Ceria Interlayer Research Articles

Lanthanum-doped ceria interlayer between electrolyte and cathode for solid oxide fuel cells

ABSTRACT Gadolinia-doped ceria (GDC) is widely used as an interlayer for preventing the formation of SrZrO3 between yttria-stabilized zirconia (YSZ) electrolyte and lanthanum strontium cobalt iron oxides (LSCF) cathode for solid oxide fuel cells (SOFCs). However, the diffusion of Gd ions to YSZ electrolyte results in the decrease in ionic conductivity due to the formation of Gd2(Zr, Ce)2O7. In the present work, we focus on lanthanum-doped ceria (LDC) as an interlayer between YSZ electrolyte and LSCF cathode. The ionic conductivity of YSZ/LDC composite was higher than that of YSZ/GDC composite in spite of the lower conductivity of LDC than GDC. The maximum power density for the microtubular SOFC with LDC interlayer was better than that with GDC interlayer. While the ohmic resistance decreased by the change from GDC to LDC interlayers, the polarization resistance for oxygen reduction reaction slightly increased as a result of distribution of relaxation times (DRT) analysis. The chemical compositions of Zr, Ce and La was gradually varied at the interface between YSZ electrolyte and LDC interlayer, which affected the both of ohmic and polarization resistances.

Improved performance of a samarium-doped ceria interlayer of intermediate temperature solid oxide electrolysis cells by doping the transition metal oxide Fe2O3.

The ionic conductivity of the interlayer in the intermediate temperature solid oxide electrolysis cell (IT-SOEC) affects the polarization resistance of the oxygen electrode. Improving the ionic conductivity of the interlayer can improve the performance of the oxygen electrode. In this work, the ionic conductivity of a samarium-doped ceria (SDC) interlayer is improved by doping the transition metal oxide Fe2O3. The experimental results show that the oxygen electrode polarization resistance of the symmetrical cell based on the SDC-Fe2O3 interlayer is 0.09 Ω cm−2 at 800 °C and under the open circuit voltage, which is obviously lower than that of the symmetrical cell based on an SDC interlayer (0.22 Ω cm−2). Besides, the electrolysis current of the SOEC based on the SDC-Fe2O3 interlayer is 0.5 A cm−2 at 800 °C and 1.5 V, which is higher than that of the SOEC based on the SDC interlayer (0.3 A cm−2). The above results show that improving the ionic conductivity of the SDC interlayer in the SOEC by doping Fe2O3 can reduce the polarization resistance of the oxygen electrode and enhance the performance of the SOEC. Thus, this work provides an effective way for improving the performance of the SDC interlayer in the IT-SOEC.

Surface analysis of lanthanum strontium cobalt oxides under cathodic polarization at high temperature through operando total-reflection X-ray absorption and X-ray fluorescence spectroscopy

The surface chemistry of perovskite-type lanthanum transition metal oxides is an important parameter in high-temperature electrochemical devices such as solid oxide fuel cells (SOFCs). Strontium segregation on the surface of lanthanum strontium cobalt oxides (LSC) may lead to poor oxygen reduction kinetics. However, previous studies have not sufficiently addressed the oxide surface chemistry, including the oxidation change of LSC under cathodic polarization. In this study, operando total-reflection X-ray fluorescence absorption and X-ray fluorescence spectroscopy were performed to investigate cathodic polarization at high temperature (773 K) and at 1 atm pressure. Dense thin-film electrodes of La0.6Sr0.4CoO3 (LSC113) on Zr0.92Y0.08O1.96 electrolytes with a gadolinium-doped ceria interlayer were examined as model SOFC cathodes. This may be the first time that predominant surface reduction of cobalt is directly observed by using LSC113 under cathodic polarization. This phenomenon is related to strontium enrichment on the LSC surface. (La0.5Sr0.5)2CoO4 decoration on LSC113 enhanced the oxygen reduction kinetics compared with that of bare LSC113. This is due to the suppression of cobalt surface reduction and Sr enrichment in the LSC113 phase.

Evolution of cathode-interlayer interfaces and its effect on long-term degradation

The degradation of electrochemical performance of solid oxide fuel cells (SOFCs) with long-term durability testing is generally evaluated in terms of increases in polarization resistances with time. Practical SOFCs normally employ an interlayer between cathode and electrolyte to improve the stability with long-term operation. To elucidate the relationship between the cathode-interlayer interfacial properties and degradation, various types of model interfaces based on La0.6Sr0.4CoO3-δ (LSC) thin film cathodes prepared on gadolinia-doped ceria (GDC) interlayers having different microstructures are investigated. Equivalent circuit analysis of the electrochemical impedance spectra reveals an increase of the interfacial resistance associated with degradation of the LSC/GDC interface with operation time. The physical origin of the interfacial resistance is attributed to the formation of yttrium-doped SrZrO3 (SZY) at the interface of LSC and GDC, a behavior which appears exacerbated for a nanocolumnar GDC interlayer. Investigation of model interfaces created with SZY inserted between LSC and GDC reveals large polarization resistances mostly dominated by the interfacial resistance, consistent with oxide ion blocking effects observed at interfaces. The role of the GDC interlayer microstructure on the long-term stability of LSC and GDC interfaces and cell performance is discussed.

A novel method to fabricate nanoporous gadolinium-doped ceria interlayer by combining wet-etching and thin film deposition

Fabrication of nanoporous doped ceria thin film by sputtering method has attracted attention for lowering operating temperature of solid oxide fuel cells and enhancing active sites for oxygen reduction reaction (ORR). However, it is difficult to obtain nanoporous structure of doped ceria by sputtering due to deposition characteristic of oxide thin film. A novel method for fabricating 300nm-thickness of nanoporous gadolinium-doped ceria (GDC) interlayer is developed by combining wet-chemical etching and co-sputtering. The nanostructure of thin film Ag-GDC cermet varied with deposition power of Ag and GDC was optimized for cathode interlayer. By etching Ag nanoparticle out of the Ag-GDC cermet interlayer, the highly porous nanostructure of GDC was obtained. The peak power density of the cell with nanoporous GDC interlayer was increased by more than a factor of 2.5 compared to that of the cell without the interlayer. Nanostructure and material properties of the GDC interlayer were analyzed by FESEM, XRD, and XPS.

In Situ Densification of Gadolinium-Doped Ceria Interlayer by Infiltration Process in SOFC

Gadolinium-doped ceria (GDC) interlayer is commonly inserted between electrolyte yttria-stabilized zirconia (YSZ) and cathode La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) to prevent the Sr migration from cathode to electrolyte and improve the stability of the solid oxide fuel cell (SOFC). However, it is difficult to make a dense GDC interlayer during the traditional process. Co, Gd and Ce nitrates solution is used as the infiltration solution to modify and in situ densify the GDC interlayer in this study. The GDC interlayer shows high density when it is pre-sintered at 1175°C and final sintered at 1300°C, and the cell shows high performance and stability, the voltage has little degradation after running for 100 hours at 800°C and 0.5Acm-2, Sr is not observed at the YSZ/GDC interface.

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.

Fabrication and Characterization of YSZ/GDC Bilayer Electrolyte Thin Films by Spray-Coating and Co-Sintering

Yttria-stabilized zirconia (YSZ) is the most popular electrolyte material for solid oxide fuel cells (SOFCs). However, when cobaltite-based perovskite cathode materials, for example lanthanum strontium cobalt ferrite (LSCF), are used, an insulating layer is easy to be formed at the cathode-electrolyte interface. For preventing the interfacial reaction, a gadolinium-doped ceria (GDC) interlayer is usually employed between YSZ and cathode. In this work, we investigated the fabrication of YSZ/GDC bilayer electrolyte thin films by a simple spray coating process and co-sintering. Dense YSZ/GDC bilayer electrolyte thin films were successfully fabricated. There is no crack and delamination observed for the YSZ/GDC bilayer. A Ni-YSZ/YSZ/GDC/LSCF-GDC anode-supported single cell with 4 µm thick YSZ layer and 2 µm thick GDC layer shows an open-circuit voltage of 1.1 V and a maximum power density of about 0.6 W/cm2 with humidified H2 as fuel and air as oxidant at 700°C.

Zero Degradation in Power Density over Long-Term SOFC Operation Based on a Novel Interlayer

Doped ceria interlayer is ubiquitously used between the cathode and electrolyte in solid oxide fuel cells to prevent the cathode/electrolyte interaction and offer a better thermal expansion match. The mechanism on how the interlayer influences the durability and activity of the cathode still remains elusive. This is particularly true in the nickelate cathode (e.g. Pr2NiO4) since the nickelate does go through a phase transition during operation. Traditional Gd-doped ceria was found to substantially accelerate the phase transition in Pr2NiO4 cathode, leading to faster cell degradation in performance. In this work, several new designs of the interlayer are adopted, including various approaches to add Pr into ceria. The addition of Pr in the ceria layer can suppress the phase decomposition of nickelate cathode and increase the performance. As a result, the cell performance exhibits zero degradation over 500 hours. Electrochemical impedance spectroscopy data and its distribution of relation times were obtained to understand the mechanism for this unique observation. Key words: Doped ceria, interlayer, zero degradation, nickelate

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.

Oxygen vacancy levels on gadolinia-doped ceria interlayer deposited by atmospheric pressure plasma jet for solid oxide fuel cells

The oxygen vacancy levels as a factor on different gadolinia-doped ceria interlayer (GDCi) films deposited on yttria stabilized zirconia (YSZ) electrolyte substrates by an atmospheric pressure plasma jet (APPJ) via precursor solution of nitrate salts are investigated. Focusing on the effect of carrier gases, scanning electron microscopy (SEM), Raman, and X-ray diffraction (XRD) are implemented for the materials characterization of the as-deposited GDCi films and sintered-GDCi films at various temperatures. The higher level of oxygen vacancies in GDCi films adhered on 8YSZ electrolyte are evidently analyzed using Ar as the carrier gas during the deposition, of which the interdiffusion resulted in the formation of (GDC + YSZ) solid solution for sintering over 1300 °C degraded the total conductivity. The deposition of GDCi films on 8YSZ by APPJ method using O2 carrier gas significantly improved the total conductivities of the whole electrolyte layers. Moreover, this study provides the useful insight into the oxygen vacancy levels on GDC films as interlayer (GDCi) to improve the values of open circuit voltage in LSM/GDCi/YSZ/Pt full-cell, as well as offering the efficiency of APPJ as one step deposition process.

In-situ Raman spectroscopy analysis of the interface between ceria-containing SOFC anode and stabilized zirconia electrolyte

The combined experimental approach for in-situ Raman spectroscopy of the inner “anode|electrolyte” interfaces in solid oxide fuel cells under current load was examined in a case study centered on the behavior of gadolinia-doped ceria (GDC) interlayers between cermet anodes and stabilized zirconia membrane. The Raman spectra of ceria at 850 °C were found sensitive enough with respect to the variations of anodic current and fuel gas mixture composition, which induce oxygen nonstoichiometry changes in GDC. Linear dependence of the Raman peak area on the open-circuit voltage makes it possible to estimate local overpotentials at the GDC interlayer|electrolyte interface under current load. The calculated local overpotentials exhibit a Tafel-like dependence on the current density, and are essentially independent on the hydrogen and water vapor partial pressures within the limits of experimental uncertainties. The relevant rate-determining mechanism may be associated with ion transfer via the GDC|zirconia interface, whilst the contributions of exchange-related processes involving gaseous phase are less significant in this interfacial zone.

Mechanism of SrZrO3 Formation at GDC/YSZ Interface of SOFC Cathode

SrZrO3 formation at the interface of gadolinia-doped ceria (GDC) interlayer and yttria-stabilized zirconia (YSZ) electrolyte is analyzed using high-resolution electron microscopy. SrZrO3 is dispersed in the inter-diffusion layer on the GDC side from the Ce/Zr border. Zr, which diffuses into the GDC grain, contributes to the formation of SrZrO3. The crystallographic relationship among the SrZrO3 grains and its neighboring GDC grains reveals that SrZrO3 is formed at the surface, at the grain boundary, and inside the grain, while maintaining a highly matched boundary with the adjacent GDC grain. The matching of the interface boundary is confirmed by the O-lattice theory, according to which the threshold Zr/Ce ratio is 13/34. If Zr/Ce ratio in the GDC grain is higher than the threshold, SrZrO3 may significantly grow into the grain. The conduction path for the oxygen ion is retained because the GDC grain containing Zr is split into the SrZrO3 grain and the less-Zr-containing GDC grain. If Zr/Ce ratio is lower than the threshold, SrZrO3 may be formed but will be limited by the amount of Zr diffusing from the adjacent region. Thus, the morphology of SrZrO3 is strongly affected by the state of GDC grains in the inter-diffusion layer.

Gadolinium doped ceria interlayers for Solid Oxide Fuel Cells cathodes: Enhanced reactivity with sintering aids (Li, Cu, Zn), and improved densification by infiltration

This paper reports the study of the densification of 20% Gd doped ceria (Ce0.8Gd0.2O1.9 (GDC)) interlayers in SOFC cathodes through two different routes: the well-known addition of sintering elements, and an innovative densification process by infiltration. First, Li, Cu, and Zn nitrates were added to GDC powders. The effect of these additives on the densification was studied by dilatometry on pellets, and show a large decrease of the sintering temperature from 1330 °C (pure GDC), down to 1080 °C, 950 °C, and 930 °C for Zn, Cu, and Li addition, respectively. However, this promising result does not apply to screen-printed layers, which are more porous than pellets and in which the shrinkage is constrained by the substrate. The second approach consists in preparing a pre-sintered GDC layer, which is subsequently infiltrated with Ce and Gd nitrates and sintered at 1250 °C to increase its density. Such an approach results in highly dense GDC interlayers. Using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) as electrode, the influence of the interlayers on the cathode performance was studied. The addition of sintering aids dramatically increases the cell resistances, most likely because the additives increase the reactivity between GDC and either Yttria Stabilized Zirconia (YSZ) or LSCF, thus losing the expected benefit related to the decrease of sintering temperatures. The interlayers prepared by infiltration do not induce additional resistances in the cell, which results in power densities of single cells 40–50% higher than those of cells prepared with commercial GDC interlayers, making this approach a valuable alternative to sintering aids.

Ridge‐Valley Nanostructured Samaria‐Doped Ceria Interlayer for Thermally Stable Cathode Interface in Low‐Temperature Solid Oxide Fuel Cell

Agglomeration of the metal electrode decreases the performance of low‐temperature solid oxide fuel cells (LT‐SOFC) during operation at elevated temperatures. Here, we report a Pt electrode/ridge‐valley nanostructured samaria‐doped ceria (SDC) layer interface with extended lifetime. LT‐SOFCs with RF‐sputtered SDC interlayers with thicknesses in the range 65–260 nm show a >20 times lower degradation of performance as compared to cells without interlayer when operated at 450 °C (1.8% h−1 vs. 45% h−1). Micromorphological analysis reveals that the coarsening of the Pt electrode is significantly suppressed by the presence of the nanostructured SDC interlayer, possibly due to a stronger bonding between this layer and the Pt grains.

Production of syngas by solid oxide electrolysis: A case study

An investigation of syngas production using solid oxide electrolysis was carried out. A conventional solid oxide cell based on Ni supporting cathode, thin yttria-stabilized zirconia electrolyte, yttria-doped ceria interlayer, and strontium-doped lanthanum cobaltite and ferrite-based perovskite anode (Ni-YSZ/YSZ/YDC/LSFC) was used for the reduction of CO2 and water to syngas. This process was assisted by H2 added to the reactant in various amounts to maintain the Ni sites in a metallic state. This was necessary to favour CO2 reaction and to avoid ohmic constraints that may derive from the occurrence of Ni re-oxidation as consequence of the presence of oxidising species like CO2 and water. The outlet gas was analysed by gas chromatography. The presence of CO and CH4 beside CO2 and H2 was detected in the outlet stream. Analysis of outlet gas composition revealed that CO was produced by both electrochemical and catalytic mechanisms. Suitable CO2 conversion was achieved using dry gas.

The Role of Interlayer on the Catalytic Activity and Performance Stability of (Pr1-xNdx)2NiO4 as Cathodes for Solid Oxide Fuel Cells

The use of doped ceria as an interlayer in solid oxide fuel cells (SOFCs) is ubiquitous, which provides better thermal expansion match and prevents interaction between the cathode (e.g. LSCF) and electrolyte (e.g. YSZ). However, the role of ceria interlayer on the cathode phase durability and catalytic activity remains obscure. This is particularly true in nickelate cathodes (e.g. Pr2NiO4). Here, we report that traditional Gd-doped ceria (GDC) interlayer substantially accelerates the phase transition in Pr2NiO4. Two new interlayer designs were investigated in order to circuvent the phase evolution by using (1) a thin Pr6O11 film deposited on the surface of GDC, and (2) a Pr-doped ceria interlayer. The latter design not only suppresses the phase transition, but also results in a 48% increase in cathode performance; and more importantly, it leads to a zero degradation over long-term measurements. Systemetic in situ and post analyses were carried out to understand the mechanism, by which the interlayer affects the cathode durability and activity. This material is based upon work supported by the U.S. Department of Energy under Award Number DE-FE0023475.

Role of the gadolinia-doped ceria interlayer in high-performance intermediate-temperature solid oxide fuel cells

In this study, Ba0.5Sr0.5Co0.8Fe0.1Zn0.1O3-δ (BSCFZ) is used as a cathode with anode-supported scandia-stabilized zirconia (ScSZ) as the electrolyte to achieve intermediate-temperature solid oxide fuel cells (SOFCs). A gadolinia-doped ceria (GDC) interlayer is used between the BSCFZ cathode and the ScSZ electrolyte to protect ScSZ against interfacial reaction with the Co-containing cathode. However, an unfavorable reaction occurs at the GDC/ScSZ interface when the sintering temperature of the GDC interlayer is higher than 1200 °C. Thus, the performance of anode-supported SOFCs with GDC interlayers prepared at different sintering temperatures has been evaluated using I–V measurements and AC impedance spectroscopy. The results demonstrate that the sintering temperature of the GDC interlayer should be optimized for both the reaction between the BSCFZ cathode and ScSZ electrolyte and the reaction at the GDC/ScSZ interface. A maximum power density of 0.78 W cm−2 has been achieved at 700 °C with a GDC interlayer sintered at 1200 °C. Furthermore, the cell with the optimized GDC interlayer has been shown to maintain excellent reliability with respect to load cycling and thermal cycling.

Achievements of NEDO Projects on SOFC Durability

The seven stacks have been examined for their durability/reliability in the NEDO SOFC projects. The main performance data of stacks have been analyzed into several contributions to voltage losses at selected time intervals, making it possible to derive degradation rates of respective parts. The current performance tests happen to be contaminated with sulfur in air supplied in CRIEPI sites, causing a more significant degradation in cathode in several stacks. Correspondingly, the ohmic loss exhibits similar degradations. Since sulfur content in CRIEPI air was higher than the average ones as confirmed by SIMS measurements by AIST and by S-TEM by Kyushu University, this situation provides weakly accelerated conditions of cathodes. Interestingly enough, this acceleration reveals differences in sensitivity of cathode against the sulfur poisoning effects. When the doped ceria interlayer was succeeded to be fabricated as dense films to prevent the Sr diffusion through doped ceria and following SrZrO3 formation as clarified by FIB-SEM and 3D reconstruction of microstructure by Kyoto University, sensitivity against Sr poisoning becomes large. Actual phenomena appearing as sulfur poisoning effects are also different from stacks to stacks; in particular, NGK INSULATOR cells exhibit peculiar behavior in the initial period of time, while Murata manufacturing cells exhibit strong dependence of air utilization, in other words, air flow rate. Kyocera cells exhibit more sensitive against sulfur after improving the ohmic parts. Although TOTO micro tubular cells do not use the doped ceria layer, a similar significant sulfur poisoning effects are recognized. For planar cells by NGK SPARK PLUG and DENSO, it can be seen effects of sulfur although other effects on the initial degradation make it difficult to extract sulfur effects alone. MHPS segment-in-series cells is free from sulfur poisoning due to their manganite-based cathode. A most plausible mechanism common to those sulfur related phenomena has been derived from considerations on all possible mechanisms of lowering or enhancing active cathodes like LSCF; this mechanism is focusing on balancing between the Sr depletion/volatilization from cathode and the Sr supplying from inside cathode by grain boundary/bulk diffusion; as a factor of enhancing the Sr depletion due to the volatilization in the form of Sr(OH)2, effects of protons or electrochemical potential of protons in the cathodes are considered as factors of controlling Sr behavior and of connecting the Sr volatilization and the microstructure of doped ceria or the thickness of the YSZ electrolyte. This new insight into the cathode degradation behavior caused by sulfur in air leads to the important understanding that cell performance durability appears as complicated interactions among cell components. This proves appropriateness of the present project of focusing on the simulation techniques for cell assemblies by Tohoku university as well as electrode performance based on 3D reconstructed microstructure by the University of Tokyo and Kyoto University. This situation also shows the appropriateness of the present project for investigating durability of those stacks which should be fabricated in a cost effectivity manner and should achieve high performance. It is hoped to show how efforts of establishing high durability can be consistent with cost reduction/high efficiency in the rest of project period of time. .

The Role of Interlayer on the Catalytic Activity and Performance Stability of (Pr1-xNdx)2NiO4 as Cathodes for Solid Oxide Fuel Cells

The use of doped ceria as an interlayer in solid oxide fuel cells (SOFCs) is ubiquitous, which provides a better thermal expansion match and prevents the interaction between cathode (e.g. LSCF) and electrolyte (e.g. YSZ). However, the role of ceria interlayer on the cathode phase durability, catalytic activity, and performance stability remains obscure. This is particularly true in nickelate cathodes (e.g. Pr2NiO4). In this work, the attention will be given to electrochemical properties in nickelate cathodes with various interlayers. Two new interlayer designs were investigated including (1) a thin Pr6O11 film deposited on the surface of GDC and (2) a Pr-doped ceria interlayer. The latter design results in a 48% increase in cathode performance; and more importantly, it leads to a zero degradation over long-term measurements. This is a tremendous improvement when compared to the performance degradation in cells with a GDC interlayer. The cathode-interlayer reaction was fully suppressed with PGCO, due to the intrinsic Pr enrichment in the interlayer, which contributed to suppressing the phase evolution in the electrode.