Role of carbonate amount and synthesis procedure in the conductivity of SDC-Na₂CO₃ composite electrolytes for solid oxide cells applications

Composite electrolytes of sodium carbonate and samarium doped ceria (SDC-Na2CO3) provide outstanding proton conductivity between 300 degree celsius and 650 degree celsius, which is extremely sensitive to the synthesis procedure and to the amount and crystallinity of the carbonate phase. Here, the role of sodium carbonate in establishing the composite conductivity is explored in relation to chemical-structural and morphological characterization methods (ICP, XRD, SEM, TEM). A coprecipitation route is properly optimized to prepare composites with <50 nm SDC particles with sodium carbonate in different amounts. The amount of carbonate, carefully quantified via elemental analysis, strongly influences the proton conductivity, while the oxygen ion conductivity is much less affected. The formulation with 27 wt% of carbonate prepared through a single-step synthesis shows the best performance, with 2.27*10(-2) S cm(-1) proton conductivity in dry hydrogen (4 % H-2 in N-2) at 600 degree celsius, and 1.73*10(-2) S cm(-1) oxygen ion conductivity in air. Interestingly, an SDC-Na2CO3 composite containing the same salt amount but produced via a double-step procedure showed lower conductivity, confirming the pivotal role of the preparation methodology in defining the composite proton conductivity. In addition, humidification is found to depress H+ conductivity, thus indicating that the preferential charge transport mechanism does not involve hydroxides, in contrast to conventional protonic ceramics. Overall, the investigation reveals a strong dependence of the composite proton conductivity on the density of the carbonate/SDC interfaces and the crystallinity of the carbonate.