Solid Oxide Fuel Cells Operating Conditions Research Articles

고체산화물 연료전지용 삼중 전도성 산화물 환원극: 리뷰

Solid oxide fuel cells (SOFCs) are at a technological level close to commercialization, which could be enabled by new material research. Especially, not only an electrolyte, but also a cathode material becomes very important to further increase electrochemical performance, due to the effort to lower operating temperature of SOFCs to intermediate range (400-600℃) to take advantage of high and low temperature operation. Unfortunately, this trend inevitably results in demand for new cathode materials with high oxygen reduction reaction activity, as well as high mechanical durability. Recently, ceramic materials which conduct oxygen ion, proton, and electron, thereby called ‘triple conducting oxide’ are being highlighted, due to their excellent material properties, to be used for cathodes of SOFCs. This paper reviews the three representative triple-conducting oxides, which were already used and tested in SOFC operating conditions.

In Situ Formation of Er0.4Bi1.6O3 Protective Layer at Cobaltite Cathode/Y2O3-ZrO2 Electrolyte Interface under Solid Oxide Fuel Cell Operation Conditions.

Bismuth-based oxides exhibit outstanding oxygen ionic conductivity and fast oxygen surface kinetics and have shown great potential as a highly active component for electrode materials in solid oxide fuel cells (SOFCs). Herein, a Nb-doped La0.6Sr0.4Co0.2Fe0.7Nb0.1O3-δ (LSCFNb) electrode with 40% Er0.4Bi1.6O3 (ESB) composite electrode was successfully fabricated by decoration method and directly assembled on barrier-layer-free yttrium-stabilized zirconia (YSZ) electrolyte cells, achieving a peak power density of 1.32 W cm-2 and excellent stability at 750 °C and 250 mA cm-2 for 100 h. ESB decoration also significantly reduces the activation energy from 214 kJ mol-1 for the O2 reduction on pristine LSCFNb electrode to 98 kJ mol-1. Further microstructural analysis reveals that there is a redistribution and migration of the ESB phase in the ESB-LSCFNb composite toward the YSZ electrolyte under the influence of cathodic polarization, forming a thin ESB layer at the cathode/YSZ electrolyte interface. The in situ formed ESB layer not only prevents the direct contact and subsequent reaction between segregated SrO and YSZ electrolytes, but also remarkably promotes the oxygen migration/diffusion at the interface for O2 reduction reaction, resulting in a remarkable increase in power output and a decrease in activation energy. The present study clearly demonstrated the in situ formation of a highly functional and active ESB protective layer at LSCFNb cobaltite cathode and YSZ electrolyte interface via ESB-decorated LSCFNb composite cathode under SOFC operation conditions.

The Chemical Evolution of the La0.6Sr0.4CoO3\u2212\u03b4 Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity

Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.

Interface formation and Mn segregation of directly assembled La0.8Sr0.2MnO3 cathode on Y2O3-ZrO2 and Gd2O3-CeO2 electrolytes of solid oxide fuel cells

The establishment of intimate electrode/electrolyte interface is very important in solid oxide fuel cells (SOFCs), because it plays a critical role in the overall cell performance and durability. In this study, Mn segregation and interface formation between directly assembled La0.8Sr0.2MnO3 (LSM) electrode and yttrium-stabilized zirconia (YSZ) or gadolinium-doped ceria (GDC) electrolytes are studied using combined focused ion beam and scanning transmission electron microscopy (FIB-STEM). In the case of LSM/YSZ and LSM/GDC electrodes, a significant reduction in the electrode ohmic resistance is observed after cathodic polarization at 900 °C and 500 mA cm−2, indicating the formation of an intimate interface. However, LSM particles start to disintegrate at the electrode/electrolyte interface with the increase of polarization time in the case of LSM/YSZ electrode. On the other hand, the LSM/GDC interface is very stable with negligible microstructure change at the interface. Mn segregation from the LSM perovskite structure is identified under the influence of polarization in both LSM/YSZ and LSM/GDC electrodes. The results demonstrate that nature of the electrolyte plays a critical role in the electrochemical activity, microstructure, morphology and stability of LSM/electrolyte interface under SOFC operation conditions.

Electrochemical and Operando Spectroscopic Studies of Sr2Fe1.5Mo0.5O6‐δ Anode Catalysts in Solid Oxide Fuel Cells Operating with Direct Alcohol Fuels

AbstractA combination of operando Raman spectroscopy and chronoamperometry was used to examine the carbon tolerance of Sr2Fe1.5Mo0.5O6‐δ (SFMO) electrode catalysts when operating with direct methanol and ethanol fuels in solid oxide fuel cells (SOFCs). Chronoamperometry studies revealed that these devices could maintain a steady power density output under typical SOFC operating conditions. High‐temperature Raman measurements of SFMO coupons exposed to methanol and ethanol (and their gas phase pyrolysis products) showed the presence of spectroscopic features associated with ordered and disordered forms of graphitic carbon. However, once SFMO was employed as an anode in an electrolyte‐supported SOFC, the graphite features disappear implying that these materials are not susceptible to carbon accumulation in functioning devices. These electrochemical and operando Raman measurements provided insight into SFMO's ability to act as an effective anode catalyst for SOFCs operating with direct alcohol fuel sources.

Material Discovery and Design Principles for Stable, High Activity Perovskite Cathodes for Solid Oxide Fuel Cells

AbstractCritical to the development of improved solid oxide fuel cell (SOFC) technology are novel compounds with high oxygen reduction reaction (ORR) catalytic activity and robust stability under cathode operating conditions. Approximately 2145 distinct perovskite compositions are screened for potential use as high activity, stable SOFC cathodes, and it is verified that the screening methodology qualitatively reproduces the experimental activity, stability, and conduction properties of well‐studied cathode materials. The calculated oxygen p‐band center is used as a first principle‐based descriptor of the surface exchange coefficient (k*), which in turn correlates with cathode ORR activity. Convex hull analysis is used under operating conditions in the presence of oxygen, hydrogen, and water vapor to determine thermodynamic stability. This search has yielded 52 potential cathode materials with good predicted stability in typical SOFC operating conditions and predicted k* on par with leading ORR perovskite catalysts. The established trends in predicted k* and stability are used to suggest methods of improving the performance of known promising compounds. The material design strategies and new materials discovered in the computational search help enable the development of high activity, stable compounds for use in future solid oxide fuel cells and related applications.

Macro-level modeling of solid oxide fuel cells, approaches, and assumptions revisited

Fuel cells are considered as major components of the future energy infrastructure in many applications due to their high efficiency, simplicity of operation, scalability, and low pollution. The inherent complexity of the internal operation of fuel cells and limitations in the experimental studies on fuel cells make the numerical simulation a vital tool for the fuel cell research and development. This field has experienced remarkable advancements in the past three decades. A wide variety of modeling approaches have been introduced in the literature. Many mathematical formulations have been employed for the macro-level modeling of solid oxide fuel cells (SOFCs). In this paper, the common fundamental bases behind different modeling approaches are identified and presented. Similarly, many assumptions have been used to simplify the modeling process. Some of the most common assumptions for modeling SOFCs are identified, and their appropriateness is reevaluated in the light of recent advancements in the experimental and numerical findings. It was found that while 0-D models cannot predict the internal dynamics of SOFCs, they are very useful for applications where the objective is to study the interaction among system components, such as SOFC hybrid plants. While several types of fuel reformers have been proposed, the identification of the most efficient technology at the operating conditions of SOFCs, particularly small-size applications, requires more research. Similarly, more research is needed to determine if the direct electrochemical reaction of carbon monoxide can be ignored in SOFC models. On the other hand, it has been experimentally proven that internal fuel reformers are thermally self-sufficient. While the assumption that the steam reforming reaction reaches chemical equilibrium has been supported by several experimental studies, the similar assumption for the water-gas shift reaction is not proven, rather there are some strong evidences against its validity. It is also proven that the methane reforming reaction reaches equilibrium when all the inlet methane moles are consumed. The presented assumptions, mathematical formulations, model constants, system operating parameters, and model validation can assist researchers in making informed decisions on their choices for future SOFC models. Also, it identifies the areas where more research, particularly experimental research, is needed to verify the validity of the assumptions.

Mechanical reliability and life prediction of coated metallic interconnects within solid oxide fuel cells

Metallic cell interconnects (IC) made of ferritic stainless steels, i.e., iron-based alloys, have been increasingly favored in the recent development of planar solid oxide fuel cells (SOFCs) because of their advantages in excellent imperviousness, low electrical resistance, ease in fabrication, and cost effectiveness. Typical SOFC operating conditions inevitably lead to the formation of oxide scales on the surface of ferritic stainless steel, which could cause delamination, buckling, and spallation resulting from the mismatch of the coefficient of thermal expansion and eventually reduce the lifetime of the interconnect components. Various protective coating techniques have been applied to alleviate these drawbacks. In the present work, a fracture-mechanics-based quantitative modeling framework has been established to predict the mechanical reliability and lifetime of the spinel-coated, surface-modified specimens under an isothermal cooling cycle. Analytical solutions have been formulated to evaluate the scale/substrate interfacial strength and determine the critical oxide thickness in terms of a variety of design factors, such as coating thickness, material properties, and uncertainties. The findings then are correlated with the experimentally measured oxide scale growth kinetics to quantify the predicted lifetime of the metallic interconnects.

CaO effect on the electrochemical performance of lanthanum strontium cobalt ferrite cathode for intermediate-temperature solid oxide fuel cell

The oxygen reduction reaction (ORR) on lanthanum strontium cobalt ferrite (LSCF) catalyst is critical for intermediate temperature solid oxide fuel cells (SOFCs). The reaction rate can be effectively improved by addition various nanoparticles including electrocatalysts such as Pd, Ag and mixed electronic-ionic conductors and electrolytes like samaria doped ceria (SDC). This work shows that ORR rate can also be improved by CaO, which is neither catalyst nor conductor. The CaO nanoparticles are deposited to porous LSCF electrodes using the infiltrating technique. No obvious reaction between CaO and LSCF is detected with X-ray diffraction analysis, indicating that CaO is chemically compatible with LSCF in the intermediate-temperature SOFC operation conditions. Impedance spectrum analysis demonstrates that the CaO nanoparticles can effectively reduce the interfacial polarization resistances for both single phase LSCF electrodes and LSCF-SDC composite electrodes. In addition, CaO nanoparticles can improve the peak power densities and reduce the total electrode resistances of single cells consisting of NiO-SDC anodes, SDC electrolytes, and LSCF based cathodes. Further, CaO can increase the oxygen surface exchange coefficient as demonstrated with electrical conductivity relaxation measurement. The improving factor is comparable to those for Rh and Pd catalysts, suggesting it is effective to increase ORR rate by infiltrating CaO nanoparticles.

Influence of Alloying Elements on the Behavior of Different Ferritic Steels as Candidate Materials for SOFC Interconnect

The efficiency of a solid oxide fuel cell (SOFC) can be improved by using a Ni-mesh between the ferritic steel interconnect and the Ni/YSZ anode. However, interdiffusion processes can lead to internal oxidation within the Ni-mesh and some microstructural changes, i.e., formation of an austenite zone and accelerated formation of σ-phase at the ferrite/austenite interface. These changes may adversely affect the performance of the cell during long-term operation. The present work focused on the influence of certain alloying elements on the overall behavior of the ferritic steel under simulated SOFC operating conditions to define conditions to minimize σ-phase formation without degrading the oxidation resistance and mechanical properties of the steel. The experimental results indicate that decreasing the amount of Cr and adjusting the amount of Nb, Si and W of the steel is a possible way to achieve that goal.

Properties of screen-printed nickel/scandia-stabilized-zirconia anodes fabricated using rheologically optimized inks during redox cycles

Cyclic reduction and oxidation (redox) of solid oxide fuel cell (SOFC) anodes generally leads to cell degradation and reduces the cell lifetime. The investigation of the sequential cyclic redox of nickel/scandia-stabilized-zirconia (Ni/ScSZ) SOFC anode films presented in this study is aimed at understanding the properties of screen-printed Ni/ScSZ anode films fabricated using a rheologically optimized ink under redox condition. An optimized NiO/ScSZ screen-printing ink containing 28 vol% of solids and 2 wt% binder was chosen for the fabrication of the anode films. The mechanical properties, DC conductivity and electrochemical performance of Ni/ScSZ films were measured between 0 and 15 cycles and related to the microstructural change of the anode. All these properties degraded with increasing redox cycles. The mechanical hardness of re-oxidized anode decreases from 8753 to 1314 MPa, while the DC conductivity of re-reduced anode decreases from 1400 to 410 S/cm at 800 °C. Also, the anode polarization resistance increases from 0.75 to 2.7 Ωcm2 at 800 °C. This degradation is related to the separation of Ni particles, which reduces the density and Ni–Ni particle connectivity. The decrease in electrochemical performance can be related to a decrease in the anode triple-phase boundary density in the fabricated films. However, all the printed films remain stable with acceptable electrical and electrochemical performance without any cracks or delamination even after 15 cycles. This indicates that ink optimization through rheological testing is an important way to prevent crack formation in printed anode films that results from redox processes in real SOFC operating conditions.

Conformal bi-layered perovskite/spinel coating on a metallic wire network for solid oxide fuel cells via an electrodeposition-based route

Abstract Solid oxide fuel cells (SOFCs) require low-cost metallic components for current collection from electrodes as well as electrical connection between unit cells; however, the degradation of their electrical properties and surface stability associated with high-temperature oxidation is of great concern. It is thus important to develop protective conducting oxide coatings capable of mitigating the degradation of metallic components under SOFC operating conditions. Here, we report a conformal bi-layered coating composed of perovskite and spinel oxides on a metallic wire network fabricated by a facile electrodeposition-based route. A highly dense, crack-free, and adhesive bi-layered LaMnO3/Co3O4 coating of ∼1.2 μm thickness is conformally formed on the surfaces of wires with ∼100 μm diameter. We demonstrate that the bi-layered LaMnO3/Co3O4 coating plays a key role in improving the power density and durability of a tubular SOFC by stabilizing the surface of the metallic wire network used as a cathode current collector. The electrodeposition-based technique presented in this study offers a low-cost and scalable process to fabricate conformal multi-layered coatings on various metallic structures.

Highly Stable Sr-Free Cobaltite-Based Perovskite Cathodes Directly Assembled on a Barrier-Layer-Free Y2 O3 -ZrO2 Electrolyte of Solid Oxide Fuel Cells.

Direct assembly is a newly developed technique in which a cobaltite-based perovskite (CBP) cathode can be directly applied to a barrier-layer-free Y2 O3 -ZrO2 (YSZ) electrolyte with no high-temperature pre-sintering steps. Solid oxide fuel cells (SOFCs) based on directly assembled CBPs such as La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ show high performance initially but degrade rapidly under SOFC operation conditions at 750 °C owing to Sr segregation and accumulation at the electrode/electrolyte interface. Herein, the performance and interface of Sr-free CBPs such as LaCoO3-δ (LC) and Sm0.95 CoO3-δ (SmC) and their composite cathodes directly assembled on YSZ electrolyte was studied systematically. The LC electrode underwent performance degradation, most likely owing to cation demixing and accumulation of La on the YSZ electrolyte under polarization at 500 mA cm-2 and 750 °C. However, the performance and stability of LC electrodes could be substantially enhanced by the formation of LC-gadolinium-doped ceria (GDC) composite cathodes. Replacement of La by Sm increased the cell stability, and doping of 5 % Pd to form Sm0.95 Co0.95 Pd0.05 O3-δ (SmCPd) significantly improved the electrode activity. An anode-supported YSZ-electrolyte cell with a directly assembled SmCPd-GDC composite electrode exhibited a peak power density of 1.4 W cm-2 at 750 °C, and an excellent stability at 750 °C for over 240 h. The higher stability of SmC as compared to that of LC is most likely a result of the lower reactivity of SmC with YSZ. This study demonstrates the new opportunities in the design and development of intermediate-temperature SOFCs based on the directly assembled high-performance and durable Sr-free CBP cathodes.

The surface evolution of La0.4Sr0.6TiO3+δ anode in solid oxide fuel cells: Understanding the sulfur-promotion effect

The ideal solid oxide fuel cells (SOFCs) can be powered by readily available hydrocarbon fuels containing impurities. While this is commonly recognized as a key advantage of SOFC, it also, together with the elevated operating temperature, becomes the main barrier impeding the in-situ or operando investigations of the anode surface chemistry. Here, using a well-designed quenching experiment, we managed to characterize the near-surface structure of La0.4Sr0.6TiO3+δ (LST) anode in SOFCs fuelled by H2S-containing methane. This new method enabled us to clearly observe the surface amorphization and sulfidation of LST under simulated SOFC operating conditions. The ∼1 nm-thick two dimensional sulfur-adsorbed layer was on top of the disordered LST, containing –S, –SH and elemental sulfur species. In SOFC test, such “poisoned” anode showed increased performances: a ten-fold enhanced power density enhancement (up to 30 mW cm−2) and an improved open circuit voltage (from 0.69 V to 1.17 V). Moreover, its anodic polarization resistance in methane decreased to 21.53 Ω cm2, a difference of 95% compared with the sulfur-free anode. Control experiments confirmed that once the adsorbed sulfur species were removed electrochemically, methane conversion slowed down simultaneously till full stop.

Blocking layer for prevention of current leakage for reversible solid oxide fuel cells and electrolysis cells with ceria-based electrolyte

Doped ceria is one of the most promising materials for an electrolyte of solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs), because it has higher ionic conductivity than stabilized zirconia. However, current leakage through the ceria-based electrolyte occurs in SOFC and SOEC operating conditions, because electron conductivity appears at low oxygen partial pressures. In order to prevent the current leakage, the BaCe0.8Y0.2O3−δ (BCY) blocking layer was inserted between Ce0.9Gd0.1O1.95 electrolyte (GDC) and Ni-GDC fuel electrode. The open circuit voltage for the cell with BCY blocking layer increased by 50–60 mV compared to that without BCY, suggesting that BCY is a proton-O2− ion mixed conductor and it has low electronic conductivity. The performance of SOFC and SOEC using the GDC electrolyte and the BCY blocking layer was better than that using a conventional yttria-stabilized zirconia electrolyte at 500 °C. The hydrogen consumption and production was successfully demonstrated in accordance with Faraday's law for the reversible SOFC–SOEC with the GDC electrolyte and BCY blocking layer.

Robust NdBa0.5Sr0.5Co1.5Fe0.5O5+δ cathode material and its degradation prevention operating logic for intermediate temperature-solid oxide fuel cells

Abstract We report solutions (durable material and degradation prevention method) to minimize the performance degradation of cell components occurring in the solid oxide fuel cell (SOFC) operation. Reliability testing is carried out with the Ni Nd0.1Ce0.9O2-δ (NDC) anode-supported intermediate temperature-SOFCs. For the cathode materials, single perovskite structured Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and double perovskite structured NdBa0.5Sr0.5Co1.5Fe0.5O5+δ (NBSCF) are prepared and evaluated under harsh SOFC operating conditions. The double perovskite NBSCF cathode shows excellent stability in harsh SOFC environments of high humidity and low flow rate of air. Furthermore, we propose the concurrent fuel and air starvation mode, in which the cell potential is temporarily reduced due to the formation of both fuel-starvation (in the anode) and air-depletion (in the cathode) concurrently under a constant load. This is carried out in order to minimize the performance decay of the stable NBSCF-cell through the periodic and extra reduction of a H 2 O (and a O 2 ) in the anode. The operating-induced degradation of SOFCs, which are ordinarily assumed to be unrecoverable, can be completely circumvented by the proposed periodical operation logic to prevent performance degradation (concurrent fuel-starvation and air-depletion mode).

(Invited) Characterization of Thermally and Cathodic Polarization Induced LSM Electrode/YSZ Electrolyte Interface of Solid Oxide Fuel Cells By Combined FIB-STEM Techniques

In high temperature solid oxide fuel cells (SOFCs), electrode/electrolyte interfaces play an ultimate role in the electrocatalytic activity and durability of the cells. In this study, thermally and electrochemically induced electrode/electrolyte interfaces were investigated on pre-sintered and in situ assembled La0.8Sr0.2MnO3 (LSM) electrode on Y2O3-ZrO2 (YSZ) electrolyte, using focus ion beam and high resolution scanning transmission electron microscopy (FIB-STEM). The results indicate that thermally induced interface is characterized by convex contact rings with depth of 100-400 nm and diameter in agreement with the particle size of pre-sintered LSM electrodes, while the cathodic polarization induced interfaces on in situ assembled electrodes are characterized by particle-shaped contact marks (20-50 nm in diameter). The results show that the electrochemical activity of the cathodic polarization induced electrode/electrolyte interface is comparable to the thermally induced one for the O2 reduction reaction under SOFC operation conditions. The FIB-STEM analysis clearly shows that along the rim of the contact rings of pre-sintered LSM electrode, there is formation of a thin layer of La and Sr on the YSZ electrolyte surface, while Mn diffuses into YSZ and the distribution of Mn inside YSZ electrolyte follows closely the profile of the contact rings. The preliminary results provide direct evidences that Mn cation inter-diffusion plays a critical role in the formation of LSM/YSZ interface of SOFCs.

Praseodymium Doped Ceria Oxygen Surface Exchange Coefficient and Film Stress Measurements Via the Curvature Relaxation Technique

Introduction Praseodymium doped ceria (PCO) is a mixed ionic electronic conductor (MIEC) that can be used as a cathode material in solid oxide fuel cells (SOFCs). Under SOFC operating conditions, PCO undergoes the following reaction [1]: ½ O2 + 2Pr’ Ce + VӦ à 2PrCe + Oo x The kinetics of this oxygen exchange reaction are represented by the oxygen surface exchange coefficient (kchem ). In this work, simultaneous kchem and stress measurements on PCO thin films atop yttria-stabilized zirconia (YSZ) substrates were performed from 625-725 oC using the curvature relaxation (κR) technique. Experimental Methods Phase pure Pr0.1Ce0.9O1.95 powder was prepared through glycine nitrate combustion and subsequent powder calcination in air. (001) oriented 9.5% YSZ substrates (Crystec, GmbH) were pre-annealed at 1450oC for 20 hours. PCO thin films were sputtered onto YSZ substrates at room temperature and then sintered for 1 hour at 1100oC. The bilayer structures were tested using a multi beam optical stress sensor (MOSS) from 625oC to 725oC with 25oC increments. The κR technique determines kchem by fitting a solution of Fick’s 2nd Law to the curvature response of mechano-chemically active film | inert substrate bilayers reacting to sudden changes in oxygen partial pressure (synthetic air (21% O2 - 79% Ar) and 10% synthetic air - 90% Ar) [2,3]. Film stress values were calculated using Stoney’s equation [2,3]. In order to compare the kchem values with those in the literature, electrically-measured PCO kq values from the literature [4] were turned into kchem values by multiplying them by the corresponding PCO thermodynamic factor from Ref [5]. Results Figure 1 shows how the kchem value measured via the κR technique compares with literature [4,5] values. Conclusions The PCO kchem and activation energies measured here are significantly different than those reported in the literature. Although additional work is needed before these effects can be explained in full, these discrepancies may be caused by the different synthesis techniques (sputtering vs. pulsed laser deposition) used to produce the two films shown in Figure 1. Acknowledgements This material is based upon work supported by the Department of Energy under Award Number DE-FE0023315.

Study of the entropy generation in a SOFC for different operating conditions

In the present work, the behavior of a MOLB-type (mono-block layer built) SOFC (solid oxide fuel cells), with different electrolyte thicknesses and operating conditions at the inlet, such as mass fraction, temperature and velocity, is numerically investigated. An entropy generation analysis is carried out in the fuel cell and it is evaluated how the operating conditions and electrolyte thicknesses affect the prediction of the thermodynamic irreversibility. For this investigation a three-dimensional CFD model was developed using the finite volume method derived from a control-volume formulation that takes into account the heat transfer, the transport of species and the electrochemical reactions. Different simulations are performed and the contribution of the local entropy generation rate is computed. The results show different trends for the current density, temperature, species, activation loss, ohmic loss and concentration loss throughout the fuel cell, and indicate that the prediction of thermodynamic irreversibilities is strongly affected by the choice of the operating conditions (e.g. inlet temperature) and the electrolyte thickness. Both local and global entropy generation rates show strong variations. The results of this work are useful in helping the designer to select the different operating conditions of a MOLB-type SOFC with lower irreversibilities.

Thermal Properties of Perovskite-Type Oxides La0.6Sr0.4Co1-XFexO3-δ (0 ≤ x ≤ 1.0)

Introduction Solid oxide fuel cells (SOFCs) are an electrochemical conversion system with high efficiency and low-emission of pollution. In order to develop a commercial SOFC system with high performance and long-term stability, it is important to understand temperature distribution in the cell and stack [1]. For this purpose, thermal properties of SOFC components should be evaluated particularly under SOFC operating conditions, e.g. at elevated temperatures and under various oxygen partial pressures. Although there exist several reports on the thermal properties of the SOFC components, most of the data were measured at room temperature or in some specific atmospheric condition. [2-3] Thus, we aimed to investigate thermal properties of SOFC components as functions of temperature and oxygen partial pressure. Perovskite-type oxides La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) are selected for this work. They are known to exhibit oxygen nonstoichiometry changes depending on temperature and oxygen potential. [4-7] The variation in oxygen vacancy concentration is known to influence electrochemical properties and mechanical properties. However, little is known about the effect on the thermal properties although. There exist several reports on the thermal properties of the LSCF. In this study, thermal diffusivities, heat capacities and thermal conductivities have been investigated by using the laser flash method and TG-DSC at high temperatures up to 1273 K under controlled oxygen atmospheres. Dependencies of the thermal properties of LSCF on temperature, oxygen partial pressure and the material composition will be discussed in terms of oxygen nonsotichiometry and crystal structures. Experimental Powders of La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) were prepared by a conventional Pechini method. Obtained powders were hydrostatically pressed at 150 MPa into compacts, and then sintered in air at 1573K-1473K for 6h. For the thermal diffusivity measurement, the sintered compacts were cut into rectangles (c.a. 10 x 10 x 1 mm). For the TG-DSC measurement, the sintered compacts were cut smaller than Pt crucible. The phase of sintered samples was characterized by XRD to investigate any phase change after sintering. Thermal diffusivities and heat capacities of the LSCF were measured by using the laser flash method (LFA 457 Micro Flash, NETZSCH) and TG-DSC (STA 447, NETZSCH) in the oxygen partial pressure range from 10-4 to 0.2 bar and in the temperature range from R.T. to 1173 K. In order to control the atmospheres, a gas mixing system and an oxygen sensor are additionally attached to the laser flash system. The thermal conductivities (λ) of LSCF were calculated by using Eq(1). α : thermal diffusivity of LSCF with respect to temperature, ρ : the density of sintered LSCF sample and Cp: heat capacity of the LSCF as a function of temperature. λ = α·ρ·Cp (1) Results and Discussion In this study, the thermal properties of perovskite-type oxides La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) were investigated. The thermal diffusivities of LSCF were dependence of temperature as well as oxygen partial pressure and Co contents ratio in intermediate temperatures (873K to 1173K).[Fig.1] The thermal diffusivity of LSCF decreased gradually as p(O2) decreased at all investigated temperatures, and decreased as temperature increased in all investigated p(O2) range. The thermal diffusivity depended on oxygen partial pressure more significant in lower oxygen partial pressure and at higher temperature. The thermal diffusivity increases with Co contents ratio increases and each composition increases with temperature through a maximum, then decreases. Heat capacity and thermal conductivity of LSCF (x=0, 0.8, 1.0) were almost constant from R.T to 873K, however increasing with increasing temperature over 873K. However, measured heat capacity compare with the Neumann-Kopp rule-specific heat of LSCF (x=0, 0.8, 1.0) were not consistent. LSCF is known to show the oxygen nonstoichiometry changes at relatively higher temperatures. Therefore, we suggest these dependenciese could be interpreted by the change in the oxygen nonstoichiometry, meaning that the thermal properties of LSCF were significantly affected by the oxygen nonstoichiometry.