A Theoretical Study of the Carbon/Carbonate/Hydroxide (Electro-) Chemical System in a Direct Carbon Fuel Cell

Both the hydroxide and the carbonate melt are proposed and tested by researchers trying to develop a DCFC (Direct Carbon Fuel Cell). It is well known that the hydroxide melt is not stable due to the carbon dioxide formed in the fuel cell reaction. The hydroxide ion reacts with CO₂ to form carbonate ions and water. From this reaction it is clear that in either approach the melt is a mixture of carbonate and hydroxide depending on the partial pressures of water and CO2 above the melt. Therefore a good insight in the equilibria present in the melts is essential for understanding and optimizing the DCFC. Following the method introduced by Smith and Missen a complete and independent set of equilibria describing the chemical equilibrium in the melt can be obtained using linear algebra. Using the modification proposed by Coleman and White also electrochemical equilibria are included. This is done for the cathode as well as the anode environment of a DCFC with a carbonate and/or hydroxide melt as electrolyte. Hereby the open cell voltage for a DCFC including the Boudouard equilibrium could be calculated. It was found that the OCV increases as a function of temperature even more rapidly than the standard potential for the electrochemical oxidation of carbon to CO, which also has a positive slope due to a positive entropy change of the overall reaction. This extra high OCV is an additional argument for developing the DCFC in particular a DCFC at high temperatures in which predominantly CO is produced. Since CO can easily be shifted to hydrogen in a water gas shift reaction with steam, coproduction of hydrogen and power can be obtained using carbon and high temperature heat as energy inputs.