SOFC

Solid oxide fuel cells employ a solid oxide material as electrolyte and are, thus, more stable than the molten carbonate fuel cells as no leakage problems due to a liquid electrolyte can occur. SOFC is a straightforward two-phase gas-solid system, so it has no problems with water management, flooding of the catalyst layer, or slow redox kinetics. On the other hand, it is difficult to find suitable materials that have the necessary thermal and stability properties for operating at high temperatures. 

As for MCFC, internal reforming in SOFCs is possible over the anode catalyst partial oxidation reactions and direct oxidation of the fuel have also been found to occur [24—28]. Different concepts for solid oxide fuel cells have been developed over the years. Flat plates have an easier stack possibility, while tubular designs have a smaller sealing problem. Monolithic plates and even single-chamber designs have been considered and investigated for SOFC use [29-31]. 

A different type of SOFC design is under development by SulzerHexis. The HEXIS (heat exchanger integrated stack) can be used for small cogeneration plants. The interconnect in this case serves as a heat exchanger as well as a current collector and is made by Plansee (Reutte, Austria) (see interconnect materials (ICMs)). Thermal spray coatings on the current collector can improve the stability of the system and performances were tested up to 3000 h [32]. 

The components of SOFC can be made in different ways. The main differences between the preparation techniques consist of the fact that the whole ceU can be made self-supporting (i.e., the electrode/electrolyte assembly supports the stmeture of the cell and no substrate is used) or supported whereby the electrodes and electrolyte are cast onto a substrate. In the anode-supported planar SOFC concept, with a 20 pm thin electrolyte layer, the operation temperature can be reduced significantly, for example, to 800 °C [34]. This reduces the material requirements considerably. 

From the beginning of SOFC development, it was found that LaSrMnOs (LSM) electrodes had a high activity for oxygen reduction at high temperatures and were stable under SOFC operation conditions. These LSM cathodes have been improved over time and it has been seen that an yttria stabilization of the cathodes improves the performance [35]. Single-phase LSM cathodes show a low oxide diffusion coefficient, so it is better to use a two-phase cathode that results in a lower overpotential for the oxygen reduction reaction. 

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AFC = all line fuel ceU MCFC = molten carbonate fuel ceU PAFC = phosphoric acid fuel ceU PEFC = polymer electrolyte fuel ceU and SOFC = solid oxide fuel ceU. 

Fig. 4. Schematic representation of the cross section of tubular configuration for SOFC.

An electrochemical vapor deposition (EVD) technique has been developed that produces thin layers of refractory oxides that are suitable for the electrolyte and cell interconnection in SOFCs (9). In this technique, the appropriate metal chloride (MeCl ) vapor is introduced on one side of a porous support tube, and H2/H2O gas is introduced on the other side. The gas environments on both sides of the support tube act to form two galvanic couples, ie. 

The tubular design is the most advanced SOFC technology. Tests of a nominal WFC 25-kW SOFC unit were started in 1992 at Rokko Island near Osaka, Japan in a joint program by Kansai Electric Co., Osaka Gas, and Tokyo Gas Co. This unit consists of 1152 cells, 50-cm length, which are contained... 

For high temperature fuel ceUs, there is stiU a strong need to develop lower cost materials for ceU components. In the case of SOFCs, improved fabrication processes and materials that permit acceptable performance in fuel ceUs at lower operating temperatures are also highly desirable. 

FIG. 27-67 Pol arization cuitcs at different teiiiperatnres for 50-cni active length thin-wall SOFCs. 

FIG. 27-68 Configiir ation of the tiilmlar SOFC. (Covriesij of Weslirij Iiouse Eleclnc Corporn-liori.)... 

FIG. 27-69 SOFC 25-kW system packige. (Coviiosij of WesliTij Jiouse EJeclnc Corpomlion.)... 

The measured pressure drops were slightly greater than literature data would indicate for packed carbon beds. However, they are certainly not prohibitive and a successful outcome of the Westinghouse trial of the SOFC guard bed is anticipated. 

Because this design has relatively low power density, recent work has focused on a monolithic SOFC, since this could have faster cell chemistry kinetics. The very high temperatures do, however, present sealing and cracking problems between the electrochemically active area and the gas manifolds. 

Conceptually elegant, the SOFC nonetheless contains inherently expensive materials, such as an electrolyte made from zirconium dioxide stabilized with yttrium oxide, a strontium-doped lanthanum man-gaiiite cathode, and a nickel-doped stabilized zirconia anode. Moreover, no low-cost fabrication methods have yet been devised. 

One leading prototype of a high-temperature fuel cell is the solid oxide fuel cell, or SOFC. The basic principle of the SOFC, like the PEM, is to use an electrolyte layer with high ionic conductivity but very small electronic conductivity. Figure B shows a schematic illustration of a SOFC fuel cell using carbon monoxide as fuel. 

Unlike the PEM, the ionic conduction occurs for the oxygen ion instead of the hydrogen ion. SOFCs are made of ceramic materials like zirconium (Z = 40) stabilized by yttrium (Z = 39). High-temperature oxygen conductivity is achieved by creating oxygen vacancies in the lattice structure of the electrolyte material. The halfcell reactions in this case are... 

Figure C shows an electron photomicrograph of a broken planar SOFC. The thick portion on the left is the porous anode structure. This is an anode-supported cell, meaning that in addition to collecting current and supporting the anode reaction, the anode layer stiffens the whole cell. The layer on the right is the cathode, and the interface between the two is the thin electrolyte. One of the challenges of this design is to ensure that the rates of expansion of the cathode and the anode match. If the anode expands faster than the cathode, the planar cell tends to curl like a potato chip when the temperature changes.

Reactions (3.9) to (3.11) proceed rapidly to equilibrium in most anodic solid oxide fuel cell (SOFC) environments and thus H2 (Eq. 3.8) rather than CH4 is oxidized electrochemically resulting in low polarization losses. Upon doubling the stoichiometric coefficients of equation (3.8), summing equations (3.8) to (3.11) and dividing the resulting coefficients by two one obtains ... 

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

This reaction is of great technological interest in the area of solid oxide fuel cells (SOFC) since it is catalyzed by the Ni surface of the Ni-stabilized Zr02 cermet used as the anode material in power-producing SOFC units.60,61 The ability of SOFC units to reform methane "internally", i.e. in the anode compartment, permits the direct use of methane or natural gas as the fuel, without a separate external reformer, and thus constitutes a significant advantage of SOFC in relation to low temperature fuel cells. 

Solid oxide fuel cell, SOFC anodes, 97 catalysis in, 98,410 cathodes, 96... 

SOFC Anode, cathode, electrolyte Powder synthesis... 

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

This presentation reports some studies on the materials and catalysis for solid oxide fuel cell (SOFC) in the author s laboratory and tries to offer some thoughts on related problems. The basic materials of SOFC are cathode, electrolyte, and anode materials, which are composed to form the membrane-electrode assembly, which then forms the unit cell for test. The cathode material is most important in the sense that most polarization is within the cathode layer. The electrolyte membrane should be as thin as possible and also posses as high an oxygen-ion conductivity as possible. The anode material should be able to deal with the carbon deposition problem especially when methane is used as the fuel. 

It was also found [8] that the sintering conditions have significant effects on the resistivity of the Smo.iCeo.gOi.g material. As shown in Fig. 4, the overall resistivity decreases with lower sintering temperature and attains a minimum at the sintering temperature of 1100-1200 °C, which is about 31 ohm-cm at 700 °C measurement. This makes the Smo.2Ceo.801,9 material capable of working as SOFC s electrolyte at temperatures lower than 700 C to avoid possible reduction of cerium (4+) and thus suitable for intermediate-temperature SOFC. 

In the search of high-performance SOFC anode, doped ceria have been evaluated as possible anode materials [9,10]. Comparing Ni-samaria-doped ceria (SDC) with Ni-YSZ, the Ni-SDC anode exhibits higher open-circuit voltages and a lower degree of polarization with either methanol as the fuel, as shown in Fig. 5, or methane as the fuel, as shown in Fig. 6. It was found that the depolarization ability of the anode is associated with the catalytic activity, the electrical conductivity, and the oxygen ionic conductivity of the anode materials [9]. It was also found that the anodic polarization and electro-catalytic activity strongly depend on the Ni content in the anode, and the optimum result for the Ni-SDC anode is achieved with 60... 

Over the anode, the hydrogen and CO produced via reactions (1) and (2) are then oxidized at the anode by reacting with the oxygen species transported from the cathode. The catalysis of the fuel such as methane at the anode and oxygen at the cathode becomes increasingly important with demanding catalytic activity as the SOFC operation temperature decreases, which is the aim under intensive research efforts. 

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