*1.1. Polarizations of Fuel Cells*

The energy conversion efficiency of a SOFC is mainly controlled by interface charge transfer, ohmic and diffusion impedances (or polarizations), a well-known electrochemical performance presented in Figure 4. General SOFC consists of two porous electrodes and an electrolyte assembled as the micrograph shown in the inset of Figure 4. Well-controlled gas diffusion, electron and ion conduction/diffusion are all considered key points in a SOFC operation to obtain a high peak power.

**Figure 4.** Typical electrochemical performance of a SOFC with I-V and I-P curves. The insect is a general microstructure of full cell (SOFC).

The electrochemical performance of SOFC is greatly determined by the species of H2 and O2 reaction processes in the porous electrode, such as oxygen adsorption, dissociation, surface diffusion to the TPB, ionization and incorporation into the electrolyte and bulk paths. Fewer TPBs exist at the electrode/electrolyte interfaces. The electrochemically active interface region is expected to extend a few micrometers from the electrolyte to the electrode and plays essential role in terms of the performance and durability of SOFCs [15,16].

Due to the significance of the electrode/electrolyte interface and electrocatalytic activity enhancement required, many studies pay efforts on the extension and enlargement of the TPB area [17,18]. The electrochemical reactions mainly proceed at the TPB sites of gas (H2 or O2), electrode and electrolyte. Theoretical calculations and experimental results have showed that a composite electrode should exhibit low charge transfer polarization by spreading the electrochemically active area within the volume of the electrode [17,19,20]. For example, the TPB length was measured by multiplying the average length of the cathode particle at the interface with the number of particles per unit area. The results suggested a three-dimensional distribution of TPB in LSM-YSZ composite cathode leads to a significant drop of cell overpotential [21,22]. Therefore, the manipulation of the electrode/electrolyte interface microstructures actually plays a crucial role in determining the overall cell performance and durability.

The interface charge transfer polarization results from TPBs between anode and electrolyte, and between cathode and electrolyte materials. The diffusion polarization originates from the internal structure including grains connection and porosity of an electrode. The ohmic polarization occurs from electrolyte material and external conducting connector structures. One significant bottleneck in the development of an intermediate temperature solid oxide fuel cell (ITSOFC), which is operated at 500~800 ◦C, is that the solid electrolyte exhibits low oxygen ionic conductivity at such a temperature, e.g., 600 ◦C. Apart from lowering the thickness of electrolytes, realizing the capability to acquire a high ionic conductivity composition of electrolytes is a major objective for the ITSOFC. It is obvious that the ionic conductivity can be enhanced via ceria doped with appropriate aliovalent cationic dopants. Co-doping has been certified to successfully increase the electrical characteristics of ceria-based electrolytes [23]. The (La0.75Sr0.2Ba0.05)0.175Ce0.825O1.891 (LSBC) is a typical electrolyte for ITSOFCs utilized and developed in our research group [24]. When the ceria-based electrolyte material is ready selected for an ITSOFC, the lowering interface charge transfer and diffusion impedances are determined mainly by the electrode materials selection and manipulating the microstructures of electrodes. Thus, how to extend and enlarge TPBs area are significant topics.

Composite electrodes by mixing ionic and electronic conducting materials are used to improve electrodes' performance. Such composite electrodes including anode and cathode help to enhance the properties of mixed electronic–ionic conductors and the intercomponent compatibility [7,25–31]. The composite electrodes are good ideas utilized by a variety of mixing and impregnation or infiltration methods to develop efficient anode and cathodes effectively. The mixed electrodes, e.g., Ni/YSZ anode and YSZ/LSM cathode in conventional SOFC extending TPBs, already reduce the interface charge transfer impedance and improve the electrochemical performance, as the schematic electrode microstructures shown in Figure 3.
