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.     

Characterization of a Cu‐La0.75Sr0.25Cr0.5Mn0.5O3 ‐CeO2/La0.75Sr0.25Cr0.5Mn0.5O3‐YSZ/Ni‐ScSZ Three‐Layer Structure Anode in Thin Film Solid Oxide Fuel Cell Running on Methane Fuel

Anodes for Solid Oxide Fuel Cell that is capable of directly using hydrocarbon without external reforming have been of great interest recently. In this paper, a three‐layer structure anode running on methane is fabricated by tape casting and screen printing method. The slurry of catalyst layer Cu‐LSCM‐CeO₂ (with weight ratios of 1.5:7.0:1.5, 2.0:7.0:1.0, 2.15:7.0:0.85 and 2.25:7.0:0.75, weight ratios of Cu/CeO₂ is 1:1, 2:1, 2.5:1 and 3:1, respectively) is screen‐printed on LSCM‐YSZ support layer and Ni‐ScSZ active layer. Thus, LSCM‐YSZ/Ni‐ScSZ anodes with Cu‐LSCM‐CeO₂ catalyst layer (denoted as LSCM‐YSZ1010, LSCM‐YSZ2010, LSCM‐YSZ2510 and LSCM‐YSZ3010, respectively) are obtained. Single cells with three‐layer structure anode are also fabricated and measured, of which the maximum power density reaches 491 and 670 mW cm⁻² for the cell with LSCM‐YSZ2510 anode running on methane at 750 °C and 800 °C, respectively. No significant degradation in performance has been observed after 240h of cell testing when LSCM‐YSZ2510 anode is exposed to methane at 750 °C. Very little carbon deposition is detected on the anode, suggesting that carbon deposition is limited during cell operation. Consequently, Cu‐LSCM‐CeO₂ catalyst layer on the surface of LSCM‐YSZ support layer makes it possible to have good stability for long‐term operation in methane due to very little carbon deposition.     

Study of Carbon Deposition Behavior on Cu–Co/CeO2–YSZ Anodes for Direct Butane Solid Oxide Fuel Cells

Electro‐catalytic activity of Cu–Co/CeO₂–YSZ anodes towards oxidation of H₂ and n‐C₄H₁₀ fuels and carbon depositions are investigated using different Cu–Co loadings. Cu–Co/CeO₂–YSZ anode based SOFCs with YSZ as electrolyte and LSM/YSZ as cathode were prepared by tape casting and wet impregnation methods and performance was analyzed using I–V characteristics and impedance spectroscopy. The Cu–Co/CeO₂–YSZ anodes with Cu–Co loading of 10, 15, and 25 wt.% produced power density of 60, 197, and 400 mW cm^–2 in H₂ and 190, 225, and 275 mW cm^–2 in n‐C₄H₁₀ at 800 °C. The power density is increased with the increase in Cu–Co loading in Cu–Co/CeO₂–YSZ anodes. The electrochemical impedance spectra shows less ohmic and polarization resistance for 25 wt.% Cu–Co loading in comparison to 10 and 15 wt.% Cu–Co. Scanning electron microscopy and high resolution transmission electron microscopy shows that the carbon fibers formed are hollow in nature with 70 nm size, whereas, thermal gravimetric analysis and X‐ray diffraction points out that they are amorphous in nature. The performance degradation of Cu–Co/CeO₂–YSZ anodes in n‐C₄H₁₀ in 16 h is attributed to increasing amount of carbon deposition with time, which is contrary to our earlier observation in Cu‐Fe/CeO₂–YSZ anode.     

Evaluation of a Cu/YSZ and Ni/YSZ Bilayer Anode for the Direct Utilization of Methane in a Solid‐Oxide Fuel Cell

Carbon deposition is an issue when operating solid oxide fuel cells (SOFC) on fuels other than hydrogen, and so a variety of strategies have been used to prevent carbon accumulation on the anodes. In this paper, we describe a bilayer anode that contains a functional layer consisting of Ni/YSZ and a conduction layer consisting of Cu/YSZ. The anode‐supported button cells were fabricated using a uni‐axially pressing technique to produce the anode, followed by impregnation with Cu. The cells were tested at 1,023 K in dry CH₄ and their performance compared to that of a typical Ni/YSZ anode. The Cu does not catalyze the cracking of methane and as such less carbon deposits in the conduction layer resulting in anode stability for over 100 h. The limitation with using Cu in the anode is the temperature of operation.     

Comparison Between Anode‐Supported and Electrolyte‐Supported Ni‐CGO‐LSCF Micro‐tubular Solid Oxide Fuel Cells

Two types of micro‐tubular hollow fiber SOFCs (MT‐HF‐SOFCs) were prepared using phase inversion and sintering; electrolyte‐supported, based on highly asymmetric Ce_0.9Gd_0.1O_1.95(CGO) HFs and anode‐supported based on co‐extruded NiO‐CGO(CGO)/CGO HFs. Electroless plating was used to deposit Ni onto the inner surfaces of the electrolyte‐supported MT‐HF‐SOFCs to form Ni‐CGO anodes. LSCF‐CGO cathodes were deposited on the outer surface of both these MT‐HF‐SOFCs before their electrochemical performances were compared at similar operating conditions. The performance of the anode‐supported MT‐HF‐SOFCs which delivered ca. 480 mW cm^–2 at 600 °C was superior to the electrolyte‐supported MT‐HF‐SOFCs which delivered ca. six times lower power. The contribution of ohmic and electrode polarization losses of both FCs was investigated using electrochemical impedance spectroscopy. The electrolyte‐supported MT‐HF‐SOFCs had significantly higher ohmic and electrode polarization ASR values; this has been attributed to the thicker electrolyte and the difficulties associated with forming quality anodes inside the small (<1 mm) lumen of the electrolyte tubes. Further development on co‐extruded anode‐supported MT‐HF‐SOFCs led to the fabrication of a thinner electrolyte layer and improved electrode microstructures which delivered a world leading 2,400 mW cm^–2. The newly made cell was investigated at different H₂ flow rates and the effect of fuel utilization on current densities was analyzed.     

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.     

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.     

Optimisation and Evaluation of La0.6Sr0.4CoO3 – δ Cathode for Intermediate Temperature Solid Oxide Fuel Cells

In this work, La_0.6Sr_0.4CoO_{3 – δ}/Ce_{1 – x}Gd_xO_{2 – δ} (LSC/GDC) composite cathodes are investigated for SOFC application at intermediate temperatures, especially below 700 °C. The symmetrical cells are prepared by spraying LSC/GDC composite cathodes on a GDC tape, and the lowest polarisation resistance (R_p) of 0.11 Ω cm² at 700 °C is obtained for the cathode containing 30 wt.‐% GDC. For the application on YSZ electrolyte, symmetrical LSC cathodes are fabricated on a YSZ tape coated on a GDC interlayer. The impact of the sintering temperature on the microstructure and electrochemical properties is investigated. The optimum temperature is determined to be 950 °C; the corresponding R_p of 0.24 Ω cm² at 600 °C and 0.06 Ω cm² at 700 °C are achieved, respectively. An YSZ‐based anode‐supported solid oxide fuel cell is fabricated by employing LSC/GDC composite cathode sintered at 950 °C. The cell with an active electrode area of 4 × 4 cm² exhibits the maximum power density of 0.42 W cm^–2 at 650 °C and 0.54 W cm^–2 at 700 °C. More than 300 h operating at 650 °C is carried out for an estimate of performance and degradation of a single cell. Despite a decline at the beginning, the stable performance during the later term suggests a potential application.     

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.     

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     

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.     

A Preliminary Study on WO3‐Infiltrated W–Cu–ScYSZ Anodes for Low Temperature Solid Oxide Fuel Cells

Preparation and electrochemical characterization of WO₃‐infiltrated 0.48W–0.52Cu–ScYSZ (WCS) anode for solid oxide fuel cell are reported. The DC conductivity of a WO₃ ceramic was 1,200 and 24 S cm^–1 in reducing and oxidizing atmospheres, respectively, at 650 °C. WCS porous backbones in the form of symmetric cells were prepared by screen printing of WO₃–CuO–ScYSZ ink and subsequent sintering at 1,300 °C for 1 h in 9% H₂/N₂. Analysis of the sintered backbone by X‐ray diffraction showed the metallic W and Cu phases. Precursor solutions of WO₃ or CuO were infiltrated into porous WCS backbones to form the anode. The electrochemical performance of these anodes measured by impedance spectroscopy showed polarization resistances of 11 and 6.5 Ω cm² for WO₃ and CuO infiltrated anodes, respectively, at 600 °C in humidified hydrogen. Activation energy values of 86.8 and 96.5 kJ mol^–1 were obtained for WO₃ and CuO infiltrated WCS anodes, respectively. The microstructure of the tested anodes showed well‐dispersed sub‐micron particles of WO₃ in the WCS backbone whereas CuO infiltration resulted in a dense microstructure.     

Low Temperature Electrode Materials Synthesized by Citrate Precursor Method for Solid Oxide Fuel Cells

Homogeneous nanocrystalline NiO–Ce_0.9Ln_0.1O_2–δ (Ln = La, Sm, Gd, and Pr) composite anode and nanocrystalline Ce_0.9Gd_0.1O_2–δ electrolyte material have been successfully synthesized by citrate precursor method. LSCF has been synthesized by conventional solid state method and used as cathode material in our studies. The synthesized powders have been characterized by powder X‐ray diffraction, microscopy, and surface area studies. The average crystallite size of the anode materials has been found to be in the range of 5–15 nm by HRTEM. Highly dense electrolyte and porous electrode materials have been observed by FESEM and confirmed by BET surface area studies. Three cells have been fabricated successfully. Based on the performance of the three cells containing three different anode materials we have achieved better electrochemical characteristics in Ni–GDC with maximum power density of 302 mW cm^–2 and open circuit voltage of 1.012 V at 500 °C. The difference in the performance of the cells containing Ni–GDC as compared to Ni–LDC and Ni–SDC anode is due to changes in the microstructure and crystallite size of anode which affects the electrochemical performance of the cells. The performances of all the cells containing nanocomposite powders are suitable anode materials for low temperature SOFCs.     

Preliminary Study on Direct Operation of LT‐SOFCs Based on GDC Electrolyte Film With DME Fuel

Direct operation of anode‐supported cone‐shaped tubular low temperature solid oxide fuel cells (LT‐SOFCs) based on gadolinia‐doped ceria (GDC) electrolyte film with dimethyl ether (DME) fuel was preliminarily investigated in this study. The single cell exhibited maximum power densities of 500 and 350 mW cm^–2 at 600 °C using moist hydrogen and DME as fuel, respectively. A durability test of the single NiO‐GDC/GDC/LSCF‐GDC cell was performed at a constant current of 0.1 A directly fuelled with DME for about 200 min at 600 °C. The results indicate that the single cell coking easily directly operated in DME fuel. EDX result shows a clear evidence of carbon deposition in the anode. Further studies are needed to develop the novel anti‐carbon anode materials, relate the carbon deposition with anode microstructure and cell‐operating condition.     

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.     

New Stack Design of Micro‐tubular SOFCs for Portable Power Sources

Micro‐tubular solid oxide fuel cells (SOFCs) have high thermal stability and higher volumetric power density, which are considered to be ideal features for portable power sources and auxiliary power units for automobile. Here, we report a new stack design using anode supported micro‐tubular SOFCs with 2 mm diameter using Gd doped CeO₂ (GDC) electrolyte, NiO‐GDC anode and (La, Sr)(Co, Fe)O₃ (LSCF)‐GDC cathode. The new stack consists of three bundles with five tubular cells, sealing layers and interconnects and fuel manifolds. The performance of the stack whose volume is 1 cm³ was shown to be 2.8 V OCV and maximum power output of 1.5 W at 500 °C, applying air only by natural convection. The results also showed strong dependence of the fuel flow rates on the stack performance, which was correlated to the gas diffusion limitation.     

Influence of Deposition Temperature of GDC Interlayer Deposited by RF Magnetron Sputtering on Anode‐Supported SOFC

Gadolinia‐doped ceria (GDC) film, as a barrier layer to prevent chemical reaction between yttria‐stabilized zirconia (YSZ) electrolyte and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ (BSCF)–GDC cathode, is deposited by radio frequency (RF) magnetron sputtering on YSZ electrolyte, and the influence of deposition temperature on Ni–YSZ/YSZ/GDC/BSCF–GDC single cell performance is investigated. The SEM results show that the GDC film deposited at 30 °C exhibits a porous structure, whereas the GDC film deposited at 400 °C has a dense structure. The single cells show excellent performance when the deposition temperature is above 250 °C, whereas the single cells show poor performance when the deposition temperature is below 200 °C. The large difference in cell performance occurs from their large difference in polarization resistance. The porous structure of GDC interlayer, which cannot well prevent the reaction between BSCF and YSZ, is responsible for the poor performance of the cells with GDC interlayer deposited at a temperature below 200 °C.     

Integration of Spin‐Coated Nanoparticulate‐Based La0.6Sr0.4CoO3–δ Cathodes into Micro‐Solid Oxide Fuel Cell Membranes

Thin cathodes for micro‐solid oxide fuel cells (micro‐SOFCs) are fabricated by spin‐coating a suspension of La_0.6Sr_0.4CoO_3–δ (LSC) nanoparticulates obtained by salt‐assisted spray pyrolysis. The resulting 250 nm thin LSC layers exhibit a three‐dimensional porous microstructure with a grain size of around 45 nm and can be integrated onto free‐standing 3 mol.% yttria‐stabilized‐zirconia (3YSZ) electrolyte membranes with high survival rates. Weakly buckled micro‐SOFC membranes enable a homogeneous distribution of the LSC dispersion on the electrolyte, whereas the steep slopes of strongly buckled membranes do not allow for a perfect LSC coverage. A micro‐SOFC membrane consisting of an LSC cathode on a weakly buckled 3YSZ electrolyte and a sputtered Pt anode has an open‐circuit voltage of 1.05 V and delivers a maximum power density of 12 mW cm^–2 at 500 °C.     

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.     

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.