Modeling and Optimization of Anode‐Supported Solid Oxide Fuel Cells on Cell Parameters via Artificial Neural Network and Genetic Algorithm

An artificial neural network (ANN) and a genetic algorithm (GA) are employed to model and optimize cell parameters to improve the performance of singular, intermediate‐temperature, solid oxide fuel cells (IT‐SOFCs). The ANN model uses a feed‐forward neural network with an error back‐propagation algorithm. The ANN is trained using experimental data as a black‐box without using physical models. The developed model is able to predict the performance of the SOFC. An optimization algorithm is utilized to select the optimal SOFC parameters. The optimal values of four cell parameters (anode support thickness, anode support porosity, electrolyte thickness, and functional layer cathode thickness) are determined by using the GA under different conditions. The results show that these optimum cell parameters deliver the highest maximum power density under different constraints on the anode support thickness, porosity, and electrolyte thickness.     

Redox Tolerance of Thin and Thick Ni/YSZ Anodes of Electrolyte‐Supported Single‐Chamber Solid Oxide Fuel Cells under Methane Oxidation Conditions

Redox tolerance of 50 and 500 μm thick Ni/YSZ (yttria‐stabilized zirconia) anodes supported on YSZ electrolytes were studied under single‐chamber solid oxide fuel cell conditions. Open circuit voltage, electrochemical impedance spectra, and discharge curves of the cells were measured under different methane/oxygen ratios at 700 °C. For the cell with the thin anode, a significant degradation accompanying oscillatory behaviors was observed, whereas the cell based on the thick anode was much more stable under the same conditions. In situ local anode resistance (R_s) results indicated that the Ni/NiO redox cycling was responsible for the oscillatory behaviors, and the cell degradation was primarily caused by the Ni reoxidation. Reoxidation of the thick anode took place at a low methane/oxygen ratio, but the anode can be recovered to its original state by switching to a methane‐rich environment. On the contrary, the thin anode was unable to be regenerated after the oxidation. Microstructure damage of the anode was attributed to its irreversible degradation.     

Analysis of Design Parameters in Anode‐Supported Solid Oxide Fuel Cells Using Response Surface Methodology

Achieving high performance from a solid oxide fuel cell (SOFC) requires optimal design based on parametric analysis. In this paper, design parameters, including anode support porosity, thicknesses of electrolyte, anode support, and cathode functional layers of a single, intermediate temperature, anode‐supported planar SOFC, are analyzed. The response surface methodology (RSM) technique based on an artificial neural network (ANN) model is used. The effects of the cell parameters on its performance are calculated to determine the significant design factors and interaction effects. The obtained optimum parameters are adopted to manufacture the single units of an SOFC through tape casting and screen‐printing processes. The cell is tested and its electrochemical characteristics, which show a satisfactory performance, are discussed. The measured maximum power density (MPD) of the fabricated SOFC displays a promising performance of 1.39 W cm^–2. The manufacturing process planned to fabricate the SOFC can be used for industrial production purposes.     

Electrochemical Performance of a Ni and YSZ Composite Synthesised by Ultrasonic Spray Pyrolysis as an Anode for SOFCs

The electrochemical performance of an anode material for a solid oxide fuel cell (SOFC) depends highly on microstructure in addition to composition. In this study, a NiO–yttria‐stabilised zirconia (NiO–YSZ) composite with a highly dispersed microstructure and large pore volume/surface area has been synthesised by ultrasonic spray pyrolysis (USP) and its electrochemical characteristics has been investigated. For comparison, the electrochemical performance of a conventional NiO–YSZ is also evaluated. The power density of the zirconia electrolyte‐supported SOFC with the synthesised anode is ∼392 mW cm^–2 at 900 °C and that of the SOFC with the conventional NiO–YSZ anode is ∼315 mW cm^–2. The improvement is ∼24%. This result demonstrates that the synthesised NiO–YSZ is a potential alternative anode material for SOFCs fabricated with a zirconia solid electrolyte.     

Comparison Between Anode‐Supported and Electrolyte‐Supported Ni‐CGO‐LSCF Micro‐tubular Solid Oxide Fuel Cells

Two types of micro‐tubular hollow fiber SOFCs (MT‐HF‐SOFCs) were prepared using phase inversion and sintering; electrolyte‐supported, based on highly asymmetric Ce_0.9Gd_0.1O_1.95(CGO) HFs and anode‐supported based on co‐extruded NiO‐CGO(CGO)/CGO HFs. Electroless plating was used to deposit Ni onto the inner surfaces of the electrolyte‐supported MT‐HF‐SOFCs to form Ni‐CGO anodes. LSCF‐CGO cathodes were deposited on the outer surface of both these MT‐HF‐SOFCs before their electrochemical performances were compared at similar operating conditions. The performance of the anode‐supported MT‐HF‐SOFCs which delivered ca. 480 mW cm^–2 at 600 °C was superior to the electrolyte‐supported MT‐HF‐SOFCs which delivered ca. six times lower power. The contribution of ohmic and electrode polarization losses of both FCs was investigated using electrochemical impedance spectroscopy. The electrolyte‐supported MT‐HF‐SOFCs had significantly higher ohmic and electrode polarization ASR values; this has been attributed to the thicker electrolyte and the difficulties associated with forming quality anodes inside the small (<1 mm) lumen of the electrolyte tubes. Further development on co‐extruded anode‐supported MT‐HF‐SOFCs led to the fabrication of a thinner electrolyte layer and improved electrode microstructures which delivered a world leading 2,400 mW cm^–2. The newly made cell was investigated at different H₂ flow rates and the effect of fuel utilization on current densities was analyzed.     

Interaction of a ceria‐based anode functional layer with a stabilized zirconia electrolyte: Considerations from a materials perspective

We report on the materials interaction of gadolinium‐doped ceria (GDC) and yttria‐stabilized zirconia (YSZ) in the context of high‐temperature sintering during manufacturing of anode supported solid oxide fuel cells (AS–SOFC). While ceria‐based anodes are expected to show superior electrochemical performance and enhanced sulfur and coking tolerance in comparison to zirconia‐based anodes, we demonstrate that the incorporation of a Ni–GDC anode into an ASC with YSZ electrolyte decreases the performance of the ASC by approximately 50% compared to the standard Ni–YSZ cell. The performance loss is attributed to interdiffusion of ceria and zirconia during cell fabrication, which is investigated using powder mixtures and demonstrated to be more severe in the presence of NiO. We examine the physical properties of a GDC–YSZ mixed phase under reducing conditions in detail regarding ionic and electronic conductivity as well as reducibility, and discuss the expected impact of cation intermixing between anode and electrolyte.     

Planar Metal‐Supported SOFC with Novel Cermet Anode

Metal‐supported solid oxide fuel cells are expected to offer several potential advantages over conventional anode (Ni‐YSZ) supported cells. For example, increased resistance against mechanical and thermal stresses and a reduction in material costs. When Ni‐YSZ based anodes are used in metal supported SOFC, elements from the active anode layer may inter‐diffuse with the metallic support during sintering. This work illustrates how the inter‐diffusion problem can be circumvented by using an alternative anode design based on porous and electronically conducting layers, into which electrocatalytically active materials are infiltrated after sintering. The paper presents the electrochemical performance and durability of the novel planar metal‐supported SOFC design. The electrode performance on symmetrical cells has also been evaluated. The novel cell and anode design shows a promising performance and durability at a broad range of temperatures and is especially suitable for intermediate temperature operation at around 650 °C.     

Scaling up aqueous processing of A-site deficient strontium titanate for SOFC anode supports

All ceramic anode supported half cells of technically relevant scale were fabricated in this study, using a novel strontium titanate anode material. The use of this material would be highly advantageous in solid oxide fuel cells due to its redox tolerance and resistance to coking and sulphur poisoning. Successful fabrication was possible through aqueous tape casting of both anode support and electrolyte layers and subsequent lamination. Screen printing of electrolyte layers onto green anode tapes was also attempted but resulted in cracked electrolyte layers upon firing. Microstructural, electrical and mechanical properties of anode supports and half cells will be discussed. The use of two different commercial titanate powders with nominal identical, but in reality different stoichiometries, strongly affect electrical and mechanical properties. Careful consideration of such variations between powder suppliers, and batches of the same supplier, is critical for the successful implementation of ceramic anode supported solid oxide fuel cells.     

Strength of Anode‐Supported Solid Oxide Fuel Cells

Nickel oxide and yttria doped zirconia composite strength is crucial for anode‐supported solid oxide fuel cells, especially during transient operation, but also for the initial stacking process, where cell curvature after sintering can cause problems. This work first compares tensile and ball‐on‐ring strength measurements of as‐sintered anodes support. Secondly, the strength of anode support sintered alone is compared to the strength of a co‐sintered anode support with anode and electrolyte layers. Finally, the orientation of the specimens to the bending axis of a co‐sintered half‐cell is investigated. Even though the electrolyte is to the tensile side, it is found that the anode support fails due to the thermo‐mechanical residual stresses.     

Micro‐Scale Modeling of a Functionally Graded Ni‐YSZ Anode

A mathematical model was developed to investigate the coupled transport and electrochemical reactions in a nickel‐yttrial‐stablized zirconia (Ni‐YSZ) anode for use in solid oxide fuel cells (SOFCs). The modeling results were consistent with experimental data from the literature. Comparison between conventional non‐graded (uniform random composites) and two types of functionally graded electrodes (FGE), namely particle size graded and porosity graded SOFC anodes were conducted to evaluate the potential of FGE for SOFC. Improved performance of both types of FGE was observed due to reduced mass transport resistance and increased volumetric reactive surface area close to the electrode‐electrolyte interface. It was found that the particle size graded SOFC anode showed the best performance. This paper demonstrates that the SOFC performance could be enhanced by modifying the microstructures of the electrodes. The results presented in this paper provide a better understanding of the working mechanisms of SOFC electrodes and could serve as an important reference for design optimizations.     

Characterisation of Ni–YSZ‐Cermets with Respect to Redox Stability

In anode‐supported solid oxide fuel cells (SOFCs), air break‐in on the anode side can result in reoxidation of metallic nickel. The volume expansion caused by Ni oxidation generates stresses within the substrate, the anode and the electrolyte. Those stresses exceed the stability of the components, potentially promoting crack growth. Therefore, either the SOFC degrades continuously after each redox‐cycle or the membrane electrode assembly (MEA) fails completely if the electrolyte cracks. The influence of several reoxidation parameters on the mechanical integrity of Ni–YSZ‐anodes after reoxidation was investigated using different types of samples. All samples were SFEs (substrate–functional layer–electrolytes), consisting of Ni–YSZ‐substrate, Ni–YSZ‐anode and YSZ‐electrolyte. Investigations were carried out on freestanding SFEs and SFEs attached to steel plates (Crofer22APU, Thyssen Krupp V. D. M., Material Data Sheet No. 4046, Edition of December 2006) with a glass sealing. The results show a big influence of the degree of oxidation, homogeneity of oxidation, the operating temperature and the incident flow on the behaviour and the mechanical integrity of the reoxidised SFEs. The time of oxidation and the gas flow rate were influencing parameters, whereas the influence of the porosity was insignificant. The behaviour of the SFEs upon reoxidation also changes dramatically when comparing freestanding samples with attached samples.     

Direct-methane anode-supported solid oxide fuel cells fabricated by aqueous gel-casting

Direct methane Solid Oxide Fuel Cells (SOFCs) operated under catalytic partial oxidation (CPOX) conditions are investigated, focusing on the processing of the anode support and the anode deactivation caused by carbon deposition. Anode-supported SOFCs based on gadolinium-doped ceria (GDC) electrolyte, and NiO-GDC anode support were fabricated by the gel-casting method. Suitable aqueous slurries formulations of NiO–GDC were prepared, starting NiO-GDC nanocomposite powders, agarose as gelling agent and rice starch as pore former. Electrochemical and mechanical tests evidenced that the support of 550 ± 50 µm thickness and 10 wt% pore former is a good candidate for direct-methane SOFCs. The cells operating under stoichiometric conditions of CPOX reached a performance of 0.64 W·cm^?2 at 650 ºC, a very close value to that measured under humidified hydrogen (0.71 W·cm^?2). The best electrochemical stability of the cell is achieved at a CH₄/O₂ ratio of 2.5, showing no evidence of carbon deposition and reducing nickel re-oxidation significantly.     

PrBaFe2O5+δ promising electrode for redox-stable symmetrical proton-conducting solid oxide fuel cells

The proton conduction behavior and electrochemical performance of PrBaFe₂O_5+δ (PBF) are firstly studied for use as promising anode materials in symmetrical proton-conducting solid oxide fuel cells (PCFCs). This study focuses on the investigation of protonic defect formation in PBF and its properties as an electrode, including its chemical stability, electrochemical performance and redox stability. PBF is found to have proton conductivity at intermediate temperatures, which contributes to improved hydrogen oxidation reaction and water formation reaction at the anode and cathode, respectively. Hence, the symmetrical PCFC exhibits a peak power density of 301 mW?cm^?2 and total resistance of 0.77 Ω?cm² at 700 °C. Further, it shows excellent redox-cycle stability upon cycling between fuel and air under a current load. The electrochemical analysis and redox cycling tests of the PBF anode demonstrate that PBF is an attractive candidate for alternative material for symmetrical PCFCs.     

Fabrication and Performance of Solid Oxide Fuel Cells with La0.2Sr0.7TiO3–δ as Anode Support and La2NiO4+δ as Cathode

A novel fabrication process for solid oxide fuel cells (SOFCs) with La_0.2Sr_0.7TiO_3–δ (LST_A–) as anode support and La₂NiO_4+δ (LNO) as cathode material, which avoids complicated impregnation process, is designed and investigated. The LST_A– anode‐supported half cells are reduced at 1,200 °C in hydrogen atmosphere. Subsequently, the LNO cathode is sintered on the YSZ electrolyte at 1,200 °C in nitrogen atmosphere and then annealed in situ at 850 °C in air. The results of XRD analysis and electrical conductivity measurement indicate that the structure and electrochemical characteristics of LNO appear similar before and after the sintering processes of the cathode. By using La_0.6Sr_0.4CoO_3–δ (LSC) as current collector, the cell with LNO cathode sintered in nitrogen atmosphere exhibits the power density at 0.7 V of 235 mW cm⁻² at 800 °C. The ohmic resistance (R_S) and polarization resistance (R_P) are 0.373 and 0.452 Ω cm², respectively. Compared to that of the cell with the LNO cathode sintered in air, the sintering processes of the cell with the LNO cathode sintered in nitrogen atmosphere can result in better electrochemical performance of the cell mainly due to the decrease in R_S. The microstructures of the cells reveal a good adhesion between each layer.     

A Degradation Model for Solid Oxide Fuel Cell Anodes Due to Impurities in Coal Syngas: Part II Estimation of Tolerance Limits

An engineering analysis based on calibrated numerical predictions was performed to estimate the minimum allowable impurity concentrations in coal syngas intended to be used in Solid Oxide Fuel Cells (SOFCs) operating for over 10,000 h. Arsine and phosphine, impurities that are known to have the most deleterious effects on the cell performance due to their affinity to have strong relations with the anode catalyst by formation of secondary phases, were investigated. Time to failure was taken as the operation time when 60% performance loss is incurred, estimated by the previously developed one‐dimensional degradation model. Limiting concentrations were determined for arsine and phosphine fuel contaminants for electrolyte and anode supported SOFCs. Predicted lifetimes for single cells can provide a basis for estimation of SOFC stack lifetimes operating on coal syngas. Extrapolation of results from the numerical simulations based on accelerated laboratory tests at relatively higher concentrations can provide guidance into predicting the cell failure at low impurity concentrations.     

结构参数对质子交换膜燃料电池阴极有序催化层的影响

建立了质子交换膜燃料电池阴极有序催化层的稳态数学模型,目的是研究催化层厚度、铂载量、电解质体积含量和碳载体直径等设计参数对催化层性能的影响。模型方程涉及质子、电子和氧气的传递以及电化学反应等过程。计算结果与实验数据吻合。模拟表明,一定范围内较薄的催化层有利于性能提高,但厚度太小反而不利;提高电解质体积含量和铂载量可以明显改进催化层工作特性,但存在优化值,高于此值,催化层性能迅速下降;较细的碳载体直径会适当改善催化层性能。     

Effects of electrolyte pattern on mechanical and electrochemical properties of solid oxide fuel cells

In order to enhance the electrochemical performance and reduce the operation temperature of a conventional electrolyte supported solid oxide fuel cell (SOFC), a three layered electrolyte with various geometry is designed and fabricated. Novel three layered electrolytes comprise a dense and thin scandia alumina stabilized zirconia (ScAlSZ) electrolyte layer sandwiched between two hallow ScAlSZ electrolyte layers each having the same thickness as the support but machined into a filter like architecture in the active region with circular, rectangular and triangular cut off patterns. The percent of thin electrolyte layer in the active region is kept constant as 30% for all designs in order to investigate the effect of pattern geometry on the mechanical properties and the performance of the electrolytes. Single cells based on novel electrolytes are manufactured and electrochemical properties are evaluated. A standard electrolyte and electrolyte supported cell are also fabricated as a base case for comparison. Although the electrolyte having triangular patterns has the highest peak power at all operation temperatures considered, it exhibits the lowest flexural strength.     

Effects of Microstructural Functional Layers on Flat‐Tubular Solid Oxide Fuel Cells

The functional layer of a flat‐tubular solid oxide fuel cell (SOFC) is examined using a three‐dimensional microscale electrode model. SOFC electrodes essentially include two types of layers: a structural layer and a functional layer. The structural layers, which are the anode support layer and the cathode current collector layer, are composed of large particles with a high porosity that facilitates gas diffusion. The functional layers consist of small particles with a low porosity that increases the triple phase boundary (TPB) reaction area and reduces the activation overpotential. In the model, the particle diameter and functional layer thickness are adjusted and analyzed. The effects of the two parameters on the performance of the functional layer are monitored in the contexts of several multilateral approaches. Most reactions occurred near the electrode–electrolyte interface; however, an electrode design that included additional TPB areas improved the electrode performance. The role of the functional layer in a flat‐tubular SOFC is examined as a function of the functional layer particle size and thickness. The performance of a cell could be enhanced by preparing a functional layer using particles of optimal size and thickness, and by operating the device under conditions optimized for these parameters.     

Numerical analysis of thermal and electrochemical phenomena for anode supported microtubular SOFC

A 2D model considering momentum, heat/species transport and electrochemical phenomena, has been proposed for tubular solid oxide fuel cell. The model was validated using experimental polarization curves and the good agreement with the experimental data was attained. The temperature distributions show that temperature varies severely at the tube inlet than at the tube outlet. The heat generation and transfer mechanisms in electrodes, electrolyte and electrochemical reaction interface were investigated. The results show that the overall electrochemical reaction heat is produced at cathode/electrolyte interface, and a small portion of the heat is consumed at anode/electrolyte interface. The heat produced at cathode/electrolyte interface is about five times as much as that consumed at anode/electrolyte interface. Overwhelming part of the heat transfer between cell and outside occurs at cathode external surface. Most current flow goes into anode from a very small area where the current collectors locates. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

A Mathematical Model of SOFC: A Proposal

The mathematical model of the solid oxide fuel cell (SOFC) is presented. The new approach for modeling the voltage of SOFC is proposed. Electrochemical, thermal, electrical, and flow parameters are collected in the 0D mathematical model. The aim was to combine all cell working conditions in as a low number of factors as possible and to have the factors relatively easy to determine. A validation process for various experimental data was made and adequate results are shown. The presented model was validated for various fuel mixtures in relatively wide ranges of parameters as well as for various cell design parameters (e.g. electrolyte thickness, anode porosity, etc.). A distinction is made between the “design‐point” and “off‐design operation”.