A novel low-pressure injection molding technique for fabricating anode supported solid oxide fuel cells

A low pressure injection molding (LPIM) technique is successfully developed to fabricate porous NiO–YSZ anode substrates for cone-shaped tubular anode-supported solid oxide fuel cells (SOFCs). The porosity and microstructure of the anode samples prepared with different amount of pore formers are investigated through the Archimedes method and SEM analysis. Experimental results show that with 15 wt.% paraffin as plasticizer, porosity of the NiO–YSZ substrates sintered at 1400 °C is proportional to the amount of graphite as pore former, and proper porosities can be obtained with or without 5 wt.% graphite. NiO–YSZ/YSZ/LSM–YSZ single cells are assembled and tested to demonstrate the feasibility of the LPIM technique. At 800 °C, with moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant, the cell with the anode substrate fabricated with 5 wt.% pore former shows a maximum power density of 531 mW cm⁻², while the cell without any pore former, 491 mW cm⁻². Two of the single cells (without graphite) are applied to assemble a two-cell-stack which gives an open circuit voltage of 1.75 V and a maximum output power of 5.32 W, at operating temperature of 800 °C.     

Characterisation of electrical performance of anode supported micro-tubular solid oxide fuel cell with methane fuel

The reduction and operation of Ni–YSZ anode-supported tubular cells on methane fuel is described. Cells were reduced on pure methane from 650 °C to 850 °C, varying reduction time and methane flow rate. The effect on electrochemical performance with methane fuel was then investigated at 850 °C after which temperature-programmed oxidation (TPO) was employed to measure carbon deposition. Results showed that carbon deposition was minimized after certain reduction conditions. The conclusion was that 30 min reduction at 650 °C with 10 ml min⁻¹ methane reduction flow rate led to the highest current output over 1.2 A cm⁻² at 0.5 V when the cell operated at 850 °C between 10 ml min⁻¹ and 12.5 ml min⁻¹ methane running flow rate. From these results, it is evident that solid oxide fuel cell (SOFC) performance can be substantially improved by optimising preparation, reduction and operating conditions without the need for hydrogen.     

Analysis of solid oxide fuel cell system concepts with anode recycling

The main drivers for anode recirculation are the increased fuel efficiency and the independence of the external water supply for the fuel pre-reforming process. Different system flow-schemes have been defined and a set of parameters have been elaborated as basis for various calculations. Taking into account the combinations of layout, cell type, fuel utilization, fuel, and recycling ratio the total number of cases modeled was about 220. All calculated SOFC systems are on a high level of electrical net efficiency in the range of 50–66%. The electrical and thermal efficiencies are mainly influenced by the fuel utilization. The layout itself, the choice of fuel gas or the type of cell have minor effects on the system efficiency.     

Fracture strength of micro-tubular solid oxide fuel cell anode in redox cycling experiments

The maximum fracture strength of Ni/8YSZ anodes exposed to several redox cycles is compared. The anodes were fabricated using fine and coarse particle size powders. Fine-structured powders show a 77% increase in mechanical strength after exposure to three redox cycles. The coarse-structured material did not produce similar results and redox cycling resulted in gradual decrease in the mechanical stability of the supports.     

Effectiveness of anode in a solid oxide fuel cell with hydrogen/oxygen mixed gases

A porous Ni/YSZ cermet in mixed hydrogen and oxygen was investigated for its ability to decrease oxygen activity as the anode of a single chamber SOFC. A cell with a dense 300 μm YSZ electrolyte was operated in a double chamber configuration. The Ni–YSZ anode was exposed to a mixture of hydrogen and oxygen of varying compositions while the cathode was exposed to oxygen. Double chamber tests with mixed gas on the anode revealed voltage oscillations linked to lowered power generation and increased resistance. Resistance measurements of the anode during operation revealed a Ni/NiO redox cycle causing the voltage oscillations. The results of these tests, and future tests of similar format, could be useful in the development of single chamber SOFC using hydrogen as fuel.     

Performance of an anode support solid oxide fuel cell manufactured by microwave sintering

An effective and facile method has been developed to manufacture anode support solid oxide fuel cells in a multimode domestic microwave oven with selective susceptors. Anode support substrate pellets are prepared by an uniaxial pressing method, and then a thin YSZ electrolyte film is coated by a spray coating method. The electrolyte thickness is kept less than 10 m. The anode supported electrolyte is co-sintered being sandwiched by two spacers and two susceptors in the microwave oven. A cathode is then screen-printed onto the sintered dense electrolyte film and sintered again in the microwave oven with only one spacer and one susceptor. The whole solid oxide fuel cell is sintered at lower temperatures compared to conventional thermal sintering temperature. The performance of the present solid oxide fuel cell is measured in an intermediate temperature range of 650–800 °C. The maximum power densities of 0.09, 0.12, 0.2 and 0.26 W cm⁻² are obtained at operating temperatures of 650, 700, 750 and 800 °C, respectively.     

Fabrication of anode support for solid oxide fuel cell using zirconium hydroxide as a pore former

A novel method of fabricating NiO-YSZ (yttria stabilized zirconia) anode substrates is developed using a composite pore former, i.e., PMMA (polymethyl-methacrylate) and carbon black or zirconium hydroxide Zr(OH)₄. By utilizing a composite pore former, both the shrinkage and porosity, which must be compatible with that of the electrolyte film and sufficient for the fuel supply and exhaust, are easily tailored. Carbon black and the inorganic pore former (Zr(OH)₄) affect the shrinkage of the anode substrate more effectively than its porosity, while the polymer spheres (PMMA) adjust the porosity more effectively. In particular, the successful use of zirconium hydroxide as a fine pore former, instead of carbon black, suggests that other zirconium or nickel compound derivatives may be used as pore formers.     

Microstructure and polarization characteristics of anode supported tubular solid oxide fuel cell with co-precipitated and mechanically mixed Ni-YSZ anodes

An anode support tubular solid oxide fuel cell (SOFC) is fabricated and the dependence of its polarization resistance on anode microstructural parameters is investigated by means of stereology and concept of contiguity (c-c) theory. Nickel yttria-stabilized zirconia (Ni-YSZ) anode supported cell with YSZ electrolyte, lanthanum-strontium-manganite (LSM)-YSZ composite cathode, and LSM cathode layers is fabricated by dip coating. Submicrometer resolution images of anode microstructure are successfully obtained by low voltage SEM-EDX and quantified by stereological analysis. Cell voltage measurements and impedance spectroscopy are performed at temperatures of 650 and 750 °C with hydrogen and nitrogen mixture gas as a fuel. A quantitative relationship between polarization resistance and microstructural parameters such as circularity, three-phase boundary length, contiguity, etc. is investigated using the concept of contiguity (c-c) theory. The effectiveness of correlating polarization resistance of anode supported tubular SOFC using stereology and c-c theory is evaluated.     

Efficiency gain of solid oxide fuel cell systems by using anode offgas recycle - Results for a small scale propane driven unit

The transfer of high electrical efficiencies of solid oxide fuel cells (SOFC) into praxis requires appropriate system concepts. One option is the anode-offgas recycling (AOGR) approach, which is based on the integration of waste heat using the principle of a chemical heat pump.The AOGR concept allows a combined steam- and dry-reforming of hydrocarbon fuel using the fuel cell products steam and carbon dioxide. SOFC fuel gas of higher quantity and quality results. In combination with internal reuse of waste heat the system efficiency increases compared to the usual path of partial oxidation (POX).The demonstration of the AOGR concept with a 300 Wel-SOFC stack running on propane required: a combined reformer/burner-reactor operating in POX (start-up) and AOGR modus; a hotgas-injector for anode-offgas recycling to the reformer; a dynamic process model; a multi-variable process controller; full system operation for experimental proof of the efficiency gain.Experimental results proof an efficiency gain of 18 percentage points (η·POX = 23%, η·AOGR = 41%) under idealized lab conditions. Nevertheless, further improvements of injector performance, stack fuel utilization and additional reduction of reformer reformer O/C ratio and system pressure drop are required to bring this approach into self-sustaining operation.     

Fabrication of solid oxide fuel cell supported on specially performed ferrite-based perovskite cathode

La_0.6Sr_0.4FeO₃ (LSF) is a promising material for cathode support application because its thermal expansion coefficient (TEC) is matching with typical electrolytes and it has sufficient level of ionic and electronic conductivity. In this paper, the LSF was used as a support for fabrication dense and nanocrystalline film of (Ce_0.8Gd_0.2O_1.95) CGO. The LSF support was fabricated by iso-axial pressing of powder, which was prepared using modified Pechini method. Several fabrication parameters were altered in order to obtain support with required density, conductivity and strength. This included different sintering temperatures, addition of pore former and variation of compaction pressure. The LSF powders were examined by X-ray diffractometry and did not show any other phases. The electrical conductivity and density of LSF supports were investigated in order to select optimal support for CGO film deposition. The electrical measurements indicated that porosity highly influence the electronic conductivity of LSF. A low temperature and cost-effective method, called net shape technique, was used to deposit CGO film on LSF. Impedance spectroscopy measurements (IS) of obtained structure showed that electrical conductivity of CGO film and calculated activation energy is in good agreement with literature data. SEM images indicated that the film has no cracks and is about 4 μm thick.     

Scalable fabrication process of thin-film solid oxide fuel cells with an anode functional layer design and a sputtered electrolyte

The fabrication process for anode-supported thin-film solid oxide fuel cells (SOFCs) was investigated by using scalable and cost-effective methods. The anode functional layer (AFL) was introduced on the surface of the substrate to stably deposit the thin-film electrolyte. In previous studies, the AFL has been generally designed to increase the catalytic activity; however, in this study, additional design parameters including the roughness and density were controlled to achieve a pinhole-free thin-film electrolyte and structural stability. Through the developed process, button and large-sized cells were fabricated, and the electrochemical performance evaluation showed potential power density and impedance values at relatively low operating temperature. Microstructural analyses showed that each layer of the AFL, electrolyte, and cathode was uniformly coated on the substrate. The thin-film electrolyte was densely deposited without cracks or pinholes. The electrochemical performance and microstructure confirmed that the developed thin-film SOFCs are reliable and reproducible without costly processes or materials.     

The properties and performance of micro-tubular (less than 2.0 mm O.D.) anode suported solid oxide fuel cell (SOFC)

Tubular solid oxide fuel cells (SOFCs) have many desirable advantages compared to other SOFC applications. Recently, micro-tubular SOFCs were studied to apply them into APU systems for future vehicles. In this study, electrochemical properties of the micro-tubular SOFCs (1.6 mm O.D.) have been characterized. Electrochemical analysis showed excellent performance with a maximum power density of 1.3 W/cm² at 550 °C. The impedance information gained at cell operating temperatures of 450, 500, and 550 °C showed individual cell ohmic resistances of 1.0, 0.6, and 0.2 Ω respectively. Within the operating temperature range of 450-550 °C, the ceria based micro-tubular SOFCs (cathode length: 8 mm) were found to have power densities ranging between 0.263 and 1.310 W/cm². The mechanical properties of the tubes were also analyzed through internal burst testing and monotonic compressive loading on a c-ring test specimen. The two testing techniques are compared and related, and maximum hoop stress values are reported for each of the fabrication parameters. This study showed feasible electrochemical properties and mechanical strength of micro-tubular SOFC for APU applications.     

Study on degradation of solid oxide fuel cell anode by using pure nickel electrode

In this study, the interactions between Ni and YSZ in solid oxide fuel cell anode and the influence of glass seal to anode performances have been investigated by pure Ni anode sintered on YSZ pellet. The evolution of Ni-YSZ interface in 100 h galvanostatic polarization in hydrogen is studied with different humidities in hydrogen. Electrochemical impedance spectroscopy was applied to analyze the time variation of the anode electrochemical characteristics. The interface microstructural changes were characterized by scanning electron microscopy. The influence of bulk gas humidity, gas-sealing material and Ni coarsening on anode durability was studied. The degradation of pure Ni anode is considered to be determined by the competition among the mechanisms of silicon deposition, YSZ interface morphological change and Ni coarsening.     

Microstructural analysis of a solid oxide fuel cell anode using focused ion beam techniques coupled with electrochemical simulation

Porous composite electrodes play a critical role in determining the performance and durability of solid oxide fuel cells, which are now emerging as a high efficiency, low emission energy conversion technology for a wide range of applications.In this paper we present work to combine experimental electrochemical and microstructural characterisation with electrochemical simulation to characterise a porous solid oxide fuel cell anode. Using a standard, electrolyte supported, screen printed Ni-YSZ anode, electrochemical impedance spectroscopy has been conducted in a symmetrical cell configuration. The electrode microstructure has been characterised using FIB tomography and the resulting microstructure has been used as the basis for electrochemical simulation. The outputs from this simulation have in turn been compared to the results of the electrochemical experiments.A sample of an SOFC anode of 6.68 μm × 5.04 μm × 1.50 μm in size was imaged in three dimensions using FIB tomography and the total triple phase boundary density was found to be 13 μm⁻². The extracted length-specific exchange current for hydrogen oxidation (97% H₂, 3% H₂O) at a Ni-YSZ anode was found to be 0.94 × 10⁻¹⁰, 2.14 × 10⁻¹⁰, and 12.2 × 10⁻¹⁰ A μm⁻¹ at 800, 900 and 1000 °C, respectively, consistent with equivalent literature data for length-specific exchange currents for hydrogen at geometrically defined nickel electrodes on YSZ electrolytes.     

Solid oxide fuel cell bi-layer anode with gadolinia-doped ceria for utilization of solid carbon fuel

Pyrolytic carbon was used as fuel in a solid oxide fuel cell (SOFC) with a yttria-stabilized zirconia (YSZ) electrolyte and a bi-layer anode composed of nickel oxide gadolinia-doped ceria (NiO-GDC) and NiO-YSZ. The common problems of bulk shrinkage and emergent porosity in the YSZ layer adjacent to the GDC/YSZ interface were avoided by using an interlayer of porous NiO-YSZ as a buffer anode layer between the electrolyte and the NiO-GDC primary anode. Cells were fabricated from commercially available component powders so that unconventional production methods suggested in the literature were avoided, that is, the necessity of glycine-nitrate combustion synthesis, specialty multicomponent oxide powders, sputtering, or chemical vapor deposition. The easily-fabricated cell was successfully utilized with hydrogen and propane fuels as well as carbon deposited on the anode during the cyclic operation with the propane. A cell of similar construction could be used in the exhaust stream of a diesel engine to capture and utilize soot for secondary power generation and decreased particulate pollution without the need for filter regeneration.     

Interactions of nickel/zirconia solid oxide fuel cell anodes with coal gas containing arsenic

The performance of anode-supported and electrolyte-supported solid oxide fuel cells was investigated in synthetic coal gas containing 0-10 ppm arsenic at 700-800 °C. Arsenic was found to interact strongly with nickel, resulting in the formation of nickel-arsenic solid solution, Ni₅As₂ and Ni₁₁As₈, depending on temperature, arsenic concentration, and reaction time. For anode-supported cells, loss of electrical connectivity in the anode support was the principal mode of degradation, as nickel was converted to nickel arsenide phases that migrated to the surface to form large grains. Cell failure occurred well before the entire anode was converted to nickel arsenide, and followed a reciprocal square root of arsenic partial pressure dependence that is consistent with a diffusion-based rate-limiting step. Failure occurred more quickly with electrolyte-supported cells, which have a substantially smaller nickel inventory. For these cells, time to failure varied linearly with the reciprocal arsenic concentration. Failure occurred when arsenic reached the anode/electrolyte interface, though agglomeration of nickel reaction products may have also contributed. Test performed with nickel/zirconia coupons showed that arsenic was essentially completely captured in a narrow band near the fuel gas inlet. Arsenic concentrations of ∼10 ppb or less are estimated to result in acceptable rates of fuel cell degradation.     

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.     

Comparison of ultra-fast microwave sintering and conventional thermal sintering in manufacturing of anode support solid oxide fuel cell

Ultra-fast microwave sintering in a multi-mode domestic microwave oven with selective susceptor and spacer has been proved to be an effective and facile method in the manufacturing of anode support solid oxide fuel cell (SOFC). Two anode support SOFCs were fabricated by using ultra-fast microwave sintering and conventional thermal sintering techniques, separately. The performances of the two cells were measured and compared in a temperature range of 700-800 °C. The microstructures of the two cells after the measurements were compared qualitatively based on SEM images. FIB-SEM technique was used to reconstruct the 3-D microstructure of both anode and cathode. The quantitative comparison of 3-D reconstructions shows the application potential and the advantages of microwave technique in SOFCs manufacturing.     

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.     

A transient analysis of a micro-tubular solid oxide fuel cell (SOFC)

A two-dimensional, axisymmetric transient computational fluid dynamics model is developed for an intermediate temperature micro-tubular solid oxide fuel cell (SOFC), which incorporates mass, species, momentum, energy, ionic and electronic charge conservation. In our model we also take into account internal current leak which is a common problem with ceria based electrolytes. The current density response of the SOFC as a result of step changes in voltage is investigated. Time scales associated with mass transfer and heat transfer are distinguished in our analysis while discussing the effect of each phenomenon on the overall dynamic response. It is found that the dynamic response is controlled by the heat transfer. Dynamic behavior of the SOFC as a result of failure in fuel supply is also investigated, and it is found that the external current drops to zero in less than 1 s.