Academic literature on the topic 'Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC)'

Fuel cell is an energy converter device that generates electricity through electrochemical reaction between hydrogen and oxygen. An example of fuel cell is the solid oxide fuel cell (SOFC) which uses a ceramics based solid electrolyte. Due to the use of ceramics, SOFC normally operates at high temperatures up to 1000 °C. This high operating temperature makes SOFC known for its efficient energy conversion capability and excellent fuel flexibility. However, despite the advantages, the extreme temperatures limit the uses of SOFC. High operation temperature leads to long term operational issues in durability and cell degradation. Yttria stabilized zirconia, YSZ is a commonly used material for electrolyte in high temperature SOFCs. However, YSZ electrolyte is unable to perform well when the operating temperature is reduced to intermediate-low zones below 800 °C. Thus, development of new materials for SOFC components is needed whereby the production of electrolyte materials becomes one of the main scopes for research in intermediate-low temperature SOFCs. Apart from the synthesis of new materials, another approach in increasing the ionic conductivity of intermediate-low temperature SOFC is through the fabrication of a bilayered electrolyte. As such, this review article focuses on the potential of bilayered electrolyte for intermediate-low temperature SOFCs

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 electrolytes for construction of the intermediate-temperature solid oxide fuel cells, IT-SOFC, have been reviewed. Yttrium stabilized tetragonal zirconia polycrystals, YTZP, as a potential electrolyte of IT-SOFC have been highlighted. The experimental results involving structural, microstructural, electrical properties based on our own studies were presented. In order to study aluminum diffusion in YTZP, aluminum oxide was deposited on the surface of 3 mol.% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP). The samples were annealed at temperatures from 1523 to 1773 K. Diffusion profiles of Al in the form of mean concentration vs. depth in B-type kinetic region were investigated by secondary ion mass spectroscopy (SIMS). Both the lattice (DB) and grain boundary (DGB) diffusion were determined.

Solid oxide fuel cell (SOFC) has significant advantages of clean and quiet operation while providing a relatively high efficiency owing to enhanced reaction kinetics at high operating temperature. The high operating temperature of SOFC, typically around 800 – 1000°C helps to enable internal reforming of hydrocarbons and negate effects of impurities in small quantities in the fuel. However, this limits the application of SOFC only to stationary applications due to the long period needed to reach this temperature range. A high temperature operation is also not ideal in terms of cost reduction and long-term stability of the cell components. Hence, lowering the operating temperature of SOFC is crucial for reduction of cost production and commercialization, which enables SOFC to have a wider range of application areas inclusive of portable and mobile ones. Building a high-performance SOFC with small volume is essential as the underlying criteria for these small-scale portable applications. Therefore, careful design and fabrication methods of SOFC operating on intermediate temperatures with high power outputs need to be considered. The intermediate temperature operation of the fuel cell not only increases the overall lifespan of cell but also allows for longer operation with a lower degradation rate compared to high temperature operation. Furthermore, a modified intermediate temperature stack design can accommodate a wider range of applications compared to the tubular and planar stack designs. This paper reviews the development of SOFC stack designs aimed at intermediate temperature operation towards achieving high performance and the benefits of each design.

To identify critical parameters upon variable operational temperatures in a planar SOFC, an experimentally agreeable model was established. The significance of temperature effect on the performance of SOFC components was investigated, and the effect of activation energy during the development of intermediate electrode materials was evaluated. It is found the ionic conductivity of electrolytes is identified to be unavoidably concerned in the development of the intermediate-temperature SOFC. The drop of the ionic conductivity of the electrolyte decreases the overall current density 63% and 80% at temperatures reducing to 700 °C and 650 °C from 800 °C. However, there exists a critical value on the defined ratio between the electric resistance of the electrolyte in the overall internal resistance of SOFC, above which the further increase in the ionic conductivity would not significantly improve the performance. The lower the operational temperature, the higher critical ratio of the electrical resistance in the overall internal resistance of the cell. The minimal decrease in the activation energy during the development of intermediate electrode materials can significantly enhance the overall performance. Considering the development trend toward the intermediate temperature SOFC, advanced electrode material with the decreased activation energy should be primarily focused. The result provides a guidance reference for developing SOFC with the operational temperature toward the intermediate temperature.

Project ABSOLUTE (advanced battery solid oxide fuel cell linked unit to maximize efficiency), aims to combine a sodium-nickel chloride battery and an intermediate temperature solid oxide fuel cell (IT-SOFC) to form an all-electric hybrid package that surpasses the efficiency and performance of a purely fuel cell driven vehicle, as well as extending the range of a purely battery driven electric vehicle. This paper discusses the project background, the ABSOLUTE hybrid concept, the methodology adopted, the vehicle types and drive cycles that best suit the hybrid and system control considerations. Results from a battery and IT-SOFC system model are presented.

The main obstacle to solid oxide fuel cells (SOFCs) implementation is the high operating temperature in the range of 800–1,000 °C so that it has an impact on high costs. SOFCs work at high temperatures causing rapid breakdown between layers (anode, electrolyte, and cathode) because they have different thermal expansion. The study focused on reducing the operating temperature in the medium temperature range. SmBa0.5Sr0.5Co2O5+δ (SBSC) oxide was studied as a cathode material for IT-SOFCs based on Ce0.8Sm0.2O1.9 (SDC) electrolyte. The SBSC powder was prepared using the solid-state reaction method with repeated ball-milling and calcining. Alumina grinding balls are used because they have a high hardness to crush and smooth the powder of SOFC material. The specimens were then tested as cathode material for SOFC at intermediate temperature (600–800 °C) using X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), electrochemical, and scanning electron microscopy (SEM) tests. The X-ray powder diffraction (XRD) pattern of SBSC powder can be indexed to a tetragonal space group (P4/mmm). The overall change in mass of the SBSC powder is 8 % at a temperature range of 125–800 °C. A sample of SBSC powder showed a high oxygen content (5+δ) that reached 5.92 and 5.41 at temperatures of 200 °C and 800 °C, respectively. High diffusion levels and increased surface activity of oxygen reduction reactions (ORRs) can be affected by high oxygen content (5+δ). The polarization resistance (Rp) of samples sintered at 1000 °C is 4.02 Ωcm2 at 600 °C, 1.04 Ωcm2 at 700 °C, and 0.42 Ωcm2 at 800 °C. The power density of the SBSC cathode is 336.1, 387.3, and 357.4 mW/cm2 at temperatures of 625 °C, 650 °C, and 675 °C, respectively. The SBSC demonstrates as a prospective cathode material for IT-SOFC

Reducing a high-operating temperature of solid oxide fuel cell (SOFC) to intermediate temperature SOFC (IT-SOFC, 500-750ºC) poses a great challenge in the sense of developing solid electrolyte at intermediate temperature range. In response to this, we report a novel composite La9.33Si6O26 (LSO) - Ce0.85Gd0.15O1.925 (CGO) in this study. The synthesis of LSO-CGO composite was carried out by combining LSO with CGO (9:1, 8:2, and 7:3 in weight ratio) using solid state reaction method. In order to get a dense pellet, all of the products were sintered at 1500°C for 3 h. The X-ray diffraction pattern of sintered pellets show typical patterns for both of LSO and CGO which indicate that the composite was successfully formed. The highest conductivity was detected in 7LSO-3CGO, i.e. 2.10×10-3 S cm-1 at 700 ○C and also has low activation energy (0.60 eV). This result suggests that the LSO-YSZ composites are good oxide ion conductors and may potentially be used as an alternative solid electrolyte in IT-SOFC technology.

A new trend in recent years is to reduce the solid oxide fuel cell (SOFC) operating temperature to an intermediate range by employing either a thin electrolyte, or new materials for the electrolyte and electrodes. In this paper, a numerical investigation is presented with focus on modeling and analysis of transport processes in planar intermediate temperature (IT, between 600 and 800°C) SOFCs. Various transport phenomena occurring in an anode duct of an ITSOFC have been analyzed by a fully three-dimensional calculation method. In addition, a general model to evaluate the stack performance has been developed for the purpose of optimal design and/or configuration based on specified electrical power or fuel supply rate.

An intermediate temperature solid oxide fuel cell (SOFC) is developed and its performance is investigated experimentally and theoretically. In the experimental program, a gadolinium doped ceria based membrane electrode group is developed with the tape casting and screen printing methodology and characterized. An experimental setup is devised for the performance measurement of SOFCs and the performance of produced cells is investigated over a range of parameters including the electrolyte thickness, the sintering temperature, the operation temperature etc. The experimental setup is then further modified to measure the temperature distribution in the large SOFC single cells. The effects of operating parameters on the temperature distribution are investigated and the parameter spaces leading high efficiency without cracking the ceramic membrane are identified. In theoretical study a mathematical model is developed to represent the fluid flow, the heat transfer, the species transport and the electrochemical reaction in intermediate temperature of solid oxide fuel cells.The differential equations are solved numerically with a commercial CFD code which employs a control volume based approach. The temperature distribution and species distribution during theSOFC operation is analyzed. The effects of operation parameters on critical SOFC characteristics and the performance are numerically investigated over a range of parameter space. The experimental and numerical results are compared to validate the mathematical model. The mathematical model is found to agree reasonable with experimental data.

La fabrication de cellules de piles à combustible IT-SOFC de large dimension par un nouveau procédé simple et peu coûteux est présentée dans ce manuscrit. L’optimisation de ce nouveau procédé en regard de l’utilisation d’agents de porosité, d’épaisseur de couches et de température de frittage a été réalisée. Les résultats des tests électrochimiques sur des cellules de surface active 10 cm2 réalisés dans le dispositif Fiaxell semi-ouvert ont été détaillés pour différentes cellules. Des tests de performance de longue durée ont également été menés sur le dispositif Fiaxell, présentés et discutés. La préparation et la réalisation d’un nouveau banc de test de stack a également été mené et présenté dans ces travaux
The fabrications of large area IT-SOFC planar cell by new simple and cost effective process were explained. The optimization of the new process with respect to pore formers, thickness of layers, sintering temperature were performed. The electrochemical results of 10cm2 performed in Fiaxell open flange set up were detailed with respect to different configuration. Long term ageing performance tests of single cells were conducted in Fiaxell device and results are discussed. Preparation of new test bench and stacking process performed till now were briefed

Proton conducting intermediate temperature (600oC-400oC) solid oxide fuel cells (IT-SOFC) have many potential advantages for clean and efficient power generation from readily available hydrocarbon fuels. However, it still has many unsolved problems, especially on the anode where the fuel got oxidized and the cathode where oxygen got reduced. In this study, for the anode, the effects of hydrogen sulfite (H2S) and carbon dioxide (CO2) as fuel contaminants were studied on the nickel (Ni) based cermet anode of proton conducting IT-SOFC using proton conducting electrolyte of BaZr0.1Ce0.7Y0.1Yb0.1O3 (BZCYYb). Both low-ppm level H2S and low-percentage level CO2 caused similar poisoning effects on the anode reaction. The H2S poisoning effect was also found to be much less than on oxide-ion conducting SOFC, which is attributed to the absence of water evolution for the anode reaction in proton conducting SOFC. In addition, the H2S/CO2 poisoning mechanisms were investigated using X-ray diffraction, energy dispersive spectroscopy (EDS), Raman spectroscopy, and secondary ion mass spectroscopy (SIMS). For H2S, other than possible sulfur dissolution into BZCYYb, no bulk reaction was found, suggesting sulfur adsorption contributes to the reduced performance. For CO2, reaction with BZCYYb to form BaCO3 and CeO2 is identified and is believed to be the reason for the sudden worsening in CO2 poisoning as temperature drops below ~550oC. For the cathode, several representative SOFC cathodes including silver (Ag), La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), LSCF-BZCYYb composite, and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) were evaluated based on BZCYYb electrolyte. LSCF give similar high interfacial resistance as Ag, while LSCF-BZCYYb composite cathode shows lower interfacial resistance, suggesting LSCF behaves like pure electronic conductor cathode in this case. For BSCF, it shows smallest interfacial resistance and the charge transfer process appears to accelerate with the introduction of H2O, while oxygen adsorption/transport seem to slow down due to adsorbed H2O. Furthermore, CO2 was shown to cause poisoning on the BSCF cathode, yet the poisoning was significantly reduced with the co-presence of water. The results suggest that although BSCF seem to display mixed proton-electronic conduction, its strong affinity to H2O may inhibit the oxygen reduction reaction on the cathode and new cathode materials still need to be designed.

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Bachelors
Engineering and Computer Science
Mechanical Engineering

Interfaces et durabilité des électrodes de pointe pour l'énergie (PAC et EHT)L'objectif de cette thèse concerne l'élaboration, par atomisation électrostatique, d'une électrode à oxygène à architecture innovante, basée sur un composite Ce0.9Gd0.1O1.95 (CGO) - La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) possédant un gradient de composition ou une composition homogène. Cette électrode a été déposée sur un substrat de zircone yttriée (YSZ = 8 % mol. Y2O3-ZrO2) sur laquelle, a été intercalée au préalable une couche barrière mince et dense de CGO. Cette électrode possède une microstructure innovante, à porosité élevée permettant d'obtenir une grande surface active qui devrait conduire à l'amélioration des performances électrochimiques. Le comportement électrique de l'électrode a été étudié par spectroscopie d'impédance en fonction de la température et sous air. Une description microstructurale détaillée a été effectuée à l'aide d'un modèle de reconstruction 3D obtenu par -MEB équipé d'une sonde ionique focalisée et par nanotomographie X. Ces propriétés microstructurales ont été reliées aux propriétés électriques. Les propriétés mécaniques et tribologiques de cette électrode composite ont été déterminées par des tests du scotch et ultra-microindentation. Finalement, des tests de durabilité ont été effectués sur une électrode de grande taille possédant une surface active de 45 cm2 jusqu'à 800 h à environ 770°C, dans une cellule complète de configurations PAC et fonctionnant respectivement sous H2 et un mélange H2/H2O
Interfaces and durability of advanced electrodes for energy (IT-SOFC and SOEC)The objective of this PhD thesis is to fabricate advanced oxygen electrode based on Ce0.9Gd0.1O1.95 (CGO) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) with graded and homogeneous composition onto yttria-stabilized zirconia (YSZ = 8 mol. % Y2O3-doped ZrO2) electrolyte using electrostatic spray deposition. A thin and dense layer of CGO was inserted between LSCF and YSZ to serve as a barrier diffusion layer. The novel microstructure with high porosity and large surface area is expected to improve the electrochemical performances. The electrical behavior of the electrode was investigated by impedance spectroscopy versus temperature in air. A detailed microstructural description was performed by 3D reconstructed model from FIB-SEM and X-ray nanotomography and related to electrical properties. The mechanical analysis was performed by scratch and ultramicroindentation tests. Finally, durability tests were performed on the electrode with 45 cm2 oxygen active area, up to 800 h at around 770°C, in full cell SOFC and SOEC configurations operating respectively in H2 and H2/ H2O mixture

The work presented in this thesis focuses on the synthesis of long axis A-site ordered perovskites, which have ordered oxygen vacancies. The materials discussed have also been assessed as potential cathodes for solid oxide fuel cells (SOFCs), targeting an intermediate temperature (IT) SOFC operating range of 500 800 °C. In chapter 3 of this thesis, a 16ap phase (ap = 1 perovskite unit, ABO3-δ), Y2.24Ba2.28Ca3.48Fe7.44Cu0.56O21-δ, was first observed in a powder X-ray diffraction (PXRD) pattern that resulted from the attempted Cu doping of a 10ap phase (Y0.9Ba1.7Ca2.4Fe5 xCuxO13 δ). This phase was indexed as ap √2 × 16ap × ap √2 by selective area electron diffraction (SAED). Mössbauer spectroscopy identified that Fe3+ existed in three different coordination environments and a Rietveld refinement was carried out using combined powder synchrotron (S) XRD and powder neutron diffraction (ND) data. High Angle Annular Scanning Transmission Electron Diffraction (HAADF-STEM) confirmed the A-site ordering from the refinement. The 16ap phase exhibited good thermal stability, CO2 stability and chemical compatibility with state of the art electrolytes (GDC, SDC and LSGM), as well as a close matching thermal expansion coefficient (TEC) with the same electrolytes. Although possessing low electronic conductivity, 3.5 S.cm-1 at 750 °C, a good area specific resistance (ASR) of 0.12 Ω.cm2 at 750 °C was achieved, within the IT-SOFC operating range. In chapter 4, a 10ap phase, Y0.9Ca2.4Sr1.7Fe5O13-δ (YCSFO), was discussed. The space group Imma was identified from SXRD data, while the structure was closely related to brownmillerite. A Rietveld refinement carried out with the addition of SXRD data collected at the K absorption edge for Sr determined A-site ordering. The ASR of YCSFO was three times higher than that of the 16ap phase, highlighting the ordering of oxygen vacancies. The final phases investigated in chapter 5 of this thesis belong to a family of Co doped 10ap phases (Y0.9Ba1.7Ca2.4Fe5 xCoxO13 δ). A range of compositions were synthesised by varying the cation ratios, in order to obtain high Co content phase pure samples. The highest Co content was x = 1.85 for compositions Y1.24Ba1.85Ca1.91Fe3.15Co1.85O13-δ (annealed in O2) and Y1.6Ba1.8Ca1.6Fe3.15Co1.85O13-δ. AC impedance measurements carried out showed that increased Co content reduced the ASR, with the values at 700 °C of 0.19 Ω.cm2 and 0.23 Ω.cm2 respectively.

The aim of this thesis is to propose the design process and considerations to be employed in the fabrication of a high-volumetric-power-density intermediate temperature solid oxide fuel cell (IT-SOFC), as well as the necessary characterization and analysis techniques for such a device. A novel hexagonal honeycomb design will be proposed with functionally graded electrodes and an alternative electrolyte – a previously unexplored configuration based on attained research. The potential use of CFD software to investigate mass and heat transport properties of an SOFC having such a design shall be discussed, as well as the utility of experimental methods such as the generation of a polarization curve and the use of SEM to characterize electrochemical performance and microstructure, respectively. Fabrication methods shall also be evaluated, and it will be shown that the proposed design is not only feasible but meets the goal of designing an SOFC with a power density of 2 W/cm3 operating at or below 650 C.

Esistono attualmente varie ragioni per cui ampio interesse scientifico e tecnologico è rivolto verso sistemi di generazione di energia alternativi rispetto ai metodi convenzionali (quali i sistemi a turbine o i motori a combustione interna). Dal punto di vista ecologico, cresce il bisogno di ridurre la produzione di sostanze inquinanti per far fronte a uno sviluppo sostenibile. Da un punto di vista socio-economico, invece, aumenta il bisogno di far fronte a un continuo aumento della richiesta di energia, mentre nello stesso tempo le principali fonti di energia, quali i combustibili fossili, si stanno esaurendo. E infine, da un punto di vista socio-politico, la scarsità delle attuali fonti di energia sta creando drammatiche tensioni tra le varie aree economiche del mondo. La diminuzione della dipendenza mondiale dai combustibili fossili e l’introduzione di forme di generazione di energia alternative potrebbero sanare tale situazione. Il concetto di energia alternativa è stato introdotto già da vari anni. Ci sono diverse fonti d energia alternativa, come l’energia solare, l’energia eolica o la fusione nucleare. Un diverso approccio consiste nello sviluppo di sistemi di generazione di energia alternativi, che siano in grado di lavorare con alti rendimenti e di limitare al massimo la produzione di inquinanti. Allo stato attuale i motori a combustione interna presentano un’efficienza totale del 20-30%. Questo significa che solo il 20-30% dell’energia termica contenuta nel gasolio viene utilizzata come lavoro meccanico. Alti rendimenti si traducono, invece, in costi ridotti per unità di lavoro prodotto. Le celle a combustibile sono sistemi di conversione di energia alternativi i quali permettono la conversione diretta dell’energia chimica dei reagenti in energia elettrica, producendo al contempo basse emissioni inquinanti. Tra i diversi tipi di celle a combustibile, le celle a ossidi solidi (SOFCs) presentano vari vantaggi; tra i primi, lavorando ad alta temperatura (800-100°C) queste celle raggiungono valori di rendimento molto alti, permettono l’uso di diversi combustibili e l’unica emissione inquinante rilevante è quella di CO2, la quale rimane comunque un terzo di quella emessa da un motore a combustione interna a parità di kW/h prodotti. Tuttavia le alte temperature di lavoro comportano anche degli svantaggi: materiali costosi, elevati stress termici, difficoltà nel sigillare la cella, lunghi tempi di accensione e spegnimento del sistema. Per risolvere questi problemi la ricerca è orientata nell’abbassare la temperatura di funzionamento delle SOFCs nel cosiddetto range di temperature intermedie (400-700°C). Abbassare la temperatura di funzionamento si traduce in un peggioramento delle performance dei vari componenti della cella, e per questo lo studio di nuovi materiali risulta essenziale nella prospettiva di rendere le SOFC commercializzabili. Lo scopo di tale lavoro di tesi è appunto lo studio di materiali elettrolitici ed elettrodici che presentino buone proprietà conduttive a temperature di funzionamento intermedie e che allo stesso tempo siano chimicamente stabili. Nel capitolo 1A della tesi si presentano i principi basilari di funzionamento di una SOFC e una breve illustrazione dei materiali più studiati in letteratura sia per le alte e intermedie temperature di funzionamento. In particolare, tra i materiali ceramici con buone proprietà conduttive a basse temperature si trovano i conduttori protonici. Nel Capitolo 2A vengono illustrate le principali proprietà chimico-fisiche ed elettrochimiche di tali materiali ceramici. Molti ossidi perovskitici presentano conduzione protonica a temperature intermedie quando esposti ad atmosfera di idrogeno e/o vapore acqueo. Tuttavia nessuno di questi ossidi presenta contemporaneamente le due proprietà essenziali richieste ad un buon elettrolita: alta conducibilità ionica e buona stabilità chimica. La seconda parte della tesi presenta i risultati del lavoro sperimentale svolto, il quale è stato rivolto alla preparazione e caratterizzazione di conduttori protonici ceramici elettrolitici con alta conducibilità e buona stabilità chimica e allo sviluppo di elettrodi a - hoc per tali elettroliti. Il Capitolo 1B riporta l’ottimizzazione di una tecnica di sintesi sol gel per produrre i seguenti conduttori protonici: BaZr0.8Y0.2O3-δ (BZY) e BaCe0.8Y0.2O3-δ (BCY). Attraverso il metodo di sintesi ottimizzato si sono sintetizzate fasi singole dei suddetti composti. Le basse temperature di calcinazione richieste dal processo hanno portato a polveri di particelle nanometriche. I due composti sono stati sinterizzati in forma di pasticche circolari e caratterizzati elettricamente mediante spettroscopia di impedenza. Inoltre sono stati svolti test termici in flusso di anidride carbonica per valutare la stabilità chimica dei due composti, osservando una buona stabilità solo nel caso del BZY. Tuttavia le performance in cella di tale elettrolita si sono rilevate insufficienti rispetto ai target richiesti per la commercializzazione. Nel Capitolo 2B si è cercato di implementare le prestazioni del BZY sostituendo nel sito B della struttura perovskitica diverse quantità di Ce. Gli elettroliti cosi prodotti sono stati analizzati ai raggi X, sotto il punto di vista della stabilità chimica e della conducibilità elettrica. Il miglior compromesso tra stabilità chimica e conducibilità elettrica è risultato il composto con stechiometria BaZr0.5Ce0.3Y0.2O3-δ. Un ulteriore miglioramento della conduzione elettrica rispetto al BaZr0.5Ce0.3Y0.2O3-δ, pur mantenendo un’ottima stabilità chimica, è stato ottenuto realizzando un elettrolita “a doppio strato”, il quale è descritto nel Capitolo 3B. Una pasticca spessa 1 mm di BCY è stata protetta con uno strato sottile (circa un micron) di BZY cresciuto tramite la tecnica di deposizione a laser pulsato. Questo nuovo elettrolita ha presentato elevata conducibilità e buone prestazioni in cella in termini di stabilità chimica e densità potenza fornita. Nel Capitolo 4B si sono invece investigati elettrodi funzionali per tali elettroliti a conduzione protonica. Un catodo composito e stato realizzato unendo un conduttore misto ionico/elettronico, La1-xSrxCo1-yFeyO3-δ (LSCF), e un conduttore misto protonico/elettronico BaCe0.9Yb0.1O3-δ (10YbBC). L’uso di catodi compositi aumenta i siti di reazione al catodo, diminuendo quindi le cadute di potenziale dovute alle reazioni catodiche.
There are increasing reasons to explore alternatives to conventional energy generation methods (that is to say coal-fired steam turbine and gasoline internal combustion engine). From an ecological point of view, there is the need to reduce the polluting by-products of conventional energy generation. From a socio-economical standpoint, the worldwide demand for energy continues to rise as more and more nations join the group of the industrialized countries, while hydrocarbon fuels go to exhaustion. Finally, from a socio-political perspective, the situation described above has created several and often dramatic tensions between different world economic areas, as evidenced by frequent wars. Lowering the global dependence on oil might reduce such tensions. However, despite all of this, changes in the energy generation methods are extremely slow, as evidenced by the wide (if we cannot say total) use of the internal combustion engine. The concept of alternative energy has been introduced a long time ago. Several different sources of energy are proposed, which can have the potential to replace conventional generation methods. Popular examples include solar radiation, wind motion, and nuclear fusion. Each of these technologies has its own set of problems that have slowed down its commercialization, but much research is being conducted to overcome these problems. In fact, the research towards the development of alternative, highly efficient, eco-friendly energy production technologies is expanding. There is a general push towards higher efficiencies. At present, automobiles based on internal combustion engines have an overall efficiency of about 20-30%. That is, only 20-30% of the thermal energy content of the gasoline is converted into useful mechanical work and the rest is wasted. Higher efficiencies translate into reduced energy costs per unit of work done. Fuel cells, an alternative energy technology, have received growing interest in recent years since they represent one of the most promising energy production systems to reduce pollutant emissions. They are electrochemical devices that allow the direct conversion of chemical energy into electrical energy. Among the different type of fuel cells, solid oxide fuel cells (SOFCs) offer great promise as a clean and efficient technology for energy generation and provide significant environmental benefits. They produce negligible hydrocarbons, CO or NOx emissions, and, as a result of their high efficiency, about one-third less CO2 per kW/h than internal combustion engines. Unfortunately, the current SOFC technology based on a stabilized zirconia electrolyte requires the cell to operate from 700 to 1000°C to avoid unacceptable ohmic losses. These high operating temperatures demand specialized (expensive) materials for fuel cell interconnectors, long start-up time, and large energy input to heat the cell up to the operating temperature. Therefore, if fuel cells could be designed to give a reasonable power output at intermediate temperatures (IT, 400-700°C), tremendous benefits may result. In particular, in the IT range ferrite steel interconnects can be used instead of expensive and brittle ceramic materials. In addition, sealing becomes easier and more reliable; rapid start-up is possible; thermal stresses (namely, those caused by thermal expansion mismatches) are reduced; electrode sintering becomes negligible. Combined together, all these improvements result in reduced initial and operating costs. Therefore, the major trend in the present research activities on SOFCs is the reduction of the operating temperature. The problem is that lowering the operating temperatures lowers the electrolyte conductivity, whereas the electrode polarization greatly increases, reducing the overall fuel cell performance. Considering the described scenario, it is clear how the study of materials assumes a considerable role in lowering SOFC operating temperature. Making SOFCs commercially competitive with conventional energy generation methods means developing a highly efficient and environmental friendly energy production device to provide for a global sustainable energy system. IT-SOFCs represent not only a laboratory research activity, but a great challenge for the entire society. The purpose of the present dissertation is the development of a stable highly-conductive electrolyte and performing electrodes for lower temperature SOFCs. Chapter 1A presents the physico-chemical principles of SOFCs functioning, the demands imposed on the components materials, together with a literature survey on the state of-the art technology. Starting from more “conventional” oxygen ion conducting electrolytes, the need for reducing the operation temperature leads to a discussion on the properties of proton conducting materials as a feasible alternative to reach the goal of fabricating an IT-SOFCs. Chapter 2A describes the main properties of ceramic proton conductors. Several perovskite-type oxides, such as doped BaCeO3, SrCeO3, BaZrO3, and SrZrO3, show proton conductivity in the IT range when exposed to hydrogen and/or water vapour containing atmospheres. They are generally known as high temperature proton conductors (HTPCs). The main challenge in the field of HTPC is to find a compound that concurrently satisfies two of the essential requirements for fuel cell application, namely high proton conductivity and good chemical stability under fuel cell operating conditions. The second part of this dissertation describes the experimental results achieved during the research carried out. In view of the considerations given in Chapter 2a, Chapter 1B describes the optimization of the sol-gel procedure to prepare BaZr0.8Y0.2O3-δ (BZY) proton conductor electrolyte. Producing BZY powders with controlled compositional homogeneity and microstructure using a proper synthesis method could improve the electrochemical performance of this electrolyte. The optimized sol–gel procedure allowed the reduction of the diffusion path up to a nanometric scale, and thus required lower calcination temperatures. Nanocrystalline single-phase powders of BZY were produced at temperatures as low as 1100 °C. The same sol-gel procedure was also used to synthesize BaCe0.8Y0.2O3-δ (BCY) proton conductor electrolyte achieving also in this case nanometric particles powder at the calcination temperature of 100°C. The performance of the synthesized BZY and BCY proton conductors were examined in terms of chemical stability. After exposure to CO2 at high temperatures, the synthesized BZY powders presented good chemical and microstructural stability, differently from BCY which strongly decomposed after the CO2 treatment. Electrical conductivity and fuel cell performance were investigated only for the stable BZY electrolyte, however without achieving the required performance for practical application. Chapter 2 presents the application of the optimized synthetic procedure to the preparation of different proton conductor electrolytes. To further improve the electrochemical performance of barium zirconate electrolyte, the B-site of the BZY perovskite structure was doped with Ce producing several BaZr0.8-xCexY0.2O3-δ compounds (0.0≤x≤0.8). The prepared samples were analyzed in terms of chemical stability in CO2 environment, electrical conductivity, microstructural characteristics, and finally under fuel cell tests. Among the tested electrolytes, the BaZr0.5Ce0.3Y0.2O3-δ composition represented the best compromise between electrical performance and chemical stability. In fact it was able to maintain almost the same chemical stability of BZY, but with improved, more than twice, fuel cell performance. Chapter 3 describes a further improvement of the HTPC electrolyte performance. To obtain a highly conductive and chemically stable proton conductor electrolyte, a sintered Y-doped barium cerate (BCY) pellet was protected with a thin BZY layer, grown by pulsed laser deposition. The overall performance of the bilayer electrolyte turned out to be of great interest for practical use in IT-SOFCs application. The promising performance of this bilayer electrolyte rose from the very good crystallographic matching at the interface between the two materials, as well as the microstructure properties of the protecting layer in terms of uniformity, density and filling factor. However, while the bilayer conductivity was only slightly smaller than the conductivity of the BCY pellet, the measured fuel cell performances were negatively affected by the interface of the Pt electrodes with the BZY layer. For this reason the development of a superior cathode is crucial to make IT-SOFCs based on proton conductors competitive with the more established SOFCs using oxygen-ion conductor electrolytes. Chapter 4 focuses on the optimization of composite cathodes for application in IT-SOFC based on HTCP electrolytes. To explore different cathode materials with respect to the most commonly used for proton conductor electrolytes, such as platinum or cobalto-ferrites, the area specific resistance (ASR) of composite cathodes was investigated. Firstly, BaCe0.9Yb0.1O3-δ (10YbBC) and SrCe0.9Yb0.1O3-δ (10YbSC) were tested as cathode materials since they show mixed protonic-electronic conductivity. However, the ASR of the interface of these cathode materials with Y-doped barium cerate proton conductor electrolyte was extremely large, probably because of their too low partial electronic conductivity. For this reason, La1-xSrxCo1-yFeyO3-δ (LSCF), which presents high electronic conductivity, was combined with 10YbSC or 10YbBC to form composite cathodes. LSCF was chosen also because it allows faster oxygen surface exchange being a mixed O2-/e- conductor. The lowest ASR values were achieved with the composite cathode made of LSCF and 10YbBC in a1:1 ratio. Single phase Pt and LSCF cathodes were tested and it was found that they showed higher interfacial resistance than LSCF/10YbBC(1:1) composite cathode. This finding clearly suggests the importance of the proton conductor phase within the electrode, which actually should increase the triple phase boundary (TPB) density and so improve the cathode performance. The good performance observed for LSCF/10YbBC(1:1) composite cathode make it a cheaper and more efficient alternative to the Pt cathode that can actually improve the performance of IT-SOFCs based on proton conductor electrolytes.

The purpose of this work is to perform a three-dimensional and stationary numerical study of the heat transfer phenomenon in the planar anode-supported solid oxide fuel cells operating at intermediate temperature (IT-P-AS-SOFC). With particular interest to evaluate and localize the maximum and minimum temperatures in a single cell during their stable operation according to two geometrical configuration types, repetition, and symmetry of the cell stages to determine the best configuration that minimizes and produces more homogeneous thermal stresses and logically improves their lifetime and performance. The considered heat sources are mainly due to electrical overpotentials (Ohm, activation, and concentration). The results are obtained according to a FORTRAN code based on the proposed model that is numerically modeled using the finite difference method. From the obtained result analysis, the achieved temperature values by IT-P-AS-SOFC with cell stages repetition are greater than obtained by IT-P-AS-SOFC with cell stages symmetry.

Pr0.3Sr0.7Co0.3Fe0.7O3−δ (PSCF3737) was prepared and characterized as a cathode material for intermediate temperature-operating solid oxide fuel cell (IT-SOFC). X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and electrical property measurement were performed to study cathode performance of the material. XPS and EXAFS results proved that oxygen vacancy concentration was decreased and lattice constants of the perovskite structure material was increased by doping Fe up to 70 mol% at B-site of the crystal structure, which also extended the distance between oxygen and neighbor atom. Thermal expansion coefficient (TEC) of PSCF3737 is smaller than that of Pr0.3Sr0.7CoO3−δ (PSC37) due to lower oxygen vacancy concentration. PSCF3737 showed better cathode performance than PSC37. It might be due good adhesion by a smaller difference of TEC between Gd0.1Ce0.9O2 (CGO91) and electrode. Composite material PSCF3737-CGO91 showed better compatibility of TEC than PSCF3737. However, PSCF3737-CGO91 did not represent higher electrochemical property than PSCF3737 due to a decline of reaction sites by CGO91.

Recently we have started a project to develop 1 kW-class SOFC system for Residential Power Generation (RPG) application supported by Korean Government. For a 1 kW-class SOFC stack that can be operated at intermediate temperatures, we started to develop anode-supported, planar type SOFC to have advantages for commercialization of SOFCs considering mass production and using cost-effective interconnects such as ferritic stainless steels. Anode-supported single cells with thin electrolyte layer of YSZ were fabricated and their small stacks were built and evaluated. The size of anode-supported single cells after final sintering was about 5 × 5 cm2, and the thickness of electrolyte and the cathode layer were about 20 μm and 30 μm, respectively. I-V and AC impedance characteristics of these single cells and small stacks were evaluated at intermediate temperature (650 ∼ 750°C) by using hydrogen gas as a fuel. We have already carried out long-term performance test for YSZ thin electrolyte single cell for above 26,000 h (3 years) at 750°C, applying 0.76 V with power density of 200 mW/cm2. Using these YSZ thin electrolyte 5 × 5 cm2 cells and Inconel interconnect plates coated by silver paste, the 15-cell and 60-cell short stack were prepared. The initial stack (15 cell) voltage at 150 mW/cm2 was 12.5 V in hydrogen as fuel of 120 sccm/cell at 750°C and decreased to about 10.9 V at 500 h of operation time. It was then stabilized and kept until 4,000 h with a degradation rate of 10 mV/(1000 h, 1 cell). AC impedance of this small stack and microstructure of cell components were analyzed during and after the operation. Furthermore thin electrolyte cells and ferritic stainless steel interconnects were built into a 4-cell stack and the small stack was operated at 650°C for cost-effective planar SOFC RPG system. I-V and AC impedance characteristics of the small stack were evaluated at 650°C by using hydrogen as a fuel.

Operational temperature around 800°C is desirable for solid oxide fuel cells (SOFC) due to alleviation of many serious problems, associated with high temperature, i.e., high degradation rate and cost of balance of plant components along with the need for expensive ceramic interconnect. This paper is concerned with the performance of hybrid cycles employing the intermedium temperature SOFC and a gas turbine. The calculations are performed with Aspen Plus® for a system in a size of 500 kW, using methane as fuel. The simulation tool is completed by a mathematical model of the fuel cell. Cell geometry is chosen to represent the type of cells developed at Risø National Laboratory. For the stand alone SOFC, introduction of the metallic interconnect gave an overall performance improvement. A maximum electric efficiency of more than 70% for the system was calculated at low pressure ratios.

Reducing operation temperature of Solid Oxide Fuel Cell (SOFC) may provide many advantages for material selections of sealing, interconnects and Balance-Of-Plant (BOP). This study focus on the advanced performance of another cathode material and the performance according to structure change about Pr1−xSrxCoO3−δ compositions (PSCs, x = 0, 0.3, 0.5 and 0.7). High temperature XRD measurement and electrochemical impedance methods were used to study the characteristics of the material as a cathode material for Intermediate Temperature-operating Solid Oxide Fuel Cell (IT-SOFC) application. Lattice parameters and crystal structures of PSCs as well as Area Specific Resistance (ASR) of PSCs on solid oxide electrolytes are discussed at various x values (x = 0, 0.3, 0.5 and 0.7) and temperatures. One of various compositions of PSC showed 0.17 Ωcm2 of ASR on 10% Gd-doped cerium oxide at 700°C.

We are continuing a national project to develop 1 kW-class SOFC system for Residential Power Generation (RPG) application supported by Korean Government. For intermediate temperature operation, we chose anode-supported, planar type SOFC design to have advantages for commercialization of SOFCs considering mass production and using cost-effective interconnects such as ferritic stainless steels. Anode-supported single cells with thin electrolyte layer of YSZ or ScSZ, respectively, were fabricated and their small stacks were built and evaluated. The size of anode-supported single cells finally sintered was about 10 × 10 cm2, and the thickness of electrolyte and the cathode layer was about 20μm and 30μm, respectively. The I-V and AC impedance characteristics of these single cells and small stacks were evaluated at intermediate temperature (650 ∼ 800°C) by using hydrogen gas as a fuel. We have already carried out long-term performance test for YSZ thin electrolyte single cell for above 33,000 h (3.8 years) at 750°C, applying 0.76 V with power density of 200 mW/cm2. Using these YSZ thin electrolyte 10 × 10 cm2 cells and Inconel interconnect plates coated by silver paste, the 15-cell and 60-cell short stack were prepared. The initial stack voltage at 150 mW/cm2 was 12.5 V in hydrogen as fuel of 120 sccm/cell at 750°C and decreased to about 10.9 V at 500 h operation time. It was then stabilized until 4,000 h with a degradation rate of 10 mV/(1000h, 1 cell). AC impedance of this small stack and microstructure of cell components were analyzed during and after the operation. Furthermore ScSZ thin electrolyte 10 × 10 cm2 cells and ferritic stainless steel interconnects were built into a 5-cell stack and the small stack was operated at 650°C for cost-effective planar SOFC RPG system. I-V and AC impedance characteristics of the small stack were evaluated at 650°C by using hydrogen gas or methane gas as fuel.

In this work, the performance of solid oxide fuel cells at an intermediate temperature (600°C) that uses reformate gas and butane as fuel sources is investigated. Anode materials consisting of Ni and Ce0.9Gd0.1O2 (CGO91) and Ni and Y0.08Zr0.92O2 (8YSZ) are tested as steam reforming catalysts. Anode materials using NiO/CGO91 steam to carbon ratio of 3 and butane as the fuel source result in the better performance. However, even if the gas hourly space velocity is very low and NiO/CGO91 is used as the anode, the conversion of butane is not 100%. Additives are added to the NiO/CGO91 materials to increase the conversion of butane. Among the additives tested, the Rh is the most effective, resulting in 100% of butane conversion and no carbon deposition. Moreover, Rh added NiO/CGO91 SOFC single cell have a very low degradation rate when butane is directly used in conditions of steam to carbon ratio 3.

Combined Heat and Power (CHP) generation units based on intermediate temperature (600∼800°C) solid oxide fuel cell (SOFC) modules have been collaboratively developed by Mitsubishi Materials Corporation and The Kansai Electric Power Co., Inc. Currently, hydrocarbon fuel utilising units designed to produce modular power outputs up to 10 kWe-AC with overall efficiencies greater than 80% (HHV) are being tested. A unique seal-less stack concept is adopted to build SOFC modules accommodating multiple stacks incorporated of stainless steel separators and disk-type planar electrolyte-supported cells. In order to advance the current technology to achieve improved levels of efficiency and reliability, through design iterations, computational modelling tools are being heavily utilised. This contribution will describe the results of coupled computational fluid dynamics (CFD) analysis performed on our fourth-generation 1 kW class SOFC stack. A commercially available CFD code is employed for solving the governing equations for conservation of mass, momentum and energy. In addition, a local electrochemical reaction model is coupled to the rest of the transport processes that take place within the SOFC stack. It is found that the CFD based multi-physics model is capable of providing necessary and proper guidance for identifying problem areas in designing multi-cell SOFC stacks. The stack performance is also estimated by calibrating the computational model against data obtained by experimental measurements.

Abstract The previous results have shown that dense bismuth oxidebased electrolytes can be fabricated simply by plasma spraying owing to their low melting point. In this study, the Bi2O3– Er2O3–WO3 electrolyte of high ionic conductivity was deposited by the cost-effective plasma spraying to assemble the SOFC for examining its electrochemical performance. The SOFC cell consisted of FeCr24.5 metal support, NiO-YSZ anode, 10 mol% scandium oxide-stabilized zirconium oxide (ScSZ) electrolyte, (Bi2O3)0.705 (Er2O3)0.245 (WO3)0.05 (EWSB) electrolyte, and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) cathode. The ScSZ electrolyte interlayer was introduced between the anode and EWSB electrolyte to hinder the reduction of EWSB in the anode environment. NiO-YSZ, ScSZ, EWSB, and LSCF were deposited by plasma spraying on the metal support which was prepared by a press-forming-sintering process. The NiO-YSZ/ ScSZ/ EWSB/ LSCF single cell assembled with the as-sprayed ScSZ presented an open circuit potential of 0.90V at 600 °C and the maximum power density of 1130 mW cm-2 at 750 °C, 450 mW cm-2 at 650 °C, and 128 mW cm-2 at 550 °C. The plasma sprayed ScSZ electrolyte was then densified through impregnating using yttrium and zirconium nitrate solutions followed by annealing treatment. The single cell assembled with the densified ScSZ presented an open circuit potential up to 1.004V at 600 °C and the maximum power density of 1356 mW cm-2 at 750 °C, 619 mW cm-2 at 650 °C, and 163 mW cm-2 at 550 °C. The performance of the cell was significantly improved by the post-spray densification treatment of the ScSZ electrolyte. The present result shows that the high performance NiO-YSZ/ScSZ/EWSB/LSCF cell at intermediate temperatures can be successfully fabricated by plasma spraying.

This paper analyzes the dynamic behaviour of a 5 kW fuel cell system based on planar co-flow Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) stack, with internal reforming. The system is composed by the SOFC stack, a combustor of the cell exhausts, two heat exchangers for fuel and air preheating and the related control valves, where the air temperature at the stack exit and the fuel utilization is controlled by means of a PI (proportional integral) device. The model of the stack is based on a lumped parameters dynamic model of a single cell, which is composed of the fuel and air channels, the electrochemically active three layer region representative of the anode, the cathode and the electrolyte. The stack model is first used here for a qualitative steady-state validation, reproducing the cell characteristic curve. Then it is presented the dynamic model of the system, which has been implemented using an a-causal software based on the open-source Modelica modelling language, which allows for future integration in complex power-plant configurations. After a description of the plant layout and of the dynamic model, we present and discuss the results obtained by applying the PI controls to different load changes and with different tuning of the controller parameters, evidencing the amplitudes of load changes, the extent of the transient phase to the new steady-state conditions, the internal cell temperature distribution and the thermal gradients along the PEN structure, giving the possibilities to adapt the control system to the requirements of specific SOFC technologies.