Broadband electric spectroscopy of sulfonated polyetherether ketone and polybenzimidazole-based proton-conducting materials

The current state of the art of polymer electrolytes for application in low-temperature fuel cells consists in perfluorinated ionomers such as Nafion®, Aquivion®, Aciplex® and others. In these materials, the fluorocarbon backbone chain is functionalized by perfluoroetheral chains tipped with acid –SO3H groups. The resulting systems are characterized by a very good proton conductivity and an excellent chemical and electrochemical stability. However, perfluorinated ionomers have a series of important drawbacks, including: (a) a relatively high permeability to reagents; (b) a very high production cost; (c) the necessity of a high hydration degree to conduct protons efficiently; and (d) the difficulty to operate at temperatures higher than 80-90°C. In order to overcome these problems, one solution is to replace the perfluorinated ionomers with other proton-conducting electrolytes. Two of the most promising systems are: (a) sulphonated polyaromatic membranes such as sulfonated polyetherether ketone (SPEEK); and (b) polybenzimidazole (PBI)-based systems doped with a mineral acid such as H3PO4 or H2SO4. SPEEK is cheap, its sulfonation degree (DS) can be controlled easily during the preparation of the material and has a very low permeability to the reagents. For these reasons, SPEEK is particularly suitable for applications in fuel cell systems fed with alcohols. On the other hand, PBI-based systems doped with mineral acids are known to conduct protons in anhydrous conditions up to a very high temperature (T ≈ 250-280°C). Thus, several fundamental and technological problems arising during the engineering of the final fuel cell power plant can be successfully solved. In this work, the electric properties of a few representatives of both families of protonconducting polymer electrolytes are investigated in detail. In particular, it is studied the effect of increasing DS on the electric properties of SPEEK, and how the conductivity of poly-2,2’(2,6-pyridine)-5,5’-bibenzimidazole (PBI5N) is affected by doping with H3PO4 and 10wt% of imidazole-functionalized silica. The electric properties of the materials are investigated by broadband electric spectroscopy (BES) in the frequency and temperature ranges of 10-2 Hz to 10 MHz and 5 to 155°C, respectively. The analysis of the real and imaginary components of the electric conductivity and permittivity spectra allows to rationalize the mechanism of proton conduction in terms ofrelaxation events and multiple interfacial polarization events. The comparison of all the results determined on both families of materials allows to study the interplay between the chemical composition of the materials and their electrical properties, also evidencing the presence of multiple phases in the investigated systems.