Preparation and characterization of new non-fluorinated polymeric and composite membranes for PEMFCs

Abstract

In this work the sulfonated derivative of an amorphous polyetheretherketone, known as SPEEK-WC, has been utilized for the preparation of ion exchange membranes by solvent evaporation method. In order to improve the membrane properties for applications as solid electrolyte in Polymer Electrolyte Membrane Fuel Cells (PEMFCs), hybrid membranes have also been developed by immobilization of inorganic heteropolyacids (HPAs) in the SPEEK-WC matrix. The polymeric and composite membranes have been characterized by chemical–physical and electrochemical analyses, investigating the effect of the sulfonation level of the polymer, the type and the loading of the inorganic additive. For comparison, the same characterization methods have been carried out on a commercial Nafion 117 membrane. SPEEK-WC membranes, both polymeric and composite, are characterized by a lower permeability to water, methanol, O 2 and H 2, in comparison with Nafion. These results are particularly interesting in the perspective of their application in PEMFCs. The contribution of the solubility and diffusivity to the gas transport through the membranes in the dry state has been identified by pure gas permeability measurements according to the time lag method. The proton conductivity has been investigated by impedance spectroscopy as a function of the temperature, evidencing a higher capacity of the SPEEK-WC based membranes in comparison with Nafion, to retain water molecules at high temperature. Moreover, in the case of the Nafion membranes, the thermally induced formation of a sulfonic anhydride by the condensation of two SO 3H group of the polymer accompanied by the loss of a water molecule, has been demonstrated by FT-IR analyses. The composite SPEEK-WC membranes have a higher proton conductivity than the pure polymeric samples at the same sulfonation level, because the HPAs, in particular H 4SiW 12O 40· nH 2O, provide a better pathway for protons hopping, reducing the resistance to the proton transport.