Direct Carbon Fuel Cells - Wetting Behavior of Carbon in Molten Carbonate

The Direct Carbon Fuel Cell (DCFC), which uses solid carbon as fuel and molten carbonate as electrolyte, has had resurgence of interest due to very high electrochemical conversion efficiency, nearly 100%, no requirement of fuel reforming, and potential for CO₂capture and sequestration. At the cathode, carbon dioxide is converted into carbonate ions. The main reaction at the anode is generation of carbon dioxide from carbon and carbonate, the net cell reaction being oxidation of carbon to carbon dioxide. Additional reactions that may be occur at the anode are a two-electron reaction resulting in co-generation of carbon dioxide and carbon monoxide and the Boudouard reaction leading to conversion of carbon and carbon dioxide to carbon monoxide, the so-called “carbon corrosion”. The performance of DCFC can be appreciably limited by this reaction. The reverse Boudouard reaction has not been studied in solid-melt-gas systems. It is important to study this reaction in conjunction with wetting behavior of carbon in molten carbonate.

With this in mind, the wetting behavior of carbon electrode in molten carbonate, comprised of a Li-K eutectic mixture, was studied in this study at different temperatures. The wetting behavior is complicated due to reactions occurring at the carbon surface before or after the start of wetting. Before wetting, carbon rods were exposed to a gas atmosphere with varying partial pressures of carbon dioxide to examine the extent of Boudouard reaction and the resulting structural changes in carbon rods. This reaction modifies the lateral surface of carbon electrode due to carbon corrosion, which leads to reduction in interfacial tension between solid carbon and molten carbonate after dipping carbon rods in molten carbonate.

Wetting behavior of carbon rods was studied for a period of 24 hours. The effects of temperature and pre-exposure to varying levels of carbon dioxide on speed of formation of liquid (molten carbonate) meniscus, propagation of liquid film thereafter, and the ultimate length of liquid film were studied. During the wetting process, bubble evolution occurred at the surface of carbon electrode not only below the meniscus, but also above the meniscus. This indicates that wetting of carbon electrode in molten carbonate is influenced not only by capillary forces but also by the reverse Boudouard reaction. The reverse Boudouard reaction is promoted at higher temperature, leading to changes to greater extent in the surface structure and therefore solid-liquid and solid-vapor interfacial tensions.

The lateral surfaces of graphite rods before wetting and after wetting, liquid films formed on the graphite rods, and cross-sections of cylindrical carbon rods above and below the meniscus were analyzed using SEM. The liquid films and circular cross-sections of carbon rods were also analyzed using energy-dispersive X-ray spectroscopy (EDX) and distributions and variations in oxygen and potassium were examined. Oxide ions are generated prior to and during wetting of carbon rods and play an important role in surface modification of these prior to and during wetting.

A reaction mechanism based on the experimental observations is proposed. The roles of Boudouard reaction, including equilibrium characteristics, and oxide ions on surface modification of carbon rods before and after wetting and the implications thereof on wetting behavior of carbon rods are explained satisfactorily by the reaction mechanism.