Stability and performance of infiltrated La0.8Sr0.2CoxFe1−xO3 electrodes with and without Sm0.2Ce0.8O1.9 interlayers

The chemical stability of composite electrodes produced by the infiltration of La_0.8Sr_0.2Co_xFe_1−xO₃ (LSCF) into a porous yttria-stabilized zirconia (YSZ) scaffold were investigated as a function of the Co:Fe ratio in the LSCF and the LSCF calcination temperature. XRD and impedance spectroscopy results indicate that for an LSCF calcination temperature of 1123 K, reactions between the LSCF and YSZ do not occur to a significant extent. Reactions producing La₂Zr₂O₇ and SrZrO₃ at the interface were observed, however, for a calcination temperature of 1373 K and x values greater than 0.2. In addition to determining the conditions for which reactions between LSCF and YSZ occur, the effectiveness of infiltrated SDC interlayers in preventing reactions at the LSCF-YSZ interface and their influence on the overall performance of LSCF/YSZ composite electrodes was studied.     

Solid oxide fuel cell cathodes prepared by infiltration of LaNi0.6Fe0.4O3 and La0.91Sr0.09Ni0.6Fe0.4O3 in porous yttria-stabilized zirconia

SOFC composite electrodes of yttria-stabilized zirconia (YSZ) and either LaNi_0.6Fe_0.4O₃ (LNF) or La_0.91Sr_0.09Ni_0.6Fe_0.4O₃ (LSNF) were prepared by infiltration to a loading of 40 wt% of the perovskite into porous YSZ using aqueous solutions of the nitrate salts. XRD measurements indicated that the perovskite structures were formed following calcination at 850 °C, at which temperature the LNF and LSNF form small particles that coat the YSZ pores. Heating to 1100 °C causes the particles to form a dense film over the YSZ but caused no solid-state reaction. Calcination of an LNF-YSZ composite to 1200 °C led to an expansion of the LNF lattice, suggesting introduction of Zr(IV) into the perovskite; further heating to 1300 °C caused the formation of La₂Zr₂O₇. For 850 °C calcination, the electrode performance of both LNF-YSZ and LSNF-YSZ composites was similar to that reported for composites of YSZ and La_0.8Sr_0.2FeO₃ (LSF), with a current-independent impedance of approximately 0.1 Ω cm² at 700 °C in air. For 1100 °C calcination, both LNF-YSZ and LSNF-YSZ composites exhibited impedances that decreased strongly under both anodic and cathodic polarization. The implications of these results for preparing electrodes based on LNF and LSNF are discussed.     

An experimental investigation into micro-fabricated solid oxide fuel cells with ultra-thin La0.6Sr0.4Co0.8Fe0.2O3 cathodes and yttria-doped zirconia electrolyte films

Thin-film solid oxide fuel cells (SOFCs) were fabricated with both Pt and mixed conducting oxide cathodes using sputtering, lithography, and etching. Each device consists of a 75–150 nm thick yttria-stabilized zirconia (YSZ) electrolyte, a 40–80 nm porous Pt anode, and a cathode of either 15–150 nm dense La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) or 130 nm porous Pt. Maximum powers produced by the cells are found to increase with temperature with activation energies of 0.94–1.09 eV. At 500 °C, power densities of 90 and 60 mW cm⁻² are observed with Pt and LSCF cathodes, respectively, although in some conditions LSCF outperforms Pt. Several device types were fabricated to systematically investigate electrical properties of components of these fuel cells. Micro-fabricated YSZ structures contacted on opposite edges by Pt electrodes were used to study temperature-dependent in-plane conductivity of YSZ as a function of lateral size and top and bottom interfaces. Si/Si₃N₄/Pt and Si/Si₃N₄/Au capacitor structures are fabricated and found to explain certain features observed in impedance spectra of in-plane and fuel cell devices containing silicon nitride layers. The results are of relevance to micro-scale energy conversion devices for portable applications.     

Preparation and characterization of graded cathode La0.6Sr0.4Co0.2Fe0.8O3−δ

Perovskite oxide La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF6428), a wonderful electronic–ionic conductor could be used as cathode of solid oxide fuel cell (SOFC). Graded cathode with coarse layer and fine layer, could improve the diffusion rate and electrochemical reaction activity of oxidant. The fabrication and properties of graded LSCF6428 cathode were discussed in this paper. First, pure perovskite LSCF6428 powders were prepared by citrate–EDTA method (CEM), citrate method (CM) and solid phase synthesis (SPS). The powders with higher specific surface area and smaller grain size are easier to be sintered and densified. Single LSCF6428 cathode with thickness of 30 μm was prepared by SPS powders, the porosity of cathode was high about 30% and pore size was about 5 μm. Graded LSCF6428 cathode including 30 μm outer layer and 10 μm inner layer was prepared by SPS and CM powders, respectively. Clear double-layer cathode was observed by SEM, which combined tightly and transited gradually. Porosity of outer layer is high about 30% and pore size is about 1–5 μm; inner layer is finer and pore size is about 0.2–1 μm. Based on the above research, 300 μm yttria stabilized zirconia (YSZ) electrolyte supported cell with single LSCF6428 cathode and double-layer LSCF6428 cathode were prepared, and the properties of two type cells were tested in H₂. Power density of graded cell is 197 mW cm⁻² at 950 °C, and improved about 46% comparing that of single layer LSCF6428 cell (135 mW cm⁻²).     

Silver infiltrated La0.6Sr0.4Co0.2Fe0.8O3 cathodes for intermediate temperature solid oxide fuel cells

Porous La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) electrodes on anode support cells were infiltrated with AgNO₃ solutions in citric acid and ethylene glycol. Two types of solid oxide fuel cells with the LSCF–Ag cathode, Ni–YSZ/YSZ/LSCF–Ag and Ni–Ce_0.9Gd_0.1O_1.95(GDC)/GDC/LSCF–Ag, were examined in a temperature range 530–730 °C under air oxidant and moist hydrogen fuel. The infiltration of about 18 wt.% Ag fine particles into LSCF resulted in the enhancement of the power density of about 50%. The maximum power density of Ni–YSZ/YSZ/LSCF was enhanced from 0.16 W cm⁻² to 0.25 W cm⁻² at 630 °C by infiltration of AgNO₃. No significant degradation of out-put power was observed for 150 h at 0.7 V and 700 °C. The Ni–GDC/GDC/LSCF–Ag cell showed the maximum power density of 0.415 W cm⁻² at 530 °C.     

Preparation of La0.6Sr0.4Co0.2Fe0.8O3 nanoceramic cathode powders for solid oxide fuel cell (SOFC) application

The La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) nanoceramic powders were prepared by sol–gel process using nitrate based chemicals for SOFC applications since these powders are considered as more promising cathode materials for SOFC. The citric acid was used as a chelating agent and ethylene glycol as a dispersant. The powders were calcined at 650 °C/6 h, 900 °C/3 h and 1150 °C/2 h in air using Thermolyne 47900 furnace. These powders were characterized by employing SEM/EDS, XRD, porosimetry and TGA/DTA techniques.     

Structural and electrical properties of selected La1−xSrxCo0.2Fe0.8O3 and La0.6Sr0.4Co0.2Fe0.6Ni0.2O3 perovskite type oxides

In this paper, the structural and transport properties of selected La_1−xSr_xCo_0.2Fe_0.8O₃ (LSCF) perovskites and La_0.6Sr_0.4Co_0.2Fe_0.6Ni_0.2O₃ (LSCFN64262) perovskite are presented. Crystal structure of the samples was characterized by means of X-ray studies with Rietveld method analysis. DC electrical conductivity and thermoelectric power were measured at a wide temperature range (80–1200 K) in air. For La_0.2Sr_0.8Co_0.2Fe_0.8O₃ (LSCF2828) and La_0.4Sr_0.6Co_0.2Fe_0.8O₃ (LSCF4628) perovskites a maximum observed on electrical conductivity dependence on temperature exists at about 750 K. It can be associated with an appearance of oxygen vacancies and implies a mixed ionic-electronic transport. A growing amount of oxygen vacancies at higher temperatures causes a decrease in the electrical conductivity due to a recombination mechanism associated with lowering of the average valence of 3d metals. A similar characteristic was found for LSCFN64262 perovskite, which also exhibits a relatively high electrical conductivity.     

Long term behaviors of La0.8Sr0.2MnO3 and La0.6Sr0.4Co0.2Fe0.8O3 as cathodes for solid oxide fuel cells

This work studies the electrochemical performance and stability of La_0.8Sr_0.2MnO₃ (LSM) and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathodes in a AISI441 interconnect/cathode/YSZ electrolyte half-cell configuration at 800 °C for 500 h. Ohmic resistance and polarization resistance of the cathodes are analyzed by deconvoluting the electrochemical impedance spectroscopy (EIS) results. The LSM cathode has much higher resistance than the LSCF electrode even though the respective cathode resistance either decreases or stays stable over the long term thermal treatment. During the 500 h thermal treatment, dramatic elemental distribution changes influence the electrochemical behaviors of the cathodes. Chromium diffusion from the interconnect into the LSM electrode at triple phase boundaries (TPBs) leads to segregation of Sr away from La and Mn. For the LSCF cathode, Sr and Co segregation is dominant. The fundamental processes at the TPBs are proposed. Overall, LSCF is a much preferred cathode material because of its much smaller resistance for the 500 h thermal treatment time.     

Electrochemical characteristics of an La0.6Sr0.4Co0.2Fe0.8O3–La0.8Sr0.2MnO3 multi-layer composite cathode for intermediate-temperature solid oxide fuel cells

An La_0.6Sr_0.4Co_0.2Fe_0.8O₃–La_0.8Sr_0.2MnO₃ (LSCF–LSM) multi-layer composite cathode for solid oxide fuel cells (SOFCs) was prepared on an yttria-stabilized zirconia (YSZ) electrolyte by the screen-printing technique. Its cathodic polarization curves and electrochemical impedance spectra were measured and the results were compared with those for a conventional LSM/LSM–YSZ cathode. While the LSCF–LSM multi-layer composite cathode exhibited a cathodic overpotential lower than 0.13 V at 750 °C at a current density of 0.4 A cm⁻², the overpotential for the conventional LSM–YSZ cathode was about 0.2 V. The electrochemical impedance spectra revealed a better electrochemical performance of the LSCF–LSM multi-layer composite cathode than that of the conventional LSM/LSM–YSZ cathode; e.g., the polarization resistance value of the multi-layer composite cathode was 0.25 Ω cm² at 800 °C, nearly 40% lower than that of LSM/LSM–YSZ at the same temperature. In addition, an encouraging output power from an YSZ-supported cell using an LSCF–LSM multi-layer composite cathode was obtained.     

Low-temperature electrochemical characterization of dense ultra-thin lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.8Fe0.2O3) cathodes synthesized by RF-sputtering on nanoporous alumina-supported Y-doped zirconia membranes

Dense ultra-thin nanocrystalline La_0.6Sr_0.4Co_0.8Fe_0.2O₃ (LSCF) films with thickness of ∼50 nm, have been sputtered on nanoporous anodic alumina-supported nanocrystalline thin film yttria-stabilized zirconia and patterned by photolithography into microelectrodes. This approach enables low-temperature (425-550 °C) electrochemical properties of dense ultra-thin nanocrystalline LSCF to be characterized. The results reveal that the electrochemical resistance of nanocrystalline ultra-thin LSCF is dominated by the oxygen surface exchange reaction at the electrode surface with an activation energy of 1.1 eV. Area-specific resistance of LSCF was obtained and the results are of potential relevance to utilizing nanostructured oxide cathodes for micro-SOFCs operated at low temperatures.     

Nanostructured La0.6Sr0.4Co0.8Fe0.2O3/Y0.08Zr0.92O1.96/La0.6Sr0.4Co0.8Fe0.2O3 (LSCF/YSZ/LSCF) symmetric thin film solid oxide fuel cells

Functional all-oxide thin film micro-solid oxide fuel cells (μSOFCs) that are free of platinum (Pt) are discussed in this report. The μSOFCs, with widths of 160 μm, consist of thin film La_0.6Sr_0.4Co_0.8Fe_0.2O₃ (LSCF) as both the anode and cathode and Y_0.08Zr_0.92O_1.96 (YSZ) as the electrolyte. Open circuit voltage and peak power density at 545 °C are 0.18 V and 210 μW cm⁻², respectively. The LSCF anodes show good lattice and microstructure stability and do not form reaction products with YSZ. The all-oxide μSOFCs endure long-term stability testing at 500 °C for over 100 h, as manifested by stable membrane morphology and crack-free microstructure.     

Electrode performance of a new La0.6Sr0.4Co0.2Fe0.8O3 coated cathode for molten carbonate fuel cells

To improve the cathode performance in molten carbonate fuel cells (MCFCs), Lanthanum Strontium Cobalt Ferrite (La_0.6Sr_0.4Co_0.2Fe_0.8O₃, LSCF) of perovskite structure was coated on a porous Ni plate by a vacuum suction method. The electrochemical performance of modified cathode was examined and compared with that of uncoated conventional cathode via single cell operation and electrochemical impedance analysis (EIS). The cell voltage of the single cell using the LSCF coated cathode, measured at 650 °C with current density of 150 mA/cm² is 0.837 V and it is higher than that of the cell with uncoated conventional cathode, 0.805 V. The higher performance and the lower charge transfer resistance were obtained at 600–700 °C after LSCF coating. The lower activation energy of oxygen reduction reaction was also obtained. The lower activation energy of oxygen reduction reaction after LSCF coating shows that LSCF on lithiated NiO cathode plays a role of catalyst on the oxygen reduction reaction in cathode.     

New insights in the polarization resistance of anode-supported solid oxide fuel cells with La0.6Sr0.4Co0.2Fe0.8O3 cathodes

In this study, the polarization resistance of anode-supported solid oxide fuel cells (SOFC) with La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathodes was investigated by I-V sweep and electrochemical impedance spectroscopy under a series of operating voltages and cathode environments (i.e. stagnant air, flowing air, and flowing oxygen) at temperatures from 550 °C to 750 °C. In flowing oxygen, the polarization resistance of the fuel cell decreased considerably with the applied current density. A linear relationship was observed between the ohmic-free over-potential and the logarithm of the current density of the fuel cell at all the measuring temperatures. In stagnant or flowing air, an arc related to the molecular oxygen diffusion through the majority species (molecular nitrogen) present in the pores of the cathode was identified at high temperatures and high current densities. The magnitude of this arc increased linearly with the applied current density due to the decreased oxygen partial pressure at the interface of the cathode and the electrolyte. It is found that the performance of the fuel cell in air is mainly determined by the oxygen diffusion process. Elimination of this process by flowing pure oxygen to the cathode improved the cell performance significantly. At 750 °C, for a fuel cell with a laser-deposited Sm_0.2Ce_0.8O_1.9 (SDC) interlayer, an extraordinarily high power density of 2.6 W cm⁻² at 0.7 V was achieved in flowing oxygen, as a result of reduced ohmic and polarization resistance of the fuel cell, which were 0.06 Ω cm² and 0.03 Ω cm², respectively. The results indicate that microstructural optimization of the LSCF cathode or adoption of a new cell design which can mitigate the oxygen diffusion limitation in the cathode might enhance cell performance significantly.     

Electrochemical study of La0.6Sr0.4Co0.8Fe0.2O3 during oxygen evolution reaction

In this paper, oxygen evolution reaction (OER) mechanism in La_0.6Sr_0.4Co_0.8Fe_0.2O₃ was investigated in KOH solution by electrochemical impedance spectroscopy (EIS) and voltammetric measurements. The Tafel slopes and reaction orders evaluated in this paper are consistent with the B. O’Grady’s Path for oxygen evolution on oxides. The activation energy for OER in La_0.6Sr_0.4Co_0.8Fe_0.2O₃ was 28.3 kJ mol⁻¹. The obtained apparent porosity of La_0.6Sr_0.4Co_0.8Fe_0.2O₃ electrode is 48% and the roughness factor is around 1.6 × 10⁴. The polarization resistance of La_0.6Sr_0.4Co_0.8Fe_0.2O₃ is much low compared with other similar oxides. This can be due the high roughness and high porosity in addition to the low active energy for the process.     

Characterization of Cu,Ag and Pt added La0.6Sr0.4Co0.2Fe0.8O3−δ and gadolinia-doped ceria as solid oxide fuel cell electrodes by temperature-programmed techniques

Cu, Ag and Pt added La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and gadolinia-doped ceria (GDC) were analyzed by the temperature-programmed techniques for their characteristics as either the cathode or the anode of the solid oxide fuel cells (SOFCs). Temperature-programmed oxidation using CO₂ was used to characterize the cathode materials while temperature-programmed reduction (TPR) using H₂ and TPR using CO were used to characterize the anode materials. These techniques can offer an easy screening of the materials as the SOFC electrodes. The effects of adding Cu, Ag and Pt to LSCF for the cathodic reduction activity and the anodic oxidation activity are different—Cu > Ag > Pt for reduction and Pt > Cu > Ag for oxidation. The CO oxidation activities are higher than the H₂ oxidation activities. Adding GDC to LSCF can increase both reduction and oxidation activities. The LSCF–GDC composite has a maximum activity for either reduction or oxidation when LSCF/GDC is 2 in weight.     

Interlayer-free nanostructured La0.58Sr0.4Co0.2 Fe0.8O3−δ cathode on scandium stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells

LSCF powders with a specific surface area of 25.2 m² g⁻¹ and an average particle size of 89 nm are synthesized by the polymerizable complex method. The use of nanocrystalline LSCF powders allows the fabrication of an interlayer-free nanoporous cathode on top of an ScSZ electrolyte at a low temperature at which non-electrocatalytic secondary phases cannot form. The electrochemical performance of the interlayer-free cathode depends largely on the sintering temperature. A cathode sintered at below 750 °C lacks sufficient mechanical adhesion to the electrolyte, while the electrode surfaces are locally densified when sintered at above 800 °C. Impedance spectroscopy combined with microstructural evidence reveals that the optimum sintering temperature for LSCF is 750 °C. This avoids excess densification and grain growth, and results in the lowest polarization resistance (0.048 Ω cm² at 750 °C).     

GdBaCo2O5+x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo₂O_5+x (GBCO) was evaluated as a cathode for intermediate-temperature solid oxide fuel cells. A porous layer of GBCO was deposited on an anode-supported fuel cell consisting of a 15 μm thick electrolyte of yttria-stabilized zirconia (YSZ) prepared by dense screen-printing and a Ni–YSZ cermet as an anode (Ni–YSZ/YSZ/GBCO). Values of power density of 150 mW cm⁻² at 700 °C and ca. 250 mW cm⁻² at 800 °C are reported for this standard configuration using 5% of H₂ in nitrogen as fuel. An intermediate porous layer of YSZ was introduced between the electrolyte and the cathode improving the performance of the cell. Values for power density of 300 mW cm⁻² at 700 °C and ca. 500 mW cm⁻² at 800 °C in this configuration were achieved.     

Fabrication and characterization of a co-fired La0.6Sr0.4Co0.2Fe0.8O3−δ cathode-supported Ce0.9Gd0.1O1.95 thin-film for IT-SOFCs

A dense membrane of Ce_0.9Gd_0.1O_1.95 on a porous cathode based on a mixed conducting La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ was fabricated via a slurry coating/co-firing process. With the purpose of matching of shrinkage between the support cathode and the supported membrane, nano-Ce_0.9Gd_0.1O_1.95 powder with specific surface area of 30 m² g⁻¹ was synthesized by a newly devised coprecipitation to make the low-temperature sinterable electrolyte, whereas 39 m² g⁻¹ nano-Ce_0.9Gd_0.1O_1.95 prepared from citrate method was added to the cathode to favor the shrinkage for the La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ. Bi-layers consisting of <20 μm dense ceria film on 2 mm thick porous cathode were successfully fabricated at 1200 °C. This was followed by co-firing with NiO–Ce_0.9Gd_0.1O_1.95 at 1100 °C to form a thin, porous, and well-adherent anode. The laboratory-sized cathode-supported cell was shown to operate below 600 °C, and the maximum power density obtained was 35 mW cm⁻² at 550 °C, 60 mW cm⁻² at 600 °C.     

Reactivity between La(Sr)FeO3 cathode,doped CeO2 interlayer and yttria-stabilized zirconia electrolyte for solid oxide fuel cell applications

Detailed X-ray diffraction (XRD) analysis of two different Sr-doped LaFeO₃ cathodes, YSZ electrolyte and two Sm/Gd-doped CeO₂ interlayer and their mixtures were used to evaluate the formation of undesired secondary reaction compounds. The analysis of room temperature X-ray diffraction data of the mixtures indicates the crystallization of strontium and/or lanthanum zirconates between the cathode and the electrolyte materials and no detected reaction between the cathode and the interlayer materials.     

Performance degradation of impregnated La0.6Sr0.4Co0.2Fe0.8O3+Y2O3 stabilized ZrO2 composite cathodes of intermediate temperature solid oxide fuel cells

Composite cathodes of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and Y₂O₃ stabilized ZrO₂ (YSZ) are fabricated by impregnating the porous YSZ scaffold pre-formed on YSZ electrolyte substrate with a solution containing La, Sr, Co and Fe in desired composition. The performance stability of the cathodes is evaluated in air at 750 °C for up to 120 h by electrochemical impedance spectroscopy under the condition of open circuit. An insignificant small amount of resistive phase SrZrO₃ is formed at 800 °C during cathode preparation; however, its volume is not further increased at 750 °C for 120 h, as indicated by the XRD results. The cathode polarization resistance (R_p) increases from 0.17 to 0.30 Ωcm² after the 120 h test mainly due to the increase of the low frequency polarization resistance (R_p2), which characterizes the low frequency processes in the reaction of oxygen reduction. The morphology change of the well connected LSCF particles to dispersive and flattened configuration accounts for the increase of the R_p2 and in turn the degradation of cathode performance.