Papers by Keyword: Solid Oxide Fuel Cell (SOFC)

Abstract: Fundamental studies on durability and reliability of SOFC modules/stacks had started as one of the new NEDO project from FY2005 and the organization had been partly modified at FY2008 and FY2013[1]. The main target is to make sure the long term durability within 10 % of degradation rate in the range of 40,000 h (5 years), and reliability during 250 times of starting up and ending operations. In this project, the effect of impurities on degradation of SOFCs was examined for 5 different stacks/modules (Kyocera, Tokyo gas, TOTO, MMC-KEPCO and MHI) at AIST. During the operation of SOFC stacks and modules, several kinds of impurities are carried continuously with air and fuels and react with cell components. The concentration of impurity in cell components was carefully investigated before and after long term operation by using a secondary ion mass spectrometer (SIMS) as a main analytical tool with conventional analytical techniques such as a scanning electron microscopy (SEM) and an electron probe micro analyzer (EPMA). Many approaches were carried out to investigate the effect of gaseous impurities as listed below. (1)The concentration of impurity in cell components was carefully investigated before and after long term operation. (2)Chemical reactivity between impurities and cell components are examined from the view point of thermodynamics. (3)In order to understand the mechanism of the degradation due to impurities, the effect of impurities on degradation of laboratory scale button cell were examined. (4)Cation diffusion were carefully investigated in some diffusion couples to clear the mechanism on formation of secondary phases between the cathode and electrolyte. (5)Acceleration tests on degradation by impurities were also carried out by using a button cell to predict the degradation rate depending on the contamination of impurities in the cathode. The results of the project is partly introduced in this report.

Abstract: The influence of residual stress, thermal stress and chemically induced expansion stress etc... on the fracture damage of solid oxide fuel cells (SOFCs) were investigated by using nondestructive testing method and numerical stress-strain analyses under operating conditions. In order to estimate stress-deformation behavior of cell/stack of SOFCs, mechanical properties of SOFC elements were evaluated under controlled high temperature and oxygen partial pressure conditions. In addition to deformation and mechanical damage behavior were observed by using acoustic emission method.

Abstract: A micro scale model of a solid oxide fuel cell (SOFC) involving the mass transfer together with the electrochemical reaction, the electron and ion transports through respective cylindrical shaped electron-and ion-conducting particles inside the electrodes was mathematically developed. The predicted cell performance was showed according to the operating and design condition. The effects of micro-scale electrode geometry on the cell performance were also taken into account. This present study reveals the working mechanisms of SOFC at the micro-scale level, while demonstrating the use of micro-scale relations to enhance the SOFC performance. The accuracy of the presented model was validated by comparing to already existing experimental results from the available literatures.

Abstract: The effect of Sr doped La_4-xSr_xNi₃O_10±δ (x = 0, 0.05, 0.1, 0.2 and 0.3) has been investigated as an intermediate temperature SOFC cathode material. The Ruddlesden-Popper (RP) compositions of n = 3 were successfully synthesized via citrate gel method. The single phase of sintered La_4-xSr_xNi₃O_10±δ (x = 0, 0.05, 0.1 and 0.2) powders was confirmed as an orthorhombic structure. However, this structure changes to more symmetry with an increasing amount of Sr dopant. With x = 0.3, the phases of La_1.7Sr_0.3NiO₄ as n = 1 RP and NiO appear without n = 3 RP. La_3.95Sr_0.05Ni₃O_10±δ shows the highest electrical conductivity with a value of 140 S/cm at room temperature. Nevertheless, the electrical conductivity of La_4-xSr_xNi₃O_10±δ decreases and further decreases with increasing amount of Sr. The conductivity values at 200°C of La_4-xSr_xNi₃O_10±δ (x =0, 0.05 and 0.1) are 89, 113 and 101 S/cm, respectively. For x = 0.3, the conductivity decreases lower than x 0.1 with its value of 36 S/cm because of the low conductivity phase of n = 1 RP.

Abstract: Ba_0.2Sr_0.8Co_0.9Nb_0.1O_3-δ (BSCN0.2)-xGd_0.1Ce_0.9O_1.95 (GDC) (x = 10, 20, 30 and 40 wt.%) composite cathodes were investigated for the potential application in the IT-SOFCs. The results of chemical compatibility measurement show that a small number of Gd and/or Ce ions may melt into the lattice of BSCN0.2 to form BSCN0.2-GDC solid solution. Thermal expansion coefficients effectively reduced by the incorporation of GDC. The electrochemical performance of BSCN0.2-xGDC composite cathodes increased with increasing x from 10 to 30 wt.%. When x = 30 wt.%, the area specific resistances were only 0.040 and 0.017 Ω cm² at 750 and 800^oC, respectively. This improved electrochemical performance is attributed to the good thermal expansion match between BSCN0.2-xGDC composite cathode and GDC electrolyte, and the increased oxygen vacancy concentration. With further increasing x, the electrochemical performance of the composite cathode decreased. This result may be due to the ambipolar resistance model of porous composite cathode and the poor electrical conductivity of BSCN-40GDC. The maximum power densities of a BSCN0.2-30GDC/La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ/NiO-Sm_0.2Ce_0.8O_1.9 single-cell achieve 537 and 722 mW cm^-2 at 750 ^oC and 800^oC, respectively. These results indicate that the BSCN0.2-30GDC composite cathode is a promising candidate for IT-SOFC.

Abstract: La_4-xSr_xNi₃O_10±δ (x = 0, 0.05, 0.1, 0.2, 0.3, 0.5 and 1) compositions synthesized via citrate gel method have been investigated as a candidate cathode for intermediate temperature SOFC. The n = 3 RP single phase of La_4-xSr_xNi₃O_10±δ (x = 0, 0.05, 0.1 and 0.2) can be achieved after calcined at 1000°C for 4 hrs in air. The further addition of Sr with x 0.3 leads to obtain La_2-xSr_xNiO_4±δ and NiO as the second phase. La₄Ni₃O_10±δ based material shows the highest electrical conductivity with a value of 124 S/cm at room temperature. With Sr dopant, both of transition temperature and the electrical conductivity of La₄Ni₃O_10±δ decrease and further decrease with increasing the amount of Sr. The electrical conductivity at 250°C of La_4-xSr_xNi₃O_10±δ (x = 0, 0.05 and 0.1) is 100, 94 and 75 S/cm respectively. For x = 0.3, 0.5 and 1, it reduces to the values of 28, 7 and 4 S/cm, respectively because of the lower conductivity phase of La_2-xSr_xNiO_4±δ.

Abstract: As solid oxide fuel cell (SOFC) has operating temperatures ranging between 973 K for intermediate temperature operation and 1273 K for high temperature operation, an advantage of the hot exhaust gas from SOFC can be used to drive a fuel processor for hydrogen production. In this study, the heat integration of a SOFC integrated with ethanol steam reformer, which is very highly endothermic reaction needed the large amount of energy supply, has been performed to improve the efficiency of SOFC system. In the conceptual design for heat integration, the pinch analysis is used. Under 1200 K of SOFC operating temperature and 973 K of reformer temperature, the hot exhaust gas leaving the SOFC is sufficient for heating requirements for the heat exchanger network and for the additional electricity generation from gas turbine. An energy integrated SOFC system presents a total electricity generation from SOFC and GT of 818 kW of which 386 kW is required for air compressor so an overall electricity production and efficiency are 432 kW and 35.0%, respectively.

Abstract: Solid oxide fuel cell (SOFC) is well known as power and heat generation device which converts chemical energy directly from fuel into electricity. SOFC operate at high temperature becomes obstacle for SOFC which reducing ionic conductivity material of current electrolyte, reduce lifetime of cell components, high fabrication cost, limited durability and performance issues. This introduce to environment pollution and decrease the SOFC lifetime. The fabrication of durability and stability composite cathode are comprised from mixing of perovskite La₀.₆Sr₀.₄CO₀.₂Fe₀.₈ (LSCF) powders with nanoscale ionically conducting ceramic electrolyte materials, SDC-carbonate (SDCc) was overcome this problems. Powder preparation and composite cathode fabrication must consider which as main factors in the development of durability and stability of LSCF-SDCc composite cathode. Powders must in nanoscale to enhance the conductivity and decrease the interfacial polarization resistance and the composite cathode should in nanoporous morphology for achieve high power density over than 500 h and remarkable durability. Calcination also plays in important role and its operations will effects to the SOFC durability and performance. The necessary to prolong the lifetime and increase the SOFC performance has lead to development of durability and stability of SOFC. This paper reviews the durability and stability of the composite cathode and focus on the challenges in material technology.

Abstract: Solid oxide fuel cells (SOFC) are the current research having several potential to obtain high efficiency, high energy–density power generation which operated at relatively higher temperature. Yttrium oxide (Y₂O₃) contributions at high temperature are accelerating to the development oxide layer of FeCr alloy. The aim of this research is to investigate the microstructure of Fe/Cr added with Y₂O₃ acting as a reactive element. The purpose is to improve macrostructure of Fe/Cr powders which can be applied at steel industry. In this study the mixing process of Fe/Cr and Y₂O₃ powder was conducted via ultrasonic treatment at a frequency of 22 kHz, and at two different holding time of 2.5 h and 3.5 h. The particle size of chromium (Cr) can be reduced by ultrasonic treatment at from 60µm to 30µm through threshing the cluster of Cr particle. It shows that the ultrasonic vibration effectively removes oxides and other contaminates on a surface coating. Therefore, homogeneity of the parent material, segregation, and uniform distribution of second phase were increased.

Abstract: The correlation between calcination temperature and properties (physical and electrochemical) of composite cathodes comprising lanthanum strontium cobaltite ferrite (LSCF) with samarium-doped ceria carbonate (SDCC) has been investigated. LSCF-SDC carbonate (LSCF-SDCC) composite cathode powders prepared via ball-milling were calcined at various temperatures in the range of 700850 °C. X-ray diffraction (XRD) results confirmed that the applied calcination temperatures do not affect the chemical compatibility and the LSCF perovskite cubic structure of the composite powders. FTIR spectra verified the presence of carbonates in the composite powders after calcination. The increment of the calcination temperature reduced the surface area of the particle from 10.9 m²/g to 6.5 m²/g. The electrochemical results revealed that the resistance of LSCF-SDC carbonate composite cathodes is dominated by the oxygen surface exchange reaction at the electrode surface. 750 °C was identified as the most appropriate calcination temperature for the LSCF-SDC carbonate powder when the cathode electrode showed the lowest resistance with conductivity value of 0.95 x 10^-3 Scm^-1. The findings are of potential relevance to utilizing the LSCF-SDC carbonate cathodes for low temperature solid oxide fuel cells (LT-SOFC).