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.

Automated summary

This study compares the electrode reaction mechanisms of 100 cm2 class molten carbonate cells operated in electrolysis cell and fuel cell modes using a reactant gas addition method. The results show that the hydrogen electrode is under a gas-phase mass transfer-controlled process in both modes, while the oxygen electrode overpotential is mainly contributed by O2 species. The addition of CO2 raises the overpotential in both modes, especially in fuel cell mode due to reduced O2 partial pressure. As a result, the total overpotential in electrolysis cell mode is less than in fuel cell mode.