MCFC

Molten earbonate fuel cell systems can have the energy conversion effieieneies up to 50%, or up to 70% when combining the fuel cell with other power generators [19]. MCFCs ean operate on a wide range of different fuels and are not prone to CO or CO2 eontamination as is the case for low-temperature cells. For stationary power, molten carbonate fuel eells ean play an important role in power conversion units. 

Cathodes for MCFCs are usually NiO made by an anodic oxidation of a Ni sinter or by an in situ oxidation of Ni metal during the cell start-up time [18,20]. NiO cathodes are active enough for oxygen reduction at high temperatures, so a Pt-based metal is not necessary. A problem with the NiO cathode occurs as over time the NiO particles grow as they creep into the molten carbonate melt that reduces the active surface area and can cause short-circuiting of the cell. One of the solutions for this problem is the addition of small amounts of magnesium metal to the cathode and the electrolyte for stability. Also, the use of a different electrolyte that decreases the dissolution of the NiO cathode is possible. 

Alternatives for MCFC cathodes have been found in doped lithium oxide materials such as LiFeOi, Li2Mn03, and LiCo02 and also in combination with NiO materials to form double-layered electrodes. A tape casting of a NiO/LiCo02 double layer electrode improved the stability tremendously. The oxygen reduction reaction is improved at these double layer cathodes and the resistance is reduced [21]. 

NiAl or NiCr metals have been employed as MCFC anodes. These materials are used because Ni metal anodes are not stable enough under MCFC operating conditions as Ni creeps out [18,20]. Cermet (ceramic metal) materials avoid sintering, pore growth, and shrinkage of the Ni metal so that a loss of surface area does not occur. A low- cost process needs to be found, however, as these materials are still expensive to fabricate. 

The electrolyte for MCFCs is a molten carbonate that is stabilized by an alumina-based matrix. Initially, Li2C03/K2C03 (Li/K) carbonate materials were used as electrolytes. Degradation of electrode materials is a problem in this electrolyte. A Li/Na melt provides the advantage of a slightly more alkaline system in which the cathode and anode dissolution is lower as it prevents a dendritic growth of Ni metal. Li/Na electrolytes are expected to have a longer endurance and a lower decay rate than Li/K melts. 

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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. 

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Steam reforming of CH is commonly carried out at 750 to 900°C, thus at the lower operating temperature of MCFCs a high activity catalyst is required. The internal reforming of methane in IRMCFCs, where the steam-reforming reaction... 

Fig. 3. Schematics of gas manifolds for MCFC stacks (a) internally manifolded fuel cell stack (b) externally manifolded fuel cell stack.

Table 5. Performance Status of Continuous MCFC Technologies. ...

Programs to develop MCFC technology are also under way in Europe. Ansaldo SpA (Italy) is setting up faciUties to produce 1-m cells in an automated process, and their goal is to test 100-kW stacks in 1994. The 100-kW stack is also to be tested by IBERDROLA in Spain as part of a complete power plant system. Two Dutch companies. Stork and Royal Schelde, have joined with the Dutch government to form Brandstofcel Nederland (BCN), which plans to test a 50-kW MCFC and two 250-kW MCFC stacks in 1994. 

Hydrogen use as a fuel in fuel cell appHcations is expected to increase. Fuel cells (qv) are devices which convert the chemical energy of a fuel and oxidant directiy into d-c electrical energy on a continuous basis, potentially approaching 100% efficiency. Large-scale (11 MW) phosphoric acid fuel cells have been commercially available since 1985 (276). Molten carbonate fuel cells (MCFCs) ate expected to be commercially available in the mid-1990s (277). 

Molten Carbonate Fuel Cell The electrolyte in the MCFC is a... 

The PAFC is, however, suitable for stationary power generation, but faces several direct fuel cell competitors. One is the molten carbonate fuel cell (MCFC), which operates at "650°C and uses an electrolyte made from molten potassium and lithium carbonate salts. Fligh-teinperature operation is ideal for stationary applications because the waste heat can enable co-generation it also allows fossil fuels to be reformed directly within the cells, and this reduces system size and complexity. Systems providing up to 2 MW have been demonstrated. 

On the negative side, the MCFC suffers from sealing and cathode corrosion problems induced by its high-temperature molten electrolyte. Thermal cycling is also limited because once the electrolyte solidifies it is prone to develop cracks during reheat-... 

MCFC Cathode Protective coatings to decrease dissolution rate... 

Electrolyte loss occurring in long-term operation of MCFC is another problem to be solved for practical application of MCFC. For commercialization, the MCFC should show stable performance over 40,000 hours. Electrolyte loss in MCFC is caused by various factors, e.g., corrosion of components, creepage, reaction with cell components and direct evaporation. These... 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

In order to describe the geometrical and structural properties of several anode electrodes of the molten carbonate fuel cell (MCFC), a fractal analysis has been applied. Four kinds of the anode electrodes, such as Ni, Ni-Cr (lOwt.%), Ni-NiaAl (7wt.%), Ni-Cr (5wt.%)-NijAl(5wt.%) were prepared [1,2] and their fractal dimensions were evaluated by nitrogen adsorption (fractal FHH equation) and mercury porosimetry. These methods of fractal analysis and the resulting values are discussed and compared with other characteristic methods and the performances as anode of MCFC. 

The wetting ability of the anode electrode was evaluated as the contact angle measured by the capillary rise method. The value of fractal dimension of anode electrode of MCFC was calculated by use of the nitrogen adsorption (fractal FHH equation) and the mercury porosimetry. 

The wetting ability of an MCFC electrode is closely related to the performance of cell operation especially including electrochemical reaction, and can be expressed as contact angle between electrolyte and electrode. The surface energy of 3 phases, geometric structure of anode electrode and... 

MCFC CH4, Coal 650 C 100-5000 50-55 150-300 10,000-40,000 1250 Base load and intermediate load power generation, cogeneration... 

PAFC, phosphoric acid fuei ceii MCFC, moiten carbonate fuei ceii SOFC, soiid oxide fuei ceii PEMFC, proton exchange membrane fuei ceii DMFC, direct methanoi fuei ceii AFC, alkaiine fuel cell. 

Figure 28. Isotherms of the shear viscosities of (Li, Na)2C03. (Reprinted from Y. Sato, T. Yamamura, H. Zhu, M. Endo, T. Yamazaki, H. Kato, and T. Ejima, Viscosities of Alkali Carbonate Melts for MCFC, in Carbonate Fuel Cell Technology, D. Shores, H. Mam, I. Uchida, and J. R. Selman, eds., p. 427, Fig. 9, 1993. Reproduced by permission of the Electrochemical Society, Inc.)...

High-temperature molten-carbonate fuel cells (MCFCs). The electrolyte is a molten mixture of carbonates of sodium, potassium, and lithium the working temperature is about 650°C. Experimental plants with a power of up to... 

Just as the aqueous, alkaline fuel cell can be adopted to C02 separation and concentration, the molten carbonate fuel cell (MCFC) can function in this application as well. Recall that the MCFC cathode operates with the net reaction... 

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