## 4 Conclusions  
A methodology was proposed to follow the ageing of electrodes and membranes of a chlor-alkali membrane electrolyser. The developed methodology is based on the potential difference and current density histories of an industrial bipolar electrolyser data collected every 15-min for about approximately eight years.  
The NaCl electrolysis data was organized monthly and the values outside of the Tafel region were removed, which permitted to calculate the average of all cells' potential and the current density.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \(\mathbf{R^{2}}\) & & **Coefficient** & \(\mathbf{p}\)**-Value** \\ \hline \(E_{j,0}\) vs. \(t_{\mathrm{m2}}\) & 0.82 & intercept & 3.2204 & \textless{}0.05 \\  & & & slope & 0.0026 & \textless{}0.05 \\ \(E_{j,1,0}\) vs. \(t_{\mathrm{m1}}\) & 0.67 & intercept & 3.2495 & \textless{}0.05 \\  & & & slope & 0.0021 & \textless{}0.05 \\ \(\eta^{\prime}\alpha_{,2}\) vs. \(t_{\mathrm{m2}}\) & 0.72 & intercept & 0.7142 & \textless{}0.05 \\  & & & slope & 0.0022 & \textless{}0.05 \\ \(\eta^{\prime}\alpha_{,1}\) vs. \(t_{\mathrm{m1}}\) & 0.60 & intercept & 0.0029 & \textless{}0.05 \\  & & & intercept & 0.2855 & \textless{}0.05 \\ \(\eta^{\prime}\text{act}\) vs. \(t_{\mathrm{e}}\) & 0.04 & slope & 0.00013 & \textless{}0.05 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Regression analysis of the \(E_{ij,0}\), \(\eta_{\Omega\,1}\)′ and \(\eta_{\mathrm{act}}\)′ as a function of the respective service times.  
The monthly open circuit cell potential was obtained from potential and current density curves of the electrolyser. The cell potential was then normalized taking into account the initial current density (when occurs the membranes or membranes and electrodes replacements) and depending on the type of elements used; this procedure allows removing from the experimental data the potential variations associated to small current fluctuations. The normalization made possible to associate the increase in ohmic and activation overpotentials with the electrodes and membranes service time. In the considered case study, the calculated average relative error of the activation overpotential was 6% and of the ohmic overpotential was 3%. The overpotential histories associated to the membrane (ohmic overpotential) and to the electrodes (activation overpotentials) allow for deciding the best instant for servicing actions. This decision, therefore, balances the maintenance costs with the over-energy consumption and the risks of a complete failure of the membranes/electrodes.  
Conceptualization, A.M. and S.V.; Methodology, A.M and J.P.; Investigation, F.F.; Resources, J.P. and A.M.; Writing-Original Draft Preparation, F.F.; Writing-Review & Editing, S.V and A.M; Supervision, A.M., S.V. and J.P.; Funding Acquisition, A.M.  
This work was supported by the (i) POCI-01-0145-FEDER-006939 (_Laboratorio de Engenharia de Processos, Ambiente, Biotecnologia e Energia_, UID/EQU/00511/2013)--funded by FEDER through COMPETE2020--_Programa Operacional Competitividade e Internacionalizzacao (POCI)--_and by national funds through FCT--_Fundacao para a Ciencia e a Tecnologia_ and (ii) NORTE-01-0145-FEDER-000005--LEPABE-2-ECO-INNOVATION, funded by FEDER--_Fundacao Europate de Desenvolvimento Regional, through COMPETE2020--Programa Operacional Competitividade e Internacionalizzacao (POCI) and Programa Operacional Regional do Norte (NORTE2020)_. This work was also supported by the Associate Laboratory for Green Chemistry- LAQY, which is financed by national funds from FCT/MCTES (UID/QUI/50006/2019).  
[PERSON] acknowledges FCT--_Fundacao para a Ciencia e Tecnologia_ and _Bondalti Chemicals_, _S.A._ for the co-financed PhD grant, PD/BDE/114351/2016.  
The authors declare no conflict of interest.