Direct Operation of IP‐Solid Oxide Fuel Cell with Hydrogen and Methane Fuel Mixtures under Current Load Cycle Operating Condition

The effects of methane concentration and current load cycle on the performance and durability of integrated planar solid oxide fuel cell (IP‐SOFC) obtained from Rolls Royce Fuel Cell Systems Ltd (RRFCS) has been investigated. The IP‐SOFC was operated with hydrogen–methane fuel mixture with up to 20% methane concentration at 900 °C for short term operation of the cells with high methane concentration increased the voltage of the IP‐SOFC due to increase in Gibbs free energy. However, it degraded the performance of the IP‐SOFC in long term operation due to carbon deposition on the anode surface. The current load cycle tests were carried out with 95% H₂–5% CH₄ and 80% H₂–20% CH₄ fuel mixtures at 900 °C with a constant current of 1 A. At low methane concentration, the decrease in the IP‐SOFC voltage was observed after operating nine current load cycles (384 h). At higher methane concentration, the voltage of IP‐SOFC decreased by almost 30% just after one current load cycle (48 h) due to faster carbon deposition. So future work is therefore required to identify viable alternative materials and optimum operating conditions.     

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.     

Temperature Measurement and Distribution Inside Planar SOFC Stacks

In this work, a kind of thin K‐type thermocouple and self‐developed CAS‐I sealant were used to assembly solid oxide fuel cell (SOFC) stacks and temperatures of unit cells inside a planar SOFC stack were measured. The open circuit voltage testing of the stack and characterization of the interface between sealant and components suggested excellent sealing effect by applying the developed method. The effect of discharging direct‐current on temperature and temperature distribution inside the designed SOFC stack was investigated. The results showed that the discharging current had a great impact and the gas flow rate had a slight impact on the temperatures of unit cells. Temperature distribution of unit cells inside the stack was much non‐uniform and there is a significant temperature difference between various components of the stack and heating environment. The relationship between temperatures and cell performance showed that the worse the cell performance, the higher the cell surface temperature. When the stack was discharged at a constant current and the temperature of cell surface was over 950 °C, the higher the temperature, the more drop the corresponding voltage.     

Development of an In Situ Surface Deformation and Temperature Measurement Technique for a Solid Oxide Fuel Cell Button Cell

A novel experimental technique is developed to measure the in situ surface deformation and temperature of a solid oxide fuel cell (SOFC) anode surface along with the cell electrochemical performance. The experimental setup consists of a NexTech Probostat^™ SOFC button cell test apparatus integrated with a Sagnac interferometric optical method and an infrared sensor for in situ surface deformation and temperature measurements, respectively. The button cell is fed with hydrogen or simulated coal syngas under SOFC operating conditions. The surface deformation is measured over time to estimate the anode structural degradation. The cell surface transient temperature is also monitored with different applied current densities under hydrogen and simulated coal syngas. The experimental results are useful to validate and develop SOFC structural durability and electrochemical models.     

阳极孔隙率对固体氧化物燃料电池性能影响的数值分析

基于商用计算流体动力学软件及开发的燃料电池多孔介质内多组分流动和扩散、传热传质、电化学反应、电流场等复杂的物理过程的计算程序,对采用不同孔隙率阳极的平板式阳极支撑固体氧化物燃料电池(planar-electrode-support solid oxide fuel cell,PES-SOFC)的性能进行数值计算,得到不同阳极孔隙率下单电池内部各气体组分浓度、温度、电势、电流、电流密度等参数的分布。由计算结果可知,在阳极孔隙率为0.3～0.4之间时,以氢气为燃料的该类型SOFC单电池表现出较好的气体扩散和电流传导特性,相应输出电压也较高。     

Three‐Dimensional Computational Fluid Dynamics Modeling of a Planar Solid Oxide Fuel Cell

A three‐dimensional computational fluid dynamics model was developed to study the performance of a planar solid oxide fuel cell (SOFC). The governing equations were solved with the finite volume method. The model was validated by comparing the simulation results with data from literature. Parametric simulations were performed to investigate the coupled heat/mass transfer and electrochemical reactions in a planar SOFC. Different from previous two‐dimensional studies the present three‐dimensional analyses revealed that the current density was higher at the center along the flow channel while lower under the interconnect ribs, due to slower diffusion of gas species under the ribs. The effects of inlet gas flow rate and electrode porosity on SOFC performance were examined as well. The analyses provide a better understanding of the working mechanisms of SOFCs. The model can serve as a useful tool for SOFC design optimization.     

Mathematical Modelling of Proton‐Conducting Solid Oxide Fuel Cells and Comparison with Oxygen‐Ion‐Conducting Counterpart

Proton‐conducting solid oxide fuel cells (H‐SOFC), using a proton‐conducting electrolyte, potentially have higher maximum energy efficiency than conventional oxygen‐ion‐conducting solid oxide fuel cells (O‐SOFC). It is important to theoretically study the current–voltage (J–V) characteristics in detail in order to facilitate advanced development of H‐SOFC. In this investigation, a parametric modelling analysis was conducted. An electrochemical H‐SOFC model was developed and it was validated as the simulation results agreed well with experimental data published in the literature. Subsequently, the analytical comparison between H‐SOFC and O‐SOFC was made to evaluate how the use of different electrolytes could affect the SOFC performance. In addition to different ohmic overpotentials at the electrolyte, the concentration overpotentials of an H‐SOFC were prominently different from those of an O‐SOFC. H‐SOFC had very low anode concentration overpotential but suffered seriously from high cathode concentration overpotential. The differences found indicated that H‐SOFC possessed fuel cell characteristics different from conventional O‐SOFC. Particular H‐SOFC electrochemical modelling and parametric microstructural analysis are essential for the enhancement of H‐SOFC performance. Further analysis of this investigation showed that the H‐SOFC performance could be enhanced by increasing the gas transport in the cathode with high porosity, large pore size and low tortuosity.     

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.     

固体氧化物燃料电池的变工况特性

以大面积电池和千瓦级电堆为研究对象,在确定的燃料成分、流量、和工作温度下,系统研究了电流阶梯变化、电流脉冲变化、电堆热启停以及冷热循环(冷启停)等工况下电堆的输出性能。结果表明:在小电流区域,电堆的电压和功率能够快速跟踪电流变化;在大电流区域,电池的电压出现波动和弛豫,电堆的功率也出现弛豫。热启停实验结果表明,SOFC电堆对电流的on-off变化具有足够的耐受性,一定数量的热启停不会导致电堆性能的明显衰减。而冷热循环会导致应力释放,引起接触电阻变化,从而使电堆性能衰减,5次以上热循环可使应力释放趋于缓和。     

Fabrication and Operation of Tubular Segmented‐In‐Series (SIS) Solid Oxide Fuel Cells (SOFC)

A tubular segmented‐in‐series (SIS) solid oxide fuel cell (SOFC) sub module for intermediate temperature (700–800 °C) operation was fabricated and operated in this study. For this purpose, we fabricated porous ceramic supports of 3 YSZ through an extrusion process and analyzed the basic properties of the ceramic support, such as visible microstructure, porosity, and mechanical strength, respectively. After that, we fabricated a tubular SIS SOFC single cell by using dip coating and vacuum slurry coating method in the case of electrode and electrolyte, and obtained at 800 °C a performance of about 400 mW cm^–2. To make a sub module for tubular SIS SOFC, ten tubular SIS SOFC single cells with an effective electrode area of 1.1 cm² were coated onto the surface of the prepared ceramic support and were connected in series by using Ag + glass interconnect between each single cell. The ten‐cell sub module of tubular SIS SOFC showed in 3% humidified H₂ and air at 800 °C a maximum power of ca. 390 mW cm^–2.     

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.     

Preparation of SOFC anode composites by spray pyrolysis

Lowering the SOFC working temperature would also be greatly attractive, but low temperature working SOFCs require high-performance anodes. The cermet SOFC anodes, which are composed of nickel and samarium doped ceria, were prepared by spray pyrolysis (SP), because SP produces spherical particles with small size distributions. SP-derived particles of NiO, SDC, and NiO/SDC composite had a round shape and comprised nanometer-sized primary grains. The cermet anodes were prepared by using SP-derived NiO/SDC composite particles or mixing SP-derived NiO and SDC particles. The anode prepared with the composite particles showed higher SOFC cell performance than that with the mixed ones. The composite particles had high surface areas and a capsule-type form. The outer shell would be composed of SDC and the inner core was NiO. The capsule-type composite particles would depress aggregation of Ni or NiO during reduction from NiO to Ni metals, and this depression would enhance SOFC anode performance.     

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.     

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.     

管式固体氧化物燃料电池机理模型与性能分析

针对Siemens-Westinghouse公司阴极支撑型(AES)管式固体氧化物燃料电池,耦合电极内部离子传导、电子传导、气体扩散、热量传递及电化学反应过程,建立了全面考虑活化极化、欧姆极化与浓差极化损失的管式SOFC横截面方向二维微观机理模型。模型计算结果与文献中实验数据吻合较好,模拟结果表明：电池横截面方向的组分浓度和电流密度的分布与SOFC的运行工况密切相关。连接器的存在和尺寸对电池工作性能均有较强影响。对于所研究的阴极支撑型SOFC,电池性能会受到氧气在多孔阴极中扩散过程的限制,改善多孔电极的微观结构可有效提高电池运行性能。     

Microstructural characterization of spray-dried NiO-8YSZ particles as plasma sprayable anode materials for metal-supported solid oxide fuel cell

The plasma spray method is widely used to produce NiO-8YSZ (composed of nickel oxide (NiO) and 8 mol% yttria-stabilized zirconia) anode layers in metal-supported solid oxide fuel cell (SOFC). Flowability control of microsized particles is important for achieving consistent performance of the SOFC anode layer. When microsized particles are fabricated via spray drying and sintering, the most significant factors that influence flowability are their sizes, distribution, and surface conditions. Thus, the aim of this study is to analyze the fabrication conditions for microsized NiO-8YSZ cermet particles made from a nanoscale, sinterable NiO-8YSZ dispersion solution by using an appropriate spray-drying and sintering process. The characteristics of the as-sprayed and sintered NiO-8YSZ composite particles (such as size, distribution, roughness, and nanostructure) were analyzed via field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), particle size distribution (PSD), Brunauer–Emmett–Teller (BET) surface area, and atomic force microscopy (AFM). The as-sprayed microsized NiO-8YSZ particles became smaller and more uniformly distributed as the rotational speed used for spray drying increased. As a result of sintering, the extent of shrinkage of as-sprayed microsized NiO-8YSZ particles generated at high RPMs was lower than that of particles formed at low RPMs. No significant difference was observed in the distribution of the nanosized NiO and 8YSZ particles at different rotational speeds. Furthermore, the highest BET surface areas were observed for particles generated at 8000 RPM before sintering at 13.74 m²/g. After sintering, the highest BET surface area was 0.94 m²/g for particles generated at 16,000 RPM. Differences in nanostructure and surface roughness between as-sprayed and sintered microsized NiO-8YSZ particles were identified via AFM. This study is expected to provide important fundamental information useful for optimizing SOFC efficiency by promoting flowability control during the production of SOFC anodes via plasma spraying.     

Formation of chromium containing impurities in (La,Sr)MnO3 solid-oxide-fuel-cell cathodes under stack operating conditions and its effect on performance

In this study, we present an investigation of the chromium-related electrical performance degradation of anode-supported SOFCs with a LSM/YSZ composite cathode. A traditional ferritic interconnect steel with high chromium content is established as the primary chromium source and chromium poisoning of the cathode is carried out at relevant SOFC operating conditions. The prolonged influence of the gaseous chromium species on the cell performance and cathode microstructure under constant current conditions was examined quantitatively. Physical deposition of chromium(III) compounds (mainly spinel-type (Cr,Mn)₃O₄ phases) was observed at the electrochemically active region adjacent to the electrolyte only under realistic constant current conditions. The microstructural degradation associated with the formation of secondary phases correlated directly with the performance degradation, the effective chromium partial pressure and the current density. Furthermore, the influence of the presence of a number of protecting layers on the interconnect steel was evaluated with regard to the cathode poisoning. It was shown that chromium-induced degradation was reduced drastically when an additional manganese reservoir layer and a Cr getter layer were applied.     

Anode-Supported Tubular Micro-Solid Oxide Fuel Cell

A tubular anode-supported "micro-solid oxide fuel cell" (μSOFC) has been developed for producing high volumetric power density (VPD) SOFC systems featuring rapid turn on/off capability. An electrophoretic deposition (EPD)-based, facile manufacturing process is being refined to produce the anode support, anode functional and electrolyte layers of a single cell. μSOFCs (diameter <5 mm) have two main potential advantages, a substantial increase in the electrolyte surface area per unit volume of a stack and also rapid start-up. As fuel cell power is directly proportional to the active electrolyte surface area, a μSOFC stack can substantially increase the VPD of an SOFC device. A decrease in tube diameter allows for a reduction in wall thickness without any degradation of a cell's mechanical properties. Owing to its thin wall, a μSOFC has an extremely high thermal shock resistance and low thermal mass. These two characteristics are fundamental in reducing start-up and turn-off time for the SOFC stack. Traditionally, SOFC has not been considered for portable applications due to its high thermal mass and low thermal shock resistance (start-up time in hours), but with μSOFCs' potential for rapid start-up, new possibilities for portable and transportable applications open up.     

Fabrication of a Flat Tubular Segmented‐in‐series Solid Oxide Fuel Cell Using Decalcomania Paper

A flat tubular segmented‐in‐series (SIS) solid oxide fuel cell (SOFC) was fabricated using decalcomania paper. The performance of a two‐cell stack with 4.5‐mm‐wide electrodes was investigated in a temperature range of 650–800 °C. The decalcomania paper allowed fabrication of the SIS‐SOFC on all sides of the flat tubular support and achieve an effective electrode area larger than that obtained using typical SOFC fabrication techniques such as screen printing or slurry coating. SEM observations revealed that each component layer was flat, uniformly thick, and well adherent to adjacent layers. Measured values of open circuit voltages were very close to the theoretical values; confirming that the processing technique utilizing decalcomania paper is suitable for SIS‐SOFC fabrication. The power densities of the two‐cell‐stack were 437.4, 375.6, 324.6, and 257.1 mW cm⁻² at 800, 750, 700 and 650 °C, respectively.     

The Effect of Humidity and Oxygen Partial Pressure on LSM–YSZ Cathode

Two series of anode supported solid oxide fuel cells (SOFC) were prepared, one with a composite cathode layer of lanthanum strontium manganite (LSM) and yttria stabilized zirconia (YSZ) on top and the other further has a LSM current collector layer on top. The fuel cells were heat treated at 1,000 °C in air or nitrogen choosing dry or humid (70% steam) conditions. XRD, SEM, and XPS investigations were performed on the various samples. The most severe modifications were observed in humid nitrogen atmosphere at low oxygen partial pressure. LSM surface change goes along with a decrease of manganese concentration and strontium enrichment on the surface of the materials. Formation of monoclinic zirconia and zirconate phases was also observed. These results give a closer insight into possible degradation mechanisms of SOFC composite cathode materials in dependence of humidity and oxygen partial pressure.