Stress analysis of solid oxide fuel cell anode microstructure reconstructed from focused ion beam tomography

The degradation and ultimately lifetime of solid oxide fuel cells (SOFCs) is determined in part by the stresses generated within the different layers of the device. For fully dense materials such as the electrolyte, when modelling these stresses on a macro-scale the material properties can be considered to be homogeneous (evenly distributed) allowing the prediction of volume average stresses due to differential thermal expansion in the layer. However, detailed stress analysis of real, multiphase porous layers such as those found in SOFC electrodes, on the micron and sub-micron scale has not been possible to date as detailed geometry and convenient methods to generate a finite element model have not been available.In this paper we present work that combines microstructural characterisation of a porous solid oxide fuel cell anode with three dimensional stress analysis to inspect the stresses within the individual phases of the anode, and at phase boundaries. The electrode microstructure has been characterised using focused ion beam (FIB) tomography and the resulting microstructure used to generate a solid mesh of three dimensional tetrahedral elements. A temperature field was applied to simulate the heating of the sample from room temperature (298 K) to operating temperature (1073 K). The maximum principal stress in the nickel phase was found to exceed the yield strength, while the minimum principal stress in the yttria-stabilized zirconia (YSZ) phase was found to exceed the characteristic strength of that volume of YSZ, indicating that the probability of failure of the YSZ matrix is significant.     

3D phase-differentiated GDL microstructure generation with binder and PTFE distributions

In this work, a new framework and model for the digital generation and characterization of the microstructure of gas diffusion layer (GDL) materials with localized binder and polytetrafluoroethylene (PTFE) distributions were developed using 3D morphological imaging processing. This new generation technique closely mimics manufacturing processes and produces complete phase-differentiated (void, fiber, binder, and PTFE) digital 3D microstructures in a cost- and time-effective manner for the first time. The results for the digital generation of Toray TGP-H-060 with 5 and 0 wt.% PTFE were in close agreement with confocal laser scanning microscope (CLSM) images as well as 3D X-ray tomography studies. The resulting structure can be readily used for analyzing transport processes utilizing commercial CFD software.     

Viable image analyzing method to characterize the microstructure and the properties of the Ni/YSZ cermet anode of SOFC

The quantitative analysis of the microstructure and related physicochemical properties of the anode substrate are essential for the identification of the optimum fabrication conditions for the best performance of the SOFC unit cell. However it is not easy to characterize the correlation of the microstructure and the property of the Ni/YSZ cermet anode due to its very complicated structural features. Moreover, it is not a simple matter to differentiate all of the phases in the Ni/YSZ anode via simple microscopic observations. In this study, we developed an image analyzing method to differentiate each constituent phase of the Ni/YSZ cermet, enabling us to investigate the microstructural evolution of the Ni/YSZ anode and the characterization of the correlation between the microstructure and the properties of the Ni/YSZ anode.     

Solid oxide fuel cell: Materials for anode,cathode and electrolyte

Solid oxide fuel cell technology is the technology which can be driving force to change the course of action of the modern era due to its optimal power generation features with maximum electrical efficiency for automobiles and household devices. Fuel cells can be best described as electrochemical devices that make use of fuel oxidation to convert chemical energy into electrical energy and also lower the amount of oxidant simultaneously.A typical SOFC consists of a cathode, anode and an electrolyte constituting a single cell. These single cells are stacked together for a bigger assembly to produce higher degree of power. The solid electrolyte fills the gap between the cathode and anode transporting O²⁻ ions only. This leaves out electrons as transporting medium, which then pass through the cell via external circuit. Out of the two electrodes, oxidation of fuel takes place at the anode and reduction of oxygen takes place at the cathode. The SOFCs operate at higher temperatures of 600–1200°C producing heat as a byproduct of high quality, actively encouraging quick electrocatalysis utilizing non-precious metals and allowing internal restructuration. The SOFC can also work with high purity hydrogen for proton transport other than O²⁻ ion transport. There are many ceramic materials which have been engineered to act as efficient electrolyte materials. Yttria-stabilized zirconia (YSZ) is the most widely used material as solid electrolyte in SOFC.The present review presents a detailed overview of the SOFC related materials and devices and is an effort to present various reported works in a concise manner.     

Crack propagation of planar and corrugated solid oxide fuel cells during cooling process

The corrugated solid oxide fuel cell (SOFC) can effectively improve energy density and transformation efficiency compared with conventional planar SOFC, but its stability and durability have not been systematically analyzed. The residual stress of SOFC may lead to crack initiation and propagation during cooling process, so stress distributions of planar and corrugated SOFCs are simulated to analyze the location of crack initiation. The materials of electrolyte, anode, and cathode in this paper are yttria‐stabilization zirconia (YSZ), Ni‐YSZ, and strontium‐doped lanthanum manganite (LSM), respectively. The result shows that the edge of cell is more prone to cracking. Therefore, precracks including edge crack and middle crack are introduced into anode‐electrolyte interfaces to investigate crack propagation of two types of SOFCs during cooling process. For corrugated SOFC, the cracks propagate more slowly, and the cell is less prone to interfacial delamination compared with planar SOFC. In addition, the interface energy release rates are obtained to further analyze crack propagation of two types of SOFCs, and the corrugated SOFC has lower energy release rate. The research in this paper provides guidance for stability analysis and lays a foundation for future mechanical analysis of corrugated SOFC.     

Parametric and transient analysis of non-isothermal, planar solid oxide fuel cells

A multidimensional, model of non-isothermal planar solid oxide fuel cells (SOFCs) including detailed coupled mass and charge transport phenomena, has been developed. The dusty-gas model has been used, in this a comprehensive SOFC model, and has been explicitly written/constructed, for the first time in the COMSOL multiphysics modelling framework to describe mass transport in the porous electrode and detailed charge conservation equations have been taken into account. As we have shown in a recent publication [9] the incorporation of the dusty-gas model results in more accurate predictions of the SOFC behaviour compared to mass transport models based on Fick’s law or Stefan-Maxwell multi-component diffusion. Our model allows prediction of the species composition profiles, temperature profiles, electronic and ionic voltage and current density distributions, and polarisation curves in a single cell. SOFC dynamics have also been considered including responses to step changes in the operating conditions. The model is implemented in two-spatial dimensions, however, the underlying theory is independent of the geometry used. Extensive parametric analysis has been performed and the corresponding SOFC behaviour has been analysed through the resulting polarisation curves. It is shown that SOFCs exhibit higher power outputs at increased operating temperatures and pressures. It was also found that the electrodes’ porosity and tortuosity have a smaller effect on power output. Furthermore, step changes in the inlet temperatures were found to induce slower dynamic behaviours than step changes in the operating voltage.     

Three-dimensional computational fluid dynamics modeling of anode-supported planar SOFC

Modeling plays a very important role in the development of fuel cells and fuel cell systems. The aim of this work is to investigate the electrochemical processes of a Solid Oxide Fuel Cell (SOFC) and to evaluate the performance of the proposed SOFC design. For this aim a three-dimensional Computational Fluid Dynamics (CFD) model has been developed for an anode-supported planar SOFC with corrugated bipolar plates serving as gas channels and current collector. The conservation of mass, momentum, energy and species is solved by using the commercial CFD code FLUENT in the developed model. The add-on FLUENT SOFC module is implemented for modeling the electrochemical reactions, loss mechanisms and related electric parameters throughout the cell. The distributions of temperature, flow velocity, pressure and gaseous (fuel and air) concentrations through the cell structure and gas channels is investigated. The relevant fuel cell variables such as the potential and current distribution over the cell and fuel utilization are calculated and studied. The modeling results indicate that, for the proposed SOFC design, reasonably uniform distributions of current density over the active cell area can be achieved. The geometry of the cathode gas channel has a substantial effect on the oxygen distribution and thus the overall cell performance. Methods for arriving at improved cell designs are discussed.     

Effect of characteristic lengths of electron,ion, and gas diffusion on electrode performance and electrochemical reaction area in a solid oxide fuel cell

A precise evaluation of the active reaction zone in the electrodes is important to design an effective solid oxide fuel cell (SOFC). A scale analysis and one‐dimensional numerical simulations are conducted to obtain a better understanding of the electrochemical reaction zone in a SOFC anode. In the scale analysis, the characteristic lengths of the electron, oxide ion, and gas transports are evaluated from their conservation equations. Relative comparisons of the characteristic lengths show that the transport phenomena in the SOFC anode are primarily governed by the oxide‐ion conduction under standard operating conditions. The gas diffusion may affect the extent and the location of the active reaction zone at high temperature and/or low reaction gas concentration conditions. The one‐dimensional numerical simulations for an anode provided detailed information such as the electric potential of electron‐ and ion‐conducting phases, the gas concentration, and local charge‐transfer current distributions. It is found that the electrochemical reaction actively occurs in the vicinity of the anode–electrolyte interface. The effective thickness increases as the characteristic length of the ion conduction is increased resulting in better power generation performance. The effective thickness is also increased when the gas‐diffusion length is short. The cell performance is, however, lowered in this case because the low gas diffusivity yields the increase of the ohmic loss of ion conduction as well as the concentration overpotential. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20373     

天然气水蒸气重整与SOFC耦合应用

固体氧化物燃料电池（SOFC）系统具有高能源效率和使用可再生燃料的可能性,将在未来的可持续能源系统中发挥重要作用。过去几年燃料电池的发展很快,但在成本、稳定性和市场份额方面,该技术仍处于早期发展阶段。在以天然气为燃料的SOFC系统中,燃料的重整过程和燃料利用水平都可能影响系统运行的稳定性、热量和能量平衡,从而影响系统的使用寿命、输出功率和效率。因此,对燃料重整过程的设计与控制对有效的SOFC电池运行具有重要意义。对天然气在SOFC系统中的重整器配置方式（包括外重整和内重整）、重整参数和重整燃料利用方式进行了详细的综述分析,并对未来天然气SOFC系统的发展进行了展望。     

Cool-down time of solid oxide fuel cells intended for transpotation application

To provide answers to the concern as to how quickly the temperature of solid oxide fuel cells (SOFCs) for transportation application will drop, a thermal analysis of the cool-down time of an SOFC stack during vehicle idle or stand-by has been carried out. Because a large amount of thermal energy is stored in high-temperature SOFC stacks, it is important to select suitable thermal insulations to reduce heat loss. Three typical kinds of thermal insulating materials have been selected in the present calculations. The results indicate that a high-performance, vacuum-multifoil thermal insulation can be applied to significantly reduce heat loss and to maintain temperature uniformity across a cell stack. Consequently, the cool-down time from 1000 to 800°C is extended from 2 h (with a 5 cm thick conventional material) to about 31 h (with a 1 cm thick high-performance material).     

Simulation of sintering kinetics and microstructure evolution of composite solid oxide fuel cells electrodes

A three-dimension (3D) kinetic Monte Carlo (kMC) model is developed to study the sintering kinetics and microstructure evolution of solid oxide fuel cell (SOFC) composite electrodes during the co-sintering processes. The model employs Lanthanum Strontium Manganite (LSM) – Yttria-stabilized Zirconia (YSZ) composites as the example electrodes but can be applied to other materials. The sintering mechanisms include surface diffusion, grain boundary migration, vacancy creation, and annihilation. A morphological dilation method is used to generate the initial LSM–YSZ compacts as the input structures for the kMC simulation. The three-phase boundary (TPB) length, porosity, and tortuosity factor of the composite cathodes are calculated during kMC sintering. Simulation results are compared with literature data and good agreement is found. Parametric study is conducted to investigate the effects of particle size, size distribution, and sintering temperature on sintering kinetics as well as the evolution of electrode microstructures. The kMC model is capable of simulating the initial and a part of intermediate sintering stages of SOFC electrodes by considering various sintering mechanisms simultaneously. It can serve as a useful tool to design and optimize the sintering processes for composite SOFC electrodes.     

Application of a detailed dimensional solid oxide fuel cell model in integrated gasification fuel cell system design and analysis

Integrated gasification fuel cell (IGFC) systems that combine coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally sensitive utilization of coal for power production. Most IGFC system analysis efforts performed to-date have employed non-dimensional SOFC models, which predict SOFC performance based upon global mass and energy balances that do not resolve important intrinsic constraints of SOFC operation, such as the limits of internal temperatures and species concentrations. In this work, a detailed dimensional planar SOFC model is applied in IGFC system analysis to investigate these constraints and their implications and effects on the system performance. The analysis results further confirm the need for employing a dimensional SOFC model in IGFC system design. To maintain the SOFC internal temperature within a safe operating range, the required cooling air flow rate is much larger than that predicted by the non-dimensional SOFC model, which results in a larger air compressor design and operating power that significantly reduces the system efficiency. Options to mitigate the challenges introduced by considering the intrinsic constraints of SOFC operation in the analyses and improve IGFC design and operation have also been investigated. Novel design concepts that include staged SOFC stacks and cascading air flow can achieve a system efficiency that is close to that of the baseline analyses, which did not consider the intrinsic SOFC limitations.     

Progress and challenges of cathode contact layer for solid oxide fuel cell

The contact layer is to provide and maintain a stable conductive path between interconnects and electrodes in a planar solid oxide fuel cell (SOFC) stack, which effects contact resistance and stack power loss. The materials used in the contact layer are well developed. This article focuses on cathode contact materials. The role and material requirements of the cathode contact layer in the SOFC stack are reviewed. The common cathode contact layer materials are listed, and their electrical properties as the cathode contact layer are discussed. Due to the evolution of the SOFC stack, the preparation technology and form of the cathode contact layer have changed. Finally, the future challenges of the cathode contact layer are presented.     

The effect of HCl in syngas on Ni-YSZ anode-supported solid oxide fuel cells

The Ni-YSZ cermet anode of the solid oxide fuel cell (SOFC) has excellent electrochemical performance in a clean blended synthetic coal syngas mixture. However, chloride, one of the major contaminants existing in coal-derived syngas, may poison the Ni-YSZ cermet and cause degradation in cell performance. Both hydrogen chloride (HCl) and chlorine (Cl₂) have been reported to attack the Ni in the anode when using electrolyte-supported SOFCs. In this paper, a commercial anode-supported SOFC was exposed to syngas with a concentration of 100 ppm HCl under a constant current load at 800 °C for 300 h and 850 °C for 100 h. The cell performance was evaluated periodically using electrochemical methods. A unique feature of this experiment is that the active central part of the anode was exposed directly to the fuel without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the 100 ppm concentration of HCl causes about 3% loss of performance for the Ni-YSZ anode-supported cell during the 400 h test. Permanent changes were noted in the surface microstructure of the nickel particles in the cell anode.     

Three-dimensional micro/macroscale simulation of planar,anode-supported,intermediate-temperature solid oxide fuel cells: I. Model development for hydrogen fueled operation

Intermediate-temperature solid oxide fuel cells (IT-SOFCs) are promising SOFC technologies that can solve many problems of high-temperature SOFCs (HT-SOFCs), such as the stringent restriction on material selection, accelerated degradation of electrode activity, limitation in thermal cycling, and requirement for long start-up times. In this study, a comprehensive three-dimensional micro/macroscale model is developed for simulating planar, anode-supported IT-SOFCs fueled with hydrogen. Many constitutive sub-models for electrode microstructure, detailed charge-transfer processes, and heat/mass transport in three-dimensional interconnect plate/gas channel geometries are combined to investigate the performance and operating characteristics of IT-SOFCs with rather standard materials (such as nickel, YSZ, LSM, and stainless steel). The current−voltage performance curves are presented along with the contribution of activation, concentration, ohmic, and contact overpotentials to total potential loss. In addition, the spatial distributions of temperature, current density, and species concentrations are also investigated for co- and counter-flow configurations. The results clearly demonstrate the capabilities of the present three-dimensional micro/macroscale model as an accurate and efficient design tool for optimizing the operating conditions, electrode microstructures, and cell geometries of planar, anode-supported IT-SOFCs.     

Progress report on the catalyst layers for hydrocarbon-fueled SOFCs

Solid oxide fuel cell (SOFC) is an energy conversion device that can directly convert the chemical energy of carbonaceous fuels into electricity. Solving the problem of carbon deposition in the conventional nickel-based anode is essential to improving the performance of SOFC when operating on carbonaceous fuels. Although impressive progress has been made in the development of alternative anode materials, nickel-based anodes with superior catalytic activity for carbonaceous fuels are still the most promising anode for the commercialization of SOFCs. The deposition of a catalyst layer with high catalytic activity for carbonaceous fuels over the nickel-based anode has been demonstrated as an effective way to enhance the performance and long-term stability of hydrocarbon-based SOFC. This review introduces the working principles of the catalyst layers, discusses the recent progress of the catalyst layer materials for hydrocarbon-fueled SOFC and issues of the different catalyst layer materials. Finally, some of the future prospects and challenges of the catalyst layers are summarized in this review article.     

Pr2NiO4-Ag composite as cathode for low temperature solid oxide fuel cells: Effects of silver loading methods and amounts

Active cathode materials for low temperature solid oxide fuel cells (SOFC) below 600 °C are urgently required due to the sluggish oxygen reduction reaction (ORR) kinetics at reduced temperature. In this work, a detailed experimental fabrication and characterization of silver modified Pr₂NiO₄ composite material for low temperature SOFC cathode catalyst with superior ionic conducting ceria-carbonate composite electrolyte was carried out. Pr₂NiO₄ was prepared by a co-precipitation method with NaOH as precipitant, and it was composited with silver to improve the electrode activity toward ORR through three various methods of impregnation, solid-state mixing and freeze-drying, respectively. Effects of Ag loading on the electrochemical activity were systematically investigated. It was found that composite materials originated from impregnation method presented the optimal material microstructure, and 15% Ag loaded composite gave the lowest area specific resistance of 0.45 Ω cm² at 600 °C, which is reduced by around 300% compared with previous work, indicating that impregnated Pr₂NiO₄-15Ag composite is a promising cathode catalyst for low temperature SOFC with ceria-carbonate composite electrolyte.