Electrode reactions on a molten carbonate Gd-doped CeO2 composite electrolyte

The complex chemical and electrochemical reactions involving CO2 and O2 at the cathode generally determine the overall performance of molten carbonate fuel cells (MCFCs) [1,2]. Experimental reports on the cathodic reactions consider merely the ions in the bulk molten carbonate (e.g. peroxide, superoxide, and peroxycarbonate ions, in addition to carbonate ions). Recently Czelej et al. used DFT calculations to investigate the MCFC cathode reactions at the perimeter of a triple phase boundary (TPB) of electronically conducting NiO, ionically conducting molten electrolyte, and pore space with reactant gases [1,2]. They showed a sequential Mars-van Krevelen and Eley-Rideal mechanism involving cathodic transformation of CO2 to CO32- via direct interaction of CO2 with the O-terminated octopolar NiO(111). Electrode reactions on a composite electrolyte with molten carbonate and ceramic oxide ion conductor have so far not been studied, however. In the present work, Gd-doped ceria (GDC) based composite membranes prepared by infiltration of Li2CO3-Na2CO3 eutectic molten phase in a pre-sintered porous GDC matrix were studied in the temperature range 575–675oC by electrochemical impedance spectroscopy (EIS) to characterize electrode reactions on them. A symmetrical cell configuration comprising two painted porous gold electrodes was used for the study. Three time constants associated with electrode processses were identified. The slowest process, with an activation energy of 130 kJ/mol showed a positive partial pressure dependency of exchange current density for O2, whereas the dependency was negative for CO2. Possible mechanisms are derived and discussed. References 1. K. Czelej et al., J. Mater. Chem. A 5 (2017) 13763-13768. 2. K. Czelej et al., Appl. Catal. B 222 (2018) 73–75.