Protic ionic liquids/poly(vinylidene fluoride) composite membranes for fuel cell application

Abstract

Poly(vinylidene fluoride), PVDF, membranes have attracted considerable attention as polymer electrolytes for fuel cells. This study explores the effect of solvent on the spherulite size and the crystallinity of the polymeric membranes. Based on Hansen solubility parameters theory, the mixture of DMC and DMSO was selected among a dozen of solvents for the preparation of PVDF membranes by thermally induced phase separation. The addition of two protic ionic liquids (PILs), bis(2-ethyl hexyl) ammonium hydrogen phosphate [EHNH2][H2PO4], and imidazolium hexanoate [Im][Hex] to PVDF membranes at concentrations (10% < wPIL < 50%) has been investigated by SEM, FTIR, DSC, TGA, EIS, and DMA. The inclusion of ionic liquids into the polymer matrix influences structural parameters (degree of crystallinity and electroactive phases), thermal stability, proton conductivity and mechanical properties of the membranes. The membranes become transparent regardless type of ionic liquid employed. A small amount of ionic liquids increases the degree of crystallinity and facilitates the production of polar β and γ crystals. The proton conductivity mechanism (Grotthuss) is dependent on the ionic liquid structure (due to its self-organization in water) and the content in the PVDF membrane, as well as the membrane water uptake. Different behavior has been observed for the two ionic liquids, which stresses the challenge on selecting an appropriate cation and anion combination. The obtained composite membranes exhibited excellent mechanical performance and reduced elastic modulus, with respect to the pure polymer matrix. These results indicate that PVDF/IL composite membranes have a high potential for PEMFC applications.