Microtubular Solid Oxide Fuel Cells Research Articles

Investigation of micro-tube solid oxide fuel cell fabrication using extrusion method

Extrusion is one of the most effective and inexpensive methods used in the production of ceramic tubes for tubular or micro-tubular solid oxide fuel cell (SOFC) applications. In this method, the parameters such as the viscosity of the ceramic slurry, the extrusion speed and the die temperature need to be optimized for a high performance. In this study, anode supported micro-tubular solid oxide fuel cells are successfully fabricated via a specially designed vertical-type piston extruder machine. The die design enables the production of micro-tubular SOFCs with outer diameters from 3 to 4.5 mm. The die temperature is determined to be the most important process parameter and the suitable die temperature is ranging 40–70 °C depending on the slurry content. The electrolyte layer is coated on the anode support tube by vacuum assist dip coating technique and co-sintering is applied with a home-made porous sintering apparatus to avoid dimensional anomalies. The effects of the parameters such as the composition of the electrolyte solution, the vacuum pressure and the immersion time on the electrolyte thickness are investigated. It is found that the electrolyte thickness decreases when the immersion time and vacuum pressure are reduced. Moreover, the thickness of the electrolyte is found to be depended on the content of the electrolyte solution. The effect of the pre-sintering temperature on the electrolyte quality is also investigated. The sintering temperatures of 1000 °C and 1100 °C provide a similar and desired electrolyte microstructure. A peak power density of 140 mW cm−2 is obtained at 700 °C from the final cell.

Fabrication and Characterization of High Performance Intermediate Temperature Micro-Tubular Solid Oxide Fuel Cells

A phase inversion-based dual-layer co-extrusion approach was advanced to fabricate micro-channel array structured micro-tubular solid oxide fuel cells (MT-SOFCs). The inner graphite layer was used as a sacrificial layer to eliminate the middle sponge-like layer and inner surface dense skin layer, which were normally formed using the single layer extrusion method. As a result, the micro-channel array generated in the outer layer was a more open structure and facile fuel/gas diffusion was obtained. The fuel/gas permeability of such an anode substrate Ni-Ce0.8Sm0.2O1.9 (Ni-SDC) was ∼9 times that of an anode substrate fabricated using the phase inversion-based single layer extrusion method. The open circuit voltages (OCVs) of the corresponding cell Ni-SDC/SDC/PrBaCo2O5+δ were 0.89 V–0.85 V at 500–600°C, much higher than those of other SDC electrolyte based MT-SOFCs. The peak power density of the cell was ∼1484 mW cm−2 at 600°C, approximately 1.5 times that of a similar cell with an anode substrate fabricated from the single layer extrusion method. This is also the highest performance among the SOFCs with the same material system in open literature. The fuel utilization rate was also significantly improved in different degrees depending on the supplied inlet fuel flow rates.

Direct Utilization of Methane in Microtubular-SOFC

This paper analyzes the possibilities for application of micro-tubular SOFC (mSOFC) with partial oxidation catalyst (CPOX) located at the cell inlet for direct utilization of hydrocarbons. Applied system reduced coke deposition without integration into the cell structure and allowed application of traditional Ni:zirconia cermet for direct hydrocarbons utilization. The key problem in the proposed Microtubular Solid Oxide Fuel Cell (mSOFC) system is where the part of the cell is outside of the hot zone. This is caused by the temperature distribution (TD). Despite the good resistance to thermal shock, the long-term operation with unequal TD can cause cell damage. The TD was measured along and across the cell (internally and externally) under different operating conditions. H2 and CH4 were used as fuels in the first instance. Different mixtures of fuel and air and different forms of the catalyst were tested for the CPOX of hydrocarbons. Different forms/geometries of the catalyst were tested to optimize the system design. The paper mainly focuses on solving problems with temperature distribution along and across the mSOFC cell.

P25] Assessment of temperature and pressure dependence of molar volume and phase diagrams of Al, Cu, Si, and Zn

The performance of a novel micro-tubular solid oxide fuel cell (SOFC) with an inert support and an integrated current collector for the inner electrode was improved by controlling its microstructural features. Multi-step dip coating and co-sintering methods were used to fabricate the cell containing porous yttria-stabilized zirconia (YSZ), Ni, Ni–YSZ, YSZ, strontium-doped lanthanum manganite (LSM)–YSZ, and LSM as the inert support, anode current collector, anode, electrolyte, cathode, and cathode current collector, respectively. To enhance gas diffusion through the YSZ support by properly tailoring its porosity, a combination of micro-crystalline cellulose and polymethyl methacrylate pore formers was used. Additionally, the porosity of the Ni current collector was improved and the LSM–YSZ cathode was sufficiently thick for high oxygen reduction activity. Owing to its optimized microstructure, the micro-tubular SOFC delivered excellent power output with maximum power densities of 710, 591, 445, and 316 mW cm−2 at 850, 800, 750, and 700 °C, respectively. The effect of redox cycling on cell performance was investigated by alternately exposing the anode to fuel and air atmospheres. The cell had good tolerance toward the redox phenomenon with no apparent degradation in its performance up to 10 redox cycles.

3D Printing and Prototyping Manifolds for Microtubular Solid Oxide Fuel Cells (mSOFCs)

3D Printing and Prototyping Manifolds for Microtubular Solid Oxide Fuel Cells (mSOFCs)

Modeling of thermal stresses in a microtubular Solid Oxide Fuel Cell stack

A modeling study was carried out to analyze thermal stresses in a microtubular Solid Oxide Fuel Cell (mSOFC) stack and to estimate thermal expansion of the fuel cells inside the stack. A joint analysis by Computational Fluid Dynamics (CFD) and Computational Structural Mechanics Finite Element Method (FEM) was performed. Temperature profiles generated by the thermo-hydrodynamic model were applied in the thermo-mechanical model to calculate thermal stress distributions in the mSOFC stack. The results yield maximum thermal axial elongation equal to 1.34 mm for the mSOFC stack, while the maximum radial elongation was equal to 0.496 mm. Modeled maximum equivalent (von Mises) stress was equal to 538 MPA in the contact areas of the cylindrical housing and manifold on the fuel inlet side. Based on comparison of the total axial stresses and the residual ones with the material strength it was noticed that the anode and electrolyte layers should not be critically deformed, but there is a risk of damage for cathode layers at chosen fuel cell configurations. A high risk of damage was also noticed for the outer housing, near contact points with manifolds as well as at the air distributor due to large number of cut-outs in the material.

Development of Ceria-Based Microtubular Solid Oxide Fuel Cells

It is well known that gadolinia-doped ceria (GDC) has high oxide-ion conductivity for electrolyte, and it improves electrochemical activities for fuel oxidation and oxygen reduction for electrodes of solid oxide fuel cell (SOFC). In this work, we investigated the usage of ceria for cathode, anode and electrolyte of microtubular SOFCs for low temperature operation and direct hydrocarbon utilization.

ChemInform Abstract: Low‐Temperature Solid‐Oxide Fuel Cells

AbstractReview: 50 refs.

Electrochemical and microstructural properties of Ni–(Y2O3)0.08(ZrO2)0.92–(Ce0.9Gd0.1)O1.95 anode-supported microtubular solid oxide fuel cells

The nickel–zirconia cermet is widely used as an anode of solid oxide fuel cells (SOFCs). On the other hand, the nickel–ceria based anode indicates high electrochemical activity for hydrogen oxidation and hydrocarbon reforming. In this study, electrochemical and microstructural properties of microtubular SOFCs with Ni-based composite anodes containing yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) are investigated electrochemically using impedance spectroscopy (EIS) and microstructurally using focused ion beam-scanning electron microscopy (FIB-SEM). The solid solution of YSZ and GDC was easily formed after mechanical mixing and sintering at 1400°C. The electrical conductivity and mechanical strength for the Ni–YSZGDC composite anodes are low relative to Ni–YSZ due to poor sinterability. The GDC-containing anodes show improved electrochemical activity for hydrogen oxidation, despite having lower three-phase boundary densities. Distribution of relaxation times (DRT) analysis of the EIS data shows that the concentration polarization is lower for the Ni–GDC anode, due to a higher measured pore volume. The maximum power density for the cell with the Ni–YSZGDC composite anode was higher than those with the Ni–YSZ and Ni–GDC anodes.

Performance improvement and redox cycling of a micro-tubular solid oxide fuel cell with a porous zirconia support

The performance of a novel micro-tubular solid oxide fuel cell (SOFC) with an inert support and an integrated current collector for the inner electrode was improved by controlling its microstructural features. Multi-step dip coating and co-sintering methods were used to fabricate the cell containing porous yttria-stabilized zirconia (YSZ), Ni, Ni–YSZ, YSZ, strontium-doped lanthanum manganite (LSM)–YSZ, and LSM as the inert support, anode current collector, anode, electrolyte, cathode, and cathode current collector, respectively. To enhance gas diffusion through the YSZ support by properly tailoring its porosity, a combination of micro-crystalline cellulose and polymethyl methacrylate pore formers was used. Additionally, the porosity of the Ni current collector was improved and the LSM–YSZ cathode was sufficiently thick for high oxygen reduction activity. Owing to its optimized microstructure, the micro-tubular SOFC delivered excellent power output with maximum power densities of 710, 591, 445, and 316 mW cm−2 at 850, 800, 750, and 700 °C, respectively. The effect of redox cycling on cell performance was investigated by alternately exposing the anode to fuel and air atmospheres. The cell had good tolerance toward the redox phenomenon with no apparent degradation in its performance up to 10 redox cycles.

A Thermo Fluid and Thermo Mechanical Modelling of a Microtubular Solid Oxide Fuel Cell Stack for Unmanned Aerial Vehicles

This work presents a three-dimensional thermo fluid and thermo mechanical computational analysis of a microtubular solid oxide fuel cell (SOFC) stack as part of a microtubular SOFC generator. The latter powers a lithium-ion battery as a microtubular SOFC hybrid power system to extend the flight endurance of a mini unmanned aerial vehicle. The microtubular SOFC stack and balance of plant components, comprising microtubular SOFC generator, were designed based on computational fluid dynamics (CFD) assumptions. The stack design process has been further upgraded with thermo mechanical finite element analysis (FEA) simulation to capture deformations and stresses in the stack due to thermal expansion of microtubular SOFCs and manifolds. The developed microtubular SOFC stack will be built based on CFD and FEA predictions and tested to determine the suitability of the microtubular SOFC hybrid power system for this application.

Performance Characteristics of a Micro-tubular Solid Oxide Fuel Cell Operated with a Fuel-rich Methane Flame

A micro-tubular solid oxide fuel cell (SOFC) is directly combined with a fuel-rich methane flame in a simple, "no-chamber" setup. The multi-element diffusion flat flame burner (MEDB) is served as a fuel reformer and implements the quick starting process for the flame fuel cell. Experiments are performed based on novel anode supported micro-tubular SOFC cells and a multi-element diffusion flame burner for various equivalence ratios and flow rates. The SOFC is directly heated up by the flame from room temperature to operating temperature in less than 120 s. The maximum power generated by the fuel cell reached 0.45 W at the equivalence ratio of 1.2. The cell was operated for 8 hours at the equivalence ratio of 1.2 at a fixed voltage of 0.45 V without significant performance degradation. A transient two-dimensional SOFC model is developed to analyze the transient temperature field and the associated thermal stresses of the SOFC when the cell is suddenly exposed to a high temperature flame. The thermal shock resistance of the direct flame micro-tubular SOFC is analyzed and the result shows the excellent performance characteristics of the micro-tubular cell in the aspect of thermal shock resistance. Figure 1

A Novel Micro-Tubular Solid Oxide Fuel Cell with a Porous Zirconia Support for Intermediate-Temperature Operation

To improve current collection efficiency and prevent cell degradation under reduction–oxidation (redox) cycling, we have proposed a novel micro-tubular solid oxide fuel cell (SOFC) with a configuration of inert support/current collector/inner electrode/electrolyte/ outer electrode/current collector. In this work, we applied multi-step dip coating and low-temperature co-sintering methods to fabricate a micro-tubular SOFC based on the proposed design using yttria-stabilized zirconia (YSZ), Ni, Ni-scandia-stabilized zirconia (ScSZ), ScSZ, strontium-doped lanthanum manganite (LSM)–ScSZ, and LSM as the inert support, anode current collector, anode, electrolyte, cathode, and cathode current collector, respectively. The electrochemical performance of the fabricated cell was characterized in an intermediate temperature range, which allows for the use of cheap stack materials and also slows down the performance degradation mechanisms. Our results indicate that the performance of the proposed micro-tubular SOFC can be improved considerably in the intermediate-temperature range using conventional materials when YSZ is substituted with ScSZ as the electrolyte and ion-conducting phase in the electrodes.

Protonic Conducting Ceramic Hollow Fiber Based on BaCe0.95Tb0.05O3-d perovskite Membrane for Hydrogen Permeation and MT-SOFC

BaCe0.95Tb0.05O3-d (BCTb) thin membranes with perovskite structure were prepared on porous Ni-BCTb hollow fiber by a combined phase inversion with dip-coating technique. After co-sintering at 1500°C, the dense BCTb with a thickness of 15μm is gas-tight, and the N2 permeance of the reduced dual-layer hollow fiber is 3×10− 7 mol·m− 2·s− 1·Pa− 1 at 5 atm. Microtubular solid oxide fuel cell (MT-SOFC) with the configuration of Ag/BCTb/Ni-BCTb possesses high performance. The OCV is reduced with the operation temperature increase from 500 to 800°C,and the variation is significant at 700°C. Meanwhile, the maximum power density is up to 552 mW cm-2 at 700°C, higher than the value at other operation temperature increase. These imply that the serious electron leakage occurs above 700°C, and thus the Hydrogen permeation was carried out. The hydrogen permeation flux is as high as 0.61mLmin-1 cm-2at 850°C.

Influence of Convective Heat Transfer by Air Flow on Local Current/Temperatures along Microtubular Solid Oxide Fuel Cells In-situ Identified by Electrodesegmentation Method for Co- and Counter-flow Configurations

Introduction Major advantages of solid oxide fuel cells (SOFC), such as employing non-precious metals as catalyst, tolerating various fuels, and releasing high quality waste heat, are virtues of high operation temperature (873-1273 K). On the other hand, thermo-mechanical stresses, as the primary problem of the SOFC systems develop due mainly to the heat treatment and operation at high temperatures. For both planar and tubular designs, a number of numerical studies addressing the thermo-mechanical stresses have been published; the thermal expansion coefficient mismatch, residual stresses, and the spatial surface temperature variations during the operation, have been stated to be the primary reasons [1-3]. To estimate the stresses stemming particularly from the spatial temperature variations, researches have been relying on the local temperatures computed numerically with thermo-electrochemical models [1,2,4,5,6]. However, as repeatedly stated, due to the difficulty in conducting experiments at high temperatures, the models were hardly validated in terms of surface temperatures. We thus measured the local current and temperatures along a microtubular SOFC with the electrode-segmentation method. Given the surplus heat generated during the cell operation is commonly removed by excess air flow method, and this method likely affects the temperature distribution along the cells, we conducted experiments for co- and counter-flow configurations under various air flow rates. Experimental Setup The tubular anode substrate of the cell was made of NiO/YSZ (65:35 wt %). Upon coating with the 8YSZ electrolyte colloid by dip-coating, we sintered the tube at 1693 K for two hours. With a special mask designed for the segmentation of the cell, we brush-coated the La0.7Sr0.3MnO3/YSZ cathode slurry (10:3 wt %) on the electrolyte surface by a cotton swab and subsequently sintered at 1323 K for two hours. Eventually, we connected the silver wires and thermocouples to the regarding segments for local current/voltage, and temperature measurements. As depicted in Fig. 1, we fed fuel and air to the anode and cathode, respectively, in the co-flow configuration. For counter-flow configuration, we exchanged the fuel inlet and outlet, retaining inlet and outlet of the air flow. Note that we keep the segment labels as on in Fig. 1 throughout the discussion, despite the switch of the flow configuration. Though the inlet air temperature was 298 K, the fuel flow was pre-heated before entering the cell. Prior to fuel supply, we sustained the cathode surface temperatures of the segments at 1073 K by an electric furnace. Results and Discussion The temperature distribution profile along the cell alters with the gas flow configuration. With the rising air flow rate at 0.6 V in the co-flow configuration, the midstream and downstream segment temperatures arise, while the upstream segment temperature decreases with a relatively steeper slope. This entails a larger maximum temperature difference along the cell. We observe the same trend in the local currents, however, the slope of the current variations in all the segments are rather small and resemble each other. With the increasing air flow rate, at 0.6V in the counter-flow configuration, all the segment temperatures drop down, where the slope of the upstream segment is relatively higher than the other segments. As a result, the maximum temperature along the cell becomes larger. The midstream and downstream segment currents increase with rather small slopes, whereas the upstream segment current remains nearly constant. Conclusions The in-situ identified current/temperature distribution profiles for co- and counter –flow configurations differ from each other considerably. With the increasing air flow rate, the maximum temperature difference along the cell grows in both flow configurations; where the total current output variation is rather small. The counter-flow configuration exhibits larger maximum temperature difference along the cell comparing to the co-flow case. With these findings, we can deduce larger thermo-mechanical stresses at high air flow rates in the counter-flow configuration; which we will explore numerically on the basis of local temperatures in-situ measured here.

Direct Current Distribution Measurement of an Electrolyte-Supported Planar Solid Oxide Fuel Cell under the Rib and Channel by Segmented Electrodes

Solid oxide fuel cells have been of practical concern as a high efficiency power generation device. Problems of the SOFCs include current distribution that decays the total cell performance and efficiency, and causes electrode degradation chemically and thermo-mechanically. In the case of the planar SOFCs, the fuel/oxidant distributions and current collecting resistance cause current and temperature distributions over the electrodes under the separator ribs and flow channels. Optimized design of the separator is hence required to improve the power generation characteristics and durability of practical fuel cell stacks. Although there have been a number of numerical analyses, very few experimental investigations confirming in-situ current distributions to reveal the influence of the separator structure have been reported. The aim of the present study is thus to measure in-plane spatial current variations of an electrolyte-supported planar SOFC with segmented cathodes under the rib and the flow channel. We used the planar cell having three segmented cathodes (Fig. 1) assembled with segmented cathode separators for electrical insulation. The segmented cathodes were placed opposing to a rib and a set of parallel flow channels of the anode separator. The cell was composed of NiO-10Sc1CeSZ anode, 10Sc1CeSZ electrolyte, GDC interlayer, and LSCF-GDC cathode. The electrode area was 1.4 cm2(2.8 x 0.5 cm) each. The anode and cathode separator made of SUS430 had the flow channels with a width of 3 mm, a depth of 1 mm, and a length of 2.8 cm. Silver mesh was employed for the current collection of both sides. Current voltage (I-V) measurements were carried out under voltage control using three electric loads to reproduce the electrode potential of a single cell[1]. The anode and cathode were electrically connected with the four-terminal method. The anode NiO was reduced to Ni by feeding H2/N2 mixture gas for 2 hours prior to measurements. During measurements, anode and cathode were fed upward with mixtures of H2/N2and dried air at constant flow rates, respectively. The cell was maintained at 800 °C by an electric furnace at open circuit voltage (OCV) We prepared two types of the cathode separators. One of them had the same flow channel configuration as the anode separator where the rib and flow channels faces each other. The other had additional flow channels opposing to the anode rib. With the former cathode separator, the overpotential at the electrodes under the anode rib was significantly large compared with the flow channel part, whereas the latter case exhibited smaller overpotentials under the rib. This indicates that oxygen transport becomes poor under the wider rib in the cathode side. The latter case also indicated that the electrode under the rib exhibited smaller overpotential than the flow channel part when fuel is sufficient, owing to the smaller contact resistance under the rib by the compression. We also calculated current and hydrogen/oxygen partial pressure distributions by finite element simulation (COMSOL Multiphysics). The current distributions can be calculated so that they agree with the in-situ current distributions derived by the segmented cathodes from setting the exchange current densities, electrode porosities, electrolyte ion conductivities, and electrode ion/electron conductivities. Thereby oxygen starvation under the wider cathode rib and larger electron conduction resistance for the flow channel part were numerically evaluated to explain the in-situ currents for the segmented cathodes. We will present the results for the case of H2 starvation. [1] Ö. Aydin, T. Koshiyama, H. Nakajima, T. Kitahara, In-situ Diagnosis and Assessment of Longitudinal Current Variation by Electrode-Segmentation Method in Anode-Supported Microtubular Solid Oxide Fuel Cells, J. Power Sources, Vol. 279, 218–223 (2015).Fig. 1 Segmented cathodes on the planar SOFC. Figure 1

Development of Micro Power Generator Using LPG-Fueled Microtubular Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) can, in principle, directly use hydrocarbon fuels such as methane and liquefied petroleum gas (LPG). It was previously found that nickel-gadolinia doped ceria (Ni-GDC) anode has high durability under LPG utilization in SOFCs. However, the performance under direct LPG utilization was low compared to hydrogen fuel, because anode polarization resistance increased by internal reforming. Carbon was slightly deposited on the Ni-GDC anode after direct LPG utilization for 100 h at 650 oC. In this work, a catalytic partial oxidation reformer was developed for LPG utilization, and high performance was obtained using reformate gas as well as hydrogen without carbon deposition on the Ni-GDC anode for 100 h at 650 oC. Furthermore, microtubular SOFC stacks were developed using mass production technologies, and a proto-type 200 W micro power generator was demonstrated via NEDO project entitled "Technology Development for Promoting SOFC Commercialization".

Multi Length-Scale Quantification of Hierarchical Microstructure in Designed Microtubular SOFC Electrodes

Extrusion techniques can be used to produce microtubular SOFCs with a wide range of hierarchical microstructures. It is believed that by controlling the pore structure of the thicker supporting electrode, the transport properties can be optimised. X-ray micro-tomography has been used to image the finger-like voids in a microtubular anode. These structures are on the order of 10 microns in diameter; however, smaller characteristic lengthscales can be imaged using FIB/SEM, which reveals the packing of the sintered particles, as well as the pore structure within the particles. Quantifying the tortuosity factor at each of these lengthscales offers insight into the effect of tailoring the pores. The tortuosity factor of porous SOFC electrodes is used to account for the apparent reduction in species diffusion caused by the geometry of the network. It affects both the electrical transport in the solid phases as well as gas diffusion in the pores. The tortuosity factor is ideally calculated by solving the steady-state diffusion equation; however in systems spanning several orders of magnitude it is very computationally expensive. The figure shows the distribution of a scalar field in an interdigitated void structure, calculated using a finite element method. This work combines the calculated tortuosity factors of each lengthscale to try and understand the significance of the hierarchical pore structure. The straight, radial pores of the microtubes imaged were found to augment the system significantly compared to an isotropic network of equivalent porosity. The importance of recasting the tortuosity factor as a function of distance from the electrolyte for hierarchical pore systems is also discussed. Figure 1

Development of Ceria-Based Microtubular Solid Oxide Fuel Cells

Gadolinia-doped ceria (GDC) has higher oxide-ion conductivity than a conventional yttria-stabilized zirconica (YSZ). GDC is one of the most desirable materials as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs) operated below 600 °C. A nickel-GDC cermet anode has high catalytic and electrochemical activities against fuel oxidation in SOFCs. For example, direct butane utilization was successfully demonstrated for Ni-GDC anode-supported microtubular SOFCs. Furthermore, an La0.6Sr0.4Co0.2Fe0.8O3(LSCF)-GDC composite cathode shows high performance at intermediate temperature. We applied GDC materials to electrolyte, anode and cathode for microtubular SOFCs.

A Review of the Importance and Present Status of Micro-Tubular Solid Oxide Fuel Cells

The generation of environmental-friendly energy is now one of the major demand of the world for healthy future. Fuel cell is one of the prime candidate in this regard which convert chemical energy of a fuel gas very efficiently and directly into electrical energy. This chapter describes the concept, impact of anode and electrolyte morphology, thickness, diameter, and fabrication of a micro-tubular solid oxide fuel cell (SOFC). The chapter describes the anode, cathode, and electrolyte of the cell components in more detail and their importance of each is regarding their size and thickness. Advantages of micro-tubular SOFCs with respect to the other fuel cell technologies are compared. The chapter describes the potential for directly running off hydrocarbon fuels and the design and operation of micro-tubular SOFCs on bio-fuel specifications and materials’ requirements. The chapter also discuss fabrication technology of micro-tubular single cell by using commercially available raw materials.