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.     

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.     

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.     

Photo-excitation enhanced high temperature conductivity and crystallization kinetics in ultra-thin La0.6Sr0.4Co0.8Fe0.2O3−δ films

The synthesis of high performance nanostructured oxide electrodes is critical to advancement of energy technologies such as intermediate temperature solid oxide fuel cells. In this communication, we demonstrate that photo-excitation during crystallization of nanostructured 60-nm-thick La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ films leads to a significant improvement in electrical conductivity. Crystallization kinetics is also enhanced by photo-excitation while the crystallization onset temperature remains similar.     

Nano-structured cathodes based on La0.6Sr0.4Co0.2Fe0.8O3−δ for solid oxide fuel cells

Lanthanum-based iron- and cobalt-containing perovskite is a promising cathode material because of its electrocatalytic activity at a relatively low operating temperature in solid oxide fuel cells (SOFCs), i.e., 700-800 °C. To enhance the electrocatalytic reduction of oxidants on La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), nanocrystalline LSCF materials are successfully fabricated using a complexing method with chelants and inorganic nano dispersants. When inorganic dispersants are added to the synthesis process, the surface area of the LSCF powder increases from 18 to 88 m² g⁻¹, which results in higher electrocatalytic activity of the cathode. The performance of a unit cell of a SOFC with nanocrystalline LSCF powders synthesized with nano dispersants is increased by 60%, from 0.7 to 1.2 W cm⁻².     

Comparative study of perovskites as cathode contact materials between an La0.8Sr0.2FeO3 cathode and a Crofer22APU interconnect in solid oxide fuel cells

Perovskites of different compositions were tested as cathode contact material between an La_0.8Sr_0.2FeO₃ cathode and a Crofer22APU interconnect by resistance measurements at 800 °C. The materials tested were LaNi_0.6Fe_0.4O₃ and La_0.8Sr_0.2FeO₃ which are also used as cathodes; La_0.8Sr_0.2Mn_0.5Co_0.5O₃ and La_0.8Sr_0.2Mn_0.1Co_0.3Fe_0.6O₃, selected for comparing perovskites with different Mn contents; and La_0.8Sr_0.2Co_0.75Fe_0.25O₃ and La_0.8Sr_0.2Co_0.75Cu_0.25O₃ for comparing perovskites with high Co content and two possible partial substitutions of the Co. The initial area-specific contact resistance (ASR) was found to depend on the electrical conductivity of the measured perovskites. Time evolution of the ASR depended on the interactions between the contact material and the interconnect, showing the highest degradation rates for LaNi_0.6Fe_0.4O₃ and La_0.8Sr_0.2FeO₃. Chromium from the interconnect reacted with the Sr-containing perovskites forming SrCrO₄. With the contact material without strontium chromium-containing perovskites were formed. A reduced interfacial reaction was achieved by application of a MnCo_1.9Fe_0.1O₄ spinel protection layer on Crofer22APU in terms resulting in low and stable ASR.     

In situ sinterable cathode with nanocrystalline La0.6Sr0.4Co0.2Fe0.8O3−δ for solid oxide fuel cells

A nanocrystalline powder with a lanthanum based iron- and cobalt-containing perovskite, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), is investigated for solid oxide fuel cell (SOFC) applications at a relatively low operating temperature (600-800 °C). A LSCF powder with a high surface area of 88 m² g⁻¹, which is synthesized via a complex method with using inorganic nano dispersants, is printed onto an anode supported cell as a cathode electrode. A LSCF cathode without a sintering process (in situ sintered cathode) is characterized and compared with that of a sintering process at 780 °C (ex situ sintered cathode). The in situ sintered SOFC shows 0.51 A cm⁻² at 0.9 V and 730 °C, which is comparable with that of the ex situ sintered SOFC. The conventional process for SOFCs, the ex situ sintered SOFC, including a heat treatment process after printing the cathodes, is time consuming and costly. The in situ sinterable nanocrystalline LSCF cathode may be effective for making the process simple and cost effective.     

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.     

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.     

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.     

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.     

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).     

Thermal, electrical and electrochemical characteristics of Ba1−xSrxCo0.8Fe0.2O3−δ cathode material for intermediate temperature solid oxide fuel cells

Ba_1−xSr_xCo_0.8Fe_0.2O_3−δ (x = 0.3-0.9) perovskite oxides have been studied as cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The structural characteristics, temperature dependent weight loss, thermal expansion, electrical conductivity, and electrochemical properties in combination with YSZ electrolyte together with an SDC buffer layer were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG), dilatometry, DC four probe conductivity measurement and electrochemical impedance spectroscopy (EIS) techniques respectively. XRD study revealed the lattice parameter and unit cell volume decrease with increase in Sr⁺² content at the A-site. TEC and electrical conductivity were found to increase with increasing Sr⁺² content. Electrical conductivity was found to be dependent on the thermal history of the samples. Polarization resistance of the samples with SDC buffered YSZ electrolyte decreased with increasing Sr⁺² content which was ascribed to the higher conductivity with improved oxygen adsorption/desorption and oxygen ions diffusion processes. The intrinsic oxygen reduction reaction rate also increased with Sr⁺² content at the A-site. The exchange current for intrinsic oxygen reduction reaction at 700 °C was found to be 50.0 mA cm⁻² for Ba_0.3Sr_0.7Co_0.8Fe_0.2O_3−δ; a value which is about 50% higher than that for Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ, a widely studied cathode material. Therefore, the present composition may be a potential cathode material for IT-SOFC application.     

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.     

Synthesis of La0.6Ca0.4Co0.8Ir0.2O3 perovskite for bi-functional catalysis in an alkaline electrolyte

The amorphous citrate precursor method was employed to prepare perovskite of La_0.6Ca_0.4Co_0.8Ir_0.2O₃ as a bi-functional electrocatalyst for oxygen reduction and evolution in an alkaline electrolyte. The X-ray diffraction pattern of the as-synthesized powders exhibited a majority phase identical to that of La_0.6Ca_0.4CoO₃, indicating successful incorporation of Ir⁴⁺ at the Co cation sites. Scanning Electron Microscope images demonstrated a foam-like microstructure with a surface area of 13.31 m² g⁻¹. For electrochemical characterization, the La_0.6Ca_0.4Co_0.8Ir_0.2O₃ particles were supported on carbon nanocapsules (CNCs) and deposited on commercially available gas diffusion electrodes with a loading of 2.4 mg cm⁻². In current–potential polarizations, La_0.6Ca_0.4Co_0.8Ir_0.2O₃/CNCs revealed more enhanced bi-functional catalytic abilities than La_0.6Ca_0.4CoO₃/CNCs. Similar behaviors were observed in galvanostatic profiles for oxygen reduction and evolution at current densities of 50 and 100 mA cm⁻² for 10 min. Moreover, notable changes from zeta potential measurements were recorded for La_0.6Ca_0.4Co_0.8Ir_0.2O₃ relative to La_0.6Ca_0.4CoO₃. In lifetime determinations, where a repeated 3 h sequence of oxygen reduction/resting/oxygen evolution/resting was imposed, La_0.6Ca_0.4Co_0.8Ir_0.2O₃/CNCs delivered a stable and sustainable behavior with moderate degradation.     

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.     

Interface reactivity study between La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathode material and metallic interconnect for fuel cell

Interface reactivity between La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathode material and metallic interconnect (Crofer22APU) was investigated in laboratory air at 700 °C. Due to the interconnect geometry, two interfaces have been analysed: (i) interconnect rib/cathode interface (physically in contact); (ii) the interface under the channel of interconnect. In both cases, formation of a parasite phase was observed after various ageing treatments (20 h, 100 h and 200 h). However, the growth of the determined SrCrO₄ parasite phase depends on interface type and on ageing time. Two different mechanisms have been established in function of interface type: (i) SrCrO₄ phase was formed after solid state diffusion of Cr from metallic interconnect to the cathode; (ii) gas phase reaction induced formation of SrCrO₄ under the channel of interconnect. Finally, the influence of a chemical etching on cathode/interconnect reactivity was evaluated.     

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.     

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.     

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.