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The tape casting and electrophoretic deposition processes are amenable to scale-up, and thin electrolyte structures (0.25-0.5 mm) can be produced. The ohmic resistance of an electrolyte structure24 and the resulting ohmic polarization have a large influence on the operating voltage of MCFCs (15). FCE has stated that the electrolyte matrix encompasses 70 of the ohmic loss (16) of the cell. At a current density of 160 mA/cm2, the voltage drop (∆Vohm) of an 0.18 cm thick electrolyte structure, with a specific conductivity of ~0.3/ohm-cm at 650 °C, was found to obey the relationship (14), ∆Vohm (V) = 0.5t (6-6) where ∆Vohm is in volts and t is the thickness in cm. Later data confirm this result (16). With this equation, it is apparent that a fuel cell with an electrolyte structure of 0.25 cm thickness would operate at a cell voltage that is 35 mV higher than that of an identical cell with an electrolyte structure of 0.18 cm thickness because of the lower ohmic loss. Thus, there is a strong incentive for making thinner electrolyte structures to improve cell performance. The electrolyte composition affects the performance and endurance of MCFCs in several ways. Higher ionic conductivities, and hence lower ohmic polarization, are achieved with Li-rich electrolytes because of the relative high ionic conductivity of Li2CO3 compared to that of Na2CO3 and K2CO3. However, gas solubility and diffusivity are lower, and corrosion is more rapid in Li2CO3. The major considerations with Ni-based anodes and NiO cathodes are structural stability and NiO dissolution, respectively (9). Sintering and mechanical deformation of the porous Ni-based anode under compressive load lead to performance decay by redistribution of electrolyte in a MCFC stack. The dissolution of NiO in molten carbonate electrolyte became evident when thin electrolyte structures were used. Despite the low solubility of NiO in carbonate electrolytes (~10 ppm), Ni ions diffuse in the electrolyte towards the anode, and metallic Ni can precipitate in regions where a H2 reducing environment is encountered. The precipitation of Ni provides a sink for Ni ions, and thus promotes the diffusion of dissolved Ni from the cathode. This phenomenon becomes worse at high CO2 partial pressures (17, 18) because dissolution may involve the following mechanism: NiO + CO2 → Ni2+ + CO=3 (6-7) The dissolution of NiO has been correlated to the acid/base properties of the molten carbonate. The basicity of the molten carbonate is defined as equal to -log (activity of O=) or -log aM2O, where a is the activity of the alkali metal oxide M2O. Based on this definition, acidic oxides are associated with carbonates (e.g., K2CO3) that do not dissociate to M2O, and basic oxides are formed with highly dissociated carbonate salts (e.g., Li2CO3). The solubility of NiO in binary 24. Electrolyte structures containing 45 wt% LiAlO2 and 55 wt% molten carbonate (62 mol% Li2CO3-38 mol% K2CO3) have a specific conductivity at 650°C of about 1/3 that of the pure carbonate phase (15). 6-7PDF Image | Fuel Cell Handbook (Seventh Edition)
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