Microtubular Solid Oxide Fuel Cells Research Articles

Experimental investigation on the effect of anode functional layer on the performance of anode supported micro-tubular SOFCs

In this study, anode supported micro-tubular solid oxide fuel cells (SOFCs) are fabricated by extrusion method and the effects of powder size, thickness and sintering temperature of the anode functional layer (AFL) on the electrochemical performance is experimentally investigated. For this purpose, four different commercial NiO powders are tested as initial powder for the fabrication of the anode functional layer. The thickness of AFL is also considered by varying the number of coatings. After deciding the optimum initial NiO powder size used in AFL and AFL thickness, the effect of pre-sintering temperature is examined. The performance tests are performed at an operating temperature of 800 °C under hydrogen and air. The microstructures of the samples are also investigated by a scanning electron microscope. The best peak power density is obtained as ∼0.5 W/cm2 from the cell having a single layer anode functional layer pre-sintered at 1250 °C prepared by NiO powders with 4 m2/g surface area.

Local heat transfer enhancement by recirculation flows for temperature gradient reduction in a tubular SOFC

ABSTRACT The large temperature gradient in solid oxide fuel cells (SOFCs) severely degrades their operation reliability. In this study, we have developed a numerical model to examine the effect of local heat transfer enhancement on the temperature gradient reduction in a counter-flow type micro-tubular SOFC. The local heat transfer is enhanced by placing turbulators in the SOFC air channel. We show that turbulators in air channels could induce recirculation flow, which increases the local convective heat transfer coefficient and facilitates the heat dissipation from electrodes to air stream. A spatial temperature gradient diagram is proposed to analyze the influences of turbulator radii, locations and pitches on the cell axial temperature gradient reduction. The turbulator effectively decreases the largest temperature gradient and reduces the region where the temperature gradient is higher than 30℃/cm. We also compare the temperature profile and air-side pressure drop between tubular SOFCs with turbulators and thin air channels. The SOFC with turbulators is demonstrated to achieve comparable heat transfer performance but at a lower air-side pressure drop than that with a thin air channel. Our work demonstrates the effectiveness of local heat transfer enhancement on SOFC temperature gradient reduction and proposes a new diagram to quantitatively evaluate the impact of the thermal management methods through local heat transfer enhancement.

A New Design Using Metal Wire Brushes as the Current Collector and Catalyst Carrier for Internal Reforming of Micro-Tubular SOFCs

In this study, a novel current collecting method of inserting metal wire brushes into anode-supported micro-tubular solid oxide fuel cells (MT-SOFCs) was designed and demonstrated. The proposed design enabled a tight and stable contact between the wires and the interior surface of the anode, which led to good performance of the SOFC. Different metal wire materials were utilized, and copper–zinc wire brushes (CZWBs) showed better performance than stainless steel wire brushes (SSWBs). The MT-SOFCs using CZWBs achieved a maximum power density of 290 mW cm−2 and a minimum ohmic resistance of 0.66 Ω∙cm2 at 700 °C in a hydrogen stream. The brush size was shown to have little effect on the ohmic resistance. A long-term discharge test (108 h) was also carried out. The feasibility of using metal wire brushes as internal reforming catalyst carriers was experimentally verified. The SSWBs loaded with Pt catalyst could realize effective reforming of methane, demonstrating the potential of the proposed design to use hydrocarbon fuels for power generation.

High-performance NdSrCo2O5+δ–Ce0.8Gd0.2O2-δ composite cathodes for electrolyte-supported microtubular solid oxide fuel cells

NdSrCo2O5+δ (NSCO) is a perovskite with an electrical conductivity of 1551.3 S cm−1 at 500 °C and 921.7 S cm−1 at 800 °C and has a metal-like temperature dependence. This perovskite is used as the cathode material for Ce0.8Gd0.2O2-δ (GDC)-supported microtubular solid oxide fuel cells (MT-SOFCs). The MT-SOFCs fabricated in this study consist of a bilayer anode, comprising a NiO–GDC composite layer and a NiO layer, and a NSCO–GDC composite cathode. Three cell designs with different outer tube diameters, GDC thicknesses, and NSCO/GDC ratios are designed. The MT-SOFC with an outer tube diameter of 1.86 mm, an electrolyte thickness of 180 μm, and a 5NSCO–5GDC composite cathode presents the best performance. The flexural strength of the aforementioned cell is 177 MPa, which is sufficient to confer mechanical integrity to the cell. Moreover, the ohmic and polarization resistance values of the cell are 0.22 and 0.09 Ω cm2 at 700 °C, respectively, and 0.15 and 0.03 Ω cm2 at 800 °C, respectively. These results indicate that the NSCO-GDC composite exhibits high electrochemical activity. The maximum power densities of the cell at 700 and 800 °C are 0.46 and 0.67 W cm−2, respectively, exceeding those of existing electrolyte-supported MT-SOFCs with similar electrolyte thicknesses.

Micro-Tubular Solid Oxide Fuel Cells with One Closed-End Fabricated Via Dip-Coating and Co-Firing

Tubular solid oxide fuel cells (SOFCs) with one closed-end configuration have high tolerance of thermal cycling owing to the capability of expanding and shrinking freely in response to thermal stresses. Additionally, tubular cells with one closed-end have simple sealing requirements, and enable making the stacks with uniform temperature distribution. In this study, anode supported micro-tubular SOFCs with one closed-end were manufactured successfully by dip-coating and co-firing methods using carbon rods with round end as a template. A dense electrolyte layer was obtained at a co-firing temperature of 1400 °C, followed by cathode deposition and sintering. The prepared micro-tubular SOFCs comprised of Ni-yttria-stabilized zirconia (YSZ), Ni–scandia-stabilized zirconia (SSZ), strontium-doped lanthanum manganite (LSM)-SSZ, and LSM as the anode support, anode functional layer, electrolyte, cathode functional layer, and cathode current collector, respectively. The electrochemical performance and microstructure of fabricated closed-end micro-tubular SOFC was evaluated by electrochemical characterization and scanning electron microscopy. The developed dip-coating method is fast in turnaround time and enables fabricating the one closed-end tubes of various lengths and diameters by co-firing.

Numerical Study on Effects of Structure Parameters and Operating Conditions on the Micro-Tubular SOFC Stack Performance

A multi-physical field coupled three dimensional model of large-scale micro-tubular solid oxide fuel cell (MTSOFC) stacks was established in this study. The effects of structural parameters and operating conditions, namely configuration of the tube array, diameter, length, distance between tubes, the flow rate and flow path of air, on the stack performance were studied. The triangular array is more compact in space and can achieve higher volume specific power than the square array. However, the more concentrated arrangement also results in a higher local temperature and temperature difference, which is challenging for long-term stability. According to the simulation results, smaller tube diameter and distance between tubes lead to higher volume specific power, with the former having a marginal effect. A longer tube can reduce the axial temperature gradient, which is conducive to long-term operation stability. Increasing the air flow rate of the cathode is the most effective method to reduce the highest temperature. Furthermore, the co-flow arrangement is found to be more helpful to take away the heat generated by the stack than the counter-flow arrangement. Based on the simulation results, the structural parameters and operation conditions suitable for a large-scale micro-tubular SOFC stack are decided, considering the output power, long-term stability, and mechanical strength. The model is demonstrated as a powerful tool for the design and optimization of MTSOFC stacks.Key words: Micro-tubular SOFC Stack; Array Configuration; Structure Parameter; Operation Condition; Volume Specific Power

A New Design Using a Metal Wire Brush as the Current Collector of Micro-Tubular SOFCs

In this study, a novel current collecting method was designed and demonstrated by inserting a metal wire brush into the anode supported micro-tubular SOFC. The new design enables a close and stable contact between the wire and the inner wall of the anode which leads to a fine performance of the SOFC. Different kinds of metal wires were applied, where the copper wire brush (CWB) shows a better performance than the stainless steel wire brush (SSWB). At a temperature of 650℃ and a fuel feed of hydrogen, the SOFC using CWB can achieve a maximum power density of 278mW/cm2 and an ohmic resistance of 0.75 Ω∙cm2, which are both better than that of SSWB (226mW/cm2 and 0.89 Ω∙cm2). While the brush size has been approved to have little effect on the ohmic resistance and the electrochemical performance. The potential of the metal wire brush as a support for internal reforming catalysts is further verified in experiments. The metal wire brushes loaded with Pt and Ni catalysts can realize effective reforming of hydrocarbon fuels, which makes the new design potential to use methane, propane, butane and other fuels for power generation. Key words: Micro-tubular SOFC; Current collecting; Metal wire brush; Internal reforming

Micro-Tubular Solid Oxide Fuel Cells with One Closed-End Fabricated Via Dip-Coating and Co-Firing

The tubular solid oxide fuel cells (SOFCs) with one closed-end configuration have high thermal-cycling tolerance owing to the capability of expanding and shrinking freely in response to thermal stresses. The tubular cells with one closed-end have simple sealing requirements, and enable making the stacks with uniform temperature distribution. In this study, anode supported micro-tubular SOFCs with one closed-end were manufactured successfully by dip-coating and co-firing methods using carbon rods as a template. A dense electrolyte layer was obtained at a co-firing temperature of 1400°C, followed by cathode deposition and sintering. The prepared micro-tubular SOFCs were comprised of Ni-yttria-stabilized zirconia, Ni–scandia-stabilized zirconia (SSZ), SSZ, strontium-doped lanthanum manganite (LSM)-SSZ, and LSM as the anode support, anode functional layer, electrolyte, cathode functional layer, and cathode current collector, respectively. The developed dip-coating method is fast in turnaround time and enables fabricating the one closed-end tubes of various lengths and diameters by co-firing.

A New Design Using a Metal Wire Brush as the Current Collector of Micro-Tubular SOFCs

In this study, a novel current collecting method was designed and demonstrated by inserting a metal wire brush into the anode supported micro-tubular SOFC. The new design enables a tight and stable contact between the wire and the inner wall of the anode which leads to a fine performance of the SOFC. Different kinds of metal wires were applied, where the copper wire brush (CZWB) shows a better performance than the stainless steel wire brush (SSWB). At a temperature of 700℃ and a fuel feed of hydrogen, the SOFC using CZWB can achieve a maximum power density of 290 mW/cm2 and an ohmic resistance of 0.66 Ω∙cm2. And the brush size has been approved to have little effect on the ohmic resistance. A 108 h long-time discharge test was carried on, resulting with a degradation rate of 1.728‰/h and indicating a relative stability.

Numerical Comparison of Planar and Micro-Tubular Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is a highly efficient power generation system that can be designed in many different geometries including planar and tubular types of fuel cell. Although their operating performance is based on the same principles, parameters may be unique for each geometry. This study investigates numerically effects of cell configuration on performance by developing a finite element method (FEM) based SOFC models. The effects of configuration on the fuel cell are investigated in terms of voltage, the average cell current density, and the average cell power. These effects are illustrated with polarization and power curves by using COMSOL software. As the most convenient results of this study, although both types have their own advantages, it can be concluded that the performance of the SOFC increases with the planar geometry compared to micro-tubular type under the studied operating conditions.

Internal current collection and thermofluidynamic enhancement in a microtubular SOFC

A low-cost, durable and simple internal current collector is presented for microtubular SOFCs (µT-SOFC). The internal design does not require removal of external cell layers, and subsequent loss of active area required to expose the interior electrode for contacting. The brush-like, high surface area current collector device is adapted from heat exchanger turbuliser technology produced by CALGAVIN Ltd. The effectiveness of the hiTRAN® turbuliser as a µT-SOFC current collector is explored and its thermofluidynamic effects on the cell described, as well as coating of the hiTRAN® design to reduce electrical contact resistance. The final design achieved a current density at 0.7 V of 0.38 A.cm−2 and peak power density of 0.27 W.cm−2, 4.3 times higher than the original design and 3.3 times higher than the state-of-the-art with the same base materials.

Lanthanum-doped ceria interlayer between electrolyte and cathode for solid oxide fuel cells

ABSTRACT Gadolinia-doped ceria (GDC) is widely used as an interlayer for preventing the formation of SrZrO3 between yttria-stabilized zirconia (YSZ) electrolyte and lanthanum strontium cobalt iron oxides (LSCF) cathode for solid oxide fuel cells (SOFCs). However, the diffusion of Gd ions to YSZ electrolyte results in the decrease in ionic conductivity due to the formation of Gd2(Zr, Ce)2O7. In the present work, we focus on lanthanum-doped ceria (LDC) as an interlayer between YSZ electrolyte and LSCF cathode. The ionic conductivity of YSZ/LDC composite was higher than that of YSZ/GDC composite in spite of the lower conductivity of LDC than GDC. The maximum power density for the microtubular SOFC with LDC interlayer was better than that with GDC interlayer. While the ohmic resistance decreased by the change from GDC to LDC interlayers, the polarization resistance for oxygen reduction reaction slightly increased as a result of distribution of relaxation times (DRT) analysis. The chemical compositions of Zr, Ce and La was gradually varied at the interface between YSZ electrolyte and LDC interlayer, which affected the both of ohmic and polarization resistances.

Investigation of Rapid, Moderate Temperature Change Thermal Cycles of a Micro-Tubular Flame-Assisted Fuel Cell

Abstract Despite many efforts and improvements over the last few decades, two of the major challenges facing solid oxide fuel cells (SOFCs) are slow heating rates to operating temperature (typically < 5 °C · min−1) and a limited ability to thermal cycle (<200 cycles). Recently, a novel hybrid setup that combines a fuel-rich combustion reformer with a SOFC was developed and utilized to investigate rapid heating, cooling, and thermal cycling of a micro-tubular SOFC. In this work, 3000 moderate thermal cycles are conducted at a heating rate exceeding 140 °C · min−1 and a cooling rate that exceeded 100 °C · min−1. The open-circuit voltage (OCV) was analyzed over the 150 h test, and a low degradation rate of ∼0.0008 V per 100 cycles per fuel cell was observed in the absence of the current collector degradation. Unlike a previous test, which was conducted at lower temperatures, significant degradation of the current collector was observed during this test. Electrochemical impedance spectroscopy shows that degradation in the SOFC was due to increases in ohmic losses, activation losses at the cathode, and increased concentration losses.

The Development of Current Collection in Micro-Tubular Solid Oxide Fuel Cells—A Review

Micro-tubular solid oxide fuel cells (µT-SOFCs) are suited to a broad range of applications with power demands ranging from a few watts to several hundred watts. µT-SOFCs possess inherently favourable characteristics over alternate configurations such as high thermo-mechanical stability, high volumetric power density and rapid start-up times, lending them particular value for use in portable applications. Efficient current collection and interconnection constitute a bottleneck in the progression of the technology. The development of current collector designs and configuration reported in the literature since the inception of the technology are the focus of this study.

Micro-Tubular Solid Oxide Fuel Cell Polarization and Impedance Variation With Thin Porous Samarium-Doped Ceria and Gadolinium-Doped Ceria Buffer Layer Thickness

Abstract Porous buffer layers for anode-supported solid oxide fuel cells (SOFCs) have been investigated for many years with different thicknesses of the buffer layer in each study. In this work, micro-tubular SOFCs having samarium-doped ceria (SDC) and gadolinium-doped ceria (GDC) buffer layers are compared using the current–voltage technique, electrochemical impedance spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The thickness of the porous SDC and GDC buffer layer is investigated systematically with the thickness varying between 0.3 and 2.0 μm. The power density varies between 212 and 1004 mW/cm2 for samples having different SDC buffer layer thickness. Comparable changes occur for the SOFCs with a GDC buffer layer, but less variation in polarization losses resulted. Variation in electrochemical performance varies due to changes in ohmic resistance, cathode activation polarization, and interfacial reactions between the cathode and electrolyte materials.

Development of high-performance anode/electrolyte/cathode micro-tubular solid oxide fuel cell via phase inversion-based co-extrusion/co-sintering technique

A complete set of triple-layer (anode/electrolyte/cathode) hollow fiber for high temperature micro-tubular solid oxide fuel cell (MT-SOFC) consisting of nickel oxide (NiO) – yttria-stabilized zirconia (YSZ)/YSZ/lanthanum strontium manganite (LSM) – YSZ has been successfully fabricated in this study. A simplified fabrication technique of phase inversion-based co-extrusion/co-sintering has yielded a perfectly bounded sandwich structure with free-delamination and defect layers. The effect of co-sintering temperatures (1300 °C–1450 °C) on the morphologies, elemental distributions, electrolyte gas-tightness, mechanical strength, electrochemical performance and the impedance spectra test are well-inspected. The increase of co-sintering temperature has significant effects on the anode finger-like micro-channels shrinkage where the voids become very sharp-thin structure; and developing a thin gas-tight electrolyte layer. Whereas, rapid co-sintering rate (10 °C min−1) and large particle size of 3–5 μm (micron) of YSZ has hindered the formation of fully dense cathode layer resulting from higher co-sintering temperature. Correspondingly, with only 0.1116 Ωcm2 value of area-specific resistance (ASR), a maximum power density has increased from 0.34 W cm−2 to 0.75 W cm−2 with 1.05 V OCV at 700 °C when the co-sintering temperature ranging from 1400 °C to 1450 °C; which comparable with single-layer counterpart.

Investigation of startup, performance and cycling of a residential furnace integrated with micro-tubular flame-assisted fuel cells for micro-combined heat and power

Solid Oxide Fuel Cells (SOFCs) offer advantages for micro-Combined Heat and Power (μCHP), but currently suffer from slow startup (>1 h) and limited thermal cycling which reduces the applications, energy savings and economics. In this work, a micro-Tubular SOFC stack is integrated into a residential furnace to create a micro-Tubular Flame-assisted Fuel Cell (mT-FFC) μCHP system. A high power density of 202 mW cm−2 is reported operating in synthesis gas generated from fuel-rich combustion of natural gas/air. Unlike previous reports, instabilities in the polarization are attributed to low temperature of the oxygen reduction reaction at the cathode. The mT-FFC stack achieved peak power density in 6 min after ignition. 200 thermal cycles at an average heating rate of 215 °C.min−1 and average cooling rate of 176 °C.min−1 were conducted and a low degradation rate of 0.0325 V per 100 cycles per fuel cell was achieved. Low NOx emissions (10 ppm) and high combined efficiency is reported.

Effect of non-solvent from the phase inversion method on the morphology and performance of the anode supported microtubular solid oxide fuel cells

The microstructure of the anode in anode-supported solid oxide fuel cells has significant influence on the cell performance. In this work, microtubular Ni-yttria stabilized zircona (Zr0.8 Y0.2O2, YSZ) anode support has been prepared by the phase inversion method. Different compositions of non-solvent have been used for the fabrication of the Ni-YSZ anode support, and the correlation between non-solvent composition and characteristics of the microstructure of the anode support has been investigated. The presence of ethanol or isopropanol in the non-solvent can inhibit the growth of the finger-like pores in the anode support. With the increase of the concentration of ethanol or isopropanol in the non-solvent, a thin dense layer can be observed on the top of the prepared tubular anode support. In addition, the mechanism of pore formation is explained based on the inter-diffusivity between the solvent and the non-solvent. The prepared microtubular solid oxide fuel cells (MT-SOFCs) have been tested, and the influence of the anode microstructure on the cell electrochemical performance is analyzed based on a polarization model.

A study of the influence of Li-containing additives in microtubular SOFC components based on Gd-doped ceria on the effectiveness of the co-firing method

In this work, the method of co-firing was applied to obtain NiO/GDC – GDC half-cell for MT SOFC. Microtubular anode preforms of NiO/10GDC (60/40 wt%) composition were formed using the phase-inversion technique. The paste of electrolyte material 10GDC was deposited on the anode preforms by the dip-coating method. Some amount of lithium salts (3 wt%) were added into anode material and 10GDC paste, and their effect on the formation of the gas-tight electrolytic layer was investigated. The Li-containing sintering additives were shown to increase shrinkage upon the co-firing process and allow one to lower the temperature of obtaining the gas-tight electrolytic layer by about 150 °C.

Investigation of microcombustion reforming of ethane/air and micro-Tubular Solid Oxide Fuel Cells

Thermal partial oxidation, or fuel-rich combustion, is a non-catalytic option for reforming hydrocarbons to synthesis gas for direct conversion in micro-Tubular Solid Oxide Fuel Cells (SOFCs). A number of studies have demonstrated the potential of using heat recirculation to sustain combustion at high equivalence ratios, where the concentration of synthesis gas can be maximized, but few have connected the fuel-rich combustion reforming to SOFCs to understand how the reforming effects the electrochemical reactions. This study investigates microcombustion of ethane/air at equivalence ratios from 1.0 to 5.0, flow rates of 10–250 mL min−1 and maximum wall temperatures of 800 °C, 900 °C and 1000 °C. The weak flame, flame with repetitive extinction and ignition (FREI) and normal flame regimes are characterized along with the exhaust composition at each condition. Micro-Tubular SOFCs (mT-SOFCs) open circuit voltage, polarization and power density are found to be effected by FREI. High fuel utilization of ~64% is achieved. Long term testing and comparison with a H2 baseline is reported.