Thermodynamic analysis of carbon deposition and electrochemical oxidation of methane for SOFC anodes

Solid oxide fuel cell (SOFC) reactions with a diluted dry methane feed were studied from chemical equilibrium analysis. The rate of carbon deposition and the composition of an anode gas mixture in equilibrium were predicted a s a function of current density by minimizing the total Gibbs free energy o f a system with 21 chemical species, including a condensed phase for carbon deposition. The equilibrium diagram is generally explained with three over all methane reactions of decomposition, partial oxidation, and direct oxida tion. A significant amount of carbon deposition is predicted from a very hi gh equilibrium conversion rate of methane via decomposition at the open cir cuit, and the carbon deposition is rapidly reduced as current applies to th e cell. There is a threshold current density above which no carbon depositi on is observed. This threshold current value distinguishes the range of cur rent density with clearly different equilibrium behaviors. Partial oxidatio n of methane is considered the major reason for the reduction of carbon dep osition in the current density range below the threshold value. Direct oxid ation of methane is considered dominant at current densities much higher th an the threshold value. Steam reforming does not seem to be influential in the entire range of current density. (C) 2001 The Electrochemical Society.