Numerical modelling of methane-powered micro-tubular, single-chamber solid oxide fuel cell

An experimentally validated, two-dimensional, axisymmetric, numerical model of micro-tubular, single-chamber solid oxide fuel cell (MT-SC-SOFC) has been developed. The model incorporates methane full combustion, steam reforming, dry reforming and water-gas shift reaction followed by electrochemical oxidation of produced hydrogen within the anode. On the cathode side, parasitic combustion of methane along with the electrochemical oxygen reduction is implemented. The results show that the poor performance of single-chamber SOFC as compared to the conventional (dual-chamber) SOFC (in case of micro-tubes) is due to the mass transport limitation on the anode side. The gas velocity inside the micro-tube is far too low when compared to the gas-chamber inlet velocity. The electronic current density is also non-uniform over the cell length, mainly due to the short length of the anode current collector located at the cell outlet. Furthermore, the higher temperature near the cell edges is due to the methane combustion (very close to the cell inlet) and current collection point (at the cell outlet). Both of these locations could be sensitive to the silver current collecting wire as silver may rupture due to cell overheating.     

Structural stability of silver under single-chamber solid oxide fuel cell conditions

In the present study, structural stability of silver under single-chamber conditions has been examined. Micro-tubular cells made of conventional solid oxide fuel cell materials (Ni-YSZ/YSZ/LSM) with silver paste and silver current-collecting wires (for both electrodes) were prepared. The cells were operated with methane/air mixture of 25/60 mL min⁻¹, furnace temperature of 750 °C, and at an operating voltage of 0.5 V. The results showed increasing porosity in the current-collecting silver wire with time, leading to rupture, finally. It is postulated that the porosity formation could be due to the formation of silver oxide which is highly unstable (volatile) at operating temperature considered in this study. Furthermore, vaporization and melting of silver due to cell overheating under mixed-reactant conditions is expected. Based on experimental evidences, it is concluded that silver may not be a good choice to be employed under the above specified operating conditions, as it lacks long-term structural stability.     

A parametric analysis of a micro-tubular, single-chamber solid oxide fuel cell (MT-SC-SOFC)

A parametric analysis is carried out in order to investigate the effect of different microstructural and operating parameters on the performance of a micro-tubular, single-chamber solid oxide fuel cell (MT-SC-SOFC). The results show that the cathode morphology (its microstructure and material) are important factors to consider. Other parameters such as inlet velocity (flow rate) and mixing ratio are also very effective in improving the cell performance but these parameters should be carefully controlled in order to avoid their counter-effects, like, lower fuel utilization, anode coking and oxidation-reduction. There are some other parameters such as, operating pressure, electrode porosity, permeability and cathode radiative emissivity, which have minimal effect in performance enhancement.     

Cell temperature measurements in micro-tubular, single-chamber, solid oxide fuel cells (MT-SC-SOFCs)

Anode supported, micro-tubular, solid oxide fuel cells made of nickel, yttria-stabilized zirconia (Ni-YSZ) anode, yttria-stabilized zirconia (YSZ) electrolyte and lanthanum strontium manganite (LSM) cathode have been prepared and operated under single-chamber conditions. Four different cells with varying cathode location/size, i.e. inlet, center, outlet and full size have been compared. The highest temperature rise of ∼93 °C and the highest power density of ∼36 mW cm⁻² (at a furnace temperature of 750 °C with methane/air = 25/60 mL min⁻¹) was observed in the case of cathode-inlet configuration. The scanning electron microscope (SEM) analysis shows that both the anode and cathode were badly damaged near the cell inlet in case of cathode-inlet configuration. On the contrary, both of the electrodes remained undamaged in case of cathode-outlet configuration.     

Micro-tubular, solid oxide fuel cell stack operated under single-chamber conditions

A micro-tubular, solid oxide fuel cell stack has been developed and operated under single-chamber conditions. The stack, made of three single-cells, arranged in triangular configuration, was operated between 500 and 700 °C with varying methane/air mixtures. The results show that the operating conditions for the stack differ significantly than the single-cell operation reported in our earlier study. The stack operated at 600 °C with methane/oxygen mixture of 1.0 gives stable performance for up to 48 h, whereas for the single-cell, this mixing ratio was not suitable. The increase in the inert gas flow rate improves the stack performance up to a certain extent, beyond that; the power output by the stack reduces due to extensive dilution of the reactants. It is concluded that both, the operating conditions and the addition of inert gas, need to be tuned according to the number of cells present within the stack.     

Mixed-reactant, micro-tubular solid oxide fuel cells: An experimental study

Anode-supported, micro-tubular solid oxide fuel cells were prepared and operated, utilizing mixed-reactant (methane and air mixture) supply. The cells were composed of conventional materials, i.e. nickel, yttria-stabilized zirconia (Ni-YSZ) as anode supported material, yttria-stabilized zirconia (YSZ) as electrolyte, and lanthanum strontium manganite (LSM) as cathode material. The cells were operated at various temperatures in between 550 and 800 °C with varying methane/air ratio (1:1-1:4.76). Cell performance was found to be strongly dependent on flow rate and mixing ratio. At 750 °C, the maximum open circuit voltage (OCV) of the cell was 1.05 V at a methane/air ratio of 1:4.76, with a maximum power output of 122 mW cm⁻². The degradation test shows 0.05% performance loss per 24 h, thereafter, fluctuations in current density were observed due to oxidation-reduction cycles over nickel surface. It is therefore concluded that although the methane/air ratio of 1:4.76 gives the best performance but the long-term performance is not guaranteed under such conditions.     

Alternative perovskite materials as a cathode component for intermediate temperature single-chamber solid oxide fuel cell

This paper exploits the suitability of three perovskite materials Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), GdBaCo₂O_5+δ (GBC) and Ba_0.5Sr_0.5Mn_0.7Fe_0.3O_3−δ (BSMF) as SOFC cathodes in the single-chamber configuration operating at the intermediate temperature range. TG analysis showed high thermal stability depending on the crystalline phases of the materials. The catalytic activity of these three materials for hydrocarbon conversion was investigated under a realistic feed, i.e. with hydrocarbon, oxygen, water and carbon dioxide. Electrochemical impedance spectroscopy of the various cathodes tested in symmetric cell configuration revealed a B-site dependence of the electrode catalytic activity for oxygen reduction. High temperature (1000 °C) powder reactivity tests over a gadolinium doped-ceria (CGO) and perovskite cathode revealed excellent chemical compatibility of BSMF and CGO. Catalytic tests associated with thermal and structural characterization attest to the suitability of these materials in the single-chamber configuration.     

A three-dimensional numerical model of a single-chamber solid oxide fuel cell

The aim of this work is to analyze the hydrodynamic/electrochemical performance of a solid oxide fuel cell operating on nitrogen diluted hydrogen/oxygen mixture. In this respect, a three-dimensional numerical model of a single-chamber solid oxide fuel cell (SC-SOFC) is developed. The model incorporates the coupled effects of fluid flow in a rectangular duct with mass transport in porous electrodes, selective electrochemical reactions (i.e. hydrogen oxidation on anode and oxygen reduction on cathode) on individual electrodes while operating on nitrogen diluted hydrogen–oxygen mixture. Results show the effect of depletion of gaseous mixture due to hydrogen and oxygen consumption along the flow direction. The model can predict hydrodynamic/electrochemical effects by varying the porosity of the gas diffusion electrodes/catalyst layers. The model is formulated in COMSOL Multiphysics 3.4, a commercial Finite Element Method (FEM) based software package.     

Experimental optimization of the fabrication parameters for anode-supported micro-tubular solid oxide fuel cells

A systematic optimization of several parameters significant in the fabrication of anode-supported micro-tubular solid oxide fuel cell via extrusion and dip coating is presented in this study. Co-sintering temperature of anode-support and electrolyte, the vehicle type and solid powder content used in electrolyte dip-coating slurry, electrolyte submersion time, cathode sintering temperature, powder ratio in the cathode functional layer, submersion time for the cathode functional layer and, submersion time and coating number of the anode functional layer are studied in this respect and optimized in the given order according to the performance tests and microstructural analyses. The performance of the micro-tubular cell is significantly improved to 0.49 Wcm⁻² at 800 °C after the optimizations, while that of the base cell is only 0.136 Wcm⁻². 12-cell micro-tubular stack is also constructed with the optimized cells and the stack is tested. Each cell in the stack is found to show very close performance to the single-cell performance and the stack with a maximum power of ~26 W at an operating temperature of 800 °C is therefore evaluated to be successful.     

A right-angular configuration for the single-chamber solid oxide fuel cell

A right-angular configuration for the single-chamber solid oxide fuel cell (SC-SOFC) has been proposed and operated successfully in a methane-oxygen mixture (CH₄:O₂ = 2:1) with a total flow rate of 300 mL min⁻¹. It was fabricated by attaching the Ni/yttria-stabilized zirconia (YSZ) anode and the Sm_0.2Ce_0.8O_1.9-impregnated La_0.7Sr_0.3MnO₃ (LSM) cathode on two mutually perpendicular planes of a YSZ electrolyte substrate. Effect of the relative position of the electrodes on the ohmic resistance has been investigated. It is shown that the cell exhibits the smallest ohmic resistance when the two electrodes are symmetrically located on the two planes. Compared with the conventional coplanar SC-SOFC, this configuration can make full use of the edge area of the electrolyte substrate and shorten the conductive channel of oxygen ion, leading to a remarkable reduction in ohmic resistance, an elevation of the open-circuit voltage, and, ultimately, an improved performance. The simple stack, consisting of two right-angular cells connected in series on an electrolyte, generated an open-circuit voltage 1.4 V at 700 °C and a maximum power 14.9 mW at 800 °C.     

Influence of patterned electrode geometry on performance of co-planar,single-chamber,solid oxide fuel cell

Co-planar, single-chamber, solid oxide fuel cells (SC-SOFCs) with linearly patterned electrode structures on one surface of the electrolyte are fabricated via a robo-dispensing method. The SC-SOFCs with various electrode patterns are prepared to investigate the relationship between electrode geometry and cell performance. The open-circuit voltage (OCV) for cells with a single electrode pair is unaffected by the anode-to-cathode distance. By contrast, for cells with multiple electrode pairs, increasing the number of electrode pairs leads to a gradual decrease in OCV. These observations confirm that the inter-mixing of product gases causes a loss in OCV and power density, which in turn reduces the oxygen partial pressure gradient between the anode and cathode. Keeping the electrode pairs apart by ∼4 mm permits cells with complex electrode geometry to exhibit higher OCVs and power densities.     

An anode-supported micro-tubular solid oxide fuel cell with redox stable composite cathode

A NiO–YSZ anode-supported hollow fiber solid oxide fuel cell (HF-SOFC) has been fabricated with redox stable (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ–Sm_0.2Ce_0.8O_1.9–YSZ (LSCM–SDC–YSZ) composite cathode. The characterization of NiO–YSZ hollow fibers prepared by the phase inversion method is focused on the microstructure, porosity, bending strength and electrical conductivity. A thin YSZ electrolyte membrane (about 10 μm) can be prepared by a vacuum-assisted dip-coating process and is characterized in terms of microstructure and gas-tightness. The performance of the as-prepared HF-SOFC is investigated at 750–850 °C with humidified H₂ as fuel and ambient air as the oxidant. The peak power densities of 513, 408 and 278 mW cm⁻² can be obtained at 850, 800 and 750 °C, respectively, and the corresponding interfacial polarization resistances are 0.14, 0.29 and 0.59 Ω cm². The high performance at intermediate-to-high temperatures could be attributed to thin electrolyte and proper composite cathode with low interfacial polarization resistance. The low interfacial polarization resistance suggests potential applications of LSCM–SDC–YSZ composite oxides as the redox stable cathode. This investigation indicates that the redox stable LSCM–SDC–YSZ is a promising cathode material system for the next generation YSZ-based HF-SOFC. The results will be expected to open up a new phase of the research on the micro-tubular SOFCs.     

Thermal stress modeling of anode supported micro-tubular solid oxide fuel cell

An anode-supported micro-tubular solid oxide fuel cell (SOFC) is analyzed by a two-dimensional axisymmetric numerical model, which is validated with the experimental I-V data. The temperature distribution generated by the thermo-electrochemical model is used to calculate the thermal stress field in the tubular SOFC. The results indicate that the current transport in the anode is the same at every investigated position. The stress of the micro-tubular cell occurs mainly because of the residual stress due to the mismatch between the coefficients of thermal expansion of the materials of the membrane electrode assembly. The micro-tubular cell can operate safely, but if there is an interfacial defect or a high enough tensile stress applied at the electrolyte, a failure can arise.     

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.     

Review of the micro-tubular solid oxide fuel cell: Part I. Stack design issues and research activities

Fuel cells are devices that convert chemical energy in hydrogen enriched fuels into electricity electrochemically. Micro-tubular solid oxide fuel cells (MT-SOFCs), the type pioneered by K. Kendall in the early 1990s, are a variety of SOFCs that are on the scale of millimetres compared to their much larger SOFC relatives that are typically on the scale of tens of centimetres. The main advantage of the MT-SOFC, over its larger predecessor, is that it is smaller in size and is more suitable for rapid start up. This may allow the SOFC to be used in devices such as auxiliary power units, automotive power supplies, mobile electricity generators and battery re-chargers.The following paper is Part I of a two part series. Part I will introduce the reader to the MT-SOFC stack and its applications, indicating who is researching what in this field and also specifically investigate the design issues related to multi-cell reactor systems called stacks. Part II will review in detail the combinations of materials and methods used to produce the electrodes and electrolytes of MT-SOFC's. Also the role of modelling and validation techniques used in the design and improvement of the electrodes and electrolytes will be investigated. A broad range of scientific and engineering disciplines are involved in a stack design. Scientific and engineering content has been discussed in the areas of thermal-self-sustainability and efficiency, sealing technologies, manifold design, electrical connections and cell performance optimisation.     

Efficiency and fuel utilization of methane-powered single-chamber solid oxide fuel cells

The single-chamber solid oxide fuel cell (SC-SOFC) is a simplification of the conventional dual-chamber SOFC and has great potential for meeting portable power generation needs. While the high energy density of hydrocarbon fuels makes SC-SOFC a promising candidate as a power source for scenarios where portability is most preferred, the low efficiency and fuel utilization reported by many experimental groups have presented a major barrier keeping it from real application.     

Enhanced performance of a single-chamber solid oxide fuel cell with an SDC-impregnated cathode

The electrochemical performance of anode-supported single-chamber solid oxide fuel cells (SC-SOFCs) with and without SDC-impregnated cathodes was compared in a diluted methane–oxygen mixture. These cells were made of conventional materials including yttrium-stabilized zirconia (YSZ) thin film, a Ni + YSZ anode and a La_0.7Sr_0.3MnO₃ (LSM) cathode. Our results showed that the cell performance was greatly enhanced with the SDC-impregnated LSM cathode. At a furnace temperature of 750 °C, the maximum power density was as high as 404 mW cm⁻² for a CH₄ to O₂ ratio of 2:1, which was 4.0 times higher than the cell with a pure LSM cathode (100 mW cm⁻²). The overall polarization resistance of the impregnated cell was 1.6 Ω cm², which was much smaller than that of the non-impregnated one (4.2 Ω cm²). The impregnation introduced SDC nanoparticles greatly extended the electrochemical active zone and hence greatly improved the cell performance.     

A new design of metal supported micro-tubular solid oxide fuel cell with sandwich structure

Micro-tubular solid oxide fuel cell (MT-SOFC) is considered as a promising choice for portable applications. In this work, we developed a novel metal supported MT-SOFC with porous 430 stainless steel support| 430 stainless steel-SSZ| SSZ| porous SSZ sandwich structure by dip coating and one-step co-sintering technology. The metal supported MT-SOFC showed good connection between each function layer and exhibited a significant maximum power density of 271 mW cm^?2 at 800 °C. Although the power density showed about 19.6% off after 14 thermal cycles between 600 °C and 800 °C, and about 5% per 100 h degradation rate during the 200 h long-term stability test at 700 °C and 0.7 V, the structure of the single cell could be maintained well and no crack and Sr diffusion was observed. As the result, the ohmic resistance of the cell kept unchanged during the thermal cycling and long-term test. The relatively fast degradation was attributed to the Ni and LSM particles coarsening and agglomeration, which will be improved in the further work. This work presented a low-cost and simple way to fabricate the metal supported MT-SOFCs with good electrochemical performance.     

Study of oxygen reduction mechanism on Ag modified Sm1.8Ce0.2CuO4 cathode for solid oxide fuel cell

Different amount of metal silver particles are infiltrated into porous Sm_1.8Ce_0.2CuO₄ (SCC) scaffold to form SCC–Ag composite cathodes. The chemical stability, microstructure evolution and electrochemical performance of the composite cathode are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and AC impedance spectroscopy respectively. The composite cathode exhibits enhanced chemical stability. The metal Ag remains un-reacted with SCC and Ce_0.9Gd_0.1O_1.95 (CGO) at 800 °C for 72 h. The polarization resistance of the composite cathode decreases with the addition of metal Ag. The optimum cathode SCC-Ag05 exhibits the lowest area specific resistance (ASR, 0.43 Ω cm²) at 700 °C in air. Investigation shows that metal Ag accelerates the charge transfer process in the composite cathode, and the rate limiting step for electrochemical oxygen reduction reaction (ORR) changes to oxygen dissociation and diffusion process.     

Monte-Carlo simulation and performance optimization for the cathode microstructure in a solid oxide fuel cell

A 3D micro-scale model is developed to simulate the transport and electrochemical reaction in a composite cathode. This model takes into account the details of the specific cathode microstructure such as random pore structure, active TPB (three phase boundary) site distribution, particle size and composition and their interrelationship to the charge transfer and mass transport processes. Especially, the pore structure and mass diffusion were incorporated into this model. Influence of the microsturcture parameters on the performance was investigated by numerical simulations.