The properties and performance of micro-tubular (less than 2.0 mm O.D.) anode suported solid oxide fuel cell (SOFC)

Tubular solid oxide fuel cells (SOFCs) have many desirable advantages compared to other SOFC applications. Recently, micro-tubular SOFCs were studied to apply them into APU systems for future vehicles. In this study, electrochemical properties of the micro-tubular SOFCs (1.6 mm O.D.) have been characterized. Electrochemical analysis showed excellent performance with a maximum power density of 1.3 W/cm² at 550 °C. The impedance information gained at cell operating temperatures of 450, 500, and 550 °C showed individual cell ohmic resistances of 1.0, 0.6, and 0.2 Ω respectively. Within the operating temperature range of 450-550 °C, the ceria based micro-tubular SOFCs (cathode length: 8 mm) were found to have power densities ranging between 0.263 and 1.310 W/cm². The mechanical properties of the tubes were also analyzed through internal burst testing and monotonic compressive loading on a c-ring test specimen. The two testing techniques are compared and related, and maximum hoop stress values are reported for each of the fabrication parameters. This study showed feasible electrochemical properties and mechanical strength of micro-tubular SOFC for APU applications.     

Development of cube-type SOFC stacks using anode-supported tubular cells

Tubular SOFCs have shown many desirable characteristics such as high thermal stability during rapid heat cycling and large electrode area per unit volume, which can accelerate to realize SOFC systems applicable to portable devices and auxiliary power units for automobile. So far, we have developed anode-supported tubular SOFCs with 0.8–2 mm diameter using Gd-doped CeO₂ (GDC) electrolyte, NiO-GDC anode and (La, Sr)(Co, Fe)O₃ (LSCF)-GDC cathode. In this study, a newly developed cube-type SOFC stack which consists of three SOFC bundles was designed and examined. The bundle consists of three 2 mm diameter tubular SOFCs and a rectangular shaped cathode support where these tubular cells are arranged in parallel. The performance of the stack whose volume is less than 1 cm³ was shown to be 2.8 V OCV and over 1 W at 1.6 V under 500 °C. Cathode loss factor due to current collection from cathode matrix was also estimated using a proposed model.     

Electrochemical characterizations of microtubular solid oxide fuel cells under a long-term testing at intermediate temperature operation

We report the long-term stability of a microtubular solid oxide fuel cell (SOFC) operable at ∼500 °C. The SOFC consists of NiO-Gd doped ceria (GDC) as the anode as well as the tubular support, GDC as an electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)-GDC as the cathode. A single tubular cell with a diameter of approximately 1.8 mm and an effective electrode length of approximately 20 mm generated 150 mW cm⁻² and 340 mW cm⁻² at 500 °C and 550 °C, respectively, under the operation conditions of 0.7 V and humidified H₂ fuel flow. The cell exhibited good stability with a degradation rate of 0.25%/100 h under operation conditions of 200 mA and 0.75 V.     

Development of LSCF–GDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte

La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) powder was prepared by glycine–nitrate combustion method. The electrochemical properties of porous LSCF cathodes and LSCF–Gd_0.1Ce_0.9O_1.95 (GDC) composite cathodes were evaluated at intermediate/low temperatures of 500–700 °C. The polarization resistance of pure LSCF cathode sintered at 975 °C for 2 h was 1.20 Ω cm² at 600 °C. The good performance of pure LSCF cathode is attributed to its unique microstructure—small grain size, high porosity and large surface area. The addition of GDC to LSCF cathode further reduced the polarization resistance. The lowest polarization resistance of 0.17 Ω cm² was achieved at 600 °C for LSCF–GDC (40:60 wt%) composite cathode. An anode-supported solid oxide fuel cell (SOFC) was prepared using LSCF–GDC (40:60 wt%) composite as cathode, GDC film (49-μm-thick) as electrolyte, and Ni–GDC (65:35 wt%) as anode. The total electrode polarization resistance was 0.27 Ω cm² at 600 °C, which implies that LSCF–GDC (40:60 wt%) composite cathode used in the anode-supported SOFC had a polarization resistance lower than 0.27 Ω cm² at 600 °C. The cell generated good performance with the maximum power density of 562, 422, 257 and 139 mW/cm² at 650, 600, 550 and 500 °C, respectively.     

Fabrication and performance of gadolinia-doped ceria-based intermediate-temperature solid oxide fuel cells

Anode-supported solid oxide fuel cells (SOFC) based on gadolinia-doped ceria (GDC) are developed in this study. A carbonate co-precipitation method is used to synthesize the nano-sized GDC powders. A dense GDC electrolyte thin film supported by a Ni–GDC porous anode is fabricated by dry-pressing and spin-coating processes, respectively. In comparison with dry pressing, it is easy to prepare a thinner electrolyte film by the novel spin-coating method. Cell performance is examined using humidified (3% H₂O) hydrogen as fuel and air as oxidant in the temperature range of 500–700 °C. Cell performance is strongly dependent on the electrolyte thickness. With a porous Ni–GDC anode, a dense 19-μm GDC electrolyte film and a porous La_0.6Sr_0.4Co_0.2Fe_0.8O₃–GDC cathode, the cell exhibits maximum power densities of 130, 253, 386 and 492 mW cm⁻² at 500, 550, 600 and 650 °C, respectively. It is also found that at the low operating temperature about 500 °C, the cell resistance is significantly dominated by the electrode polarization resistance.     

Characterisation of electrical performance of anode supported micro-tubular solid oxide fuel cell with methane fuel

The reduction and operation of Ni–YSZ anode-supported tubular cells on methane fuel is described. Cells were reduced on pure methane from 650 °C to 850 °C, varying reduction time and methane flow rate. The effect on electrochemical performance with methane fuel was then investigated at 850 °C after which temperature-programmed oxidation (TPO) was employed to measure carbon deposition. Results showed that carbon deposition was minimized after certain reduction conditions. The conclusion was that 30 min reduction at 650 °C with 10 ml min⁻¹ methane reduction flow rate led to the highest current output over 1.2 A cm⁻² at 0.5 V when the cell operated at 850 °C between 10 ml min⁻¹ and 12.5 ml min⁻¹ methane running flow rate. From these results, it is evident that solid oxide fuel cell (SOFC) performance can be substantially improved by optimising preparation, reduction and operating conditions without the need for hydrogen.     

Silver-perovskite composite SOFC cathodes processed via mechanofusion

Atomized silver spheres (≈20–50 μm diameter) were coated with 1 μm thick layers of (La_0.6Sr_0.4)_0.98Co_0.2Fe_0.8O₃ or Sm_0.5Sr_0.5CoO₃ via a mechanofusion dry processing method. The materials were subsequently assessed as solid oxide fuel cell cathodes on anode-supported YSZ electrolytes at 650–750 °C. The materials were subject to significant electrochemical conditioning during initial cell operation, and factors, such as temperature and operating voltage, affecting the conditioning rate are discussed. Post-conditioned power densities (at 0.7 V) were typically 550–650, 400–450 and 300–350 mW cm⁻² at 750, 700 and 650 °C, respectively. Though power degradation rates of ≈7.5 and 4.5% (per 1000 h) were observed at 750 and 700 °C, respectively, no degradation was detected over almost 2000 h of testing at 650 °C.     

Current collecting efficiency of micro tubular SOFCs

In this study, current collecting efficiency of the micro tubular solid oxide fuel cell (SOFC) was estimated to determine optimum size of the micro tubular SOFC. Two models for collecting current from single terminal (ST) and double terminal (DT) of anode tube were proposed and used to calculate the current collecting efficiency as functions of anode thickness, tube length and operating temperature. It was shown that design of the cell geometry and current correcting method are significantly important to achieve high performance micro tubular SOFC stacks. The efficiency loss estimated from the DT model was about 2–4-fold lower than those of obtained from the ST model. The DT model was shown to be more effective for higher operating temperature and the tube length.     

New insights in the polarization resistance of anode-supported solid oxide fuel cells with La0.6Sr0.4Co0.2Fe0.8O3 cathodes

In this study, the polarization resistance of anode-supported solid oxide fuel cells (SOFC) with La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) cathodes was investigated by I-V sweep and electrochemical impedance spectroscopy under a series of operating voltages and cathode environments (i.e. stagnant air, flowing air, and flowing oxygen) at temperatures from 550 °C to 750 °C. In flowing oxygen, the polarization resistance of the fuel cell decreased considerably with the applied current density. A linear relationship was observed between the ohmic-free over-potential and the logarithm of the current density of the fuel cell at all the measuring temperatures. In stagnant or flowing air, an arc related to the molecular oxygen diffusion through the majority species (molecular nitrogen) present in the pores of the cathode was identified at high temperatures and high current densities. The magnitude of this arc increased linearly with the applied current density due to the decreased oxygen partial pressure at the interface of the cathode and the electrolyte. It is found that the performance of the fuel cell in air is mainly determined by the oxygen diffusion process. Elimination of this process by flowing pure oxygen to the cathode improved the cell performance significantly. At 750 °C, for a fuel cell with a laser-deposited Sm_0.2Ce_0.8O_1.9 (SDC) interlayer, an extraordinarily high power density of 2.6 W cm⁻² at 0.7 V was achieved in flowing oxygen, as a result of reduced ohmic and polarization resistance of the fuel cell, which were 0.06 Ω cm² and 0.03 Ω cm², respectively. The results indicate that microstructural optimization of the LSCF cathode or adoption of a new cell design which can mitigate the oxygen diffusion limitation in the cathode might enhance cell performance significantly.     

Fabrication and characterization of micro tubular SOFCs for operation in the intermediate temperature

Tubular SOFC systems appear to be well-suited to accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. Our goal is to develop innovative processing method to fabricate new micro tubular SOFCs with sub-millimeter diameter and its stack module which enable to generate high volumetric power density. In this study, micro tubular SOFCs under 1 mm diameter have been successfully fabricated and tested in the intermediate temperature region (550 °C or under). The cell consists of NiO–Gd doped ceria (GDC) as an anode (support tube), GDC as an electrolyte and (La, Sr)(Fe, Co)O₃ (LSCF)–GDC as a cathode. The single tubular cell with 0.8 mm diameter and 12 mm length generated over 70 mW at 550 °C with H₂ fuel, which indicates that the cell generated over 0.3 W cm⁻² at 550 °C.     

Fabrication and evaluation of Ni-GDC composite anode prepared by aqueous-based tape casting method for low-temperature solid oxide fuel cell

Large-size, 8 cm × 8 cm, NiO-Gd_0.1Ce_0.9O_1.95 (Ni-GDC) composite anodes have been successfully fabricated by aqueous-based tape casting method for anode-supported solid oxide fuel cell (SOFC). The pre-sintered anode green tape was coated with a GDC electrolyte film by spray coating method and then co-sintered together to obtain electrolyte/anode bi-layer. The cathode, which is made of La_0.8Sr_0.2Co_0.2Fe_0.8O₃-GDC (LSCF-GDC) was screen printed onto the electrolyte film and sintered to form a complete anode-supported SOFC. The performance of the cell was evaluated on an in-house developed test station between 500 and 650 °C. Due to the limitation of the test station for large-cell testing, small-size samples with dimensions of 2.5 cm × 2.5 cm were cut out from the large-cell. For the single cell with humidified hydrogen as fuel and air as oxidant, the maximum power density achieved 909, 623, 335 and 168 mW cm⁻² at 650, 600, 550 and 500 °C, respectively. Impedance analysis confirmed that the performance of single cells below 600 °C was retarded primarily due to the slow interfacial reaction kinetics at reduced temperatures. Development of catalytically active electrode materials, especially the cathode material and improvement of the electrode microstructure are thus crucial for achieving a high performance low-temperature SOFC.     

Direct liquid methanol-fueled solid oxide fuel cell

Anode coking problem of solid oxide fuel cell (SOFC) when using hydrocarbon fuels has been the major barrier for the practice and commercialization of well-developed high performance SOFC. In this work, based on fuels consideration, we chose liquid methanol as the candidate fuel for SOFC with the configuration of NiO/SDC–SDC–SSC/SDC. For comparison, traditional fuels, hydrogen and ammonia, were tested. With methanol as fuel, the maximum power densities were 698, 430 and 223 mW cm⁻² at 650, 600 and 550 °C, respectively, which were higher than that with ammonia and lower than that of hydrogen. The electrochemical properties of the cells with the three fuels were investigated by AC impedance spectroscopy. The long-term stability of the cell with methanol, methane and ethanol were also studied at a constant output voltage of 0.5 V.     

Low temperature solid oxide fuel cells with pulsed laser deposited bi-layer electrolyte

Solid oxide fuel cells (SOFC) using a pulsed laser deposited bi-layer electrolyte have been successfully fabricated and have shown very good performance at low operating temperatures. The cell reaches power densities of 0.5 W cm⁻² at 550 °C and 0.9 W cm⁻² at 600 °C, with open circuit voltage (OCV) values larger than 1.04 V. The bi-layer electrolyte contains a 6–7 μm thick samarium-doped ceria (SDC) layer deposited over a ∼1 μm thick scandium-stabilized zirconia (ScSZ) layer. The electrical leaking between the anode and cathode through the SDC electrolyte, which due to the reduction of Ce⁴⁺ to Ce³⁺ in reducing environment when using a single layer SDC electrolyte, has been eliminated by adopting the bi-layer electrolyte concept. Both ScSZ and SDC layers in the bi-layer electrolyte prepared by the pulsed laser deposition (PLD) technique are the highly conductive cubic phases. Poor conductive (Zr, Ce)O₂-based solid solutions or β-phase ScSZ were not found in the bi-layer electrolyte prepared by the PLD due to low processing temperatures of the technique. Excellent reliability and flexibility of the PLD technique makes it a very promising technique for the fabrication of thin electrolyte layer for SOFCs operating at reduced temperatures.     

An intermediate-temperature direct ammonia fuel cell with a molten alkaline hydroxide electrolyte

This paper describes the development and testing of a direct ammonia fuel cell utilizing a molten alkaline hydroxide electrolyte at temperatures between 200 and 450 °C. The advantages of a molten hydroxide fuel cell include the use of a highly conductive and very low-cost electrolyte, inexpensive base metal electrocatalysts, a wide operating temperature range, fuel flexibility, and fast electrode kinetics. The direct use of ammonia in such a fuel cell, even at temperatures as low as 200 °C, is made possible due to the very chemically aggressive nature of the melt. A test cell was constructed using a KOH–NaOH eutectic mixture and produced approximately 40 mW cm⁻² of power at 450 °C while operating on a stream of pure ammonia fed to the anode and compressed ambient air fed to the cathode.     

A novel design and performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack

A novel design of cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack is presented in this paper. The cone-shaped tubular anode substrates are fabricated by slip casting technique and the yttria-stabilized zirconia (YSZ) electrolyte films are deposited onto the anode tubes by dip coating method. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 7 μm is successfully obtained. The single cell, NiO-YSZ/YSZ (7 μm)/LSM-YSZ, provides a maximum power density of 1.78 W cm⁻² at 800 °C, using moist hydrogen (75 ml min⁻¹) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC is fabricated. Its typical operating characteristics are investigated, particularly with respect to the thermal cycling test. The results show that the two-cell-stack has good thermo-mechanical properties and that the developed segmented-in-series SOFC stack is highly promising for portable applications.     

Novel fabrication technique of hollow fibre support for micro-tubular solid oxide fuel cells

In this work, a cerium-gadolinium oxide (CGO)/nickel (Ni)-CGO hollow fibre (HF) for micro-tubular solid oxide fuel cells (SOFCs), which consists of a fully gas-tight outer electrolyte layer supported on a porous inner composite anode layer, has been developed via a novel single-step co-extrusion/co-sintering technique, followed by an easy reduction process. After depositing a multi-layers cathode layer and applying current collectors on both anode and cathode, a micro-tubular SOFC is developed with the maximum power densities of 440-1000 W m⁻² at 450-580 °C. Efforts have been made in enhancing the performance of the cell by reducing the co-sintering temperature and improving the cathode layer and current collection from inner (anode) wall. The improved cell produces maximum power densities of 3400-6800 W m⁻² at 550-600 °C, almost fivefold higher than the previous cell. Further improvement has been carried out by reducing thickness of the electrolyte layer. Uniform and defect-free outer electrolyte layer as thin as 10 μm can be achieved when the extrusion rate of the outer layer is controlled. The highest power output of 11,100 W m⁻² is obtained for the cell of 10 μm electrolyte layer at 600 °C. This result further highlights the potential of co-extrusion technique in producing high quality dual-layer HF support for micro-tubular SOFC.     

Fabrication,microstructure and properties of a YSZ electrolyte for SOFCs

Yttria stabilized zirconia (YSZ) has widely been used as an electrolyte in solid oxide fuel cell (SOFC) stacks. The microstructure and properties of YSZ related to the fabrication process are discussed in this paper. For the named two-step sintering process, uniform and hexagonal grains with a size of 1–4 μm were obtained from the adobe following tape calendaring (TCL). Elliptical and hexagonal grains with a size of 0.4–3 μm were obtained from the adobe of tape casting (TCS) using the three-step process. The electrical conductivities of YSZ with different grain sizes were measured via the four-probe DC technique and grain conductivities and grain boundary conductivities of YSZ were investigated by impedance spectroscopy. YSZ electrolytes with a grain size of 0.1–0.4 μm had the highest electrical conductivity in the range of 500–1000 °C, especially at medium and low temperatures 550–800 °C. As the YSZ grain size becomes small, the thickness of the intergranular region decreased greatly. The YSZ electrolytes with sub-micrometer grain sizes, high ion conductivity and low sintering temperatures are important to the electrode-supported SOFC, on which the dense YSZ electrolyte films are optimized at 10 μm.     

Reducing the operation temperature of a solid oxide fuel cell using a conventional nickel-based cermet anode on dimethyl ether fuel through internal partial oxidation

Dimethyl ether (DME)-oxygen mixture as the fuel of an anode-supported SOFC with a conventional nickel-cermet anode for operating at reduced temperatures is systematically investigated. The results of the catalytic tests indicate that sintered Ni-YSZ has high activity for DME partial oxidation, and DME conversion exceeds 90% at temperatures higher than 700 °C. Maximum methane selectivity is reached at 700 °C. Cell performance is observed between 600 and 800 °C. Peak power densities of approximately 400 and 1400 mW cm⁻² at 600 and 800 °C, respectively, are reached for the cell operating on DME-O₂ mixture. These values are comparable to those obtained using hydrogen as a fuel, and cell performance is reasonably stable at 700 °C for a test period of 340 min. SEM results demonstrate that the cell maintains good geometric integrity without any delimitation of respective layer after the stability test, and EDX results show that carbon deposition occurrs only at the outer surface of the anode. O₂-TPO analysis shows that carbon deposition over the Ni-YSZ operating on DME is greatly suppressed in the presence of oxygen. Internal partial oxidation may be a practical way to achieve high cell performance at intermediate-temperatures for SOFCs operating on DME fuel.     

The interaction of biomass gasification syngas components with tar in a solid oxide fuel cell and operational conditions to mitigate carbon deposition on nickel-gadolinium doped ceria anodes

The combination of biomass gasification with solid oxide fuel cells (SOFCs) is gaining increasing interest as an efficient and environmentally benign method of producing electricity and heat. However, tars in the gas stream arising from the gasification of biomass material can deposit carbon on the SOFC anode, having detrimental effects to the life cycle and operational characteristics of the fuel cell. This work examines the impact of biomass gasification syngas components combined with benzene as a model tar, on carbon formation on Ni/CGO (gadolinium-doped ceria) SOFC anodes. Thermodynamic calculations suggest that SOFCs operating at temperatures > 750 °C are not susceptible to carbon deposition from a typical biomass gasification syngas containing 15 g m⁻³ benzene.However, intermediate temperature SOFCs operating at temperatures < 650 °C require threshold current densities well above what is technologically achievable to inhibit the effects of carbon deposition. SOFC anodes have been shown to withstand tar levels of 2-15 g m⁻³ benzene at 765 °C for 3 h at a current density of 300 mA cm⁻², with negligible impact on the electrochemical performance of the anode. Furthermore, no carbon could be detected on the anode at this current density when benzene levels were <5 g m⁻³.     

Effects of operating conditions on the performance of a micro-tubular solid oxide fuel cell (SOFC)

A parametric analysis is carried out to study the effects of the operating conditions on the performance and operation of a micro-tubular solid oxide fuel cell. The computational fluid dynamics model incorporates mass, momentum, species and energy balances along with ionic and electronic charge transfers. Effects of temperature, fuel flow rate, fuel composition, anode pressure and cathode pressure on fuel cell performance are investigated. Polarization curves are compared to allow an understanding of the effects of different operating conditions on the performance of the fuel cell. Effects of anode flow rate on fuel cell efficiency and fuel utilization are also investigated. Moreover, influence of operating temperature on the internal electronic current leaks is outlined. Temperature distributions, current density profiles and hydrogen mole fraction profiles are also utilized to have a better understanding of the spatial effects of operating parameters. It is predicted that at 550 °C, for an output current demand of 0.53 A cm⁻², fuel cell needs to generate 0.65 A cm⁻² ionic current density where the difference in these values is attributed to internal current leaks. On the other hand for temperatures lower than 500 °C, the effect of electronic leakage currents are not significant.