Cathode Materials For IT-SOFCs Research Articles

Hydride reduced LaSrCoO 4− δ as new cathode material for Ba(Zr 0.1Ce 0.7Y 0.2)O 3 based intermediate temperature solid oxide fuel cells

Cathodes with high catalytic activities are usually required for the preparation of high performance intermediate temperature (600–800 °C) solid oxide fuel cells (IT-SOFCs). In the present study, a new kind of cathode material for IT-SOFCs based on Ba(Zr 0.1Ce 0.7Y 0.2)O 3 (BZCY) electrolytes is prepared through solid state reduction of the K 2NiF 4 structured LaSrCoO 4− δ with CaH 2. Structural analysis by XRD reveals that the cell parameters of LaSrCoO 4− δ become larger after the reduction. And oxygen stoichiometry of 3.57 is determined by iodometric titration for the CaH 2 reduced phase (marked as H-LaSrCoO 4− δ ). The electrochemical properties of both the H-LaSrCoO 4− δ –BZCY and the LaSrCoO 4− δ –BZCY composite cathodes are investigated through ac impedance spectroscopy and dc polarization measurements. At 750 °C, the H-LaSrCoO 4− δ –BZCY cathode exhibits a polarization resistance of 0.229 Ω cm 2, which is about one third smaller than that of the LaSrCoO 4− δ –BZCY cathode. Meanwhile, cathodic overpotential of 25 mV is obtained for the H-LaSrCoO 4− δ –BZCY cathode under a current density of 100 mA cm −2 at 750 °C. This value is much lower than 70 mV of the LaSrCoO 4− δ –BZCY cathode obtained at the same condition. Subsequent study on the cathodic reaction process implies that the better electrochemical properties of the H-LaSrCoO 4− δ –BZCY cathode can be attributed to the higher oxygen vacancy concentration in the H-LaSrCoO 4− δ lattice that enhances some key steps of the oxygen reduction reaction (ORR).

In-situ Investigation of the Chromium Induced Degradation of the Oxygen Exchange Kinetics of the IT-SOFC Cathode Material La0.6Sr0.4CoO3-δ

The long-term stability of the oxygen exchange properties of the IT-SOFC cathode material La0.6Sr0.4CoO3-d vs. chromium poisoning is investigated at 600°C by in-situ measurements of the chemical surface exchange coefficient kchem and the chemical diffusion coefficient Dchem of oxygen by applying the dc-conductivity relaxation method. The as-prepared sample shows high values of kchem=3x10-4 cm s-1 and Dchem=1x10-6 cm2s-1, which are only negligibly affected by the treatment in a dry atmosphere for 1000 h without and with the presence of a Cr-source, respectively. However, humidification of the carrier gas results in a strong decrease in both kinetic parameters. Post-test surface analysis of the degraded samples by X-ray photoelectron spectroscopy identifies Cr- and Si-poisoning as well as significant Sr-enrichment within 150 nm depth as the origin of the degradation in a humid atmosphere.

In-Situ Investigations of the Chromium Induced Degradation of the Oxygen Exchange Kinetics of the IT-SOFC Cathode Materials LSC and LSCF

not Available.

Characterization of the La1−xBaxCoO3−δ (0 ≤ x ≤ 1) System as Cathode Material for IT-SOFC

The influence of temperature and procedure on the phase relationships of LaxBa1−xCoO3−δ with 0 ≤ x ≤ 1.0, oxygen nonstoichiometry, thermal and chemical expansion contributions, in air, and the electrochemical response of electrodes formed by three porous layers LaxBa1−xCoO3−δ /(50:50wt%)LaxBa1−xCoO3−δ+Ce0.9Gd0.1O1.95/Ce0.9Gd0.1O1.95 have been investigated. The crystal structures of samples with 0 ≤ x ≤ 0.5 and x = 1.0 were refined satisfactorily with a single phase. The cubic perovskite could be obtained for the x = 0.5−0.7 furnace-cooled samples (∼5°C/min). This phase was found unstable for x = 0.6 and 0.7, in air, at T ≤ 1000°C. For x = 0.8 and 0.9 a mixture of phases has been obtained. The expansion coefficient reaches a maximum of 26 × 10−6 K−1 for x = 0.6 and a minimum of 16 × 10−6 K−1 for x = 0.8 and 0.9 for ΔT = 20−900°C. AC impedance spectroscopy measurements were carried out in the temperature range of 400 ≤ T ≤ 700°C. A minimum polarization resistance Rp ∼ 0.065 – 0.075 Ω cm2 was obtained for 0.4 ≤ x ≤ 0.7 at T = 600°C, which compares very well with the Rp value reported for composites prepared with cobaltites.

Preparation of Nanocomposite GDC/LSCF Cathode Material for IT-SOFC by Induction Plasma Spraying

Homogeneous mixtures of Ce0.8Gd0.2O1.9 (GDC) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) nanopowders were successfully synthesized using induction plasma by axial injection of a solution. The resulting nanocomposite powders consisted of two kinds of nanopowders with different mass ratio of GDC/LSCF, such as 3/7 and 6/4. The morphological features, crystallinity, and the phases of the synthesized powders were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), local energy-dispersive x-ray spectroscopy (EDS) analysis, and x-ray diffraction (XRD). The nanopowders are almost globular in shape with a diameter smaller than 100 nm and their BET specific areas are around 20 m2 g−1. The GDC and LSCF phases are well distributed in the nanopowders. In addition, suspensions, made with the as-synthesized composite nanopowders and ethanol, were used to deposit cathode coatings using suspension plasma spray (SPS). Micro-nanostructures of the coatings are discussed. The coatings are homogeneous and porous (51% porosity) with cauliflower structures.

Potentiality of Ba2Co9O14 as cathode material for IT-SOFC on various electrolytes

In the research of new cathode materials for Solid Oxide Fuel Cells, the cobaltite Ba2Co9O14 shows attractive properties. Its good electronic conductivity (σ=100–240S.cm−1 between 450 to 650°C) as well as a reversible partial reducibility without structural modification, evidenced above 400°C, in favor of a possible O2− mobility, suggests a possible mixed conductivity. These properties make it a potential material for SOFC cathode application. A good chemical compatibility between Ba2Co9O14 and the classical electrolytes YSZ and CGO is evidenced. However, the cathode adhesion on both electrolytes is problematic due to its greater thermal expansion. Various composites [Ba2Co9O14–electrolyte] cathodes have been considered to improve the adhesion. Particular attention is paid on 70%wt. Ba2Co9O14–30%wt. YSZ and 70%wt. Ba2Co9O14–30%wt. CGO cathodes on dense YSZ and CGO, respectively. The cathode layers were deposited by hand-painting or screen-printing. The impedance spectroscopy carried out on symmetrical cells under air reveals that the deposition technique affects the polarization resistance. In the series of samples considered in our study, the screen-printed layer 70%wt. Ba2Co9O14–30%wt. CGO has the lowest polarization resistance: Rp(750°C)=0.5Ω.cm2. The activation energies are about 1.1eV in any case.

Effects of chromium poisoning on the long-term oxygen exchange kinetics of the solid oxide fuel cell cathode materials La 0.6Sr 0.4CoO 3 and Nd 2NiO 4

The influence of chromium poisoning on the long-term stability of the oxygen exchange kinetics of the promising IT-SOFC cathode materials La 0.6Sr 0.4CoO 3−δ (LSC) and Nd 2NiO 4+δ (NDN) is investigated in-situ by dc-conductivity relaxation experiments. The as-prepared LSC and NDN samples show high chemical oxygen surface exchange coefficients k chem. After the deposition of a 10 nm thick Cr-layer onto the surface, k chem of LSC decreases to 50% of the initial value. Additional chromium deposition of 20 nm on LSC leads to a further decrease of k chem to 27% of the initial value. In contrast, the effect of a 10 nm thick Cr-layer on k chem of NDN is negligible. Even with additional 20 nm of chromium and a total testing time of 1750 h, the nickelate retains a k chem of 60% of the initial value. X-ray photoelectron spectroscopy (XPS) of the degraded. LSC shows a significantly altered surface cation composition with Sr-enrichment down to 30 nm depth while XPS analysis of the degraded NDN reveals a thin surface zone of approximately 30 nm containing nickel and chromium. In contrast to LSC, the changes in the surface composition of NDN due to Cr-poisoning ultimately had only a minor influence on the surface exchange properties.

Chemical compatibility of Sr-doped La 2CuO 4 cathode material with LSGM solid oxide electrolyte

The chemical reactivity of a novel cathode material for IT-SOFC, La 2 − x Sr xCuO 4 − δ , with the oxide-ion conductor La 0.83Sr 0.17Ga 0.83Mg 0.17O 3 − w is investigated using XRD and ESEM/EDX techniques. La 2 − x Sr xCuO 4 − δ samples (x = 0.1 and 0.2) are prepared by citrate auto-combustion synthesis. The behaviour of mixtures of La 2 − x Sr xCuO 4 − δ cathode material and La 0.83Sr 0.17Ga 0.83Mg 0.17O 3 − w (LSGM) electrolyte (1:1 weight ratio) annealed at 1000 °C for 24–48 h is studied. Peak broadening and shifting in the diffraction patterns are explained as diffusion of cations from one phase to the other; these findings are corroborated by the ESEM/EDX analysis. No diffraction peaks from reaction products are found. These results showed that the use of La 2 − x Sr xCuO 4 − δ (x = 0.1 and 0.2) cathode material in contact with LSGM electrolyte is not advantageous for SOFC application.

Evaluation of Ba 0.6Sr 0.4Co 0.9Nb 0.1O 3− δ mixed conductor as a cathode for intermediate-temperature oxygen-ionic solid-oxide fuel cells

A perovskite-type Ba 0.6Sr 0.4Co 0.9Nb 0.1O 3− δ (BSCN) oxide is investigated as the cathode material of oxygen-ionic solid-oxide fuel cells (SOFCs) with Sm 0.2Ce 0.8O 3− δ (SDC) electrolyte. Powder X-ray diffraction and SEM characterization demonstrate that solid phase reactions between BSCN and SDC are negligible at temperatures up to 1100 °C. The results of thermal-expansion and electrical conductivity measurements indicate the introduction of Ba 2+ into the A-site of SrCo 0.9N 0.1O 3− δ (SCN) led to a decrease in the thermal-expansion coefficient (TEC) and electrical conductivity of the compound. A TEC of 14.4 × 10 −6 K −1 is observed for BSCN within a temperature range of 200–500 °C. The chemical diffusion coefficient ( D chem) and surface exchange constant ( k ex) of BSCN and SCN are obtained using an electrical conductivity relaxation technique and BSCN prove to have higher D chem and k ex than SCN. An area-specific resistance of 0.1 Ω cm −2 is achieved for BSCN cathodes at 600 °C based on symmetric cells test. Peak power density of ∼1150 mW cm −2 is reached for a thin-film electrolyte cell with BSCN cathode at 600 °C, which is higher than a similar cell with SCN cathode (∼1008 mW cm −2). BSCN is a promising cathode material for oxygen-ionic IT-SOFCs.

Long-term stability of the oxygen exchange properties of (La,Sr) 1 − z(Co,Fe)O 3 − δ in dry and wet atmospheres

The long-term degradation of the surface oxygen exchange kinetics of the IT-SOFC cathode material La 0.58Sr 0.4Co 0.2Fe 0.8O 3 − δ (LSCF) is investigated in dry and wet O 2–Ar atmospheres at 600–700 °C. Conductivity relaxation measurements indicate high chemical surface exchange coefficients ( k chem) in dry O 2–Ar atmosphere at 600–800 °C with E a( k chem) ∼ 200 kJ mol − 1 . However, during 1 kh an exponential decrease of k chem is observed at 600 °C in dry atmosphere. After switching to a H 2O-containing atmosphere a further step-wise decrease of k chem by a factor of 2 occurs, followed by an approximately linear long-term degradation with a rate of ∼ 10% kh − 1 . At 700 °C a pronounced decrease of the surface exchange coefficient by 2 orders of magnitude is found after switching from dry to wet atmospheres. Post-test XPS analysis indicates that the deterioration of the catalytic activity of LSCF originates in significant changes of the surface cation composition (especially Sr enrichment) and silicon poisoning.

Preparation and electrical properties of Ca-doped La2NiO4+δ cathode materials for IT-SOFC

Ca-doped La(2)NiO(4+δ) is synthesized via the nitrate-citrate route. The effects of Ca substitution for La on the sinterability, lattice structure and electrical properties of La(2)NiO(4+δ) are investigated. Ca-doping is unfavorable for the densification process of La(2-x)Ca(x)NiO(4+δ) materials. The introduction of Ca leads to the elongation of the La-O(2) bond length, which provides more space for the migration of oxygen ion in La(2)O(2) rock salt layers. The substitution of Ca increases remarkably the electronic conductivity of La(2-x)Ca(x)NiO(4+δ). With increasing Ca-doping level, both the excess oxygen concentration and the activation energy of oxygen ion migration decrease, resulting in an optimization where a highest ionic conductivity is presented. Ca-doping is charge compensated by the oxidation of Ni(2+) to Ni(3+) and the desorption of excess oxygen. The substitution of Ca enhances the structural stability of La(2)NiO(4+δ) material at high temperatures and renders the material a good thermal cycleability. La(1.7)Ca(0.3)NiO(4+δ) exhibits an excellent chemical compatibility with CGO electrolyte. La(2-x)Ca(x)NiO(4+δ) is a promising cathode alternative for solid oxide fuel cells.

The Electrochemical and Thermal Performances of Ca3Co4O9-δ as a Cathode Material for IT-SOFCs

With the aim of application to cathode material in IT-SOFC, 2D misfit layered calcium cobalt oxide Ca3Co4O9-d was studied using XRD, SEM, thermal dilatometory and impedance spectroscopy. Chemical reactivity of Ca3Co4O9-d with electrolyte indicated good compatibility with CGO against generating impunity phase CaZrO3 with YSZ. Thermal expansion coefficient of pure Ca3Co4O9-d exhibited 10.0-11.0×10-6 K-1, which is similar to that of CGO electrolyte. Indeed, we verified a good adherence between electrode thin layer and the electrolyte. Electrochemical property of Ca3Co4O9-d pellet showed good performances, i.e. ASR values are 3.0 and 24.5 Ωcm2 at 700 and 600 ºC in ambient air respectively, in our experimental conditions.

Electrochemical Behaviour of La2Ni0.6Cu0.4O4+δ Oxide on SDC Electrolyte as Cathode for IT-SOFC

La2Ni0.6Cu0.4O4+δ, a mixed conductor oxide with K2NiF4-type structure, was evaluated as cathode material for IT-SOFC based on Sm0.2Ce0.8O1.9 (SDC) electrolyte. A single cell with La2Ni0.6Cu0.4O4+δ as cathode, SDC as electrolyte and Ni-SDC as anode was fabricated and tested with humidified H2 as fuel and stationary air as oxidant. The electrochemical behavior was analyzed by current voltages curves, ac impedance spectroscopy and load demands at 750, 800 and 850° C. The maximum power density reached by the cell was 120, 166 and 217 mW/cm2 at 750, 800 and 850° C using a 0.4 mm-thick electrolyte. In addition, the cell performance was very stable at 800 ºC for a current density of 421 mA/cm2 demanded for 40 h and at 750 ºC for different current loads (from 84 to 336 mA/cm2) for 80 h. The use of a thinner SDC electrolyte encourages to obtain better performances.

La(Ni,Fe)O3 Stability in the Presence of Cr Species- solid-state Reactivity Study

The solid-state reactivity between La(Ni0.6Fe0.4)O3 (LNF) and Cr2O3 in the IT-SOFC operating temperature range of 600-800 ºC has been investigated using XRD analysis. Because LNF material can be used as a cathode, as a current collecting layer and/or an interconnect coating, a thorough investigation of its chemical stability is of importance for SOFC systems utilizing ferritic stainless steel interconnects. This study demonstrates the intrinsic instability of LNF when exposed to direct contact with Cr2O3 at 800 ºC. It has been observed that Cr enters the perovskite phase, replacing first Ni and then Fe already after 200 h. The rate of the Cr reaction with LNF depends on temperature and exposure time. After 1000 h at 600 ºC no reaction products could be detected with XRD. Although this is a promising result, long-term testing under fuel cell operating conditions is needed to prove LNF as a viable IT-SOFC cathode material.

The Electrochemical and Thermal Performances of Ca3Co4O9-δ as a Cathode Material for IT-SOFCs

not Available.

Polarization Resistances of (Ln1−xSrx)CoO3 (Ln=Pr, Nd, Sm, and Gd x=0, 0.3, 0.5, 0.7, and 1) as Cathode Materials for Intermediate Temperature-operating Solid Oxide Fuel Cells

This study focuses on cathode performances of (Ln1−xSrx)CoO3−δ (Ln=Pr, Nd, Sm, and Gd, x=0, 0.3, 0.5, 0.7, and 1) for intermediate temperature-operating solid oxide fuel cells (IT-SOFCs, 600–800°C). (Ln1−xSrx)CoO3−δ powders as cathode materials for IT-SOFC, which can be operated between 600°C and 800°C were synthesized by the glycine-nitrate-process (GNP) method. The impedance spectroscopy method was used to investigate area specific resistance (ASR) and electrochemical properties of (Ln1−xSrx)CoO3−δ (Ln=Pr, Nd, Sm, and Gd, x=0, 0.3, 0.5, 0.7, and 1). Pr0.5Sr0.5CoO3−δ and Pr0.3Sr0.7CoO3−δ showed to 0.15 Ω cm2 of ASR at 700°C and Nd0.5Sr0.5CoO3−δ to 0.14 Ω cm2 at 700°C. Sm0.5Sr0.5CoO3−δ showed the lowest ASR values of 0.10 Ω cm2 on 10% Gd-doped cerium oxide at 700°C.

Advanced Electrochemical Properties of LnBa0.5Sr0.5Co2O5 + δ (Ln = Pr , Sm, and Gd) as Cathode Materials for IT-SOFC

Excellent area-specific-resistance (ASR) values have been exhibited by cathode materials with a Sr-doped layer perovskite type structure and therefore show themselves to be possible candidates for intermediate-temperature-operating solid oxide fuel cell (IT-SOFC, 600–800°C) applications. (SBSCO) electrode was sintered onto gadolinia-doped ceria (, CGO91) at to form symmetrical cells and exhibited an ASR value of at . The lowest ASR value was observed when the composite cathode of of SBSCO and of CGO91 (SBSCO50) was used in conjunction with an interlayer of CGO91 applied between the electrode and stabilized electrolyte. These were at and at , respectively. The coefficient of thermal expansion (CTE) of SBSCO was at . However, the CTE of the composite cathode of SBSCO50 was shown to be at , this being more compatable with the other components within the cell.

Progress in understanding and development of Ba0.5Sr0.5Co0.8Fe0.2O3−δ-based cathodes for intermediate-temperature solid-oxide fuel cells: A review

Abstract Solid-oxide fuel cells (SOFCs) convert chemical energy directly into electric power in a highly efficient way. Lowering the operating temperature of SOFCs to around 500–800 °C is one of the main goals in current SOFC research. The associated benefits include reducing the difficulties associated with sealing and thermal degradation, allowing the use of low-cost metallic interconnectors and suppressing reactions between the cell components. However, the electrochemical activity of the cathode deteriorates dramatically with decreasing temperature for the typical La0.8Sr0.2MnO3-based electrodes. The cathode becomes the limiting factor in determining the overall cell performance. Therefore, the development of new electrodes with high electrocatalytic activity for oxygen reduction becomes a critical issue for intermediate-temperature (IT)-SOFCs. Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite oxide was first reported as a potential IT-SOFC cathode material in 2004 by Shao and Haile. After that, the BSCF cathode has attracted considerable attention. This paper reviews the current research activities on BSCF-based cathodes for IT-SOFCs. Emphasis will be placed on the understanding and optimization of BSCF-based materials. The issues raised by the BSCF cathode are also presented and analyzed to provide some guidelines in the search for the new generation of cathode materials for IT-SOFCs.

Evaluation of the La 0.75Sr 0.25Mn 0.8Co 0.2O 3− δ system as cathode material for ITSOFCs with La 9Sr 1Si 6O 26.5 apatite as electrolyte

In the past years, a major interest has been devoted to decrease the working temperature of solid oxide fuel cells (SOFCs) down to about 700 °C. Apatite materials (La 10− x Sr x Si 6O 27− x/2 ) are attractive candidates for solid electrolytes, with a high ionic conductivity at 700 °C, a chemical and a dimensional stability for a pO 2 ranging from 10 −25 to 0.2 atm. A perovskite oxide (La 0.75Sr 0.25Mn 0.8Co 0.2O 3− δ ) has been used as a cathode material. Symmetrical cathode/electrolyte/cathode cells were fabricated by stacking layers obtained by tape casting of apatite and perovskite powders and co-sintering at 1400 °C for 2 h in air. Impedance spectroscopy measurements were performed on these cells in order to determine the electrode resistance. It has been shown that the latter decreases with the porosity content of the cathode and with the use of a composite material (apatite/perovskite) instead of a simple perovskite.

Ru-Pyrochlores: Compositional Tuning for Electrochemical Stability as Cathode Materials for IT-SOFCs

Using XRD, impedance spectroscopy, and XPS, it was shown that both Bi2Ru2O7 and Pb2Ru2O6.5, generally announced as attractive electrode materials for IT-SOFCs, chemically react with CGO. In addition, a never-mentioned time-instability under electrochemical measurements with YZS as electrolyte mediates the polarization tests. For Pb2Ru2O6.5, long time experiments (stabilization time > 900 h, previously to complementary electrochemical tests), yields strong increasing of the resistance polarization. Concerning Bi2Ru2O7, a partial transformation into Bi3Ru3O11 is observed after impedance measurements. The investigation of the Bi2−xMxRu2O7−δ (M = Sr, Pb) solid solutions was performed with the aim to improve the catalytic/electric properties. For low x values, it shows a time and chemical stabilization of the electrode/electrolyte cells, able to preserve high electrode performances of these materials. For higher substitution, the chemical/electrochemical instability reappears. According to our polarization mea...