Cathode Materials For IT-SOFCs Research Articles

Characteristics of Cu and Mo-doped Ca3Co4O9−δ cathode materials for use in solid oxide fuel cells

In this study, Cu and Mo ions were doped in Ca3Co4O9−δ to improve the electrical conductivity and electrochemical behavior of Ca3Co4O9−δ ceramic and the performance of a solid oxide fuel cell (SOFC) single cell based on NiO-SDC/SDC/doped Ca3Co4O9−δ-SDC were examined. Cu substitution in the monoclinic Ca3Co4O9−δ ceramic effectively enhanced the densification, slightly increased the grain size, and triggered the formation of some Ca3Co2O6; however, no second phase was found in porous Mo-doped Ca3Co4O9−δ ceramics even when the sintering temperature reached 1050°C. Substitution of Cu ions caused slight increase in the Co3+ and Co4+ contents and decrease in the Co2+ content; however, doping with Mo ions showed the opposite trend. Doping the Ca3Co4O9−δ ceramic with a small amount of Cu or Mo increased its electrical conductivity. The maximum electrical conductivity measured was 218.8Scm−1 for the Ca3Co3.9Cu0.1O9−δ ceramic at 800°C. The Ca3Co3.9Cu0.1O9−δ ceramic with a coefficient of thermal expansion coefficient of 12.1×10−6K−1 was chosen as the cathode to build SOFC single cells consisting of a 20μm SDC electrolyte layer. Without optimizing the microstructure of the cathode or hermetically sealing the cell against the gas, a power density of 0.367Wcm−2 at 750°C was achieved, demonstrating that Cu-doped Ca3Co4O9−δ can be used as a potential cathode material for IT-SOFCs.

Electrostatic spray deposition of Ca3Co4O9 + δ layers to be used as cathode materials for IT-SOFC

In this paper, the deposition of Ca3Co4O9+δ layers on CGO substrates using electrostatic spray deposition (ESD) technique, starting from calcium and cobalt nitrates as precursor salts, is reported. The microstructure was investigated as a function of process parameters such as nozzle-to-substrate distance, solvent composition, substrate temperature, flow rate and deposition time, based on an upper and a lower value. Films with controlled microstructures were obtained after annealing at 880°C for 2h in air. The formation of Ca3Co4O9+δ was confirmed by X-ray diffraction, which also showed evidence of Co3O4 traces. As shown by in-situ X-ray diffraction, Ca3Co4O9+δ starts to form at 625°C and decomposes at 950°C, but the transformation is reversible. Interestingly, the initial morphology of the films was maintained after annealing at either 700 or 880°C, with the appearance of faceted crystals forming a gypsum flower-like microstructure. AC impedance spectroscopy was carried out at intermediate temperatures (600–800°C) under air on 2 batches of symmetrical cells based on ESD and screen-printed electrodes, respectively. This work confirmed the beneficial impact of the ESD technique. The area-specific resistance was improved by at least 23% at 600°C and 40% at 800°C for the sample prepared by ESD compared to the reference one prepared by screen printing. This improvement was explained by a better interface between the electrode and the electrolyte.

Complex modulation of the crystal structure of a layered perovskite. A promising solid-oxide-fuel-cell cathode

Excellent performance as a cathode material for IT-SOFCs is found in (GdBa)0.8Ca0.4Co0.6Fe1.4O6−δ related to a complex crystal superstructure based on ordering of Gd, Ba and Ca in combination with different coordination polyhedral of Co and Fe and layered-location of anion vacancies.

Evaluation of Ba deficient NdBaCo2O5+δ oxide as cathode material for IT-SOFC

Neodymium- and cobalt-based layered perovskite oxides with increasing Ba deficiency (NdBa1-xCo2O5+δ, x=0.0÷0.2) are prepared via solid state firing. The chemical and electrochemical properties of these materials are characterized via XRD, SEM, TG-DTA, 4-probe conductivity measurement and EIS tests on symmetric cells. The maximum tolerated Ba deficiency is slightly below 10%. The compounds crystallize in an ordered layered structure with orthorhombic Pmmm (for x=0 and 0.5) and tetragonal P4/mmm (for x=0.10) crystal lattice. All the compounds show high total conductivity (between 400 and 600S/cm at 700°C). A remarkable tenfold decrease of the area specific resistance is observed at decreasing the Ba content from 0 to 10%, with the compound at 10% Ba deficiency reaching the best performance (0.1Ω*cm2 at 700°C). Detailed equivalent circuit analysis on EIS tests at varying temperature (550 to 700°C) and O2 content (100% to 5% v/v) reveals that the limiting steps are the transport of oxygen ions within the cathodic phase and across the electrolyte interface at high frequency, and the formation of an oxygen ion at intermediate frequency. The effect of Ba on the electrochemical mechanism is associated with a promotion of the bulk diffusion steps at high frequency.

Evaluation of La1.8−xPrxSr0.2CuO4−δ oxides as cathode materials for IT-SOFCs

Conducting properties, thermal expansion, and chemical stability of La1.8−xPrxSr0.2CuO4−δ (x = 0.2; 0.4) related to the K2NiF4-type structure have been investigated in order to estimate their appropriateness to be used as a cathode material for solid oxide fuel cells. Electrical conductivity of La1.8−xPrxSr0.2CuO4−δ demonstrates metallic-like behavior in the temperature range of 100–900 °С at variable oxygen partial pressure (pO2 = 10−3–1 atm), and reaches 50 S/cm at 750 °C in air. The cuprates display thermal expansion coefficients (TECs) of about 14·10−6 K−1 that are compatible with La0.8Sr0.2Ga0.85Mg0.15O3−δ (LSGM) and Ce0.9Gd0.1O1.95 (GDC) solid electrolytes. The electrocatalytic activity study of La1.6Pr0.2Sr0.2CuO4−δ electrode deposited on GDC as well as LSGM shows that the lowest polarization resistance is achieved in case of deposition of the electrode on GDC electrolyte.

Influence of La/Pr Ratio on the Ageing Properties of La2-X Pr x NiO4+δ as Cathodes in IT-SOFCs

The present study is focused on alternative oxygen electrodes for IT-SOFCs using Metal Supported Cells (MSCs) conditions. Ln2NiO4+δ (Ln = La, Pr) compounds with the K2NiF4-type structure act as cathode materials for IT-SOFC due to their MIEC properties. Pr2NiO4+δ (PNO) shows excellent electrochemical properties at intermediate temperature while La2NiO4+δ (LNO) exhibits higher chemical stability. Thus, the properties of La2 -x Pr x NiO4+δ (LPNO) mixed nickelates were investigated with the aim to find the best compromise between chemical stability and electrochemical performances. Herein, the chemical stability of the nickelates under air at operating temperatures as well as the evolution of the polarization resistances, R p, during ageing (i dc = 0 and i dc ≠ 0 conditions) were studied for duration up to one month. At i dc = 0 condition, the ageing of the half cells shows no change in R p. A different behavior is observed under i dc ≠ 0 conditions with an increase in R p in SOFC mode while interestingly, R p remains stable in SOEC mode.

Optimization of La1−xSrxCoO3−δ perovskite cathodes for intermediate temperature solid oxide fuel cells through the analysis of crystal structure and electrical properties

Lanthanide doped SrCoO3−δ oxides, Ln1−xSrxCoO3−δ (Ln = La, Pr, Nd, Sm, and Gd), have been extensively investigated on the basis of their good ionic and electronic conductivity and high performance as IT-SOFC cathode materials. La1−xSrxCoO3−δ (LSCs) is one of the most common materials for IT-SOFCs due to its good electrical properties and high catalytic activity, and favourable thermal stability over wide ranges of temperature and oxygen partial pressure. The role of lanthanum in LSC, however, has not been systematically studied. In this study, we systematically investigated the effect of the La in La1−xSrxCoO3−δ cathodes on the structure, electrical properties, and electrochemical performance of the cathode. La0.4Sr0.6CoO3−δ is confirmed as the most suitable cathode candidate material for IT-SOFCs in terms of electrochemical performance.

Performance of cobalt-free perovskite La0.6Sr0.4Fe1−xNbxO3−δ cathode materials for proton-conducting IT-SOFC

A novel series of cobalt-free perovskite cathode materials La0.6Sr0.4Fe1−xNbxO3−δ (0.00⩽x⩽0.20) are prepared by a solid-state reaction method. The X-ray diffraction results indicate the doping limit of Nb in La0.6Sr0.4Fe1−xNbxO3−δ is at least x=0.15. The conductivities of the cathode materials are measured as functions of temperature (300–800°C) and oxygen partial pressure (1–10−10atm). It is discovered that the Nb-doping for Fe-site of La0.6Sr0.4FeO3−δ distinctly improved the electrical conductivity of La0.6Sr0.4FeO3−δ; the oxides displayed a semiconductor-like conduction behavior in the temperature region. The highest conductivity is observed for La0.6Sr0.4Fe0.9Nb0.1O3−δ (LSFN10) to be 132Scm−1 in air at 800°C. A single cell using LSFN10 cathode and NiO–BaZr0.1Ce0.7Y0.2O3−δ (NiO–BZCY) composite anode supported BZCY electrolyte membrane (ca. 30μm in thickness) is assembled and tested at 500–700°C with H2 as fuel and air as oxidant. The peak power density and the interfacial polarization resistance (Rp) are 503mWcm−2 and 0.17Ωcm2 at 700°C, respectively. The above results indicate that La0.6Sr0.4Fe0.9Nb0.1O3−δ is a new promising cathode material for proton-conducting IT-SOFC.

Effects of the electrode configuration, phase relationship and microstructure on the polarization resistance of La0.3Ba0.7CoO3−δ as cathode material for IT-SOFC

The polarization resistance of La0.3Ba0.7CoO3−δ (LBC7) as cathode material was studied by impedance spectroscopy measurements in the temperature range of 450 ≤ T ≤ 750 °C, in air, on samples prepared by the solid state reaction (SSR) method and a wet chemical (WC) route, using symmetrical cells with single-layer and graded composition electrodes. While the LBC7 sample prepared by the SSR method and cooled at a rate of 10 °C/min turned out to be single phase with cubic symmetry, the sample prepared by the WC route is a mixture of the cubic phase and the 2H hexagonal phase. The lowest polarization resistance at 600 °C, Rp ∼ 0.06 Ω cm2, was obtained for the graded composition electrodes. Our results suggest that the electrode configuration is a key factor in order to reduce the polarization resistance of the LBC7 electrode material.

Synthesis and Electrochemical Characterization of T* La0.84Sm0.96Sr0.2CuO4 as a Cathode Material for IT-SOFC

not Available.

Optimization of Transport Properties of A-Site Ordered LnBa1-xSrxCo2-yFeyO5+δ Perovskite-Type Cathode Materials

Choice of an appropriate chemical composition in a group of A-site ordered LnBa1-xSrxCo2-xFexO5+δ (Ln - selected lanthanides, Y) oxides may allow to obtain novel, potential cathode materials for IT-SOFC having desirable physicochemical properties, which can surpass those of typical Ln1-xSrxCo1-yFeyO3-δ perovskites. Layered ordering in A sublattice (Ln-Ba1-xSrx), resulting from a significant difference between ionic radii, is observed for compounds having smaller Ln cations, even for a substantial substitution of Ba by Sr. Materials with a high Co content exhibit very high mixed ionic-electronic conductivity and substantial deviation from oxygen stoichiometry, while increase of Fe content improves stability and decreases thermal expansion. The observed relationship between structural properties, oxygen nonstoichiometry and transport properties is discussed.

Improved electrochemical performance by doping cathode materials Sr2Fe1.5Mo0.5−xTaxO6−δ (0.0 ≤ x ≤ 0.15) for Solid State Fuel Cell

To date great efforts have been devoted to the development of solid oxide fuel cell (SOFC) devices that is able to run at intermediate temperature (IT) and to retain good electrochemical performance as in the high temperature regime. To this end, Tantalum (Ta) is doped into Sr2Fe1.5Mo0.5−xTaxO6−δ (0.0 ≤ x ≤ 0.15) to further improve the electrochemical performance as the cathode materials for IT-SOFC. Pure orthorhombic Sr2Fe1.5Mo0.5−xTaxO6−δ (0.0 ≤ x ≤ 0.15) samples, with space group Pnma (no. 62), are synthesized by a two-step method and identified via the powder X-ray Rietveld refinements. X-ray photoelectron spectroscopy and thermogravimetry results consistently show that the number of oxygen vacancy increases with the increasing of Ta content. The thermal expansion compatibility of the samples is found to match the electrolyte of La0.9Sr0.4Ga0.1Mg0.2O3−δ (LSGM) better than that of cobalt-containing electrode compounds. Finally, the higher oxygen vacancy concentration leads to the enhanced ionic conductivity although electrical conductivity is slightly decreased due to the oxygen vacancy blocking. Therefore, Ta-doped Sr2Fe1.5Mo0.5O6−δ is proved to be a promising candidate for future cathode materials of IT-SOFC.

Synthesis and Electrochemical Characterization of T* La0.84Sm0.96Sr0.2CuO4 as a Cathode Material for IT-SOFC

The synthesis and electrochemical characterisation of T* based La0.84Sm0.96Sr0.2CuO4 (LSSCu) has been carried out in order to use as a cathode material for intermediate temperature solid oxide fuel cell application. XRD studies shows, the phase pure material and matches with the JCPDF (# 79-1861), belong to space group of P4/nmm. The electrical conductivity value decreases from 52 Scm-1 at room temperature to 18.3 Scm-1 at 850 oC showing metallic behaviour in all temperature range. A decrease in conductivity, decreasing the partial pressure of oxygen implying the above material is p-type conductor. The Coefficient of thermal expansion value measured from Dilatometry is 14.4 * 10 -6 K-1 which matches with La0.9Sr0.1Ga0.8Mg0.2 (LSGM). Symmetrical cell testing results shows that the area specific resistance is 0.02 Ω.cm2 at 800 oC when the 1:1 weight ratio of the LSSCu and LSGM composite was screen printed on LSGM electrolyte and sintered at 900 oC for 1 hr.

Investigation of GdBaCo2−xFexO6−δ (x = 0, 0.2) – Ce0.8Sm0.2O2 composite cathodes for intermediate temperature solid oxide fuel cells

The double perovskites GdBaCo2−xFexO6−δ (x = 0, 0.2) and composites (100 − y) GdBaCo2−xFexO6−δ (x = 0, 0.2) – y Ce0.8Sm0.2O2 (y = 10–50 wt.%) were investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT–SOFCs). Chemical compatibility of GdBaCo2−xFexO6−δ (x = 0, 0.2) with solid electrolyte Ce0.8Sm0.2O2, thermal expansion, DC conductivity and electrochemical performance of (100 − y) GdBaCo2−xFexO6−δ (x = 0, 0.2) – y Ce0.8Sm0.2O2 (y = 10–50 wt.%) were studied. Partial substitution of Fe for Co was shown to lead to decrease of double perovskite GdBaCo2−xFexO6−δ reactivity with the solid electrolyte Ce0.8Sm0.2O2. Polarization resistance of cathodes studied was found to depend significantly on firing temperature. Variation of solid electrolyte content in (100 − y) GdBaCo2−xFexO6−δ (x = 0, 0.2) – y Ce0.8Sm0.2O2 (y = 10–50 wt.%) composites was shown to allow to optimize their electrochemical performance. Cathode materials of 80 wt.% GdBaCo2O6−δ – 20 wt.% Ce0.8Sm0.2O2 and 65 wt.% GdBaCo1.8Fe0.2O6−δ – 35 wt.% Ce0.8Sm0.2O2 were found to have the lowest polarization resistances and reasonable values of thermal expansion coefficient (TEC) and, therefore, can be considered as promising cathode materials for IT-SOFCs.

Synthesis and Electrical Transport Property of Sr and Cu-Doped LaFeO<sub>3</sub>-Based Cathode Material for IT-SOFC

In order to accelerate the commercialization of SOFCs technology, the key is the development of high performance cathode materials operated at intermediate temperature. Sr and Cu doped rare earth oxide La1-xSrxFe1-yCu.yO3-δ (x=0.1, 0.3 ; y=0.1, 0.2, denoted as LSFCu-11, LSFCu-31 and LSFCu-32 ) were synthesized by solid state reaction method (SSR). The formation process, phase structure and microstructure of the synthesized samples were characterized using TG/DSC, XRD and SEM. The thermal expansion coefficients (TEC) of the samples were analyzed by thermal dilatometry. The electrical conductivities of the samples were measured with DC four-terminal method from 25 to 950 °C. The results indicate that the samples exhibit a single phase with orthorhombic and hexagonal perovskite structure after sintered at 1200 °C for 4h. The electrical conductivity of the samples increases with temperature up to a maximum value, and then decreases gradually. The small polaron hopping is regarded as the conducting mechanism for synthesized samples at T 550 °C. The negative temperature dependence occurring at higher temperature is due to the creation of oxygen vacancies for charge balance. LSFCu-32 has higher mixed conductivity (> 100 S·cm-1) at intermediate temperature and can meet the demand of cathode material for IT-SOFC. In addition, the average TECs of LSFCu-11, LSFCu-31 and LSFCu-32 are 1.22 × 10-6 K-1 , 1.30 × 10-6 K-1 and 1.34 × 10-6 K-1 respectively.

Electrical conductivity, thermal expansion and electrochemical performances of Ba-doped SrCo0.9Nb0.1O3−δ cathodes for IT-SOFCs

Perovskite-type oxides BaxSr1−xCo0.9Nb0.1O3−δ (BSCNx, x = 0.0–0.8) were synthesized and investigated as cathodes for IT-SOFCs. Ba doping improves chemical compatibility between BSCNx oxides and Ce0.9Gd0.1O1.95 (GDC) electrolyte. Effects of Ba doping on electrical conductivity, thermal expansion and electrochemical performances were systematically elucidated and discussed. Both thermal expansion coefficient (TEC) and polarization resistance (Rp) decrease with increasing Ba doping level up to x = 0.6, attain a minimum at x = 0.6 and then increase with further increasing x > 0.6. The decrease of TEC with the incorporation of Ba can be attributed to the weakened chemical expansion and the decrease of Rp with Ba is due to the increase of oxygen vacancy concentration and oxygen vacancy diffusion coefficient. With a 300 μm-thick GDC as electrolyte and BSCN0.6 as the cathode, the maximum power density of a single-cell achieves 778 mW cm−2 at 800 °C. All these results indicate that the BSCN0.6 oxide is a promising cathode material for IT-SOFC.

A Bifunctional Air Electrode Catalyzed by Ba0.5Sr0.5Co0.8Fe0.2O3–δ for Zinc-Air Batteries

not Available.

Evaluation of Ln2CuO4 (Ln: La, Pr, Nd) oxides as cathode materials for IT-SOFCs

In this work an evaluation of Ln2CuO4 (Ln: La, Pr, Nd) cuprates is presented in terms of their possible application as cathode materials in Solid Oxide Fuel Cells working in an intermediate temperature range (600–800°C). The materials were synthesized by a soft chemistry EDTA-based method. Structural and microstructural characterization, measurements of transport properties and oxygen nonstoichiometry, determination of chemical stability in relation to Ce0.8Gd0.2O1.9 electrolyte, as well as mass relaxation studies for determination of oxygen diffusion coefficient D and surface exchange coefficient K in Pr2CuO4 and measurements of electrochemical properties of electrolyte-supported cells with Pr2CuO4 cathodes were performed. Pr2CuO4 and Nd2CuO4 oxides were found to possess tetragonal I4/mmm structure, in case of La2CuO4, orthorhombic Bmab one was observed. High temperature XRD data revealed anisotropic thermal expansion of Pr2CuO4, smaller along c-axis. Based on reduction experiments performed on TG apparatus, the initial slight oxygen excess δ present in the compounds at room temperature was evaluated: La2CuO4.03(2), Pr2CuO4.05(2) and Nd2CuO4.04(2). Additionally, it was observed that up to 900°C in air the oxygen content in the materials does not change considerably. Pr2CuO4 exhibits activated character of the electrical conductivity as a function of temperature, with values relatively high for cuprates, exceeding 100Scm−1 in 500–800°C range, as well as negative values of Seebeck coefficient from room temperature (RT) up to 800°C. Electrical conductivities of La2CuO4 and Nd2CuO4 are much lower in the temperature range of interest. Using mass relaxation technique it was possible to simultaneously evaluate D and K in 600–900°C temperature range. Activation energy of D was calculated to be 0.13(2)eV. Results of chemical stability studies in relation to Ce0.8Gd0.2O1.9 electrolyte revealed moderately good chemical compatibility of Pr2CuO4 oxide, while La2CuO4 and Nd2CuO4 stability proved to be insufficient. Using custom-made electrolyte-supported SOFC cells with Pr2CuO4-based cathode, it was possible to obtain power densities exceeding 0.1Wcm−2 at 800°C and 0.25Wcm−2 at 900°C.

Characterization of layered perovskite oxides NdBa1−xSrxCo2O5+δ (x = 0 and 0.5) as cathode materials for IT-SOFC

The crystal structure, thermal expansion, and electrochemical properties of the layered perovskite series NdBa1−xSrxCo2O5+δ (x = 0 and 0.5) were investigated to study the effects of substituting Sr for Ba on a layered perovskite oxide.A tetragonal layered perovskite structure was observed in NdBaCo2O5+δ (NBCO), and a tetragonal structure was also observed in NdBa0.5Sr0.5Co2O5+δ (NBSCO) when Sr was substituted into NBCO. The thermal expansion coefficients (TECs) of NBSCO and NBCO were both bigger than 22 × 10−6 K−1 at 530 °C. However, the TEC values for 50 wt% of NBSCO and 50 wt% of Ce0.9Gd0.1O2−δ (CGO91) (NBSCO:50) were 13.2 × 10−6 K−1 at 600 °C and 13.7 × 10−6 K−1 at 700 °C; for 50 wt% of NBCO and 50 wt% of CGO91 (NBCO:50) they were 12.4 × 10−6 K−1 at 600 °C and 12.5 × 10−6 K−1 at 700 °C. The cathode polarizations (Rp) of NBSCO and NBCO on a CGO91 electrolyte were observed to be 0.25 and 0.11 Ωcm2, respectively, at 700 °C. The Rps of composite cathodes in NBSCO:50 and NBCO:50 were 0.061 and 0.038 Ωcm2, respectively, at 700 °C.

In Situ Investigations of the Chromium-Induced Degradation of the Oxygen Surface Exchange Kinetics of IT-SOFC Cathode Materials La0.6Sr0.4CoO3−δ and La0.58Sr0.4Co0.2Fe0.8O3−δ

The long-term stability of the oxygen exchange properties of the IT-SOFC cathode materials La0.6Sr0.4CoO3−δ and La0.58Sr0.4Co0.2Fe0.8O3−δ vs. chromium poisoning is investigated at 600°C by in-situ measurements of the chemical surface exchange coefficient of oxygen kchem using the dc-conductivity relaxation method. The as-prepared samples show high values of kchem, which decrease moderately during the treatment in a dry atmosphere during 1000 h without and with the presence of a Cr-source. Humidification of the carrier gas results in a strong decrease in kchem of both materials. Post-test surface analysis of the degraded specimens by X-ray photoelectron spectroscopy identifies Cr- and Si-poisoning as well as Sr- and O-enrichment in depth ranges of about 100 nm as the origin of the degradation in a humid atmosphere. These results can be correlated with the strong enhancement of the gas phase transport of volatile Cr- and Si-species in the presence of H2O(g). It can be concluded that La0.6Sr0.4CoO3−δ (and to a lesser extent La0.58Sr0.4Co0.2Fe0.8O3−δ) cathodes retain a relatively high stability of the chemical surface exchange coefficients of oxygen at 600°C in dry atmospheres, even in the vicinity of Cr- and Si-sources. However, significant performance degradation of (La,Sr)(Co,Fe)O3−δ-based cathodes has to be expected in humid atmospheres.