Effect of gas-phase transport in molten carbonate fuel cell

Electrode reaction characteristics involving gas-phase transport effect have been investigated with several 100 cm2 class molten carbonate fuel cells (MCFCs). Although the MCFCs operate on gas-phase reactants at relatively high temperature, most of studies on the electrode reaction kinetics have been confined within kinetic-control and liquid-phase mass-transfer regions. To evaluate the gas-phase transport effect in the MCFC, an inert gas step addition (ISA) method was devised in this work. The ISA varies reactant flow rate for an electrode by adding an inert gas, which results in an overpotential shift. Since gas-phase mass-transfer resistance should be a function of reactant flow rate, correlation of the overpotential shifts with the reactant flow rates yields valuable information regarding the gas-phase transport effect in the electrode. The ISA was performed at both the anode and cathode with respect to reactant gas flow rates, addition amounts of inert gas, inert gas species, and currents applied to the cell. The overpotential shifts for both the anode and cathode were found to be inversely proportional to the square root of the reactant flow rate, indicating gas-phase mass-transfer control of both these electrodes. Especially, the overpotential shift values for the anode are much larger than those for the cathode, which suggests that the anode is under severe gas-phase mass-transfer control. From the partial pressure dependence of the overpotential shifts in the cathode, the cathode was found to be a combined gas and liquid-phase mass-transfer control system.