Fabrication of micro-tubular solid oxide fuel cells with a single-grain-thick yttria stabilized zirconia electrolyte

This study discusses the fabrication and electrochemical performance of micro-tubular solid oxide fuel cells (SOFCs) with an electrolyte consisting a single-grain-thick yttria stabilized zirconia (YSZ) layer. It is found that a uniform coating of an electrolyte slurry and controlled shrinkage of the supported tube leads to a dense, crack-free, single-grain-thick (less than 1 μm) electrolyte on a porous anode tube. The SOFC has a power density of 0.39 W cm⁻² at an operating temperature as low as 600 °C, with YSZ and nickel/YSZ for the electrolyte and anode, respectively. An examination is made of the effect of hydrogen fuel flow rate and shown that a higher flow rate leads to better cell performance. Hence a YSZ cell can be used for low-temperature SOFC systems below 600 °C, simply by optimizing the cell structure and operating conditions.     

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.     

Characterization of metal-supported axial injection plasma sprayed solid oxide fuel cells with aqueous suspension plasma sprayed electrolyte layers

A method for manufacturing metal-supported SOFCs with atmospheric plasma spraying (APS) is presented, making use of aqueous suspension feedstock for the electrolyte layer and dry powder feedstock for the anode and cathode layers. The cathode layer was deposited first directly onto a metal support, in order to minimize contact resistance, and to allow the introduction of added porosity. The electrolyte layers produced by suspension plasma spraying (SPS) were characterized in terms of thickness, permeability, and microstructure, and the impact of substrate morphology on electrolyte properties was investigated. Fuel cells produced by APS were electrochemically tested at temperatures ranging from 650 to 750 °C. The substrate morphology had little effect on open circuit voltage, but substrates with finer porosity resulted in lower kinetic losses in the fuel cell polarization.     

Effects of anode and electrolyte microstructures on performance of solid oxide fuel cells

To improve the performance of anode-supported solid oxide fuel cells (SOFCs), various types of single cells are manufactured using a thin-film electrolyte of Yttria stabilized zirconia (YSZ) and an anode functional layer composed of a NiO–YSZ nano-composite powder. Microstructural/electrochemical analyses are conducted. Single-cell performances are highly dependent on electrolyte thickness, to the degree that the maximum power density increases from 0.74 to 1.12 W cm⁻² according to a decrease in electrolyte thickness from 10.5 to 6.5 μm at 800 °C. The anode functional layer reduced the polarization resistance of a single cell from 1.07 to 0.48 Ω cm² at 800 °C. This is attributed to the provision by the anode layer of a highly reactive and uniform electrode microstructure. It is concluded that optimization of the thickness and homogeneity of component microstructure improves single-cell performances.     

Solid oxide fuel cells with dense yttria-stabilized zirconia electrolyte membranes fabricated by a dry pressing process

Dense yttria-stabilized zirconia (YSZ) electrolyte films were successfully fabricated onto anode substrates using a modified dry pressing process. The film thickness was uniform, and could be readily controlled by the mass of the nanocrystalline YSZ powders. The electrolyte films adhered well to the anode substrates by controlling the anode composition. An anode-supported solid oxide fuel cell (SOFC) with a dense YSZ electrolyte film of 8 μm in thickness was operated at temperatures from 700 to 800 °C using humidified (3 vol% H₂O) hydrogen as fuel and air as oxidant. An open circuit voltage of 1.06 V and a maximum power density of 791 mW cm⁻² were achieved at 800 °C. The results indicate that the gas permeation through the electrolyte film was negligible, and that good performance can be obtained by this simple and cost-effective technique which can significantly reduce the fabrication cost of SOFCs.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

A novel electrolyte for intermediate solid oxide fuel cells

Scheelite-type, La_xCa_1−xMoO_4+δ electrolyte powders, are prepared by the sol-gel process. The crystal structure of the samples is determined by employing the technique of X-ray diffraction (XRD). According to XRD analysis, the continuous series of La_xCa_1−xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions have the structure of tetragonal scheelite. Their lattice parameters are greater than that of the original sample, and increase with increasing values of x in the La-substituted system. Results of sinterability and electrochemical testing reveal that the performances of La-doped calcium molybdate are superior to that of pure CaMoO₄. La_xCa_1−xMoO_4+δ ceramics demonstrate higher sinterability. The La_0.2Ca_0.8MoO_4+δ sample that achieved 96.5% of the theoretical density was obtained after being sintered at 1250 °C for 4 h. The conductivity increases with increasing lanthanum content, and a total conductivity of 7.3 × 10⁻³ S cm⁻¹ at 800 °C could be obtained in the La_0.2Ca_0.8MoO_4+δ compound sintered at 1250 °C for 4 h.     

Thermodynamic analysis of ammonia fed solid oxide fuel cells: Comparison between proton-conducting electrolyte and oxygen ion-conducting electrolyte

A thermodynamic analysis has been performed to compare the theoretical performance of ammonia fed solid oxide fuel cells (SOFCs) based on proton-conducting electrolyte (SOFC-H) and oxygen ion-conducting electrolyte (SOFC-O). It is found that the ammonia fed SOFC-H is superior to SOFC-O in terms of theoretical maximum efficiency. For example, at a fuel utilization of 80% and an oxygen utilization of 20%, the efficiency of ammonia fed SOFC-H is 11% higher than that of SOFC-O. The difference between SOFC-H and SOFC-O becomes more significant at higher fuel utilizations and higher temperatures. This is because an SOFC-H has a higher hydrogen partial pressure and a lower steam partial pressure than an SOFC-O. In addition, an increase in oxygen utilization is found to increase the efficiency of ammonia fed SOFCs due to an increase in oxygen molar fraction and a reduction in steam molar fraction. With further development of new ceramics with high proton conductivity and effective fabrication of thin film electrolyte, the SOFC based on proton-conducting electrolyte is expected to be a promising approach to convert ammonia fuel into electricity.     

Development of 1 kW class solid oxide fuel cell stack using anode-supported planar cells

We have developed a 1 kW class solid oxide fuel cell (SOFC) stack composed of 50 anode-supported planar 120-mm-diameter SOFCs. Intermediate plates, which exhibited negligible deformation under operating conditions, were placed in the stack to cancel out the cumulative error related to the position and angle of the stack parts. The stack provided an electrical conversion efficiency of 54% (based on the lower heating value (LHV) of the methane used as a fuel) and an output of 1120 W when the fuel utilization, current density, and operating temperature were 67%, 0.28 A cm⁻², and 1073 K, respectively. The stack operated stably for almost 700 h.     

Design and fabrication of stair‐step‐type electrolyte structure for solid oxide fuel cells

The design and the fabrication of novel stair‐step electrolyte based on yttria stabilized zirconia are presented. The novel electrolyte has gradually reduced oxide ion transport paths achieved by the stair‐step design. The mechanical and electrochemical performance of the novel electrolyte are investigated and compared to those of standard electrolyte support. Three‐point bending tests indicate that the fracture displacement and force measured for the novel electrolyte are 11% and 32% less than those of the standard electrolyte support, respectively. However, the cell based on the novel electrolyte exhibits 40% higher electrochemical performance than the standard electrolyte supported cell at an operation temperature of 700 °C. Impedance analyses revealed that the enhanced cell performance is mainly due to the decrease in the ohmic resistance of the cell achieved by the novel electrolyte design. In addition, the electrode resistances are found to be decreased due to the increased electrochemical reaction zones since the contact area between the novel electrolyte and both electrodes are increased by the novel electrolyte design. Moreover, the cell with novel electrolyte produced 0.47 Wcm^?2 peak power at 750 °C while the standard electrolyte supported cell shows almost the same power output at around 800 °C. Thus, novel designed electrolyte also offers some amount of reduction in the operation temperature of solid oxide fuel cells. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel electrolytes for solid oxide fuel cells with improved mechanical properties

The improvement of the mechanical properties of novel structured electrolytes with triangular cut off geometry in the active region is presented by filleting the tips of triangles. The effect of fillet radius on the bending strength of the yttria stabilized zirconia electrolyte was investigated with a commercial finite element code implementing the calculated Weibull stress through the experimental stress–strain curve determined via tensile tests. The model was verified with the experimental three point bending test results for the electrolyte with unfilleted triangular cut off patterns. Ten different fillet radii ranging from 0.05 mm to 0.5 mm were considered in the simulations. The fracture displacement was found to increase with increasing fillet radius as expected. Since the electrolyte with fillet radius of 0.5 mm was found to show the highest flexural strength, single cell based on this electrolyte was fabricated and the cell performance was measured. It was found that the strength of the novel electrolyte with partly reduced thickness can be increased by 26.2% with sacrificing only 10.2% decrease in the performance. Since the final cell still showed 22.2% higher peak performance than the standard electrolyte supported cell, 10.2% decrease in the cell performance compared to the cell having unfilleted triangular cut off patterns is acceptable.     

Coal gas fuel utilization effects on electrolyte supported solide oxide fuel cell performance

Solid oxide fuel cells (SOFCs) transform the energy of the fuel instantly into electric energy with a large fuel option. Coal, which is a local energy source, is a preferred fuel despite its negative features because it is cheap and abundant. The use of coal and coal-based fuels in SOFCs has recently attracted considerable attention. In this study, performance analysis of the SOFC has been performed experimentally by using hydrogen, generator gas (contained 12% H₂), and water-gas (contained 50% H₂) in an electrolyte-supported SOFC (ES-SOFC). The numerical modelling of the fuel cell had been previously performed. In addition, the effect of inlet gas fuel flow rates on the ES- SOFC has been investigated numerically in this study. The temperature effect on the performance of ES-SOFC has been examined experimentally. It is seen that the performance of SOFCs fueled hydrogen is favorable than fueled water gas and generator gas. This is because of the higher hydrogen substance in the water gas measure against the other gas. In addition, it is seen that the increase in temperature increases the performance with positive effects on the reactions. It is also concluded that the performance of SOFC increases when inlet fuel flow rates increase.     

Carbon monoxide-fueled solid oxide fuel cell

This study explored CO as a primary fuel in anode-supported solid oxide fuel cells (SOFCs) of both tubular and planar geometries. Tubular single cells with active areas of 24 cm² generated power up to 16 W. Open circuit voltages for various CO/CO₂ mixture compositions agreed well with the expected values. In flowing dry CO, power densities up to 0.67 W cm⁻² were achieved at 1 A cm⁻² and 850 °C. This performance compared well with 0.74 W cm⁻² measured for pure H₂ in the same cell and under the same operating conditions. Performance stability of tubular cells was investigated by long-term testing in flowing CO during which no carbon deposition was observed. At a constant current of 9.96 A (or, 0.414 A cm⁻²) power output remained unchanged over 375 h of continuous operation at 850 °C. In addition, a 50-cell planar SOFC stack was operated at 800 °C on 95% CO (balance CO₂), which generated 1176 W of total power at a power density of 224 mW cm⁻². The results demonstrate that CO is a viable primary fuel for SOFCs.     

Study on GDC-LSGM composite electrolytes for intermediate-temperature solid oxide fuel cells

The electrolyte materials Ce_0.9Gd_0.1O_1.9 (GDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) were synthesized by means of glycine-nitrate processes, respectively, then GDC-LSGM composite electrolytes were prepared by mixing GDC and LSGM. The GDC and LSGM powders were mixed in the weight ratio of 95:5, 90:10 and 85:15 and named as GL9505, GL9010 and GL8515. Their structures and ionic conductivities were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and AC impedance spectroscopy. The grain sizes of GDC-LSGM composites could be increased distinctly and the grain boundary resistance could be significantly decreased by small addition of LSGM. The experimental results show that the GDC-LSGM composites exhibit excellent ionic conductivity and could significantly enhance the fuel cell performances. The open circuit voltages are higher in the cell with composite electrolytes than in the cell with single GDC as electrolyte at the working temperature. Among these electrolytes, GL9505 has the highest ionic conductivity and the maximum power density.     

Thin films for micro solid oxide fuel cells

Thin film deposition as applied to micro solid oxide fuel cell (μSOFC) fabrication is an emerging and highly active field of research that is attracting greater attention. This paper reviews thin film (thickness ≤1 μm) deposition techniques and components relevant to SOFCs including current research on nanocrystalline thin film electrolyte and thin-film-based model electrodes. Calculations showing the geometric limits of μSOFCs and first results towards fabrication of μSOFCs are also discussed.     

The effect of electrolyte type on performance of solid oxide fuel cells running on hydrocarbon fuels

A two-dimensional model is developed to simulate the performance of methane fueled solid oxide fuel cells (SOFCs), focusing on the effect of electrolyte type on SOFC performance. The model considers the heat and mass transfer, direct internal reforming (DIR) reaction, water gas shift reaction (WGSR), and electrochemical reactions in SOFCs. The electrochemical oxidation of CO in oxygen ion-conducting SOFC (O-SOFC) is considered. The present study reveals that the performance of H-SOFC is lower than that of O-SOFC at a high temperature or at a low operating potential, as electrochemical oxidation of CO in O-SOFC contributes to power generation. This finding is contrary to our common understanding that proton conducting SOFC (H-SOFC) always performs better than O-SOFC. However, at a high operating potential of 0.8 V or at a lower temperature, H-SOFC does exhibit better performance than O-SOFC due to its higher Nernst potential and higher ionic conductivity of the electrolyte. This indicates that the proton conductors can be good choices for SOFCs at intermediate temperature, even with hydrocarbons fuels. The results provide better understanding on how the electrolyte type influences the performance of SOFCs running on hydrocarbon fuels.     

An analytical model for solid oxide fuel cells with bi-layer electrolyte

A theoretical model for a solid oxide fuel cell (SOFC) with a bi-layer electrolyte is developed and analytical solutions of various important relationships, such as I–V relationship, distribution of oxygen partial pressure in the bi-layer electrolyte, leakage current density etc. are obtained. Based on the assumptions of constant ionic conductivity and reversible electrodes, the model takes into considerations of transports of both ions and electrons in the electrolyte. The modeling results are compared with both experimental data and results from other models in the literature and very good agreements are obtained.     

Characteristics of electrolyte supported micro-tubular solid oxide fuel cells with GDC-ScSZ bilayer electrolyte

In this study, a gadolinia-doped ceria (GDC)-supported micro tubular SOFC (T-SOFC) was fabricated using extrusion and dip-coating techniques (Cell A). The effects of inserting a scandium-stabilized zirconia (ScSZ) layer as an electron blocking layer between the GDC layer and the GDC-Ni anode layer were also explored (Cell B). The microstructures and electrochemical performances of Cell A and Cell B were investigated and compared. The layer thicknesses of the GDC and ScSZ bi-layer electrolytes were approximately 285 μm and 8 μm respectively. With the inserted ScSZ layer, both the ohmic resistance and the polarization resistance significantly increased at all the operating temperatures. The increase in the ohmic resistance of Cell B was predominantly due to the interfacial resistance, while the substantial escalation in the polarization resistance was mainly because of the low bulk oxygen diffusion process in the ScSZ layer and the smaller charge transfer processes occurring at the interfaces. The OCV of Cell B showed a slight decrease from 1.06 to 0.98 V and that of Cell A experienced an obvious decline from 0.92 to 0.76 V as the temperature rose from 650 to 800 °C. The ScSZ layer of Cell B successfully inhibited the OCV loss caused by the electronic conduction in GDC. The maximum power densities (MPDs) of Cell A at 650, 700, 750, and 800 °C were 0.20, 0.27, 0.33, and 0.36 Wcm⁻², and those of Cell B 0.16, 0.23, 0.32, and 0.42 Wcm⁻². The MPD of Cell B was improved at temperatures above 750 °C but remained inferior to that of Cell A below 750 °C. This is due to the fact that, as operating temperature increased above 750 °C, the benefit of the higher OCV in Cell B surpassed the deficiency of the higher cell resistance, thereby leading to a higher MPD.     

Effect of alumina additions on the anode|electrolyte interface in solid oxide fuel cells

The longevity of a solid oxide fuel cell (SOFC) stack is curtailed by the fragility of its ceramic components. At Ceramic Fuel Cells Limited (CFCL), 15 wt.% alumina is added to the commonly used 10 mol% Y₂O₃–ZrO₂ (YSZ) electrolyte to improve both the fracture toughness and grain-boundary conductivity of the electrolyte. This study investigates the effect of such addition of alumina on the anode|electrolyte interface; more specifically, which reactions occur with the Al₂O₃ at the interface and how these reactions influence fuel cell performance. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the formation of NiAl₂O₄ in the alumina regions in the electrolyte. The NiAl₂O₄ is observed to grow into the adjacent grain boundaries to form an interconnected NiAl₂O₄ network up to 4 μm deep into the electrolyte. Impedance spectroscopy shows that the formation of NiAl₂O₄ does not affect the grain bulk ionic conductivity. The grain-boundary conductivity is markedly reduced at low temperatures. However, at the high SOFC operating temperature at CFCL (850 °C) the contribution of the grain-boundary conductivity to the total conductivity is diminished, and the NiAl₂O₄ is found not to have an effect on the total electrolyte conductivity and is deemed not to be a detrimental reaction.