## 3 Results  
### Impact of a Cyclic Compressive Stress  
First, accelerated stress tests coupling mechanical and chemical stresses were performed by applying a cyclic compressive stress to mimic the swelling/shrinkage sequences generated by the water content changes of the membrane during a transient fuel cell operation. A 5 MPa pressure was exerted on the membrane while a degrading solution was circulated through the aging cell for 8 h: a 3 vol % H2O2 solution for mild conditions and a Fenton solution with an optimal concentration of Fenton's reagents for aggressive conditions [2, 28]. The fluoride emission rates (FER) were evaluated by analyzing the circulating solutions collected at the end of each aging test and compared with those measured for pure chemical stress tests to estimate the impact of mechanical fatigue on the polymer chemical decomposition (Figure 2).  
As observed in the case of ex situ chemical stress tests, the degradation rate obtained after mechanical-chemical stress tests for both XL and NR211 membranes was higher with a Fenton solution exposure than with the H2O2 solution exposure, which indicated that the mechanical fatigue did not predominate over the chemical degradation, i.e., shifting from hydrogen peroxide to the Fenton solution had a higher impact than adding a mechanical stress. Nonetheless, coupling mechanical and chemical stresses accelerated the membrane degradation because a FER twice to thrice higher was obtained for both membranes by applying a 5 MPa compression while circulating a Fenton solution instead of exposing membranes to the Fenton solution only. Furthermore, the XL membrane was significantly degraded by the conjoint chemical and mechanical stress exposure despite the presence of an additional mechanical reinforcement and radical scavengers in comparison with its non-reinforced analogue, the NR211 membrane.  
Supplementary experiments were then carried out by increasing the pressure level from 5 to 10 MPa and the aging duration from 8 to 20 h to evaluate the degradation behavior of XL and NR211 membranes in more severe conditions (Table 2). These new tests were carried out using Fenton solutions only.  
The degradation rate of both membranes increased significantly when the pressure level was doubled: the FER of XL was almost twice higher while a slight increase of 22% was noticed for NR211. Furthermore, by extending the aging duration from 8 to 20 h, the FER was considerably amplified for both XL and NR211 membranes, indicating an acceleration of the degradation. Moreover, when more severe operating conditions were applied on the membranes, the results showed that both membranes were degraded in equal proportion, which could suggest that the mitigation strategies to improve XL durability were insufficient to effectively prevent the chemical decomposition of the polymer at a  
Figure 2: Fluoride emission rates (FER) of NafionTM XL and NR211 membranes after chemical aging (to the left) and after chemical-mechanical aging (to the right).  
higher pressure level or after long-term aging. It is thus possible that the PTFE-based reinforcement layer and/or the cerium-based radical scavengers were no longer able to inhibit the chemical decomposition of the polymer when the mechanical fatigue became too important, thus leading to a degradation behavior through fluoride emission rates similar to that of the non-reinforced NR211 membrane. However, in the light of our current results, it cannot be concluded definitely if one strategy prevailed over the other or whether both were involved in preventing the membrane from degradation.