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.     

Thermal stress and thermo-electrochemical analysis of a planar anode-supported solid oxide fuel cell: Effects of anode porosity

A 3D integrated numerical model is constructed to evaluate the thermal-fluid behavior and thermal stress characteristics of a planar anode-supported solid oxide fuel cell (SOFC). Effects of anode porosity on performance, temperature gradient and thermal stress are investigated. Using commercial Star-CD software with the es-sofc module, simulations are performed to obtain the current-voltage (I-V) characteristics of a fuel cell as a function of the anode porosity and the temperature distribution within the fuel cell under various operating conditions. The temperature field is then imported into the MARC finite element analysis (FEA) program to analyze thermal stresses induced within the cell. The numerical results are found to be in good agreement with the experimental data. It is shown that the maximum principal stress within the positive electrode-electrolyte-negative electrode (PEN) increases at a higher current and a higher temperature gradient. It is recommended that the temperature gradient should be limited to less than 10.6 °C mm⁻¹ to maintain the structural integrity of the PEN.     

Application of infrared thermal imaging to the study of pellet solid oxide fuel cells

The application of infrared thermal imaging to the study of solid oxide fuel cells is demonstrated. The temperature increase accompanying polarisation of gadolinium doped ceria pellet cells is measured and the effect of temperature increase on polarisation characteristics is modelled. Temperature increases of the order of 2.5 °C were measured for heavily loaded pellet cells. Measurement accuracy of 0.1 °C and spatial resolution of 0.5 mm allow temperature distribution heterogeneity to be clearly discerned. A total heat transfer coefficient is derived from experimental results that allow the development of a model that predicts the extent of self-heating. For pellet fuel cells, self-heating is not expected to have a large effect on the polarisation characteristics; however, for thin electrolytes and high current density the effect becomes appreciable.     

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.     

Basic properties of a liquid tin anode solid oxide fuel cell

An unconventional high temperature fuel cell system, the liquid tin anode solid oxide fuel cell (LTA-SOFC), is discussed. A thermodynamic analysis of a solid oxide fuel cell with a liquid metal anode is developed. Pertinent thermochemical and thermophysical properties of liquid tin in particular are detailed. An experimental setup for analysis of LTA-SOFC anode kinetics is described, and data for a planar cell under hydrogen indicated an effective oxygen diffusion coefficient of 5.3 × 10⁻⁵ cm² s⁻¹ at 800 °C and 8.9 × 10⁻⁵ cm² s⁻¹ at 900 °C. This value is similar to previously reported literature values for liquid tin. The oxygen conductivity through the tin, calculated from measured diffusion coefficients and theoretical oxygen solubility limits, is found to be on the same order of that of yttria-stabilized zirconia (YSZ), a traditional SOFC electrolyte material. As such, the ohmic loss due to oxygen transport through the tin layer must be considered in practical system cell design since the tin layer will usually be at least as thick as the electrolyte.     

Investigation of the electrochemical active thickness of solid oxide fuel cell anode

Determination of the electrochemical active thickness (EAT) is of paramount importance for optimizing the solid oxide fuel cell (SOFC) electrode. However, very different EAT values are reported in the previous literatures. This paper aims to systematically study the EAT of SOFC anode numerically. An SOFC model coupling electrochemical reactions with transport of gas, electron and ion is developed. The microstructure features of the electrode are modeled based on the percolation theory and coordinate number theory. Parametric analysis is performed to examine the effects of various operating conditions and microstructures on EAT. Results indicate that EAT increases with decreasing exchange current density (or decreasing TPB length) and increasing effective ionic conductivity. In addition to the numerical simulations, theoretical analysis is conducted including various losses in the electrode, which clearly shows that the EAT highly depends on the ratio of concentration related activation loss R_act,con to ohmic loss R_ohmic. The theoretical analysis explains very well the different EATs reported in the literature and is different from the common understanding that the EAT is controlled mainly by the ionic conductivity of electrode.     

Three dimensional stress analysis of solid oxide fuel cell anode micro structure

One of the most common problems in solid oxide fuel cells (SOFCs) is the delamination and thus the degradation of electrode/electrolyte interface which occurs in the consequences of the stresses generated within the different layers of the cell. Nowadays, the modeling of this problem under certain conditions is one of the main issues for the researchers. The structural and thermo-physical properties of the cell materials (i.e. porosity, density, Young's modulus etc.) are usually assumed to be homogenous in the mathematical modeling of solid oxide fuel cells at macro-scale. However, during the real operation, the stresses created in the multiphase porous layers might be very different than those at macro-scale. Therefore, micro-level modeling is required for an accurate estimation of the real stresses and the performance of SOFCs. This study presents a microstructural characterization and a finite element analysis of the delamination and the degradation of porous solid oxide fuel cell anode and electrode/electrolyte interface under various operating temperatures, compressing forces and material compositions by using the synthetically generated microstructures. A multi physics computational package (COMSOL) is employed to calculate the Von Misses stresses in the anode microstructures. The maximum thermal stress in the electrode/electrolyte interface and three phase boundaries is found to exceed the yield strength at 900 °C while 800 °C is estimated as a critical temperature for the delamination and micro cracks due to thermal stress generated. The thermal stress decreases in the grain boundaries with increasing content of one of the phases (either Ni or YSZ) and the porosity of the electrode. A clamping load higher than 5 kg cm⁻² is also found to exceed the shear stress limit.     

Micro modeling of solid oxide fuel cell anode based on stochastic reconstruction

A novel modeling scheme of SOFC anode based on the stochastic reconstruction technique and the Lattice Boltzmann Method (LBM) is proposed and applied to the performance assessment and also to the optimization of anode microstructures. A cross-sectional microscopy image is exploited to obtain a two-dimensional phase map (i.e., Ni, YSZ and pore), of which two-point correlation functions are used to reconstruct a three-dimensional model microstructure. Then, the polarization resistance of the reconstructed anode is obtained by the LBM simulation. The predicted anodic polarization resistance for a given microstructure and its sintering temperature dependence are in good agreement with the literature data. Three-dimensional distributions of potential and current can be obtained, while and the effect of working temperature is discussed. The proposed method is considered as a promising tool for designing SOFC anodes.     

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.     

Ni and metal aluminate mixtures for solid oxide fuel cell anode supports

Mixtures of nickel and metal aluminate (Ni–MAl₂O₄ [M = Fe, Co, Ni and Cu]) were fabricated, and their electrical conductivities, microstructures and thermal expansions were measured. During the sintering of these mixtures, MAl₂O₄ reacts with NiO to form NiAl₂O₄ and MO_x which are thought to be the reasons for the differences in the microstructures and electrical properties. Except for FeAl₂O₄, Ni–MAl₂O₄ mixtures show metallic conductivity behavior and their electrical conductivities are sufficient for cell operation. Their thermal expansion coefficients are much lower than conventional Ni-YSZ mixtures and closer to the 8YSZ electrolyte. The peak power densities of single cells supported with Ni–NiAl₂O₄ and Ni–CoAl₂O₄ are 410 and 440 mW cm⁻² at 850 °C, respectively, which are lower than 490 mW cm⁻² of Ni-YSZ. This is due to the polarization resistances of functional anode layer. The Ni–CuAl₂O₄-supported cell has no electrical performance because of Cu migration and segregation.     

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

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

Mesoscale-structure control at anode/electrolyte interface in solid oxide fuel cell

To enhance the power density of a solid oxide fuel cell, a mesoscale-structure control of an electrode/electrolyte interface was proposed; here, the mesoscale means a size range of 10-100 μm, which is larger than the microscale of the electrode particles but smaller than the macroscale of the cell geometries. Therefore, the mesoscale structure does not only change the local thickness of the electrolyte and electrode but also enlarge the electrode/electrolyte interface area, and thus influence the cell performance. First, to find effective conditions for the mesoscale-structure control, a preliminary theoretical analysis in a conventional flat cell was performed focusing on the ratio of the ion-conducting resistance to the reaction resistance. In the light of this basic knowledge, as a second step, the effects of the mesoscale structure on an anode side of an electrolyte-supported cell were studied numerically and experimentally. A 2D numerical simulation based on an equivalent electrical circuit model and the dusty-gas model was carried out. As a result, the mesoscale-grooved structure was found to be effective for enhancement of the power generation, if the groove scale is sufficiently larger than that of the active reaction region of the electrode. Qualitatively similar results were obtained from the experiments using a segmented electrode with both flat and mesoscale-grooved surface in a button-type cell.     

Thermal impact performance study for the thermal management of ammonia-fueled single tubular solid oxide fuel cell

With the substantial improvement of the direct ammonia fuel cells performance, it has become the key to the further development of ammonia fuel cells to deeply understand the heat and mass transfer process inside the cell and to study the thermal impacts generation mechanism during cell operation. In this paper, a whole-cell model of single tubular direct ammonia cracking solid oxide fuel cell (SOFC) is established, and the generation mechanism of thermal impacts inside the cell is analysed in a data-driven method. The model includes the coupling of chemical-electrochemical reactions, local current, local temperature, mass flow and energy transfer inside the cell. It's identified from model simulations that the key to the thermal impact optimization of direct ammonia cracking SOFCs is to reduce the effect of the excessively fast and unbalanced ammonia cracking reaction on the cell. Both introducing the ammonia pre-reforming reaction and improving the activation energy of the ammonia cracking reaction can increase the overall average temperature of the cell and improve the temperature distribution. The 96% ammonia pre-reforming SOFCs can improve the extreme temperature difference in the anode from 37.71 K to 0.52 K at the operating temperature of 800 °C. Increasing activation energy of ammonia cracking reaction by 1.5 times can also make the ammonia cracking reaction rate distribution more uniform at the fuel channel, it can improve the extreme temperature difference in the anode to 4.49 K. This study can enrich the basic theory and research methods of thermal management of direct ammonia cracking SOFCs, and provide theoretical support for further improving cell performance.     

Nickel/gadolinium-doped ceria anode for direct ethanol solid oxide fuel cell

This report investigates the properties of nickel/gadolinium-doped ceria (Ni/GDC) as anode material for bio-ethanol fueled SOFC. The Ni/GDC cermets with 18 and 44 wt.% Ni were prepared by a hydrothermal method. Ethanol decomposition, steam reforming, and partial oxidation of ethanol were studied using a fixed-bed reactor at 1123 K. Carbon was formed only under dry ethanol for both catalysts. The addition of water or oxygen to the feed inhibited the formation of carbon. Ni/GDC was used as the anode current collector layer and as a catalytic layer in single cells tests. No deposits of carbon were detected in single cells with Ni/GDC catalytic layer after 50 h of continuous operation under direct (dry) ethanol. This result was attributed to the catalytic properties of the Ni/GDC layer and the operation mechanism of gradual internal reforming, in which the oxidation of hydrogen provides the steam for ethanol reforming, thus avoiding carbon deposition.     

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.     

Performance analysis of solid oxide fuel cell using reformed fuel

Fuelling SOFC with reformed fuel can be beneficial due to it being cheaper compared to pure hydrogen. A biomass fuel can be easily modeled as a reformed fuel, as it can be converted into H₂ and CO using gasification or biodegradation, the main composition of product from a reformer. Hence in this study it is assumed that feed to the fuel cell contains only H₂ and CO. A closed parametric model is formulated. Performance is analyzed with changes in temperature, pressure and fuel ratio; considering the possible voltage losses, like ohmic, activation, mass transfer and fuel crossover. Performance curves consisting of operating voltage, fuel utilization, efficiency, power density and current density are developed for both pure hydrogen and mixture of CO and H₂. Variations of open circuit voltage with temperature, power density with current density, operating voltage with current density and maximum power density with fuel utilization are also evaluated.     

Sulfur-poisoned Ni-based solid oxide fuel cell anode characterization by varying water content

The 0.2% hydrogen sulfide (H₂S) poisoning of Ni/YSZ anode-supported solid oxide fuel cells (SOFCs) is investigated by varying water content in fuel. The degradation extent of the cell voltage decreases with increasing water content (from 0% to 10%). Water can be generated at the anode side through an electrochemical reaction, and the content of water product augments upon the addition of fuel. However, our results indicate that an increase in fuel utilization significantly aggravates H₂S poisoning behavior. The results of X-ray analysis suggest that the presence of water in fuel cannot affect the final forms of nickel sulfides, but microstructural inspection reveals different attack modes upon the injection of vapor (from 0 to 10%) into fuel containing 0.2% H₂S.     

Evaluation of the redox stability of segmented-in-series solid oxide fuel cell stacks

The tolerance for the reduction and oxidation (redox) reactions of the segmented-in-series solid oxide fuel cells (SIS-SOFCs) has been investigated. In conventional anode-supported solid oxide fuel cells (SOFCs), the anode and the substrate are typically prepared from Ni-YSZ-based materials which exhibit a significant dimensional change because of the redox reaction and cannot retain their structure. The substrate of the SIS-SOFCs is prepared from Ni-doped MgO-based material, which has a high redox tolerance, and the SIS-SOFC exhibits a good performance after the redox cycles.The degradation rate is approximately 0.15% per cycle in a redox condition of start-and-stop operation without fuel supply. In the other redox condition (when the fuel supply is interrupted for 1.5 min), the voltage of the SIS-SOFCs remains almost constant. However, the voltage of SIS-SOFCs decreases with an increase in the reoxidation time of the interruption in the fuel supply. The high redox tolerance is attributed to the fact that the diffusion coefficients, mean free path, and existence of the Ni particles in the substrate can effectively deter the oxidation of the anode.     

Fabrication of hydrogen electrode supported cell for utilized regenerative solid oxide fuel cell application

A utilized regenerative solid oxide fuel cell (URSOFC) provides the dual function of performing energy storage and power generation, all in one unit. When functioning as an energy storage device, the URSOFC acts like a solid oxide electrolyzer cell (SOEC) in water electrolysis mode; whereby the electric energy is stored as (electrolyzied) hydrogen and oxygen gases. While hydrogen is useful as a transportation fuel and in other industrial applications, the URSOFC also acts as a solid oxide fuel cell (SOFC) in power generation mode to produce electricity when needed. The URSOFC would be a competitive technology in the upcoming hydrogen economy on the basis of its low cost, simple structure, and high efficiency. This paper reports on the design and manufacturing of its anode support cell using commercially available materials. Also reported are the resulting performance, both in electrolysis and fuel cell modes, as a function of its operating parameters such as temperature and current density. We found that the URSOFC performance improved with increasing temperature and its fuel cell mode had a better performance than its electrolysis mode due to a limited humidity inlet causing concentration polarization. In addition, there were great improvements in performance for both the SOFC and SOEC modes after the first test and could be attributed to an increase in porosity within the oxygen electrode, which was beneficial for the oxygen reaction.     

Thermal stability of a new solid oxide fuel/electrolyzer cell seal glass

Long-term thermal stability of sealing glass is critical for hermetic seals of solid oxide fuel and electrolyzer cell stacks. In this work, a SrO–La₂O₃–Al₂O₃–SiO₂ glass (SABS-0 glass) has been evaluated as a high temperature sealant by thermal treatment. Powder and bulk SABS-0 glasses are studied in both air and H₂/H₂O atmospheres at 800 °C for up to 1000 h. Weight measurements show negligible SABS-0 glass vaporization during the thermal treatment. Both SABS-0 powder and bulk samples show some surface devitrification but the SABS-0 glass bulk remains amorphous at all the thermal treatment conditions. On the polished bulk SABS-0 surface, needle-shaped crystals are observed for both the air and the H₂/H₂O thermal treatment conditions. Polishing is believed to be the initiator for the SABS-0 glass surface devitrification. The crystalline phases, indentified as silicates and aluminates, increase with the thermal treatment time. However, the crystalline phases on the polished SABS-0 glass surface are very limited and only exist on the very surface of both the air and the H₂/H₂O atmosphere treated samples. The SABS-0 glass has excellent thermal stability in solid oxide fuel/electrolyzer operating environments and is a promising sealant material for such applications.