Reaction characteristics of molten carbonate cell operated in fuel cell and electrolysis modes with reactant gas addition method

This work compares the electrode reaction mechanisms of 100 cm² class molten carbonate cells (MCCs) operated in electrolysis cell (EC) and fuel cell (FC) modes using a reactant gas addition (RA) method. The RA method reveals essential information on an electrode reaction mechanism by measuring the overpotential of an electrode resulting from adding a reactant. The hydrogen electrode (HE) is revealed to be under a gas-phase mass transfer-controlled process in both modes. In addition, the HE overpotential at an inlet composition of H₂: CO₂: H₂O = 0.3: 0.3: 0.4 atm is caused mainly by H₂ species in FC mode, while CO₂ contributes the majority in EC mode due to the production of H₂ and consumption of CO₂ by the water–gas shift reaction. On the other hand, most of the oxygen electrode (OE) overpotential is contributed by O₂ species in both modes. The overpotential induced by O₂ species was larger in FC mode than EC mode because EC mode generates O₂ and provides less mass transfer resistance of O₂ species in the liquid phase. The addition of CO₂ to the OE raised overpotential in both modes. The overpotential was especially large in FC mode due to the reduced O₂ partial pressure and relatively low in EC mode because of O₂ generation. Therefore, the total overpotential in EC mode is less than in FC mode.