Anode-supported Planar Solid Oxide Fuel Cells Research Articles

Beyond Traditional fuel cells: Development and a comprehensive analysis of mechanically Robust metal mesh-supported solid oxide fuel cell

At elevated operating temperatures, a large temperature gradient can cause irreparable damage to the solid oxide fuel cells (SOFCs) stack, eventually interrupting the durability of the stack. Metal substrate support could be used to overcome this challenge. However, the application of metal substrate support poses various challenges such as different thermal expansion coefficients, pore diameters, and complex fabrication techniques. Therefore, a first-ever novel and mechanically strong ferritic stainless-steel metal mesh-supported SOFC design is developed to mitigate these challenges. The metal mesh of 200 μm thickness is laminated with tape-casted green films of anode-support, anode functional layer (AFL), and electrolyte films of SOFC. The iso-static pressure of 300 MPa exhibits a firm attachment of the green films of SOFC with the metal mesh. Subsequently, the metal mesh-supported planar SOFC exhibits 3.3 times higher flexural strength compared to the conventional commercial anode-supported planar SOFC. The nano-CuO is added to constituent layers as a sintering aid to attain the maximum density at a lower sintering temperature of 1100 °C. The result shows that the practical application of the metal mesh-supported cell technology has a great potential to overwhelm the mechanical durability of SOFCs.

Numerical Investigation of Heat/Flow Transfer and Thermal Stress in an Anode-Supported Planar SOFC

To elucidate the thermofluid reacting environment and thermal stress inside a solid oxide fuel cell (SOFC), a three-dimensional SOFC model is implemented by using the finite element method in the commercial software COMSOL Multiphysics®, which contains both a geometric model of the full-cell structure and a mathematical model. The mathematical model describes heat and mass transfer, electrochemical reactions, internal reforming reactions, and mechanical behaviors that occur within the cell. A parameter study is performed focusing on the inlet fuel composition, where humidified hydrogen and methane syngas (the steam-to-carbon ratio is 3) as well as the local distribution of temperature, velocity, gas concentrations, and thermal stress are predicted and presented. The simulated results show that the fuel inlet composition has a significant effect on the temperature and gas concentration distributions. The high-temperature zone of the hydrogen-fueled SOFC is located at the central part of units 5, 6, and 7, and the maximum value is about 44 K higher than that of methane syngas-fueled SOFC. The methane-reforming and electrochemical reactions in the anode active layer result in a significant concentration gradient between the anode support layer and the active layer of the methane syngas-fueled SOFC. It is also found that the thermal stress distributions of different fuel inlet compositions are rather different. The maximum stress variation gradient between electrode layers of hydrogen SOFC is larger (44.2 MPa) than that of methanol syngas SOFC (14.1 MPa), but the remaining components have a more uniform stress distribution. In addition, the electrode layer of each fuel SOFC produces a significant stress gradient in the y-axis direction, and stress extremes appear in the corner regions where adjacent assembly components are in contact.

Multiphysical field simulation of exchange current density effection on solid oxide fuel cell (SOFC) with methane mixture fuel

In order to understand the performance and internal parameter distribution of SOFC fueled by methane mixture in detail, the paper uses numerical simulation method to conduct three-dimensional multiphysics coupling simulation on a single-channel anode-supported planar SOFC. The uniqueness of the three-dimensional model is that it employs the anode exchange current density from the methane gas mixture and considers the electrochemical oxidation of carbon monoxide. The numerical model proposed in this work provides more accurate simulation results. The results show that compared with the model that uses the exchange current density equation from humidified hydrogen for simulation, the model that uses the exchange current density equation from methane mixture to simulate the performance curve is closer to the experimental value. When using methane gas mixture as fuel, the maximum temperature difference along the flow direction is 10 K and the maximum power density can reach 0.482 W/cm2. In the thickness direction of the SOFC, the current density is not much different at the inlet and outlet. Along the direction of flow, the current density decreases near the inlet and outlet. Reducing the rate of fuel flow or increasing gas inflow temperature can improve SOFC performance.

Optimization of Cathode Structure for Anode-Supported Planar Solid Oxide Fuel Cell

Abstract It is always an important research direction to improve the performance of solid oxide fuel cells (SOFCs) through structural optimization. For anode-supported SOFC, the thin cathode thickness results in an uneven distribution of oxygen concentration in the porous electrode, which limits the output performance. In this paper, a three-dimensional model of the anode-supported planar SOFC was established, and the thickness of the cathode diffusion layer in the area covered by the connector was increased to improve the uniformity of oxygen distribution in the cathode. The results show that increasing the thickness of the cathode diffusion layer under the connector can significantly improve the overall output performance of SOFC. The uniformity of oxygen concentration distribution in the cathode is improved, the local current density under the connector is increased, and the maximum output power density of the cell is increased by 29.14%. The results show that the output performance of the SOFC and the uniformity of gas distribution in the porous electrode can be improved by increasing the thickness of the cathode diffusion layer under the connector in the structure design and practical application of the SOFC. The research of this paper provides a reference for the optimization design of SOFC.

Three-Dimensional Modeling of Anode-Supported Planar SOFC with Corrugated Electrolyte

The aim of this work is to analyze the effect of corrugated electrolyte on the cell performance. The corrugated electrolyte is applied to the traditional anode-supported planar solid oxide fuel cell (SOFC) based on the theory of mesoscale electrode-electrolyte interface modification. Three-dimensional models of SOFCs with electrolytes with different corrugation heights and periods are developed. In order to better understand the influence mechanism of corrugated electrolyte, not only the effect of the corrugated electrolyte on the overall cell performance is analyzed, but also the local gas distribution, concentration polarization, activation polarization, exchange current density and electron current density are discussed. The results show that the corrugated electrolyte leads to a decrease in the local exchange current density, but the enlargement of the activation reaction zone leads to an increase in the total charge transfer, thereby resulting in an improvement in cell performance. The enhancement of average current density with an expansion factor of 2 has a maximum value of 56.9% at an operating voltage of 0.7 V.

Development of High-Performance Anode-Supported Planar SOFC with Large Area by 4-Layer Co-Firing Process

Solid oxide fuel cell (SOFC) technology has attratced great attention because of higher efficiency, fuel flexibility and eco-friendly stationary power generation system. However, its commercialization is hindered due to the lack of durability and high cost. Herein, a facile and exceptionally reliable 4-layered anode supported SOFCs with a large area (12cm×12cm) was fabricated in a single step co-firing process at 1400 oC, in which tape-casted green films of NiO-YSZ, NiO-1CeScSZ, 1CeSxSZ, and GDC were laminated by isostatic press. The fabrication parameter such as viscosity of the slurries and mechanical strengths of the green films, etc. were optimized. As a result, finely porous anode layer, and highly dense 1CeScSZ electrolyte and GDC buffer layers on a robust porous anode-support were formed successfully. The LSC cathode was screen-printed on the GDC layer. The possibility of mass production of this process are verified through the fabrication of large area cells with an active area 100cm2. The single cell showed high power output of 38 W at total current 50A at 700°C and exhibits low degradation rate of 0.2% kh-1 at 25A for long-term test of 1000 hrs. This facile strategy is time-efficient, reproducible, cost-effective, and highly practical to produce SOFC system satisfying both high-performance and durability.

Simulation and Analysis of Thermal Stress and Sintering Warpage of Planar Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is one of the new energy conversion technologies producing electricity and heat from fuel and oxidant through an electrochemical reaction. The high operating temperature has many advantages including high efficiency, flexible fuel adaptability and low emissions. However, it has also some drawbacks, e.g., thermal expansion mismatches among the involved materials, particularly when the temperature reaches around 1400 oC during the sintering process, which may cause non-uniform distribution of thermal stress and even further deformation and warpage observed in the cell manufacturing steps.A three-dimensional CFD (computational fluid dynamics) simulation model is developed to study the thermal stress distributed in the sintered function layers of the anode-supported planar SOFC unit cells (10×10 cm2) using a finite element method. Five layers are included, i.e., cathode and its separation layer (GDC/LSCF, 70 µm), anode support and active layers (Ni/YSZ, 550µm), and electrolyte layer (YSZ, 30 µm) between them. In terms of the thermal stress and deformation, the predicted results are presented and discussed for the sintered unit cells with different angels located around the four cornels.It is found that the maximum thermal stress occurs at the interface between the electrode and electrolyte; the magnitude and distribution of thermal stress at the interfaces are closely related to the material thermal properties; the maximum deformation about 13 mm in the thickness direction is predicted for the 90-angel shaped cornels at the sintering temperature 1400 oC, which is bigger than that for the case with the circular shaped cornels; the deformation magnitude depends not only on the maximum thermal stress, but also the difference between the maximum and minimum thermal stress. The findings and predicted results may be applied for optimization of design and sintering conditions for SOFC unit cells.AcknowledgementsThis work is supported by the National Key Research and Development Project of China (2018YFB1502204, 2018YFB1502203, 2018YFB1502205), the Ningbo major special projects of the Plan “Science and Technology Innovation 2025” (2018B10048). Figure 1

Development of High-Performance Anode-Supported Planar SOFC with Large Area by 4-Layer Co-Firing Process

Solid oxide fuel cell (SOFC) technology has attracted great attention because of higher efficiency, fuel flexibility, and eco-friendly stationary power generation system. However, its commercialization is hindered due to the lack of durability and high-cost. Herein, a facile and exceptionally reliable 4-layered anode supported SOFCs with a large area (12cm×12cm) was fabricated in a single step co-firing process at 1400°C, in which tape-casted green films of NiO-YSZ, NiO-1CeScSZ, 1CeScSZ, and GDC were laminated by isostatic press. The fabrication parameter, such as viscosity of the slurries and mechanical strengths of the green films, were optimized. As a result, finely porous anode layer, and highly dense 1CeScSZ electrolyte and GDC buffer layers on a robust porous anode-support, were formed successfully. The possibility of mass production is verified through the fabrication of large area cells with an active area 100 cm2. The single cell showed high-power output of 28 W at total current 50 A at 700°C and exhibits low degradation rate of 0.2%/kh at 25A for long-term test of 1000 hrs. This facile strategy is time-efficient, reproducible, cost-effective, and highly practical to produce SOFC system satisfying both high-performance and durability.

Acceleration tests: Degradation of anode-supported planar solid oxide fuel cells at elevated operating temperatures

As the solid oxide fuel cell (SOFC) technology matures, durability under real operating conditions is considered as one of the most critical issues for commercialization. The severe conditions encountered in practical operation include a large temperature gradient and generation of local hot spots within stacks. Herein, we report the degradation mechanisms of anode-supported planar SOFCs supplied by Posco Energy at elevated operating temperatures. A simple comparison of the voltage reduction rates at different operating temperatures does not appropriately represent the degree of degradation, because the rapid deterioration of the cell components at high temperatures is compensated for by the fast reaction and transport kinetics. A combination of impedance interpretation and post-mortem analysis reveals the major degradation processes that are distinctively accelerated by increasing temperature, including the chemical interaction between the cathode and electrolyte, the enlargement of the interfacial pores, the coarsening of the fine particles in the composite electrodes, the formation of interfacial cracks and Cr poisoning. Systematic analysis presented in this study provides guidelines for counteracting the unexpected temperature increase, and the database established under various extreme conditions would form the groundwork for achieving the lifetime goals of commercial SOFC systems.

In-Situ Analysis of the in-Plane Current Distribution Difference between Electrolyte-Supported and Anode-Supported Planar Solid Oxide Fuel Cells by Segmented Electrodes

In the planar solid oxide fuel cell (SOFC), the fuel/oxidant distributions cause current and temperature distributions over the electrodes under the separator (interconnector) ribs and flow channels. Optimized designs of the separator to improve the output power and chemical/thermo-mechanical durabilities of practical stacks require numerical models validated by insitu current distributions measured. We have therefore clarified the difference of in-situ measurements inplane spatial current variations between an anode-supported planar SOFC and electrolyte-supported by segmented cathodes opposing the anode rib and flow channels. We focus on the effect of the rib width on the spatial current distribution. We model the current and hydrogen partial pressure distributions by finite element modeling so that the model agrees with the insitu measurements by the segmented cathodes with determining the exchange current densities, electrode porosities, electrolyte ion conductivities, and electrode ion/electron conductivities.

Segmented Electrode Analysis of an Anode-Supported Planar Solid Oxide Fuel Cell for the Diagnosis of Marine Power Applications

In planar solid oxide fuel cells (SOFCs), the fuel/oxidant distributions cause current and temperature distributions over the electrodes, giving chemical and thermo-mechanical cell degradations as well as decays in the total cell performance and efficiency. We focus on the development of real-time abnormal-diagnosis method to improve the reliability and durability required for the long-term safety and stable operation of the marine SOFC. We therefore address in-situ measurements of inplane spatial current and impedance distributions along the flow channels of an anode-supported planar SOFC by segmented cathodes opposing to up, mid, and downstream parts of the anode.

Solid oxide fuel cell interconnect design optimization considering the thermal stresses

The mechanical failure of solid oxide fuel cell (SOFC) components may cause cracks with consequences such as gas leakage, structure instability and reduction of cell lifetime. A comprehensive 3D model of the thermal stresses of an anode-supported planar SOFC is presented in this work. The main objective of this paper is to get an interconnect optimized design by evaluating the thermal stresses of an anode-supported SOFC for different designs, which would be a new criterion for interconnect design. The model incorporates the momentum, mass, heat, ion and electron transport, as well as steady-state mechanics. Heat from methane steam reforming and water–gas shift reaction were considered in our model. The results examine the relationship between the interconnect structures and thermal stresses in SOFC at certain mechanical properties. A wider interconnect of the anode side lowers the stress obviously. The simulation results also indicate that thermal stress of coflow design is smaller than that of counterflow, corresponding to the temperature distribution. This study shows that it is possible to design interconnects for an optimum thermal stress performance of the cell.

Three-Dimensional Computational Fluid Dynamics Modelling for a Planar Solid Oxide Fuel Cell of a New Design

The aim of the work is to investigate the electrochemical processes of a planar Solid Oxide Fuel Cell (SOFC) and to estimate the performance of a novel SOFC design. The design involves cross-flow bipolar plates. Flow channels were designed to connect pairs of four orifices at the corners of the SOFC plates. Each of the bipolar plates has an air channel system on one side and a fuel channel system on the other side. One pair of channels in adjacent plates is used for the air flow along the cathode electrode, the other for the fuel flow along the anode of each cell. A schematic of the design proposed by Bossel [1] is shown in Fig. 1. Figure 1. A schematic of the proposed planar SOFC design: (a) cathode side view, (b) anode side view [1] A three-dimensional Computational Fluid Dynamics (CFD) model for an anode-supported planar SOFC with complex bipolar plates has been developed using the ANSYS Fluent code with additional Fuel Cell Tools module. The conservation equations of mass, momentum, energy and species as well as electrochemical models were solved for the fuel cell. The distributions of temperature and gas flow through the gas channels and PEN (positive electrode/electrolyte/negative electrode) structure were studied. The results identified the most susceptible areas for significant increase of the temperature at high current density, which can lead to hot spots formation and destruction of the electrodes. Knowledge of such parameters as the flow fields, pressure losses or temperature variation over the fuel cell area allowed to validate the effectiveness of the new SOFC design and is useful in further design modification. Acknowledgments The research programme leading to these results received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement no [325323]. Acknowledgments are due to the partners of SAFARI project. The work was also financed from the Polish research funds awarded for the project No. 3043/7.PR/2014/2 of international cooperation within SAFARI in years 2014-2016. [1] U. Bossel, Rapid startup SOFC modules, Energy Procedia, 28, 2012, 48-56. DOI: 10.1016/j.egypro.2012.08.039. Figure 1

Contribution of properties of composite cathode and cathode/electrolyte interface to cell performance in a planar solid oxide fuel cell stack

Solid oxide fuel cells (SOFCs) with distinct cathode materials usually differ in output performance. In this study, 2 μm-thick Pt voltage probes are embedded into the cathode/electrolyte interface. The effects of the electrical properties and cathode/electrolyte interfaces of LSCF-GDC and LSM-YSZ composite cathodes on cell performance are investigated in situ for anode-supported planar SOFCs. Results show that the voltage and maximum output power density measured by the probes on both sides of the LSCF-GDC and LSM-YSZ composite cathodes are 7% and 4%, respectively, of those of the corresponding cell during instantaneous current–voltage testing. The enhanced LSCF cell performance is mainly attributed to the rough GDC/LSCF-GDC interface that is responsible for the three-dimensional contact between the GDC layer and LSCF-GDC cathode particles and increases the triple-phase boundary (TPB) length. The LSM-YSZ cathode performance degradation is attributed to the variation in polarization resistance caused by cathode particle growth. However, the primary factor for the degradation of LSCF-GDC cathode performance is structural instability, such as inner cracks.

Experimental Characterization of a Direct Methane Flame Solid Oxide Fuel Cell Power Generation Unit

A direct flame solid oxide fuel cell unit was designed and built in this work for potential combined heat and power generation. The direct combination of a methane flat flame and a solid oxide fuel cell (SOFC) in a simple, “no-chamber” setup was implemented. Direct flame fuel cell experiments were performed using an anode-supported planar SOFC with 50-mm-long square anode and 30-mm-long square cathode. At the SOFC anode, a multi-element diffusion flame burner with an outlet of 55 mm by 55 mm square provided methane/air rich flames for various equivalence ratios and flow rates. The maximum power generated by the direct flame fuel cell unit reached 0.35 W with the power density of 388 W/m2 at the equivalence ratio of 1.2. The unit was operated for 3 hours at the equivalence ratio of 1.1 at a fixed voltage of 0.4 V with the current deteriorating from 0.29 A to 0.25 A. The system efficiency of the power generation unit was analyzed by considering the combustion efficiency, the fuel utilization efficiency and the SOFC electrical efficiency. The fuel utilization efficiency was found to be the dominant factor of the total efficiency.

Analysis of Design Parameters in Anode‐Supported Solid Oxide Fuel Cells Using Response Surface Methodology

AbstractAchieving high performance from a solid oxide fuel cell (SOFC) requires optimal design based on parametric analysis. In this paper, design parameters, including anode support porosity, thicknesses of electrolyte, anode support, and cathode functional layers of a single, intermediate temperature, anode‐supported planar SOFC, are analyzed. The response surface methodology (RSM) technique based on an artificial neural network (ANN) model is used. The effects of the cell parameters on its performance are calculated to determine the significant design factors and interaction effects. The obtained optimum parameters are adopted to manufacture the single units of an SOFC through tape casting and screen‐printing processes. The cell is tested and its electrochemical characteristics, which show a satisfactory performance, are discussed. The measured maximum power density (MPD) of the fabricated SOFC displays a promising performance of 1.39 W cm–2. The manufacturing process planned to fabricate the SOFC can be used for industrial production purposes.

Actuator Limitations in Spatial Temperature Control of SOFC

A high performance multi-input multi-output feedback controller has been developed to minimize solid oxide fuel cell (SOFC) spatial temperature variation during load following. Cathode flow rate and its inlet temperature are used to minimize spatial temperature variations in the SOFC electrode electrolyte assembly for significant load perturbations. We focus on control design in the presence of nonideal actuation. This includes the effects of fuel processing delays, cathode inlet thermal delays, and parasitic power associated with the blower supplying air to the cathode. The controller, based on energy-to-peak minimization synthesis, is applied to a dynamic model of an anode-supported coflow planar SOFC stack. The results indicate that many of the problems associated with realistic and imperfect actuation can be addressed with relatively standard control synthesis modifications, but fuel flow delays can compromise power following significantly. Finally, a strategy that relies primarily on partial internal reformation for power following addresses many of the difficulties associated with reformer delays.

Performance evaluation of an integrated small-scale SOFC-biomass gasification power generation system

The combination of biomass gasification and high-temperature solid oxide fuel cells (SOFCs) offers great potential as a future sustainable power generation system. In order to provide insights into an integrated small-scale SOFC-biomass gasification power generation system, system simulation was performed under diverse operating conditions. A detailed anode-supported planar SOFC model under co-flow operation and a thermodynamic equilibrium for biomass gasification model were developed and verified by reliable experimental and simulation data. The other peripheral components include three gas-to-gas heat exchangers (HXs), heat recovery steam generator (HRSG), burner, fuel and air compressors. To determine safe operating conditions with high system efficiency, energy and exergy analysis was performed to investigate the influence through detailed sensitivity analysis of four key parameters, e.g. steam-to-biomass ratio (STBR), SOFC inlet stream temperatures, fuel utilization factor (Uf) and anode off-gas recycle ratio (AGR) on system performance. Due to the fact that SOFC stack is accounted for the most expensive part of the initial investment cost, the number of cells required for SOFC stack is economically optimized as well. Through the detailed sensitivity analysis, it shows that the increase of STBR positively affects SOFC while gasifier performance drops. The most preferable operating STBR is 1.5 when the highest system efficiencies and the smallest number of cells. The increase in SOFC inlet temperature shows negative impact on system and gasifier performances while SOFC efficiencies are slightly increased. The number of cells required for SOFC is reduced with the increase of SOFC inlet temperature. The system performance is optimized for Uf of 0.75 while SOFC and system efficiencies are the highest with the smallest number of cells. The result also shows the optimal anode off-gas recycle ratio of 0.6. Regarding with the increase of anode off-gas recycle ratio, there is a trade-off between overall efficiencies and the number of SOFC cells.

Modeling of Overpotentials in an Anode-Supported Planar SOFC Using a Detailed Simulation Model

In this paper results from a parameter study of an anode-supported solid oxide fuel cell (SOFC) are presented. The effects on performance, current-voltage (I-V) characteristics, polarization voltages, and diffusion coefficients are modeled for different temperatures, electrolyte thickness, porosities, and pore sizes. The analysis is carried out for a planar SOFC with YSZ electrolyte, LSM cathode, and Ni-YSZ anode, with thicknesses 20, 50, and 500 μm respectively, and with co-flow geometry. The predicted performance is validated with measured data found in the literature with good agreement. Standard equations for binary and Knudsen diffusion in porous media, concentration overpotentials, and Ohm’s law are used in the modeling. Activation overpotential is predicted by use of temperature dependent linear equations at both the anode and cathode sides. It is found that both ohmic and activation overpotentials decrease considerably with increasing temperature, while concentration overpotentials increase moderately with increasing temperature. The effect on concentration overpotentials can be explained by the reduced gas density with increased temperature, despite the increasing diffusion coefficient. Furthermore, it was found that increasing the pore sizes decreases concentration overpotentials. At low pore size the Knudsen diffusion coefficient is a bottleneck for the diffusion coefficient since it is much lower than the binary diffusion coefficient. It has been demonstrated that by increasing the pore size the Knudsen diffusion coefficient is improved. The effect of porosity has much in common with the effect of pore size; increasing porosity leads to decreased concentration overpotential due to the improved diffusion coefficient. As a natural phenomenon for anode-supported cells, most of the concentration overpotentials take place at the anode side due to its thick structure despite the high diffusion coefficient of hydrogen. It must be underlined that in this study the effect of different porosities and pore sizes is modeled at the anode and cathode substrates only without taking into account the length of the three phase boundary (LTPB) near the electrolyte interface. However, since these microstructural parameters can have an impact on the LTPB, they can also have an impact on the activation overpotentials. This is not considered here and will be taken into account in future work.

SOFC fuel cell heat production: Analysis

In the present work the effect of various heat sources on the temperature field within an Anode Supported Planar Solid Oxide Fuel Cell (ASP-SOFC) is studied. In order to describe the thermal behavior within the SOFC during its operation, the coupling of the mass and energy transport phenomena along with the electrochemistry is required. More precisely, the subject of the present analysis is the visualization of the temperature field and the location of the highest temperatures within an ASP-SOFC fed with hydrogen and air. The studied parameters are: i) the temperature values of the reactants and ii) the different types of the heat sources; due to the over potentials, the Joule effect and the water formation. The complex system of the governing equations is numerically solved with the finite differences method and the temperature field within each domain of the ASP-SOFC is calculated via a mathematical model implemented in FORTRAN language. The mathematical model predictions for the temperature gradient within the ASP-SOFC under the influence of the studied parameters are thoroughly discussed.