New results on the visco-elastic behaviour of ionomer membranes and relations between T-RH plots and proton conductivity decay of Nafion~® 117 in the range 50-140 ℃

摘要

The colligative properties of acidic solution inside Nafion~® 117 membranes have been investigated, in a large temperature range, by two different methods. 1) The matrix counter-pressure index of the ionomer, n_c, was first determined and then its relations with the molar fraction and molality of the inner proton solution were found. 2) In order to calculate the density of the inner proton solutions, a factor F_e proportional to the solution electro-restriction, was experimentally found. The molarity and molality of the inner proton solution, and hence the inner osmotic pressure, were then calculated. The obtained data confirmed the viscous behaviour of Nafion~® 117 membranes. Furthermore, the comparison between the molality of inner proton solution obtained with the above different methods showed that to consider a solvation shell for the protons (first method) is equivalent to consider an electro-restriction of the solutions (second method). In other words, hydration number and electro-restriction factor are strictly related and the equation relating them is reported. Finally, T-RH plots at constant n_c values show that the believing concerning the inability of Nafion~® membranes to be employed in fuel cells at temperature higher than about 80-90 ℃ is not completely true.