## 3 Experimental Assessment of Mixed Metal Oxide Systems for TCES
### Co-Fe-O Mixed Oxide System  
The effects of the Fe incorporation to Co\({}_{3}\)O\({}_{4}\) (initial phase: (Fe\({}_{x}\)Co\({}_{1-x}\))\({}_{3}\)O\({}_{4}\)) was studied for amounts of Fe up to x(Fe) = 0.4 (Figure 1), as there is no phase transition for this system below 1200 \({}^{\circ}\)C in air for x(Fe) \(>\) 0.6 [35], which is the upper limit usually admitted for TCES application. Compared to pure Co\({}_{3}\)O\({}_{4}\), the studied compositions feature lesser oxygen storage capacity (OSC) with increasing iron content (Figure 1a), which is partly ascribed to an increasing proportion of spinel phase which is not converted into monoxide for 0.1 \(<\) x(Fe) \(<\) 0.6, and to the presence of residual oxygen storage in the monoxide phase introduced by the iron addition.  
A linear correlation between the variations in oxygen mass loss/gain and the reaction enthalpy is evidenced in Figure 2. It is in accordance with the experimental data obtained for the reaction enthalpy of Co-Fe-O mixed oxides.  
A decrease in reaction enthalpy was also observed with the increasing addition of Fe, from small amounts up to x(Fe) = 0.4 (51 kJ/kg, against 597 kJ/kg for pure Co\({}_{3}\)O\({}_{4}\)) (Figure 1b). [PERSON] et al. (2014) previously described that the incorporation of iron to cobalt oxide may contribute to the Co\({}_{3}\)O\({}_{4}\) stability when added in small amounts (10%), by enhancing the microstructural stability of cobalt oxide while retaining suitable reaction enthalpies, whereas larger amounts of added iron oxide gradually reduce the reaction enthalpy of the redox systems. In contrast, a minor loss in oxygen exchange capacity upon cycling was evidenced for lower Fe contents 0.05 \(<\) x(Fe) \(<\) 0.1 [35]. Concerning the Co-Fe-O system, the gravimetric energy storage density of the mixed oxides is lower than that of both pure cobalt oxide and iron oxide [27; 35].  
A noticeable decrease in the temperature gap between the reduction and oxidation reactions is also observed with the addition of Fe from x(Fe) = 0.1 to x(Fe) = 0.4 (Figure 1c), which is convenient to reduce the amount of energy required to switch between the charge and the discharge steps of the system. Overall, the transition temperature for both reduction and oxidation is increased with the addition of Fe to Co\({}_{3}\)O\({}_{4}\).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**TCES System**} & \multicolumn{3}{c}{**Onset Temperature (\({}^{\circ}\)C)**} & **Temperature** & **Conversion** & **Exp. Reaction** & **Phase Transition \& \\ \cline{2-9}  & **Reduction** & **Oxidation** & **Gap (\({}^{\circ}\)C)** & **(\%)** & **Enthalpy (kJ/kg)** & **Redox Couple** \\ \hline \multirow{8}{*}{_Mn-Cu-O_ x(Cu)} & 0.05 & 900 & no re-ox. & - & 21.2 & 126 & \(B\) & \(H\) \\  & 0.1 & 877 & no re-ox. & - & 10.7 & - & \(Mn^{III}\) & \(\rightarrow\) & \(Mn^{II}\) \\  & 0.2 & 811 & no re-ox. & - & 1.6 & - & & \\ \cline{1-1}  & 0.3 & 976 & 100 & 64 & 90.8 & - & - & \\ \cline{1-1}  & 0.4 & 960 & 969 & 9 & 96.9 & - & \(S\) & \(D\) \\  & 0.5 & 962 & 886 & 76 & 88.2 & - & \(Cu^{II}\) & \(\rightarrow\) & \(Cu^{I}\) \\  & 0.8 & 954 & 1000 & 46 & 89.7 & 354 & \(Cu^{II}\) & \(\rightarrow\) & \(Cu^{I}\) \\ \hline \multirow{8}{*}{_Mn-Fe-O x(Fe)_} & 0.1 & 990 & 774 & 216 & 31 & 168 & \(B\) & \(H\) \\  & 0.15 & 981 & 870 & 111 & 99.1 & 171 & \(Mn^{III}\) & \(\rightarrow\) & \(Mn^{II}\) \\ \cline{1-1}  & 0.2 & 980 & 860 & 120 & 98.2 & 194 & & \\ \cline{1-1}  & 0.3 & 984 & 866 & 118 & 94.1 & 195 & \(B\) & \(S\) \\ \cline{1-1}  & 0.4 & 998 & 903 & 95 & 95.3 & 189 & \(Mn^{III}\) & \(\rightarrow\) & \(Mn^{II}\) \\ \cline{1-1}  & 0.5 & 1014 & 937 & 77 & 94.7 & 189 & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._