PrBa0.5Sr0.5Co2O5+δ layered perovskite cathode for intermediate temperature solid oxide fuel cells

Layered perovskite oxides have ordered A-cations localizing oxygen vacancies, and may potentially improve oxygen ion diffusivity and surface exchange coefficient. The A-site-ordered layered perovskite PrBa_0.5Sr_0.5Co₂O_5+δ (PBSC) was evaluated as new cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The material was characterized using electrochemical impedance spectroscopy in a symmetrical cell system (PBSC/Ce_0.9Sm_0.1O_1.9 (SDC)/PBSC), exhibiting excellent performance in the intermediate temperature range of 500-700 °C. An area-specific-resistance (ASR) of 0.23 Ω cm² was achieved at 650 °C for cathode polarization. The low activation energy (Ea) 124 kJ mol⁻¹ is comparable to that of La_0.8Sr_0.2CoO_3−δ. A laboratory-scaled SDC-based tri-layer cell of Ni-SDC/SDC/PBSC was tested in intermediate temperature conditions of 550 to 700 °C. A maximum power density of 1045 mW cm⁻² was achieved at 700 °C. The interfacial polarization resistance is as low as 0.285, 0.145, 0.09 and 0.05 Ω cm² at 550, 600, 650 and 700 °C, respectively. Layered perovskite PBSC shows promising performance as cathode material for IT-SOFCs.     

Characterization of NdSrCo1−xFexO4+δ (0 ≤ x ≤ 1.0) intergrowth oxide cathode materials for intermediate temperature solid oxide fuel cells

NdSrCo_1−xFe_xO_4+δ (0 ≤ x ≤ 1.0) intergrowth oxides have been investigated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). All the cathodes prepared by a glycine nitrate process (GNP) indicated single phase intergrowth oxides. The introduction of Fe for Co leads to decrease TEC values and electrical conductivity, and increase polarization resistance and oxygen content. The polarization resistance of NdSrCoO_4+δ composition is 0.16 Ω cm² at 800 °C in air atmosphere, which is the best electrochemical performance compared with other compositions.     

LaBaCuFeO5+δ-Ce0.8Sm0.2O1.9 as composite cathode for solid oxide fuel cells

The performance of the LaBaCuFeO_5+δ-Ce_0.8Sm_0.2O_1.9 (LBCF-SDC) composite cathodes was studied in this paper. Electrical conductivity, thermal expansion and electrochemical properties were investigated by four probing DC technique, dilatometry, AC impedance and polarization techniques, respectively. The thermal expansion coefficients of the LBCF-SDC were between (16.3 and 13.4) × 10⁻⁶ K⁻¹ from 30 to 850 °C, which was lower value than LBCF (17.0 × 10⁻⁶ K⁻¹). AC Impedance spectroscopy measurements of LBCF-SDC/SDC/LBCF-SDC test cell were carried out. Polarization resistance values for the LBCF-SDC10 cathode was as low as 0.097 Ω cm² at 750 °C.     

Investigation of layered perovskite NdBa0.5Sr0.25Ca0.25Co2O5+δ as cathode for solid oxide fuel cells

Layered perovskite oxides with and without Ca-doped NdBa_0.5Sr_0.25Ca_0.25Co₂O_5+δ (NBSCaCO) and NdBa_0.5Sr_0.5Co₂O_5+δ (NBSCO) are studied to investigate the effects of Ca doping on the crystal structure, thermal behavior, electrical and electrochemical properties. Both NBSCO and NBSCaCO are tetragonal structure with P4/mmm space group. The average thermal expansion coefficient (TEC) value is reduced from 23.3?×?10^?6 K^?1 to 19.8?×?10^?6 K^?1 during 30–800?°C. The electrical conductivities are increased by Ca doping. Both electrical conductivities of NBSCO and NBSCaCO are higher than 600?S·cm^?1 over 30–800?°C. Substitution of Sr with Ca can effectively improve the electrochemical properties of NBSCaCO. From 650?°C to 800?°C, the area specific resistance (ASR) of NBSCaCO are decreased from 0.62 to 0.062?Ω?cm² and the corresponding output power density are increased from 258 to 812?mW?cm^?2. On the basis of these results, Ca doped layered perovskite NBSCaCO can be a good cathode candidate material for SOFC application.     

Microstructure of Ba1−xLaxTiO3−δ ceramics sintered by Spark Plasma Sintering

Nano-sized Ba_1−xLa_xTiO₃ (0.00 ≤ x ≤ 0.14) powders were prepared by a coprecipitation method and calcined at 850 °C in air. The corresponding ceramics were obtained by Spark Plasma Sintering (SPS) at 1050 °C. These ceramics are oxygen deficient and are marked as Ba_1−xLa_xTiO_3−δ. Both powders and ceramics were characterized by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The effect of lanthanum concentration on the densification behavior, on the structure and the microstructure of the oxides was investigated. Average grain sizes are comprised between 54 (3) nm and 27 (2) nm for powders, and 330 (11) nm and 36 (1) nm for ceramics according to the La-doping level. Powders crystallize in the cubic (or pseudo-cubic) perovskite phase. The structure of ceramics consists in a mixture of cubic (or pseudo-cubic) and tetragonal perovskite type phases. As the lanthanum content increases, the tetragonality of the samples decreases, as well as the grain size.     

Electrical conductivity of La1−xCaxFeO3−δ solid solutions

La_1−xCa_xFeO_3−δ solid solutions (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) were investigated. The samples were prepared by the polymerizable complex route and characterized by X-ray diffraction and complex impedance spectroscopy techniques. Results reveal the formation of a single perovskite phase for the La_1−xCa_xFeO_3−δ (0≤x≤0.5) compositions. However, the La_0.4Ca_0.6FeO_3−δ sample is a mixture of many phases: perovskite, calcium ferrite and iron oxide. The unsubstituted lanthanum ferrite oxide, as well as the substituted samples, exhibits an orthorhombic symmetry. The direct current conductivity analyses reveal a typical negative temperature coefficient of the resistance behaviour for all the samples. The incorporation of calcium into the lanthanum ferrite lattice results in a significant improvement of the direct current conductivity. In fact, La_0.8Ca_0.2FeO_3−δ oxide shows the optimal conduction value. For all the studied compositions, a change in the activation energy is highlighted around 440 °C. This behaviour is attributed to the antiferromagnetic to paramagnetic transition of lanthanum ferrite. As for the alternating current conductivity, it obeys the Jonsher's power law. The correlated barrier hopping model is proposed to describe the transport mechanism in the studied matrix.     

Perovskite SrFe1-xTixO3-δ (x < = 0.1) cathode for low temperature solid oxide fuel cell

Stable and compatible cathode materials are a key factor for realizing the low-temperature (LT, ≤600?°C) operation and practical implementations of solid oxide fuel cells (SOFCs). In this study, perovskite oxides SrFe_1-xTi_xO_3-δ (x?< = 0.1), with various ratios of Ti doping, are prepared by a sol-gel method for cathode material for LT-SOFCs. The structure, morphology and thermo-gravimetric characteristics of the resultant SFT powders are investigated. It is found that the Ti is successfully doped into SrFeO_3-δ to form a single phase cubic perovskite structure and crystal structure of SFT shows better stability than SrFeO_3-δ. The dc electrical conductivity and electrochemical properties of SFT are measured and analysed by four-probe and electrochemical impedance spectra (EIS) measurements, respectively. The obtained SFT exhibits a very low polarization resistance (R_p), .01 Ωcm² at 600?C. The SFT powders using as cathode in fuel cell devices, exhibit maximum power density of 551?mW?cm^?2 with open circuit voltage (OCV) of 1.15?V at 600?C. The good performance of the SFT cathode indicates a high rate of oxygen diffusion through the material at cathode. By enabling operation at low temperatures, SFT cathodes may result in a practical implementation of SOFCs.     

Effect of divalent ion substitution on oxygen sensing properties of hot-spot based Eu1−xCaxBa2Cu3O7−δ and Eu1−yMgyBa2Cu3O7−δ ceramics

In this paper effects of Ca and Mg substitution on oxygen sensing properties of hot spot based Eu123 rods are reported. Eu_1−xCa_xBa₂Cu₃O_7−δ (x=0.2–0.5) and Eu_1−yMg_yBa₂Cu₃O_7−δ (y=0.2–0.5) ceramics were synthesized from oxide powders using the standard solid state method and fabricated into short rods. For Ca-substituted rods, after appearance of a visible hot spot, a constant current plateau in I–V curve was formed. The output current response of the rod in periodically changing pO₂ between 20% and 100% showed improved stability and reproducibility for x=0.4 compared to x=0.2. Improved oxygen absorption and desorption time was observed for x=0.4 compared to previously reported unsubstituted rod. On the other hand, for Mg-substituted rods the I–V behavior after formation of hot spot showed a negative slope. Faster absorption time of 3.0 s and desorption time of 6.9 s were observed for y=0.4 compared to y=0.2. The improved output current stability, reproducibility and response time is suggested to be due to changes in oxygen activation energy and increased hole concentration as a result of Ca²⁺/Mg²⁺substitutions. The Mg-substituted rods showed better performance compared to Ca-substituted rods possibly due to higher porosity and vacancy concentration.     

Spray-drying synthesized lithium-excess Li4+xTi5−xO12−δ and its electrochemical property as negative electrode material for Li-ion batteries

Li₄Ti₅O₁₂ (Fd-3m space group) materials were synthesized by controlling the lithium and titanium ratios (Li/Ti) in the range of 0.800-0.900 by using a spray-drying method, followed by calcination at several temperatures between 700 and 900 °C for large-scale production. Chemical and structure studies of the final products were done by X-ray diffraction (XRD), neutron diffraction (ND), X-ray photon electron spectroscopy (XPS), scanning electron microscopy (SEM) and inductively coupled plasma mass spectrometry (ICP-MS). The optimum synthesis condition was examined in relation to the electrochemical characteristics including charge-discharge cycling and ac impedance spectroscopy. It was found that when the spray-drying precursors at the Li/Ti ratio of 0.860 were calcined at 700-900 °C for 12 h in air, a pure Li_4+xTi_5−xO_12−δ (x = 0.06-0.08) phase with a lithium-excess composition was obtained. Based on the structural studies, it was found that the excess lithium is located at the lithium and titanium layer of the 16d site in the spinel structure (Fd-3m). These pure Li_4+xTi_5−xO_12−δ (x = 0.06-0.08) phase materials showed a higher discharge capacity of ∼164 mAh g⁻¹ at 1.55 V (vs. Li/Li⁺), between the cut-off voltage of 1.2-3.0, with an excellent cyclability and superior rate performance in comparison with the Li₄Ti₅O₁₂ phase containing impurity phases.     

Structure and thermal properties of La0.6Sr0.4Co0.2Fe0.8O3−δ-SDC carbonate composite cathodes for intermediate- to low-temperature solid oxide fuel cells

The structure and thermal properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ-SDC carbonate (LSCF-SDC carbonate) composite cathodes were investigated with respect to the calcination temperatures and the weight content of the samarium-doped ceria (SDC) carbonate electrolyte. The composite cathode powder has been prepared from La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ and SDC carbonate powders using the high-energy ball milling technique in air at room temperature. Different powder mixtures at 30 wt%, 40 wt% and 50 wt% of SDC carbonate were calcined at 750-900 °C. The findings indicated that the structure and thermal properties of the composite cathodes were responsive to the calcination temperature and the content of SDC carbonate. The absence of any new phases as confirmed via XRD analysis demonstrated the excellent compatibility between the cathode and electrolyte materials. The particle size of the composite cathode powder was ∼0.3-0.9 μm having a surface area of 4-15 m² g⁻¹. SEM investigation revealed the presence of large particles in the resultant powders resulting from the increased calcination temperature. The composite cathode containing 50 wt% SDC carbonate was found to exhibit the best thermal expansion compatibility with the electrolyte.     

Conductivity and electrochemical performance of (Ba0.5Sr0.5)0.8La0.2Fe1−xMnxO3−δ cathode prepared by the citrate-EDTA complexing method

This study reports the successful preparation of single-phase perovskite (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ (x = 0-0.2) by the citrate-EDTA complexing method. The crystal structure, thermal gravity analysis, coefficient of thermal expansion, electrical conductivity, and electrochemical performance of (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ were investigated to determine its suitability as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The lattice parameter a of (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ decreases as the amount of Mn doping increases. The coefficients of thermal expansion of the samples are in the range of 21.6-25.9 × 10⁻⁶ K⁻¹ and show an abnormal expansion at around 400 °C associated with the loss of lattice oxygen. The electrical conductivity of the (Ba_0.5Sr_0.5)_0.8La_0.2Fe_1−xMn_xO_3−δ samples decreases as the amount of Mn-doping increases. The electrical conductivity of the samples reaches a maximum value at around 400 °C and then decreases as the temperature increases. The charge transfer resistance, diffusion resistance and total resistance of a (Ba_0.5Sr_0.5)_0.8La_0.2Fe_0.8Mn_0.15O_3-δ-Ce_0.8Sm_0.2O_1.9 composite cathode electrode at 800 °C are 0.11 Ω cm², 0.24 Ω cm² and 0.35 Ω cm², respectively.     

Liquid-phase synthesis of SrCo0.9Nb0.1O3-δ cathode material for proton-conducting solid oxide fuel cells

A novel liquid-phase synthesis strategy is demonstrated for the preparation of the Nb-containing ceramic oxide SrCo_0.9Nb_0.1O_3-δ (SCN). In comparison with the traditional solid-state reaction (SSR) method, the liquid-phase synthesis route offers a couple of advantages, including a lower phase formation temperature and a smaller particle size of the SCN materials that are beneficial for applications as proton-conducting fuel cell cathode. With BaCe_0.4Zr_0.4Y_0.2O_3-δ (BCZY442) as the electrolyte and the SCN synthesized in this work as the cathode, a proton-conducting solid oxide fuel cell (SOFC) shows a peak power density of 348 mW cm^?2 at 700 °C, significantly higher than that of a SOFC fabricated with SCN cathode prepared using the SSR method, which can only deliver 204 mW cm^?2 at the same temperature. Additionally, this new synthesis strategy allows impregnation of Sr²⁺, Co³⁺and Nb⁵⁺ on the solid backbone in aqueous solution, further improving cell performance to reach a peak power density of 488 mW cm^?2 at 700 °C.     

Electrochemical characterization of gradient Sm0.5Sr0.5CoO3−δ cathodes on Ce0.8Sm0.2O1.9 electrolytes for solid oxide fuel cells

This study investigates Sm_0.5Sr_0.5CoO_3−δ (SSC)-Ce_0.8Sm_0.2O_1.9 (SDC) composite cathodes with a gradual change in composition from electrolyte to the cathode in an attempt to discover a potential approach applicable to solid oxide fuel cells (SOFCs). The gradual change in composition from electrolyte to cathode shows the decline in charge transfer resistance (R₂) and gas phase diffusion resistance (R₃). Because the value of R₃ is always larger than R₂ and R₃ significantly dominates the total cathode polarization resistance (R_P) at temperatures within the range of 750-850 °C, i.e., in this temperature range, the rate-determining step is dominated by the diffusion or dissociative adsorption of oxygen. The functionally gradient cathode with a graded interface between cathode and electrolyte reveals both a higher exchange current density (i₀) and a lower activation energy for oxygen reduction reaction (ORR), which suggests that the ORR kinetics can be improved by using the configuration of a functionally gradient cathode.     

Electrochemical properties of LaBaCo2O5+δ-Sm0.2Ce0.8O1.9 composite cathodes for intermediate-temperature solid oxide fuel cells

The electrochemical properties of LaBaCo₂O_5+δ-xSm_0.2Ce_0.8O_1.9 (LBCO-xSDC, x = 20, 30, 40, 50, 60, wt%) were investigated for the potential application in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The LBCO-50SDC composite cathode exhibited the best electrochemical performance in the LBCO-xSDC cathodes. With x = 50 wt%, the ASR was 1.308 Ω cm² at 500 °C (0.267 Ω cm² at 600 °C and 0.052 Ω cm² at 700 °C). The maximum of exchange current density i₀ was 0.5630 A cm⁻² at 700 °C. The improved electrochemical properties of LBCO-50SDC were ascribed to the porous structures of the cathode and more cathode/electrolyte/gas triple phase boundary (TPB) areas.     

Electrochemical characterization of Co-doped Sr0.8Ce0.2MnO3−δ cathodes on Sm0.2Ce0.8O1.9-electrolyte for intermediate-temperature solid oxide fuel cells

Electrochemical properties of Co-doped Sr_0.8Ce_0.2MnO_3−δ cathode were investigated at the cathode/Sm_0.2Ce_0.8O_1.9 electrolyte interface. The electrochemical impedance spectroscopy was measured under applied cathodic voltages (E = −0.4 to 0 V). At E = 0 V, the area-specific resistance decreased from 2.20 Ω cm² to 0.19 Ω cm² at 700 °C with Co doping. Under the cathodic polarization, the rate determining step of oxygen reduction process was different for both cathodes: the charge transfer for Sr_0.8Ce_0.2MnO_3−δ and the diffusion process for Sr_0.8Ce_0.2Mn_0.8Co_0.2O_3−δ. Besides, the overpotential also decreased from 124 mV to 19 mV at the current density of 0.1 A cm⁻² at 800 °C with Co doping. The improved electrochemical properties of Co-doped Sr_0.8Ce_0.2MnO_3−δ can be ascribed to the formation of more oxygen vacancies and more active sites for oxygen reduction reaction.     

Further performance improvement of Ba0.5Sr0.5Co0.8Fe0.2O3−δ perovskite membranes for air separation

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) is a promising mixed conducting ceramic membrane material for air separation. In this work, BSCF powder was synthesized by a modified Pechini sol–gel technique at relatively lower temperature. The O₂ permeation through a series of BSCF membranes has been tested at different temperatures and various O₂ partial pressure gradients. Theoretical investigation indicated that bulk diffusion and the O₂ exchange reactions on membrane surfaces jointly controlled the O₂ permeation through BSCF membranes with thickness of between 1.1 and 0.75 mm. To further improve the O₂ fluxes, effective efforts are made on membrane thickness reduction and surface modification by spraying porous BSCF layers on both surfaces. When the membrane thickness was reduced from 0.75 to 0.40 mm, the O₂ fluxes were increased by 20–60% depending on the operating conditions. The surface modification further improved the O₂ flux by another 20–40%. The high O₂ fluxes achieved in this work are quite encouraging with a maximum value reaching 6.0 mL min⁻¹ cm⁻² at 900 °C.     

Effect of Bi2O3 on the electrochemical performance of LaBaCo2O5+δ cathode for intermediate-temperature solid oxide fuel cells

The LaBaCo₂O_5+δ−x wt.% Bi₂O₃ (LBCO-xBi₂O₃, x=10, 20, 30, and 40) were prepared as composite cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs) via the conventional mechanical mixing method. The effect of Bi₂O₃ on polarization resistance, overpotential, and long-term stability of the LBCO cathode was investigated. An effective sintering aid for LBCO cathode, Bi₂O₃ not only lowers its sintering temperature by ~200 °C, but also improves the electrochemical performance within the intermediate temperature range of 600–800 °C. Electrochemical impedance spectroscopy measurements showed that the addition of 20 wt% Bi₂O₃ to LBCO exhibited the lowest area-specific resistance of 0.020 Ω cm² at 800 °C in air, which was about a seventh of that of the LBCO cathode at the same condition. At a current density of 0.2 A cm⁻², the cathodic overpotential of LBCO-20Bi₂O₃ was about 12.6 mV at 700 °C, while the corresponding value for LBCO was 51.0 mV. Compared to B₂O₃–Bi₂O₃–PbO frit, the addition of Bi₂O₃ significantly improved the long-term stability of cathode. Therefore, LBCO-20Bi₂O₃ can be a promising cathode for IT-SOFCs.     

Pr0.6Sr0.4CoO3−δ electrocatalyst for solid oxide fuel cell cathode introduced via infiltration

Effects of infiltrated Pr_0.6Sr_0.4CoO_3−δ (PSCo) electrocatalyst on SOFC cathode performance have been studied. Nano-sized particulate catalysts, deposited on surfaces of a composite cathode of Sm₂O₃ doped CeO₂ (SDC) and La_1−xSr_xCo_1−yFe_yO_3−δ (LSCF), are assumed to effectively widen active sites, or triple phase boundaries, for the oxygen reduction reaction. Area specific resistance of commercially available cells has been decreased by 36–40% with the addition of 23 wt% PSCo electrocatalyst on cathode. Analysis of the impedance spectra demonstrates that PSCo electrocatalyst plays a significant role in dissociation of oxygen molecules and adsorption of oxygen atoms into the cathode. A total of 200 h operation of the cells demonstrated that catalytic activity of PSCo has not been significantly degraded. Simultaneous operations of multiple cells using a parallel-cell testing system have made it possible to compare the performance of several cells with high reliability.     

Electrochemical performance of silver-modified Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathodes prepared via electroless deposition

Silver-modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathodes for intermediate-temperature solid-oxide fuel cells (IT-SOFCs) were prepared by an electroless deposition process using N₂H₄ as the reducing agent at room temperature. This fabrication technique together with tailored electrode porosity, modified the BSCF electrodes with silver content that varied from 0.3 to 30 wt.% without damaging the electrode microstructure. Both the Ag loading and firing temperatures were found to have a significant impact on the electrode performance, which could facilitate or block the electrochemical processes of the BSCF-based cathodes, processes that include charge-transfer, oxygen adsorption and oxygen electrochemical reduction. At an optimal Ag loading of 3.0 wt.% and firing temperature of 850 °C, an area specific resistance of only 0.042 Ω cm² at 600 °C was achieved for a modified BSCF cathode.