## 3 Results and Discussion
It shows that the ohmic overpotential increases by about 170 mV between 2012 and 2016 and 100 mV during the period from 2008 to 2012, which are within the same order of the normalized potential increase, \(E_{ij,0}\). However, it is difficult to identify a trend for the activation overpotential, since it remains mostly constant. Figure 4: Normalized potential (\(\lozenge\)), \(E_{ij,0}\), and the correspondent ohmic resistance (-), \((E-E_{0})\cdot j^{-1}\), as a function of the electrode and membrane service times, respectively \(t_{\text{e}}\) and \(t_{\text{mi}}\). Figure 3: The monthly \(E_{0}\) (\(\blacksquare\)) and \(k\)-factor (\(\blacktriangle\)) values obtained from the polarization curves analysis. Figure 6 shows \(E_{j2,0}\) and \(E_{j1,0}\) (Figure 6a), \(\eta_{\Omega,2}\)' and \(\eta_{\Omega,1}\)' (Figure 6b), and \(\eta_{\mathrm{act}}\)' (Figure 6c) as a function of the respective service time and the respective regression analyses in Table 1.  
Figure 2.0 Activation (\(\Delta\)) and ohmic overpotentials (\(\Circle\)) as a function of the electrode and membrane service times, respectively \(t_{\mathrm{e}}\) and \(t_{\mathrm{mi}}\). Figure 2.1 (**a**) Normalized potential, \(E_{j,\mu,0}\), as a function of the respective membrane service time, \(t_{\mathrm{mi}}\); (**b**) associated ohmic overpotential, \(\eta_{\Omega}\)’, as a function of the respective membrane service time, \(t_{\mathrm{mi}}\); and (**c**) associated activation overpotential, \(\eta_{\mathrm{act}}\)′, as a function of the respective electrode time, \(t_{\mathrm{e}}\). From Figure 6 and Table 1, a linear trend with time of the normalized potential and ohmic overpotentials, when the analysis is applied to each stage of the electrolyser history, is clearly observable. The observed trends show a small increase of the activation overpotential over time, which is related to the electrode catalytic activity, and of the ohmic overpotential, mostly related to the performance of the membranes. The average of the relative error, \(\varepsilon\), between the actual values of \(E_{ij,0}\), \(\eta_{\Omega\,1}\)', and \(\eta_{\mathrm{act}}\)' and the corresponding linear regression are given in Table 2, together with the actual values for each instants. The average relative error of the activation overpotential is 6% and of the ohmic overpotential is 3%. Concerning the average of the relative error of the normalized potential, it is quite small, <0.5%. It can, therefore, be concluded that the methodology presented allows for discriminating the performance over operation time of the most critical elements in an electrolyser: electrodes and membranes, which can be therefore used for scheduling their preventive maintenances. The developed methodology allows to estimate the energy cost of a given set of membranes/electrodes over its period of operation based on the overpotentials increase. Moreover, if the costs of acquisition and replacement of all elements and their respective amortization period are known, the average value of the maintenance costs can be calculated. Thus, the intersection between the maintenance cost and operating cost can be considered as the ideal replacement time for each set of membranes/electrodes.