## 3 Results and Discussion
### Mercury Intrusion Porosimetry and N\({}_{2}\)-Physisorption
Although the pore structure of a composite electrode is usually a mixture of different pore shapes, a conical pore shape model was applied to calculate \(\sigma_{pore}\) for all electrode porosities, because this shape allows the best description of the hysteresis of the Hg-intrusion and extrusion data in Figure 5a. Measurement of the specific surface area independent of the pore shape can be achieved by N\({}_{2}\)-physisorption, which can be incorporated into the evaluation of the strengths and limitations of the chosen pore shape model in the MIP results. Despite the detailed insights into the electrode microstructure and porosity from MIP experiments, a correct interpretation of the results and deviations is crucial. The measurement principle of MIP, where Hg in the capillary forms together with the metal sleeve around the dilatometer a plate capacitor and the intruded Hg-volume is proportional to the change in capacitance, is accurate up to \(\pm\)0.05 mm\({}^{3}\). But further variance regarding \(V_{Hg}\), the filling volume of Hg in the dilatometer, and changes in temperature can result in a realistic accuracy of \(\pm\)2 mm\({}^{3}\) for the determined porosity and up to \(\pm\)5 mm\({}^{3}\) for the density calculation (values given by device distributor). With \(V_{Hg}\) ranging between 87 and 260 mm\({}^{3}\) for lowest and highest electrode porosity, respectively, a relative error of 1-6%  
\begin{table}
\begin{tabular}{c c c c c} \hline \multicolumn{4}{c}{**Porosity (\%) Derived by Gauge and MIP Measurements**} & \multicolumn{2}{c}{**MIP \(\sigma_{pore}\) (m\({}^{2}\) g\({}^{-1}\))**} \\ \hline \(\varepsilon\) & \(\epsilon_{xtal}\) (\(H\beta\)) & \(\epsilon_{skel}\) (\(H\beta\)) & \(\epsilon_{app}\) (\(H\beta\)) & \\ \hline
44 & 37.8 & 36.9 & 34.4 & 1.3 \\ \hline
39 & 35.9 & 35.2 & 33.2 & 1.2 \\ \hline
30 & 28.2 & 27.5 & 26.1 & 1.5 \\ \hline
28 & 28.1 & 27.4 & 26.4 & 1.7 \\ \hline
26 & 23.2 & 22.6 & 22.1 & 1.6 \\ \hline
18 & 17.2 & 1 has to be considered. As for all analytical techniques in this study, the sample preparation can influence the accuracy of the results (e.g., cutting the electrode sheet into samples). In addition, the use of an aluminum foil as a typical cathode electrode current collector [44] can lead to additional inaccuracies since Al and Hg can form an alloy [45]. However, this barely influences the results as the native oxide layer of Al prevents a reaction with Hg before and during the MIP measurement [45], which was confirmed by a baseline measurement of the pristine aluminum current collector. With complex systems, such as composite electrodes, a constant contact angle of 140\({}^{\circ}\) between Hg and the electrode materials is considered as approximation. When comparing the PSD from MIP with results from other methods (e.g., SEM or N\({}_{2}\)-physisorption) the definition of the pore size has to be considered, since MIP measures the pore entries that are of the same size as the pore bodies (e.g., cylindrical pores) or smaller (e.g., spherical pores). Furthermore, the speed of the pressure increase/decrease can shift the pore size determination (faster = shift to smaller pore sizes). Structural heterogeneities (see Hg-entrapment) and the densification of the electrode surface, described by the CLSM results, amplifies the pore-blocking effect and shifts the PSD results to smaller pore sizes [46]. Since nearly all described effects influencing the results of MIP are similar for electrodes with the same composition, the comparison of different electrode porosities is reasonable. Furthermore, by comprehending the factors that influence PSD changes and comparing the results of MIP with other methods, such as SEM cross-sections, changes in the electrode manufacturing process can also be understood and analyzed. Overall, MIP measurements have a higher accuracy and reproducibility for porosity determination than gauge measurements.