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

Abstract

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.

Analysis of the carbon anode in direct carbon conversion fuel cells

“…6, depends on porosity and specific area, and cannot be predicted from the model. The segment originates at the open circuit potential (À1.12 V vs. SHE) which is fixed by the relative concentrations of CO and CO 2 under conditions of Boudouard equilibrium at zero current, as calculated by Hemmes and Cassir [6]. Finally, the progressive shift from the low-to the high current segment should produce a continuous increase in h coul with current density, as first reported by Hauser [12] and Weaver [15].…”

“…The kinetics of the Boudouard reaction are quite complex and the topic is reviewed by Laurendeau [33], Calo and Perkins [34], and Hemmes and Cassir [6]. Our model does not require explicit inclusion of Boudouard kinetics.…”

“…The total efficiency h total of a carbon fuel cell is defined as the ratio of electrical energy output to the heat of reaction, typically referenced to the standard enthalpy of the combustion reaction, C þ O 2 ¼ CO 2 , DH ¼ À393.5 kJ/mol-C pathway for our model is related to that accepted for the Hall cell anode in cryolite-alumina melts, except the formation of the free oxide ion results from carbonate decomposition rather than by aluminooxyfluoride decomposition. In both models, step [6] is assumed to be rate-determining.…”

“…2) remains electrosorbed on the surface and is converted to CO 2 according to reaction (2). On the HCS, the rate-determining step [6] controls the overall reaction rate (steps [1] through [9]). Since the differential resistance of the high current segment is roughly an order of magnitude lower than that of the low current segment, reaction (2) quickly dominates the stoichiometry of the carbon oxidation as the total current is increased.…”

“…Following Coleman and White [5], Hemmes and Cassir [6] made use of matrix algebra techniques to determine the minimum number of chemical equations (three) necessary to describe the complex DCFC system under constraints of equilibrium. The carbonate will dissociate at equilibrium according to:…”