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.     

Electrochemical performance and microstructure characterization of nickel yttrium‐stabilized zirconia anode

A nickel and yttrium‐stabilized zirconia (Ni‐YSZ) composite is one of the most commonly used anode materials in solid oxide fuel cells (SOFCs). One of the drawbacks of the Ni‐YSZ anode is its susceptibility to deactivation due to the formation of carbonaceous species when hydrocarbons are used as fuel supplies. We therefore initiated an electrochemical study of the influence of methane (CH₄) on the performance of Ni‐YSZ anodes by examining the kinetics of the oxidation of CH₄ and H₂ over operating temperatures of 600–800°C. Anode performance deterioration was then correlated with the degree of carbonization observed on the anode using ex‐situ X‐ray powder diffraction and scanning electron microscopy techniques. Results showed that carbonaceous species led to a significant deactivation of Ni‐YSZ anode toward methane oxidation. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

NI/NI‐YSZ Current Collector/Anode Dual Layer Hollow Fibers for Micro‐Tubular Solid Oxide Fuel Cells

A co‐extrusion technique was employed to fabricate a novel dual layer NiO/NiO‐YSZ hollow fiber (HF) precursor which was then co‐sintered at 1,400 °C and reduced at 700 °C to form, respectively, a meshed porous inner Ni current collector and outer Ni‐YSZ anode layers for SOFC applications. The inner thin and highly porous “mesh‐like” pure Ni layer of approximately 50 μm in thickness functions as a current collector in micro‐tubular solid oxide fuel cell (SOFC), aiming at highly efficient current collection with low fuel diffusion resistance, while the thicker outer Ni‐YSZ layer of 260 μm acts as an anode, providing also major mechanical strength to the dual‐layer HF. Achieved morphology consisted of short finger‐like voids originating from the inner lumen of the HF, and a sponge‐like structure filling most of the Ni‐YSZ anode layer, which is considered to be suitable macrostructure for anode SOFC system. The electrical conductivity of the meshed porous inner Ni layer is measured to be 77.5 × 10⁵ S m^–1. This result is significantly higher than previous reported results on single layer Ni‐YSZ HFs, which performs not only as a catalyst for the oxidation reaction, but also as a current collector. These results highlight the advantages of this novel dual‐layer HF design as a new and highly efficient way of collecting current from the lumen of micro‐tubular SOFC.     

Development of a novel proton conducting fuel cell based on a Ni‐YSZ support

A new proton conducting fuel cell design based on the BZCYYb electrolyte is studied in this research. In high‐performance YSZ‐based SOFCs, the Ni‐YSZ support plays a key role in providing required electrical properties and robust mechanical behavior. In this study, this well‐established Ni‐YSZ support is used to maintain the proton conducting fuel cell integrity. The cell is in a Ni‐YSZ (375 μm support)/Ni‐BZCYYb (20 μm anode functional layer)/BZCYYb (10 μm electrolyte)/LSCF‐BZCYYb (25 μm cathode) configuration. Maximum power density values of 166, 218, and 285 mW/cm² have been obtained at 600°C, 650°C, and 700°C, respectively. AC impedance spectroscopy results show values of 2.17, 1.23, and 0.76 Ω·cm² at these temperatures where the main resistance contributor above 600°C is ohmic resistance. Very fine NiO and YSZ powders were used to achieve a suitable sintering shrinkage which can enhance the electrolyte sintering. During cosintering of the support and BZCYYb electrolyte layers, the higher shrinkage of the support layer led to compressive stress in the electrolyte, thereby enhancing its densification. The promising results of the current study show that a new generation of proton conducting fuel cells based on the chemically and mechanically robust Ni‐YSZ support can be developed which can improve long‐term performance and reduce fabrication costs of proton conducting fuel cells.     

Long‐Term Stability Studies of Anode‐Supported Microtubular Solid Oxide Fuel Cells

A long‐term stability study of an anode‐supported NiO/YSZ‐YSZ‐LSM/YSZ microtubular cell was performed, under low fuel utilization conditions, using pure humidified hydrogen as fuel at the anode side and air at the cathode side. A first galvanometric test was performed at 766 °C and 200 mA cm^–2, measuring a power output at 0.5 V of ∼250 mW cm^–2. During the test, some electrical contact breakdowns at the anode current collector caused sudden current shutdowns and start‐up events. In spite of this, the cell performance remains unchanged. After a period of 325 h, the cell temperature and the current density was raised to 873°C and 500 mA cm^–2, and the cell power output at 0.5 V was ∼600 mW cm^–2. Several partial reoxidation events due to disturbance in fuel supply occurred, but no apparent degradation was observed. On the contrary, a small increase in the cell output power of about 4%/1,000 h after 654 h under current load was obtained. The excellent cell aging behavior is discussed in connection to cell configuration. Finally, the experiment concluded when the cell suffered irreversible damage due to an accidental interruption of fuel supply, causing a full reoxidation of the anode support and cracking of the thin YSZ electrolyte.     

Redox Tolerance of Thin and Thick Ni/YSZ Anodes of Electrolyte‐Supported Single‐Chamber Solid Oxide Fuel Cells under Methane Oxidation Conditions

Redox tolerance of 50 and 500 μm thick Ni/YSZ (yttria‐stabilized zirconia) anodes supported on YSZ electrolytes were studied under single‐chamber solid oxide fuel cell conditions. Open circuit voltage, electrochemical impedance spectra, and discharge curves of the cells were measured under different methane/oxygen ratios at 700 °C. For the cell with the thin anode, a significant degradation accompanying oscillatory behaviors was observed, whereas the cell based on the thick anode was much more stable under the same conditions. In situ local anode resistance (R_s) results indicated that the Ni/NiO redox cycling was responsible for the oscillatory behaviors, and the cell degradation was primarily caused by the Ni reoxidation. Reoxidation of the thick anode took place at a low methane/oxygen ratio, but the anode can be recovered to its original state by switching to a methane‐rich environment. On the contrary, the thin anode was unable to be regenerated after the oxidation. Microstructure damage of the anode was attributed to its irreversible degradation.     

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.     

Fabrication and Power Densities of Anode‐Supported Solid Oxide Fuel Cells with Different Ni‐YSZ Anode Functional Layer Thicknesses

We investigated an appropriate preparation condition for anode‐supported SOFCs: (La,Sr)MnO₃/cathode functional layer/YSZ/Ni‐YSZ were fabricated with and without a Ni‐YSZ anode functional layer (AFL) via the tape‐casting method, where the AFL thicknesses were controlled from approximately 20 to 80 μm. The warpage depended on the co‐sintering temperature of the electrolyte/AFL/anode‐support half‐cells, indicating that similar shrinkage of the electrolyte/AFL/anode support is significant for lower warpages. The electrical properties of SOFCs with AFLs were compared to those of SOFCs without AFLs. In this regard, the use of an AFL decreased the ohmic and activation polarization resistances due to both the decrease in contact resistance between the electrolyte and the AFL and the increase in three‐phase boundaries. However, the polarization diffusion increased when an AFL was employed, because AFL layers are denser than the anode support. The maximum power densities of samples with AFL were higher than those of SOFCs without AFLs, indicating that the decrease in both ohmic and activation‐polarization resistances is more significant for improving the power densities, as compared to the concentration polarization resistance.     

Effects of Anode Microstructure on Mechanical and Electrochemical Properties for Anode‐Supported Microtubular Solid Oxide Fuel Cells

The effects of anode microstructure on mechanical and electrochemical properties were investigated for anode‐supported microtubular solid oxide fuel cells (SOFCs). The anode microstructures can be varied by the change in pore formers. For example, the acrylic resin pore former was burnt more rapidly at lower temperature than the graphite pore former during sintering. The acrylic resin pore former can introduce macropores with a diameter of several micrometers in nickel–yttria‐stabilized zirconia (Ni–YSZ) anode. The walls of the macropores were packed with the nickel and YSZ particles. Although the Ni–YSZ anode microtube using the 10 wt% acrylic resin pore former was compatible with high porosity and mechanical strength, the maximum fuel utilization was limited to 72%. On the other hand, the graphite pore former can produce a relatively uniform distribution of micropores with a diameter of several hundred nanometers. The mechanical strength was reduced with a rise in porosity for the Ni–YSZ microtube using the graphite pore former in comparison with the acrylic resin. However, a high fuel utilization of 93% was realized for the microtubular SOFCs using the 10 wt% graphite pore former in spite of lower porosity than the acrylic resin. The selection of a pore former is important to obtain higher power generation efficiency for anode‐supported microtubular SOFCs.     

Fabrication and Characterization of Anode‐Supported Low‐Temperature SOFC Based on Gd‐Doped Ceria Electrolyte

Large‐size, 9.5 cm × 9.5 cm, Ni‐Gd_0.1Ce_0.9O_1.95 (Ni‐GDC) anode‐supported solid oxide fuel cell (SOFC) has been successfully fabricated with NiO‐GDC anode substrate prepared by tape casting method and thin‐film GDC electrolyte fabricated by screen‐printing method. Influence of the sintering shrinkage behavior of NiO‐GDC anode substrate on the densification of thin GDC electrolyte film and on the flatness of the co‐sintered electrolyte/anode bi‐layer was studied. The increase in the pore‐former content in the anode substrate improved the densification of GDC electrolyte film. Pre‐sintering temperature of the anode substrate was optimized to obtain a homogeneous electrolyte film, significantly reducing the mismatch between the electrolyte and anode substrate and improving the electrolyte quality. Dense GDC electrolyte film and flat electrolyte/anode bi‐layer can be fabricated by adding 10 wt.% of pore‐former into the composite anode and pre‐sintering it at 1,100 °C for 2 h. Composite cathode, La_0.6Sr_0.4Fe_0.8Co_0.2O₃, and GDC (LSCF‐GDC), was screen‐printed on the as‐prepared electrolyte surface and sintered to form a complete single cell. The maximum power density of the single cell reached 497 mW cm^–2 at 600 °C and 953 mW cm^–2 at 650 °C with hydrogen as fuel and air as oxidant.     

Planar Metal‐Supported SOFC with Novel Cermet Anode

Metal‐supported solid oxide fuel cells are expected to offer several potential advantages over conventional anode (Ni‐YSZ) supported cells. For example, increased resistance against mechanical and thermal stresses and a reduction in material costs. When Ni‐YSZ based anodes are used in metal supported SOFC, elements from the active anode layer may inter‐diffuse with the metallic support during sintering. This work illustrates how the inter‐diffusion problem can be circumvented by using an alternative anode design based on porous and electronically conducting layers, into which electrocatalytically active materials are infiltrated after sintering. The paper presents the electrochemical performance and durability of the novel planar metal‐supported SOFC design. The electrode performance on symmetrical cells has also been evaluated. The novel cell and anode design shows a promising performance and durability at a broad range of temperatures and is especially suitable for intermediate temperature operation at around 650 °C.     

Curvature Reversal and Residual Stress in Solid Oxide Fuel Cell Induced by Chemical Shrinkage and Expansion

Structural stability of layered functional ceramic composites is challenged by curvature effects and residual stresses caused by the thermal mismatch and chemical strains. In this study, a phenomenon of curvature reversal is found in the half‐cell structure of solid oxide fuel cell (SOFC) during the reduction of the half‐cell from NiO‐YSZ to Ni‐YSZ. An analytical model is derived to study the curvature and residual stress caused by the chemical shrinkage and expansion of anode. With reducing to Ni‐YSZ, the curvature of the half‐cell changes from the initial direction to an opposite direction, then back to the initial direction. This curvature reversal is inevitable during reduction while the thickness ratio of electrolyte to anode is between 0 and 0.102. The residual stress in electrolyte, calculated by the analytical model, is well agreement with the experiment result using X‐ray stress analysis. The YSZ layer is always subjected to compressive stress in despite of curvature reversal existing in half‐cell. It is impossible to get the residual stress by measuring the curvature unless the half‐cell was reduced completely.     

Improved Electrodes/Electrolyte Interfaces for Solid Oxide Fuel Cell by Using Dual‐Sized Powders in Electrolyte Slurry

The dense electrolyte film with the rough surfaces for solid oxide fuel cell (SOFC) was fabricated on NiO/yttria‐stabilized zirconia (YSZ) anode substrate by using dual‐sized YSZ powders without additional effort to roughen electrolyte film. The dual‐sized YSZ powders consisted of the fine YSZ powder and the coarse YSZ powder at different weight ratios. Incorporation of the coarse YSZ powder into the fine YSZ powder is in order to increase the surface roughness of electrolyte film, and the surface roughness obviously increased with the increase of coarse YSZ powder. The rough surfaces resulted in an enlargement of the electrochemical active area. It was found that electrode polarization was reduced evidently and cell electrochemical performance was enhanced, as the surface roughness increased. However, the excessive coarse YSZ powder was not beneficial for densification of electrolyte film and thus the open‐circuit voltage (OCV) was declined. The cell with 17 wt.% coarse YSZ powder in the electrolyte exhibited the best performance and the maximum power density was 1,930 mW cm^–2 at 800 °C.     

Modeling and Parametric Study of a Single Solid Oxide Fuel Cell by Finite Element Method

A 2D isothermal axisymmetric model of an anode‐supported solid oxide fuel cell has been developed. The model, which is based on finite element approach, comprises electronic and ionic charge balance, Butler–Volmer charge transfer kinetic, flow distribution and gas phase mass balance in both channels and porous electrodes. The model has been validated using available experimental data coming from a single anode‐supported cell comprising Ni–YSZ/YSZ/LSM–YSZ as anode, electrolyte and cathode, respectively. Hydrogen and steam were used as fuel inlet and air as an oxidant. The validation has been carried out at 1 atm, 1,500 ml min^–1 air flow rate and three different operating conditions of temperature and fuel flow rate: 1,073 K and 400 ml min^–1, 1,073 K and 500 ml min^–1, and 1,003 K and 400 ml min^–1. The polarization and power density versus current density curves show a good agreement with the experimental data. A parametric analysis has been carried out to highlight which parameters have main effect on the overall cell performance as measured by polarization curve, especially focusing on the influence of two geometrical characteristics, temperature and some effective material properties.     

An Investigation on Dip‐Coating Technique for Fabricating Anode‐Supported Solid Oxide Fuel Cells

To cut down the energy consumption and processing period, Ni‐YSZ anode‐supported thin YSZ electrolyte SOFCs have been fabricated by an improved dip‐coating technique, in which the electrolyte layer was dip‐coated on green anode substrates instead of prefired ones, followed by co‐firing. The film formation mechanisms of dip‐coating were analyzed with liquid entrainment and capillary effect assumptions. According to the mechanisms, a typical YSZ electrolyte slurry formula of conventional dip‐coating technique was modified for the improved technique by increasing the binder content and the solid loading. With the improved dip‐coating technique, along with an optimized electrolyte slurry formula, a SOFC with dense electrolyte, revealing a maximum power density of 460 mW/cm² at 800°C, was obtained. Factors affecting the coating layers were also investigated by SEM and AC impedance analyses.     

Nickel–Iron Anode Substrate for Smart Solid Oxide Fuel Cells with a Self‐Protecting Function Against Reoxidation

Development of highly reliable solid oxide fuel cells (SOFCs) is strongly requested, and the introduction of a self‐protecting function is an ideal approach to increase the reliability of SOFCs. A highly porous (>33%) Ni–Fe metal substrate, which has well‐developed nanopores, is prepared by reduction of NiO–Fe₂O₃. In an oxidizing atmosphere, a thin layer of Fe₂O₃ forms on the surface of the substrate. As a result, the porous morphology changes at the surface and becomes denser. This morphological change occurs only at the surface and prevents oxidation of Ni in the bulk of the substrate. Furthermore, the surface morphology returns to its original state following reduction. Therefore, despite the fact that Ni is readily oxidized, Ni metal phase is sustained in the Ni–Fe bimetallic alloy substrate even after 480 h oxidation in air. The cell power density is also stably sustained after a few reduction–reoxidation cycles. Here, we report that Ni–Fe bimetal alloy substrate exhibits a self‐protecting function against reoxidation of the substrate, which would otherwise lead to a permanent failure of the cell.     

Development and Fabrication of a New Concept Planar‐tubular Solid Oxide Fuel Cell (PT‐SOFC)

The paper reports a new concept of planar‐tubular solid oxide fuel cell (PT‐SOFC). Emphasis is on the fabrication of the required complex configuration of Ni‐yttria‐stabilised zirconia (YSZ) porous anode support by tert‐butyl alcohol (TBA) based gelcasting, particularly the effects of solid loading, amounts of monomers and dispersant on the rheological behaviour of suspension, the shrinkage of a wet gelcast green body upon drying, and the properties of final sample after sintering at 1350 °C and reduction from NiO‐YSZ to Ni‐YSZ. The results show that the gelcasting is a powerful method for preparation of the required complex configuration anode support. The anode support resulted from an optimised suspension with the solid loading of 25 vol% has uniform microstructure with 37% porosity, bending strength of 44 MPa and conductivity of 300 S cm^—1 at 700 °C, meeting the requirements for an anode support of SOFC. Based on the as‐prepared anode support, PT‐SOFC single cell of Ni‐YSZ/YSZ/LSCF has been fabricated by slurry coating and co‐sintering technique. The cell peak power density reaches 63, 106 and 141 mW cm^—2 at 700, 750 and 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.     

Infiltration of SOFC Stacks: Evaluation of the Electrochemical Performance Enhancement and the Underlying Changes in the Microstructure

Experimental SOFC stacks with 10 SOFCs (LSM‐YSZ/YSZ/Ni‐YSZ) were infiltrated with CGO and Ni‐CGO on the air and fuel side, respectively in an attempt to counter degradation and improve the output. The electrochemical performance of each cell was characterized (i) before infiltration, (ii) after infiltration on the cathode side, and (iii) after the infiltration of the anode side. A significant performance enhancement was observed after the infiltration with CGO on the cathode, while the infiltration of the anode side with Ni‐CGO had no significant effect on the electrochemical performance. After testing the cells were characterized by SEM and TEM/EELS. A thin layer of CGO nanoparticles around the LSM‐YSZ back bone structure was found after infiltration. On the anode side nano sized Ni particles were found embedded in a CGO layer formed around the Ni‐YSZ structure. EELS analysis showed that the oxidation state of the Ce ions is identical on the air and the fuel side.     

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).     

Performance of mix‐impregnated CeO2‐Ni/YSZ Anodes for Direct Oxidation of Methane in Solid Oxide Fuel Cells

CeO₂‐Ni/YSZ anodes for methane direct oxidation were prepared by the vacuum mix‐impregnation method. By this method, NiO and CeO₂ are obtained from nitrate decomposition and high temperature sintering is avoided, which is different from the preparation of conventional Ni‐yttria‐stabilised zirconia(YSZ) anodes. Impregnating CeO₂ into the anode can improve the cell performance, especially, when CH₄ is used as fuel_. The investigation indicated that CeO₂‐Ni/YSZ anodes calcined at higher temperature exhibited better stability than those calcined at lower temperature. Under the testing temperature of 1,073 K, the anode calcined at 1,073 K exhibited the best performance. The maximum power density of a cell with a 10 wt.‐%CeO₂‐25 wt.‐%Ni anode calcined at 1,073 K reached 480 mW cm^–2 after running on CH₄ for 5 h. At the same time, high discharge current favoured cell operation on CH₄ when using these anodes. No obvious carbon was found on the CeO₂‐Ni anode after testing in CH₄ as revealed from SEM and corresponding linear EDS analysis. In addition, cell performance decreased at the beginning of discharge testing which was attributed to the anode microstructure change observed with SEM.