**1. Introduction**

The fuel cell (FC) belongs to an electrochemical reactor that straightly converts the chemical potential of a fuel into electrical energy. Therefore, the FC capability is not restricted by the Carnot cycle count, which is the regulation for some mechanical facilities (e.g., steam turbine or internal cumbustion engine). The power conversion efficiency of a fuel cell is about 45% principally such as proton exchange membrane fuel cell (PEMFC). Taner's group [1,2] has studied hydrogen gas that entered the PEMFC anode through the catalyst anode to form the proton. The protons pass through electrolyte membrane to generate electricity due to the released electrons passing through external circuit to the cathode side underwent reduction interactions of the oxygen gas. This experimental study proves that the H2 pressure drop and water management at cathode side are the important factors to affect the fuel cell performance. If combining the heat recovery system, a solid oxide fuel cell (SOFC) can reach the high power efficiency of 80% [3] with no water treatement issues because of higher 100 ◦C operation temperatures. Thus, fuel cells possess

**Citation:** Wang, R.-T.; Chang, H.-Y.; Wang, J.-C. An Overview on the Novel Core-Shell Electrodes for Solid Oxide Fuel Cell (SOFC) Using Polymeric Methodology. *Polymers* **2021**, *13*, 2774. https://doi.org/ 10.3390/polym13162774

Academic Editor: Arunas Ramanavicius

Received: 29 June 2021 Accepted: 16 August 2021 Published: 18 August 2021

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characteristics of high performance of energy conversion, environment-friendly, multiple choices of fuels and high waste heat recovery rate. Figure 1 displays that the SOFC has the supreme efficiency of power conversion among various fuel cells and demonstrates the SOFC-combined cycle system achieving the highest power generation efficiency, which can be employed in a wide range of power outputs (large-scale power generation of kW~MW). It is indicated in Figure 1 marked with red circles. High power output lets SOFC suite to large power generation capacity systems, for example, various power plants and container ships. The SOFC is also a good candidate for large distributed energy systems [4,5].

**Figure 1.** Total power generation efficiency of various fuel cells for the electric power output [4].

There are three consisting parts in a SOFC that are anode, electrolyte and cathode. A schematic diagram of a general planar design of SOFC is shown in Figure 2. The planar cells can be electrolyte-supported, electrode-supported and metal-supported types. Planar patterns provide a number of potential benefits, containing easier and cheaper fabricating procedures and higher power densities than tubular devices [6,7]. Thus, the planar SOFC is an excellent candidate for high power requirement such as power plant, cargo and container ships [8]. The operation of a planar fuel cell stack requires bipolar plates to connect the membrane electrode assembly (MEA) and external circuit or to combine another MEA. The bipolar plates allow fuel and oxidant flow through their channels [9]. The electrochemical capability and performance of planar SOFCs is extremely based on the materials of components, the microstructure of the electrodes and the geometric parameters of the cell [10].

**Figure 2.** Planar solid oxide fuel cell stack with MEA indication [9].

The typical oxygen ion conducting SOFC is Ni/YSZ cermet for anode, 8 mol% Y2O3 stabilized ZrO2 (YSZ) for electrolyte and perovskite La1−xSrxMnO3 (LSM) for cathode. Such a schematic diagram of SOFC is shown in Figure 3 [11]. The operation mechanism is that the cathode material receives the electrons from the external circuit to reduce oxygen molecules into oxygen ions (O2 + 4e<sup>−</sup> → 2O2−) at triple-phase boundaries (TPBs) of cathode/electrolyte; then the O2<sup>−</sup> is conveyed into electrolyte, the electrolyte material transports the oxygen ions to the TPBs of anode/electrolyte to oxidize the fuel H2 (O2<sup>−</sup> + H2 → H2O + 2e−), which comes from the anode side, then the electrons are transported to the current collector and further to external circuit, which is indicated in left picture of Figure 3. The reaction products are pure water and heat. The electrolyte material also plays a role to prohibit the electrons from the anode through electrolyte into the cathode.

**Figure 3.** Schematic diagram for SOFC structure (left) and operation mechanism (right) [11].

Generally, the operation temperatures are in the range of 800–1000 ◦C for ZrO2-based electrolyte [12], which is known as high temperature solid oxide fuel cell (HTSOFC). In the Ni-YSZ anode system, the reduced Ni metal network acts as electron conducting path and the YSZ can decrease the thermal expansion mismatch between Ni metal and YSZ ceramic electrolyte. The agglomeration and coarsening of Ni particles during SOFC operation at high temperature of 800–1000 ◦C is a major concern, since Ni particles sintering results in loss of active surface area and decreased conductivity of the anode. When using hydrocarbon fuels in a Ni-based anode of SOFCs, the major concerns are the anode catalyst coking and sulfur poisoning since H2S exists in most of fossil fuels. The Ni-based anode catalyst can be easily deactivated by carbon deposits and suffers irreversible sulfur poisoning [13].

Decreasing the operating temperature to below 800 ◦C (under 600 ◦C is even better) permits the application of fuels involving methane and butane without pre-forming a hydrogen fuel. The advantages of a lower operating temperature incorporate the wide selections of materials, interface matching and long-term stability of the cell system and cost-effective operation and fabrication [14].
