Strength of Anode‐Supported Solid Oxide Fuel Cells

Nickel oxide and yttria doped zirconia composite strength is crucial for anode‐supported solid oxide fuel cells, especially during transient operation, but also for the initial stacking process, where cell curvature after sintering can cause problems. This work first compares tensile and ball‐on‐ring strength measurements of as‐sintered anodes support. Secondly, the strength of anode support sintered alone is compared to the strength of a co‐sintered anode support with anode and electrolyte layers. Finally, the orientation of the specimens to the bending axis of a co‐sintered half‐cell is investigated. Even though the electrolyte is to the tensile side, it is found that the anode support fails due to the thermo‐mechanical residual stresses.     

Electrochemical Performance of a Ni and YSZ Composite Synthesised by Ultrasonic Spray Pyrolysis as an Anode for SOFCs

The electrochemical performance of an anode material for a solid oxide fuel cell (SOFC) depends highly on microstructure in addition to composition. In this study, a NiO–yttria‐stabilised zirconia (NiO–YSZ) composite with a highly dispersed microstructure and large pore volume/surface area has been synthesised by ultrasonic spray pyrolysis (USP) and its electrochemical characteristics has been investigated. For comparison, the electrochemical performance of a conventional NiO–YSZ is also evaluated. The power density of the zirconia electrolyte‐supported SOFC with the synthesised anode is ∼392 mW cm^–2 at 900 °C and that of the SOFC with the conventional NiO–YSZ anode is ∼315 mW cm^–2. The improvement is ∼24%. This result demonstrates that the synthesised NiO–YSZ is a potential alternative anode material for SOFCs fabricated with a zirconia solid electrolyte.     

Compilation of mechanical properties for the structural analysis of solid oxide fuel cell stacks. Constitutive materials of anode-supported cells

The mechanical failure of one cell is sufficient to lead to the end of service of a solid oxide fuel cell (SOFC) stack. Therefore, there is growing interest in gaining knowledge on the mechanical properties of the cell materials for stress analysis.This study compiles available data from the literature on the mechanical properties of the most common materials used in intermediate-temperature anode-supported cells: nickel and yttria-stabilized zirconia (Ni–YSZ) anodes, YSZ electrolytes, yttria (YDC) or gadolinia-doped ceria (GDC) compatibility layers and lanthanum strontium manganite (LSM) or lanthanum strontium cobalt ferrite (LSCF) cathodes. The properties for the simulation of stresses, i.e. coefficient of thermal expansion (CTE), Young's modulus, Poisson's ratio, creep behaviour and strength are reported, with an emphasis on temperature and porosity dependence and the evolution upon aging or cycling when available. Measurements of our Ni(O)–YSZ anode material includes the CTE (oxidised and reduced state), Young's modulus and strength at room temperature (oxidised and reduced) and 1073 K (oxidised).     

Interaction of a ceria‐based anode functional layer with a stabilized zirconia electrolyte: Considerations from a materials perspective

We report on the materials interaction of gadolinium‐doped ceria (GDC) and yttria‐stabilized zirconia (YSZ) in the context of high‐temperature sintering during manufacturing of anode supported solid oxide fuel cells (AS–SOFC). While ceria‐based anodes are expected to show superior electrochemical performance and enhanced sulfur and coking tolerance in comparison to zirconia‐based anodes, we demonstrate that the incorporation of a Ni–GDC anode into an ASC with YSZ electrolyte decreases the performance of the ASC by approximately 50% compared to the standard Ni–YSZ cell. The performance loss is attributed to interdiffusion of ceria and zirconia during cell fabrication, which is investigated using powder mixtures and demonstrated to be more severe in the presence of NiO. We examine the physical properties of a GDC–YSZ mixed phase under reducing conditions in detail regarding ionic and electronic conductivity as well as reducibility, and discuss the expected impact of cation intermixing between anode and electrolyte.     

The stability of direct carbon fuel cells with molten Sb and Sb–Bi alloy anodes

The long‐term stability of direct carbon fuel cells, based on solid oxide fuel cells with molten Sb and Sb–Bi anodes, was examined for operation with activated charcoal, rice starch, and bio‐oil fuels at 973 K. With intermittent stirring of the fuel–metal anode interface, the anode performance was stable, and reasonable power densities (～250 mW/cm²) were achieved for periods up to 250 h. With Sc‐stabilized zirconia, severe thinning of the electrolyte occurred in regions of high current flow. No electrolyte thinning was observed with yttria‐stabilized zirconia as the electrolyte operating at the same current densities. © 2012 American Institute of Chemical Engineers AIChE J, 59: 3342–3348, 2013     

Temperature and Impurity Concentration Effects on Degradation of Nickel/Yttria‐stabilised Zirconia Anode in PH3‐Containing Coal Syngas

Degradation of the Ni/yttria‐stabilised zirconia (YSZ) anode of the solid oxide fuel cell has been evaluated in the coal syngas containing different PH₃ concentrations in the temperature range from 750 to 900 °C. Thermodynamic equilibrium calculations show that PH₃ in the coal syngas gas is converted mostly to P₂O₃ at 750–900 °C. The phosphorous impurity reacts with the Ni‐YSZ anode to form phosphates. The P‐impurity poisoning leads to the deactivation of the Ni catalyst and to the reduction in the electronic conductivity of the anode. The impurity poisoning effect on the anode is exacerbated by increase in the temperature and/or the PH₃ concentration.     

A Performance Prediction Tool for Solid Oxide Fuel Cells after Single Redox Cycle

The effects of anode support fabrication parameters on the cell performance and the redox behavior of the cell are investigated experimentally and theoretically. In the experimental program, an yttria stabilized zirconia based anode supported membrane electrode group (MEG) is developed via the tape casting, co‐sintering and screen printing methodologies. For comparison, various anode supported cells with different electrolyte thickness and anode support porosities are also fabricated. In the theoretical study, a mathematical model is developed to represent the fluid flow, the heat transfer, the species transport and the electrochemical reaction in solid oxide fuel cells. In addition, a redox model representing the mechanical damage in the electrochemical reaction zones due to redox cycling is developed by defining a damage function as a function of strain and a damage coefficient. The effects of anode support porosity and the electrolyte thickness on the cell performance and redox stability of the cells are numerically investigated. The experimental results are compared with the numerical results to validate the mathematical model. Finally, a predictive tool, which is valid for the ranges of the cell fabrication parameters investigated, is developed to estimate the electrochemical performance after single redox cycle.     

Characterisation of Ni–YSZ‐Cermets with Respect to Redox Stability

In anode‐supported solid oxide fuel cells (SOFCs), air break‐in on the anode side can result in reoxidation of metallic nickel. The volume expansion caused by Ni oxidation generates stresses within the substrate, the anode and the electrolyte. Those stresses exceed the stability of the components, potentially promoting crack growth. Therefore, either the SOFC degrades continuously after each redox‐cycle or the membrane electrode assembly (MEA) fails completely if the electrolyte cracks. The influence of several reoxidation parameters on the mechanical integrity of Ni–YSZ‐anodes after reoxidation was investigated using different types of samples. All samples were SFEs (substrate–functional layer–electrolytes), consisting of Ni–YSZ‐substrate, Ni–YSZ‐anode and YSZ‐electrolyte. Investigations were carried out on freestanding SFEs and SFEs attached to steel plates (Crofer22APU, Thyssen Krupp V. D. M., Material Data Sheet No. 4046, Edition of December 2006) with a glass sealing. The results show a big influence of the degree of oxidation, homogeneity of oxidation, the operating temperature and the incident flow on the behaviour and the mechanical integrity of the reoxidised SFEs. The time of oxidation and the gas flow rate were influencing parameters, whereas the influence of the porosity was insignificant. The behaviour of the SFEs upon reoxidation also changes dramatically when comparing freestanding samples with attached samples.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Gadolinia Doped Ceria Thin Films Prepared by Aerosol Assisted Chemical Vapor Deposition and Applications in Intermediate‐Temperature Solid Oxide Fuel Cells

Furthermore, deposition at such low temperatures is promising for processing of thin film assemblies. The preparation of bi‐layer electrolytes of yttria stabilized zirconia and gadolinia doped ceria thin films by aerosol assisted chemical vapor deposition is demonstrated. Gadolinia doped ceria films as thin as 150 nm are applied as barrier layers between yttria stabilized zirconia electrolyte and La_0.6Sr_0.4CoO_3–δ cathode in anode supported solid oxide fuel cells. High power densities above 850 mW cm^–2 at 650 °C are only obtained with these barrier layers, indicating that the GDC thin films effectively inhibit the formation of unwanted interface reactions.     

Structural and electrical characterisation of silica-containing yttria-stabilised zirconia

Zirconia stabilised by yttria has a high oxide ion conductivity at high temperature and, therefore, is currently used as electrolyte in Solid Oxide Fuel Cells. Silica is normally avoided in this material because formation of amorphous silica phases along the grain boundaries causes an increased grain boundary impedance. The present study examines the effect of SiO₂ and SiO₂ with Mn-oxide on the structure and resistivity of yttria stabilised zirconia electrolyte materials. During fabrication of Solid Oxide Fuel Cells, Mn readily diffuses from the manganite-based cathode into the electrolyte. It is shown that a grain boundary phase which causes an insulating layer in the grain boundaries is formed when both SiO₂ and Mn-oxide coexist in the samples, whereas such effects are much less pronounced when only SiO₂ is present.     

Novel Glass‐Ceramic Composition as Sealant for SOFCs

This work deals with the design, the characterization, and testing of a novel glass‐ceramic to be used as sealant for planar solid oxide fuel cells and its compatibility with Mn_1.5Co_1.5O₄‐coated Crofer22APU. Thermal, sintering, and crystallization behavior and thermo mechanical properties of the sealant are reviewed and discussed, indicating therefore that these compositions can be deposited at 850°C and provide an excellent compatibility with both the Mn_1.5Co_1.5O₄‐coated Crofer22APU and the anode‐supported electrolyte. In particular, Mn_1.5Co_1.5O₄‐coated Crofer22APU/sealant/anode‐supported‐electrolyte joined samples have been submitted to thermal tests (in air atmosphere) from RT to 800°C (SOFC operating temperature) up to 500 h. No interactions, cracks formation, or failure were observed at the Mn_1.5Co_1.5O₄‐coated Crofer22APU/sealant interface and between the glass‐ceramic and the anode‐supported‐electrolyte after 500 h of thermal tests in air atmosphere.     

Fabrication and performance of advanced multi-layer SOFC cathodes

Multilayered cathodes for solid oxide fuel cells are presented. The cathodes are composed of strontium doped lanthanum manganate and yttria stabilised zirconia, the ratio of which is increased with increasing distance from a supporting zirconia electrolyte. Some cathodes additionally carry a number of layers with a graded transition from manganite to cobaltite to add an electronically highly efficient current collector. The cathodes were prepared by spray painting and low temperature sintering. Electrochemical measurements revealed a performance up to 0.2 Ω cm² at 750°C for the first generation of these cathodes. The electrochemical performance was found to be influenced further by the microstructure at the interface close to the solid electrolyte and the quality of electrical contact thorough the sintered electrode.     

Sequential Tape Casting of Anode‐Supported Solid Oxide Fuel Cells

A novel route was developed to fabricate anode‐supported solid oxide fuel cells with a high throughput and low manufacturing costs. In contrast to classical manufacturing routes, this novel route starts with the tape casting of the thin electrolyte followed by the tape casting of the anode and anode support. All three layers were cast green‐on‐green and finally sintered to yield a gas‐tight electrolyte. By carefully selecting the raw materials for all three layers, it is possible to manufacture near‐net‐shape half‐cells. The half‐cells were characterized with respect to thickness, microstructure, bending behavior, electrolyte gas leakage, shrinkage, electrolyte residual stresses, and mechanical strength. Finally, the cathode was screen‐printed and fired, and the full cell characteristics were obtained in single‐cell and stack tests. Additionally, a scale‐up to cell sizes of 200 × 200 mm² was verified. Electrolyte and anode thickness were around 20 μm, and the support was cast to 300–500 μm. The helium leak rates were better than the necessary internal threshold, and the characteristic bending strength obtained was in the range of 150–200 MPa. The single‐cell tests revealed current densities of 1.0 A cm^–2 at 700 mV and 800 °C (H₂/air). A first stack test proved their stackability and operational functionality.     

Micro‐Raman spectroscopy of indentation induced phase transformation in nanozirconia ceramics

Abstract

Abstract

Micro‐Raman spectroscopy has been employed as an effective technique to determine the phase transformations in nanostructured yttria stabilised zirconia (YSZ) ceramics with different yttria contents. Samples have been prepared with varying mean grain sizes by a slip casting route followed by a microwave assisted two‐step sintering cycle starting with aqueous nanozirconia suspensions. Indents were generated using a Vickers pyramidal indenter at different loads, and the resulting phase transformations were mapped using micro‐Raman spectroscopy. The results were compared to those of a commercial submicrometre 3?mol.% YSZ. The amount of transformation was found to be much lower for nanozirconia compared to the submicrometre zirconia with similar yttria content.     

Compatibility Issues of Yttria‐Stabilized Zirconia Solid Oxide Membrane in the Direct Electro‐Deoxidation of Metal Oxides

Direct electro‐deoxidation of metal oxides has become quite popular in the production of metals and alloys. In this process, metal oxide cathode is directly reduced to a metal in a molten CaCl₂ salt bath. The anode material used is graphite. Over the years, graphite is reported to cause numerous process difficulties. Recently, based on the solid oxide membrane technology, yttria‐stabilized zirconia (YSZ) has been tested as oxygen ion conducting membrane for the anode. The success of using a membrane implies its long‐term stability in the bath. In this paper, it is seen that YSZ chemically degrades in a static melt of CaCl₂ or CaCl₂–CaO. The degradation occurs by leaching of yttria into solution leading to the formation of monoclinic zirconia which, being porous, reacts with the molten electrolyte to form calcium zirconate. However, on application of voltage, YSZ degrades via a different mechanism. The metallic calcium produced during electrolysis increases the electronic conductivity of the salt, apparently leading to the electrochemical reduction of zirconia to ZrO_2?x. As a result, localized pores are formed which allow the infiltration of salts. Addition of yttria to the salt is seen to prevent both the chemical and electrochemical degradation of the membrane.     

Interaction Study of Yttria‐Based Glasses with High‐Temperature Electrolyte for SOFC

The chemical interaction study of AO–SiO₂–B₂O₃–Y₂O₃ (A = Ba, Sr) (BaY, SrY) glass with high‐temperature electrolyte yttria‐stabilized zirconia (YSZ 8 mol%) is reported as a function of different heat treatment durations. The as‐prepared glass with 10 mol% of yttria shows limited amount of crystallization at 800 °C. Due to this yttria‐based glasses BaY and SrY have been chosen to make diffusion couples with high‐temperature electrolyte and interconnect material. These diffusion couples have been heat treated at 850 °C, for 100, 200, and 500 h. The heat‐treated diffusion couples have been characterized using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Microstructural analysis of diffusion couples shows absence of any undesired oxides and detrimental reaction products at the interface. The glass has shown good bonding characteristics and absence of cracks, pores, or any kind of delamination from YSZ. Apart from this, SrY and BaY glass seals have also shown good adhesion characteristics with Crofer 22 APU, even after 500 h at 850 °C. The morphology and microstructure of the glass matrix suggest limited amount of devitrification in the glass.     

Performance Evaluation of Solid Oxide Fuel Cells with Thin Film Electrolyte Fabricated by Binder‐Assisted Slurry Casting

A gas‐tight yttria‐stabilized zirconia (YSZ) electrolyte film was fabricated on porous NiO–YSZ anode substrates by a binder‐assisted slurry casting technique. The scanning electron microscope (SEM) results showed that the YSZ film was relatively dense with a thickness of 10 μm. La_0.8Sr_0.2MnO₃ (LSM)–YSZ was applied to cathode using a screen‐print technique and the single fuel cells were tested in a temperature range from 600 to 800 °C. An open circuit voltage (OCV) of over 1.0 V was observed. The maximum power densities at 600, 700, and 800 °C were 0.13, 0.44, and 1.1 W cm^–2, respectively.     

Oxygen Dissociation and Interfacial Transfer Rate on Performance of SOFCs with Metal‐Added (LaSr)(CoFe)O3‐(Ce,Gd)O2 – δ Cathodes

A solid oxide fuel cell (SOFC) unit is constructed with Ni‐Ce_0.9Gd_0.1O_{2 – δ} (GDC) as the anode, yttria‐stabilised zirconia (YSZ) as the electrolyte and Pt, Ag or Cu‐added La_0.58Sr_0.4Co_0.2Fe_0.8O_{3 – δ} (LSCF)–GDC as the cathode. The current–voltage measurements are performed at 800 °C. Cu addition leads to best SOFC performance. LSCF–GDC–Cu is better than LSCF–GDC and much better than GDC as the material of the cathode interlayer. Cu content of 2 wt.‐% leads to best SOFC performance. A cathode functional layer calcined at 800 °C is better than that calcined at higher temperature. Metal addition increases the O₂ dissociation reactivity but results in an interfacial resistance for O transfer. A balance between the rates of O₂ dissociation and interfacial O transfer is needed for best SOFC performance.     

Sintering Effect on Material Properties of Electrochemical Reactors Used for Removal of Nitrogen Oxides and Soot Particles Emitted from Diesel Engines

In the present work, 12‐layered electrochemical reactors (comprising five cells) with a novel configuration including supporting layer lanthanum strontium manganate (LSM)‐yttria stabilised zirconia (YSZ), electrode layer LSM‐gadolinia‐doped cerium oxide (CGO) and electrolyte layer CGO were fabricated via the processes of slurry preparation, tape casting and lamination and sintering. The parameters of porosity, pore size, pore size distribution, shrinkage, flow rate of the sintered reactors and the electrical conductivities of the supporting layer and the electrode in the sintered reactors were characterised. The effect of sintering temperature on microstructures and properties of the sintered samples was discussed, and 1,250 °C was determined as the appropriate sintering temperature for reactor production based on the performance requirements for applications. Using the present ceramic processing route, porous, flat and crack‐free electrochemical reactors were successfully achieved. The produced electrochemical reactors have the potential application in the removal of NO_x and soot particles emitted from the diesel engines.