## 3 Results
### Heat Generation  
The reaction rate and charge transport also affect the heat generated by the cell during discharging. Based on Equation (17), the volumetric heat generation rate, q, has been calculated. The temperature dependency of the open circuit voltage is assumed to be equal for nickel and iron reactions and can be found in the literature as \(\frac{\partial E_{\text{COV,M}}}{\partial T}=2.16\times 10^{-4}V/K\)[28].  
Figure 8a shows that the overall heat generated by the cell increases up to a \(DoD\) of 0.7, until it suddenly drops down only to increase again. This fits very well with the two reaction plateaus shown in Figure 4. The slope of the overall heat generation can be related to a change in the path length that ions need to overcome to reach the side of reaction. Therefore, in Figure 8b, the individual heat generation terms from the right side of Equation (17) are plotted. Figure 8b indicates that the reversible part of the reaction heat stays constant over the entire discharging process, which is consistent since the provided discharge current stays constant as well. However, the ionic path length increases with the progress of discharging and therefore the related Ohmic heat also increases. For a discharge rate of 1/8C, Figure 4a shows that at \(DoD=0.7\), the terminal voltage reaches the iron OCV. At this time, the reaction front moves back to the separator interface and the ionic path length decreases instantaneously. Due to the shorter ionic path length, the Ohmic heat by ionic transport as well as the irreversible reaction part drop significantly. Therewith, the overall heat generation reflects the transfer point from one reaction plateau to the other by a sudden drop. Additionally, Figure 8b shows that the Ohmic heat due to electron transport in the solid matrix is negligible compared to the other contributors. This is similar to the impact of the electron resistance. Figure 8c gives some interesting insights into the heat generation of a single segment of the cathode (segment position in the middle of the cathode). For \(DoD>0.6\), the reaction in the observed segment stops and heat is generated only from the charge transport. At this point, the reversible reaction decreases to zero, indicating that all active material has been used up by the discharging reaction. At the same time, the heat due to ionic transport propagates with a constant value. In this DoD range (\(DoD=0.6\)-0.7), all ions need to pass the examined segment to reach the side of the reaction. The slope exhibits again a sudden change in the moment the transfer point between OCVs is reached. At \(DoD=0.7\), the reaction of the second active material is enabled and consequently the reaction heat increases while the ionic transport  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Symbol** & **Value** & **Additional** \\ \hline \(T\) & 573.15 K & \\ \(j_{0}\) & 0.14 A cm\({}^{-3}\) & [15] \\ \(j_{0,A}\) & 5 A cm\({}^{-3}\) & [15] \\ \(h_{K}\) & 21 cm & Estimated value \\ \(r_{K}\) & 1.8 cm & Estimated value \\ \(d_{B}\) & 0.15 cm & Estimated value \\ \(\sigma_{Ni}\) & \(1/(6.24\cdot 10^{-6}(1+0.0069\cdot(T-(20+273.15))))\) & [34] \\ \(\sigma_{rx}\) & \(1/(9.71\cdot 10^{-6}(1+0.0065\cdot(T-(20+273.15))))\) & [34] \\ \(x\) & \(0.145\cdot 1.827\mu m_{B}+(0.5715\cdot 6.358\mu m_{B})\cdot 10^{-3}\cdot T\) & [14] \\ \(m_{B}\) & \(0.5436\cdot 1.972\cdot 10^{-4}\cdot T+2.346\cdot 10^{-7}\cdot T^{2}\) & Molar ratio in the electrolyte [14] \\ \(mel_{\text{atil}}\) & 0.25776 & Metal utilization \\ \hline \hline \end{tabular}
\end{table}
Table 3: List of model parameters.