Role of wetting and transport in performance of molten carbonate fuel cells

Various issues related to the performance of Li/Na carbonate as an alternative electrolyte for molten carbonate fuel cells (MCFCs) are explored in this thesis. Li/Na carbonate shows superior performance (especially long-term) compared to state-of-the-art Li/K electrolyte, but with greater temperature sensitivity below the standard 650°C operating temperature. This has been ascribed to de-wetting of the porous electrodes at low temperatures. Satisfactory performance can be restored by using alkaline earth carbonate additives, which are believed to relate to wetting.;The wetting characteristics of molten Li/Na carbonate containing alkaline earth additives were experimentally measured and the dependence of the contact angle on polarization was thoroughly examined for various temperatures and gas atmospheres. The complex dependence of contact angle on polarization can be interpreted in terms of a superimposed electrocapillary and transport effects. Since a quantitative interpretation of the wetting behavior of these mixtures requires knowledge of their surface tension, as a function of composition and temperature, a method to predict the surface tension of molten alkali-alkaline earth carbonate mixtures, as a function of composition and temperature, has been developed based on the molecular theory of corresponding states.;Migrational separation due to differences in cationic mobility is commonly observed during current passage in molten carbonate mixtures, and this might be responsible for the improved wetting observed upon polarization, as found experimentally in this work. To check this, a 2-D transport model based on concentrated-solution theory was applied to analyze the movement of ions in and near the meniscus. By means of this model, the effect of differences in cationic mobility and of ionic transport in general, on current distribution, reaction rate, and electrolyte composition (cationic segregation) in the meniscus region was quantified, and corresponding surface tension gradients over the meniscus surface predicted.;Finally, an electrolyte distribution model which accounts for the wetting within electrode pores, was combined with an agglomerate-type performance model to study the effect of wetting and microstructure on electrode performance. This model helps in designing an electrode whose structure is capable of minimizing the electrolyte movement and keeping the electrolyte filled at an optimum value during MCFC operation.