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.     

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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

Preparation of dense and uniform La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) films for fundamental studies of SOFC cathodes

Thin films of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) were deposited on (1 0 0) silicon and on GDC electrolyte substrates by rf-magnetron sputtering using a single-phase oxide target of LSCF. The conditions for sputtering were systematically studied to get dense and uniform films, including substrate temperature (23–600 °C) background pressure (1.2 × 10⁻² to 3.0 × 10⁻² mbar), power, and deposition time. Results indicate that to produce a dense, uniform, and crack-free LSCF film, the best substrate temperature is 23 °C and the argon pressure is 2.5 × 10⁻² mbar. Further, the electrochemical properties of a dense LSCF film were also determined in a cell consisting of a dense LSCF film (as working electrode), a GDC electrolyte membrane, and a porous LSCF counter electrode. Successful fabrication of high quality (dense and uniform) LSCF films with control of thickness, morphology, and crystallinity is vital to fundamental studies of cathode materials for solid oxide fuel cells.     

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).     

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.     

LaNi0.6Fe0.4O3-Ce0.8Sm0.2O1.9-Ag composite cathode for intermediate temperature solid oxide fuel cells

LaNi_0.6Fe_0.4O₃ (LNF), LNF-Sm_0.2Ce_0.8O_1.9 (SDC), and LNF-SDC-Ag cathodes on SDC electrolytes were investigated at intermediate temperatures using AC impedance spectroscopy. Results show that adding 50 wt.% SDC into LNF yields a significant low area specific resistance (ASR) which was found to be 0.92 Ω cm² at 700 °C. Infiltrating 0.3 mg/cm² Ag into LNF-50 wt.% SDC can improve the electronic conductivity and oxygen exchange reaction activity, and thereby remarkably decrease the ASRs. The ASR value of the LNF-SDC-Ag cathode is as low as 0.18 Ω cm² at 700 °C, and 0.46 Ω cm² at 650 °C. The long-term test shows that the LNF-SDC-Ag cathode may be a promising candidate for solid oxide fuel cells operating at temperatures lower than 650 °C.     

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).     

Performance and structural stability of Gd0.2Ce0.8O1.9 infiltrated La0.8Sr0.2MnO3 nano-structured oxygen electrodes of solid oxide electrolysis cells

Effect of Gd_0.2Ce_0.8O_1.9 (GDC) infiltration on the performance and stability of La_0.8Sr_0.2MnO₃ (LSM) oxygen electrodes on Y₂O₃-stabilized ZrO₂ (YSZ) electrolyte has been studied in detail under solid oxide electrolysis cell (SOEC) operating conditions at 800 °C. The incorporation of GDC nanoparticles significantly enhances the electrocatalytic activity for oxygen oxidation reaction on LSM electrodes. Electrode polarization resistance of pristine LSM electrode is 8.2 Ω cm² at 800 °C and decreases to 0.39 and 0.09 Ω cm² after the infiltration of 0.5 and 1.5 mg cm⁻² GDC, respectively. The stability of LSM oxygen electrodes under the SOEC operating conditions is also significantly increased by the GDC infiltration. A 2.0 mg cm⁻² GDC infiltrated LSM electrode shows an excellent stability under the anodic current passage at 500 mA cm⁻² and 800 °C for 100 h. The infiltrated GDC nanoparticles effectively shift the reaction sites from the LSM electrode/YSZ electrolyte interface to the LSM grains/GDC nanoparticle interface in the bulk of the electrode, effectively mitigating the delamination at the LSM/YSZ interface. The results demonstrate that the GDC infiltration is an effective approach to enhance the structural integrity and thus to achieve the high activity and excellent stability of LSM-based oxygen electrode under the SOEC operating conditions.     

Optimization of La0.6Ca0.4Fe0.8Ni0.2O3-Ce0.8Sm0.2O2 composite cathodes for intermediate-temperature solid oxide fuel cells

Sample of nominal composition La_0.6Ca_0.4Fe_0.8Ni_0.2O₃ (LCFN) was prepared by liquid mix method. The structure of the polycrystalline powder was analyzed with X-ray powder diffraction data. This compound shows orthorhombic perovskite structure with a space group Pnma. In order to improve the electrochemical performance, Sm-doped ceria (SDC) powder was added to prepare the LCFN-SDC composite cathodes. Electrochemical characteristics of the composites have been investigated for possible application as cathode material for an intermediate-temperature-operating solid oxide fuel cell (IT-SOFC). The polarization resistance was studied using Sm-doped ceria (SDC). Electrochemical impedance spectroscopy measurements of LCFN-SDC/SDC/LCFN-SDC test cell were carried out. These electrochemical experiments were performed at equilibrium from 850 °C to room temperature, under both zero dc current intensity and air. The best value of area-specific resistance (ASR) was for LCFN cathode doped with 10% of SDC (LCFN-SDC9010), 0.13 Ω cm² at 850 °C. The dc four-probe measurement exhibits a total electrical conductivity over 100 S cm⁻¹ at T ≥ 600 °C for LCFN-SDC9010 composite cathode.     

Preparation and characterization of La0.8Sr0.2Ga0.83Mg0.17O3 electrolyte by polyol method for solid oxide fuel cells

Perovskite type La_0.8Sr_0.2Ga_0.83Mg_0.17O₃ powders were prepared via simple polyol method for the first time in literature. Obtained material was characterized by using XRD, SEM, Impedance Spectroscopy and density measurements. Pure LSGM powders were achieved after heat treatment at 1100 °C for 12 h. 87% relative density was obtained after pressing of these powders under 1 MPa and sintering at 1250 °C for 12 h. Average particle size was calculated as 1.77–2.4 μm from SEM micrographs. Overall conductivity of the LSGM pellet was found to be 0.0014 S/cm at 850 °C with impedance analysis and showed that the preparation process needs improvements.     

Microstructural improvements of the gradient composite material Pr0.6Sr0.4Fe0.8Co0.2O3/Ce0.8Sm0.2O1.9 by employing vertically aligned carbon nanotubes

Ionic–electronic conductor composite materials are widely employed to improve the ionic conductivity of solid oxide fuel cell electrodes, making them more compatible with the solid oxide fuel cell materials. The addition of an ionic conductor implies, however, a decrease of the cathode active area for the oxygen reduction reactions. In addition, the ionic conductor particles are usually dispersed and isolated, limiting the enhanced properties to certain areas. In order to avoid this drawback and improve the ionic conductivity of the electrode, vertically aligned carbon nanotubes (VACNTs) have been synthesized and employed as template for the deposition of Sm_0.2Ce_0.8O₂ (SDC) particles. Subsequently, these SDC-VACNTs have been coated with a Pr_0.6Sr_0.4Fe_0.8Co_0.2O₃ (PSFC) perovskite phase, leading to the formation of the desired composite material. Attending to the obtained results, it is proved that the new composite material exhibits a significantly better electrochemical performance, due to a better distribution of the ionic conductor and the burning of the carbon nanotubes (CNTs), which result in the improvement of the microstructure.     

Characteristics of La0.8Sr0.2Ga0.8Mg0.2O3-δ-supported micro-tubular solid oxide fuel cells with LaCo0.4Ni0.6-xCuxO3-δ cathodes

In this study, micro-tubular solid oxide fuel cells (T-SOFCs) with extruded La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) electrolyte as the mechanical support and LaCo_0.4Ni_0.6O_3-δ (LCNO) or LaCo_0.4Ni_0.4Cu_0.2O_3-δ (LCNCO) as cathodes were prepared and characterized. Partial substitution of Cu for the Ni-ion positions in the LCNO lattices was found to significantly enhance the densification and accelerate the grain growth. The porosity-corrected electrical conductivity was significantly increased from 1275 S/cm for LCNO ceramic to 1537 S/cm for LCNCO ceramic, because the acceptor doping was compensated by the formation of hole carriers that produced additional polarons and significantly augmented the electrical conductivity. SOFCs with three configurations were built in this study, including Cell A that had a lanthanum-doped ceria (LDC) buffer layer inserted between the LSGM electrolyte and the LCNCO cathode, Cell B that used an LCNO-LSGM composite cathode, and Cell C that featured an LCNCO-LSGM composite cathode. Among the three cells, Cell C with 263 μm of LSGM electrolyte possessed the lowest Ohmic resistance of 0.89 Ω cm², a polarization resistance of 0.69 Ω cm², and the highest maximum power density of 178 mW cm^?2 at 750 °C.     

Low temperature deposited (Ce,Gd)O2−x interlayer for La0.6Sr0.4Co0.2Fe0.8O3 cathode based solid oxide fuel cell

Application of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ perovskites cathode in solid oxide fuel cell (SOFC) can benefit from its high electrocatalytic activity at 600-800 °C. However, due to the chemical and mechanical incompatibility between the LSCF cathode and state-of-the-art yttria stabilized zirconia (YSZ) electrolyte, a ceria-based oxide barrier interlayer is usually introduced. In this work, gadolinia doped ceria (GDC) interlayers are prepared by screen printing (SP), electron beam evaporation (EB) and ion assisted deposition (IAD) methods. The microstructures of the GDC interlayers show great dependence on the deposition methods. The 1250 °C-sintered SP interlayer exhibits a porous microstructure. The EB method generates a thin and compact interlayer at a low substrate temperature of 250 °C. With the help of additional energetic argon and oxygen ions bombardment on the deposited species, the IAD method yields the densest GDC interlayer at the same substrate temperature, which leads to the best electrochemical performance of LSFC-based SOFC.