Electrochemical Study Of Ceria-carbonate Composite Electrolyte For Low Temperature Solid Oxide Cells

Reversible solid oxide cell?RSOC?is a novel energy conversion device with the advantages of high efficiency,no pollution and modularization.RSOC can be operated in solid oxide fuel cell?SOFC?mode that can directly convert chemical energy to electricity.It also can be operated in solid oxide electrolysis cell?SOEC?mode to convert electricity and heat energy into chemical energy for storage.Highly ion conductive ceria-carbonate composite is one type of highly promising electrolyte materials for low temperature SOFCs.Composite electrolyte with controllable/ideal microstructure,such as large oxide-carbonate interface and homogeneous element/phase distribution are desirable to further enhance electrical properties and to study the ionic conduction mechanism.In this work,we first report the successful synthesis of element/phase well-distributed,interfacial strongly coupled Sm_0.2Ce_0.8O₂-Na₂CO₃?NSDC?nanocomposite with different residual carbonate contents by in-situ one-pot one-step citric acid-nitrate combustion.The resultant NSDC shows distinct properties over those prepared by conventional methods and improved ionic,especial proton conductivity.The latter reaches to 0.045 S·cm^-1 at 650 ^oC,around 5 times higher than that of the oxide proton conductors.The electrolyte supported SOFCs based on resultant nanocomposite electrolyte NSDC9010 gave the best output of 281.5 mW·cm^-2 at 600?with LiNiO₂ symmetric catalysts among all composite electrolyte based SOFCs.We also correlate the excellent ionic conductivity and fuel cell performance to the unique core-shell structure and phase distribution induced by the one-step method,the strong coupling verified by the Raman spectroscopy characterization results and the optimal carbonate interfacial layer thickness by intentionally adjusting carbonate content and distribution.In addition,SOFC exhibits no observable degradation in performance for more than 8 h of continuous operation under real operation condition.Depending on electrolyte materials,two types of SOECs oxygen ion conducting SOECs?oxygen-SOECs?and proton conducting SOECs?proton-SOECs?are frequently investigated in the literature.Here we report our new findings in exploring a SOEC based on a mixed O^2-/H⁺-ion conductor that can transport both oxygen ion and proton at the same time,which is denoted as“Hybrid-SOEC”.The effect of the opetional parameters such as steam concentration and running temperature on the SOEC performance is systematically studied.When NSDC9010 was used as an electrolyte,the Hybrid SOEC shows the highest efficiency,demonstrating a current density of 0.35 A·cm^-2 at 1.3 V and 600 ^oC under 10%humidified hydrogen at fuel electrode and 10%humidified air at positive electrode,19%and 15%higher than that of oxygen oxygen-SOEC and proton-SOECs,respectively.Moreover,the Hybrid SOEC exhibits no observable degradation in performance for more than 27 h of continuous operation,implying a robust system for hydrogen production.