Broadband electric spectroscopy of proton conducting SPEEK membranes

Many papers have focused on the thermal properties, conductivity and fuel cell performance of sulfonated poly(ether ether ketone) (SPEEK) membranes, but the electrical properties have not been extensively studied. In this work, the electric properties of SPEEK electrolytes are studied with broadband electric spectroscopy to elucidate the relationship between the degree of sulfonation and the conductivity and to explore the mechanism of long-range conductivity. SPEEK membranes exhibit two polarization phenomena that contribute to the overall conductivity: "bulk" and interfacial conductivities. The "bulk" conductivity increases with increasing degrees of sulfonation due to an increase in the concentration of charge carriers and a higher hydrophilicity that allows increased water uptake. The interfacial conductivity is the result of the accumulation of charge at the interfacial regions between the hydrophobic and hydrophilic domains of the SPEEK membranes. The bulk and interfacial conductivities can be divided into two temperature regimes: one at temperatures below 75 oC that exhibits Arrhenius behaviour and the other at temperatures above 75 oC that follows a Vogel-Tamman-Fulcher (VTF) trend. In the Arrhenius region, proton transport occurs primarily via a Grotthus-like mechanism where protons move between water molecules and acid groups. In the VTF region, segmental motion is critical in the long-range proton conduction process as the mean hopping distance increases along with the temperature due to loss of water.