Tape Casting of Anode Supports for Solid Oxide Fuel Cells at Forschungszentrum Jülich

This contribution describes the development of tape casting for solid oxide fuel cells (SOFCs) anode supports starting with the characterization of the powders and ending with manufacturing of cells for stack testing. After casting the support, full cells were prepared by screen printing and sintering of the functional layers. The results of single‐cell and stack tests of the novel SOFC will be discussed. The new cell showed excellent electrochemical performance in single‐cell tests with more than 1.5 A/cm² (800°C, 0.7 V). Furthermore, stack tests showed no significant difference from earlier standard cells when operated at 800°C with a current density of 0.5 A/cm².     

Five-layer reverse tape casting of IT-SOFC

Tape casting is a well-established method for manufacturing thin ceramic layers with controllable thickness and porosity. This study investigates the potential of 10Sc1CeSZ material for the electrolyte and anode layers for intermediate-temperature solid oxide fuel cells (IT-SOFC) in an anode-supported cell (ASC) geometry. In order to use La_0.6Sr_0.4Co_0.2Fe_0.8 Oxide (LSCF) cathode material, a Gd_0.2Ce_0.8 Oxide (GDC) barrier layer is needed; however, thermal expansion coefficient mismatch results in delamination of the GDC from the electrolyte during high temperature sintering when fabricated by conventional tape casting procedures. For the first time, ASCs have been manufactured by a five-layer tape casting technique; barrier layer, novel composite layer, electrolyte, anode functional layer, and anode substrate. Ni-ScCeSZ composite cells were tested between 650 and 800°C in H₂:N₂ fuel (85% H₂) on the anode and air on the cathode to yield a maximum power density of .46 W/cm². These results demonstrate the feasibility of this new five-layer tape casting technique to produce IT-SOFC.     

Analysis of Design Parameters in Anode‐Supported Solid Oxide Fuel Cells Using Response Surface Methodology

Achieving high performance from a solid oxide fuel cell (SOFC) requires optimal design based on parametric analysis. In this paper, design parameters, including anode support porosity, thicknesses of electrolyte, anode support, and cathode functional layers of a single, intermediate temperature, anode‐supported planar SOFC, are analyzed. The response surface methodology (RSM) technique based on an artificial neural network (ANN) model is used. The effects of the cell parameters on its performance are calculated to determine the significant design factors and interaction effects. The obtained optimum parameters are adopted to manufacture the single units of an SOFC through tape casting and screen‐printing processes. The cell is tested and its electrochemical characteristics, which show a satisfactory performance, are discussed. The measured maximum power density (MPD) of the fabricated SOFC displays a promising performance of 1.39 W cm^–2. The manufacturing process planned to fabricate the SOFC can be used for industrial production purposes.     

Influence of doctor blade gap on the properties of tape cast NiO/YSZ anode supports for solid oxide fuel cells

In this study, various tape cast NiO/YSZ anode support layers with similar geometric properties are fabricated by varying the doctor blade from 100?µm to 200?µm with an increment of 25?µm. The mechanical properties of the anode support layers are investigated by three point bending tests of 30 samples for each doctor blade gap. The reliability curves of the flexural strength data are also obtained via two-parameter Weibull distribution method. The effects of the doctor blade gap on the microstructure and the electrochemical performance of the anode support layers are determined via SEM investigations and single cell performance-impedance tests, respectively. The apparent porosities of the samples are also measured by Archimedes’ principle. The results indicate that the doctor blade gap or the resultant tape thickness influences the microstructure of tape cast NiO/YSZ anode supports significantly, yielding different mechanical and electrochemical characteristics. At a reliability level of 70%, the highest flexural strength of 110.20?MPa is obtained from the anode support layer with a doctor blade gap of 175?µm and the 16?cm² active area cell with this anode support layer also exhibits the highest peak performance of 0.483?W/cm² at an operating temperature of 800?°C. Thus, a doctor blade gap of 175?µm is found to have such a microstructure that provides not only better mechanical strength but also higher electrochemical performance.     

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.     

Scaling up aqueous processing of A-site deficient strontium titanate for SOFC anode supports

All ceramic anode supported half cells of technically relevant scale were fabricated in this study, using a novel strontium titanate anode material. The use of this material would be highly advantageous in solid oxide fuel cells due to its redox tolerance and resistance to coking and sulphur poisoning. Successful fabrication was possible through aqueous tape casting of both anode support and electrolyte layers and subsequent lamination. Screen printing of electrolyte layers onto green anode tapes was also attempted but resulted in cracked electrolyte layers upon firing. Microstructural, electrical and mechanical properties of anode supports and half cells will be discussed. The use of two different commercial titanate powders with nominal identical, but in reality different stoichiometries, strongly affect electrical and mechanical properties. Careful consideration of such variations between powder suppliers, and batches of the same supplier, is critical for the successful implementation of ceramic anode supported solid oxide fuel cells.     

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.     

Single‐step fabrication of an anode supported planar single‐chamber solid oxide fuel cell

We present single‐step‐co‐sintering manufacture of a planar single‐chamber solid oxide fuel cell (SC‐SOFC) with porous multilayer structures consisting of NiO/CGO, CGO and CGO‐LSCF as anode, electrolyte, and cathode, respectively. Their green tapes were casted with 20 μm thickness and stacked into layers of anode, electrolyte, and cathode (10:2:2), then hot‐pressed at 2 MPa and 60°C for 5 minutes (deemed optimal). Subsequently, hot laminated layers were cut into 40 × 40 mm cells and co‐sintered up to 1200°C via different sintering profiles. Shrinkage behavior and curvature developments of cells were characterized, determining the best sintering profile. Hence, anode‐supported SC‐SOFCs were fabricated via a single‐step co‐sintering process, albeit with curvature formation at edges. Subsequently, anode thickness was increased to 800 μm and electrolyte reduced to 20 μm to obtain SOFCs with drastically reduced curvature with the help of a porous alumina cover plate.     

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.     

Preparation of BaZr0.1Ce0.7Y0.2O3–δ Based Solid Oxide Fuel Cells with Anode Functional Layers by Tape Casting

Solid oxide fuel cells (SOFCs) based on the proton conducting BaZr_0.1Ce_0.7Y_0.2O_3–δ (BZCY) electrolyte were prepared and tested in 500–700 °C using humidified H₂ as fuel (100 cm³ min^–1 with 3% H₂O) and dry O₂ (50 cm³ min^–1) as oxidant. Thin NiO‐BZCY anode functional layers (AFL) with 0, 5, 10 and 15 wt.% carbon pore former were inserted between the NiO‐BZCY anode and BZCY electrolyte to enhance the cell performance. The anode/AFL/BZCY half cells were prepared by tape casting and co‐sintering (1,300 °C/8 h), while the Sm_0.5Sr_0.5CoO_3–δ (SSC) cathodes were prepared by thermal spray deposition. Well adhered planar SOFCs were obtained and the test results indicated that the SOFC with an AFL containing 10 wt.% pore former content showed the best performance: area specific resistance as low as 0.39 Ω cm² and peak power density as high as 0.863 W cm^–2 were obtained at 700 °C. High open circuit voltages ranging from 1.00 to 1.12 V in 700–500 °C also indicated negligible leakage of fuel gas through the electrolyte.     

Strength of Anode‐Supported Solid Oxide Fuel Cells

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

The rapid one-step fabrication of bilayer anode for protonic ceramic fuel cells by phase inversion tape casting

The bilayer anode fabricated by phase inversion tape casting has an excellent microstructure for protonic ceramic fuel cell compared with the dry pressing method. The large diameter and straight hole structure facilitates the fuel gas transportation thus eliminates the concentration polarization loss. But a dense skin layer (70 μm) results in a power density of only 150 mWcm⁻² at 600 ℃. The anode added with 10 wt% corn starch could eliminate the skin layer, but cause a mismatch between electrolyte suspension and anode. To resolve the mismatch between electrolyte and anode, an anode functional layer (AFL) is employed with different content of corn starch (10−40 wt%). Finally, the single cell with optimized bilayer anode has a maximum power density of 574 mWcm⁻² at 650 ℃. This work provides an easy and rapid method for the preparation of planar protonic ceramic fuel cell with satisfactory performance at intermediate temperature range.     

Optimization of anode structure for intermediate temperature solid oxide fuel cell via phase‐inversion cotape casting

A functional layer and a porous support that together constitute an anode for a solid oxide fuel cell were simultaneously formed by the phase‐inversion tape casting method. Two slurries, one composed of NiO and yttria‐stabilized zirconia (YSZ) powders and the other of NiO, YSZ, and graphite were cocasted and solidified by immersion in a water bath via the phase‐inversion mechanism. The as‐formed green tape consisted of a sponge‐like thin layer and a fingerlike thick porous layer, derived from the first slurry and the second slurry, respectively. The former acted as the anode functional layer (AFL), while the latter was used as the anode substrate. The AFL thickness was varied between 20 and 60 μm by adjusting the blade gap for the tape casting. Single cells based on such NiO‐YSZ anodes were prepared with thin YSZ electrolytes and YSZ‐(La_0.8Sr_0.2)_0.95MnO_3?δ (LSM) cathodes, and their electrochemical performance was measured using air as oxidant and hydrogen as fuel. The maximum power densities obtained at 750°C were 720, 821, and 988 mW cm^?2 with the AFL thickness at 60, 40, and 20 μm, respectively. The satisfactory electrochemical performance was attributed to the dual‐layer structure of the anode, where the sponge‐like AFL layer provided plenty of triple‐phase boundaries for hydrogen oxidation, and the fingerlike thick porous substrate allowed for facile fuel transport. The phase‐inversion tape casting developed in this study is applicable to the preparation of other planar ceramic electrodes with dual‐layer asymmetric structure.     

Planar Solid Oxide Fuel Cells Using PSZ,Processed by Sequential Aqueous Tape Casting and Constrained Sintering

A process for obtaining planar anode‐supported solid oxide fuel cells was developed. Aqueous‐based slurries were prepared and sequentially deposited via tape casting to form half cell tapes consisting of the electrolyte, functional, and structural anode. Sintering of the three‐layered tapes was done in two stages: presintering circular samples of 25 mm diameter in free conditions first, and then sintering them using zirconium disks as light loads (90 Pa), to obtain half cells having 20 mm and 3.8 m^?1 in diameter and curvature, respectively. Active materials for the electrolyte, anode, and cathode were partially stabilized zirconia (PSZ), Ni and LSM, respectively. Finally, thicknesses of complete cells were 400, 30, 30, and 80 μm for the structural anode, functional anode, electrolyte, and cathode, respectively. The cells were tested in a no‐chamber (direct‐flame) setup evaluating electrochemical performance and shock thermal resistance. Open circuit voltage was 830 mV at 560°C using methanol as fuel in a burner with porous media to modify the shape of the flame. Cells were also strong enough to resist the rapid temperature changes during several no‐chamber tests.     

Layered YSZ/SCSZ/YSZ Electrolytes for Intermediate Temperature SOFC Part I: Design and Manufacturing

(Sc₂O₃)_0.1(CeO₂)_0.01(ZrO₂)_0.89 (SCSZ) ceramic electrolyte has superior ionic conductivity in the intermediate temperature range (700–800 °C), but it does not exhibit good phase and chemical stability in comparison with 8 mol% Y₂O₃–ZrO₂ (YSZ). To maintain high ionic conductivity and improve the stability in the whole electrolyte, layered structures with YSZ outer layers and SCSZ inner layers were designed. Because of a mismatch of coefficients of thermal expansion and Young's moduli of SCSZ and YSZ phases, upon cooling of the electrolytes after sintering, thermal residual stresses will arise, leading to a possible strengthening of the layered composite and, therefore, an increase in the reliability of the electrolyte. Laminated electrolytes with three, four, and six layers design were manufactured using tape‐casting, lamination, and sintering techniques. After sintering, while the thickness of YSZ outer layers remained constant at ∼30 μm, the thickness of the SCSZ inner layer varied from ∼30 μm for a Y–SC–Y three‐layered electrolyte, ∼60 μm for a Y–2SC–Y four‐layered electrolyte, and ∼120 μm for a Y–4SC–Y six‐layered electrolyte. The microstructure, crystal structure, impurities present, and the density of the sintered electrolytes were characterized by scanning and transmission electron microscopy, X‐ray and neutron diffraction, secondary ion mass spectroscopy, and water immersion techniques.     

Fabrication of 80 mm diameter-sized solid oxide fuel cells using a water-based NiO–YSZ slurry

Solid oxide fuel cells (SOFCs) are very attractive for their high energy conversion efficiency and low emissions. Generally, a supported layer of SOFCs is fabricated by tape casting, using an organic solvent. Recently, a slurry based on water instead of an organic solvent has been sought in order to avoid environmental pollution. In this study, the anode of SOFCs was fabricated by aqueous tape casting, and the electrolyte and the cathode were deposited by screen printing. The I–V characteristics of the cell thus obtained were evaluated. As a result, an 80 mm diameter-sized cell with a power density of 0.33 W/cm² at 800 °C was successfully fabricated by controlling sintering conditions.     

The Influence of Temperature and Composition on the Operation of Al Anodes for Aluminum‐Air Batteries

The influence of composition and temperature on the anode polarization and corrosion rate of pure Al and Al‐In anodic alloys in 8M NaON electrolyte has been investigated. High current density (more than 800 mA cm⁻²) and faradaic efficiency over 97% were observed for all investigated alloys at 60 °C. Lower temperature provides lower current density (200–300 mA cm⁻² at 40 °C, and less than 100 mA cm⁻² at 25 °C). Different formation of the product reaction layers was observed for pure aluminum and Al–0.41In alloy, leading to the different polarization character of the samples. The comparison of two Al‐In alloys with similar composition has been carried out. Al–0.45In alloy having a coarse‐grained structure had a more positive no‐current potential and lower value of anode limiting current (200 mA cm⁻² vs. 300 mA cm⁻²) compared with the fine‐grained Al–0.41In alloy, as well as greater parasitic corrosion rate and greater no‐current corrosion. The current‐voltage, power and discharge characteristics of the aluminum‐air cell with Al–0.41In anode and gas diffusion cathode have been investigated. Open circuit voltage of the cell is 1.934 V and the maximum power density of the cell is 240 mW cm⁻² at the voltage of 1.3 V.     

Porous asymmetric microfiltration membranes shaped by combined alumina freeze and tape casting

An assembled asymmetric alumina microfiltration membrane with high performance was prepared by combining freeze and tape casting techniques followed by two sintering steps. Freeze casting was used for manufacturing of the porous support layer with a highly interconnected pore network. Tape casting was applied on the top layer to form a pre-membrane with smaller pore size and controlled thickness, which was set on the sintered support. Morphology influences were investigated for different solid loadings, additives content and the assembled layer membrane structures. No delamination among the layers was observed. The assembled ceramic membrane had an average pore size between 30 and 50 μm together with a top surface layer around 0.35 μm, which is suitable to the microfiltration separation process. Porosity in the range of 26–50 % and water flux of 11–32 m³ m^?2 h^?1 bar^?1 were reached for samples prepared with two sintering steps at 1600 and 1300 °C for 2 h.     

A comparison of the tape casting of α- and β-alumina

This work concerns the manufacture of planar cell configurations for molten sodium batteries in energy storage devices such as vehicle batteries and stationary storage cells. Tape casting of beta-alumina electrolyte could provide a low-cost mass production route but intriguingly there are only a few reports of tape casting using beta-alumina directly as the raw powder. We first compared tape casting of α-alumina and beta-alumina using polyvinyl butyral (PVB) in conventional formulations. While it is relatively easy to obtain homogeneous α-alumina tape cast sheet, beta-alumina resulted in adhesion to the substrate and cracking. These problems were shown to be attributable to particle characteristics. When the binder was changed to polymethylmethacrylate (PMMA), tape casting of beta-alumina and three different α-alumina powders was facilitated, producing a formulation that was more tolerant to different powder types. Screening of several commercial dispersants provided two which were effective with PMMA and a conventional MEK/ethanol dual solvent.