"Navigating the impurity-obstructive electronegative interfacial carbonate on the hybrid ceria/carbonate interface for solid oxide fuel cells."

Ceria-carbonate composite electrolytes have received much attention in recent years for application in low-temperature solid oxide fuel cells (LT-SOFCs). The principal advantage of carbonate in composite electrolytes is its role in conductivity enhancement, proposed through several different mechanisms. This study navigates the scarcely studied obstruction caused by electronegative interfacial carbonate to impurity intrusion into the ceria lattice, with some interesting facts being noted. The methodically examined structure-property relationship of systematically annealed doped ceria, incorporated with carbonate at the grain boundary region, revealed a change in the nature of carbonate especially when annealed between 600 –800 °C. Owing to the electronegativity difference between carbonate and host ceria, the added impurities (magnesium and lithium) showed preferential bonding with the interfacial carbonate evidenced by a change in peak positions for carbonate in the Raman spectra. This was further supported by an increase in defect concentration for samples that were annealed at 600 °C, which unexpectedly declined when the annealing temperature was increased to 800 °C, as demonstrated by the quantitative analysis of Raman spectra. Electrochemical impedance spectroscopy (EIS) indicated lower activation energy in the presence of carbonate, leading to reduced electrolyte resistance, thereby resulting in enhanced interfacial conductivity. Volatilizing the interfacial carbonate by annealing the prepared samples at higher temperatures (1200 °C) resulted in a second-stage incorporation of impurities, especially lithium, in the ceria phase. The fuel cell performance of selected high-temperature annealed samples showed impressive results and suitability for fuel cell applications provided the materials prove to display long-term stability. Therefore, long-term stability studies with microstructural characterization would be needed as the carbonate-containing electrolytes may show residual porosity after sintering, which could affect gas-tightness and long-term stability.