## 4 Conclusions  
The degradation behavior of both composite Nafion(tm) XL and non-reinforced Nafion(tm) NR211 membranes was investigated after various chemical-mechanical aging tests. In this regard, an innovative device able to couple mechanical fatigue and an aggressive chemical environment exposure to the membrane was elaborated in order to mimic fuel cell operating conditions with an ex situ approach. It is therefore required to develop new accelerated stress tests coupling mechanical and chemical stresses to evaluate efficiently and rapidly the durability of PFSA membranes in conditions close to those experienced during fuel cell operation. This study is believed to offer new understandings about the impact of conjoint mechanical and chemical stresses on the degradation of Nafion(tm) membranes.  
The first results demonstrated that the degradation behavior of both XL and NR211 membranes was not modified when chemical stress was coupled to a mechanical stress: the FER remained higher by exposing the membrane to the Fenton solution instead of the H\({}_{2}\)O\({}_{2}\) solution only. However, adding a mechanical fatigue to the chemical stress significantly accelerated the polymer decomposition.  
The comparison of XL and NR211 degradation behaviors seemed to indicate that XL was slightly more stable than NR211 against conjoint chemical and mechanical stresses. This discrepancy could be attributed to the presence of an additional PTFE-based reinforcement providing a better mechanical strength and dimensional stability to the membrane and/or to the presence of radical scavengers preventing the polymer decomposition against radical attacks. Nevertheless, the mitigation strategies developed for XL did not prevent the polymer decomposition after the application of severe mechanical solicitations (i.e., long-term aging or a high pressure level) because the degradation rates of XL were comparable with those obtained with the non-reinforced NR211.  
The custom-made device and the associated accelerated aging protocols presented in this study constitute a first step towards larger investigations to provide new insights about the chemical-mechanical aging of PFSA membranes and opens up various perspectives. For instance, introducing gas diffusion layers (GDL) between the flow field plates and the membrane in our aging cell or modifying the operating procedure to expose the membrane to a gaseous H\({}_{2}\)O\({}_{2}\) environment instead of an aqueous solution could better mimic the mechanical constraint exerted on membranes during fuel cell operation. Finally, one can also imagine lengthening the aging duration to more than a few tens of hours to further test the membrane durability.  
Conceptualization: M.R., A.E.K., J.-C.P., J.D., J.-Y.M. and O.L.; methodology: M.R., A.E.K., J.-C.P., K.M., J.D. and O.L.; validation: M.R. and K.M.; investigation: M.R.; data curation: M.R.; writing--original draft preparation, M.R.; writing--review and editing: A.E.K., J.-C.P. and O.L.; visualization, M.R. and K.M.; supervision: [PERSON], [PERSON]-C.P. and O.L.; project administration: O.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors would like to acknowledge Pr. [PERSON] for the access to the electromechanical universal testing machine.  
**Conflicts of Interest:** The authors declare no conflict of interest.