SOFC Specifications

FZJ followed the principles of this quality system. By adopting the principles of the ISO 9000 system, it can be assured that any system that was implemented will be fully compatible with other existing and possible future SOFC-specific standards. 

As already pointed out, the need for standardizing SOFC measurements and QA is strongly recommended for approaching extant problems with consistency and repeatability and for rehabihty of data. The cell testing procedure and the QA system used at FZJ were developed starting in 2004. AU the aspects were entered in the QA system following the outlines of the ISO 9000 series standards, from SOFC specifications until reporting of the experimental data. It was found that only measurements performed under such specifications and with optimized experimental conditions resulted in consistent and reliable tests and measurement results. 

There are a number of informative reviews on anodes for SOFCs [1-5], providing details on processing, fabrication, characterization, and electrochemical behavior of anode materials, especially the nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes. There are also several reviews dedicated to specific topics such as oxide anode materials [6], carbon-tolerant anode materials [7-9], sulfur-tolerant anode materials [10], and the redox cycling behavior of Ni-YSZ cermet anodes [11], In this chapter, we do not attempt to offer a comprehensive survey of the literature on SOFC anode research instead, we focus primarily on some critical issues in the preparation and testing of SOFC anodes, including the processing-property relationships that are well accepted in the SOFC community as well as some apparently contradictory observations reported in the literature. We will also briefly review some recent advancement in the development of alternative anode materials for improved tolerance to sulfur poisoning and carbon deposition. 

With an emphasis on scale electrical conductivity (surface stability as well), a number of new alloys have been recently developed specifically for SOFC interconnect applications. The one that has received wide attention is Crofer 22 APU, an FSS developed by Quadakkers et al. [136, 137] at Julich and commercialized by Thyssen Krupp of Germany. Crofer 22 APU, which contains about 0.5% Mn, forms a unique scale, as shown in Figure 4.6, comprised of a (Mn,Cr)304 spinel top layer and a chromia sublayer [137-139], The electrical conductivity of (Mn,Cr)304 has been reported... 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

SOFC are produced with either tubular or planar stack configurations investments for planar design are a rough estimate, as no prototypes exist. Specific investments for PAFC are in the range 4000- 4500/kW (IEA, 2007). For further fuel-cell R D needs see IEA (2005). 

The main catalysis challenges related to SOFC anodes can be summarized as the specification of an anode material or mix of materials having the following properties ... 

Hence, catalysis related challenges for SOFC cathode are the development of cathode specifications, i.e., material and microstructure, having high catalytic activity for oxygen reduction at 600 °C, high electron and ion conductivity, and a low sensitivity for poisoning by volatile Cr species. Again, as for the anode, cost and compatibility related requirements have to be considered. 

The voltage losses in SOFCs are governed by ohmic losses in the cell components. The contribution to ohmic polarization (iR) in a tubular cell" is 45% from cathode, 18% from the anode, 12% from the electrolyte, and 25% from the interconnect, when these components have thickness (mm) of 2.2, 0.1, 0.04 and 0.085, respectively, and specific resistivities (ohm cm) at 1000°C of 0.013, 3 X 10, 10, and 1, respectively. The cathode iR dominates the total ohmic loss despite the higher specific resistivities of the electrolyte and cell interconnection because of the short conduction path through these components and the long current path in the plane of the cathode. 

As can be seen, there is a reasonably large range in the value of K between these references. As the SOFC technology matures, these differences may reconcile to a more cohesive set of values. In the interim, the following single average combination of the above K values may help the reader if no specific information is available. 

Because SOFCs operate at high temperature, they are capable of internally reforming fuel gases (i.e., CH4 and other light hydrocarbons) without the use of a specific reforming catalyst (i.e., anode itself is sufficient), and this attractive feature of high temperature operation of SOFCs has recently been experimentally verified. Another important aspect of SOFCs is that recycle of CO2 from the spent fuel stream to the inlet oxidant, as required by MCFCs, is not necessary because SOFCs utilize only O2 at the cathode. 

The European Union sponsors the Real SOFC program, which aims to raise the durability of p lanarSOFCstackstoa level acceptable for stationary applications to find materials, manufacturing routes and standards suitable for low cost production and, to reduce specific weight and volume of SOFC stacks. The consortium includes 25 of Europe s leading research and industrial organisations. 

Germany - has been involved in demonstrating high temperature fuel cells (MCFC, SOFC) for stationary applications over the past decade. The national ZIP program nc udes projects specifically related to fuel cells, such as programs for the development and demonstration of fuel cells for residential applications (2-5 kW PEM). The ZIP program concentrates on further development and demonstration of six 250 kW MCFC block power plants and the 125 kW SOFC block power plant. 

Start up company HT-Ceramics, which has developed a specific (anode supported thin layer electrolyte) SOFC system. The PEM research is carried out by the Paul Scherrer Institute in collaboration with the Federal Institute of Technology Zurich and the University of applied science in Biel. The Paul Scherrer Institute is also researching DMFC systems, while the Federal Institute of Technology Lausanne is involved in research activities for HTcceramics. 

However, these advantages may be offset by the use of high temperature seals and the fact that the components will be subject to more stress. Besides mechanical stress due to seals, thermal stresses may appear because of temperature gradients and cycling. Few results related to the performance of this type of SOFCs have been published. An area power density of 0.12 W/cm2 has been reported without any specification of oxidant and fuel composition and utilization [117]. 

Interestingly, research has started on single chamber SOFC (SC-SOFC) concepts. However, the SC-SOFC exhibits inherently low power density and is therefore primarily of academic interest. It has the potential to relax cell component requirements and probably to ease manufacture. The principle of SC-SOFC is that it is fed by an air fuel mixture which flows onto the PEN contained in a single compartment, avoiding the use of gas separator plates and high temperature sealants. The fluid may flow simultaneously or sequentially along the electrodes. Both electrodes are either built onto the same side of the electrolyte some distance apart or on opposite sides. Low temperature operation would apparently suppress direct combustion of the air fuel mixture provided the electrode materials chosen are highly selective towards their respective catalytic reactions. SC-SOFC stacks may hold prospects in specific applications where the reaction products are the prime focus. 

When, on the other hand, the model is used as a tool for designing or improving a specific component of the fuel cell, it is important that the model is capable of providing very detailed information on the performance-related variables in that specific component. Examples of such analyses are copious in the literature (e.g. [4-8]), and most of them are developed at single cell level, with particular emphasis on one particular component or cell characteristic. Chan et al. [4], for example, applied an SOFC single cell model for analyzing the effect of the electrodes and... 

The so-called micromodels are models of a particular component, or of a part of a cell component, conducted at molecular or atomistic level. Due to the high level of detail related to the material properties and characteristics, the information provided by such models is usually limited to the specific phenomenon analyzed, and provides only limited indications on the resulting fuel cell performance and operating conditions. However, the results of such models play a fundamental role in understanding, analyzing and designing improved solutions for SOFC. Moreover, the results of such analyses may be used as an input for macro-models, i.e. models conducted at fuel cell level. 

The mass diffusive flux m, of Equation (3.2) generally depends on the operating conditions, such as reactant concentration, temperature and pressure and on the microstructure of material (porosity, tortuosity and pore size). Well established ways of describing the diffusion phenomenon in the SOFC electrodes are through either Fick s first law [21, 34. 48, 50, 51], or the Maxwell-Stefan equation [52-55], Some authors use more complex models, like for example the dusty-gas model [56] or other models derived from this [57, 58], A comparison between the three approaches is reported by Suwanwarangkul et al. [59], who concluded that the choice of the most appropriate model is very case-sensitive, and should be selected, according to the specific case under study. 

The micro-tubular SOFCs considered are depicted in Figure 4.19. Specifically, Figure 4.19 shows the anode (supporting structure), the anode plus the electrolyte, and the final single cells. More details about the production process, the cell properties and characteristics can be found in [13-15],... 

As for the power density of SOFC systems, a comparison between the different kinds of cells has been made (Vora, 2006). The measured specific power (at 1000 K, fuel utilization ratio of 80% and 0.65 V) of a tubular SOFC bundle (24 cells) is around 0.13 in W/cm3, whereas that of an HPD5 bundle (six cells) is of 0.17 in W/cm3. The Delta9 configuration, with bundles of nine cells, reaches over 0.4 W/cm3. A comparison is shown in Figure 7.9. 

The SOFC model introduced in this section only solves the energy equation and the current conservation. The necessary information concerning fluid dynamics and diffusion of species is set through specific assumptions. 

In Part Two, the reader is provided with practical examples of how the general equations defined in part one can be simplified/adapted to specific cases. The examples cover the most widely employed single cell designs, for steady-state and dynamic conditions, and evolve towards stack and system modehng. Finally, Chapter 10 introduces the reader to the problem of mechanical stresses in SOFC, and shows an approach for modeling mechanical stresses induced by the operating conditions. 

Abstract Single-chamber solid oxide fuel cells (SC-SOFCs) immerse the entire cell in a mixture of fuel and oxidizer gases within a single chamber, which eliminates the need for high temperature sealant, simplifies construction, and increases reliability over traditional double-chamber cells. However, there are challenges, such as low fuel utilization and electrode catalytic selectivity, that need to be overcome. This brief review paper looks at recent improvements in materials, processing, and operation of SC-SOFCs, which are rapidly approaching the performances of the double-chamber fuel cells and may become attractive for specific fuel cell applications. 

Figure 11. Cathode events in an SOFC fuel cell (schematic). (As regards the free enthalpy profde (1) the configuration effect is included in the free enthalpy in the case of gas diffusion (1), while otherwise the nonconfigurational value (2, 3) is shown.71) For the purpose of a simple presentation, it is assumed that the oxygen is completely ionized when it enters the electrolyte. Cf. text for a more specific discussion.71...

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