## 3 Results
### Cell Voltage for Discharge and Charge Cycle  
When a current is applied to the cell, the cell voltage changes as a function of current direction, current intensity and the state of charge of the cell. The voltage curve is therefore an important parameter of cell performance. Accordingly, the charge and discharge voltages were simulated and afterwards the model-based predictions were compared with experimental data. Figure 4a shows the voltage curve over the depth of discharge (DoD) for a discharge rate of 1/8C, which corresponds to a discharge current of \(I=-5.125\) A. For small \(DoDs\) (\(<\)0.6), the modelled curve (black line) shows a flat voltage progression at about 2.52 V, followed by a sharp drop in voltage at \(DoD=0.75\) to a level of about 2.31 V, leading to a second plateau. The formation of two voltage plateaus can also be observed in the experimental data (green curve). The nickel chloride reaction exhibits a higher equilibrium voltage (Equation (1)) and can therefore be assigned to the first plateau. The equilibrium voltage of the iron reaction is only reached when the cell voltage decreases, so that the iron reaction (Equation (2)) dominates for the second plateau. For the first plateau, the model and experimental curves lay comparatively close to each other. However, at \(DoD>0.8\), the second plateau of the experimental curve appears later in comparison to the modelled one. In addition, the experimental curve exhibits a much flatter curve with several voltage steps between 2.3 and 2.4 V at the transition voltage plateau. Recent studies have shown that, besides the direct reduction of \(MCl_{2}\) to \(M\), intermediate complexes are formed in a voltage window of 2.3-2.4 V [32]. This complex formation is not considered = in the presented model, but may be the reason for the voltage steps in the transition region of the experimental curve. Furthermore, the experimental end-of-discharge voltage lies significantly above the model-based curves. The experimental curve ends at 2.2 V, whereas the model curve has a much steeper voltage drop. This discrepancy can be related to the unknown exact amount of active material and thus the capacity in the commercial cell. A precise matching of the DoD between the experiment and model is therefore not possible.  
Figure 4b shows the corresponding charge curve, also for a C-rate of 1/8C. Analogous to the discharge curve, two plateaus can be identified in the model-based voltage curve (black curve). For a fully discharged cell (\(DoD=1.0\)), the voltage has a value of about 2.4 V and rises linearly at first, then jumps to a level of about 2.6 V at \(DoD=0.83\), which is followed by another voltage plateau. Finally, the voltage raises again abruptly at the end of the charge cycle. For charging, the first plateau at \(DoD=0.85\)-1.0 can be assigned to the oxidation of iron (Equation (2)), where for voltages above \(E_{OCV,Ni}\), the oxidation of nickel starts, leading to a second plateau. In the experimental curve (green curve), the charge cycle was terminated at an end-of-charge voltage of 2.7 V. The experimental curve also shows an offset from the model over the entire charge cycle. This suggests that the overall internal resistance of the real cell is not correctly reflected in the model. Missing details of the commercial cell, such as the exact dimensions, can be explained by one reason. Similar to the discharging experiment, the voltage steps in the transition region between the two plateaus appear most probably due to complex formation [32]. Another reason for the voltage deviation is the separator shape of the model. Compared to the coverleaf separator in the validation cell, a cylindrical-shaped separator is assumed in themodel. However, in general, qualitatively similar voltage curves are found for between the experiment and model.