Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells

A comparison of three solid oxide electrolyte fabrication processes, namely dip coating, screen printing and tape casting, for planar anode supported solid oxide fuel cells (SOFCs) is presented in this study. The effect of sintering temperature (1325–1400 °C) is also examined. The anode and cathode layers of the anode-supported cells, on the other hand, are fabricated by tape casting and screen printing, respectively. The quality of the electrolytes is evaluated via performance measurements, impedance analyses and microstructural investigations of the cells. It is found that the density of the electrolyte increases with the sintering temperatures for all fabrication methods studied. The results also show that with the process and fabrication parameters considered in this study, both dip coating and screen printing do not yield a desired dense electrolyte structure as proven by open circuit potentials measured and SEM photos. The cells with tape cast electrolytes, on the other hand, provide the highest performances regardless of the electrolyte sintering and cell operating temperatures. The best peak performance of 0.924 W/cm² is obtained from the cell with tape cast electrolyte sintered at 1400 °C. SEM investigations and measured open circuit potentials reveal that almost fully dense electrolyte layer can be obtained with a tape cast electrolyte particularly sintered at temperatures higher than 1350 °C. Impedance analyses indicate that the main reason behind the significantly higher performances is due to not only increased electrolyte density but a decrease in the interface resistance of the anode functional and electrolyte layer is also responsible. This can be explained by the load applied during the lamination step in the fabrication of the tape cast electrolyte, providing better powder compaction and adhesion.     

Fabrication of multi-layered solid oxide fuel cells using a sheet joining process

Ceria-based electrolytes can be used in solid oxide fuel cells (SOFCs) that operate at intermediate-temperature due to their high ionic conductivity. However, the difficulty in fabricating a thin and dense ceria-based electrolyte on an anode support is well-known. In this study, a new sheet joining process is suggested to produce a thin and dense ceria-based electrolyte for anode-supported SOFCs. The main idea used with the sheet joining process is a combination of a sheet cell fabricated by tape casting, and an anode pellet, fabricated by pressing. The maximum power density of a single cell, fabricated by the sheet joining process, is 0.315 W cm⁻² at 600 °C in a power generation test when Pr_0.3Sr_0.7Co_0.3Fe_0.7O_3−δ was used as the cathode material. The performance durability of a single cell is tested over 1000 h of operation in which a dense electrolyte was observed to survive.     

Anode supported micro-tubular SOFC fabricated with mixed particle size electrolyte via phase-inversion technique

Cerium-gadolinium oxide is a promising material for electrolytes of intermediate temperature solid oxide fuel cells (IT-SOFCs) due to its high electrical conductivity at relatively lower temperatures of 400–700 °C. However, a high sintering temperature of up to 1550 °C is typically required to produce dense CGO electrolyte, eventually leading to an interfacial interdiffusion between the electrolyte and electrode components as well as generate a highly resistive interface which reduces ionic conductivity. Lowering the sintering temperature of the electrolyte will greatly benefit the fabrication of SOFCs. This study examines the effectiveness of introducing nano size CGO particles as an approach to get dense CGO electrolyte at lower sintering temperature. A series of dope suspensions with 0–50% nano size loading were prepared to observe rheology and measure viscosity. Then, 30% loading was selected and casting into flat sheet via phase-inversion technique. The flat sheet was characterized by morphology, surface roughness and mechanical strength tests. The suspension was extruded into dual-layer hollow fiber (DLHF) as well. The electrolyte/anode dual-layer hollow fibers (DLHFs) half-cell of micro-tubular solid oxide fuel cells (MT-SOFCs) were prepared via phase inversion based co-extrusion/co-sintering technique. The developed half-cell was characterized by morphological and gas tightness tests which further compared them with fully micron ones. The results show that the incorporation of 30% nanoparticle yielded to dense and tight CGO layers sintered at temperature 1450 °C, which about 50 °C lower than those reported previously for 100% micron particles. The I–V measurements demonstrated the maximum power density of 0.66 Wcm^?2 at temperatures 500 °C using 100% H₂ as fuel. Therefore, this approach is able to reduce the energy cost for the microstructural control of the prepared fiber and thus is recommended for the fabrication of low-cost dual-layer hollow fiber micro tubular SOFCs.     

Multilayer tape casting of large-scale anode-supported thin-film electrolyte solid oxide fuel cells

Tape casting is conventionally used to prepare individual, relatively thick components (i.e., the anode or electrolyte supporting layer) for solid oxide fuel cells (SOFCs). In this research, a multilayer ceramic structure is prepared by sequentially tape casting ceramic slurries of different compositions onto a Mylar carrier followed by co-sintering at 1400 °C. The resulting half-cells contains a 300 μm thick NiO–yttria-stabilized zirconia (YSZ) anode support, a 20 μm NiO–YSZ anode functional layer, and an 8 μm YSZ electrolyte membrane. Complete SOFCs are obtained after applying a Gd_0.1Ce_0.9O₂ (GDC) barrier layer and a Sm_0.5Sr_0.5CoO₃ (SSC) -GDC cathode by using a wet-slurry spray method. The 50 mm × 50 mm SOFCs produce peak power densities of 337, 554, 772, and 923 mW/cm² at 600, 650, 700, and 750 °C, respectively, on hydrogen fuel. A short stack including four 100 mm × 150 mm cells is assembled and tested. Each stack repeat unit (one cell and one interconnect) generates around 28.5 W of electrical power at a 300 mA/cm² current density and 700 °C.     

Novel fabrication technique of hollow fibre support for micro-tubular solid oxide fuel cells

In this work, a cerium-gadolinium oxide (CGO)/nickel (Ni)-CGO hollow fibre (HF) for micro-tubular solid oxide fuel cells (SOFCs), which consists of a fully gas-tight outer electrolyte layer supported on a porous inner composite anode layer, has been developed via a novel single-step co-extrusion/co-sintering technique, followed by an easy reduction process. After depositing a multi-layers cathode layer and applying current collectors on both anode and cathode, a micro-tubular SOFC is developed with the maximum power densities of 440-1000 W m⁻² at 450-580 °C. Efforts have been made in enhancing the performance of the cell by reducing the co-sintering temperature and improving the cathode layer and current collection from inner (anode) wall. The improved cell produces maximum power densities of 3400-6800 W m⁻² at 550-600 °C, almost fivefold higher than the previous cell. Further improvement has been carried out by reducing thickness of the electrolyte layer. Uniform and defect-free outer electrolyte layer as thin as 10 μm can be achieved when the extrusion rate of the outer layer is controlled. The highest power output of 11,100 W m⁻² is obtained for the cell of 10 μm electrolyte layer at 600 °C. This result further highlights the potential of co-extrusion technique in producing high quality dual-layer HF support for micro-tubular SOFC.     

Achieving high mechanical-strength CH4-based SOFCs by low-temperature sintering (1100 °C)

Despite much progress achieved in the past decades in the process of advancing the low-temperature sintering technologies for Solid oxide fuel cells (SOFCs), such as via the structure design of the electrode materials, the practical application of low-temperature sintered SOFCs (with disqualified mechanical strength) remains challenging. In this work, first, we demonstrate that the appropriate amount of CuO as sintering aids can successfully reduce the co-firing temperature of conventional micron size NiO-YSZ (yttrium-stabilized zirconia (Y₂O₃)_0.08–(ZrO₂)_0.92) anode from about 1400 °C to only 1100 °C. Second, the quantitative structure-activity relationship among the mechanical strength (low-temperature sintering ability) of anode cermets with the inclusion of CuO contents and the densification of YSZ electrolyte was synthetically evaluated, and the optimal Cu–NiO-YSZ anode composition demonstrates almost the equal mechanical strength when compared with the traditional NiO-YSZ anode (sintering at 1400 °C). At last, by comprehensive assessment, 8%Cu–52NiO-40YSZ (8%CuO–NiO-YSZ) shows excellent low-temperature sintering ability, high mechanical strength, optimal power output, and anti-carbon deposition when using as hydrocarbon-based anode for SOFCs.     

An ultra-fast fabrication technique for anode support solid oxide fuel cells by microwave

An effective and facile technique has been developed for high temperature anode-electrolyte co-sintering of anode support solid oxide fuel cells by using microwave activated sparking plasma. A high sintering temperature of 1600 °C can be achieved in a few minutes time by discharging effect. Anode support substrate pellet is uniaxially pressed, and then dip-coated with a 10 μm yttria stabilized zirconia electrolyte layer. After the microwave co-sintering, La_0.8Sr_0.2MnO_x cathode is screen-printed onto electrolyte and sintered by conventional thermal method. The cell has stably operated in 3% humidified hydrogen for more than 130 h.     

Fabrication and properties of anode-supported tubular solid oxide fuel cells

Tubular solid oxide fuel cell (SOFC) systems have many desirable characteristics over their planar counter-parts. Anode-supported tubes provide an excellent platform for individual cells. They allow for a thin electrolyte layer to be applied to the outside of the tube, which helps to minimize polarization losses. This paper describes the fabrication of nickel–zirconia (Ni–YSZ)-based anode tubes via extrusion of a plastic mass through a die of the required dimensions. The anode tubes were then dried and fired. Tests were performed on the tubes to determine the effects of firing temperature on porosity to allow for a pinhole-free electrolyte coating to be applied. Thin layer coating techniques, including vacuum-assisted dip coating and painting, were compared. Ni–YSZ anode-supported tubular SOFCs with a gas-tight thin YSZ electrolyte layer were then realized. Microstructure of the anode support, electrolyte and cathode thin films was also examined.     

The effect of porosity gradient in a Nickel/Yttria Stabilized Zirconia anode for an anode-supported planar solid oxide fuel cell

In this paper, a graded Ni/YSZ cermet anode, an 8 mol.%YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode were used to fabricate a solid oxide fuel cell (SOFC) unit. An anode-supported cell was prepared using a tape casting technique followed by hot pressing lamination and a single step co-firing process, allowing for the creation of a thin layer of dense electrolyte on a porous anode support. To reduce activation and concentration overpotential in the unit cell, a porosity gradient was developed in the anode using different percentages of pore former to a number of different tape-slurries, followed by tape casting and lamination of the tapes. The unit cell demonstrated that a concentration distribution of porosity in the anode increases the power in the unit cell from 76 mW cm⁻² to 101 mW cm⁻² at 600 °C in humidified hydrogen. Although the results have not been optimized for good performance, the effect of the porosity gradient is quite apparent and has potential in developing superior anode systems.     

Measurement of the temperature distribution in a large solid oxide fuel cell short stack

During the operation of solid oxide fuel cells (SOFCs), nonhomogeneous electrochemical reactions in both electrodes and boundary conditions may lead to a temperature gradient in the cell which may result in the development of thermal stresses causing the failure of the cell. Thus, in this study, effects of operating parameters (current density, flow configuration and cell size) on the temperature gradient of planar SOFCs are experimentally investigated. Two short stacks are fabricated using a small (16 cm² active area) and a large size (81 cm² active area) scandia alumina stabilized zirconia (ScAlSZ) based electrolyte supported cells fabricated via tape casting and screen printing routes and an experimental set up is devised to measure both the performance and the temperature distribution in short stacks. The temperature distribution is found to be uniform in the small short stack; however, a significant temperature gradient is measured in the large short stack. Temperature measurements in the large short stack show that the temperature close to inlet section is relatively higher than those of other locations for all cases due to the high concentrated fuel resulted in higher electrochemical reactions hence the generated heat. The operation current is found to significantly affect the temperature distribution in the anode gas channel. SEM analyses show the presence of small deformations on the anode surface of the large cell near to the inlet after high current operations.     

Porous nickel-iron alloys as anode support for intermediate temperature solid oxide fuel cells: II. Cell performance and stability

Porous nickel–iron alloy supported solid oxide fuel cells (SOFCs) are fabricated through cost-effective ceramic process including tape casting, screen printing and co-sintering. The cell performance is characterized with humidified hydrogen as the fuel and flowing air as the oxidant. Effects of iron content on the cell performance and stability under redox and thermal cycle are investigated from the point of view of structural stability. Single cells supported by nickel and nickel–iron alloy (50 wt % iron) present relatively high discharge performance, and the maximum power density measured at 800 °C is 1.52 and 1.30 W cm^?2 respectively. Nickel supported SOFC shows better thermal stability between 200 and 750 °C due to its dimensional stable substrate under thermal cycles. Posttest analysis shows that a dense iron oxide layer formed on the surface of the nickel-iron alloy during the early stage of oxidation, which prevents the further oxidation of the substrate as well as the functional anode layer, and thus, making nickel-iron supported SOFC exhibits better redox stability at 750 °C. Adding 0.5 wt % magnesium oxide into the nickel-iron alloy (50 wt% iron) can inhibit the metal sintering and reduce the linear shrinkage, making the single cell exhibit promising thermal stability.     

Simulation and analysis of sintering stress and warpage displacement in anode supported planar solid oxide fuel cells

During the sintering process of solid oxide fuel cells (SOFCs), the mismatch in the thermophysical properties of materials can lead to excessive local thermal stress and warpage. By establishing a 3D multiphysics model, the stress distribution and displacement during sintering are studied. The results show that when the anode and electrolyte thicknesses are 0.2 mm and 0.02 mm, respectively, the maximum sintering stress is 38.8 MPa, which is 48% lower than the maximum value of all simulation results. In this study, when the anode thickness is 0.7 mm and the electrolyte thickness is 0.008 mm, the maximum warpage displacement is the smallest at 0.14 mm. A sintering preparation method for partially coated cells is proposed. These results can be used to optimize the sintering process of SOFCs and greatly reduce the sintering stress and warpage of SOFCs.     

Improvement of the performances of tubular solid oxide fuel cells by optimizing co-sintering temperature of the NiO/YSZ anode-YSZ electrolyte double layers

The effects of co-sintering temperature on anode microstructure, electrolyte film microstructure, and final cell performance of tubular solid oxide fuel cells (SOFCs) were fully studied. The co-sintering of the NiO/YSZ anode-YSZ electrolyte double layers at temperature ranging from 1350 to 1400 °C for 5 h was carried out. Porosity and electrical conductivity were measured to examine the anodes microstructure, and the electrolyte films microstructure were characterized by scanning electronic microscope (SEM). A higher open current voltage (OCV) value of 0.99 V was achieved by co-sintering the cell at 1400 °C indicating denser electrolyte film, while the maximum power density of the cell co-sintered at 1380 °C was achieved with 322 mW cm⁻² at 800 °C, which was higher than that (241.3 mW cm⁻²) of the cell co-sintered at 1400 °C because of better anode microstructure.     

A cathode-supported solid oxide fuel cell prepared by the phase-inversion tape casting and impregnating method

Cathode-supported Solid Oxide Fuel Cells (SOFCs) have unique advantages of stability and operating life, but the commercialization process is limited by manufacturing cost and poor electrochemical performance. In this paper, a cathode-supported SOFC with 3YSZ-LSM95| porous 8YSZ| dense 8YSZ| porous 8YSZ sandwich structure was successfully fabricated by phase-inversion tape casting and co-sintering method. The cathode support demonstrated finger shaped macropore with high porosity. The long-term stability of symmetric cells with and without impregnated LSC nanoparticals was evaluated and no obvious degregadion were observed. The peak power densities of single cell reached 464, 209, 271 and 144 mW cm^?2 at 850, 800, 750 and 700 °C respectively when Ni nano-particles as the anode catalyst and LSC nano-particles as the cathode catalyst, showing a significant improvement in electrochemical performance compared with non-LSC cell. Additionally, the distribution of relaxation times (DRT) method was empoyed to analysis the polarization process at high-resolution, for better understanding the mechanism of electrochemical reaction of cells. The results indicated the impregnated LSC particles can increase the triple phase-boundaries (TPBs) for fast oxygen reduction reaction and improve the electrochemical performance. However, the optimization of anode and cathode are needed in the future work.     

Aqueous tape casting technique for the fabrication of Sc0.1Ce0·01Zr0·89O2+Δ ceramic for electrolyte-supported solid oxide fuel cell

Despite some the advantages of the solid oxide fuel cell (SOFC), one of the greatest challenges that hinders the SOFC from rising to dominance in the field of power generation is its high fabrication cost. As a solution, the tape casting process has been widely used to fabricate low-cost, uniform and thin SOFC electrolytes. Compared to organic-based tape casting, aqueous-based tape casting is a much more environmentally friendly technique. In this work, a large-area electrolyte-supported solid oxide fuel cell was fabricated by this technique together with sintering. A 10 cm × 10 cm and 0.17 mm thick supported Sc_0.1Ce_0·01Zr_0·89O_2+△ (SSZ) electrolyte was obtained with good flatness, low ohmic resistance and high open-circuit voltage.     

Improved performance of ammonia-fueled solid oxide fuel cell with SSZ thin film electrolyte and Ni-SSZ anode functional layer

Ammonia offers several advantages over hydrogen as an alternative fuel. However, using ammonia as a hydrogen source for fuel cells has not been received enough attention. In present paper, Scandia-stabilized Zirconia (SSZ) thin film electrolyte and Ni-SSZ anode functional layer were developed by tape casting in order to obtain high power output performance in ammonia, the results of a SOFC running on ammonia were described and its performance was compared with that when running on hydrogen. In order to improve the performance of the cell at higher temperatures, the anode was modified by iron through infiltration. A direct comparison of the performance of the modified cell running on either hydrogen or ammonia showed that the cell in ammonia generated slightly higher power densities at 700 and 750 °C. The performance in ammonia, using the anode catalyst, was comparable to that in hydrogen indicating ammonia could be treated as a promising alternative fuel by selecting an appropriate catalyst.     

Tape casting fabrication,co-sintering and optimisation of anode/electrolyte assemblies for SOFC based on BIT07-Ni/BIT07

BaIn_0.3Ti_0.7O_2.85 (BIT07) is an electrolyte material for SOFC due to its high ionic conductivity level and its compatibility with mixed ionic and electronic conductor (MIEC) cathode materials, such as LSCF and Nd₂NiO_4+δ. BIT07 is also compatible with NiO to form a cermet as anode material. In this study, the electrolyte material has been prepared by tape casting and characterised. The coupled influences of the powder grain size and the firing temperature have been investigated and optimised to yield a dense electrolyte at 1300 °C. Then anode-supported half-cells (electrolyte/anode), based on BIT07/BIT07-Ni have been prepared by tape casting and co-sintered. The composition and the microstructure of the cermet anode (BIT07 grain size, amount and nature of pore-forming agent) have been optimised to achieve an area surface resistance (ASR) value of about 0.15 Ω cm² at 700 °C under wet reducing atmosphere. The stability of the most performing anode has been followed during 500 h and the observed degradation seems to be due a loss of nickel percolation.     

Metal-supported solid oxide fuel cells with in-situ sintered (Bi2O3)0.7(Er2O3)0.3–Ag composite cathode

Metal-supported solid oxide fuel cells (SOFCs) containing porous 430L stainless steel support, Ni-YSZ anode and YSZ electrolyte were fabricated by tape casting, laminating and co-firing in a reduced atmosphere. (Bi₂O₃)_0.7(Er₂O₃)_0.3–Ag composite cathode was applied by screen printing and in-situ sintering. The polarization resistances of the composite cathode were 1.18, 0.48, 0.18, 0.09 Ω cm² at 600, 650, 700 and 750 °C, respectively. A promissing maximum power density of 568 mW cm⁻² at 750 °C was obtained of the single cell. Short-term stability was measured as well.     

Application of sol gel spin coated yttria-stabilized zirconia layers for the improvement of solid oxide fuel cell electrolytes produced by atmospheric plasma spraying

Due to its high thermal stability and purely oxide ionic conductivity, yttria-stabilized zirconia (YSZ) is the most commonly used electrolyte material for solid oxide fuel cells (SOFCs). Standard electrolyte fabrication techniques for planar SOFCs involve wet ceramic techniques such as tape-casting or screen printing, requiring sintering steps at temperatures above 1300 °C. Plasma spraying (PS) may provide a more rapid and cost efficient method to produce SOFCs without sintering. High-temperature sintering requires long processing times and can lead to oxidation of metal alloys used as mechanical supports, or to detrimental interreactions between the electrolyte and adjacent electrode layers. This study investigates the use of spin coated sol gel derived YSZ precursor solutions to fill the pores present in plasma sprayed YSZ layers, and to enhance the surface area for reaction at the electrolyte-cathode interface, without the use of high-temperature firing steps. The effects of different plasma conditions and sol concentrations and solid loadings on the gas permeability and fuel cell performance have been investigated.     

A general electrolyte–electrode-assembly model for the performance characteristics of planar anode-supported solid oxide fuel cells

A general electrode–electrolyte-assembly (EEA) model has been developed, which is valid for different designs of solid oxide fuel cells (SOFCs) operating at different temperatures. In this study, it is applied to analyze the performance characteristics of planar anode-supported SOFCs. One of the novel features of the present model is its treatment of electrodes. An electrode in the present model is composed of two distinct layers referred to as the backing layer and the reaction zone layer. The other important feature of the present model is its flexibility in fuel, having taking into account the reforming and water–gas shift reactions in the anode. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved using finite volume method. The model can predict all forms of overpotentials and the predicted concentration overpotential is validated with measured data available in literature. It is found that in an anode-supported SOFC, the cathode overpotential is still the largest cell potential loss mechanism, followed by the anode overpotential at low current densities; however, the anode overpotential becomes dominant at high current densities. The cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Even at low fuel utilizations, the anode concentration overpotential becomes significant when chemical reactions (reforming and water–gas shift) in the anode are not considered. A parametric study has also been carried out to examine the effect of various key operating and design parameters on the performance of an anode-supported planar SOFCs.