Characterisation of proton exchange membranes using a high-pressure gas membrane rupture test

Abstract

The purpose of this research was to investigate the effect of operational environmental parameters of electrochemical hydrogen energy systems on the mechanical and viscoelastic properties of Proton Exchange Membranes through the use of a high-pressure gas membrane rupture test rig. A biaxial tensile testing method was proposed to characterise the viscoelastic properties that affect the mechanical durability of proton exchange membranes. It served as a good representation of the operational environment found within electrochemical hydrogen energy systems, replicating stresses induced on the constrained membranes. Through the use of a highpressure gas membrane rupture test the rupture pressure and membrane defection were recorded, enabling the determination of the Young's modulus. The values obtained from the biaxial testing were compared to results obtained through uniaxial tensile testing at the same environmental conditions and agreement between the two methods was obtained. It was observed that the Young's modulus remains constant for all Nafion® materials at a fixed environmental condition, regardless of the thickness of the membrane specimen. The highpressure membrane rupture test was used to determine the Young's modulus of Nafion® membranes at three temperatures (20 ℃, 50 ℃ and 80 ℃) and four relative humidity levels (35 %, 50 %, 70 % and 90 %). The results showed that the Young's modulus decreases with increased temperature and RH with the change in temperature having a significantly larger effect. The biaxial tensile testing was also used for the determination of the ultimate membrane stress at the point of rupture. By using a mathematical model proposed by Schomburg (2011) it was possible to show that during the membrane rupture test there are no influence of bending moments on the total stress of the membrane. It was also shown that all initial residual stresses are negligibly small. Nafion® 1110 membrane samples were found to have a higher rupture pressure at sub-zero temperatures than at the studied temperature larger than 0 ℃. It was also shown that the properties of the membrane remain constant for the two temperatures. Nafion® 1110 membranes were subjected to ion exchange with cations (Na+, Mg2+ and Fe3+). An increase in the Young's modulus was observed with the presence of foreign cations as a result of reduced moisture uptake. Reinforced membranes were ruptured at 90 % RH and 50 ℃ with the rupture pressures compared to Nafion® membranes with similar thicknesses at the same environmental conditions. The rupture pressure of the reinforced membranes showed a nearly 100 % increase in strength compared to that of the Nafion® membranes. It is therefore clear that the e-PTFE layer of the reinforced membranes strongly improves the mechanical strength of the specimen. Unhydrolyzed perfluorinated membranes were partially hydrolysed for up to 46 hours to investigate the effect of the equivalent weight of the membrane specimen on the mechanical strength. These tests showed that the equivalent weight of the specimens decreased as the hydrolysis time increased, which in turn resulted in an increase of the rupture pressure of the specimen at 50 % RH and 50 ℃.