electrochemical performance was examined in air for Au/L5S15 M/GDC/YSZ/GDC/L5S15 M/Au symmetrical cell. For the oxygen reduction reaction, the electron transfer between the electrode and oxygen, and the incorporation of oxygen ions into the electrode are the main controlling processes of oxygen reduction reaction for electrodes sintered at 1150 and 1200 °C. Increasing the sintering temperature up to 1250 °C increases the Sr diffusion through GDC interlayer, which leads to Sr accumulation at the GDC/YSZ interface with SrZrO3 formation and subsequent cell performance degradation. The lowest area specific resistance (ASR) values was obtained for the L5S15 M electrode sintered at 1150 °C.

Influence of electrolyte substrates on the Sr-segregation and SrSO4 formation in La0.6Sr0.4Co0.2Fe0.8O3_δ thin films

Abstract

Direct Visualization of Oxygen Incorporation for Oxygen Reduction on Porous Mixed-Conducting LaSrCoO3 Cathode

Oxygen incorporation for the oxygen reduction reaction at a porous mixed-conducting LaSrCoO3 (LSC) electrode was experimentally investigated using technique of oxygen isotope (18O) labelling coupled with subsequent quenching and secondary ion mass spectrometer (SIMS) imaging. The isotopic oxygen exchange experiments of porous LSC cathode were conducted under current density conditions of 0 (OCV), 0.05 and 0.24 A cm−2. Distribution images of incorporated 18O inside solid oxide microstructures and analyzed concentration profiles along the LSC cathode thickness are presented. The 18O distribution image and concentration profile under an applied current density clearly reflect extension of active reaction zone over surface area of LSC, as well as distribution of oxygen chemical potential in the porous LSC cathode. The non-uniform profiles under influence of applied currents revealed that oxygen chemical potential distribution in porous mixed-conducting LSC cathode strongly depends on an applied current density. The analyzed profiles also showed a significant interfacial barrier to the oxygen diffusion across the interface between the GDC interlayer and the YSZ electrolyte due to the unfavorable diffusion of Sr from the LSC to the GDC/YSZ interface. The present results provide the first experimental evidence of oxygen chemical potential distribution in the porous mixed-conducting LSC cathode.

Barium- and Strontium-Containing Anode Materials toward Ceria-Based Solid Oxide Fuel Cells with High Open Circuit Voltages

Developing ceria-based solid oxide fuel cells (SOFCs) with low cost, high power density, and, in particular, high working efficiency is of great significance to practical applications. In this work, Ba- and Sr-containing composites (Ni–Ba1–xSrxCe0.7Zr0.1Y0.2O3−δ) were for the first time proposed and evaluated as anodes for Ce0.8Sm0.2O1.9 (SDC)-based SOFCs. Both Ba and Sr diffusion occurs at elevated temperatures during fabricating anode-supported half-cells; correspondingly, a thin electron-blocking interlayer is formed in situ at the anode/electrolyte interface. The presence of Ba ensures the formation of a BaCeO3-based electron-blocking layer, which has a very high ion transport number and completely eliminates the internal short circuit current across the SDC electrolyte. Sr incorporation can substantially promote the sintering activity of the anode and electrolyte and, hence, reduces the sintering temperature of the half-cells to 1150 °C. The electrochemical performance of the SDC-based cells varies s...

Effects of applied current density and thermal cycling on the degradation of a solid oxide fuel cell cathode

Effects of applied current density and thermal cycles on the durability of a solid oxide fuel cell (SOFC) cathode have been studied. SOFC half-cells with and without a gadolinium-doped ceria (GDC) interlayer were fabricated and tested for 1000 h at 900 °C under various current densities and thermal cycles. Performance degradation of the half-cells was assessed by increment of the area specific resistance (ASR). Initially, the ASR of the half-cells without the GDC interlayer decreased for around 150 h due to cathode activation and thereafter increased. A rapid increase in the ASR was observed at higher applied current density, which is attributed to delamination of the electrolyte/cathode interface due to the formation of Sr zirconates, and microstructural change in the cathode. However, these adverse effects were prevented by the GDC interlayer. The half-cells with the GDC interlayer exhibited a smaller degradation rate as compared to that without the GDC interlayer. During the thermal cycling test, ASR values of all GDC interlayer thickness cells increased with an increasing number of thermal cycles. The thermally cycled cell with a GDC interlayer thickness of 3.4 μm showed a lower degradation rate due to the dense GDC interlayer, which resulted in less interfacial resistance and prevented elemental diffusion towards the electrolyte. However, the half-cells with GDC interlayer thickness of 2.4 and 4.5 μm showed a higher increase in the ASR due to relatively higher Sr diffusion and delamination of the cathode/GDC interlayer interface, respectively.

Persistent luminescent borosilicate glasses using direct particles doping method

Persistent luminescence (PeL) was obtained, from the first time to the best of our knowledge, from borosilicate bulk glasses. The glasses were prepared using direct doping method. Commercial PeL SrAl2O4:Eu2+,Dy3+ microparticles (MPs) were added in the borosilicate glass after melting. The persistent luminescence can be augmented when casting the glass 3min after adding the MPs at 950°C. Although the borosilicate glasses exhibit persistent luminescence, the glass melt has a corrosive behavior on the MPs leading to the diffusion of Al and Sr into the glasses.

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.

Enhanced stability of solid oxide fuel cells by employing a modified cathode-interlayer interface with a dense La0.6Sr0.4Co0.2Fe0.8O3-δ thin film

In La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode/Gd-doped ceria (GDC)/yttria-stabilized zirconia (YSZ)-electrolyte based solid oxide fuel cells (SOFCs), one of the key issues affecting performance and long-term stability is the apparent deactivation of LSCF cathode by the presence of secondary phases such as SrZrO3 at the interfaces. Herein, we report that by modifying the cathode-interlayer interface with a dense LSCF thin film, the severe cation interdiffusion is suppressed especially the fast gas or surface diffusion of Sr into adjacent GDC-interlayer/YSZ-electrolyte resulting in the significant reduction of SrZrO3 formation at the interfaces improving cell stability. In order to understand the present results, the interface chemistry is carefully considered and discussed. The results show that modification of cathode-interlayer interfaces is an important strategy for improving the lifetime of SOFCs.

Sr and Zr transport in PLD-grown Gd-doped ceria interlayers

Understanding the nature of cation diffusion through the reaction barrier layer provides important information on the long-term stability of solid oxide fuel cells. In this work, model LSCF/GDC/YSZ diffusion triplets were fabricated by pulsed laser deposition. To understand the role of the barrier microstructure on cation transport, epitaxial GDC barrier layers were prepared on (100) and (111) single crystal YSZ substrates. The GDC films successfully prevented severe Sr diffusion into YSZ electrolyte. However, an active Zr transport was observed especially on the (100)-oriented GDC interlayer which is accompanied with severe pore formation. Long-term annealing at high temperature revealed nanosized SrZrO3 grains at the LSCF/GDC(100) interface. SIMS and TEM analyses indicate that Zr is primarily transported through the dislocations. By varying the GDC film thickness systematically between 10nm and 1.0μm, we demonstrate by high-resolution reciprocal space mapping that the strain thickness levels in GDC films are different. The (111)-oriented GDC film is strained to the YSZ for thicknesses up to 200nm, which is several times greater than the (100)-oriented GDC films indicating that strained layers inhibit the Zr transport. The presence of a network of dislocations in GDC provides the fast diffusion pathway for Zr.

Cationic Interdiffusion at the SOFC Electrolyte/Cathode Interface in La2Mo2O9/La0.8Sr0.2MnO3‐δ

AbstractIn this work cation diffusion between a La2Mo2O9 (LM) ionic conductor and the conventional Solid Oxide Fuel Cell (SOFC) cathode material La0.8Sr0.2MnO3‐δ (LSM), was probed using secondary ion mass spectrometry (SIMS), and diffusion coefficients of Sr, Mo and Mn cations within both materials evaluated. Diffusion coefficients extracted from samples with a Sr solution deposited on the LM pellets and from a Mo solution deposited on LSM pellets were found to be orders of magnitude higher than the cross‐diffusion through the interface between two dense pellets in direct contact. These differences may be due to uncertainty in determining the interface position, or to a real dependence on the source of the diffusing cation. In the most favorable case, that of pellets in direct contact, extrapolation of diffusion coefficients down to a typical SOFC operating temperature, 800 °C, show that Mo diffusion in LSM (diffusion coefficient ∼ 10−14 cm2.s−1) is much higher than Sr or Mn diffusion in LM, and incompatible with use in a SOFC device, unless an efficient buffer layer is used.