New Stack Design of Micro‐tubular SOFCs for Portable Power Sources

Micro‐tubular solid oxide fuel cells (SOFCs) have high thermal stability and higher volumetric power density, which are considered to be ideal features for portable power sources and auxiliary power units for automobile. Here, we report a new stack design using anode supported micro‐tubular SOFCs with 2 mm diameter using Gd doped CeO₂ (GDC) electrolyte, NiO‐GDC anode and (La, Sr)(Co, Fe)O₃ (LSCF)‐GDC cathode. The new stack consists of three bundles with five tubular cells, sealing layers and interconnects and fuel manifolds. The performance of the stack whose volume is 1 cm³ was shown to be 2.8 V OCV and maximum power output of 1.5 W at 500 °C, applying air only by natural convection. The results also showed strong dependence of the fuel flow rates on the stack performance, which was correlated to the gas diffusion limitation.     

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.     

NI/NI‐YSZ Current Collector/Anode Dual Layer Hollow Fibers for Micro‐Tubular Solid Oxide Fuel Cells

A co‐extrusion technique was employed to fabricate a novel dual layer NiO/NiO‐YSZ hollow fiber (HF) precursor which was then co‐sintered at 1,400 °C and reduced at 700 °C to form, respectively, a meshed porous inner Ni current collector and outer Ni‐YSZ anode layers for SOFC applications. The inner thin and highly porous “mesh‐like” pure Ni layer of approximately 50 μm in thickness functions as a current collector in micro‐tubular solid oxide fuel cell (SOFC), aiming at highly efficient current collection with low fuel diffusion resistance, while the thicker outer Ni‐YSZ layer of 260 μm acts as an anode, providing also major mechanical strength to the dual‐layer HF. Achieved morphology consisted of short finger‐like voids originating from the inner lumen of the HF, and a sponge‐like structure filling most of the Ni‐YSZ anode layer, which is considered to be suitable macrostructure for anode SOFC system. The electrical conductivity of the meshed porous inner Ni layer is measured to be 77.5 × 10⁵ S m^–1. This result is significantly higher than previous reported results on single layer Ni‐YSZ HFs, which performs not only as a catalyst for the oxidation reaction, but also as a current collector. These results highlight the advantages of this novel dual‐layer HF design as a new and highly efficient way of collecting current from the lumen of micro‐tubular SOFC.     

Preliminary Study on Direct Operation of LT‐SOFCs Based on GDC Electrolyte Film With DME Fuel

Direct operation of anode‐supported cone‐shaped tubular low temperature solid oxide fuel cells (LT‐SOFCs) based on gadolinia‐doped ceria (GDC) electrolyte film with dimethyl ether (DME) fuel was preliminarily investigated in this study. The single cell exhibited maximum power densities of 500 and 350 mW cm^–2 at 600 °C using moist hydrogen and DME as fuel, respectively. A durability test of the single NiO‐GDC/GDC/LSCF‐GDC cell was performed at a constant current of 0.1 A directly fuelled with DME for about 200 min at 600 °C. The results indicate that the single cell coking easily directly operated in DME fuel. EDX result shows a clear evidence of carbon deposition in the anode. Further studies are needed to develop the novel anti‐carbon anode materials, relate the carbon deposition with anode microstructure and cell‐operating condition.     

Novel Ag–Glass Composite Interconnect Materials for Anode‐Supported Flat‐Tubular Solid Oxide Fuel Cells Operated at an Intermediate Temperature

We developed novel Ag–glass composite interconnect materials for anode‐supported flat‐tubular solid oxide fuel cells (SOFCs) operated at 700 °C by optimization of the glass content. For this purpose, the variations of phase stability, area specific resistance (ASR), microstructure, gas leak rate, cell performance, and open circuit voltage (OCV) were determined for the Ag–glass composite materials with respect to the glass content. The Ag–glass composite materials maintain phase stability without chemical reactions. The ASR increased as the glass content increases due to glass existing as an insulator between the Ag phases. All the composite materials showed dense coating layers on the anode support and had a low gas leak. The cell performance and OCV were measured to identify the optimum composition of the Ag–glass composites. Our results confirm that Ag–glass composites are suitable for high performance interconnects in anode‐supported flat‐tubular fuel cells operated below 700 °C.     

The Use of Methane‐Containing Syngas in a Solid Oxide Fuel Cell: A Comparison of Kinetic Models and a Performance Evaluation

The nickel‐based anodes of solid oxide fuel cells (SOFCs) can catalytically reform hydrocarbons, which make natural gas, gasification syngas, etc., become potential fuels in addition to hydrogen. SR and water–gas shift (WGS) often occur inside SOFCs when operated on these fuels. Their reaction rates affect the partial pressures of hydrogen and carbon monoxide, the local temperatures and the related Nernst voltages. Consequently, the reaction rates affect the electrochemical reactions in the fuel cell. Three different kinetic models were used to characterize methane SR in a tubular SOFC; the results of each model were evaluated and compared. The polarizations of the fuel cell results of these models were validated against experimental data. The performance of a fuel cell operated with different fuels and based on a selected kinetic model was further studied in terms of the anode oxygen partial pressure, the thermo‐electrochemical distribution, and the system level performance.     

Advances in Mixed‐Reactant Fuel Cells

The mixed‐reactant fuel cell (MRFC) is a new concept, in which a mixture of aqueous fuel and gaseous oxygen (or air) flows directly through a porous anode‐electrolyte‐cathode structure or through a strip‐cell with an anode‐electrolyte‐cathode configuration. These structures can be single cells or parallel stacks of cells and may be in a planar, tubular or any other geometry. Selectivity in the electrocatalysts for MRFCs is mandatory to minimize mixed‐potential at the electrodes, which otherwise would reduce the available cell voltage and compromise the fuel efficiency. MRFC offers a cost effective solution in fuel cell design, since there is no need for gas‐tight structure within the stack and, as a consequence, considerable reduction in sealing, manifolding and reactants delivery structure is possible. In recent years, significant advances have been made in MRFCs, using methanol as a fuel. This paper reviews the status of mixed reactant fuel cells and reports some recent experimental data for methanol fuel cell systems.     

Influences of feedstock and plasma spraying parameters on the fabrication of tubular solid oxide fuel cell anodes

This study developed a tubular solid oxide fuel cell (SOFC) anode support layer via atmospheric plasma spraying, which is considered one of the most promising methods for producing SOFCs because of its faster deposition rate and lower cost compared with other film formation processes. Plasma spraying can replace the traditional use of extrusion technology to manufacture the anode base tube, eliminating the need for high-temperature sintering steps. In this study, commercially available powders were used to make the anode of a tubular SOFC from NiO/yttria-stabilized zirconia (YSZ) powder, and Na₂CO₃ and polymethyl methacrylate were tested as pore-forming agents. The anode composite powder was sprayed on the graphite base pipe, and the final product was changed by altering the spraying parameters and anode powder ratio. The direct current (DC) resistance measurements showed that the conductivity of the Ni/YSZ tubular anode formed with higher power plasma spraying could reach 428.55?S/cm at 800?°C. The experimental results showed that the power and parameters of atmospheric plasma spraying could affect the porosity and electron conductivity of tubular SOFC anodes.     

原位析出纳米合金的PrBaFe2O6-δ 基阳极构筑及其在固体碳燃料电池中的应用研究

开发高性能阳极材料对于固体碳为燃料的固体氧化物燃料电池（solid oxide fuel cells, SOFCs）的发展意义重大。本文研究了原位析出Fe以及FeNi合金的PrBaFe₂O_6-δ （PBF）基层状双钙钛矿材料在SOFCs中的应用。通过溶胶-凝胶法制备了Ni掺杂的 (PrBa)_0.95Fe_1.7Ti_0.2Ni_0.1O_6-δ （PBFTN）阳极材料。XRD表明合成的材料呈现钙钛矿结构且在阳极还原性气氛下保持稳定。XRD、SEM、TEM、XPS结果表明在还原性气氛下材料表面析出大量均匀分布的纳米金属颗粒。当采用纯的纳米活性炭为燃料时,电解质支撑型的以PBFTN为阳极的单电池在800℃下实现了698 mW·cm^-2的最大功率密度,性能十分优异,表明其是一种具有潜力的SOFCs阳极材料。     

Fabrication and Power Densities of Anode‐Supported Solid Oxide Fuel Cells with Different Ni‐YSZ Anode Functional Layer Thicknesses

We investigated an appropriate preparation condition for anode‐supported SOFCs: (La,Sr)MnO₃/cathode functional layer/YSZ/Ni‐YSZ were fabricated with and without a Ni‐YSZ anode functional layer (AFL) via the tape‐casting method, where the AFL thicknesses were controlled from approximately 20 to 80 μm. The warpage depended on the co‐sintering temperature of the electrolyte/AFL/anode‐support half‐cells, indicating that similar shrinkage of the electrolyte/AFL/anode support is significant for lower warpages. The electrical properties of SOFCs with AFLs were compared to those of SOFCs without AFLs. In this regard, the use of an AFL decreased the ohmic and activation polarization resistances due to both the decrease in contact resistance between the electrolyte and the AFL and the increase in three‐phase boundaries. However, the polarization diffusion increased when an AFL was employed, because AFL layers are denser than the anode support. The maximum power densities of samples with AFL were higher than those of SOFCs without AFLs, indicating that the decrease in both ohmic and activation‐polarization resistances is more significant for improving the power densities, as compared to the concentration polarization resistance.     

原位析出纳米合金的PrBaFe2O6-δ 基阳极构筑及其在固体碳燃料电池中的应用研究

开发高性能阳极材料对于固体碳为燃料的固体氧化物燃料电池（solid oxide fuel cells, SOFCs）的发展意义重大。本文研究了原位析出Fe以及FeNi合金的PrBaFe₂O_6-δ （PBF）基层状双钙钛矿材料在SOFCs中的应用。通过溶胶-凝胶法制备了Ni掺杂的 (PrBa)_0.95Fe_1.7Ti_0.2Ni_0.1O_6-δ （PBFTN）阳极材料。XRD表明合成的材料呈现钙钛矿结构且在阳极还原性气氛下保持稳定。XRD、SEM、TEM、XPS结果表明在还原性气氛下材料表面析出大量均匀分布的纳米金属颗粒。当采用纯的纳米活性炭为燃料时,电解质支撑型的以PBFTN为阳极的单电池在800℃下实现了698 mW·cm^-2的最大功率密度,性能十分优异,表明其是一种具有潜力的SOFCs阳极材料。     

Co-extrusion of multilayered ceramic micro-tubes for use as solid oxide fuel cells

Current methods to manufacture tubular solid oxide fuel cells (SOFCs) involve multiple steps of extrusion, layer deposition and sintering, leading to high manufacturing costs. The aim of the work presented in this paper is to reduce the cost of manufacturing SOFCs. This is achieved by developing a method for manufacturing a five-layered micro-tubular structure by a multi-billet co-extrusion process. With the implementation of continuous screw extrusion equipment, this co-extrusion process could easily be adapted into a fully continuous manufacturing process.The co-extrusion process presented initially involves rheologically unifying five pastes made up of individual powder compositions. It is shown that it is possible to formulate the pastes with in an optimum solids loading region where the die land rheological properties are relatively insensitive to small variations in solids loading, thus allowing for a more stable process.These pastes are then extruded as billets from separate extrusion barrels through a single nozzle. This uses a novel die design which does not require the use of a central mandrel to form the tubular structure. The sintered structure comprises four Ni/YSZ anode layers and a YSZ electrolyte layer, each layer being approximately 60 μm thick, forming a tube with an outer diameter of 3 mm and an inner diameter of 2.4 mm.     

Catalytic Influence of Oxide Component in Ni‐Based Cermet Anodes for Ammonia‐Fueled Solid Oxide Fuel Cells

Recently, the promising prospect of ammonia as a hydrogen carrier for solid oxide fuel cells (SOFCs) has attracted significant interests. In this work, the effects of temperature, fuel content, and total flow rate of anode gas on the performance of Ni/yttria‐stabilized zirconia (Ni/YSZ) anode for ammonia‐fueled SOFCs were investigated. Based on obtained results, the utilization route of ammonia on Ni/YSZ anode was discussed; the results of electrochemical experiments were related with the catalytic decomposition bahavior of ammonia over Ni/YSZ. Moreover, the catalytic activity for ammonia decomposition and anode performance of Ni/samarium‐doped ceria (Ni/SDC) and Ni/yttrium‐doped barium cerate (Ni/BCY) were also investigated. Among these anode materials, Ni/BCY exhibited the highest ammonia decomposition activity and anode performance for ammonia‐fueled SOFCs at intermediate temperatures.     

氨燃料固体氧化物燃料电池性能研究进展与展望

氨是一种零碳燃料,也是富氢载体,具有较大储运优势。固体氧化物燃料电池(solid oxide fuel cell, SOFC)是一种清洁高效发电装置,在分布式发电、热电联供、储能调峰等领域有广阔应用前景,氨气可直接用作SOFC阳极燃料以实现高效、清洁、低成本发电。首先简介了质子传导型和氧离子传导型氨SOFC的工作原理,电解质、电极材料的选择以及氨气在阳极的分解过程。其次总结了氨SOFC的实验研究现状,以单电池最大功率密度为评价指标,综述了不同电解质/电极材料、电解质厚度、操作温度等因素下两种传导类型的氨SOFC的性能表现,并分析了造成电池性能差异的原因。之后介绍了氨SOFC当前面临的挑战,最后对氨SOFC未来研究方向、热电联供系统的应用进行了展望。     

Tubular solid oxide fuel cells fabricated by a novel freeze casting method

Freeze casting is an established method for fabricating porous ceramic structures with controlled porosity and pore geometries. Herein, we developed a novel freeze casting and freeze drying process to fabricate tubular anode supports for solid oxide fuel cells (SOFCs). Freeze casting was performed by injecting aqueous anode slurry to a dual-purpose freeze casting and freeze drying mold wrapped with peripheral coils for flowing a coolant. With the use of an ice barrier layer, proper control of the experimental setup, and adjustments in the drying temperature profile, complete drying of the individual anode tubes was achieved in 4 hours. The freeze-cast anode tubes contained radially aligned columnar pore channels, thus significantly enhancing the gaseous diffusion. SOFC single cells with conventional Ni/yttria-stabilized zirconia/strontium-doped lanthanum manganite materials were prepared by dip coating the thin functional layers onto the anode support. Single cell tests showed that the concentration polarization was low owing to the highly porous anode support with directional pores. With H₂/N₂ (1:1) fuel, maximum power densities of 0.47, 0.36, and 0.27 W/cm² were recorded at 800°C, 750°C, and 700°C, respectively. Our results demonstrate the feasibility of using freeze casting to obtain tubular SOFCs with desired microstructures and fast turn-around times.     

原位浸渍法制备NiO-BZCYYb阳极支撑SOFCs及电化学性能

采用原位浸渍法一步烧结成型制备了NiO-BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ（BZCYYb）/SDC/LSCF管状结构阳极支撑型SDC电解质膜固体氧化物燃料电池（SOFCs）。以加湿H₂（约含有体积分数为3%的水）为燃料,空气为氧化剂,研究了电池的电化学性能、热循环性能和工作电压下运行的稳定性。结果表明：电池在600、650、700、750、800℃的开路电压分别为1.084、1.074、1.067、1.058、1.046 V;最大输出功率密度分别为0.12、0.25、0.38、0.54和0.70 W·cm^-2。单电池在700℃和0.7 V连续放电测试过程中稳定运行,没有明显的下降和衰退。单电池经历了11次热循环,输出功率稳定,能够经受住重复启动考验。     

Barium Borosilicate Sealing Glasses Synthesized by a Sol–Gel Process: Chemical Interactions with a Stainless Steel and Gas‐Tightness of a SOFC

Several glasses synthesized by sol–gel route and based on the BaO–B₂O₃–X–Al₂O₃–SiO₂ (X = CaO, MgO) glass system have been investigated to evaluate their applicability as sealant for solid oxide fuel cell (SOFC). Chemical interactions with K41X stainless steel and hydrogen‐tightness of these materials were evaluated after operations at high temperatures over 1,000 h in air atmosphere. Formation of a new phase at the steel–glass interface and formation of porosity in the glass were observed and determined as critical problems over mid‐term operations. The role of MgO is important to obtain a gas‐tight sealing. Application of the glass paste without binder addition was performed in order to avoid possible residual porosity related problems. The best glass was finally used as sealant between anodic and cathodic compartments in complete SOFCs operated at 760 and at 800 °C. Open circuit voltages and power densities of the cells were recorded during the first hours of operation.     

An investigation of fuel composition and flow‐rate effects in a H2S fuelled sofc: Experiments and thermodynamic analysis

Hydrogen sulphide (H₂S)‐fuelled solid oxide fuel cells (SOFCs) can potentially generate useful electrical energy while disposing of H₂S, a toxic by‐product of the fossil fuel industry, on site. Experimental results from H₂S fuelled SOFCs exhibit characteristics, for example, an unusual dependence of cell performance on fuel composition and flow‐rate, which are poorly explained in the literature. In this work we: (a) present results for experiments where the composition and flow‐rates were varied for both the fuel and oxidant streams to analyse their effect on fuel cell performance, and (b) develop and use a thermodynamic analysis to help understand these experimental results. Through this work, we shed further light on two basic questions unanswered so far, (1) Why does the flow‐rate of the fuel affect the open circuit potential of the fuel cell? (2) Which of the chemical species present in the fuel is oxidised on the anode? Our experiments and analysis suggest that H₂S, and not H₂ produced from H₂S dissociation, is preferentially electro‐oxidised on the anode in our experiments. © 2011 Canadian Society for Chemical Engineering     

Single‐step fabrication of an anode supported planar single‐chamber solid oxide fuel cell

We present single‐step‐co‐sintering manufacture of a planar single‐chamber solid oxide fuel cell (SC‐SOFC) with porous multilayer structures consisting of NiO/CGO, CGO and CGO‐LSCF as anode, electrolyte, and cathode, respectively. Their green tapes were casted with 20 μm thickness and stacked into layers of anode, electrolyte, and cathode (10:2:2), then hot‐pressed at 2 MPa and 60°C for 5 minutes (deemed optimal). Subsequently, hot laminated layers were cut into 40 × 40 mm cells and co‐sintered up to 1200°C via different sintering profiles. Shrinkage behavior and curvature developments of cells were characterized, determining the best sintering profile. Hence, anode‐supported SC‐SOFCs were fabricated via a single‐step co‐sintering process, albeit with curvature formation at edges. Subsequently, anode thickness was increased to 800 μm and electrolyte reduced to 20 μm to obtain SOFCs with drastically reduced curvature with the help of a porous alumina cover plate.     

Effect of pre-calcined ceramic powders at different temperatures on Ni-YSZ anode-supported SOFC cell/stack by low pressure injection molding

Recently, powder injection molding (PIM) has been exploited in the field of solid oxide fuel cells (SOFCs), especially for fabricating anode supports. The current study employs low pressure injection molding (LPIM) to manufacture near net shape, porous, tubular NiO-yttria stabilized zirconia (YSZ) anode supports for anode-supported SOFCs. The study investigates the effects of pre-calcining temperature of the ceramic powder on the microstructure, porosity and electrochemical performance of the cells in detail. Archimedes tests reveal that the porosity of an unreduced NiO-YSZ anode with 900 °C pre-calcined powder reaches a high of 25.9%, approaching the optimal value of 26%. Meanwhile, the anode prepared under this condition possesses more porous and homogeneous microstructures. At 800 °C, with humidified hydrogen as fuel and ambient air as the oxidant, the single cell with 900 °C pre-calcined powder delivers a maximum power density of 671 mW cm⁻² while the cell with raw powder, 555 mW cm⁻², and the cell with 1000 °C pre-calcined powder, 648 mW cm⁻². A four-cell stack is assembled by connecting four single cells in series. The stack could provide a maximum output power of 4.6 W and an open circuit voltage of 3.2 V when fuelled with humidified hydrogen at 800 °C.