Electrochemical characterization of La0.6Ca0.4Fe0.8Ni0.2O3 cathode on Ce0.8Gd0.2O1.9 electrolyte for IT-SOFC

For Solid Oxide Fuel Cells (SOFCs) to become an economically attractive energy conversion technology, suitable materials and structures which enable operation at lower temperatures, while retaining high cell performance, must be developed. Recently, the perovskite-type La_0.6Ca_0.4Fe_0.8Ni_0.2O₃ oxide has shown potential as an intermediate temperature SOFC cathode. An equivalent circuit describing the cathode polarization resistances was constructed from analyzing impedance spectra recorded at different temperatures in oxygen. A competitive electrode polarization resistance is reported for this oxygen electrode using a Ce_0.8Gd_0.2O_1.9 electrolyte, determined by impedance spectroscopy studies of symmetrical cells sintered at 800 °C and 1000 °C. Scanning electron microscopy (SEM) studies of the symmetrical cells revealed the absence of any reaction layer between cathode and electrolyte, and a porous electrode microstructure even when sintered at a temperature of only 800 °C. The performance of this cathode shows favorable oxygen reduction reaction (ORR) properties potentially making it an excellent choice for IT-SOFC application.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Structural and electrical properties of thin films of Pr0.8Sr0.2Fe0.8Ni0.2O3−δ

Pr_0.8Sr_0.2Fe_0.8Ni_0.2O_3−δ (PN22) films have been deposited at different temperatures on yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analysed by X-ray diffraction (XRD) and atomic force microscopy (AFM). A marked influence in the structural properties of the substrate temperature has been found but not of the composition. Samples deposited at temperatures below 700 °C are amorphous, with granular aspect, and with decreasing roughness with the temperature. Meanwhile, the films at 700 °C are polycrystalline and exhibit a needle-shaped surface, with the highest roughness observed. Additionally, the conducting behaviour of the films has been studied by electrochemical impedance spectroscopy (EIS) and their cathodic area specific resistance (ASR) was determined. The main part of the impedance of the testing cells is due to the electrode. The ASR values of the films of PN22 are lower than those of Pr_0.9Sr_0.1Fe_0.8Ni_0.2O_3−δ (PN12), being the lowest 0.5 Ω cm² at 850 °C for the sample PN22 deposited at room temperature.     

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.     

Sm0.5Sr0.5CoO3 + Sm0.2Ce0.8O1.9 composite cathode for cermet supported thin Sm0.2Ce0.8O1.9 electrolyte SOFC operating below 600 °C

The cathode is a key component in low temperature solid oxide fuel cells. In this study, composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSC) + 25 wt.% Sm_0.2Ce_0.8O_1.9 (SDC), was applied on the cermet supported thin SDC electrolyte cell which was fabricated by tape casting, screen-printing, and co-firing. Single cells with the composite cathodes sintered at different temperatures were tested from 400 to 650 °C. The best cell performance, 0.75 W cm⁻² peak power operating at 600 °C, was obtained from the 1050 °C sintered cathode. The measured thin SDC electrolyte resistance R_s was 0.128 Ω cm² and total electrode polarization R_p(a + c) was only 0.102 Ω cm² at 600 °C.     

Formation of Ce0.8Sm0.2O1.9 nanoparticles by urea-based low-temperature hydrothermal process

The synthesis and formation mechanism of the nano-sized Ce_0.8Sm_0.2O_1.9 particles prepared by a urea-based low-temperature hydrothermal process was investigated in this study. From ex situ X-ray diffraction and induced coupled plasma-atomic emission spectroscopy investigations, it was found that large quantities of cerium hydroxide co-precipitated with some samarium hydroxide at the initial stage of the hydrothermal process. The remaining Sm³⁺ ions in the solutions were further hydrolyzed and deposited on the surface of the cerium hydroxide-rich precipitates to form a core–shell-like structure. During the hydrothermal process, the core–shell-like structure transformed to a single cubic fluorite phase which is due to the incorporation of the deposited samarium hydroxide into the cerium oxide-rich core. Further, the average grain size of the synthesized nanocrystalline Ce_0.8Sm_0.2O_1.9 was reduced with increasing the urea concentration in the solution. The density of the disk prepared with the synthesized Ce_0.8Sm_0.2O_1.9 powders was found to increase with a decrease in the grain size of Ce_0.8Sm_0.2O_1.9. The existence of SO₄²⁻ anions in the SDC powders prepared at low-urea concentration may result in the SDC disks with low density due to their decomposition during sintering process.     

Electrochemical oxidation of catalytic grown carbon fiber in a direct carbon fuel cell using Ce0.8Sm0.2O1.9-carbonate electrolyte

The electrochemical oxidation of catalytic grown carbon fiber has been examined in a direct carbon fuel cell (DCFC). The single cell contains a composite electrolyte layer made of a samarium doped ceria (SDC) and a eutectic carbonate phase. The cathode is a mixture of lithiated NiO and the composite electrolyte, while the anode is composed of NiO and SDC powder. Catalytic carbon fiber and the eutectic carbonate is premixed and used as the feed of the anode fuel. The effects of the cell pellet configuration, cathode gas composition and the operation temperature on the DCFC performance have been examined in this work. At 700 °C, the maximum power output achieves 112 mW cm⁻² with a current density of 249 mA cm⁻². The anode off-gas is analyzed with a gas chromatograph, and the Boudouard reaction is found suppressed by the electrical field in the fuel cell operation.     

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.     

On the LiNi0.2Mn0.2Co0.6O2 positive electrode material

Layered LiNi_0.2Mn_0.2Co_0.6O₂ phase, belonging to a solid solution between LiNi_1/2Mn_1/2O₂ and LiCoO₂ most commercialized cathodes, was prepared via the combustion method at 900 °C for a short time (1 h). Structural, electrochemical and magnetic properties of this material were investigated. Rietveld analysis of the XRD pattern shows this compound as having the α-NaFeO₂ type structure (S.G. R-3m; a = 2.8399(2) ?; c = 14.165(1) ?) with almost none of the well-known Li/Ni cation disorder. SQUID measurements clearly indicate that the studied compound consists of Ni²⁺, Co³⁺ and Mn⁴⁺ ions in the crystal structure. X-ray analysis of the chemically delithiated Li_xNi_0.2Mn_0.2Co_0.6O₂ phases reveals that the rhombohedral symmetry was maintained during Li-extraction, confirmed by the monotonous variation of the potential-composition curve of the Li//Li_xNi_0.2Mn_0.2Co_0.6O₂ cell. LiNi_0.2Mn_0.2Co_0.6O₂ cathode has a discharge capacity of ∼160 mAh g⁻¹ in the voltage range 2.7-4.3 V corresponding to the extraction/insertion of 0.6 lithium ion with very low polarization. It exhibits a stable capacity on cycling and good rate capability in the rate range 0.2-2 C. The almost 2D structure of this cathode material, its good electrochemical performances and its relatively low cost comparing to LiCoO₂, make this material very promising for applications.     

A screen-printed Ce0.8Sm0.2O1.9 film solid oxide fuel cell with a Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode

Screen-printing technology was developed to fabricate Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte films onto porous NiO–SDC green anode substrates. After sintering at 1400 °C for 4 h, a gas-tight SDC film with a thickness of 12 μm was obtained. A novel cathode material of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was subsequently applied onto the sintered SDC electrolyte film also by screen-printing and sintered at 970 °C for 3 h to get a single cell. A fuel cell of Ni–SDC/SDC (12 μm)/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ provides the maximum power densities of 1280, 1080, 670, 370, 180 and 73 mW cm⁻² at 650, 600, 555, 505, 455 and 405 °C, respectively, using hydrogen as fuel and stationary air as oxidant. When dry methane was used as fuel, the maximum power densities are 876, 568, 346 and 114 mW cm⁻² at 650, 600, 555 and 505 °C, respectively. The present fuel cell shows excellent performance at lowered temperatures.     

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.     

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.     

Preparation and characterization of nanocrystalline Ce0.8Sm0.2O1.9 for low temperature solid oxide fuel cells based on composite electrolyte

Nanocrystalline Ce_0.8Sm_0.2O_1.9 (SDC) has been synthesized by a combined EDTA–citrate complexing sol–gel process for low temperature solid oxide fuel cells (SOFCs) based on composite electrolyte. A range of techniques including X-ray diffraction (XRD), and electron microscopy (SEM and TEM) have been employed to characterize the SDC and the composite electrolyte. The influence of pH values and citric acid-to-metal ions ratios (C/M) on lattice constant, crystallite size and conductivity has been investigated. Composite electrolyte consisting of SDC derived from different synthesis conditions and binary carbonates (Li₂CO₃–Na₂CO₃) has been prepared and conduction mechanism is discussed. Water was observed on both anode and cathode side during the fuel cell operation, indicating the composite electrolyte is co-ionic conductor possessing H⁺ and O²⁻ conduction. The variation of composite electrolyte conductivity and fuel cell power output with different synthesis conditions was in accordance with that of the SDC originated from different precursors, demonstrating O²⁻ conduction is predominant in the conduction process. A maximum power density of 817 mW cm⁻² at 600 °C and 605 mW cm⁻² at 500 °C was achieved for fuel cell based on composite electrolyte.