Codoped Ceria Ce_0.8M_0.1Gd_0.1O_2−δ (M = Sm³⁺, Sr²⁺, Ca²⁺) and Codoped Ceria–Na₂CO₃ Nanocomposite Electrolytes for Solid Oxide Fuel Cells

This work reports the coprecipitation synthesis and characterization of single-phase Sm3+/Sr2+/Ca2+ codoped with Gd3+ in cerium oxide formulated as Ce0.8M0.1Gd0.1O2−δ (CMGO; M = Sm3+, Sr2+, Ca2) nanopowder and their nanocomposites with sodium carbonate formulated as Ce0.8M0.1Gd0.1O2−δ–Na2CO3 (CMGO-NC). The structural properties of both the CMGO and CMGO-NC compositions were investigated by X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) techniques. The XRD results revealed that the crystallite sizes of the codoped ceria–sodium carbonate nanocomposite (CMGO-NC) were lower (∼8–15 nm) than their corresponding single-phase codoped ceria compositions. CMGO-NC has shown improved electrical conductivity in the intermediate temperature range (673–973 K), as measured by alternating-current impedance spectroscopy. The highest total electrical conductivity was found to be σ973 K = 2.3 mS·cm–1 with significantly reduced activation energy (Ea = 0.63 eV) for Ca–Gd-codoped ceria–sodium carbonate Ce0.8Ca0.1Gd0.1O1.85–Na2CO3 (CaG-NC). The morphologies of both the single-phase CaG and corresponding CaG-NC nanocomposite were investigated by field-emission scanning electron microscopy (FESEM) and HRTEM techniques in order to determine the size and shape effect by the sodium carbonate addition to the codoped ceria nanopowders. The FESEM images of the CaG-NC nanomposite showed significantly reduced particle size. The HRTEM images revealed that the secondary Na2CO3 phase was found to be an amorphous layer that has covered codoped ceria (Ce0.8Ca0.1Gd0.1O1.85) particles as a core–shell structure. The X-ray photoelectron microscopy studies support the CaG-NC composition and surface interaction between the carbonate and crystalline CaG phases. Enhancement in the net electrical conductivity of nanocomposites could be due to the reduced particle size and the core–shell morphology, which enables the superionic interfacial conduction in the composite. The CaG-NC core–shell nanocomposite is proposed to be a cost-effective variant of the solid electrolyte with improved electrical properties for operating solid oxide fuel cells at low/intermediate temperature (673–973 K).