Investigation of LSM/8YSZ cathode within an all-ceramic SOFC,Part II: Optimization of performance and co-sinterability

This paper focuses on the cathode and current collector layers of a co-sintered, all-ceramic solid oxide fuel cell (SOFC) concept. Challenges to reach good electrochemical performance have to be overcome, due to more demanding manufacturing conditions, including a relatively high co-sintering temperature. Master sintering curves show that the sintering activity of lanthanum strontium manganite (LSM) is significantly higher than that of 8-mol% yttria stabilized zirconia (8YSZ). By applying a double-layered cathode and a current collector with optimized microstructures the best electrochemical performance of the cathode is 0.26 $\mathrm{\Omega }$cm${}^2$ at 800 $°$C, evaluated from polarization resistances of 8YSZ electrolyte-supported symmetric cells post-sintered at 1150 $°$C $<T<1250$ $°$C. The cathode and current collector materials are adapted to fit the co-sintering process by adjustment of the paste compositions. Half-cells consisting of silicate mechanical support, LSM current collector, LSM mixed with 8YSZ composite cathode and 8YSZ electrolyte are co-sintered porous and defect-free at 1150 $°$C $<T<1250$ $°$C.     

Inert substrate-supported microtubular solid oxide fuel cells based on highly porous ceramic by low-temperature co-sintering

Inert substrate-supported microtubular solid oxide fuel cells (MT-SOFCs) are attractive due to their advantages, including high reduction–oxidation (redox) cycling stability and thermal cycling tolerance. A method involving sequential dip-coating, leaching, and co-sintering was developed and applied to fabricate inert substrate-supported MT-SOFCs through acid leaching nickel from the conventional Ni–yttria-stabilized zirconia (YSZ) anode. A thin current collector was deposited onto the support surface to minimize the current collection losses by collecting current from the entire surface area of the anode. A dense electrolyte could be obtained at a co-sintering temperature of 1250?°C. The produced MT-SOFC with the configuration of porous zirconia support/Ni–Scandia-stabilized zirconia (SSZ) anode current collector/Ni-SSZ anode/SSZ electrolyte/strontium-doped lanthanum manganite (LSM)-SSZ cathode/LSM cathode current collector was evaluated by electrochemical characterization tests. The inert substrate-supported MT-SOFC exhibited the maximum power densities of 616, 542, 440, and 300?mW?cm^?2 at 800, 750, 700, and 650?°C, respectively using dry hydrogen and air. In addition, the thermal cycling stability of the MT-SOFC was evaluated. The cell survived from thermal cycling tests and came out intact after 50 thermal cycles between 700?°C and 400?°C during an operation time of 50?h.     

Lowering the co-sintering temperature of cathode–electrolyte bilayers for micro-tubular solid oxide fuel cells

To prevent undesirable reactions between the cathode and electrolyte materials in cathode-supported solid oxide fuel cells (SOFCs), the co-sintering temperature of these two layers must be lowered. In the present work, we employed different strategies to lower the co-sintering temperature of cathode–electrolyte bilayers for micro-tubular SOFCs by increasing the cathode sintering shrinkage and adding sintering aids to the electrolyte. Strontium-doped lanthanum manganite (LSM) and yttria-stabilized zirconia (YSZ) were used as the cathode and electrolyte materials, respectively. To facilitate densification of the electrolyte layer by controlling the shrinkage of the cathode support, the particle size of the LSM powder was reduced by high-energy ball milling and different amounts of micro-crystalline cellulose pore former were used. Sintering aids, namely NiO and Fe₂O₃, were also added to the YSZ electrolyte to further improve its low-temperature sintering. Our results indicate that with the improvement in the cathode support shrinkage and use of the small amounts of sintering aids, the cathode–electrolyte co-sintering temperature can be reduced to 1250–1300 °C. It was also observed that the presence of the sintering aids helps to reduce the reactivity between the LSM cathode and YSZ electrolyte.     

Freestanding Micro‐SOFCs with Electrodes Prepared by Electrostatic Spray Deposition

We report a freestanding micro solid oxide fuel cell with both the anode and cathode deposited using electrostatic spray deposition (ESD) technique. The cell is consisted of dense yittria‐stabilized zirconia (YSZ) electrolyte (100 nm thick), porous lanthanum strontium manganite (LSM)–YSZ cathode (∼3 μm thick), and porous NiO‐YSZ anode (∼3 μm thick). LSM‐YSZ and NiO‐YSZ composite powders were initially prepared by glycine nitrate process and super‐critical fluid processes, respectively, and both cathode and anode layers were deposited by the ESD. The resulting freestanding micro cell exhibited an open circuit voltage close to the theoretical value of 1.09 V, and a maximum power density of 41.3 mWcm^–2 at 640 °C.     

Synthesis of LSM–YSZ–GDC dual composite SOFC cathodes for high-performance power-generation systems

This article investigates a method in further improvement of a (La_0.8,Sr_0.2)MnO₃ (LSM)-Yttria-stabilized zirconia (YSZ) dual composite cathode by adding material with high ionic conductivity such as gadolinia-doped ceria (GDC). A nano-porous composite cathode containing LSM, YSZ, and GDC was prepared by a two-step polymerizable complex (PC) method which minimizes the formation of YSZ–GDC solid solution. The structure of the resulting LSM/GDC–YSZ dual composite cathode was such that the LSM and GDC phases were present on the YSZ core particles without formation of the La₂Zr₂O₇, SrZrO₃, and GDC–YSZ solid solution. At 800 °C, the electrode polarization resistance of the LSM/GDC–YSZ dual composite cathode decreased to 0.266 Ω cm², compared with 0.385 Ω cm² for the LSM/YSZ–YSZ dual composite cathode. In addition, the Ni–YSZ anode-supported single cell using a LSM/GDC–YSZ dual composite cathode with H₂ as the fuel achieved a maximum power density of 0.65 W cm⁻² at 800 °C.     

Effect of GDC addition method on the properties of LSM–YSZ composite cathode support for solid oxide fuel cells

Equal amounts of Gd_0.1Ce_0.9O_2−δ (GDC) were added to La_0.65Sr_0.3MnO_3−δ/(Y₂O₃)_0.08(ZrO₂)_0.92 (LSM/YSZ) powder either by physical mixing or by sol–gel process, to produce a porous cathode support for solid oxide fuel cells (SOFCs). The effect of the GDC mixing method was analyzed in view of sinterability, thermal expansion coefficient, microstructure, porosity, and electrical conductivity of the LSM/YSZ composite. GDC infiltrated LSM/YSZ (G-LY) composite showed a highly porous microstructure when compared with mechanically mixed LSM/YSZ (LY) and LSM/YSZ/GDC (LYG) composites. The cathode support composites were used to fabricate the button SOFCs by slurry coating of YSZ electrolyte and a nickel/YSZ anode functional layer, followed by co-firing at 1250 °C. The G-LY composite cathode-supported SOFC showed maximum power densities of 215, 316, and 396 mW cm⁻² at 750, 800, and 850 °C, respectively, using dry hydrogen as fuel. Results showed that the GDC deposition by sol–gel process on LSM/YSZ powder before sintering is a promising technique for producing porous cathode support for the SOFCs.     

The Effect of Humidity and Oxygen Partial Pressure on LSM–YSZ Cathode

Two series of anode supported solid oxide fuel cells (SOFC) were prepared, one with a composite cathode layer of lanthanum strontium manganite (LSM) and yttria stabilized zirconia (YSZ) on top and the other further has a LSM current collector layer on top. The fuel cells were heat treated at 1,000 °C in air or nitrogen choosing dry or humid (70% steam) conditions. XRD, SEM, and XPS investigations were performed on the various samples. The most severe modifications were observed in humid nitrogen atmosphere at low oxygen partial pressure. LSM surface change goes along with a decrease of manganese concentration and strontium enrichment on the surface of the materials. Formation of monoclinic zirconia and zirconate phases was also observed. These results give a closer insight into possible degradation mechanisms of SOFC composite cathode materials in dependence of humidity and oxygen partial pressure.     

Evaluation of a Single Cell and Candidate Materials with High Water Content Hydrogen in a Generic Solid Oxide Fuel Cell Stack Test Fixture,Part II: Materials and Interface Characterization

A generic solid oxide fuel cell (SOFC) test fixture was developed to evaluate candidate materials under realistic operating conditions. A commercial 50 mm × 50 mm NiO‐YSZ anode‐supported thin YSZ electrolyte cell with lanthanum strontium manganite (LSM)/YSZ cathode was tested to evaluate the stability of candidate materials. The cell was tested in two stages at 800°C: stage I with low (~3% H₂O) humidity and stage II with high (~30% H₂O) humidity hydrogen fuel in constant voltage or constant current mode. Part I of the work, published previously, provided information regarding the generic test fixture design, materials, cell performance, and optical post‐mortem analysis. In part II, detailed microstructure and interfacial characterizations are reported regarding the SOFC candidate materials: (Mn,Co)‐spinel conductive coating, alumina coating for sealing area, ferritic stainless steel interconnect, refractory sealing glass, and their interactions with each other. Overall, the (Mn,Co)‐spinel coating was very effective in minimizing Cr migration. No Cr was identified in the cathode after 1720 h at 800°C. Aluminization of metallic interconnects also proved to be chemically compatible with alkaline‐earth silicate sealing glass. The details of interfacial reaction and microstructure development are discussed.     

Impedance studies of cathode/electrolyte behaviour in SOFC

This paper reports impedance studies of the cathode/electrolyte behaviour in solid oxide fuel cells (SOFC), based on comparative investigation of half-cells with yttria stabilized zirconia (YSZ) electrolyte and different cathode materials: lanthanum strontium manganite (LSM), and composite LSM/YSZ with low ionic conductivity as well as the electron conducting Ag, Pt and Au. For improved impedance data analysis the technique of the differential impedance analysis is applied. It ensures structural and parametric identification without preliminary assumptions about the working model. It is found that despite the low ionic conductivity of LSM, the cathode reaction of the oxide cathode materials is a two-step process including: (i) charge transfer with activation energy of the resistivity E_a increasing with the temperature and (ii) transport of oxygen ions through the bulk of the electrode (rate-limiting stage) with E_a independent on the temperature. For the metal (electron conducting) electrodes, the reaction behaviour is described with one step process with higher E_a at higher temperatures. The activation energy of the electrolyte conductivity decreases with the increase of the temperature. The observed changes in E_a for the electrolyte and the cathode reaction (the charge transfer step for the LSM-based electrodes) appear in the same temperature interval. This interesting coincidence suggests for correlation between the bulk (electrolyte) and surface conduction properties. Approaches for improvement of both the ionic conductivity and the supply with electrons in LSM should be also searched.     

Reduction of CO2 to CO at Cu–ceria-gadolinia (CGO) cathode in solid oxide electrolyser

The feasibility was investigated of using a Cu/CGO cathode for CO₂ reduction to CO in a high temperature solid oxide electrolyser (CO₂–CO, Cu/CGO|YSZ|YSZ/LSM|LSM, ambient air). An adherent layer of porous Cu/CGO electrode on YSZ electrolyte was achieved by sintering Cu/CGO paste at 1,000 °C for 5 h. Comparable performance was obtained with Ni/YSZ and Cu/CGO cathodes for CO₂ reduction at 750 °C and a 50:50 CO₂–CO feed; CO oxidation rates were faster than CO₂ reduction rates. Ohmic and polarisation resistances of the Cu/CGO electrode all decreased with decreasing CO₂:CO feed ratio. In the electrolytic mode, 100 % current efficiency for CO₂ reduction to CO was achieved on the Cu/CGO cathode at potential differences up to 1.5 V, above which the electronic conductivity of the YSZ electrolyte increased, causing a loss in effective current efficiency. Further increase in potential difference to ca. >2.3 V caused irreversible damage to the YSZ electrolyte due to its partial decomposition. No significant performance degradation, Cu sintering/migration, carbon deposition or electrode delamination was evident during 2 h of operating the electrolyser at 1.85 V and 750 °C for CO₂ reduction with a Cu/CGO cathode.     

Tailoring the O2 reduction activity on hydrangea-like La0.5Sr0.5MnO3 cathode film fabricated via atmospheric pressure plasma jet process

An atmospheric-pressure plasma jet (APPJ) is applied to prepare porous perovskite materials, particularly of lanthanum strontium manganite La_0.5Sr_0.5MnO₃ (LSM551) oxide powder and film. LSM nano powder around 50.0?nm is obtained, and characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscope. A spherical morphology with hydrangea-like shape is observed as associates to the pure tetragonal phase. LSM film is deposited onto yttria-stabilized zirconia (YSZ) electrolyte-support substrate as cathode layer for the operation in a solid oxide fuel cell at 600–900?°C operating temperatures. A series of symmetrical cells possessing high exchange current density of 30.12?mA/cm² at 800?°C. The prepared samples are assessed as an object to discover the diffusion mechanism of oxygen pathways for LSM/YSZ system based on the microstructural (particles size, and porosities) and electrochemical (kinetic and impedance) data. The mechanism of oxygen pathways is directly associated with the triple phase boundary lengthiness, in which the surface and bulk pathways occurring in APPJ-prepared LSM layer on YSZ lead to an increasing in activity of oxygen reduction reaction. Moreover, a fabrication of desirable ternary metal oxide, LSM, with highly porous structure via an advance-innovative APPJ preparation is outlined.     

Characteristics of nano-sized perovskite structured LaSrMn derived from hydrothermally synthesized amorphous LaSrMn oxide powder

Nano-sized and spherical shaped LaSrMn oxide particle was synthesized by thermal treatment at 900 °C for 3 h after hydrothermally treatment at 200 °C for 8 h, and characterized to be applied as a cathode material in solid oxide fuel cells. A spherical LSM nano-particle with sizes ranging from 100 to 120 nm was shown in SEM image and particle size distribution. The pellets of five types composed of x-wt% LSM synthesized and commercial y-wt% YSZ were manufactured and pre-sintered. Their ionic conductivities were enhanced responding to LSM concentrations, and it was the highest in 8LSM:2YSZ showing 74.0 S cm⁻¹.     

Oxygen vacancy levels on gadolinia-doped ceria interlayer deposited by atmospheric pressure plasma jet for solid oxide fuel cells

The oxygen vacancy levels as a factor on different gadolinia-doped ceria interlayer (GDCi) films deposited on yttria stabilized zirconia (YSZ) electrolyte substrates by an atmospheric pressure plasma jet (APPJ) via precursor solution of nitrate salts are investigated. Focusing on the effect of carrier gases, scanning electron microscopy (SEM), Raman, and X-ray diffraction (XRD) are implemented for the materials characterization of the as-deposited GDCi films and sintered-GDCi films at various temperatures. The higher level of oxygen vacancies in GDCi films adhered on 8YSZ electrolyte are evidently analyzed using Ar as the carrier gas during the deposition, of which the interdiffusion resulted in the formation of (GDC + YSZ) solid solution for sintering over 1300?°C degraded the total conductivity. The deposition of GDCi films on 8YSZ by APPJ method using O₂ carrier gas significantly improved the total conductivities of the whole electrolyte layers. Moreover, this study provides the useful insight into the oxygen vacancy levels on GDC films as interlayer (GDCi) to improve the values of open circuit voltage in LSM/GDCi/YSZ/Pt full-cell, as well as offering the efficiency of APPJ as one step deposition process.     

Moisture Effect on La0.8Sr0.2MnO3 and La0.6Sr0.4Co0.2Fe0.8O3 Cathode Behaviors in Solid Oxide Fuel Cells

Moisture effect on cathode behaviors is a major issue for solid oxide fuel cells servicing under severe high temperature environments. This work studies the effect of dry air and moist air on La_0.8Sr_0.2MnO₃ (LSM821) and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF6428) cathodes at 800 °C by investigating the interfacial reaction and degradation through an AISI 441 interconnect/LSM821 (LSCF6428) electrode/yttria‐stabilized zirconia (YSZ) electrolyte tri‐layer structure. Under the same processing condition, the grain size of the LSCF6428 cathode is smaller than that of the LSM821 cathode. Ohmic resistance and polarization resistance of the cathodes are analyzed by deconvoluting the electrochemical impedance spectroscopy (EIS) results. The LSCF6428 cathode has much smaller resistance than the LSM821 cathode. Moisture produces a larger effect on the ohmic resistance and polarization resistance of the LSM821 cathode than on those of the LSCF6428 cathode. More chromium diffuses from the interconnect to the cathode for both LSM821 and LSCF6428 electrodes thermally treated in moist air. Based on the structure, elemental distribution, and EIS analysis, the interaction mechanisms between the electrodes and the AISI 441 alloy interconnect are proposed.     

Dense 8 mol% yttria-stabilized zirconia electrolyte by DLP stereolithography

Solid Oxide Fuel Cells (SOFCs) are environmentally efficient energy conversion devices, but are partially limited by the complicated fabrication procedure. In this work, dense 8 mol% yttria-stabilized zirconia (8YSZ) ceramics were successfully realized through a DLP (digital light processing) stereolithography method and the electrolyte self-supported fuel cell was also tested at 800 °C. The sintering behavior of the as-printed planar samples were investigated and a fully dense ceramic can be achieved at 1450 °C. The total conductivity of the sintered 8YSZ can reach 2.18 × 10⁻² S cm⁻¹ at a test temperature of 800 °C, which is acceptable for practical application. For the electrolyte self-supported fuel cell test, a power density of 114.3 mW cm⁻² can be achieved when Ni-8YSZ cermet and La_0.8Sr_0.2MnO₃ (LSM) were used as anode and cathode. It was demonstrated that 3D printing is a promising processing technique to build up electrolyte self-supported SOFCs with desired structure for the future development.     

Fabrication of YSZ electrolyte for intermediate temperature solid oxide fuel cell using electrostatic spray deposition: II – Cell performance

Electrostatic spray deposition (ESD) was applied to fabricate a thin-layer of yttria-stabilized zirconia (YSZ) electrolyte on a solid oxide fuel cell (SOFC) anode substrate consisting of nickel-YSZ cermet. A colloidal solution of 8 mol% YSZ in ethanol was sprayed onto the substrate anode surface at 250–300 °C by ESD. After sintering the deposited layer at 1250–1400 °C for 1–2 h depending on temperature, the cathode layer, consisting of lanthanum strontium manganate (LSM), was sprayed or brush coated onto the electrolyte layer. Performance tests and AC impedance measurements of the complete cell were carried out at 800 °C to evaluate the density and conductance of the electrolyte layer formed by ESD. With a 97% H₂/3% H₂O mixture and air as fuel and oxidant gas, respectively, the open-circuit voltage (OCV) was close to theoretical and electrolyte impedance was about 0.23Ω cm². A power density of 0.45 W cm⁻² at 0.62 V was obtained. No abnormal degradation was observed after 170 h operation. The electrolyte sintering temperature and time did not significantly affect the electrolyte impedance. on leave from     

Novel Design of Microchanneled Tubular Solid Oxide Fuel Cells and Synthesis Using a Multipass Extrusion Process

A novel, microchanneled tubular solid oxide fuel cell was fabricated using a multipass extrusion process, with an outside diameter of 2.7 mm that contained 61 cells. Cell materials used in this work were 8 mol% yttria-stabilized zirconia (8YSZ), La_0.8Sr_0.2MnO₃ (LSM), and NiO–8YSZ (50:50 vol%) as electrolyte, cathode, and anode, respectively. Three stages of heat-treatment processes were applied, at 700°C in N₂ condition, at 1000°C in air, and then sintered at 1300°C for 2 h, respectively. The X-ray diffraction analysis confirmed that no reaction phases appeared after sintering. The microstructures of anode and cathode were fairly porous while the electrolyte had a dense microstructure (relative density >96%). The thickness of electrolyte, anode, and cathode were 20, 30, and 40 μm, respectively, and the diameter of the continuous channels was 150 μm.     

Reaction Kinetics and Mechanisms between La0.65Sr0.3MnO3 and 8 mol% Yttria-Stabilized Zirconia

The reaction kinetics and mechanisms between 8 mol% yttria-stabilized zirconia (YSZ) and 30 mol% Sr-doped lanthanum manganite (La_0.65Sr_0.30MnO₃, LSM) with A-site deficiency for the application of planar solid oxide fuel cells (SOFCs) were investigated. The LSM/YSZ green tapes were cofired from 1200° to 1400°C for 1 to 48 h and then annealed at 1000°C for up to 1000 h. The results showed that the diffusion of manganese cations first caused the amorphization of YSZ, and then the formation of small La₂Zr₂O₇ (LZ) or SrZrO₃ (SZ) crystals if treated for a longer time at 1400°C. The ambipolar diffusion of the Mn–O pair, transported through the migration of oxygen vacancy, plays an important role in the formation of secondary phases. The diffusion of LSM to YSZ and substitution of Mn for Zr both result in the enhanced concentration of oxygen vacancy, leading to the formation of a void-free zone (VFZ). No additional reaction products in annealed LSM/YSZ specimens, treated at 1000°C for 1000 h, were detected. The interfacial reactions, detailed reaction kinetics, and mechanisms are reported.     

Development of LSM/YSZ composite cathode for anode-supported solid oxide fuel cells

This paper presents the effect of (La,Sr)MnO₃ (LSM) stoichiometry on the polarization behaviour of LSM/Y₂O₃-ZrO₂ (YSZ) composite cathodes. The composite cathode made of A-site deficient (La_0.85Sr_0.15)_0.9MnO₃ (LSM-B) showed much lower electrode interfacial resistance and overpotential losses than that made of stoichiometric (La_0.85Sr_0.15)_1.0MnO₃ (LSM-A). The much poorer performance of the latter is believed to be due to the formation of resistive substances such as La₂Zr₂O₇/SrZrO₃ between LSM and YSZ phases in the composite electrode. A slight A-site deficiency (∼0.1) was effective in inhibiting the formation of these resistive substances. A power density of ∼1 W cm⁻² at 800 °C was achieved with an anode-supported cell using an LSM-B/YSZ composite cathode. In addition, the effects of cathodic current treatment and electrolyte surface grinding on the performance of composite cathodes were also studied.     

Fabrication of one-step co-fired proton-conducting solid oxide fuel cells with the assistance of microwave sintering

A microwave sintering technique is reported for fabricating co-sintered proton-conducting solid oxide fuel cells. With this method, high-quality ceramic electrolyte membranes can be prepared at 1100?°C, thus enabling the fabrication of entire cells in a single step. The microwave sintering method not only enhances electrolyte densification but also improves the cathode/electrolyte interface, which is critical for improving fuel cell performance. The power output of the co-sintered cell prepared under the microwave conditions (up to 449?mW?cm^?2 at 700?°C) was significantly higher than that of the cell fabricated using the traditional co-sintering method (approximately 292?mW?cm^?2 at the same temperature). Electrochemical analysis revealed that the enhanced electrolyte density and the improved cathode/electrolyte interface achieved by using the microwave sintering technique decrease both the ohmic resistance and the polarisation resistance of the cell, leading to good fuel cell performance.