## 2 Materials and Methods
### Experimental Setup and Aging Tests Parameters  
A specific custom-made device was elaborated in order to reproduce operating conditions close to those exerted on PFSA membranes in fuel cell systems and thus study the impact of conjoint chemical and mechanical stress on the membrane [2]. The chemical-mechanical aging experiments consisted of applying a compressive stress on the membrane while exposed to an oxidizing environment. In this regard, an aging cell composed of two symmetric half-cells made in 316L stainless steel was machined and a single serpentine flow channel like those of fuel cell flow field plates was engraved on their surface. The lands and channels had a width of 1 mm and a depth of 0.7 mm (Figure 1). The effective area where the membrane was exposed to both mechanical and chemical stresses was approximately 19.5 \(\times\) 39.5 mm\({}^{2}\) per half-cell.  
During the aging tests, the membrane was sandwiched between the two half-cells and the aging cell was then inserted between the clamp of an electromechanical universal  
Figure 1: Photograph of the chemical-mechanical aging device. The inset focuses on the aging cell and the adapter specifically designed for this study.  
testing machine (MTS load frame model 312.21,MTS Systems, Creteil, France). In addition, the aging cell was heated to 80 \({}^{\circ}\)C thanks to heater cartridges inserted into each half-cell close to the membrane and an EPDM (ethylene propylene diene monomer) O-ring gasket was used to guarantee perfect sealing. Prior to each aging test, it was necessary to determine the minimal pressure required to compress the gasket and thus ensure a perfect sealing. This minimal pressure corresponded in this study with the 0 MPa reference pressure level.  
The chemical stress was induced by circulating a continuous flow of degrading solution (H\({}_{2}\)O\({}_{2}\) or Fenton solution) through the flow channel of the aging cell with a flow rate of 3.0 mL/min, which corresponded with a residence time of the solution in contact with the membrane of about 10 s per half-cell. Based on the literature, mild conditions were replicated by using a 3 vol % H\({}_{2}\)O\({}_{2}\) solution while a Fenton solution containing 3 vol % of H\({}_{2}\)O\({}_{2}\) and 1 ppm of ferrous ions Fe\({}^{2+}\) was circulated to provoke more aggressive conditions [2, 28].  
A mechanical fatigue was applied on the membrane through the channel ribs in order to reproduce on the one hand the swelling/shrinkage cycles (i.e., by applying a cyclic compressive stress) or, on the other hand, the stack clamping pressure (i.e., by maintaining a static compressive stress) experienced by the membrane during fuel cell operation. The cyclic compressive stress consisted in a sinusoidal profile oscillating between 0 and 5 or 10 MPa [1, 29] with a frequency of 0.1 Hz while the static compressive stress was maintained at constant pressure level.  
According to these operating conditions, one hour of a conjoint mechanical-chemical aging test could be considered as roughly equivalent to one year of daily operation with one startup/shutdown per day (i.e., 360 swelling/shrinkage cycles). Table 1 summarizes the main operating conditions in which the various chemical-mechanical aging tests were performed.  
After each test, the membrane samples were extracted from the cell, immersed in distilled water and treated to eliminate cationic contaminants. For that purpose, the samples were soaked in a complexing solution of EDTA-Na\({}_{2}\) (0.01 mol/L) at room temperature overnight before being boiled in a nitric acid HNO\({}_{3}\) solution (1 mol/L) at 80 \({}^{\circ}\)C for 2 h and rinsed in distilled water at 80 \({}^{\circ}\)C, again for 2 h. Finally, the samples were dried in an oven at 60 \({}^{\circ}\)C for 20 h before being analyzed. The solutions that circulated through the aging cell were collected and stored for (possible) further analysis.