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.     

Co-extrusion of multilayered ceramic micro-tubes for use as solid oxide fuel cells

Current methods to manufacture tubular solid oxide fuel cells (SOFCs) involve multiple steps of extrusion, layer deposition and sintering, leading to high manufacturing costs. The aim of the work presented in this paper is to reduce the cost of manufacturing SOFCs. This is achieved by developing a method for manufacturing a five-layered micro-tubular structure by a multi-billet co-extrusion process. With the implementation of continuous screw extrusion equipment, this co-extrusion process could easily be adapted into a fully continuous manufacturing process.The co-extrusion process presented initially involves rheologically unifying five pastes made up of individual powder compositions. It is shown that it is possible to formulate the pastes with in an optimum solids loading region where the die land rheological properties are relatively insensitive to small variations in solids loading, thus allowing for a more stable process.These pastes are then extruded as billets from separate extrusion barrels through a single nozzle. This uses a novel die design which does not require the use of a central mandrel to form the tubular structure. The sintered structure comprises four Ni/YSZ anode layers and a YSZ electrolyte layer, each layer being approximately 60 μm thick, forming a tube with an outer diameter of 3 mm and an inner diameter of 2.4 mm.     

Cofiring of Thin Zirconia Films During SOFC Manufacturing

High-performance solid oxide fuel cells require a thin and gas-tight electrolyte membrane that must be coated on a porous and relatively rough support. A pretreatment of the delivered submicronmeter electrolyte powder of 8 mol%-yttria-stabilized zirconia (8YSZ) yielded a reduced sintering mismatch between the anode substrate made from NiO/8YSZ and the electrolyte coating. Furthermore, it also enhanced the powder packing inside the green film. Constrained sintering usually leads to inadequate film density and an unfavorable pore deformation and orientation. It was demonstrated that these limitations can be resolved by using a coshrinking substrate in a planar cell design. Relative densities of >97% were achieved, which are higher than those for free-standing layers. Additionally, the camber behavior was investigated in dependence of the temperature program with and without gravity effects, giving an overall suggestion for the cofiring parameters of the electrolyte.     

Tailoring the electron-blocking layer by addition of YSZ to Ba-containing anode for improvement of ceria-based solid oxide fuel cell

The in-situ fabrication of an electron-blocking layer between the Ba-containing anode and the ceria-based electrolyte is an effective approach in suppressing the internal electronic leakage in ceria-based solid oxide fuel cell (SOFC). To improve the thickness of the electron-blocking layer and to research the effect of the layer thickness on the improvement of SOFC, a Ba-containing compound (0.6NiO-0.4BaZr_0.1Ce_0.7Y_0.2O_3-δ) modified by Y stabilized zirconia (YSZ) was employed as a composite anode in this research. SEM analyses demonstrated that the thickness of the interlayer can be simply controlled by regulating the proportion of YSZ at anode. The in-situ formed interlayer in the cell with the anode modified by 20?mol% YSZ possesses a thickness of 0.9?µm which is more suitable for the cell achieving an enhanced performance.     

Influences of feedstock and plasma spraying parameters on the fabrication of tubular solid oxide fuel cell anodes

This study developed a tubular solid oxide fuel cell (SOFC) anode support layer via atmospheric plasma spraying, which is considered one of the most promising methods for producing SOFCs because of its faster deposition rate and lower cost compared with other film formation processes. Plasma spraying can replace the traditional use of extrusion technology to manufacture the anode base tube, eliminating the need for high-temperature sintering steps. In this study, commercially available powders were used to make the anode of a tubular SOFC from NiO/yttria-stabilized zirconia (YSZ) powder, and Na₂CO₃ and polymethyl methacrylate were tested as pore-forming agents. The anode composite powder was sprayed on the graphite base pipe, and the final product was changed by altering the spraying parameters and anode powder ratio. The direct current (DC) resistance measurements showed that the conductivity of the Ni/YSZ tubular anode formed with higher power plasma spraying could reach 428.55?S/cm at 800?°C. The experimental results showed that the power and parameters of atmospheric plasma spraying could affect the porosity and electron conductivity of tubular SOFC anodes.     

Effect of Nickel Oxide/Yttria-Stabilized Zirconia Anode Precursor Sintering Temperature on the Properties of Solid Oxide Fuel Cells

An NiO/yttria-stabilized zirconia (YSZ) layer sintered at temperatures between 1100° and 1500°C onto dense YSZ electrolyte foils forms the precursor structure for a porous Ni/YSZ cermet anode for solid oxide fuel cells. Conflicting requirements for the electrochemical performance and mechanical strength of such cells are investigated. A minimum polarization resistance of 0.09 Ω^.cm²at 1000°C in moist hydrogen is obtained for sintering temperatures of 1300°–1400°C. The mechanical strength of the cells decreases with increased sintering temperature because of the formation of channel cracks in the electrode layers, originating in a thermal expansion coefficient mismatch between the layers.     

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.     

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.     

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.     

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.     

Mechanical behaviour of multi-layer half-cells of microtubular solid oxide fuel cells fabricated by the co-extrusion process

Multi-layer micro-tubes consisting of four anode layers of NiO and YSZ mixture, and an electrolyte layer, YSZ, were fabricated by co-extrusion. The die designed in this study is able to extrude 5 layers around a sacrificial core, which eliminates the difficulties from the use of mandrel in processing tubes. Scanning electron microscopy (SEM) results revealed that this technique can be used for the successful fabrication of multi-layer microtubes with good bonding between the layers. 3-point bending was used to evaluate the mechanical properties of these co-extruded multi-layer samples. Moreover, thermal shock resistance of the tubes was investigated by water quenching from an elevated temperature. The results were compared with those obtained for conventionally extruded single layer samples. The co-extruded samples were found to have the highest average strength and also the highest weibull modulus and reliability. It was also found that multi-layer anode can significantly improve thermal shock resistance.     

Thin film yttria-stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition

A 500 nm thick thin film YSZ (yttria-stabilized zirconia) electrolyte was successfully fabricated on a conventionally processed anode substrate by spin coating of chemical solution containing slow-sintering YSZ nanoparticles with the particle size of 20 nm and subsequent sintering at 1100 °C. Incorporation of YSZ nanoparticles was effective for suppressing the differential densification of ultrafine precursor powder by mitigating the prevailing bi-axial constraining stress of the rigid substrate with numerous local multi-axial stress fields around them. In particular, adding 5 vol% YSZ nanoparticles resulted in a dense and uniform thin film electrolyte with narrow grain size distribution, and fine residual pores in isolated state. The thin film YSZ electrolyte placed on a rigid anode substrate with the GDC (gadolinia-doped ceria) and LSC (La_0.6Sr_0.4CoO_3?δ) layers deposited by PLD (pulsed laser deposition) processes revealed that it had fairly good gas tightness relevant to a SOFC (solid oxide fuel cell) electrolyte and maintained its structural integrity during fabrication and operation processes. In fact, the open circuit voltage was 1.07 V and maximum power density was 425 mW/cm² at 600 °C, which demonstrates that the chemical solution route can be a viable means for reducing electrolyte thickness for low- to intermediate-temperature SOFCs.     

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.     

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.     

Assembly of 8YSZ nanoparticles into gas-tight 1-2 μm thick 8YSZ electrolyte layers using wet coating methods

The application of a thin film electrolyte layer with a thickness in the micrometer range could greatly improve current solid oxide fuel cells (SOFCs) in terms of operating temperature and power output. Since the achievable minimal layer thickness with conventional powder coating methods is limited to ∼5 μm, a variety of thin film methods have been studied, but results on regular large-scale anode substrates are still lacking in the literature. In this paper, a wet coating process is presented for fabricating gas-tight 1-2 μm thick 8YSZ electrolyte layers on a regular NiO/8YSZ substrate, with a rough surface, a high porosity and a large pore size. These layers were deposited in a similar way as conventional suspension based layers, but the essential difference includes the use of coating liquids (nano-dispersion, sol) with a considerably smaller particle size (85 nm, 60 nm, 35 nm, 6 nm). Successful deposition of such layers was accomplished by means of an innovative coating process, which involves the preparation of a hybrid polyvinyl alcohol/8YSZ membrane by dip-coating or spin-coating and subsequently burning out the polymer part at 500 °C. Results from He leak tests confirmed that the sintered layers posses a very low number of defects and with values in the range 10⁻⁴-10⁻⁶ (hPa dm³)/(s cm²) the gas-tightness of the thin film layers is satisfactory for fuel cell operation. Moreover, preliminary results have also indicated a potential reduction of the sintering temperature from 1400 °C to the range 1200-1300 °C, using the presented coating process.     

Fabrication and Characterization of Thin and Dense Electrolyte-Coated Anode Tube Using Thermoplastic Coextrusion

Anode tubes coated with thin and dense electrolyte layers were fabricated by thermoplastic coextrusion, using a nickel oxide (NiO)–yttria-stabilized zirconia (YSZ) composite and YSZ as the anode and electrolyte materials, respectively. The initial feedrod with a diameter of 22 mm, comprised of three thermoplastic compounds (carbon black (core), NiO–YSZ (intermediate), and YSZ (shell)), was coextruded through various orifices having diameters of 0.5, 1, and 2 mm at 120°C, producing continuous filaments with remarkable size reductions, while preserving the cross section of the initial feedrod. After removing the binder and carbon black used as the core material, the samples were cofired at 1350°C for 3 h in air, producing ceramic tubes with small diameters, ranging from 0.4 to 1.8 mm, where the surfaces of porous NiO–YSZ layers were coated with thin and dense YSZ layers, ranging from 22 to 98 μm.     

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.     

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.     

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.     

Development and Characterization of Porous Composites for Solid Oxide Fuel Cell Anode Conduction Layers Using Ceramic-Filled Highly Porous Ni Foam

Porous Ni-yttria stabilized zirconia (YSZ) composites are the most common materials used for solid oxide fuel cell anodes. In conventional anodes, percolation of the Ni phase for acceptable conductivity requires relatively high Ni contents (i.e., >35 vol% of solids) which can reduce cell reliability due to increased coefficient of thermal expansion (CTE) mismatch with the YSZ electrolyte and damage produced by redox cycling. In this study, the incorporation of highly porous Ni foam into an anode structure was investigated in order to produce an anode conduction layer with high conductivity values at lower Ni volumes. This was done by developing techniques for pasting various YSZ based slurries into a Ni foam structure followed by sintering. The electrical conductivity, dimensional stability, and CTE of these structures were measured as a function of Ni, YSZ, and porous volume. Sintered anodes made with Ni-foam exhibited a superior combination of conductivity and CTE compared with conventional anode structures. For example, a Ni foam–YSZ composite with a nickel loading of 13 vol% has a CTE of 10.7 × 10⁻⁶ K⁻¹, but with a similar electrical conductivity to a conventional anode (∼3000 S/cm) which requires at least 30 vol% Ni resulting in a CTE >13 × 10⁻⁶ K⁻¹. Alteration of the paste composition produced a porous composite with engineered porosity and good dimensional stability.