Local Proton Conductance in the Proton Exchange Membranes: A Density Functional Study

The performance of the proton exchange membrane fuel cells (PEMFCs) is closely related to the performance of the proton exchange membranes and many researchers have focused on the development of new proton-conducting polymer membranes as an alternative to Nafion (DuPont) membranes. Recently, the multiblock sulfonated poly (arylene ether sulfone)s (SPE) containing highly sulfonated hydrophilic component seem to be very promising candidates and  have shown many advantages over Nafion. However, their proton conductivities are still lower than Nafion at low relative humidity conditions.¹ To make clear why perfluorinated polymer membrane Nafion is superior to other membranes in the proton conductivity at low humidity, we firstly compared their hydrated structures. It was found that minimum three water molecules are needed to detach a proton from the sulfonic acid group in both membranes and the SPE membrane has two types of the sulfonic acid groups: active “separated” and inactive “paired” groups for proton conductance. To explore the possibility of the proton conductance in the vehicle mechanism at low humidity, we examined the relay model (Fig. 1) of protonated water clusters between the sulfonic acid groups in Nafion and found that it needs 33 kcal mol^-1, which is common in usual reactions. This seems to indicate that relay model may contribute to the high proton conductivity of Nafion at low humidity.²

[1]   K. Miyatake, B. Bae and M. Watanabe, Polym. Chem., 2011, 2, 1919.

[2]   R. K. Singh, T. Tsuneda, K. Miyatake and M. Watanabe 2013 (Submitted)