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.     

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.     

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.     

Silver modified cathode for a micro-tubular, single-chamber solid oxide fuel cell

Micro-tubular, solid oxide fuel cells consisting of nickel, yttria-stabilized zirconia (Ni-YSZ) anode, yttria-stabilized zirconia (YSZ) electrolyte and lanthanum strontium cobaltite ferrite-gadolinium doped ceria (LSCF-GDC) cathode have been developed and operated under single-chamber conditions, utilizing methane/air mixture. The cell performance was compared with a silver modified cathode by the addition of 10wt% silver-paste in LSCF-GDC cathode. The cells with and without silver addition yielded maximum power densities of 118.75 mW cm⁻² and 61.53 mW cm⁻² at 700 °C, respectively. The results demonstrate that silver is a good candidate for enhancing the oxidation reduction kinetics via improved adsorption, desorption, dissociation and subsequent diffusion. However, long term performance of the silver modified cathode is not guaranteed under single-chamber conditions.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Analysis of a perovskite-ceria functional layer-based solid oxide fuel cell

A fuel cell based on a functional layer of perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) composited samarium doped ceria (SDC) has been developed. The device achieves a peak power density of 640.4 mW cm^?2 with an open circuit voltage (OCV) of 1.04 V at 560 °C using hydrogen and air as the fuel and oxidant, respectively. A numerical model is applied to fit the experimental cell voltage. The kinetics of anodic and cathodic reactions are modeled based on the measurements obtained by electrochemical impedance spectroscopy (EIS). Modeling results are in well agreement with the experimental data. Mechanical stability of the cell is also examined by using analysis with field emission scanning electron microscope (FESEM) associated with energy dispersive spectroscopy (EDS) after testing the cell performance.     

A durability model for solid oxide fuel cell electrodes in thermal cycle processes

Despite the intense interest in solid oxide fuel cells, many details of their durability remain a mystery. Here, we present the insight see on electrode degradation in thermal cycle processes. Our model interprets the degradation to the stresses induced by thermal expansion mismatch of the electrocatalyst and electrolyte in a composite electrode that undergoes a temperature change. Such stresses might break the particle-particle interfaces (grain boundaries), thus reduce oxygen-ionic conductivity, electronic conductivity, and three-phase boundaries within the electrode, and consequently, degrade its performance. The model formulates the degradation rate as a function of cycle number, thermal expansion coefficient, composition, and particle size, providing a remarkable ability to balance thermal expansion restriction and catalytic activity of electrode materials, to optimize the electrode structure and composition, and to predict thermal-cycle durability. The model explicitly demonstrates that, in addition to their excellent electrochemical activity, nanostructured electrodes exhibit exceptional durability in thermal cycle processes.     

Comparative performance analysis of a direct ammonia-fuelled anode supported flat tubular solid oxide fuel cell: A 3D numerical study

Solid oxide fuel cells that are designed in different geometrical structures (planar, tubular, flat-tubular, etc.) are dirt-free, quiet, and efficient cells that run using different fuels including contagions fuels. In this work, the performance of a 3D model of direct ammonia feed anode supported flat-tubular solid oxide fuel cell having six fuel supply channels was developed, investigated, and elucidated numerically in comparison with hydrogen fuels at different operating conditions using COMOSOL Multiphysics. The finding of this study is revealed that the performance of the developed model that is running with direct ammonia is better than hydrogen feed one using the same geometrical dimensions and operating parameters. It is also confirmed that direct ammonia feed anode supported flat-tubular solid oxide fuel cell has outstanding performance over the corresponding anode supported tubular solid oxide fuel cell using the same active cell surface area, gas channel length, and operating conditions. Parametric sweep analyses have been also performed on selected operating parameters and the outcomes revealed that the working temperature and the amount of reactant gases have a powerful impact on cell performance. Thus, ammonia is a green auspicious, and profitable candidate to use as a carbon-neutral fuel for anode supported flat-tubular solid oxide fuel cells in the near future.     

Asymmetric behavior of solid oxide cells between fuel cell and electrolyzer operations

The electrochemical performance of solid oxide cells (SOCs) is investigated under both fuel cell and electrolyzer operations to understand their asymmetric behavior between the two operation modes. The current–voltage and electrochemical impedance characteristics of a hydrogen-electrode-supported cell are experimentally analyzed. Also, a numerical model is developed to reproduce the cell performance and to understand the internal resistances of the cell. Partial pressures of supplied gas and load current are varied to evaluate their effects on the cell performance. The gas partial pressures of hydrogen and steam supplied to the hydrogen electrode are kept equivalent so that the cell performance can be fairly compared between the two operation modes when the same current is applied. It is found that the origin of the asymmetry is mostly from the hydrogen electrode; both activation and concentration overpotentials show asymmetric behavior particularly at high current densities. A numerical experiment is also conducted by deliberately changing parameters in the model. Asymmetry in the activation overpotential is found to be originated from the non-identical charge-transfer coefficients in the Butler–Volmer equation and also from the non-uniform gas concentration formed in the hydrogen electrode under current-biased conditions. On the other hand, asymmetry in the concentration overpotential is associated with the non-equimolar counter diffusion of hydrogen and steam caused by the effect of Knudsen diffusion. Therefore, enhancing gas transport in the hydrogen electrode and reducing the contribution of Knudsen diffusion are effective approaches to reduce asymmetry not only in the concentration overpotential but also in the activation overpotential.     

Simulation of hydrogen and coal gas fueled flat-tubular solid oxide fuel cell (FT-SOFC)

Solid oxide fuel cells (SOFCs) are considered an important technology in terms of high efficiency and clean energy generation. Flat-tubular solid oxide fuel cell (FT-SOFC) which is a combination of tubular and planar cell geometries stands out with its performance values and low costs. In this study, the performance of an FT-SOFC is analyzed numerically by using finite element method-based design as a result of changing parameters by using different fuels which are pure hydrogen and coal gas with various proportions of CO. In addition, cell performance values for different temperatures were analyzed and interpreted. Analyzes have been performed by using COMSOL Multiphysics software. The rates of CO composition used are 10%, 20%, and 40%, respectively. In addition, the air was used as the oxidizer in all cases. The cell voltage and average cell power of the FT-SOFC were examined under the 800 °C operating condition. The maximum power value and current density value were obtained as 710 W/m² and 1420 A/m² for the flat-tubular cell, respectively. As a result of the study, it was observed that the maximum cell power densities increased with increasing temperature. Analysis results showed that FT-SOFCs have suitable properties for different fuel usage and different operating temperatures. High-performance values and design features in different operating conditions are expected to make FT-SOFC the focus of many studies in the future.     

A fully-homogenized multiphysics model for a reversible solid oxide cell stack

In electrochemical devices such as solid oxide cell stacks, many physical phenomena are interacting on many different length scales in an intricate geometry. Modeling is a strong tool to understand the interior of such devices during operation, enhance their design and investigate long-term response (degradation). Computations can however be challenging as the many geometric details and coupled physical phenomena require a significant computational power, and in some cases, even state-of-the-art clusters will not be sufficient. This hinders the use of the models for the further development of the technology. In this work, we present an original type of solid oxide cell stack model, which is highly computationally efficient, resulting in computations which are two orders of magnitude faster than the conventional type of stack models with all geometric details explicitly represented. In the model presented here, the geometric details are implicitly represented by using the so-called homogenization. The resulting homogeneous anisotropic media provides the correct overall response (temperature, species, molar fractions, etc.). Local details as the mechanical stress in the electrolyte are not represented explicitly. These can be retrieved by localization through sub-models (multiscale model), in some cases without loss of computational efficiency, as demonstrated.     

A review of numerical modeling of solid oxide fuel cells

The solid oxide fuel cell (SOFC) is one of the most promising fuel cells for direct conversion of chemical energy to electrical energy with the possibility of its use in co-generation systems because of the high temperature waste heat. Various mathematical models have been developed for three geometric configurations (tubular, planar, and monolithic) to solve transport equations coupled with electrochemical processes to describe the reaction kinetics including internal reforming chemistry in SOFCs. In recent years, considerable progress has been made in modeling to improve the design and performance of this type of fuel cells. The numbers of the contributions on this important type of fuels have been increasing rapidly. The objective of this paper is to summarize the present status of the SOFC modeling efforts so that unresolved problems can be identified by the researchers.     

Fabrication and testing of coplanar single-chamber micro solid oxide fuel cells with geometrically complex electrodes

Coplanar single-chamber micro solid oxide fuel cells (SC-μSOFCs) with curvilinear microelectrode configurations of arbitrarily complex two-dimensional geometry were fabricated by a direct-write microfabrication technique using conventional fuel cell materials. The electrochemical performance of two SC-μSOFCs with different electrode shapes, but comparable electrode and inter-electrode dimensions, was characterized in a methane–air mixture at 700 °C. Both cells exhibited stable open circuit voltage and peak power density of 0.9 V and 2.3 mW cm⁻², respectively, indicating that electrode shape did not have a significant influence on the performance of these fuel cells.