## 3 Results and Discussion
### Performance of Nanostructured Lead-Acid Electrodes at \(-20\)\({}^{\circ}\)C  
A VWR cryostat was used to perform the tests at different temperatures. The conditions used were the same as those set for the test of the electrodes at 25 \({}^{\circ}\)C: 1.2 V cut-off, a C-rate of 10 C and a total of 1000 charge/discharge cycles. Figure 7 shows the results obtained from cyclization of nanostructured electrodes at a temperature of \(-20\pm 2\)\({}^{\circ}\)C.  
Figure 7a shows the charge/discharge curves. It can be observed that with the increase of the number of cycles, there was an increase of the plateau of the discharge curve; therefore, there was a greater area under the curve and hence a greater usable energy. From the charge and discharge curves, it can be observed that despite the severe conditions to which the battery has been subjected, from 500\({}^{\circ}\) to 1000\({}^{\circ}\) cycles, the behavior remains almost unchanged and it is possible to obtain a constant plateau, which means a constant capacity value of the device. The charging curves show a behavior similar to the one shown by the electrodes cycled at 25 \({}^{\circ}\)C. In fact, an initial voltage value of about 3.2 V has been measured, probably also in this case due to the rapid variation of the C-rate of the battery, which decreases with the increase of the number of cycles. In contrast to the battery cycled at 25 \({}^{\circ}\)C, in this case, the charging curves show the typical pattern of PbO\({}_{2}\) electrodes. At a temperature of \(-20\)\({}^{\circ}\)C, an efficiency of about 50% was found, as shown in Figure 7b, compared to a maximum value of 30% found with commercial electrodes [32]. At low temperature, a porous PbSO\({}_{4}\) layer is formed which, during charging, is converted into a porous mass of PAM consisting of small particles (<1.5 \(\upmu\)m) that agglomerate with each other, forming micropores. This type of structure creates a very large contact surface between the PAM and the electrolyte, ensuring good performance. After testing, both nanostructured electrodes were analyzed by SEM to evaluate their morphological changes. Figure 8a-d show the results of this analysis for both nanostructured electrodes.  
Figure 6: SEM images at different magnifications of Pb and PbO\({}_{2}\) NW electrodes after 1000 cycles at 10 C at the end of the 1000 th discharge at 25 \(\pm\) 2 \({}^{\circ}\)C: (**a**,**b**) Pb electrode and (**c**,**d**) PbO\({}_{2}\) electrode.  
Figure 8a,b show the top view of the Pb nanostructured electrode. The high surface porosity generated by the nanowire cyclization is evident. SEM analysis on the PbO\({}_{2}\) nanostructured electrode is shown in Figure 8c,d. These images show a top view of the PbO\({}_{2}\) nanostructured electrode, and the presence of sulfate agglomerates (of similar shape) with a size of about 10-12 \(\upmu\)m distributed all over the surface can be visualized, but in particular, a highly porous surface under the agglomerates can be distinguished.  
The observed morphology confirms the trend of the charge and discharge curves reported above and the possibility of a better diffusion of the electrolyte inside the active mass, thus obtaining better performance and longer life times despite the critical conditions to which the electrodes have been exposed.  
Figure 7: Electrochemical performance of nanostructured lead-acid electrodes operating at \(-20\pm 2\)\({}^{\circ}\)C and 10 C: (**a**) charging and discharging curves of different cycles, and (**b**) cycling efficiency on discharging and discharge capacity.