Development of Solid Oxide Fuel Cell Anodes Using Metal-Coated Pore-Forming Agents

Adding Ni-coated graphite particulates to a tape casting colloidal suspension is capable of creating a sintered solid oxide fuel cell anode with a critical percolation threshold for conductivity significantly lower than conventional anode materials. For example, conductivity at 800°C reached over 1200 Ω⁻¹·cm⁻¹ in these anodes at a Ni volume below the percolation threshold reported for conventional anode materials. This behavior was explained based on an "effective" Ni content, V _Nieff, which includes the graphite portion of the particle. In the green tape, V _Nieff controls the creation of a percolating network of Ni. However, when the graphite burns away, it leaves a percolated Ni network at a much lower volume fraction than would otherwise be required.     

Quantifying intermediate‐frequency heterogeneities of SOFC electrodes using X‐ray computed tomography

The electrodes in solid oxide fuel cells (SOFCs) consist of three phases interconnected in three dimensions. The volume needed to describe quantitatively such microstructures depends on several lengths scales, which are functions of materials properties and fabrication methods. This work focuses on quantifying the volume needed to represent “intermediate frequency” heterogeneities in electrodes of a commercial SOFC using X‐ray computed tomography (CT) over two different length scales. Electrode volumes of 150 μm × 150 μm × 9 μm were extracted from a synchrotron‐based micro‐CT data set, with 1³ μm³ voxels. 13.6 μm × 19.8 μm × 19.4 μm of the cathode and 26.3 μm × 24.8 μm × 15.7 μm of the anode were extracted from laboratory nano‐CT data sets, both with 65³ nm³ voxels. After comparing the variation across sub‐regions for the grayscale values from the micro‐CT, and for the phase fractions and triple phase boundary densities from the nano‐CT, it was found that the sub‐region length scales needed to yield statistically similar average values were an order of magnitude larger than those expected to capture the “high frequency” heterogeneity related to the discrete nature of the three phases in electrodes. The challenge of quantifying such electrodes using available experimental methods is discussed.     

Kinetics of crack healing and self-repair behaviors in a sealant glass for SOFC applications

A sealant is required for the solid oxide fuel cell (SOFC) to maintain hermeticity at high operating temperatures, keep fuel and oxidant from mixing, and avoid shorting of the cell stack. Glass and glass–ceramic materials are widely used as a sealant because their properties can be tailored to meet the stringent requirements of SOFC stack, but they are susceptible to cracking. In contrast, a promising concept of self-repairable glass for seals is pursued for making reliable seals that can self-repair cracks at the SOFC operating temperatures. This concept is studied through measuring crack-healing kinetics and independent measurement of glass viscosity for relating to the observed self-repair. The cracks on the glass surface are created using a Vickers indenter to achieve a well-defined crack geometry, and then the glass is exposed to elevated temperatures for different length of times to study the crack-healing kinetics. The crack-healing kinetics is compared with the predictions of our theoretical model and found to be in good agreement. In addition, glass viscosity is extracted from the healing kinetics and compared with the independent measurement of viscosity measured from the dilatometry and sintering data to further validate the crack-healing theoretical model. These results are presented and discussed.     

Fabrication,characterization, and performance of YbDSB ternary compounds for IT‐SOFC applications

(Yb₂O₃)_x(Dy₂O₃)_y(Bi₂O₃)_1?x?y (0.04≤x+y≤0.20) powders (xYbyDSB) were synthesized by modified sol‐gel Pecchini method. The powders were characterized for structural, surface morphological, thermal, and electrical properties and power density measurements by means of X‐ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis/thermal gravimetry (DTA/TG), and impedance spectroscopy, respectively. Lattice parameters and crystalline size of δ‐phase of Yb₂O₃‐ and Dy₂O₃‐doped Bi₂O₃ samples were calculated from the X‐ray diffraction data. Surface and grain properties of the related phases were determined by SEM analysis. In the investigated system, the maximum electrical conductivity was observed as σ=0.954 S cm^?1 for 6% mol Yb₂O₃ and 6% mol Dy₂O₃ at 800°C among all δ‐YbDSB systems. Cathode supported electrochemical cell was fabricated and 6Yb6DSB was used as the electrolyte. Maximum power density of single cell with an active area of 1.5 cm² is 72.50 mW/cm² at 700°C.     

固体氧化物燃料电池阳极材料Sr_2Fe_(1.5)Mo_(0.5)O_6(SFM)/Sm_(0.2)Ce_(0.8)O_(1.9)(SDC)的研究

采用机械混合法合成了Sr2Fe1.5Mo0.5O6(SFM)和Sm0.2Ce0.8O1.9(SDC)质量比为7∶3的SFM/SDC复合材料。用X射线衍射(XRD)、扫描电镜(SEM)、H2-TPR、EIS等表征手段对其进行了表征,并以SFM/SDC|La0.8Sr0.2Ga0.83Mg0.17O3(LSGM)|Ba0.5Sr0.5Co0.8Fe0.2O3(BSCF)为单电池片进行电化学测试,对其性能进行评价。结果表明,复合材料取得了较好的放电性能,即以氢气为燃料气,850、800、750℃时分别取得了630.6、548.4、426 mW/cm2最大功率密度;以甲醇为燃料,850、800、750℃时分别取得了551.6、426.8、335.3 mW/cm2最大功率密度。     

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.     

Ionic conductivity and relaxations of In-doped GDC (gadolinium doped ceria) ceramics

The effect of In doping on the sintering behaviors and electrical properties of Gd_0.1Ce_0.9O_1.95 (Gd-doped ceria, or GDC) was investigated. The solubility limit of In in GDC was determined to be ~2 at%, and the lattice parameter of GDC was found to decrease from 5.417(7) Å to 5.416(5) Å with 2 at% In dopant. The mean grain size of the sintered body decreased with increasing In content. The concentration of In did not significantly affect the conductivity of the samples; however, undoped GDC showed the highest conductivity. Cole-Cole plots showed that the activation energies of the grain boundaries and grain interiors decreased and increased, respectively, as the In concentration increased to 1 at%. The decreased grain-boundary activation energy is attributed to the segregation of the negatively charged dopant at the grain boundaries, while the increased activation energy of the grain interiors is attributed to the decreases in both the lattice parameters and binding energies with In doping.     

Orientation Effects on the High-Temperature Morphological Evolution of Pore Channels in Sapphire

Arrays of semi-infinite and of controlled-aspect-ratio pore channels, both of controlled orientation, were introduced into undoped basal-plane sapphire substrates, using microfabrication techniques, ion-beam etching, and hot pressing. The breakup of these channels via Rayleigh instabilities during a series of annealings at a temperature of 1700°C was monitored. In all cases, the channels broke up with a characteristic wavelength (λ) that was much larger than that expected for a material with isotropic surface energy, which reflected stabilization effects that were due to anisotropy of the surface energy. The break-up wavelength also was very dependent on orientation: channels that were oriented along the [11¯00] and [112¯0] directions yielded the smallest and largest pore spacings (λ-values), respectively, which is in qualitative agreement with prior observations. The critical (minimum) aspect ratio for the breakup of finite-length channels into multiple pores also is dependent upon channel orientation, and the trend mirrors that observed for semi-infinite channels. The pattern of channel evolution suggested two-fold rotational symmetry within the basal plane, where, because of the nature of the experiment, apparent six-fold symmetry is expected. Several factors that may contribute to or cause an apparent or real loss of symmetry have been discussed.     

Modeling of Chemical Vapor Infiltration Rate Considering a Pore Size Distribution

To analyze chemical vapor infiltration (CVI) rates, a simulation model (PC model) was proposed, which is capable of considering difficulties of densification in the internal regions of porous bodies and the pore closure phenomena. In the model, a fictitious tapered pore is constructed based on the pore size distribution of the preform body, and variation of the pore shape during the process is calculated. For simulation study, fictitious tapered pores were constructed using different pore size distribution functions of Gaussian, δ-function, and flat distribution, and their shapes and characteristics were compared. In the case of the Gaussian flat distribution (large dispersion), the fraction of fine pores was large and these pores became closed near their mouths. Reconversion of temporal fictitious pore shape into the pore distribution function was attempted and the PC model was successfully applied to monitor variations in the pore size distributions during the CVI process.     

La(Sr)FeO3 SOFC Cathodes with Marginal Copper Doping

(La_0.8Sr_0.2)_0.98Fe_0.98Cu_0.02O_3−δ can be sintered directly onto YSZ (without the need for a protective ceria interlayer). Though subject to an extended "burn-in" period (∼200 h), anode-supported YSZ cells using the Cu-doped LSF achieve power densities ranging from 1.3 to 1.7 W/cm² at 750°C and 0.7 V. These cells have also demonstrated 500 h of stable performance. The results are somewhat surprising given that XRD indicates an interaction between (La_0.8Sr_0.2)_0.98Fe_0.98-Cu_0.02O_3−δ and YSZ resulting in the formation of strontium zirconate and/or monoclinic zirconia. The amount and type of reaction product was found to be dependent on cathode and electrolyte powder precalcination temperatures.     

操作条件对固体氧化物燃料电池阳极反应转变的影响

研究了不同电流密度下,甲烷浓度、反应温度对甲烷在SOFC中反应由部分氧化到完全氧化转变的规律;测量了不同电流密度下,阳极出口气体产生速率;确定了甲烷浓度和电池反应温度变化时甲烷电化学反应由部分氧化转变为完全氧化的电流密度门槛值,及该门槛值与甲烷浓度、电池操作温度的变化关系.结果说明甲烷开始发生完全氧化的电流密度门槛值与甲烷浓度成正比;甲烷浓度一定,温度升高,甲烷开始发生完全氧化的电流密度的门槛值也随之提高.     

Highly promoted performance of triple-conducting cathode for YSZ-based SOFC via fluorine anion doping

One of the most important factors to commercialized yttria-stabilized zirconia (YSZ)-based solid oxide fuel cell (SOFC) is a highly active mixed-conducting cathode at reduced temperatures. Herein, we propose a new strategy of fluorine anion (F⁻) doping to enhance electrochemical performance of the H⁺/O²⁻/e triple-conducting BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY) perovskite cathode for YSZ-based SOFCs. The F⁻-doped BCFZY as BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_2.95-δF_0.05 (BCFZYF) retained the cubic structure with better symmetry. The doping of minor fluorine in oxygen-site was found to increase the oxygen exchange capability and thus oxygen reduction reaction (ORR) catalytical activity, as reflected by lower area specific resistance (ASR) of cathode on symmetrical cells. Maximum power density of 786 mWcm⁻² was achieved at 800 °C for anode-supported single cell with BCFZYF cathode, being 1.7 times higher than that with BCFZY cathode. Furthermore, the single cell with BCFZYF cathode demonstrated excellent stability without any degradation of current density over 200 h at 700 °C. The present work clarifies that the fluorine doping strategy is highly effective to promote the ORR activity of the triple-conducting BCFZY cathode for state-of-the-art oxygen-conducting SOFC.     

Investigations of ice‐structuring agents in ice‐templated ceramics

The freeze‐cast ceramics process involves the use of an aqueous slurry in which ice acts as a fugitive template, providing control of the morphology of the resultant green body, which is then sintered. Here we report the role of various additives and procedures in tailoring the morphology of ice‐templated, freeze‐cast alumina ceramics. We show that zirconium acetate (ZrAc) in the slurry can change the structure from lamellar to columnar, whereas polyvinyl alcohol can act as both a binder and as a modifier of the ice surface, and can compete with ZrAc to provide cooperative effects and a wide range of porosities. We also show that the ceramic structure can be influenced by epitaxial growth on an oriented layer of ice.     

SOFC单相氧化物MIEC阳极材料的研究进展

综述了近年来国内外固体氧化物燃料电池复合导电性能(MIEC)单相氧化物阳极材料的研究进展。阐述了钙钛矿结构、立方萤石结构以及其它诸如烧绿石结构的单相氧化物阳极材料的导电性能以及应用于电池阳极的发电性能。指出了阳极材料研究亟待解决的问题及发展方向。     

A novel concept for improved thermal management of the planar SOFC

A new design for the solid oxide fuel cell (SOFC) planar stack is proposed to minimise the thermal gradients in the cell. This design involves including a secondary air channel with flow in the counter direction to the cathodic air channel. The effectiveness of the new design is tested by means of a tank in series reactor (TSR) model of the SOFC. It is found that the new design is capable of reducing the steady state temperature difference across the cell to less than 2 K over a range of voltages, while satisfying the requirements on fuel utilisation (FU) and cell average temperature. This is achieved by manipulating the primary air channel inlet flow rate and the secondary air channel inlet temperature. More modelling and experimental studies are required to further investigate the proposed design.     

Alternative concept for SOFC with direct internal reforming operation: Benefits from inserting catalyst rod

Mathematical models of direct internal reforming solid oxide fuel cell (DIR‐SOFC) fueled by methane are developed using COMSOL® software. The benefits of inserting Ni‐catalyst rod in the middle of tubular‐SOFC are simulated and compared to conventional DIR‐SOFC. It reveals that DIR‐SOFC with inserted catalyst provides smoother temperature gradient along the system and gains higher power density and electrochemical efficiency with less carbon deposition. Sensitivity analyses are performed. By increasing inlet fuel flow rate, the temperature gradient and power density improve, but less electrical efficiency with higher carbon deposition is predicted. The feed with low inlet steam/carbon ratio enhances good system performances but also results in high potential for carbon formation; this gains great benefit of DIR‐SOFC with inserted catalyst because the rate of carbon deposition is remarkably low. Compared between counter‐ and co‐flow patterns, the latter provides smoother temperature distribution with higher efficiency; thus, it is the better option for practical applications. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Fabrication and operation of flow‐through tubular SOFCs for electric power and synthesis gas cogeneration from methane

Flow‐through type tubular solid oxide fuel cells were successfully fabricated and operated with a single‐chamber configuration for realizing the simultaneous generation of electric power and synthesis gas from methane by integrating a downstream catalyst into the fuel cell reactor. A new operation mode, which completely eliminated the gas diffusion between cathode side and anode side, is proposed. The cell showed high open‐circuit voltages of 1.02–1.08 V at the furnace temperature range of 650–800°C when operating on CH₄‐O₂ gas mixture at a molar ratio of 2:1. A peak power density of approximately 300 mW cm^?2 and a maximum power output of 1.5 W were achieved for a single cell with an effective cathode geometric surface area of 5.4 cm² at the furnace temperature of 750°C. The in‐situ initialization of the cell using CH₄‐O₂ gas mixture was also realized via applying an effective catalyst into the tubular cell. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1036–1044, 2014     

Structure and conductivity of yttria and scandia‐doped zirconia crystals grown by skull melting

In this paper a detailed study of the (ZrO₂)_1‐x(Y₂O₃)_x (x=0.025–0.15), (ZrO₂)_1‐x(Sc₂O₃)_x (x = 0.06 – 0.11) and (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y (x=0.07 – 0.11; y=0.01 – 0.04) solid solution crystals grown by skull melting technique is presented. The structure, phase composition, and ion conductivity of the obtained crystals were investigated by X‐ray diffraction, transmission electron microscopy, Raman scattering spectroscopy, and impedance spectroscopy. Maximum conductivity as (ZrO₂)_1‐x(Y₂O₃)_x and (ZrO₂)_1‐x(Sc₂O₃)_x solid solution crystals is observed for the compositions containing 10 mol% stabilizing oxide, and the conductivity of 10ScSZ is ~3 times higher than for 10YSZ. Experiments on crystal growth (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y solid solutions showed that uniform, transparent crystals 7Sc3YSZ, 7Sc4YSZ, 8Sc2YSZ, 8Sc3YSZ, 9Sc2YSZ, 9Sc3YSZ, 10Sc1YSZ, and 10Sc2YSZ are single phase crystal containing t″ phase. It is established that a necessary condition of melt growth of (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y single‐phase crystals is the total concentration of the stabilizing oxides from 10 to 12 mol%. The addition of Y₂O₃ affects the (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y solid solution conductivity different ways and depends on the Sc₂O₃ content in the starting composition. The effects of structure, phase composition, concentration, and type of stabilizing oxides on the electrical characteristics of obtained crystals are discussed.     

Critical analysis of aqueous tape casting,sintering, and characterization of planar Yttria‐Stabilized Zirconia electrolytes for SOFC

Tape casting is an established forming technique for several industries, however, researches focus more on slurry composition. In this work, the combined use of design of experiment and materials characterization techniques showed tape casting process parameters have great influence on the microstructure and mechanical properties of green tapes. Formulation and processing optimization allowed obtaining YSZ green tapes with good mechanical characteristics and homogeneous microstructure without laminating step. The optimized sintering schedule and sintering load allowed obtaining planar electrolytes with high density, tensile strength, and electrical conductivity. This work provides an environmental friendly procedure for large‐scale production of SOFCs planar electrolytes.     

Preparation of Chamottes as a Raw Material for Low‐Cost Ceramic Membranes

Low‐cost ceramic membranes are usually prepared from a mixture of natural raw materials and some organic porogen agent such as starch. The fact that porogen must be completely eliminated during firing, leaving an interconnected porous structure, impose large firing times, increasing the final price. A study about the synthesis of porous chamottes as an alternative to organic pore formers was conducted to reduce firing costs. Chamottes were obtained from mixtures of clay and starch. Different starches were used and the influence of the composition and processing variables were studied. The viability of the porous chamottes was demonstrated.