## 3 Results and Discussion
### Microstructure Characterization and Single Cell Performance
\[\text{CO}_{3}^{2-}+\text{O}_{2}\ \leftrightarrow\ \text{CO}_{5}^{2-} \tag{1}\]  
\[\text{CO}_{5}^{2-}+\text{CO}_{3}^{2-}\ \leftrightarrow\ 2\text{CO}_{4}^{2-} \tag{2}\]  
\[\text{2 CO}_{3}^{2-}+\text{O}_{2}\ \leftrightarrow\ 2\text{CO}_{4}^{2-} \tag{3}\]  
\[\text{CO}_{4}^{2-}+2\text{e}^{-}\ \leftrightarrow\ \text{CO}_{3}^{2-}+\text{O}^{2-} \tag{4}\]  
However, incorporating this mixed multi-ionic conducting SDCC composite electrolyte to electrode materials could eventually suppress the electronic conduction behavior of the LSCF cathode materials and thereby deteriorate the oxygen reduction reaction (ORR) kinetics at the surface [47]. This condition is attributable to the carbonates because they do not conduct or prevent electrons from moving from one ceria particle to another [45]. Moreover, carbonates are generally added to the doped ceria electrolytes to avoid localized electron conduction in ceria at temperatures above 600 \({}^{\circ}\)C. The electronic conduction contribution in ceria is due to the partial reduction of Ce\({}^{4+}\) to Ce\({}^{3+}\) in reducing environment, which decreases the overall cell efficiency and contributes to enhancing mechanical stability issues [26; 48]. Dissociation/ionization of adsorbed oxygen molecules required electrons to diffuse oxygen (O\({}^{2-}\)) ions through the bulk of the cathode microstructure and the interface region. This reaction can be expressed by using the Kroger-Vink notation:  
\[1/2\text{O}_{2}+2\text{e}^{-}+V_{\text{O}}^{-}\ \rightarrow\ \text{O}_{\text{O}}^{\times} \tag{5}\]  
When hydrogen and air are used as fuel, H\({}^{+}\)/O\({}^{2-}\) ion conduction occurs in the carbonate-based fuel cells, leading to superior cell performance. The possible ion electrode reaction mechanism at the cathode side can be expressed as follows [49]:  
H\({}^{+}\) ion conduction at the cathode side:  
\[1/2\text{O}_{2}+2\text{H}^{+}+2\text{e}^{-}\ \rightarrow\ \text{H}_{2} \text{O}^{2-}. \tag{6}\]  
Ion conduction at the cathode side:  
\[1/2\text{O}_{2}+2\text{e}^{-}\ \rightarrow\ \text{O}^{2-}. \tag{7}\]  
Reactions (6) and (7) show that the ORR reaction requires electrons (e\({}^{-}\)) for the ionization of adsorbed oxygen molecules, and ORR kinetics are associated with the number of oxygen molecules adsorbed on the LSCF surface [50]. The composite cathode microstructure (Figure 2c) reveals that the particle connectivity among LSCF particles was poor due to the presence of molten carbonate and SDC particles. This finding could considerably affect the electronic network over the LSCF surface, consequently blocking the conduction paths between the LSCF and SDCC particles, and decreasing the single-cell performance [20]. The results of our study proved that the single-cell performance decreased with increased SDCC composite electrolyte content. Therefore, this finding suggests that the formation of O\({}^{2-}\) ions at the surface of the composite cathode probably affected the ORR rate [16]. [PERSON] et al. also reported a similar finding for LSCF-based composite materials [51]. However, in-plane electronic conductivity for the rapid formation of O\({}^{2-}\) ions by electron conduction can be enhanced by printing or depositing a thin current-collecting layer (CCL) on the LSCF-SDCC composite cathode surface [52]. As shown in Table 2, Ag was used as a CCL on the electrode surface to improve the in-plane electronic conductivity and achieve high performance at low operating temperatures. For instance, [PERSON] et al. reported a higher power density value of 120.4 mW/cm\({}^{2}\) at 750 \({}^{\circ}\)C for a single cell fabricated with LSCF-50 wt % SDCC composite cathode by using Ag as CCL [28]. This shows that the presence of CCL could help to minimize the rate-limiting factor associated with the surface exchange reaction, and the formation and diffusion of H\({}^{+}\)/O\({}^{2-}\) ions at the composite cathode side.