Electrospinning La0.8Sr0.2Co0.2Fe0.8O3−δ tubes impregnated with Ce0.8Gd0.2O1.9 nanoparticles for an intermediate temperature solid oxide fuel cell cathode

In order to reduce the polarization resistance of the cathode, we have developed one-dimensional (1D) nanostructured La_0.8Sr_0.2Co_0.2Fe_0.8O_3−δ (LSCF) tubes/Ce_0.8Gd_0.2O_1.9 (GDC) nanoparticles composite cathodes for solid oxide fuel cell. Uniform LSCF/PVP composite nanofibers have been firstly synthesized by a single-nozzle electrospinning technique, followed by firing at 800 °C for 2 h to form one-dimensional LSCF tubes. Subsequently, the GDC phases were introduced into tube structured LSCF scaffold pre-sintered on a GDC pellet by a multi-impregnation process. Electrochemical Impedance spectra reveal that nanostructured LSCF tubes/GDC nanoparticles composite cathodes have a better electrochemical performance, achieving area-specific resistances of 4.70, 1.12, 0.27 and 0.07 Ω cm² at 500, 550, 600 and 650 °C for the composite of GDC and LSCF in a weight ratio of 0.52:1. The low ASR values are mainly related to its optimal microstructure with larger triple-phase boundaries and higher porosity. These results suggest that LSCF tube/GDC nanoparticle composite can be an alternative cathode material for intermediate temperature solid oxide fuel cell (IT-SOFC).     

Nanoscaled La0.6Sr0.4CoO3−δ as intermediate temperature solid oxide fuel cell cathode: Microstructure and electrochemical performance

Lowering the operation temperature of solid oxide fuel cells to the range of 400-600 °C has generated new concepts for materials choice, interfacial design and electrode microstructures. In this study nanometer scaled and nanoporous La_0.6Sr_0.4CoO_3−δ (LSC) was derived from metal-organic precursors as thin film cathodes of about 200 nm thickness with mean grain sizes ranging from 17 to 90 nm and porosities of up to 45%. These microstructures resulted from different processing parameters such as heating rate, calcination temperature and post calcination annealing, and made it possible to study the influence of the electrode microstructure on the electrochemical performance. Microstructural characteristics were analyzed by scanning and transmission electron microscopy and the performance was evaluated in terms of area specific polarization resistance by means of electrochemical impedance spectroscopy in a temperature range of 400-600 °C. Polarization resistances as low as 0.023 Ω cm² were measured at 600 °C, facilitated by a substantial increase of the inner surface area of the nanoscaled microstructure, resulting from low temperature processing at ≤800 °C, and by enhanced catalytic properties determined for nanoscaled La_0.6Sr_0.4CoO_3−δ prepared by metal organic deposition.     

Electrochemical performance of La0.7Sr0.3CuO3−δ–Sm0.2Ce0.8O2−δ functional graded composite cathode for intermediate temperature solid oxide fuel cells

The fabrication and electrochemical properties of graded La_0.7Sr_0.3CuO_3−δ–Sm_0.2Ce_0.8O_2−δ (LSCu–SDC) composite cathodes were investigated in this paper. The phase composition, microstructure and electrochemical properties of the electrodes were characterized using X-ray diffraction (XRD), electron microscopy, electrochemical impedance spectroscopy (EIS) and cathodic polarization examinations. The results showed that the triple-layer graded cathode had super electrochemical performance comparing with the monolayer cathode. The graded LSCu–SDC cathode showed a polarization resistance of 0.094 Ωcm², a value much lower than the monolayer LSCu cathode of 0.234 Ωcm² at 800 °C in air. The current density of the graded cathode was 0.341 A cm⁻², more than double higher than monolayer LSCu of 0.146 A cm⁻² at an overpotential of 30 mV. The improved electrochemical performance could be attributed to the improved physical and chemical compatibility of the cathode layers in graded compositions with SDC electrolyte as well as the enlargement of triple-phase boundary for oxygen reduction.     

Thermochemical compatibility and polarization behaviors of La0.8Sr0.2Co0.8Ni0.2O3−δ as a cathode material for solid oxide fuel cell

Thermochemical compatibilities with Ce_0.8Gd_0.2O_2−δ (GDC) electrolyte and electrochemical behaviors under the condition of anodic or cathodic current treatment are investigated for La_0.8Sr_0.2Co_0.8Ni_0.2O_3−δ (LSCN) cathode of solid oxide fuel cell (SOFC). X-ray diffractometer (XRD) shows that cation exchange at 1150 °C leads to the formation of solid state solution between the cathode and electrolyte. Considering thermal expansion coefficient (TEC) and conductivity, La_1−xSr_xCo_1−yNi_yO_3−δ with the composition of La_0.8Sr_0.2Co_0.8Ni_0.2O_3−δ is indicated as a promising cathode for intermediate temperature SOFC. Electrochemical measurement reveals that the performance of LSCN cathode shows reversibility under anodic with subsequent cathodic current treatment. Further, the low frequency electrode process is strongly affected by anodic current. While the high frequency arc shows independent relationship with current polarization.     

Effects of bi-layer La0.6Sr0.4Co0.2Fe0.8O3−δ-based cathodes on characteristics of intermediate temperature solid oxide fuel cells

In this study, anode-supported planar IT-SOFCs, with a thin Sm_0.2Ce_0.8O_2−δ (SDC) electrolyte film and a bi-layer cathode, are fabricated using tape-casting and screen-printing processes. The bi-layer cathode consists of a current collector La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) layer and a functional LSCF-SDC composite layer in various thicknesses. Microstructure studies reveal that the interfaces among various layers show good adhesion, except for Cell A equipped with a cathode of pure LSCF. Cell A reports the lowest ohmic (R₀) and polarization (R_P) resistances. R_P, which increases with the thickness of the LSCF-SDC composite layer in the cathode, rises rapidly as the temperature drops, particularly at temperatures ≤550 °C. This indicates the high electrical conductivity of the cathode as a major contribution to the decrease of R_P at 500 °C. The best cell performances are observed at 650 °C for all cases, in which Cell A shows a maximum power density of 1.51 W cm⁻² and an open circuit voltage of 0.80 V. Considering both of the electrical and the mechanical integrity of the single cell, insertion of the composite layer is required to guarantee a good adhesion of cathode layer to electrolyte layer. However, the thickness of the composite layer should be retained as thin as possible to minimize the R₀ and R_P and maximize the cell performance.     

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.     

LaCo0.6Ni0.4O3−δ as cathode contact material for intermediate temperature solid oxide fuel cells

LaCo_0.6Ni_0.4O_3−δ (LCN64) was prepared through the polymeric steric entrapment precursor route with Polyvinyl alcohol (PVA) as the entrapment agent and was evaluated as a contact material between the metallic interconnect and the cathode in planar intermediate temperature solid oxide fuel cell stacks (IT-SOFC). The ratio of PVA to metal nitrates and the calcination temperature of the precursor were optimized for the process. The electrical conductivity and thermal expansion coefficient (TEC) of the synthesized LCN64 and its chemical compatibility with SUS 430 were also characterized. The results indicate that 1:4 is a proper ratio of PVA to metal nitrates for process control and safety management; and calcination of the precursor at temperatures above 650 °C leads to formation of single perovskite phase LCN64. The conductivity of fully sintered LCN64 is above 1150 S cm⁻¹ in the temperature range between 100 °C and 800 °C, which is higher than those of conventional contact materials La_1−xSr_xMnO₃ (LSM) and LaNi_yFe_1−yO₃ (LNF). The average TEC is 17.22 × 10⁻⁶ K⁻¹ at temperatures below 900 °C, which is higher than those of the metallic interconnect and cell components. Mn and Cr elements contained in SUS 430 migrated into the porous LCN64 layer at 800 °C without chemically forming resistive phases.     

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.     

Electrochemical performance of La0.9Sr0.1Co0.8Ni0.2O3−δ-Ce0.8Sm0.2O1.9 composite cathode for solid oxide fuel cells

The mixed ionic and electronic conductors (MIEC) of La_0.9Sr_0.1Co_0.8Ni_0.2O_3−δ (LSCN)-Ce_0.8Sm_0.2O_1.9 (SDC) were investigated for potential application as a cathode material for solid oxide fuel cells (SOFCs) based on a SDC electrolyte. Electrochemical impedance spectroscopy (EIS) technique was performed over the temperature range of 600-850 °C to determine the cathode polarization resistance, which is represented by area specific resistance (ASR). This study systematically investigated the exchange current densities (i₀) for oxygen reduction reaction (ORR), determined from the EIS data and high-field cyclic voltammetry. The 70LSCN-30SDC composite cathode revealed a high exchange current density (i₀) value of 297.6 mA/cm² at 800 °C determined by high-field technique. This suggested that the triple phase boundary (TPB) may spread over more surface of this composite cathode and revealing a high catalytically active surface area. The activation energies (E_a) of ORR determined from the slope of Arrhenius plots for EIS and high-field techniques are 96.9 kJ mol⁻¹ and 90.4 kJ mol⁻¹, respectively.     

High performance proton-conducting solid oxide fuel cells with a stable Sm0.5Sr0.5Co3−δ-Ce0.8Sm0.2O2−δ composite cathode

A Sm_0.5Sr_0.5CoO_3−δ-Ce_0.8Sm_0.2O_2−δ (SSC-SDC) composite is employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs). BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) is used as the electrolyte, and the system exhibits a relatively high performance. An extremely low electrode polarization resistance of 0.066 Ω cm² is achieved at 700 °C. The maximum power densities are: 665, 504, 344, 214, and 118 mW cm⁻² at 700, 650, 600, 550, and 500 °C, respectively. Moreover, the SSC-SDC cathode shows an essentially stable performance for 25 h at 600 °C with a constant output voltage of 0.5 V. This excellent performance implies that SSC-SDC, which is a typical cathode material for SOFCs based on oxide ionic conductor, is also a promising alternative cathode for H-SOFCs.     

La0.99Co0.4Ni0.6O3−δ-Ce0.8Gd0.2O1.95 as composite cathode for solid oxide fuel cells

We have studied a new composite SOFC cathode consisting of LaCo_0.4Ni_0.6O_3−δ (LCN60) and Ce_0.9Gd_0.1O_1.95 (CGO). The polarisation resistance (R_P) at 750 °C and OCV was measured to 0.05 ± 0.01 Ω cm² and the activation energy was determined to be about 1 eV. The impedance spectra were modelled with an EQC model consisting of a high frequency Z_RQ circuit and a medium frequency Gerischer impedance, Z_G. The resistance of Z_G was found to decrease with approximately a factor of two as a consequence of infiltration of (La_0.6Sr_0.4)_0.99CoO₃ into the porous LCN60-CGO structure. R_P of both infiltrated and non-infiltrated LCN60-CGO cathodes is substantially lower than that of LSM-YSZ and comparable with single phase LSC cathodes at low T due to its low E_A. R_P was also found to be stable at 750 °C and OCV. The cathodes were integrated onto ScYSZ based anode supported cells which were measured to have an ASR of 0.16-0.18 Ω cm² at 750 °C.     

Investigation on scandium-doped manganate La0.8Sr0.2Mn1−xScxO3−δ cathode for intermediate temperaturesolid oxide fuel cells

Scandium-doped lanthanum strontium manganate La_0.8Sr_0.2Mn_1−xSc_xO_3−δ (LSMS) combined with YSZ as composite cathode for anode-supported solid oxide fuel cell is investigated. The LSMS powders are prepared using the modified Pechini method. The XRD and H₂-TPR results reveal that non-stoichiometric defects are introduced into the perovskite lattice of LSMS samples as a result of Sc substitution, which leads to increased oxygen ion mobility in the Sc containing samples. But high level doping of Sc may results in the segregation of the Sc₂O₃ secondary phase at elevated temperature. The cells with the LSMS-containing cathodes exhibit higher performances, especially at lower temperatures, which can be ascribed to the increased oxygen anionic vacancies in the LSMS.     

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.     

La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes infiltrated with samarium-doped cerium oxide for solid oxide fuel cells

Porous La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes are coated with a thin film of Sm_0.2Ce_0.8O_1.95−δ (SDC) using a one-step infiltration process. Examination of the microstructures reveals that small SDC particles are formed on the surface of LSCF grains with a relatively narrow size distribution. Impedance analysis indicates that the SDC infiltration has dramatically reduced the polarization of LSCF cathode, reaching interfacial resistances of 0.074 and 0.44 Ω cm² at 750 °C and 650 °C, respectively, which are about half of those for LSCF cathode without infiltration of SDC. The activation energies of the SDC infiltrated LSCF cathodes are in the range of 1.42-1.55 eV, slightly lower than those for a blank LSCF cathode. The SDC infiltrated LSCF cathodes have also shown improved stability under typical SOFC operating conditions, suggesting that SDC infiltration improves not only power output but also performance stability and operational life.     

Preparation and performance of large-area La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte for intermediate temperature solid oxide fuel cell

Pre-treated LSGM starting powders decrease in particle size, leading to an increase in the LSGM relative density and electric conductivity. The starting powders are ball-milled with the assistance of absolute ethanol to reduce the particle size, dried ultrasonically to prevent the agglomeration of the powders and pre-calcined and re-balled to increase the rate of grain growth. These improvements make it possible to obtain single phase LSGM powders with small particle size at the calcination temperature of 1300 °C. A large-area (9 cm × 9 cm) LSGM electrolyte substrate has been prepared successfully by tape casting from these powders. The LSGM electrolyte exhibits a dense structure, the relative density reaches 96%, and the electrical conductivity is 0.08 S cm⁻¹ at 800 °C.     

La0.8Sr0.2Mn1−xRuxO3−δ as a cathode material for solid oxide fuel cells

Oxides of La_0.8Sr_0.2Mn_1−xRu_xO_3−δ (LSMR) (x = 0, 0.25, 0.50, 0.75, or 1.0) were prepared to fabricate cathodes in solid oxide fuel cells. The crystal structure changed from trigonal (x = 0 or 0.25) to a mixture of trigonal and orthorhombic (x = 0.5) and to orthorhombic (x = 0.75 or 1.0). X-ray photoelectron spectroscopy analysis after electrochemical testing indicated that the relative concentrations of Ru⁴⁺ to Ru⁶⁺ and Mn⁴⁺ to Mn²⁺ influence the performance of a single cell. The transformation from Ru⁴⁺ to Ru⁶⁺ releases two electrons but that from Mn⁴⁺ to Mn²⁺ creates two electron holes (an oxygen vacancy). The relative concentrations in LSMR were determined using the stoichiometric ratio (x) of Ru, and then, the concentrations of electrons and electron holes for influencing the cathode electrochemical catalytic reactivity were estimated. x = 0.25 represented the better cell performance, and Ru may stabilize the LSMR grain size during electrochemical testing.     

Low-temperature solid oxide fuel cells with novel La0.6Sr0.4Co0.8Cu0.2O3−δ perovskite cathode and functional graded anode

The perovskite La_0.6Sr_0.4Co_0.8Cu_0.2O_3−δ (LSCCu) oxide is synthesized by a modified Pechini method and examined as a novel cathode material for low-temperature solid oxide fuel cells (LT-SOFCs) based upon functional graded anode. The perovskite LSCCu exhibits excellent ionic and electronic conductivities in the intermediate-to-low-temperature range (400-800 °C). Thin Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte and NiO-SDC anode functional layer are prepared over macroporous anode substrates composed of NiO-SDC by a one-step dry-pressing/co-firing process. A single cell with 20 μm thick SDC electrolyte on a porous anode support and LSCCu-SDC cathode shows peak power densities of only 583.2 mW cm⁻² at 650 °C and 309.4 mW cm⁻² for 550 °C. While a cell with 20 μm thick SDC electrolyte and an anode functional layer on the macroporous anode substrate shows peak power densities of 867.3 and 490.3 mW cm⁻² at 650 and 550 °C, respectively. The dramatic improvement of cell performance is attributed to the much improved anode microstructure that is confirmed by both SEM observation and impedance spectroscopy. The results indicate that LSCCu is a very promising cathode material for LT-SOFCs and the one-step dry-pressing/co-firing process is a suitable technique to fabricate high performance SOFCs.     

Characterization of Ba1.0Sr1.0FeO4+δ cathode on La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte for intermediate temperature solid oxide fuel cells

Ba_1.0Sr_1.0FeO_4+δ (BSFO) with A₂BO₄ structure as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) is synthesized through an ethylene diamine tetraacetic acid (EDTA)-citrate process, and characterized by X-ray diffraction. Field emission scanning electron microscopy shows that BSFO cathode is well attached to the La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte. The electrical conductivity measured by DC four-probe method increases as the temperature increases. A linear relationship between ln(σT) and 1000/T indicates that the conducting behavior obeys the small polaron conductivity mechanism. Electrochemical performance of BSFO cathode on LSGM electrolyte is investigated in the temperature range from 500 °C to 800 °C. The results indicate that oxygen adsorption/dissociation process dominates cathodic reaction. Furthermore, the polarization resistance of BSFO cathode decreases with increasing temperature, and declines to 1.42 Ω cm² at 800 °C. These results show that BSFO can be a promising cathode material used on LSGM electrolyte for IT-SOFCs.     

Enhanced electrochemical performance of the solid oxide fuel cell cathode using Ca3Co4O9+δ

This paper reports on the electrochemical performance of an SOFC cathode for potential use in intermediate-temperature solid oxide fuel cells (IT-SOFCs) using the oxygen non-stoichiometric misfit-layered cobaltite Ca₃Co₄O_9+δ or composites of Ca₃Co₄O_9+δ with Ce_0.9Gd_0.1O_1.95 (CGO/Ca₃Co₄O_9+δ). Electrochemical impedance spectroscopy revealed that symmetric cells with an electrode of pure Ca₃Co₄O_9+δ exhibit a cathode polarization resistance (R_p) of 12.4 Ω cm², at 600 °C in air. Strikingly, R_p of the composite CGO/Ca₃Co₄O_9+δ with 50 vol.% CGO was reduced by a factor of 19 (i.e. R_p = 0.64 Ω cm²), the lowest value reported so far for the Ca₃Co₄O₉ family of compounds. These findings together with the reported thermal expansion coefficient, good compatibility with CGO and chemical durability of this material suggest that it is a promising candidate cathode for IT-SOFCs.     

Palladium and ceria infiltrated La0.8Sr0.2Co0.5Fe0.5O3−δ cathodes of solid oxide fuel cells

La_0.8Sr_0.2Co_0.5Fe_0.5O_3−δ (LSCF) cathodes infiltrated with electrocatalytically active Pd and (Gd,Ce)O₂ (GDC) nanoparticles are investigated as high performance cathodes for the O₂ reduction reaction in intermediate temperature solid oxide fuel cells (IT-SOFCs). Incorporation of nano-sized Pd and GDC particles significantly reduces the electrode area specific resistance (ASR) as compared to the pure LSCF cathode; ASR is 0.1 Ω cm² for the reaction on a LSCF cathode infiltrated with 1.2 mg cm⁻² Pd and 0.06 Ω cm² on a LSCF cathode infiltrated with 1.5 mg cm⁻² GDC at 750 °C, which are all significantly smaller than 0.22 Ω cm² obtained for the reaction on a conventional LSCF cathode. The activation energy of GDC- and Pd-impregnated LSCF cathodes is 157 and 176 kJ mol⁻¹, respectively. The GDC-infiltrated LSCF cathode has a lower activation energy and higher electrocatalytic activity for the O₂ reduction reaction, showing promising potential for applications in IT-SOFCs.