Compressive Seals Research Articles

Modeling of a Single Repeating Unit for Protonic Ceramic Cell Applications

Protonic Ceramic Cells (PCCs) are a promising alternative to oxygen-ion conducting solid oxide cells (SOCs), offering benefits for hydrogen separation, electrolysis and fuel cell applications. The main advantages are the lower operating temperature and the capability of using/producing a pure hydrogen stream due to the protonic conductor. Recent advancements in PCCs at the laboratory level are encouraging, and the current development stage is focusing on the scale-up of the technology to cross the gap between single cells and stacks. An intermediate stage for this process is the development of single repeating units (SRU), composed of cells, sealings and interconnects. The design of the SRU is the first step of development, which consists in the selection of the optimal geometry of the interconnect-cell assembly for a peculiar cell and application which allows achieving a homogeneous temperature field and an even distribution of the reactants to the electrodes during the operation. In order to optimize the design of the geometry, a common approach is to develop numerical models to simulate the fluidic distribution in the SRU and the thermal-electrical response of the integrated interconnect-cell assembly.A numerical model for three-dimensional, steady-state simulations of an SRU geometry designed for a circular, anode-supported 40 cm2 PPC, composed of a BCZY electrolyte, NiO/BCZY anode, and a BCZY-BGLC air electrode has been developed. The numerical model implemented applies the equations for the conservation of mass, energy and momentum in fluid and porous domains coupled with activation kinetic equations in the active electrodes. In the solid domains, the model applies energy and charge conservation equations. The model calculated the distribution of chemical species and temperature, and the pressure and velocity fields in the fluid domains. In the solid domains, the distribution of temperature, electronic/ionic potentials and current is evaluated. The cell model has been first calibrated using experimental data from single cell polarizations.The structure considered for the SRU is shown in Figure 1 (cross-section). In the configuration simulated, the cell is sealed with a glass-ceramic seal placed between the electrolyte of the cell and a metallic frame (Figure 1b and Figure 1c). Metallic meshes/foams are placed in contact with the electrodes to improve the electrical contact between them and interconnect plates of the system (Figure 1d). Compressive seals (Figure 2e) are utilized between the frame and interconnects. Finally, interconnects are at the bottom and on top of the system.To optimize the cell operation, we focused on the design of the interconnects, either with channels or porous foams to improve the gas distribution. The area in which the reactant has to be distributed and the thickness of layers is imposed by the cell geometry, but it is important to properly choose a gas flow distribution geometry (flow field) of the interconnect plate to guarantee high performance of chemical reactions in the cell electrodes. Hence, the geometrical parameters selected for the optimization were those of the channels (number, length, ribs dimensions parameters). Figure 1: Main structure of the SRU with a single cell. In the sequential schematics one can observe, the cell (a), the glass sealing (b), the metallic frame (c), the metallic meshes (d), the compressive sealing (e) and eventually the interconnect with channels (f). The objective functions selected for the optimization of the SRU geometry were the maximum temperature gradient in the cell volume and the voltage losses.The model has been applied to the simulation of both electrolysis and fuel cell operation and the interconnect geometry has been optimized for these two applications. The use of a porous metallic foam in place of the channels configuration has been also evaluated. Results show that complex channel configurations could optimize the flow fields for specific operating points, but simpler geometries are more flexible for the development of a SRU platform for general PCC applications. Figure 1

Investigation on Long-Term Stability of Vermiculite Seals for Reversible Solid Oxide Cell.

A reversible solid oxide cell (RSOC) integrating solid oxide fuel (SOFC) and a solid oxide electrolysis cell (SOEC) usually utilizes compressive seals. In this work, the vermiculite seals of various thickness and compressive load during thermal cycles and long-term operation were investigated. The leakage rates of seals were gradually increased with increasing thickness and input gas pressure. The thinner seals had good sealing performance. The compressive load was carried out at thinner seals, the possible holes were squeezed, and finally the leakage rates were lower. With a fixed input gas pressure of 1 psi, 2 psi, and 3 psi, the leakage rates of 0.50 mm vermiculite remained at around 0.009 sccm/cm, 0.017 sccm/cm and 0.028 sccm/cm during twenty thermal cycles, while the leakage rates remained at around 0.011 sccm/cm for about 240 h. Simultaneously, elemental diffusions between seals and components were limited, implying good compatibility. Furthermore, the open circuit voltage (OCV) remained at around 1.04 V during 17 thermal cycles, which is close to Nernst potentials. The stack performance confirmed that the vermiculite seals can meet the structural support and sealing requirements. Therefore, the vermiculite shows good promise for application in stacks during thermal cycles and long-term operation.

Thermal cycling stability of novel hexagonal boron nitride (h-BN)/glass compressive seals for planar intermediate temperature solid oxide fuel cells

Abstract The novel h-BN/glass compressive seals were investigated for use in solid oxide fuel cell. The glass was mixed with h-BN powders to obtain composite materials using tape casting technique. Leakage rates were measured below 0.012 sccm/cm under input gas pressure of 6.8 kPa. Seals containing 40% by weight glass showed excellent thermal cycle and long-term operation stability. The leakage rates were reduced by more than half after ten thermal cycles under different input gas pressure. Also, the leakage rates gradually decreased to 0.008 sccm/cm under input gas pressure of 20.4 kPa at 750 °C during 208 h, and then remained stable. These observations can be explained by liquid B2O3 films having gradually formed on the h-BN surface. Boron oxidation effectively improved bondage for both interface and interior particles. This phenomenon was interpreted by seals’ microstructure analyses and transmission electron microscopy. Finally, an evolution model is proposed to explain the process.

Improvement of sealing performance for Al2O3 fiber-reinforced compressive seals for intermediate temperature solid oxide fuel cell

Ceramic based compressive seal is regarded as an option for sealing the planar solid oxide fuel cell (SOFC) stack. However, during the practice of SOFC stack testing, the powder-based seals are found to have weakness in keeping mechanical integrity. In consideration of enhancing mechanical properties, ceramic fibers are added to the powder-based seals and the relationship between sealing performance and the content ratio of the ceramic fiber under different inlet gas pressure is investigated. The seal containing 20 wt% ceramic fiber (F20) is found to have the best sealing performance. The leakage rate can be maintained at about 0.01 sccm/cm at 750 °C during 10 thermal cycle testing. The interface between the seal and adjacent components exhibits good contact conditions and chemical compatibility which can effectively avoid the formation of interface leakage paths. Introducing the ceramic fiber reduces the tensile strength of the seal but enhances ductility and thus improves the reliability and structural integrity of the seal. The applicability of the F20 composite seal is verified in a SOFC 1-cell stack testing.

Numerical analysis of gas tightness of seals in pSOFCs based on Lattice Boltzmann Method simulation

To better understand the leakage mechanism of compressive seals in planar solid oxide fuel cells (pSOFCs), a theoretical method of leakage prediction is proposed that combines three numerical techniques. The Lattice-Boltzmann method was introduced to simulate the nonlinear flow in rough wall interfacial gaps due to its flexibility when dealing with flow problems related to complex boundaries. A numerical three-dimensional (3D) rough-surface generation technique was applied for the geometry configuration of interface gaps, and finite element analysis was adopted for micro-contact mechanics of single asperities to determine the gap height under stress. Based on this method, the 3D flow characteristics of the gas in interface gaps were numerically studied, and two important dimensionless flow factors (the flow factor Φσ of roughness and the flow factor Φh of height) were proposed. All numerical results were fitted as dimensionless criteria for calculating the interface leakage rate. These criteria can be directly used to predict the leakage rate of a given compressive seal under specific working conditions without relying on any experimental empirical formula. Furthermore, leakage rate predictions were compared to existing experimental data and good agreement was obtained, corroborating the accuracy of the method. In addition, the effects of influencing parameters on leakage according to the rate-temperature relationship were discussed. Several important conclusions were drawn to guide the design of compressive seals for pSOFC.

Performance of Non‐Destructive Compressive Seals for Reversible Solid Oxide Cells

AbstractContrary to hardened glass ceramics, compressive non‐destructive seals for solid oxide cells (SOCs) could enable stack repairs without utilizing destructive separation methods. In light of increasing interest for high temperature steam electrolysis and energy storage, the performances of two commercially available compressive materials were investigated in humidified gas conditions: Thermiculite‐866 and AFS‐170 alumina felts. Mass loss and thermogravimetric (TGA) experiments carried out in inert, oxidizing, and reducing atmospheres at 800 °C showed no influence of the atmosphere on the alumina felts, and a reversible redox behavior for Thermiculite‐866. Thermiculite‐866 showed better cyclability compared to AFS‐170 during SOC thermal cycling experiments. Initial open circuit voltages in dry H2 were measured and compared to those obtained with commercial mica seals. The highest were recorded with Thermiculite‐866, followed by mica and AFS‐170. The species outgassed by Thermiculite‐866 were quantified, and fluorine was found to be the main component released at 800 °C.

SiO2–Al2O3–Y2O3–ZnO glass sealants for intermediate temperature solid oxide fuel cell applications

In this study, eleven alkaline earth metal and B2O3-free SiO2–Al2O3–Y2O3–ZnO glass sealants were prepared. The thermal stability, adhesion, and sealing properties of the glass systems with respect to SUS430 stainless steel (SUS430) were assessed for use in intermediate temperature solid oxide fuel cells (ITSOFCs). The glass transition points fell in the range of 694 °C and 833 °C, while the glass softening points, varying from 725 °C to 885 °C, decreased linearly with the ZnO/SiO2 ratio. Among the glasses evaluated, the YAS1-50, YAS4-50, YAS5-50, and YAS5-90 glasses coupled with SUS430 showed fine adhesion and no detectable inter-diffusion across the interface. The coefficient of thermal expansion (CTE) of the YAS5-50 glass escalated from 7.01 to 9.30 × 10−6/K with 50 wt% Bi1.5Y0.5O3 (BYO) fillers. The leak rates of the composite seals comprising the YAS5-50 glass and 50 wt% BYO fillers were measured at 700 °C after joining at 850 °C. The measurement used helium with a pressure at 1 psi across the glass seals and a slight load of 1.0 psi to minimize compressive seal effect on the glass. It appeared that a low leak rate of 0.052 sccm/cm was obtained and stayed unchanged as the system was soaked at 700 °C for 500 h, indicating a fine thermal stability against the extended duration benefited from the limited crystallization of the YAS5-50 glass. This promising long-term stability qualified the glass for use as SOFC seal.

Effect of Elastic Network of Ceramic Fillers on Thermal Cycle Stability of a Solid Oxide Fuel Cell Stack

AbstractGlass‐based seals for planar solid‐oxide fuel‐cell (SOFC) stacks are open to uncontrolled deformation and mechanical damages, limiting both sealing integrity and stack reliability, particularly in thermal cycle operations. If the glass‐based seals work like an elastomer‐based compressive seal, SOFC stacks may survive unprecedented numbers of thermal cycles. A novel composite sealing gasket is successfully developed to mimic the unique features of the elastomer‐based compressive seal by controlling the composition and packing behavior of binary ceramic fillers. A single‐cell SOFC stack undergoes more than 100 thermal cycles with little performance loss, during which the sealing integrity is lost/recovered repeatedly upon cooling and reheating, corresponding to unloading/loading of the elastomer‐based compressive seal. The thermal‐cycle responses of the SOFC stack are explained in sequence by the concurrent events of elastic deformation/recovery of ceramic filler network and corresponding redistribution of sealing glass.

A Computational Leakage Model for Solid Oxide Fuel Cell Compressive Seals

One of the key obstacles precluding the maturation and commercialization of planar solid oxide fuel cells has been the absence of a robust sealant. A computational model has been developed in conjunction with leakage experiments at Oak Ridge National Laboratory. The aforementioned model consists of three components: a macroscopic model, a microscopic model, and a mixed lubrication model. The macroscopic model is a finite element representation of a preloaded metal-metal seal interface, which is used to ascertain macroscopic stresses and deformations. The microscale contact mechanics model accounts for the role of surface roughness in determining the mean interfacial gap at the sealing interface. In particular, a new multiscale fast Fourier transform-based model is used to determine the gap. An averaged Reynolds equation derived from mixed lubrication theory is then applied to approximate the leakage flow across the rough annular interface. The composite model is applied as a predictive tool for assessing how certain physical parameters (i.e., seal material composition, compressive applied stress, surface finish, and elastic thermophysical properties) affect seal leakage rates. The leakage results predicted by the aforementioned computational leakage model are then compared with experimental results.

Thermal cycle stability of Al 2O 3-based compressive seals for planar intermediate temperature solid oxide fuel cells

Al 2O 3-based compressive seals were fabricated by tape casting with Al 2O 3 and 0–30 wt% aluminum powders, and their sealing effectiveness, thermal cycle stability between 200 and 750 °C and applicability in planar intermediate temperature solid oxide fuel cells were evaluated. The results indicate that increasing the aluminum content from 0 to 30 wt% in the seals decreases the leakage rate and increases the thermal cycle stability under various inlet gas (N 2) pressures of 3.5, 7.0 and 10.5 kPa. Especially, with the seal containing 30 wt% of aluminum (ACS3), the initial leakage rate was below 0.03 sccm cm −1 under an inlet pressure of 10.5 kPa, and the leakage rates during 96 thermal cycles were below 0.04 sccm cm −1 under the same inlet gas pressure. The interfaces in the interconnect/seal/cell assembly with the ACS3 seal retained integrity after 50 thermal cycles, demonstrating the applicability of the Al 2O 3-based compressive seals in the planar intermediate temperature SOFCs.

Comments on “Ultra-low leak rate of hybrid compressive mica seals for solid oxide fuel cells”

Comments on “Ultra-low leak rate of hybrid compressive mica seals for solid oxide fuel cells”

Long-term ageing and materials degradation of hybrid mica compressive seals for solid oxide fuel cells

Hybrid phlogopite mica seals with silver interlayers were evaluated in long-term isothermal ageing tests in a dual environment consisting of dilute hydrogen versus air at 800 °C. High-temperature leak tests with helium showed very stable leakage of 0.01–0.02 sccm cm −1 for 28,366 h under a low applied compressive stress of 82 kPa (12 psi). Post-mortem SEM and EDS analyses of the mica showed minimum degradation in terms of changes in microstructure and chemical composition, although there appeared to be some Ag migration and segregation at interstices between mica flakes. Fluorine was also found to be released from mica. Overall, the low, constant leakage through the hybrid mica/Ag seals clearly demonstrated a very promising candidate for SOFC sealing.

Glass-based Sealants for Application in Planar Solid Oxide Fuel Cell Stack

One of the major challenges for the commercialization of planar anode-supported solid oxide fuel cell (SOFC) is the development of suitable high temperature sealant. The sealants must maintain gas-tightness between the anode and the cathode compartments to avoid intermixing of fuel and oxidant at high operating temperature (700°-800°C), be an electrical insulator and chemically non-reactive with the stack components. Due to the high commercial importance of such sealants, very few sealing compositions are available in the open literature and have to be developed by the individual SOFC developer. In this context the present article gives an overview on such sealant development with particular emphasis on glass-ceramics based sealants. Other than glass-based sealants, very recent approaches such as compressive and self healing seals also show some potential as SOFC sealants. The review of literature is followed by description of the progress of work related to development of glass based sealant at our Institute for application in the developed planar anode-supported SOFC stack. Under this developmental activity, a number of glass systems having SiO2 and B2O3 as glass formers and BaO, MgO and CaO as the alkaline earth metal oxide content have been investigated and thoroughly characterized. Based on the findings, some of these glasses have been used for making seals between different types of SOFC components, e.g. ferritic stainless steel (Crofer22APU) (metallic interconnect) and 8 mol% yttria-stabilized zirconia (YSZ) (electrolyte) under various combinations, viz. metal-metal, metal-ceramic and ceramic-ceramic. A novel concept of “bi-layered sealing” consisting of two different glass compositions having graded CTE values has been implemented for the first time during actual SOFC stack operation. The performance of the developed sealants has been evaluated in several working SOFC short stacks (up to 6-cell) for a maximum period of 20 days. It is observed that the open circuit voltage (OCV) of these stacks is very near to its theoretical value and remains same during the stack testing period and thus ensured the potentiality of the developed sealant for SOFC application.

Preparation, structural and thermo-mechanical properties of lithium aluminum silicate glass–ceramics

Lithium aluminum silicate glasses of composition (wt%) 12.6Li 2O–71.7SiO 2–5.1Al 2O 3–4.9K 2O–3.2B 2O 3–2.5P 2O 5 were prepared by the melt quench technique. These glasses were converted to glass–ceramics based on DTA data. X-ray diffraction (XRD) and Fourier transform infra-red spectroscopy (FTIR) were used to discern the phases evolved in the glass–ceramics. Phase morphology was studied using scanning electron microscopy (SEM). Thermal expansion coefficient (TEC) and glass transition temperature ( T g) of all samples were measured using thermo-mechanical analyzer (TMA). It was found that 3 h dwell time at crystallization temperature yielded samples with good crystallinity with a TEC of 9.461 × 10 −6 °C −1. Glass–ceramic-to-metal compressive seal with SS-304 was fabricated using LAS glass–ceramic. The presence of metal housing and compressive stresses at the glass–ceramic-to-metal interface reduced average grain size and changed the overall microstructure.

Al<SUB>2</SUB>O<SUB>3</SUB>-based Compressive Seals for Planar Intermediate Temperature Solid Oxide Fuel Cells

Al<SUB>2</SUB>O<SUB>3</SUB>-based Compressive Seals for Planar Intermediate Temperature Solid Oxide Fuel Cells

Some properties of lithium aluminium silicate (LAS) glass-ceramics used in glass-ceramic to metal compressive seal for vacuum applications

We report here the preparation of LAS glass-ceramics and some studies on their thermo-physical properties and microstructure, for compressive seals in vacuum applications. Glass of composition 12.6Li2O-71.7SiO2-5.1Al2O3-4.9K2O-3.2B2O3-2.5P2O5 was prepared by the conventional melt quench technique. Based on differential thermal analysis (DTA) data, glass samples were nucleated at 600°C for 2hr and were then crystallized at 800°C for 2-5hr. X-ray diffraction (XRD) spectra showed lithium disilicate to be the major phase. A dwell time of 3hr yielded a sample of good crystallinity. Dilatometric measurements of this sample on a thermo-mechanical analyzer (TMA) measured a thermal expansion coefficient (TEC) of 94.61×10−7°C−1. Glass transition temperature (Tg), and dilatometric softening temperature (Tds) of the sample was recorded as 585°C and 830°C respectively. Considering TEC and Tds compression type seals were prepared with SS304 (TEC = 172×10−7°C−1) housing of length 15mm, outer diameter 30mm and inner diameter 15mm. After pre-heat treatment of the metal components, sealing was carried out under a protective atmosphere of flowing Argon gas. The finished seal was tested for leak tightness on a He leak detector. The seal was capable of withstanding a vacuum of 10−6 Torr; at a leak rate of 10−9 Torr lit s−1. Scanning electron microscopy (SEM) was carried out on LAS before and after fabrication of compressive seal to elucidate the effect of compressive stress and the presence of metal near the interface. We observe a significant difference in microstructure due to compressive stresses of sealing and due to the presence of metal. Energy dispersive analysis of X-rays (EDAX) revealed no interdiffusion of species from glass-ceramic to metal or vice-versa.

Modeling and Study of the Influence of Sealing on a Solid Oxide Fuel Cell

The properties of sealing materials are important for the performance and reliability of solid oxide fuel cells (SOFCs). Even if the properties of a sealing material can be studied separately, it remains difficult to quantify the effect of an imperfect seal on the repeat-element behavior. In this study, simulation is used to investigate the effects of an imperfect seal behavior on the performance and reliability of SOFCs. Diffusion through the sealing material and inherent local combustion of fuel are added to the computational fluid dynamics (CFD) repeat-element model, which also allows us to compute the flow field, the electrochemical reactions, and the energy equations. The results are in good agreement with experiments. The zones of parasitic combustion and local overheating are well reproduced. Furthermore, the model predicts a risk of reoxidation under polarization that is well observed. The model also shows the necessity to take into account the diffusion transport for the development of compressive seal materials, hence verifying the hypotheses made by other groups. The modeling approach presented here, which includes the imperfections of components, allows us to reproduce experiments with good accuracy and gives a better understanding of degradation processes. With its reasonable computational cost, it is a powerful tool for a design of SOFC based on reliability.

Novel Al 2O 3-based compressive seals for IT-SOFC applications

Novel compressive Al 2O 3-based seals were developed and characterized under simulated intermediate temperature solid oxide fuel cell (IT-SOFC) environment. The seals were prepared by tape casting, mainly composed of fine Al 2O 3 powder with various contents of fine Al powder addition. The leakage rates were determined at 800 °C under 0.14–0.69 MPa compressive stresses, and the stabilities were evaluated at 750 °C under constant 0.35 MPa compressive stress. The leakage rates at 800 °C were in range of 0.2–0.01 sccm cm −1, decreasing with increasing the compressive stress and Al content; Al addition significantly improved the stability, the leakage rate with 20 wt% Al addition was as low as 0.025 sccm cm −1 at 800 °C under 0.35 MPa compressive stress with a gauge pressure of 6.9 kPa, and exhibited good stability at 750 °C. Single cell test also confirmed the effectiveness of the tape cast Al 2O 3-based seal for planar IT-SOFC applications.

Comparison of infiltrated ceramic fiber paper and mica base compressive seals for planar solid oxide fuel cells

Solid oxide fuel cells using non-glass sealants have become increasingly common. In this paper, fumed silica infiltrated ceramic fiber paper with pre-compression was compared with plain and pre-compressed at 10 MPa hybrid mica as compressive seals. Leakage tests were measured under a 0.1–1.0 MPa compressive load with the pressure gradient varying from 2 to 15 kPa. The results demonstrated that the leakage rate of infiltrated fiber paper was 0.04 sccm cm −1 for a 10 kPa gradient, under 1.0 MPa compressive load, while for mica it was 0.60 and 0.63 sccm cm −1 which indicated that the infiltrated ceramic fiber paper showed a much lower leakage than mica. Long-term thermal cycling tests demonstrated that although the leakage of fumed silica infiltrated fiber paper was slightly higher than that of hybrid mica, it remained stable after 20 thermal cycles and no interlayer was needed. The mass loss of the fiber paper was 1.7 × 10 −2 mg cm −2 h −1 in a hydrogen environment at 1073 K for 200 h. The leakage of infiltrated fiber paper remained about 0.06 sccm cm −1 after reduction.

Improvements to Test Rig for Laboratory Scale Anode Supported SOFC

This work encompasses the design of a rig for the testing of laboratory pellet solid oxide fuel cells (SOFC). The unique feature of this pellet fuel cell test rig is the use of pneumatic actuator to give a simple compressive seal. This facilitates the fast setup of pellet fuel cells containing novel material for evaluation. The various design iterations are presented with reasoning for material and design choices. Initial testing of pellet fuel cells revealed a number of issues that needed addressing. These include current collection, fuel preheating, fuel pressurization cell preparation and elimination of small scale leaks. The design philosophy used to rectify these issues will be presented. To accommodate increased functionality and allow flexibility for different types of SOFC tested, anticipated future design changes will also be discussed.