*2.2. Core-Shell Cathode Preparation*

The mostly studied perovskite cathode materials contained (La, Sr)MnO3 (LSM); (La, Sr)(Co, Fe)O3 (LSCF); and (Ba, Sr)(Co, Fe)O3 (BSCF) series. The LSM material had excellent co-fired matching with a YSZ or ceria-based electrolyte in HTSOFC. Nevertheless, the electron-conductivity of LSM decreased as it was manipulated at an ITSOFC due to lowering temperature. Perovskite LSCF and BSCF with MIEC characteristics have longer-term of TPBs than electron-conductive LSM [62,63]. The electrocatalytic activity of oxygen in LSM is poorer than that in LSCF and BSCF. However, the CTE for both LSCF and BSCF are about 20 × 10−<sup>6</sup> K<sup>−</sup>1, which are fairly greater than that of the ceriabased electrolyte (~12.5 × 10−<sup>6</sup> K<sup>−</sup>1). The present thermal mismatching trouble brings about the questions in the operating temperature of the fuel cell and in co-firing with the electrolyte. Furthermore, there are still having many disadvantages for Co-based cathodes containing the large evaporation owing to the reduction of Co, high cost and the transition of Co3+ (with octahedral coordination) from a low- to a high-spin states [64–66]. Ferratebased materials without cobalt are well candidates for the cathode material since the iron is cheap and reveals nearly zero toxicity. The Ba0.5Sr0.5FeO3−<sup>δ</sup> (BSF) has the supreme electrical conductivity among ferrates [67,68] but this perovskite oxide still displays a large CTE [69]. Accordingly, it is difficult to apply for co-firing with ceria-based and zirconia electrolytes directly.

The iron perovskites display prominent conductivities of oxygen ions resulting from the lower B–O bonding energies and the facile transitions of charge carriers between the various coordination polyhedral. They are easily collapsed by the moisture from the air, also unstable at high temperatures and low partial pressures of oxygen [70]. The Ce doping in BSF provides lattice stability, better cathode–electrolyte adhesion and enhanced cell performance by increasing the TPBs in SOFC [71,72].

The as-prepared BSF particles were coated by the Ce component by utilizing an ethanol-water mixed semi-organic solution (SOS). A similar solvent removal and decomposition method was utilized in the production of hydrocolloid film [73]. The Nb was used to modify the B-site of BSF, referred to as BSNF, to enhance it with structural and environmental stability. Subsequently, the La and Ce elements were coated onto the asprepared BSNF particles also employing an ethanol–water mixed SOS. The stability of the BSF, BSNF structure and the obtained electrical properties were surveyed with respect to the influences of the Ce, La diffusion into the BSF and BSNF [72].

The calcined and pulverized BSF and BSNF powders were dispersed in absolute ethanol, individually. The aqueous solution of 20 mol% La(NO3)3·6H2O and 80 mol% Ce(NO3)3·6H2O was prepared in DI water, referred to as LC. The La(NO3)3·6H2O aqueous solution or Ce(NO3)3·6H2O aqueous solution or LC solution was added to the dispersed BSN and BSNF. Hereafter, the volume ratio of water to ethanol in every one reaction mixture was regulated to 1:9. Such SOS containing Ce, La and LC to coat the powders of BSF and BSNF were hence referred to as BS(N)F-y Ce (y = 2.5, 5 and 10 mol%), BSNF-y La and BSNF-y LC (y = 1.25, 3.75, 5, 10, 15, or 25 mol%), respectively, then each was stirred for 6 h and then dried at 80 ◦C with successive stirring. The powders were later ground and then calcined under 800 ◦C for 4 h.

The terpineol and ethyl cellulose were further mixed with these calcined powders including the BSF, BS(N)F-y Ce, BSNF-y La and BSNF-y LC. Another powder of the calcined LSBC was mixed with the binder named PB72. The ball-milling machine was employed to homogenize each binder-containing powder. After drying, these powders were granulated utilizing the mortar with a pestle and transited through a screen of 60 mesh. Each of the granulated powders was then pressed in a die employing a uniaxial press to form a disk. The pressed disks were next subjected to traditional sintering in an electric furnace (6 h at 1150 ◦C for BSF, BS(N)F-y Ce, BSNF-y La and BSNF-y LC electrodes; and 6 h at 1500 ◦C for LSBC electrolyte).
