**4. Conclusions and Future Respective**

The progress of core-shell electrodes in our efforts is demonstrated with an overview. The core-shell electrode particles and pseudo-core-shell structure are prepared by LSBC electrolyte chemical chelating solution coating and reverse impregnation processes in the anode and cathode. The pre-formed core-shell structure in electrodes, including anode and cathode, provide a feasible, convenient, cost-effective and time saving TPBs extension technique.

The prepared anode SLTN and cathode BS(N)F coarse particles were coated with LSBC electrolyte chelating solution to form core-shell structure. For SLTN-LSBC core-shell anode, the sufficient DC conductivity at intermediate temperature only needed a small amount of 1.5 or 3.0 mol% LSBC shell coating on SLTN. Too molar ratio of LSBC shell was not necessary. Similarly, the half-cell of (SLTN-x LSBC) with x < 3.0 mol% exhibited the decrease in interface charge transfer polarization (ReZ(i)) and chemical electrocatalysis polarization (ReZ(d)) effectively.

The La and Ce formed LC shell coating on BSNF electrode particles increased the peak conductivity and achieved the transition temperature of metal-like conductivity to 510 ◦C. This is beneficial for the function of a BSNF-LC cathode using in ITSOFC. A low amount of 3.75 mol% LC coating on BSNF reduced AC impedance due to the LC electrocatalytic thin shell influence on the core surface of BSNF, which provided ionic conductivity enhancement among TPBs. The LC coating on BSNF particle also enhanced its CTE matching with LSBC electrolyte. The half-cell of (BSNF-3.75 LC)/LSBC/Pt displayed a higher power density than those utilizing the BSNF or BSNF-5 Ce cathodes at 800 ◦C. The (Ba, La, Ce) and Nb replaced the A- and B-sites of strontium-ferrate could enhance the more stable BSNF perovskite structure.

The ultrasonic spray pyrolyzed LSBC electrolyte droplets impregnated into LST porous anode to form a pseudo-core-shell and continued the deposition as to resulting in a fired thickness of 2.75 μm dense LSBC electrolyte. This pseudo-core-shell anode substrate improved contact between the anode and the electrolyte without anode functional layer. The LST/iLSBC/Pt half-cell achieved a power density of 377 mW/cm<sup>2</sup> at 750 ◦C. These results proved that the activation, diffusion and ohmic resistances of the cell were effectively decreased using an ultrasonic spray pyrolysis process. The impregnated LSBC extended the TPB area and length, promoting the performance of the half-cell and also providing a beneficial and convenient unique reverse impregnation from electrolyte into porous electrode to fabricate a thin layer of electrolyte. Full cells with core-shell electrodes are prepared and studied in the next stage. The core-shell interface reaction mechanisms are also necessary investigation topics to control the electrochemical performance precisely. Advancing the core-shell technique up to applying double ions (H<sup>+</sup> and O2−) conduction will realize low temperature solid oxide fuel (or electrolyzer) cells in the future.

**Author Contributions:** Conceptualization, H.-Y.C. and J.-C.W.; data curation, R.-T.W. and H.-Y.C.; investigation, H.-Y.C. and R.-T.W.; methodology, H.-Y.C.; supervision, H.-Y.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to thank the Ministry of Science and Technology, Taiwan, for financially supporting our research under the following Grant Nos. NSC99-2221-E-019-021, NSC101- 2221-E-019 -035, NSC102-2221-E-019-010, MOST104-2221-E-019-005, MOST106-2622-E-019-004-CC3, MOST108-2221-E-019-032-MY2, MOST108-2622-E-019-005-CC3 and MOST110-2221-E-019-054-MY3. We are also grateful to the funding support from Grant Nos. NTOU-RD-AA-2011-101041, NTOU-103- 007 and USTP-NTUT-NTOU-104-03 of National Taiwan Ocean University and University System of Taipei Joint Research Program, Taiwan.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data are offered by the authors by reasonable request and the novel core-shell electrodes are available from the authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
