*3.1. Core-Shell Anode of SLTN-LSBC*

If a low ratio of LSBC (e.g., 1.5 mol%) displayed a shell, then the amounts of nanoparticles of LSBC were not clear enough to recognize. Figure 7a shows that the larger particles of the calcined SLTN core were adhered by enough distinguished LSBC nanoparticles (12 mol% LSBC) [56]. The elemental analysis on the core-shell SLTN-12 mol% LSBC powders is easily identified as the inserted EDS spectrum shown in Figure 7a. The primary components of Ti, Ce and Sr were mapped on the morphology of Figure 7a obtained from the EDS elemental analysis. Figure 7c demonstrated the Ce element existed on the nanoparticles of the shell, which further certified the core-shell formation. The Titanium (Ti) and Strontium (Sr) assigned over core-shell major body as shown in Figure 7b,d. The Ti and Sr mapping images demonstrated the core SLTN existed below the LSBC nanoparticles shell.

**Figure 7.** The FESEM images and EDS elemental analyses of the prepared core-shell anode powders, (**a**) FESEM image of SLTN-12 mol% LSBC, the inserted spectrum is its EDS analysis and the EDS elemental mappings on the SLTN-12 mol% LSBC, (**b**) Ti, (**c**) Ce and (**d**) Sr [56].

The conductivity contribution of La and Nb donors-doped perovskite structure was enhanced if the SLTN sintered in the reduction atmosphere. Figure 8 exhibits that the DC conductivity decreased with the increase in the molar ratio of shell LSBC, which demonstrated that the the resistance of shell LSBC affected the electrons hopping as the ratio of shell LSBC increased. The more shell nanoparticles coated on the core, the more favorable its role for ionic conductivity was. A ceria-based shell may increase the electrocatalytic activity but it would lower the electronic conductivity. The lesser shell coating such as 1.5 or 3.0 mol% LSBC shown in Figure 8 maintains appropriate high electronic conductivity and electrocatalytic activity simultaneously. This situation is the electron transfer at the TPB of the electronic core and ionic shell [24,47,55]. Ce3+ was possibly formed at the anode in a reduction atmosphere at the higher temperature, 700 ◦C, in Figure 8 and enhanced the electronic conductivity [74].

**Figure 8.** The conductivities of SLTN-x mole% LSBC core-shell anodes after 1300 ◦C/3 h sintering in activated carbon-reducing atmosphere as a function of measuring temperatures in H2 atmosphere.

Such few shell coatings less than 3 mol% also altered the peak semiconducting activity to a lower critical temperature (Tc) of 500 ◦C. Figure 8 exhibits that the conductivity of metallic behavior of the core-shell anode as the measuring temperature is larger than the Tc. The present achievement further proved that the shell of ion may shift the redox reaction to a lower operating temperature than the one without coating ionic shell. Figure 8 reveals that a high covering ratio of shell on core (e.g., 6.0 or 12.0 mol% LSBC) displays a degraded electrical conductivity on the core-shell anode. The lattice oxygen led to the coexistent generation of oxygen vacancies and Ti3+ ions at a high temperature and low oxygen partial pressure. More oxygen vacancies produced from a high covering ratio of the ionic shell such as 12 mol% did not distinctly contribute to the overall DC conductivities of the SLTN core [54] as to lower the total conductivities of the core shell anode and owing to the low mobility of oxygen vacancies contrasted with electrons at a temperature of higher than 700 ◦C.

In addition, measuring the AC impedance of the core-shell anode may provide testimony of the ionic shell of LSBC to enhance the electrocatalytic activity and electron transfer rate. Figure 9 exhibits the AC complex impedance analyses. Those indicate that the LSBC shell (x = 0 to 3.0) profited the impedance decrease of the core-shell anode. However, the larger covering ratio (x = 6.0) of the LSBC shell increased the impedance of the core-shell anode. This result responds to the decrease in DC conductivities for the larger amount of shell coatings. AC impedance data acquired in Figure 9 may be roughly adapted as two depressed impedance semicircles for each anode sample according to RQ equivalent circuits containing parallel resistance/CPE circuits in series [24,56,75–81]. The SOFC is full solid state device, in order to corresponding to the illustrations in Figure 4, the detail electric double layers are not considered and discussed in the work. The depressed arc at a high-frequency range is concerned with the interface charge transfer process, while the arc at a low-frequency range is related to surface diffusion processes and hydrogen dissociation. The three intersections on the ReZ axis from left to right for each x mol% LSBC shell coating express the ohmic resistance (Re), interface resistance of charge transfer (Ri), and electrocatalytic resistance having no electron transfer (Rc), respectively. The valid resistance dedication is indicated as ReZ(d) = Rc − Ri for diffusion polarization and the interface charge transfer polarization of ReZ(i) = Ri − Re. In addition to the higher than 6.0 mol% LSBC shell coating, the LSBC shell coating decreased ReZ(d) effectively and increased the ReZ(i) slightly. The present work illustrates the core-shell structure with low LSBC coating increased the electrocatalytic activity of anode due to the extension of TPBs.

**Figure 9.** The AC impedance spectra for SLTN-x mol% LSBC core-shell anodes after 1300 ◦C/3 h sintering in activated carbon-reduction atmosphere then measured at 600 ◦C in H2/N2 atmosphere [56].
