*1.3. Proposed Core-Shell Electrodes*

The SOFC synthesis needs high co-firing temperatures (often above 1000 ◦C) as to result in chemical reactions between the perovskite-based electrode, such as LSM, and the zirconia-based electrolyte; the formation of a secondary phase at the interface is usually insulating, and thus may impact the stability and performance of a SOFC [16,41–44].

In the conventional impregnated or infiltrated solution into the porous electrode scaffold, the nanoparticles are formed over the electrode scaffold surface at a relatively low temperature, necessarily (<800 ◦C), as the other way, it is not reactive when sintering under such temperatures. Therefore, the isolated nanoparticles constituted on the scaffold have comparatively disappointing electrical conductivity as a result of the lack of reactive

sintering of a normal infiltration–sintering process. In order to fill nanoparticles to cover the scaffold surface enough, the impregnation or infiltration process is required to repeat several times [45]. Such a repeated process often causes the block of porosity in the electrode and inhibits the thorough impregnation into the electrode/electrolyte interface. If the reactive temperature is so high as to enhance the reaction of impregnated nanoparticles and electrode grains, further coarsening or necking the impregnated nanoparticles or electrode grains significantly [33,46–48]. The particles agglomeration occurs to reduce the electrode porosity. Secondary phases are also generated due to the diffusion of impregnated nanoparticle composition into the electrode lattice seriously. The porosity reduction and too significant secondary phase existence will affect the TPBs function and contribute the polarization increase, also resulting in a coefficient of thermal expansion (CTE) of electrode mismatching with the electrolyte.

A core-shell structure is pre-formed during the preparation of electrode particles. A high enough amount of shell nanoparticles forming a continuous charge transport pathway increases electronic and ionic conductivity of the electrode. The electrode activity is then improved as contrasted with mechanical mixing composite electrodes and traditionally impregnated electrodes. Furthermore, the core-shell electrode configuration can successfully enhance the thermal stability by preventing more sintering and thermomechanical stress because of the CTE matching with the electrolyte. In recent decades, the core-shell electrodes of anode and cathode were developed by our research group to contribute the extending TPBs and solve the mismatching issue of the electrode and electrolyte. Significant improvement in electrocatalytic performance and in impedance have been achieved.

The core-shell is not a new terminology. It has been used for ceramics in several studies [49–51]. However, in our work, the core-shell electrodes pre-formed by chelating solution to extend TPBs were appreciated as a beneficial developing technology in future for SOFC [13]. The main objective of this work was therefore to demonstrate the progress of core-shell electrodes by our efforts to provide a feasible, convenient, cost-effective and time saving TPBs extension technique. The proposed core-shell electrodes, from fabrication by chemical chelating and solution coating processes to electrical and electrocatalytic characterization, are informed in the later parts. Furthermore, the recent developing pseudo-core-shell anode by thin electrolyte impregnation is introduced and compared with other ITSOFC research. The pseudo-core-shell or inverse impregnation is further expected to apply to double ions (H+ and O2−) conducting low temperature fuel cells.

#### **2. Core-Shell Electrodes Preparation**
