Improvement of the Electrochemical Performance of Ln0.58Sr0.4Fe0.8Co0.2O3 – δ IT‐SOFC Cathodes by Ternary Lanthanide Combinations (La‐Pr‐Sm)

Active perovskite‐based SOFC cathodes have been developed through lanthanide combination in the (La_{1 – x – y}Pr_xSm_y)_0.58Sr_0.4Fe_0.8Co_0.2O_{3 – δ} system following a ternary mixture experimental design. These compositions were prepared through a sol–gel method and characterised by electrochemical impedance spectroscopy (EIS) as symmetrical cells on GDC‐electrolyte samples in the 450–650 °C temperature range. The electrochemical properties of the single lanthanide‐based Ln_0.58Sr_0.4Fe_0.8Co_0.2O_{3 – δ} compounds were enhanced when different lanthanides were combined together in the same crystalline structure. The observed improvement does not follow a mere additional effect of the performance from the parent Ln_0.58Sr_0.4Fe_0.8Co_0.2O_{3 – δ} compounds, i.e. it does not follow a linear behaviour, and the better performance is ascribed to synergetic catalytic effects among lanthanide cations. A reduction in electrode polarisation resistance with respect to non‐substituted compositions is stated for most Ln_0.58Sr_0.4Fe_0.8Co_0.2O_{3 – δ} electrode compositions combining two or three lanthanides. Samarium addition to the electrode material involves a substantial reduction in the activation energy and the reduction degree is directly dependant on the samarium amount incorporated in the lattice. The best performing composition comprises a praseodymium‐rich lanthanum‐based electrode material. The experimental data derived from the ternary mixture design were modelled using nonlinear functions and this modelling allowed finding an electrode composition minimising the polarisation resistance while maintaining the activation energy at reduced values. Selected cathode compositions were tested in fully assembled anode‐supported cells and electrochemical characterisation supports the cooperative effect of lanthanide combination.     

The Effect of Plasma Spraying Power on La0.58Sr0.4Co0.2Fe0.8O3–δ–La0.8Sr0.2Ga0.8Mg0.2O3–δ Composite Cathode Interlayer Microstructure and Cell Performance

The metal‐supported intermediate temperature solid oxide fuel cells with a porous nickel substrate, a nano‐structured LDC (Ce_0.55La_0.45O_2–δ)–Ni composite anode, an LDC diffusion barrier layer, an LSGM (La_0.8Sr_0.2Ga_0.8Mg_0.2O_3–δ) electrolyte, an LSCF (La_0.58Sr_0.4Co_0.2Fe_0.8O_3–δ)–LSGM composite cathode interlayer and an LSCF cathode current collector are fabricated by atmospheric plasma spraying. Four different plasma spraying powers of 26, 28, 30, and 34 kW are used to fabricate the LSCF–LSGM composite cathode interlayers. Each cell with a prepared LSCF–LSGM composite cathode interlayer has been post‐heat treated at 960 °C for 2 h in air with an applied pressure of 450 g cm^–2. The current‐voltage‐power and AC impedance measurements indicate that the LSCF–LSGM composite cathode interlayer formed at 28 kW plasma spraying power has the best power performance and the smallest polarization resistance at temperatures from 600 to 800 °C. The microstructure of the LSCF–LSGM composite cathode interlayer shows to be less dense and composed of smaller dense regions as the plasma spraying power decreases to 28 kW. The durability test of the cell with an optimized LSCF–LSGM composite cathode interlayer gives a degradation rate of 1.1% kh^–1 at the 0.3 A cm^–2 constant current density and 750 °C test temperature.     

Electrochemical behavior of Ln0.6Sr0.4Co0.2Fe0.8O3−δ (Ln=Ce, Gd, Sm, Dy) materials used as cathode of IT-SOFC

The perovskite-type compounds Ln_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (Ln=Ce, Sm, Gd, Dy) used as the cathodes of intermediate temperature solid oxide fuel cell (IT-SOFC) were studied. The cells consisted of anode supported Sm-doped-ceria electrolyte bi-layer and cathode with 0.65 cm² effective area. Open-circuit voltage (OCV), V–I and P–I curves of the cells were measured over a temperature range from 400 to 800 °C, using H₂–3%H₂O as fuel and air as oxidant. Polarization potential of electrodes were measured with asymmetry three-electrode method during cell discharging. The results indicated that, Dy-SCF material cathode behaved with high catalytic activity for oxygen dissociation at low temperatures. For each cell with a particular cathode, there was a transition temperature, at which OCV of the cell reached the highest value. When temperature was higher than the transition temperature, OCV of the cell increases with decreasing temperature, whereas as temperature was lower than that, OCV decreased with lowering temperature.     

Design of BaZr0.8Y0.2O3–δ Protonic Conductor to Improve the Electrochemical Performance in Intermediate Temperature Solid Oxide Fuel Cells (IT‐SOFCs)

BaZr_0.8Y_0.2O_3–δ, (BZY), a protonic conductor candidate as an electrolyte for intermediate temperature (500–700 °C) solid oxide fuel cells (IT‐SOFCs), was prepared using a sol–gel technique to control stoichiometry and microstructural properties. Several synthetic parameters were investigated: the metal cation precursors were dissolved in two solvents (water and ethylene glycol), and different molar ratios of citric acid with respect to the total metal content were used. A single phase was obtained at a temperature as low as 1,100 °C. The powders were sintered between 1,450 and 1,600 °C. The phase composition of the resulting specimens was investigated using X‐ray diffraction (XRD) analysis. Microstructural characterisation was performed using field emission scanning electron microscopy (FE‐SEM). Chemical stability of the BZY oxide was evaluated upon exposure to CO₂ for 3 h at 900 °C, and BZY showed no degradation in the testing conditions. Fuel cell polarisation curves on symmetric Pt/BZY/Pt cells of different thicknesses were measured at 500–700 °C. Improvements in the electrochemical performance were obtained using alternative materials for electrodes, such as NiO‐BZY cermet and LSCF (La_0.8Sr_0.2Co_0.8Fe_0.2O₃), and reducing the thickness of the BZY electrolyte, reaching a maximum value of power density of 7.0 mW cm^–2 at 700 °C.     

Degradation of La0.6Sr0.4Co0.2Fe0.8O3–δ–Ce0.8Sm0.2O1.9 Cathodes on Coated and Uncoated Porous Metal Supports

The degradation of composite LSCF‐SDC cathodes on porous 430 stainless steel supports was investigated. Two degradation mechanisms were observed: a multi‐layer oxide scale, believed to consist of Cr₂O₃ and SrCrO₄, formed at the support‐cathode interface, and small amounts of chromium were detected within the cathodes. To reduce degradation, La₂O₃ and Y₂O₃ reactive element oxide coatings were deposited on the internal pore surfaces of the metal supports. The reactive element oxide coatings reduced the amount of volatile chromium that deposited in the cathodes. As a result, the degradation rates of the cathodes on coated supports were significantly lower than the degradation rates of cathodes made on uncoated metal supports. In cathode symmetrical cells, polarization resistance degradation rates as low as 2.56 × 10⁻⁶ Ω cm² h⁻¹ were observed over 100 hours on coated metal supports, compared to an average of 1.23 × 10⁻⁴ Ω cm² h⁻¹ on uncoated supports.     

Characterization of La0.6Sr0.4Co0.2Fe0.8O3–δ + La2NiO4+δ Composite Cathode Materials for Solid Oxide Fuel Cells

The structure, electrical conduction, thermal expansion and electrochemical properties of the La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ + La₂NiO_4+δ (LSCF‐LNO) composite cathodes were investigated with regard to the volume fraction of the LNO composition. No chemical reaction product between the two constituent phases was found for the composite cathodes sintered at 1,400 °C for 10 h within the sensitivity of the XRD. Compared to the performance of the LSCF cathode, the LNO composition in the composite cathode plays a role in deteriorating both electrical conductivity and electrochemical properties, however, improving the thermal expansion properties. The trade‐off between electrical conducting and thermal expansion classifies the composite cathode containing 30 volume percent (vol.%) LNO as the optimum composition. For characterizing cathode performance in a single cell, a slurry spin coating technique was employed to prepare a porous cathode layer as well as a YSZ/Ce_0.8Sm_0.2O_3–δ (SDC) electrolyte. The optimum conditions for fabricating the YSZ/SDC electrolyte were investigated. The resulting single cell with 70 vol.% LSCF‐30 vol.%LNO (LSCF‐LNO30) cathode shows a power density of 497 mW cm^–2 at 800 °C, which is lower than that of the cell with a LSCF cathode, but still within the limits acceptable for practical applications.     

Flame Spray Synthesis of Nanoscale La0.6Sr0.4Co0.2Fe0.8O3–δ and Ba0.5Sr0.5Co0.8Fe0.2O3–δ as Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells

Electrochemical performance and degradation was analysed by conductivity measurements as well as thermogravimetric analysis (TGA) under different atmospheres. CO₂ was identified as a critical parameter in terms of carbonate formation from Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ and causes a strong increase in the material resistivity, whereas La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ is unaffected. The oxygen exchange kinetic of both compositions is affected by CO₂ containing atmospheres.     

Performance of Cobalt‐free La0.6Sr0.4Fe0.9Ni0.1O3–δ Cathode Material for Intermediate Temperature Solid Oxide Fuel Cells

A series of cobalt‐free perovskite‐type cathode materials La_0.6Sr_0.4Fe_1–xNi_xO_3–δ (0 ≤ x ≤ 0.15) for intermediate temperature solid oxide fuel cells (IT‐SOFCs) are prepared by a citric‐nitrate process. The conductivities of the cathode materials are measured as functions of temperature (300–800 °C) in air by AC impedance method, and the La_0.6Sr_0.4Fe_0.9Ni_0.1O_3–δ (LSFN10) has the highest conductivity to be 160 S cm^–1 at 400 °C. A single IT‐SOFC based on LSFN10 cathode, BaZr_0.1Ce_0.7Y_0.2O_3–δ electrolyte membrane and Ni–BaZr_0.1Ce_0.7Y_0.2O_3–δ anode substrate was fabricated by a simple spin‐coating process, and the performances of the cell using hydrogen as fuel and air as the oxidant were researched by electrochemical methods at 600–700 °C. The maximum power densities of the cell are 405 mW cm^–2 at 700 °C, 238 mW cm^–2 at 650 °C, and 140 mW cm^–2 at 600 °C, respectively. The results indicate that the LSFN10 is a promising cathode material for proton conducting IT‐SOFCs.     

Microstructure and Polarization Studies on Interlayer Free La0.65Sr0.3Co0.2Fe0.8O3–δ Cathodes Fabricated on Yttria Stabilized Zirconia by Solution Precursor Plasma Spraying

Nano‐structured cathodes of La_0.65Sr_0.3Co_0.2Fe_0.8O_3–δ (LSCF) are fabricated by solution precursor plasma spraying (SPPS) on yttria stabilized zirconia (YSZ) electrolytes (LSCF‐SPPS‐YSZ). Phase pure LSCF is obtained at all plasma power. Performances of LSCF‐SPPS‐YSZ cathodes are compared with conventionally prepared LSCF cathodes on YSZ (LSCF‐C‐YSZ) and gadolinium doped ceria (GDC) (LSCF‐C‐GDC) electrolytes. High R_p is observed in the LSCF‐C‐YSZ (∼42 Ohm cm² at 700 °C) followed by LSCF‐C‐GDC (R_p ∼ 1.5 Ohm cm² at 700 °C) cathodes. Performance of the LSCF‐SPPS‐YSZ cathodes (R_p ∼ 0.1 Ohm cm² at 700 °C) is found to be even superior to the performance of LSCF‐C‐GDC cathodes. High performance in LSCF‐SPPS‐YSZ cathodes is attributed to its nano‐structure and absence of any interfacial insulating phase which may be attributed to the low temperature at the interaction point of LSCF and YSZ and low interaction time between LSCF and YSZ during SPPS process. In the time scale of 100 h, no change in the polarization resistances is observed at 750 °C. Based on the literature and from the present studies it can be stated that SOFC with YSZ electrolyte and LSCF‐SPPS‐YSZ cathode can be operated at 750 °C for a longer duration of time and good performance can probably be achieved.     

First Principles Modeling of Pd‐doped (La,Sr)(Co,Fe)O3 Complex Perovskites

(La,Sr)(Co,Fe)O₃ (LSCF) perovskites are well known promising materials for cathodes of solid oxide fuel cells. In order to reduce cathode operational temperature, doping on B‐sublattice with different metals was suggested. Indeed, as it was shown recently experimentally, doping with low Pd content increases oxygen vacancy concentration which is one of factors controlling oxygen transport in fuel cells. In this Communication, we modeled this material using first principles DFT calculations combined with supercell model. The charge density redistribution, density of states, and local lattice distortion around palladium ions are analyzed and reduction of the vacancy formation energy confirmed.     

Co‐synthesis of Sm0.5Sr0.5CoO3‐Sm0.2Ce0.8O1.9 Composite Cathode with Enhanced Electrochemical Property for Intermediate Temperature SOFCs

A 70 wt.% Sm_0.5Sr_0.5CoO₃ – 30 wt.% Sm_0.2Ce_0.8O_1.9 (SSC–SDC73) composite cathode was co‐synthesized by a facile one‐step sol–gel method, which showed lower polarization resistance and overpotential than those of physically mixed SSC–SDC73 cathode. The polarization resistance of co‐synthesized SSC–SDC73 cathode at 800 °C was as low as 0.03 Ω cm² in air. Scanning electron microscopy (SEM) images showed that the enhanced electrochemical property was mainly attributed to the smaller grains and good dispersion of SSC and SDC phases within the composite cathode, leading to an increase in three‐phase boundary length. The dependence of polarization resistance with oxygen partial pressure indicated that the rate‐limiting step for oxygen reduction reaction was the dissociation of molecular oxygen to atomic oxygen process. An anode supported fuel cell with a co‐synthesized SSC–SDC73 cathode exhibited a peak power density of 924 mW cm⁻² at 800 °C. Our results suggested that co‐synthesized composite was a promising cathode for intermediate temperature solid oxide fuel cells (IT‐SOFCs).     

Effects of Fabrication Conditions on the Crystallinity,Barium Deficiency,and Conductivity of BaZr0.8Y0.2O3–δ Films Grown by Pulsed Laser Deposition

Yttrium‐doped barium zirconate (BZY) thin films were deposited on MgO and sapphire substrates using a pulsed‐laser deposition (PLD) method with varying deposition rates. The films deposited with a low deposition rate exhibited highly oriented microstructures with little grain boundaries. The electrical conductivities of these films were higher than those of the films, deposited with high deposition rates, which showed little‐oriented polycrystalline microstructure. While the films deposited on the MgO substrates had a stoichiometric composition, those deposited on the sapphire substrates had a high barium deficiency, which was possibly due to the differences in the crystal structure and large lattice mismatch between the sapphire substrate and the BZY film. The electrical conductivity of the highly oriented BZY film, grown on MgO substrate with a low deposition rate, showed little barium deficiency and the highest conductivity value that is higher than the typical conductivity of sintered pellets.     

Evaluation of Using LaNi0.6Fe0.4O3–δ Contact Layer Between La0.6Sr0.4FeO3 Cathode and Crofer22APU Interconnect for Solid Oxide Fuel Cells

Interconnect‐cathode interfacial adhesion is important for the durability of solid oxide fuel cell (SOFC). Thus, the use of a conductive contact layer between interconnect and cathode could reduce the cell area specific resistance (ASR). The use of La_0.6Sr_0.4FeO₃ (LSF) cathode, LaNi_0.6Fe_0.4O_3–δ (LNF) contact layer and Crofer22APU interconnect was proposed as an alternative cathode side. LNF‐LSF powder mixtures were heated at 800 °C for 1,000 h and at 1,050 °C for 2 h and analyzed by X‐Ray power diffraction (XRD). The results indicated a low reactivity between the materials. The degradation occurring between the components of the half‐cell (LSF/LNF/Crofer22APU) was studied. XRD results indicated the formation of secondary phases, mainly: SrCrO₄, A(B, Cr)O₃ (A = La, Sr; B = Ni, Fe) and SrFe₁₂O₁₉. Scanning electron microscopy with energy dispersive X‐Ray spectroscopy (SEM‐EDX) and the X‐Ray photoelectron spectroscopy (XPS) analyzes confirmed the interaction between LSF/LNF and the metallic interconnect due to the Cr vaporization/migration. An increment of the resistance of ∼0.007 Ω cm² in 1,000 h is observed for (LSF/LNF/Crofer22APU) sample. However, the ASR values of the cell without contact coating, (LSF/Crofer22APU), were higher (0.31(1) Ω cm²) than those of the system with LNF coated interconnect (0.054(7) Ω cm²), which makes the proposed materials combination interesting for SOFC.     

Electrochemical Properties of La0.5Sr0.5Co0.8M0.2O3–δ (M=Mn,Fe, Ni,Cu) Perovskite Cathodes for IT‐SOFCs

The electrochemical properties of La_0.5Sr_0.5Co_0.8M_0.2O_3–δ (M=Mn, Fe, Ni, Cu) cathodes are investigated with chemical bulk diffusion coefficients (D_chem) and polarization resistances. The electrochemical performance of long‐term testing for La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ cathode was carried out to investigate its electrochemical stability. In this work, an anode‐supported single cell with a thick‐film SDC electrolyte (30 μm), a Ni‐SDC cermet anode (1 mm), and a La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ cathode (10 μm) reaches a maximum peak power density of 983 mW/cm² at 700°C. Obviously, Cu substitution for B‐site of La_0.5Sr_0.5CoO_3–δ cathode reduced thermal expansion coefficient (TEC) value and enhanced oxygen bulk diffusion and electrochemical properties. La_0.5Sr_0.5Co_0.8Cu_0.2O_3–δ is a promising cathode material for intermediate temperature solid oxide fuel cells (IT‐SOFC).     

La0.6Sr0.4Co0.8Ga0.2O3‐δ (LSCG) hollow fiber membrane reactor: Partial oxidation of methane at medium temperature

In this study, La_0.6Sr_0.4Co_0.8Ga_0.2O_3‐δ (LSCG) hollow fiber membrane reactor was integrated with Ni/LaAlO₃‐Al₂O₃ catalyst for the catalytic partial oxidation of methane (POM) reaction. The process was successfully carried out in the medium temperature range (600–800°C) for reaction of blank POM with bare membrane, catalytic POM reaction and swept with H₂:CO gas mixture. For the catalytic POM reaction, enhancement in selectivity to H₂ and CO is obtained between 650–750°C when O₂:CH₄ <1. High CH₄ conversion of 97% is achieved at 750°C with corresponding H₂ and CO selectivity of about 74 and 91%. The oxygen flux of the membranes also increased with the increase in oxygen partial pressure gradient across the membrane. The postreacted membranes were tested via XRD and FESEM‐EDX for their crystallinity and surface morphology. XPS analysis was further used to investigate the O1s, Co 2p and Sr 3d binding energies of the segregated elements from the reducing reaction environment. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3874–3885, 2013     

Degradation mechanism analysis of Ba0.5Sr0.5Co0.8Fe0.2O3‐δ membranes at intermediate‐low temperatures

The degradation of the permeation flux of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ membranes has typically been attributed to the phase transformation of the material at intermediate temperatures. In this study, the effect of the interfacial oxygen exchange steps was considered to give an overall view of the degradation mechanism. The changes in the interfacial exchange resistances, bulk resistance, and morphologies of the membranes were investigated via physical characterizations and a permeation model. The interfacial oxygen exchange resistances increased more quickly with time than bulk resistance. Meanwhile, BaSO₄ particles were detected on both surfaces of the membranes, and their contents reached maximum at 650°C. However, after the membrane surfaces were coated by Sm_0.5Sr_0.5CoO_3‐δ porous layers, the interfacial oxygen exchange resistances kept constant and the degradation rates were slowed down. The degradation was predominated by the increase of interfacial oxygen exchange resistances induced by the enrichment of BaSO₄ particles on membrane surfaces. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3879–3888, 2015     

In Situ Foaming of Porous (La0.6Sr0.4)0.98 (Co0.2 Fe0.8) O3−δ (LSCF) Cathodes for Solid Oxide Fuel Cell Applications

A binder system containing polyurethane precursors was used to in situ foam (direct foam) a (La_0.6Sr_0.4)_0.98 (Co_0.2 Fe_0.8) O_3?δ (LSCF) composition for solid oxide fuel cell (SOFC) cathode applications. The relation between in situ foaming parameters on the final microstructure and electrochemical properties was characterized by microscopy and electrochemical impedance spectroscopy (EIS), respectively. The optimal porous cathode architecture was formed with a 70 vol% solids loading within a polymer precursor composition with a volume ratio of 8:4:1 (isocyanate: PEG: surfactant) in a terpineol‐based ink vehicle. The resultant microstructure displayed a broad pore size distribution with highly elongated pore structure.     

Grain boundary conductivity of Ce0.8Ln0.2O2−δ ceramics (Ln = Y, La, Gd, Sm) with and without Co-doping

The acetylacetone precursor method was used to obtain powders of different ceria-based Ce_0.8Ln_0.2O_2−δ (Ln = Y, Gd, Sm, La) compositions, and these powders were used to prepare disks which were sintered at 1150 °C after addition of Co nitrate, or at 1500 °C, without Co. These materials were characterized by impedance spectroscopy to distinguish the bulk and grain boundary behaviours. Arrhenius representations of bulk conductivity data show a gradual slope change, indicating defect interactions on cooling to intermediate and relatively low temperatures. Though data for different doped-ceria samples converge in the high temperature range, significant differences in conductivity and activation energy were found at low temperatures. The grain boundary behaviour shows dependence on the trivalent additive (Y, La, Gd or Sm), and the highest grain boundary conductivity for samples obtained without addition of cobalt nitrate was found for samples with La. However, addition of cobalt nitrate and sintering at relatively low temperature enhances the grain boundary conductivity of the remaining materials. The highest specific grain boundary conductivity was then found for samples containing Sm. Differences in grain boundary behaviour were interpreted based on differences in segregation of the rare earth additives for samples without Co-doping and preferential segregation of Co for Co-doped samples.     

Synthesis and conductivity behaviour of proton conducting (1−x)Ba0.6Sr0.4Ce0.75Zr0.10Y0.15O3−δ‐xGDC (x=0, 0.2, 0.5) composite electrolytes

An attempt has been made here to synthesize (1?x)Ba_0.6Sr_0.4CZY‐xGDC (x=0, 0.2, 0.5) composite electrolytes and investigated their phase(s), X‐ray photo spectra (XPS) and conduction properties. All compositions possess dual phases (perovskite‐type as well as cubic fluorite structure) and show proton conduction in various atmospheres. Homogeneous formation and compatibility between phases have been confirmed from X‐ray diffraction analysis. Detailed X‐ray photoelectron spectroscopy (XPS) studies on the oxidation states of barium, strontium, gadolinium, cerium, zirconium, yttrium, and oxygen was performed. With increasing “x”, oxygen vacancy concentration increases as cerium ions in 4+ oxidation state decreases. The conduction behavior of composites depicts the protonic in nature and total activation energy lying in the range of 0.16‐0.24 eV. This study indicates that the conductivity increases with GDC content in composite electrolytes and highest conductivity is found for composite with x=0.5. These characteristics are useful to make (1?x)Ba_0.6Sr_0.4CZY‐xGDC composite electrolytes as promising candidate of central membrane for advanced fuel cell technology.