Modeling design and analysis of multi-layer solid oxide fuel cells

The thermo-mechanical analytical model proposed for different solid oxide fuel cell (SOFC) designs addresses the deformation behavior and mechanical stability of SOFCs at various thermal stresses, specifically the creep resistance and the long-term endurance beyond the elastic limit.The model considers the deformation of multi-layer SOFC in the temperature range of 600-800 °C and presents the combination of the correlated parameters for SOFC performance evaluation, stability and long-term endurance under realistic operating conditions and temperature gradients. The numerical analysis of the thermo-mechanical properties of the SOFC materials is presented in terms of mechanical behavior at failure conditions and the influence of rheological and structural properties on SOFC long-term endurance. The SOFC thermal behavior, creep parameters of the SOFC materials and long-term stability are analyzed in terms of stresses, deformations and displacements.In terms of broader impact, the algorithms for Maurice-Levi and Voltaire theorems and their validity for non-elastic, e.g. viscous-elastic, viscous-plastic, and elastic-plastic deformations were confirmed. This result allowed us to apply the stress condition of non-elastic body to the stress condition of the elastic body which is relevant to the SOFC operation at elevated temperatures.     

Electrolytes for solid oxide fuel cells

The high operating temperature of solid oxide fuel cells (SOFCs), as compared to polymer electrolyte membrane fuel cells (PEMFCs), improves tolerance to impurities in the fuel, but also creates challenges in the development of suitable materials for the various fuel cell components. In response to these challenges, intermediate temperature solid oxide fuel cells (IT-SOFCs) are being developed to reduce high-temperature material requirements, which will extend useful lifetime, improve durability and reduce cost, while maintaining good fuel flexibility. A major challenge in reducing the operating temperature of SOFCs is the development of solid electrolyte materials with sufficient conductivity to maintain acceptably low ohmic losses during operation. In this paper, solid electrolytes being developed for solid oxide fuel cells, including zirconia-, ceria- and lanthanum gallate-based materials, are reviewed and compared. The focus is on the conductivity, but other issues, such as compatibility with electrode materials, are also discussed.     

Operation of micro tubular solid oxide fuel cells integrated with propane reformers

This paper presents the operating results of micro tubular solid oxide fuel cells (MT-SOFCs) integrated with propane catalytic partial oxidation (CPOX) reformers. The cells combined with the propane CPOX reformers successfully survived 1000 continuous thermal cycles totaling 1922 h of operation with only a 0.47 mW power loss per cycle, as well as surviving 100 extreme thermal cycles with a maximum ramp rate of 1000 °C.min⁻¹ without any power loss. This excellent thermal shock resistance is due to both the well matched coefficient of thermal expansion (CTE) among all of the cells’ layers as well as the homogenous anode structure present in all of the cells. Additionally, the cells operated on propane for more than 1500 continuous hours with an average degradation rate of 0.067 mW h⁻¹ (0.58%/1000 h). This degradation was attributed to the sintering of the nickel in the anode, degradation of the current collection and the reformer. The fact that the cells showed no sign of delamination, cracking or coking after these tests also proves the successful integration of cell and CPOX reformer. Overall, the 3D printed MT-SOFCs with integrated CPOX reformers exhibited a breakthrough in terms of cell thermal cycling and long-term stability which will significantly advance the development of portable SOFCs systems.     

Seal glass for solid oxide fuel cells

Seal glass plays a crucial role in solid oxide fuel cell performance and durability. In this review paper, overall composition-structure-property relations of seal glasses are discussed from bulk glass behavior, interfacial interaction, and sealing ability point of view. A seal glass should have a combination of desired thermal, chemical, mechanical, and electrical properties in order to seal cell components and stacks and prevent gas leakage. It must be stable for ∼40,000 h at 500-1000 °C in oxidizing and reducing atmospheres and withstand ∼10,000 thermal cycles between room temperature and cell operating temperature. A SrO-La₂O₃-Al₂O₃-SiO₂ based seal glass shows the promise to meet all the desired thermophysical properties, long-term stability, and thermal cycling resistance. In this paper, the most recent advances in the field are discussed along with this glass. Future seal glass research directions for solid oxide fuel cells are also analyzed.     

Rational design of novel cathode materials in solid oxide fuel cells using first-principles simulations

The search for clean and renewable sources of energy represents one of the most vital challenges facing us today. Solid oxide fuel cells (SOFCs) are among the most promising technologies for a clean and secure energy future due to their high energy efficiency and excellent fuel flexibility (e.g., direct utilization of hydrocarbons or renewable fuels). To make SOFCs economically competitive, however, development of new materials for low-temperature operation is essential. Here we report our results on a computational study to achieve rational design of SOFC cathodes with fast oxygen reduction kinetics and rapid ionic transport. Results suggest that surface catalytic properties are strongly correlated with the bulk transport properties in several material systems with the formula of La_0.5Sr_0.5BO_2.75 (where B = Cr, Mn, Fe, or Co). The predictions seem to agree qualitatively with available experimental results on these materials. This computational screening technique may guide us to search for high-efficiency cathode materials for a new generation of SOFCs.     

Alternative anode materials for solid oxide fuel cells

The electrolyte of a solid oxide fuel cell (SOFC) is an O²⁻-ion conductor. The anode must oxidize the fuel with O²⁻ ions received from the electrolyte and it must deliver electrons of the fuel chemisorption reaction to a current collector. Cells operating on H₂ and CO generally use a porous Ni/electrolyte cermet that supports a thin, dense electrolyte. Ni acts as both the electronic conductor and the catalyst for splitting the H₂ bond; the oxidation of H₂ to H₂O occurs at the Ni/electrolyte/H₂ triple-phase boundary (TPB). The CO is oxidized at the oxide component of the cermet, which may be the electrolyte, yttria-stabilized zirconia, or a mixed oxide-ion/electron conductor (MIEC). The MIEC is commonly a Gd-doped ceria. The design and fabrication of these anodes are evaluated. Use of natural gas as the fuel requires another strategy, and MIECs are being explored for this application. The several constraints on these MIECs are outlined, and preliminary results of this on-going investigation are reviewed.     

Novel nano-network cathodes for solid oxide fuel cells

A novel nano-network of Sm_0.5Sr_0.5CoO_3−δ (SSC) is successfully fabricated as the cathodes for intermediate-temperature solid oxide fuel cells (SOFCs) operated at 500–600 °C. The cathode is composed of SSC nanowires formed from nanobeads of less than 50 nm thus exhibiting high surface area and porosity, forming straight path for oxygen ion and electron transportation, resulting in high three-phase boundaries, and consequently showing remarkably high electrode performance. An anode-supported cell with the nano-network cathode demonstrates a peak power density of 0.44 W cm⁻² at 500 °C and displays exceptional performance with cell operating time. The result suggests a new direction to significantly improve the SOFC performance.     

High-Performance composite cathodes for solid oxide fuel cells

The composite cathodes with lanthanum-based iron and cobalt-containing perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and Ce_0.9Gd_0.1O_1.95 (GDC) are investigated for solid oxide fuel cell (SOFC) applications at relatively low operating temperatures (700-800 °C). LSCFs with high surface areas of 55 m²g⁻¹ are synthesized via a complex method with inorganic nano dispersants. The fuel cell performances of composite cathodes on anode supported SOFCs are characterized with GDC materials of surface areas of 5 m²g⁻¹ (ULSA-GDC), 12 m²g⁻¹ (LSA-GDC), and 23 m²g⁻¹ (HAS-GDC). The maximum power density of the SOFCs increases from 0.68 Wcm⁻² to 1.2 Wcm⁻² at 780 °C and 0.8 V as the GDC surface area increases from 5 m²g⁻¹ to 23 m²g⁻¹. The area specific resistance of the porous composite cathodes with a HAS-GDC are 0.467 ohmcm² at 780 °C and 1.086 ohmcm² at 680 °C, while these values with an LSA-GDC are 0.543 ohmcm² and 0.945 ohmcm², respectively. The best compositions of the porous composite cathodes result from the morphologies of the GDC materials at each temperature due to the formation of an electron-oxygen ion-gas boundary.     

Novel compressive seals for solid oxide fuel cells

Traditional seals for planar solid oxide fuel cells (pSOFCs) are rigid glass and glass–ceramic, which have caused the problem of being unable to replace malfunctioning components. Non-glass sealants have become a recent trend. In this paper, fumed silica-infiltrated alumina–silica fiber paper gaskets were investigated as a novel compressive seal for planar solid oxide fuel cells. The leak rates decreased with increase of the silica-infiltration amount and the compressive load. Samples pre-stressed at 10 MPa indicated far superior sealing characteristics, with leak rates as low as 0.04 sccm cm⁻¹ at a 1 MPa compressive stress and a 10 kPa pressure gradient, and 0.05 sccm cm⁻¹ for 0.05 MPa, and a 1.4 kPa pressure gradient.     

Thin films for micro solid oxide fuel cells

Thin film deposition as applied to micro solid oxide fuel cell (μSOFC) fabrication is an emerging and highly active field of research that is attracting greater attention. This paper reviews thin film (thickness ≤1 μm) deposition techniques and components relevant to SOFCs including current research on nanocrystalline thin film electrolyte and thin-film-based model electrodes. Calculations showing the geometric limits of μSOFCs and first results towards fabrication of μSOFCs are also discussed.     

Cycling studies of solid oxide fuel cells

The purpose of this work was to study the transient performance of solid oxide fuel cells (SOFCs) under several cycling conditions, in order to understand the degradation mechanisms. Initially, the Rolls Royce Fuel Cell IP-SOFC (Integrated Planar SOFC) single tube was investigated. The objective was to cycle up to 100 times to check if degradation was occurring and to assess its extent. In this paper the results of loading cycles at nominally constant operating temperature are reported. Work on two other kinds of cycles, i.e. thermal and redox, for this type of tube has commenced and the results will be reported in the follow-up paper.     

A diesel fuel processor for stable operation of solid oxide fuel cells system: II. Integrated diesel fuel processor for the operation of solid oxide fuel cells

Post-reforming experimental results for the complete removal of light hydrocarbons from diesel reformate are introduced in part I. In part II of the paper, an integrated diesel fuel processor is investigated for the stable operation of SOFCs. Several post-reforming processors have been operated to suppress both sulfur poisoning and carbon deposition on the anode catalyst. The integrated diesel fuel processor is composed of an autothermal reformer, a desulfurizer, and a post-reformer. The autothermal reforming section in the integrated diesel fuel processor effectively decomposes aromatics, and converts fuel into H₂-rich syngas. The subsequent desulfurizer removes sulfur-containing compounds present in the diesel reformate. Finally, the post-reformer completely removes the light hydrocarbons, which are carbon precursors, in the diesel reformate. We successfully operate the diesel reformer, desulfurizer, and post-reformer as microreactors for about 2500 h in an integrated mode. The degradation rate of the overall reforming performance is negligible for the 2000 h, and light hydrocarbons and sulfur-containing compounds are completely removed from the diesel reformate.     

Glass fiber reinforced sealants for solid oxide fuel cells

Novel sealants for solid oxide fuel cells are developed by addition of glass fiber into glass-ceramic as a reinforcement material. Various sealants including three different fiberglass types and four different structural designs are fabricated. The mechanical and sealing performances of the sealants are investigated via tensile and short stack leakage tests, respectively. The tensile tests reveal that the fracture strength of the sealants varies depending on the type and number of the glass fiber used. In general, the sealants having relatively high number of glass fiber layers exhibit relatively low joining strength. The best bonding strength values are obtained from the sealants having a structure where a single glass fiber layer is sandwiched between two glass-ceramic layers. The sealing performance tests are performed for the sealants showing the highest and lowest fracture strengths in the tensile tests as well as for the sealant without glass fiber addition as a base case for a comparison. The results indicate that it is possible to obtain a gas-tight sealing at high temperatures under all pressures studied, whereas leakage occurs at room temperature for all cases considered. However, the sealing performance is found to be related with the mechanical strength of the sealants.     

An efficient ceramic-based anode for solid oxide fuel cells

A new ceramic-based multi-component material, containing one electronic conductor (Y-substituted SrTiO₃, SYT), one ionic conductor (YSZ) and a small amount (∼5 vol.%) of Ni catalyst, was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The ceramic framework SYT/YSZ shows good dimensional stability upon redox cycling and has an electrical conductivity of ∼10 S cm⁻¹ under typical anode conditions. Owing to the substantial electrocatalytic activity of the fine and well-dispersed Ni particles on the surface of the ceramic framework, the electrode polarization resistance of 5 vol.% Ni-impregnated SYT/YSZ anode reached 0.21 Ω cm² at 800 °C in wet Ar/5%H₂. Based on these results, a redox-stable anode-supported planar SOFC is expected using this anode material, thus offering great advantages over the current generation of Ni/YSZ-based SOFCs.     

A novel electrolyte for intermediate solid oxide fuel cells

Scheelite-type, La_xCa_1−xMoO_4+δ electrolyte powders, are prepared by the sol-gel process. The crystal structure of the samples is determined by employing the technique of X-ray diffraction (XRD). According to XRD analysis, the continuous series of La_xCa_1−xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions have the structure of tetragonal scheelite. Their lattice parameters are greater than that of the original sample, and increase with increasing values of x in the La-substituted system. Results of sinterability and electrochemical testing reveal that the performances of La-doped calcium molybdate are superior to that of pure CaMoO₄. La_xCa_1−xMoO_4+δ ceramics demonstrate higher sinterability. The La_0.2Ca_0.8MoO_4+δ sample that achieved 96.5% of the theoretical density was obtained after being sintered at 1250 °C for 4 h. The conductivity increases with increasing lanthanum content, and a total conductivity of 7.3 × 10⁻³ S cm⁻¹ at 800 °C could be obtained in the La_0.2Ca_0.8MoO_4+δ compound sintered at 1250 °C for 4 h.     

Micro-tubular solid oxide fuel cells and stacks

The properties and performance of micro-tubular solid oxide fuel cells are compared and the differentiating factors discussed. The best recorded power density for a single cell in the literature to date is 1.1 W cm⁻², with anode microstructure and current collection technique emerging as two key factors influencing electrical performance. The use of hydrocarbon fuels instead of pure hydrogen and methods for reducing the resultant carbon deposition are briefly discussed. Performance on thermal and reduction-oxidation (RedOx) cycling is also a critical issue for cell durability. Combining these individual cells into stacks is necessary to obtain useful power outputs. As such, issues of fluid and heat transfer within such stacks become critical, and computational modelling can therefore be a useful design tool. Experimentally tested stacks and stack models are discussed and the findings summarised. New results for a simple stack manufactured at the University of Birmingham are also given.     

Fuel utilization and fuel sensitivity of solid oxide fuel cells

Fuel utilization and fuel sensitivity are two important process variables widely used in operation of SOFC cells, stacks, and generators. To illustrate the technical values, the definitions of these two variables as well as practical examples are particularly given in this paper. It is explicitly shown that the oxygen-leakage has a substantial effect on the actual fuel utilization, fuel sensitivity and V-I characteristics. An underestimation of the leakage flux could potentially results in overly consuming fuel and oxidizing Ni-based anode. A fuel sensitivity model is also proposed to help extract the leakage flux information from a fuel sensitivity curve. Finally, the “bending-over” phenomenon observed in the low-current range of a V-I curve measured at constant fuel-utilization is quantitatively coupled with leakage flux.     

Monolithic flat tubular types of solid oxide fuel cells with integrated electrode and gas channels

A new monolithic solid oxide fuel cell (SOFC) design stacked with flatten tubes of unit cells without using metallic interconnector plate is introduced and evaluated in this study. The anode support is manufactured in a flat tubular shape with fuel channel inside and air gas channel on the cathode surface. This design allows all-ceramic stack to provide flow channels and electrical connection between unit cells without needing metal plates. This structure not only greatly reduces the production cost of SOFC stack, but also fundamentally avoids chromium poisoning originated from a metal plate, thereby improving stack stability. The fuel channel was created in the extrusion process by using the outlet shape of mold. The air channel was created by grinding the surface of pre-sintered support. The anode functional layer and electrolyte were dip-coated on the support. The cathode layer and ceramic interconnector were then spray coated. The maximum power density and total resistance of unit cell with an active area of 30 cm² at 800 °C were 498 mW/cm² and 0.67 Ωcm², respectively. A 5-cell stack was assembled with ceramic components only without metal plates. Its maximum power output at 750 °C was 46 W with degradation rate of 0.69%/kh during severe operation condition for more than 1000 h, proving that such all-ceramic stack is a strong candidate as novel SOFC stack design.     

Modeling of cracking of the glass-based seals for solid oxide fuel cell

For planar SOFCs the seal is a critical component, potential fracture in the seal needs to be investigated in order to enhance the reliability of the seal. A model based on the classical beam bending theory and the fracture theory of ceramic materials has been developed for predicting the crack extension in the seal. The model reveals that the resistance of the seal to cracking on cooling is mainly affected by two factors: the seal thickness and the CTE mismatch. Furthermore, a cracking diagram is established to reveal the effects of the seal thickness and CTE mismatch on the crack extension behavior. It shows that the ‘no cracking’ area increases with decreasing seal thickness, and larger CTE mismatch requires a thinner seal to avoid cracking. The model and the cracking diagram are experimentally validated through monitoring the leakage rate of a glass-sealed chamber, and the crack extension deduced from the measured leakage rate shows good agreement with those predicted by the model. The proposed model can serve as a useful tool in sealing design of SOFC.