## 3 Results and Discussion
### Microstructure Characterization and Single Cell Performance  
The cross-sectional microstructure of the single cell and its components are shown in Figure 2. Figure 2 shows dense morphology with no pores for the SDCC electrolyte layers, whereas the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & **Surface Area** & **Average Particle** & \multicolumn{2}{c}{**Crystallite Size (nm)**} \\  & **(m\({}^{2}\)/g)** & **Size (nm)** & **LSCF** & **SDCC** \\ \hline LSCF–30 SDCC & 7.42 & 560 & 42 & 81 \\ LSCF–40 SDCC & 4.39 & 642 & 49 & 102 \\ LSCF–50 SDCC & 4.04 & 655 & 48 & 99 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of the composite cathode LSCF–SDCC powders. Figure 3: XRD patterns of LSCF–SDCC composite cathodes with different SDCC electrolyte contents: (a) LSCF–30 SDCC, (b) LSCF–40 SDCC, and (c) LSCF–50 SDCC. anode and cathode layers exhibited loosely agglomerated and porous microstructures. The presence and distribution of the carbonate crystals in the microstructure were evident in all three layers. The carbonates formed a continuous network and homogenously dispersed on the SDC, LSCF, and NiO particles. However, identifying the individual constituent elements of LSCF, SDC, and NiO from the microstructure images of Figure 2c,d was difficult due to the presence of carbonate in the mixture and its melting characteristics. The relationship between the SDCC composite electrolyte content and the properties of the LSCF-SDCC composite cathode was studied using _I-V_ and _I-P_ measurements in air and hydrogen. A schematic setup for single-cell electrochemical tests to measure the _I-V_ and _I-P_ characteristics of the single cell is shown in Figure 4. The carbonate-based fuel cells must be operated beyond the melting temperature of the carbonate, which is between 500 \({}^{\circ}\)C and 650 \({}^{\circ}\)C [35]. Therefore, the performances of the single button cells of cells A, B, and C were investigated between 500 \({}^{\circ}\)C to 650 \({}^{\circ}\)C, as shown in Figure 5a-c. The open-circuit voltage and power density of all the fabricated cells increased with temperature [36]. The influence of the LSCF-SDCC composite cathode composition on the single-cell performance was significant at different SDCC contents. The increase in SDCC composite electrolyte content in the LSCF cathode worsened the single-cell performance at all the operating temperatures. When the electrolyte content was below 40 wt %, cell A with 30 wt % SDCC composite electrolyte exhibited the highest performance, with a measured power density value of 75.4 mW/cm\({}^{2}\) at 650 \({}^{\circ}\)C. Therefore, the LSCF-SDCC composite cathode resistance can be reduced by using lower SDCC content (30 wt %) in the cathode microstructure. However, decreasing the SDCC content below 30 wt % could cause severe thermal expansion coefficient mismatch between the cathode and electrolyte components [29]. This could induce thermo-mechanical failure between the components during the operational conditions, thus causing cell failure. Figure 4: Schematic of the SOFC testing chamber. The results obtained from this study were comparable with those of previous reports on single cells based on the LSCF composite cathode. For example, [PERSON] et al. reported a maximum density of 75 mW/cm\({}^{2}\) for a LSCF-SDC-Ag composite cathode-based fuel cell at the operating temperature of 650 \({}^{\circ}\)C [37]. In another study, [PERSON] et al. obtained maximum power densities of 35 and 60 mW/cm\({}^{2}\) at 550 \({}^{\circ}\)C and 600 \({}^{\circ}\)C, respectively [38]. However, the power density values obtained here were significantly lower than those earlier reported in the literature on the single cell based on SDCC composite electrolytes, as shown in Table 2. This difference in performance of the single cell based on SDCC composite electrolytes can be ascribed to the choice of electrode materials and carbonates and their compositions and fabrication conditions.