Microtubular Solid Oxide Fuel Cells Research Articles

Effects of Polymer Binder in Electrolyte Slurries and Their Microtubular SOFC Applications

The electrolyte slurry was prepared by mixing Gd-doped CeO2(GDC), solvent (ethanol and toluene), a polymer (polyvinyl butyral (PVB)) binder, and a dispersant (an amine system). The slurries were processed by an atomization process and coated on the top of microtubular tubes. A very smooth (with no cracks) electrolyte surface was obtained when the PVB polymer content was 8 wt. % (regular solution); however, a unique natural patchwork-type nanoporous grain boundary was obtained when the polymer content was increased to 16 wt. % (excess solution) in the same slurries. The results of this study show that polymers (binders) can be used not only to fabricate a dense electrolyte but also to generate a nanoporous grain boundary. The fabricated electrolytes have been tested for solid oxide fuel cell (SOFC) applications in the intermediate-temperature region. The microtubular cell with dense electrolytes maintained a high performance even under 600 °C.

Fabrication of Y2O3-stabilized-ZrO2(YSZ)/La0.8Sr0.2MnO3−α–YSZ dual-layer hollow fibers for the cathode-supported micro-tubular solid oxide fuel cells by a co-spinning/co-sintering technique

Y2O3-stabilized-ZrO2(YSZ)/La0.8Sr0.2MnO3-α(LSM)–YSZ dual-layer hollow fibers have been fabricated for micro-tubular solid oxide fuel cells (MT-SOFCs) using a co-spinning/co-sintering technique. The hollow fibers consist of a dense YSZ top layer supported on the porous LSM–YSZ substrate with an asymmetric structure. The electrolyte and the cathode layers are perfectly adhered to each other without observable elemental inter-diffusion. Both the mechanical strength of the hollow fibers and the compactness of the electrolyte layer are improved by increasing the sintering temperature, whereas the effective porosity of the cathode layer also decreases. The suitable sintering temperature of the YSZ/LSM–YSZ dual-layer hollow fibers to construct the electrolyte/cathode half-cells should be controlled between 1350 °C and 1400 °C. An Ni-YSZ layer of about 5 μm in thickness is dip-coated on the outer surface of the hollow fibers to serve as the anode of fuel cells. The resultant micro-tubular SOFCs yield a maximum power density of 290 mW cm−2 at 850 °C using 1.49 × 10−5 mol s−1 hydrogen as fuel and 2.23 × 10−5 mol s−1 air as oxidant, respectively. The output performance of the micro-tubular fuel cells is mainly limited by the high polarization resistance of the cathode.

A Hollow Fiber-Based Micro-Tubular SOFC for Efficient Current Collection from the Inner Electrode

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Multicriteria Assessment of the Performance of Solid Oxide Fuel Cells by Cell Design and Materials Development: Design and Modeling Approach

The performance of current solid oxide fuel cells (SOFCs) was evaluated in terms of the cell designs and the physicochemical properties of the component materials such as the electrode and electrolyte in order to demonstrate the potentials of state-of-the-art SOFC technology for the widespread use of SOFCs. A flat tubular type SOFC stack for residential use was analyzed as a standard case of a production version in terms of stack volume, weight, and material cost. The power density and power generation efficiency were also evaluated by model estimation. A microtubular type SOFC was evaluated as an example of an advanced cell design. The assessment of the cell design can pinpoint performance advantages of the microtubular type in stack volume, weight, material cost, volumetric power density, and efficiency. In addition, we attempted to demonstrate an analysis for the concurrent comparison of the impact of cell designs and material properties on cell performance by using volumetric power density as a common assessment criterion. Through the assessment with the state-of-the-art SOFC technology, it is possible to make a quantitative comparison of the significances of cell design and material property. The present assessment suggests that the development of cell design is a consistent approach to improving cell and stack performance. In this way, the proposed assessment can provide hints to a reliable research strategy for improving cell performance and realizing the widespread use of SOFCs.

Anode morphology and performance of micro-tubular solid oxide fuel cells made by aqueous electrophoretic deposition

Anode-supported micro-tubular solid oxide fuel cells (SOFCs) are manufactured by aqueous electrophoretic deposition (EPD). The process of these micro-tubular SOFCs includes consecutive aqueous EPDs of a porous anode layer (8YSZ–NiO), a dense electrolyte layer (8YSZ), and a porous cathode layer (LSM) onto a thin wire electrode, followed by stripping, drying, and a single-step co-sintering. The microstructure of the micro-tubular SOFCs, including the thickness and porosity of each layer, is controlled by the processing parameters such as solid loading, current density, deposition time, and sintering temperature. In particular, the effects of the morphology of the anode layer on the electrochemical performance of such micro-tubular SOFCs are investigated and discussed based on the microstructural and voltage–current–power analyses.

Experimental and Numerical Study of Various MT-SOFC Flow Manifold Techniques: Single MT-SOFC Analysis

Standard anode supported micro tubular-solid oxide fuel cell (MT-SOFC) stacks may provide the oxidant, in relation to the fuel, in three different manifold regimes. Firstly, “co-flow” involves oxidant outside the MT-SOFC flowing co-linearly in relation to the fuel inside. Secondly, “counter flow” involves oxidant outside the MT-SOFC flowing counter-linearly in relation to the fuel inside the MT-SOFC. Finally, “cross-flow” involves the oxidant outside the MT-SOFC flowing perpendicular to the fuel flow inside the MT-SOFC. In order to examine the effect of manifold technique on MT-SOFC performance, a combination of numerical simulation and experimental measurements was performed. Furthermore, the cathode current tap location, in relation to the fuel flow, was also studied. It was found that the oxidant manifold and the location of the cathode current collection point on the MT-SOFC tested and modeled had negligible effect on the MT-SOFC's electrical and thermal performance. In this study, a single MT-SOFC was studied in order to establish the measurement technique and numerical simulation implementation as a prerequisite before further test involving a 7 cell MT-SOFC stack.

Fabrication of electrolyte supported micro-tubular SOFCs using extrusion and dip-coating

Dense electrolyte supported micro tubular solid oxide fuel cells (T-SOFCs) were prepared in this study. Green Zr0.8Sc0.2O2 − δ (ScSZ) electrolyte micro-tubes with a thickness of 300 μm were successfully prepared by extrusion at room temperature. After firing at 1400 °C, the bare electrolyte micro-tube with a thickness of 210 μm, a diameter of 3.8 mm, and a length of 40 mm reached a relative density of 96.84% and a flexural strength of 202 MPa. To achieve a better sintering shrinkage match, the electrolyte micro-tubes were pre-sintered at 1100 °C, coated with NiO/ScSZ (60 vol.%:40 vol.%) in the inner surface of the micro-tubes by dip-coating method, and then co-fired at 1400 °C. After co-sintering, the interface of the porous anode layer and the dense electrolyte layer demonstrated good adhesion and mechanical integrity. Subsequently, a La0.8Sr0.2MnO3 − δ (LSM)/Ce0.8Gd0.2O2 − δ (GDC) (80 vol.%:20 vol.%) cathode layer was coated on the outer surface of the micro-tubes by dip-coating method and post-sintered at 1100 °C. The thicknesses of the anode and cathode layers read approximately 28 and 34 μm respectively. After thermal cycling between 30 °C and 800 °C under 97% N2–3%H2 on the anode and air on the cathode, the micro T-SOFCs showed no delamination and retained good adhesion based on the SEM results. The maximum power densities (MPD) of the single cell read 0.26 and 0.23 W cm−2 respectively at 920 and 900 °C, and the open circuit voltage (OCV) reported the same 1.08 V.

Experimental and Simulated Evaluations of Current Collection Losses in Anode-Supported Microtubular Solid Oxide Fuel Cells

Anode-supported microtubular solid oxide fuel cells (SOFCs) with a diameter on the millimeter scale can generate high electrical power density. However, the longer the cell, the larger are the losses associated with current collection, because the current path is along the length of the microtubular cell. In this study, the experimental and simulated evaluations were conducted to estimate the current collection losses of the microtubular SOFCs with various geometries (the anode thickness and length) and several species of current collectors on the cathode. The cell with 2 mm in diameter and 30 mm in cathode length without a current collector on the cathode had a large ohmic loss. The simulated evaluation using a simple equivalent circuit model suggests the low current density in the wide area without a current collector for the microtubular cell, which causes the decrease in the cell performance. The increase in the electrical conductivity of a current collector on the cathode was effective to improve the current density in the entire microtubular cell. The cathode current collection is significantly important to obtain high cell performance for the long anode-supported microtubular SOFCs.

Effect of Operating Temperature on Durability for Direct Butane Utilization of Microtubular Solid Oxide Fuel Cells

Effect of Operating Temperature on Durability for Direct Butane Utilization of Microtubular Solid Oxide Fuel Cells

I113 マイクロ円筒型固体酸化物形燃料電池の電流分布計測解析(OS-5:燃料電池・二次電池関連研究の新展開(1))

I113 マイクロ円筒型固体酸化物形燃料電池の電流分布計測解析(OS-5:燃料電池・二次電池関連研究の新展開(1))

Performance of microtubular SOFCs with infiltrated electrodes under thermal cycling

In this research, tubes consisting of a co-extruded dense YSZ electrolyte (∼10 μm) and porous NiO–YSZ anode (∼200 μm) were modified with different cathodes and anode infiltration to investigate the effects on both power and thermal cycling tolerance. Type of cathode (produced by infiltration of LSM into a porous YSZ matrix or by hand-painting of an LSM–YSZ ink), the type of pore former used in the cathode (graphite or poly (methyl methacrylate), PMMA) and the infiltration of the anode (no infiltration, or with infiltration steps using a co-precipitated SDC (Samaria doped ceria) mixture, or Ni–SDC mixture) were investigated as variables. The overall aim of this work is to produce cells that are more tolerant to thermal cycling, without sacrificing power density.Testing at 750 °C with 20 mL/min of dry hydrogen shows that anode infiltration has a particularly advantageous effect on performance, raising the peak power and reducing the degradation in peak power seen after aggressive cycling (100 °C/min heating and cooling rates). Cell power can be improved by LSM infiltration into a porous YSZ layer when the thickness of the YSZ layer is optimised and there is sufficient LSM. Infiltrated cells with cathode thicknesses of 20–40 μm functioned better than those 10–15 μm thick despite having a similar LSM/YSZ weight ratio. This may be due to the additional reaction zone available as a result of the higher LSM mass, suggesting that the reaction zone extends beyond 15 μm from the electrolyte. LSM and SDC infiltrated fine particles (50–100 nm) show good distribution and connectivity in the cathode and anode of the cells respectively. When PMMA is used as the pore former in the porous YSZ matrix, a slightly better cell performance is obtained compared with graphite as the pore former. Monitoring the power variation is found to be a more reliable tool than open circuit voltage (OCV) measurements for studying the effect of thermal cycling on cell stability.

Microstructure and performance of micro-tubular solid oxide fuel cells made by aqueous electrophoretic deposition

Abstract Anode-supported micro-tubular solid oxide fuel cells (SOFCs) were manufactured by consecutive aqueous electrophoretic depositions (EPDs) of porous anode layer (8YSZ–NiO), dense electrolyte layer (8YSZ) and porous cathode layer (LSM) onto a thin Cu wire, followed by stripping, drying, and a single-step co-sintering. The microstructure of the micro-tubular SOFC, including the thickness and porosity of each layer, was controlled by the processing parameters such as solid loading, current density, surfactant concentration and sintering temperature. The electrochemical performance of such a micro-tubular SOFC was demonstrated by the V–I–P (voltage–current–power) and impedance measurements.

Improved ceramics leading to microtubular Solid Oxide Fuel Cells (mSOFCs)

Two methods for improving ceramics in SOFCs are contrasted: first, the strengthening of the cell materials through control of nanoparticle aggregation during ceramic processing; second, the design of tubular cells and stacks with smaller diameters to avoid thermal shock damage.Nanoparticle aggregation is a natural process within a fluid dispersion of the ceramic grains, caused by the attractions of weak van der Waals forces which cause clumping to occur. Large aggregates are shown to act as defects which initiate cracks at low stress. By adding polymer and using high shear extrusion, the strength of zirconia was increased by a factor 4 compared to material prepared by conventional powder pressing methods.A much larger improvement, a factor 1000, was demonstrated by reducing the tube diameter from 25 mm to 6 mm then to 2 mm. The start-up time was thereby improved from 10 h to 22 min to less than 1 min, demonstrating that the thermal shock resistance of mSOFCs could be controlled more satisfactorily through cell design than from ceramic processing.Tests on a 250 We mSOFC system have shown that the start-up is now more limited by the balance of plant issues, such as thermal inertia, preheating fuel processing catalyst and heat exchanger, rather than the ceramic thermal shock resistance properties. The advances needed to obtain 1 min start-up of an mSOFC system are discussed.

마이크로 원통형 SOFC 제작 및 특성평가

본 연구에서는 마이크로 원통형 SOFC 지지체의 특성을 평가하기 위해 직경 3 mm의 연료극 지지체를 제조하여 지지체의 미세구조를 분석하고, 기계적 강도 및 가스투과도를 측정하였다. 다공성 연료극 지지체의 표면과 파단면의 미세구조를 분석하기 위해 SEM (Scanning Electron Microscope)을 이용하였다. 지지체의 가스투과도는 차압계를 이용하여 50, 100, 150 cc/min의 유량에서 측정하였으며, 기계적 강도는 만능 시험기를 이용하여 측정하였다. 마이크로 원통형 연료극 지지체의 기본적인 물성 평가 후 NiO-YSZ, YSZ, YSZ-LSM/LSM/LSCF로 구성된 마이크로 SOFC 단위전지를 제조하였으며, 반응온도와 연료 유량별로 성능평가를 수행하여 <TEX>$800^{\circ}C$</TEX>에서 <TEX>$1095mW/cm^2$</TEX>의 출력이 얻어짐을 확인하였다. 또한, 반응 온도에 따른 전기화학적 임피던스 특성평가를 통하여 온도가 높아질수록 전해질 이온전도도가 증가되어 ohmic 저항이 감소되고 그에 따라 마이크로 관형 SOFC 셀 성능이 증가함을 확인할 수 있었다. In present work, anode support for micro-tubular SOFC was fabricated with outer diameter of 3 mm and characterized with microstructure, mechanical properties and gas permeability. The microstructure of surface and cross section of a porous anode support were analyzed by using SEM (Scanning Electron Microscope) image. The gas permeability and the mechanical strength of anode support was measured and analysed by using differential pressure at the flow rates of 50, 100, 150 cc/min. and using universal testing machine respectively. The unit cell composed of NiO-YSZ, YSZ, YSZ-LSM/LSM/LSCF was fabricated and operated with reaction temperature and fuel flow rate and showed maximum power density of <TEX>$1095mW/cm^2$</TEX> on the condition of <TEX>$800^{\circ}C$</TEX>. The performance of single cell for micro-tubular SOFC increased with the increasing the reaction temperature due to the decrement of ohmic resistance of cell by the increment of the ionic conductivity of electrolyte through the evaluation of electrochemical impedance analysis for single cell with reaction temperature.

Investigation of Cobalt-Free Cathode Material Ba&lt;sub&gt;0.5&lt;/sub&gt;Sr&lt;sub&gt;0.5&lt;/sub&gt;Fe&lt;sub&gt;0.8&lt;/sub&gt;Cu&lt;sub&gt;0.2&lt;/sub&gt;O&lt;sub&gt;3−δ&lt;/sub&gt; for Micro-Tubular SOFC with Asymmetric Structure Anode

A cobalt-free perovskite oxide Ba0.5Sr0.5Fe0.8Cu0.2O3−δ (BSFCu) is applied as the cathode material for intermediate temperature micro-tubular solid oxide fuel cells (SOFCs) with asymmetric structure anode. The NiO-YSZ hollow fiber anode as support was prepared by the phase inversion technique. The as-prepared fiber anode shows a special asymmetric structure consisting of porous sponge-like structure in the middle and finger-like porous structure on the inner and outer side. Dense thin YSZ electrolyte membrane and SDC transition layer were deposited on the NiO-YSZ hollow fiber anode by a vacuum-assisted dip-coating and co-sintering technique. Laboratory-sized micro-tubular single cells of NiO-YSZ/YSZ+SDC/BSFCu-SDC were tested at 650-750°C with humidified H2 as the fuel and static ambient air as the oxidant. The peak power densities of 437.2, 326.5 and 214.4 mWcm-2 can be obtained at 750, 700 and 650°C, respectively. The experimental results indicate that the cobalt-free perovskite oxide BSFCu is a promising cathode material candidate for the developed intermediate temperature micro-tubular SOFC with asymmetric hollow fiber anode.

Carbon Oxidation in Fuel Cells and H2 Production

High temperature direct carbon fuel cells (DCFCs) offer potentially high efficiencies for conversion of the chemical energy of solid carbonaceous fuel(s) into electrical energy. DCFCs with molten metal anodes consume carbonaceous fuels by oxygen dissolved within the melt, which functions as an anode in an otherwise conventional solid oxide fuel cell (SOFC). Such reactors can also be used in electrolytic mode for reducing steam to hydrogen at the cathode, which enables the specific electrical energy consumption for H2 production to be decreased compared with conventional steam electrolysis. 2-D models have been developed to predict the behavior of a micro-tubular hollow fiber SOFC (ca. 1 mm ID) with a molten metal (Sn) anode, using assumed values for the anode exchange current density. Results are reported for the temperature dependence of open circuit potentials for the reactor: Sn | SnO2 | YSZ | Air measured using a pellet cell and compared with thermodynamic predictions.

Fabrication and Evaluation of Micro-Tubular SOFC Stack

We developed a fabrication processes of a micro-tubular SOFC stack using a metallic tubular interconnect. The stack was made of the 12 micro-tubular cells, which were connected in series. The open circuit voltage (OCV) was above 13V, thus the single cell showed the OCV value about 1.1V. Fuel utilization of the stack under a wet H2 flow of 20cc/min was above 90%, and the efficiency (LHV) was about 60%. In addition, the stack can be started-up within 5 min, and we succeeded the quick-start/shut down multi-cycles.

Fabrication of anode-supported Sc2O3-stabilized-ZrO2 electrolyte micro-tubular Solid Oxide Fuel Cell by phase-inversion and dip-coating

An anode-supported Sc2O3-stabilized-ZrO2 (ScSZ) electrolyte micro-tubular solid oxide fuel cell (MT-SOFC) with a cell configuration of Ni-YSZ|Ni-ScSZ|ScSZ|LSM-ScSZ|LSM has been prepared by a combination of phase-inversion and dip-coating technology. ScSZ is adopted as the electrolyte, decreasing the ohmic polarization of the cell. Porous Ni-YSZ micro-tubular anode substrate with a continuously transitional structure is prepared by the phase-inversion, and no pore former is used in the fabricating process. Ni-ScSZ is introduced between ScSZ electrolyte and Ni-YSZ micro-tubular anode substrate. LSM-ScSZ composite cathode was prepared by the bush-painting method. Single cell is successfully fabricated, exhibiting maximum power densities of 0.38, 0.54, 0.72 and 1.0Wcm−2 at 650, 700, 750 and 800°C, respectively, with humidified (3%H2O) hydrogen as fuel and stationary air as oxidant.

AC impedance characteristics for anode-supported microtubular solid oxide fuel cells

The DRT method was applied to analyze the AC impedance spectra for anode-supported microtubular solid oxide fuel cells (SOFCs). The anode microstructure has a more severe effect on the electrochemical properties of the anode-supported microtubular SOFCs because of the small inside diameter and thickness of the anode. For example, the maximum current density and fuel utilization for the cells sintered at 1400°C were lower than that at 1350°C due to the low porosity of the anode. However, it is difficult to separate the anode and cathode impedances by the general method of complex non-linear least squares (CNLS) with equivalent circuit models, because several components overlap for the impedance spectra measured between the anode and cathode without a reference electrode. The DRT method can easily separate the impedance spectra into five polarization components for the anode-supported microtubular SOFCs. The activation energy of the anode diffusion resistance was only 24–33kJ/mol. The walls of the pores in the anode were relatively well-packed with the nickel and YSZ particles for the cells made by the use of acrylic resin as a pore former. The anode diffusion resistance decreased by only a small amount with a rise of operating temperature for the anode-supported microtubular SOFCs.

Scrutiny of MT-SOFC Stack Manifolding Design Using CFD

In this work we investigated the use of Computational Fluid Dynamics (CFD) in order to describe the behav- iour of a single Micro Tubular Solid Oxide Fuel Cell (MT-SOFC) and a bundle thereof. It is the first step before building a rather necessarily complicated experimental apparatus in order to compare the predictions with experimental measure- ment. The first goal of this study was to test the suitability of commercially available CFD & SOFC modelling software, with some modified features. The second goal was to predict the effects of various fuel and oxidant manifolding tech- niques regarding temperature, species and current density distributions. A result of this paper showed that CFD is a very useful tool, when a SOFC module is incorporated for MT-SOFC stack modelling. A second result showed that the oxidant flow regime was much more important than the fuel regime in order to manipulate a single MT-SOFC's temperature pro- file. The cases investigated had a radiation model included and the differences in temperature profiles, when radiation was included and neglected, especially for MT-SOFCs with view factors to the reactor housing, was shown to be important. The CFD predictions clearly showed the benefits and advantages associated with the different forms of fuel and oxidant manifoldings. A future experimental analysis is currently being designed.