Development of functionally graded anode supports for microtubular solid oxide fuel cells by tape casting and isostatic pressing

This paper suggests an alternative method to manufacture functionally graded anode supports for microtubular solid oxide fuel cells by employing tape casting and isostatic pressing for the first time in the literature. In this regard, six different anode support strips with various pore former contents are produced by tape casting. Besides the anode supports made from uniform tapes, three-layered anode supports composed of various combination of these tapes are also fabricated by wrapping the corresponding tape(s) of the same total length on a metallic rod followed by isostatic pressing. Microtubular cells are then built on these anode supports by dip coating the other layers and evaluated by microstructural investigations and electrochemical performance tests performed under the same conditions. Porosity measurements of the homogeneous anode supports are also carried out. Microstructural examinations reveal that not only the homogeneous anode supports but porosity graded anode supports can be also successfully manufactured by the suggested method. Electrochemical tests indicate that the performance of the cells with a uniform anode support tends to increase with the anode support porosity up to ∼26% porosity then shows a decreasing trend. The highest maximum performance of 0.645 Wcm⁻² at 800 °C under 0.3 NLmin⁻¹ hydrogen and stationary air, on the other hand, is obtained from the cell with a porosity graded anode support.     

Effect of non-solvent from the phase inversion method on the morphology and performance of the anode supported microtubular solid oxide fuel cells

The microstructure of the anode in anode-supported solid oxide fuel cells has significant influence on the cell performance. In this work, microtubular Ni-yttria stabilized zircona (Zr_0.8 Y_0.2O₂, YSZ) anode support has been prepared by the phase inversion method. Different compositions of non-solvent have been used for the fabrication of the Ni-YSZ anode support, and the correlation between non-solvent composition and characteristics of the microstructure of the anode support has been investigated. The presence of ethanol or isopropanol in the non-solvent can inhibit the growth of the finger-like pores in the anode support. With the increase of the concentration of ethanol or isopropanol in the non-solvent, a thin dense layer can be observed on the top of the prepared tubular anode support. In addition, the mechanism of pore formation is explained based on the inter-diffusivity between the solvent and the non-solvent. The prepared microtubular solid oxide fuel cells (MT-SOFCs) have been tested, and the influence of the anode microstructure on the cell electrochemical performance is analyzed based on a polarization model.     

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.     

Effect of Al and Ce doping on the deformation upon sintering in sequential tape cast layers for solid oxide fuel cells

Water-based tape casting is an attractive production route for planar solid oxide fuel cells (SOFCs) due to its high productivity and reduced environmental issues. In this work planar anode supported SOFCs with thin electrolyte were prepared by water-based sequential tape casting and co-sintering. An in situ high temperature monitoring apparatus was assembled to allow the determination of free sintering shrinkage of thin green tape cast layers and to follow the curvature developed in multilayers during the entire sintering process.The instantaneous curvature developed upon co-sintering was studied as a function of the firing schedule and layer composition. It was found that by tailoring the electrode composition it is possible to reduce the shrinking rate difference between anode and electrolyte thus obtaining defect-free electrolyte, minimising the residual curvature of the half-cell and improving the electrochemical performances of the cell.     

Evaluation of metal-supported solid oxide fuel cells (MS-SOFCs) fabricated at low temperature (∼1,000 °C) using wet chemical coating processes and a catalyst wet impregnation method

The objective of this study is to evaluate metal-supported solid oxide fuel cells fabricated at low temperatures (～1000 °C) in oxidizing environments using wet chemical coating processes and a catalyst impregnation method. Typically, applying general wet chemical coating processes and heat treatment at low temperature is desirable for fabricating metal-supported solid oxide fuel cells when considering manufacturing productivity and efficiency. However, in the case of conventional anodes, a well-organized structure for high performance is rarely formed by sintering at low temperatures when using general fabrication processes. For this reason, a catalyst-impregnated anode is designed and applied to overcome the above issue. First, to evaluate the electrochemical performance of the designed anode, the area-specific resistances of half-cells are investigated. Then, the newly designed anode is applied to a single cell, and microstructural analysis and electrochemical performance measurements are performed. These results confirm that the catalysts are well distributed, that the electrolyte is fully dense and that the electrochemical performances are reasonable. Additionally, the high durability is also verified through a long-term test over 1000 h. Finally, the metal-supported solid oxide fuel cell with a catalyst-impregnated anode fabricated at low temperature is completely validated through the evaluation of a large-size single cell.     

Design of experiment approach applied to reducing and oxidizing tolerance of anode supported solid oxide fuel cell. Part II: Electrical, electrochemical and microstructural characterization of tape-cast cells

One of the major limitations of the nickel (Ni) - yttria-stabilized zirconia (YSZ) anode support for solid oxide fuel cells (SOFC) is its low capability to withstand transients between reducing and oxidizing atmospheres (“RedOx” cycle), owing to the Ni-to-NiO volume expansion. This work presents results on different anode supports fabricated by tape casting. Three compositions are prepared, as the outcome of a preceding design of experiment approach. The NiO proportion is 40, 50 and 60 wt% of the anode composite.The anode support characteristics like shrinkage during sintering, in-situ conductivity at high temperature, electrochemical performance and tolerance against RedOx cycles have been measured. Performance up to 0.72 W cm⁻² (0.62 V, 800 °C) is recorded for the 60 wt% NiO sample on small cells. The open circuit voltage is maintained within ±5 mV after 10 full RedOx cycles at 800 °C and one at 850 °C. Performances tend to be stabilized after one or multiple RedOx cycles. The microstructural observations show round Ni particles after the first reduction; after a RedOx cycle, the Ni particles include micro-porosities that are stable under humidified reducing atmosphere for more than 300 h.     

Fabrication, electrochemical characterization and thermal cycling of anode supported microtubular solid oxide fuel cells

This work describes the manufacture and electrochemical characterization of anode supported microtubular SOFC's (solid oxide fuel cells). The cells consist of a Ni-YSZ anode tube of 400 μm wall-thickness and 2.4 mm inner diameter, a YSZ electrolyte of 15-20 μm thickness and a LSM-YSZ cathode. The microtubular anode supporting tubes were prepared by cold isostatic pressing. The deposition of thin layers of electrolyte and cathode are made by spray coating and dip coating respectively. The cells were electrochemically characterized with polarization curves and complex impedance measurements using 5% H₂/95% Ar and 100% of H₂, humidified at 3% as reactant gas in the anodic compartment and air in the cathodic one at temperatures between 750 and 900 °C. The complex impedance measurements show an overall resistance from 1 to 0.42 Ω cm² at temperatures between 750 and 900 °C with polarization of 200 mA cm⁻². The I-V measurements show maximum power densities of 0.3-0.7 W cm⁻² in the same temperature interval, using pure H₂ humidified at 3%. Deterioration in the cathode performance for thin cathodes and high sintering temperatures was observed. They were associated to manganese losses. The cell performance did not present considerable degradation at least after 20 fast shut-down and heating thermal cycles.     

Characteristics of SOFC single cells with anode active layer via tape casting and co-firing

SOFC (solid oxide fuel cell) single cells with anode active layers of various thicknesses were fabricated successfully via tape casting and co-firing in order to improve their electrochemical performance and long-term stability. The mercury porosimeter and the gas permeability were measured so as to examine the effects of the anode active layer while under a gaseous flow. It was found that the anode active layers affected the microstructural characteristics as a result of the pore distribution and the gas permeation behavior. The anode active layers improved the cell performance by increasing the number of active sites in the anode. The thickness of the anode active layer was optimized at 20 μm in this work through a combination of the power density, the ohmic ASR (area specific resistance), and the cell ASR. SOFCs with the optimized active layer showed good electrochemical performance at 600–700 °C in hydrogen or hydrocarbon fuel (methane) and excellent long-term stability for 500 h.     

Effects of pore former type on mechanical and electrochemical performance of anode support microtubes in solid oxide fuel cells

The effects of pore formers added into the extrusion slurry of anode support microtubes on the mechanical and electrochemical performance of the microtubes are investigated in this study. For this purpose, several microtubular anode supports are fabricated by using various pore formers with different particle sizes. The effect of pore former content is also taken into consideration for a certain pore former type. The flexural strengths of the anode support microtubes are measured via three point bending tests and reliability analysis is performed. The porosities of the anode supports are also determined along with microstructural investigations. The results reveal that the flexural strength decreases with increasing the particle size of the pore former employed for a fixed pore former content and with increasing the pore former content for a certain pore former material considered. In addition, a number microtubular cells are fabricated based on the various microtubular anode supports and their electrochemical performances are evaluated via performance and impedance tests. The impedance results indicate that the cell performance is mainly restricted by the diffusion polarization. Among the pore former materials considered in this study, the highest cell performance for a certain pore former content of 20 vol% is measured from the cell prepared with graphite (325 mesh) pore former at all temperatures and hydrogen flowrates studied. The optimization studies display that the cell performance can be further improved by increasing the pore former content to 22.5 vol% for this pore former material.     

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.     

Effects of Sn doping on the manufacturing,performance and carbon deposition of Ni/ScSZ cells in solid oxide fuel cells

This work demonstrates the effect of tin (Sn) doping on the manufacturing, electrochemical performance, and carbon deposition in dry biogas-fuelled solid oxide fuel cells (SOFCs). Sn doping via blending in technique alters the rheology of tape casting slurry and increases the Ni/ScSZ anode porosity. In contrast to the undoped Ni/ScSZ cells, where open-circuit voltage (OCV) drops in biogas, Sn–Ni/ScSZ SOFC OCV increases by 3%. The maximum power densities in biogas are 0.116, 0.211, 0.263, and 0.314 W/cm² for undoped Ni/ScSZ, undoped Ni/ScSZ with 3 wt% pore former, Sn–Ni/ScSZ and Sn–NiScSZ with 1 wt% pore former, respectively. Sn–Ni/ScSZ reduces the effect of the drop in the maximum power densities by 26%–36% with the fuel switch. A 1.28–2.24-fold higher amount of carbon is detected on the Sn–Ni/ScSZ samples despite the better electrochemical performance, which may reflect an enhanced methane decomposition reaction.     

Design of industrially scalable microtubular solid oxide fuel cells based on an extruded support

The current work describes the adaptation of an existing lab-scale cell production method for an anode supported microtubular solid oxide fuel cell to an industrially ready and easily scalable method using extruded supports. For this purpose, Ni–YSZ (yttria stabilized zirconia) anode is firstly manufactured by Powder Extrusion Moulding (PEM). Feedstock composition, extruding parameters and binder removal procedure are adapted to obtain the tubular supports. The final conditions for this process were: feedstock solid load of 65 vol%; a combination of solvent debinding in heptane and thermal debinding at 600 °C. Subsequently, the YSZ electrolyte layer is deposited by dip coating and the sintering parameters are optimized to achieve a dense layer at 1500 °C during 2 h. For the cathode, an LSM (lanthanum strontium manganite)–YSZ layer with an active area of ∼1 cm² is deposited by dip coating. Finally, the electrochemical performance of the cell is measured using pure humidified hydrogen as fuel. The measured power density of the cell at 0.5 V was 0.7 W cm⁻² at 850 °C.     

Recent anode advances in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical reactors that can directly convert the chemical energy of a fuel gas into electrical energy with high efficiency and in an environment-friendly way. The recent trends in the research of solid oxide fuel cells concern the use of available hydrocarbon fuels, such as natural gas. The most commonly used anode material Ni/YSZ cermet exhibits some disadvantages when hydrocarbons were used as fuels. Thus it is necessary to develop alternative anode materials which display mixed conductivity under fuel conditions. This article reviews the recent developments of anode in SOFCs with principal emphasis on the material aspects. In addition, the mechanism and kinetics of fuel oxidation reactions are also addressed. Various processes used for the cost-effective fabrication of anode have also been summarized. Finally, this review will be concluded with personal perspectives on the future research directions of this area.     

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.     

Fabrication of solid oxide fuel cells (SOFCs) by solvent-controlled co-tape casting technique

A co-tape casting process has an advantage of cost-effectiveness for mass production. To fabricate solid oxide fuel cells (SOFCs) with high electrochemical performance by co-tape casting, high solid loading and binder content of tape cast slurry are required to improve particle network strength. However, high solid loading and binder content cause high viscosity of the slurry, which makes removal of air bubbles and handling difficult. In this study, a new method to fabricate uniform green tapes with high solid loading and binder content by controlling solvent ratio under vacuum condition is proposed. As a result, high solid loading and binder content with 39% improved storage shear modulus, 26% improved LVR length, tensile strength of 5.0 MPa, and packing density of 57.5% were achieved at solvent ratio of 22 wt%. To fabricate unit cells using the green tapes, thermal decomposition and shrinkage behavior are characterized, and heat treatment steps at 250 °C, 350 °C, and 500 °C and co-sintering temperature were determined at 1250 °C. A fabricated unit cell showed open circuit voltage (OCV) of 1.10 V and the maximum power density of 1.20 W cm^?2 at 800 °C. To fabricate crack-free Ф5.0 cm unit cells, the mechanical strength of the anode support tapes after thermal decomposition was measured to determine the tape compositions that can minimize cracks at the unit cell. As a result, a crack-free unit cell with a diameter of 5.0 cm was fabricated, achieving OCV of 1.05 V and power of 4.3 W at 800 °C.     

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.     

Sr0.92Y0.08TiO3−δ/Sm0.2Ce0.8O2−δ anode for solid oxide fuel cells running on methane

Yttria-doped strontium titanium oxide (Sr_0.92Y_0.08TiO_3−δ; SYT) was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The SYT synthesized by the Pechini method exhibits excellent phase stability during the cell fabrication processes and SOFC operation and good electrical conductivity (about 0.85 S/cm, porosity 30%) in reducing atmosphere. The performance of SYT anode is characterized by slow electrochemical reactions except for the gas-phase diffusion reactions. The cell performance with the SYT anode running on methane fuel was improved about 5 times by SDC film coating, which increased the number of reaction sites and also accelerated electrochemical reaction kinetics of the anode. In addition, the SDC-coated SYT anode cell was stably operated for 900 h with methane. These results show that the SDC-coated SYT anode can be a promising anode material for high temperature SOFCs running directly on hydrocarbon fuels.     

Three-dimensional numerical simulation and experimental validation on ammonia and hydrogen fueled micro tubular solid oxide fuel cell performance

The main aim of this research is to investigate the performance of ammonia-powered microtubular solid oxide fuel cells in order to use ammonia as a possible candidate for eco-friendly and sustainable power generation systems. The performance of a direct ammonia-powered cell has been elucidated and validated with the experimental results of pure hydrogen gas at Ni?de Ömer Halisdemir University Prof. T. Nejat Veziro?lu Clean Energy Research Center. For both studies, the cathode electrode is supplied with atmospheric air. The performance of anode, electrolyte, and cathode-supported microtubular solid oxide fuel cells has been compared numerically. The findings confirmed that the peak possible power densities obtained numerically using direct ammonia, hydrogen and experimentally using pure hydrogen gas are is 628.92 mW/cm², 622.29 mW/cm^2, and, 589.28 mW/cm² respectively at the same geometrical dimensions, component materials, and operating parameters. Thus, the results of this study demonstrate that simultaneous experimental and numerical studies make a great contribution to minimizing biases due to literature data during model validation. The numerical simulation also indicates that the performance of cathode supported is superior to that of anode supported cells run with hydrogen and ammonia fuel. Likewise, parametric sweep analysis asserts that the working temperature has a greater effect than operating pressure on tubular cell performance. Therefore, the results of this study advise that ammonia will become a carbon-free alternative fuel for solid oxide fuel cells in the coming years.     

Characterization of the Ni-ScSZ anode with a LSCM-CeO2 catalyst layer in thin film solid oxide fuel cell running on ethanol fuel

Solid oxide fuel cell (SOFC) running directly on hydrocarbon fuels has attracted much attention in recent years. In this paper, a dual-layer structure anode running on ethanol is fabricated by tape casting and screen-printing method, the addition of a LSCM-CeO₂ catalyst layer to the supported anode surface yields better performance in ethanol fuel. The effect that the synthesis conditions of the catalyst layer have on the performances of the composite anodes is investigated. Single cells with this anode are also fabricated, of which the maximum power density reaches 669 mW cm⁻² at 850 °C running on ethanol steam. No significant degradation in performance has been observed after 216 h of cell testing when the Ni-ScSZ13 anode is exposed to ethanol steam at 700 °C. Very little carbon is detected on the anode, suggesting that carbon deposition is limited during cell operation. Consequently, the LSCM-CeO₂ catalyst layer on the surface of the supported anode makes it possible to have good stability for long-term operation in ethanol fuel due to low carbon deposition.     

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.