Multiscale model for solid oxide fuel cell with electrode containing mixed conducting material

Solid oxide fuel cells (SOFCs) with electrodes that contain mixed conducting materials usually show very different relationships among microstructure parameters, effective electrode characteristics, and detailed working processes from conventional ones. A new multiscale model for SOFCs using mixed conducting materials, such as LSCF or BSCF, was developed. It consisted of a generalized percolation micromodel to obtain the electrode properties from microstructure parameters and a multiphysics single cell model to relate these properties to performance details. Various constraint relationships between the activation overpotential expressions and electric boundaries for SOFC models were collected by analyzing the local electrochemical equilibrium. Finally, taking a typical LSCF‐SDC/SDC/Ni‐SDC intermediate temperature SOFC as an example, the application of the multiscale model was illustrated. The accuracy of the models was verified by fitting 25 experimental I‐V curves reported in literature with a few adjustable parameters; additionally, and several conclusions were drawn from the analysis of simulation results. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3786–3803, 2015     

Modelling the 3D microstructure and performance of solid oxide fuel cell electrodes: Computational parameters

In this paper, the computational parameters for a 3D model for solid oxide fuel cell (SOFC) electrodes developed to link the microstructure of the electrode to its performance are investigated. The 3D microstructure model, which is based on Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials. Once formed, the synthetic electrodes are discretized into voxels (small cubes) of equal sizes from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three-phase boundary (TPB) length. Transport phenomena and electrochemical reactions taking place within the electrode are modelled so that the performance of the synthetic electrode can be predicted. The degree of microstructure discretization required to obtain reliable microstructural analysis is found to be related to the particle sizes used for generating the structure; the particle diameter should be at least 20–40 times greater than the edge length of a voxel. The structure should also contain at least 25³ discrete volumes which are called volume-of-fluid (VOF) units for the purpose of transport and electrochemical modelling. To adequately represent the electrode microstructure, the characterized volume of the electrode should be equivalent to a cube having a minimum length of 7.5 times the particle diameter. Using the modelling approach, the impacts of microstructural parameters on the electrochemical performance of the electrodes are illustrated on synthetic electrodes.     

Investigation of the active thickness of solid oxide fuel cell electrodes using a 3D microstructure model

A 3D microstructure model is used to investigate the effect of the thickness of the solid oxide fuel cell (SOFC) electrode on its performance. The 3D microstructure model, which is based on 3D Monte Carlo packing of spherical particles of different types, can be used to handle different particle sizes and generate a heterogeneous network of the composite materials from which a range of microstructural properties can be calculated, including phase volume fraction, percolation and three phase boundary (TPB) length. The electrode model can also be used to perform transport and electrochemical modelling such that the performance of the synthetic electrode can be predicted. The dependence of the active electrode thickness, i.e. the region of the anode, which is electrochemically active, on operating over-potential, electrode composition and particle size is observed. Operating the electrode at an over-potential of above 200 mV results in a decrease in the active thickness with increasing over-potential. Reducing the particle size dramatically enhances the percolating TPB density and thus the performance of the electrode at smaller thicknesses; a smaller active thickness is found with electrodes made of smaller particles. Distributions of local current generation throughout the electrode reveal the heterogeneity of the 3D microstructure at the electrode/electrolyte interface and the dominant current generation in the vicinity of this interface. The active electrode thickness predicted using the model ranges from 5 μm to 15 μm, which corresponds well to many experimental observations, supporting the use of our 3D microstructure model for the investigation of SOFC electrode related phenomena.     

Recent advances and prospects of symmetrical solid oxide fuel cells

Solid oxide fuel cells (SOFCs) with symmetrical electrodes have been investigated extensively because of their potential significant advantages compared to the traditional configurations, regarding manufacturing, thermomechanical compatibility with cell components, operation stability, anting sulfur poisoning and carbon deposition. Many electrodes with novel structure and properties are currently being developed and studied in recent years. In this review, we summarized the recent advances of symmetrical SOFCs on their electrode materials, applications and prospects. The electrode materials include single phased perovskite, double perovskite, perovskite derived structures and composite electrodes. The relationships between the electrode materials and relevant properties are discussed. The applications and perspectives are highlighted, providing critical and useful directions for researchers to prepare and design electrode materials rationally.     

Thermo-mechanical analysis of 3D manufactured electrodes for solid oxide fuel cells

Additive manufacturing has widened the scope for designing more performing microstructures for solid oxide fuel cells (SOFCs). Structural modifications, such as the insertion of ceramic pillars within the electrode, facilitate ion transport and boost the electrochemical performance. However, questions still remain on the related mechanical requirements during operation. This study presents a comprehensive thermal-electrochemical-mechanical model targeted to assess the stress distribution in 3D manufactured electrodes. Simulations show that a dense pillar increases the stress distribution by ca. 10 % compared to a flat electrode benchmark. The stress is generated by the material thermal contraction and intensifies at the pillar-electrolyte junction while external loads have negligible effects. An analysis on manufacturing inaccuracies indicates that sharp edges, surface roughness and tilted pillars intensify the stress; nonetheless, the corresponding stress increase is narrow, suggesting that manufacturing inaccuracies can be easily tolerated. The model points towards robust design criteria for 3D manufactured electrodes.     

A critical look into effects of electrode pore morphology in solid oxide fuel cells

Knudsen diffusion, an important form of gas transport in sub‐micro/nanoscale porous electrodes of solid oxide fuel cells (SOFCs), is evaluated typically based on the assumption of isotropic cross‐sections of electrode pores. As a consequence, errors are induced in the evaluation of gas transport and polarization loss of SOFCs with irregular, anisotropic pore morphology. Here, a numerical model is derived to investigate the impact of pore morphology on Knudsen diffusivity and effective total diffusivity in porous SOFC electrodes. Based on the model, the correlation between pore morphology and important parameters of SOFCs, including limiting current density (LCD) and concentration polarization (CP), is evaluated. As the aspect ratio of pore cross‐section increases, the gas diffusivity in SOFC electrodes decreases, and then nontrivial variations in LCD and CP are induced. This work facilitates the accurate evaluation of gas transport in SOFCs as well as the rational design of electrode microstructure of SOFCs. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2312–2317, 2017     

Effects of mixing state of composite powders on sintering behavior of cathode for solid oxide fuel cells

For efficient development of high-performance composite electrodes for solid oxide fuel cells (SOFCs), it is crucial to precisely tailor the microstructural features of the electrodes, such as their grain size, phase connectivity, and pore structure. Herein, we report the effects of the mixing state of component powders of a composite cathode composed of Sr-doped LaMnO₃ (LSM) and yttria-stabilized zirconia (YSZ) on its sintering behavior. LSM-YSZ composite powders were synthesized by a particle-dispersed glycine-nitrate process using YSZ particles as inclusions in the LSM precursor solution. The dispersion state of the YSZ particles in the solution was varied from a well-dispersed state to a highly flocculated state through adjustment of the amount of adsorbed polyethylene glycol. The dispersion state of the component powders was found to strongly impact the densification behavior of the composite, which was explained by the formation of a continuous network of the “slow-sintering” inclusion particles. A highly porous structure with phase connectivity and sufficient triple phase boundaries could be achieved by enhancing the mixing homogeneity and optimizing the mixing scale. The proposed concept provides new insights into the microstructural evolution of composites in constrained sintering, and it could potentially enable development of the ideal electrode structure for SOFCs.     

Percolation modeling investigation of TPB formation in a solid oxide fuel cell electrode-electrolyte interface

A Monte Carlo percolation model has been developed and utilized to characterize the factors controlling triple phase boundary (TPB) formation in an SOFC electrode. The model accounts for (1) electronic conductor, ionic conductor, and gas phase percolation, (2) competition between percolation of gas and electronically conducting phases, and (3) determination of continuous, though not necessarily fully percolating, paths from TPBs to the bulk phases. The model results show that physical processes near the TPB, such as sorbate transport, significantly affect TPB formation in a composite electrode. Active TPB formation is found to be most significantly dependent upon continuous and competing percolation of multiple phases. Simultaneously requiring continuous paths and accounting for non-continuous boundary conditions results in lower active TPB formation levels (up to 8% of possible sites) than presented in the literature (75% of possible sites). In addition, the varying ratio of active to potential TPB sites predicted by the current model (up to 80%) differs significantly from the constant reported in the literature (80%), which lacks analyses of three-phase percolation, gas phase paths, and gas/current collector boundary conditions. This dependence of active TPB formation on percolation of all three phases is important to understand as a basis for determining SOFC performance and optimization.     

Inverse approach to quantify multi-physicochemical properties of porous electrodes for solid oxide fuel cells

Porous electrodes are critical components of solid oxide fuel cells (SOFCs). The quantification of electrode properties is very significant for high fidelity modeling and underlying mechanism studies. In this research, an inverse approach is presented to determine multi-physicochemical properties of electrodes using cell polarization performance measurements and repulsive particle swarm optimization technique, without resorting to porous microstructure features, e.g., porosity, tortuosity, pore size. The mathematical model is developed using a proton conducting button cell test stand as the physical base and employed for inverse analysis. The approach is demonstrated using both simulated results and practical measurements. This research provides a new approach for practical SOFC analysis.     

Copper nanofiber-networked cobalt oxide composites for high performance Li-ion batteries

We prepared a composite electrode structure consisting of copper nanofiber-networked cobalt oxide (CuNFs@CoO _x ). The copper nanofibers (CuNFs) were fabricated on a substrate with formation of a network structure, which may have potential for improving electron percolation and retarding film deformation during the discharging/charging process over the electroactive cobalt oxide. Compared to bare CoO _x thin-film (CoO _x TF) electrodes, the CuNFs@CoO _x electrodes exhibited a significant enhancement of rate performance by at least six-fold at an input current density of 3C-rate. Such enhanced Li-ion storage performance may be associated with modified electrode structure at the nanoscale, improved charge transfer, and facile stress relaxation from the embedded CuNF network. Consequently, the CuNFs@CoO _x composite structure demonstrated here can be used as a promising high-performance electrode for Li-ion batteries.     

Electrochemical performance of low-temperature solid oxide fuel cells running on syngas from pyrolytic urban sludge

Low temperature solid oxide fuel cells (SOFCs) that efficiently utilize widely available hydrocarbon resources are highly desirable for cost reduction and durability purposes. In this work, SOFCs consisting of highly ionic conductive ceria-carbonate composite electrolytes and lithiated transition metal oxide symmetric electrodes are assembled and their electrochemical performances at reduced temperature (≤650 °C) are investigated using syngas fuel (44.65% H₂, 10.19% CH_4, 2.01% CO and the balanced CO₂) derived from pyrolytic urban sludge. The cell gives a peak power output of 127 mW cm^?2 at 600 °C and shows a relatively stable operation for 11 hours under constant voltage operational conditions. Though the composite electrode presents a moderately high polarization resistance toward CH₄ and CO oxidation and the electrochemical performance is highly correlated with the microstructure of ceria-carbonate electrolyte, it is interesting to see that a higher concentration of methane is obtained after the fuel cell reaction, which may suggest an alternative approach to realize the power and chemical co-generation within such a SOFC reactor. Finally, the symmetric electrode shows high resistance toward carbon deposition, possibly due to its high alkaline nature.     

双脉冲电沉积制备Ni-聚苯胺复合电极及其析氢性能的研究

采用控制双脉冲电位沉积技术制备N i-聚苯胺复合电极。扫描电镜下观察电极表面呈菜花状结构,比表面积约为普通镀N i电极的4~50倍。N i-PAN复合电极的X射线衍射谱图中分别出现了N i和PAN的特征峰。通过测试复合电极在模拟氯碱工业电解液中的阴极极化曲线,研究了N i-PAN电极的析氢性能,结果显示当电流密度为0.10 A/cm2时,析氢过电位较镀N i电极降低约350 mV。复合电极性能稳定,可作为氯碱工业用活性阴极,能显著降低能耗。     

Shaping of advanced ceramics: The case of composite electrodes for lithium batteries

Composite electrodes for electrochemical energy storage systems, such as lithium batteries, are mixtures of a ceramic powder, an electronic conducting agent, often carbon black, and a polymeric binder. Such a complex medium must provide efficient transport of electrons and ions from the current collector/electrode and electrolyte/electrode interfaces, respectively, to the grain surface of the ceramic. It seems quite obvious that the morphology within the composite electrode should have an influence on the electrode performance. However, such an issue has never been studied yet. Here we study composite electrodes prepared by the tape casting method, with the purpose to gain better understanding of the relationships between: the suspension properties, the morphology within the dried composite electrode, and the resulting electrochemical properties.     

Pyrolyzable pore-formers for the porous-electrode formation in solid oxide fuel cells: A review

Porosity is a key property that plays a crucial role in enhancing the performance of solid oxide fuel cell (SOFC) electrodes. The addition of pyrolyzable pore-formers to the electrode materials of SOFCs can generate suitable porous microstructures with the required porosity, pore sizes, and morphology. The present review provides details on the characterization and microstructural analysis, firing profile, electrical conductivity, mechanical strength, and gas permeability of the porous electrodes of SOFCs. A better understanding of these relationships can help to design optimized porous microstructures for generating higher power densities of the cells.     

3D imaging and quantification of interfaces in SOFC anodes

Solid oxide fuel cells (SOFCs) are functional electrochemical conversion devices whose performance is strongly dependent on electrode microstructure. Both the performance and lifetime of these electrochemical devices can be considerably enhanced by the ability to design better electrodes. Data acquired from high resolution 3D imaging techniques were used in the quantification of two electrode structures of different compositions. The quantified nickel-based anode data through the analysis of particle sizes with their metal–metal and ceramic–ceramic neck sizes, metal–ceramic interface sizes, volume fractions, and triple-phase boundary densities, demonstrate it is possible to understand how microstructure contributes to differences observed in electrochemical and mechanical performance; facilitating optimisation of electrode micro/nano structure towards improved performance. In doing so, new insights are gained that could be used to develop better electrodes.     

Improved electrochemical activity of Bi0.5Sr0.5FeO3-δ–Ce0.9Gd0.1O1.95composite cathode electrocatalyst for solid oxide fuel cells

Developing MIEC materials with high electrocatalytic performance for the ORR and good thermal/chemical/structural stability is of paramount importance to the success of solid oxide fuel cells (SOFCs). In this work, high-activity Bi_0.5Sr_0.5FeO_3-δ-xCe_0.9Gd_0.1O_1.95 (BSFO-xGDC, x = 10, 20, 30 and 40 wt%) oxygen electrodes are synthesized, and confirmed by XRD, SEM and EIS, respectively. The crystal structure, microstructure, electrochemical property and performance stability of the promising BSFO-xGDC composite cathodes are systematically evaluated. It is found that introducing GDC nanoparticles can obviously improve the electrochemical property of the porous composite electrode. Among all these composite cathodes, BSFO-30GDC composite cathode shows the best ORR activity. The peak power density of anode supported single cells employing BSFO-30GDC composite cathode reaches 709 mW cm^?2 and the electrode polarization resistance (R_p) of the BSFO-30GDC is about 0.14 Ω cm² at 700 °C. The analysis of the oxygen reduction kinetic indicates that the major electrochemical process of the GDC-decorated composite cathode is oxygen adsorption-dissociation. These preliminary results demonstrated that BSFO-30GDC is a prospective composite cathode catalyst for SOFCs because of its outstanding ORR activity.     

Novel bipolar electrodes for battery applications

A novel bipolar graphite felt electrode for use in redox flow batteries and other electrochemical systems is described. The new electrode features a unique approach in the design of bipolar electrodes, employing carbon black free, nonconductive polymer materials as substrates. This innovation allows a dramatic reduction of processing time and cost compared to conventional carbon polymer composite electrodes used in bipolar battery systems. The conductivity of the new electrode assembly is similar to that of conventional bipolar electrodes, however, it shows significant improvements in mechanical properties. The functionality of these novel electrodes has been evaluated in the vanadium redox battery application and the results show comparable performance with conventional composite materials. An important operational advantage, however, is that side reactions leading to the deterioration of conductive filler in the electrode substrate material (i.e., electrode delamination due to CO₂-evolution) during cell overcharging are eliminated, making these electrodes more durable than the conventional designs. To date, these bipolar electrodes have been applied in vanadium redox cells but their design and properties promise further applications in a range of other redox flow batteries and bipolar electrochemical cell systems.     

基于聚苯胺改性构建三维MXene复合材料及其电容性能

通过直接电化学法,本文利用MXene表面官能团的诱导能力,在外加电场的作用下,将苯胺单体与MXene共同修饰在不锈钢电极表面,成功制得具有三维结构的MXene/聚苯胺复合电极材料。采用SEM、XRD、XPS、FTIR和Raman光谱对复合电极材料的表面形貌、物相结构和组成进行了表征,并在1mol/L H₂SO₄中详细研究了该电极材料的电容性能。结果表明,得益于MXene的掺杂,MXene/聚苯胺复合电极表现出较好的电子传导能力和优异的电容性能,在10mV/s的扫描速率下电容可达417F/g,当扫描速率增至200mV/s时,其电容保持率为52%,比纯PANI电极高31%。该复合电极材料具有良好的循环稳定性,在1.0A/g的电流密度下循环2000次后电容保持率可维持在83.4%。此项研究工作可为三维MXene复合材料的构建提供设计思路。     

Effect of composition on the performance of cermet electrodes. Experimental and theoretical approach

This work aims to analyse the behaviour of cermet electrodes as a function of their composition, i.e. the ratio between ionic and electronic conducting particles. This is an important parameter to be considered to obtain maximum performance from this type of electrode, which is currently under study for application in oxygen sensors and solid oxide fuel cells. Experimental results of overall electrode resistance, including both ohmic and activation polarisation effects, have been obtained through electrochemical impedance spectroscopy measurements of Pt/YSZ electrodes in air. The results compare favourably with the theoretical predictions for several compositions above the percolation threshold of the electronic conductor. For this reason, the model is a useful tool for the design of optimised cermet electrodes; in particular, the experimental data show that maximum performance is attained for compositions very close to the percolation threshold of the electronic conductor, and this is in very good agreement with the modelling results.     

Three-dimensional random resistor-network model for solid oxide fuel cell composite electrodes

A three-dimensional reconstruction of solid oxide fuel cell (SOFC) composite electrodes was developed to evaluate the performance and further investigate the effect of microstructure on the performance of SOFC electrodes. Porosity of the electrode is controlled by adding pore former particles (spheres) to the electrode and ignoring them in analysis step. To enhance connectivity between particles and increase the length of triple-phase boundary (TPB), sintering process is mimicked by enlarging particles to certain degree after settling them inside the packing. Geometrical characteristics such as length of TBP and active contact area as well as porosity can easily be calculated using the current model. Electrochemical process is simulated using resistor-network model and complete Butler-Volmer equation is used to deal with charge transfer process on TBP. The model shows that TPBs are not uniformly distributed across the electrode and location of TPBs as well as amount of electrochemical reaction is not uniform. Effects of electrode thickness, particle size ratio, electron and ion conductor conductivities and rate of electrochemical reaction on overall electrochemical performance of electrode are investigated.