Doped Ceria for Solid Oxide Fuel Cells

Lower valent cation-doped CeO2 materials have attracted remarkable research interest for the electrolyte application in solid oxide fuel cells operating in the intermediate temperature range (500–700°C). At these temperatures, the oxygen-ion conductivity of gadolinium-doped ceria is about an order of magnitude higher than that of yttria-stabilized zirconia. The oxygen-ion diffusion in the cubic fluorite structure of CeO2 is dependent on several factors such as charge valence and size of dopant cation, doping amount, etc. In the literature, several conductivity trends have been reported as a function of these parameters and are explained by the atomistic computational models. This chapter describes the highlights of the various activities that have been done in this regard to provide insights into the mechanisms underlying the oxygen-ion conduction process in acceptor-doped ceria

Development of Higher Ionic Conductivity Ceria Electrolyte

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A Strategic Co-doping Approach Using Sc³⁺ and Ce⁴⁺ toward Enhanced Conductivity in NASICON-Type Na₃Zr₂Si₂PO₁₂

NASICON-framework ceramic electrolytes are crucial for realizing the promise of all-solid-state sodium-ion batteries. The present work investigates the strategic co-doping approach used to enhance the sodium-ion conductivity in NASICON-type Na3Zr2Si2PO12. Sc3+ and Ce4+ are added as co-dopants for Zr4+ such that Sc3+ content is fixed at 16.5 mol % while the Ce4+ amount is varied from 0 to 5 mol %. All the samples are fabricated using the conventional solid-state reaction with the pressureless sintering performed at 1250 °C for 5 h. Although bare Na3Zr2Si2PO12 is synthesized using the monoclinic-ZrO2 precursor, the cubic-ZrO2 precursors are utilized to prepare the other remaining compositions. The microstructure of all the samples contains cuboidal-shaped grains, with the grain size varying from 1.2 to 0.9 μm. The bare Na3Zr2Si2PO12 possesses monoclinic-ZrO2 as an impurity phase that is found absent in other samples, emphasizing the importance of using cubic-ZrO2 precursor to eliminate the formation of a poor conducting phase. Owing to the low solubility of Ce4+ in the NASICON phase, several secondary phases are formed by adding more than 1 mol % Ce4+ in Na3.33Sc0.33Zr1.67Si2PO12. Substituting 16.5 mol % Sc3+ for Zr4+ in Na3Zr2Si2PO12 improves the ionic conductivity from 0.61 to 0.96 mS·cm–1 at room temperature, which is attributed to the presence of excess Na-ions to maintain the charge neutrality in the doped composition. However, replacing 1 mol % Ce4+ for Zr4+ remarkably raises the conductivity of Na3.33Sc0.33Zr1.67Si2PO12 from 0.96 to 2.44 mS·cm–1 at 25 °C. The optimized composition of Na3.33Ce0.02Sc0.33Zr1.65Si2PO12 exhibits more than four times higher sodium-ion conductivity than the bare Na3Zr2Si2PO12 at 25 °C. The detailed electrochemical and structural characterizations of these materials and the possible reasons for the observed superionic conduction in Na3.33Ce0.02Sc0.33Zr1.65Si2PO12 are discussed