Nanocomposite membranes of sulfonated poly(phthalalizinone ether ketone)-sulfonated graphite nanofibers as electrolytes for direct methanol fuel cells

Polymer nanocomposite membranes are prepared by the dispersion of sulfonated graphite nanofibers (SGNF) in the sulfonated poly(phthalazinone ether ketone) (SPPEK) matrix. GNF are first functionalized with 4-benzene sulfonic acid, and the presence of sulfonic acid groups in GNF is confirmed by FT-IR and elemental analysis. The GNF morphology after functionalization is analysed via SEM and TEM. Nanocomposite membranes are then prepared by varying the content of SGNF in SPPEK using the solvent cast technique and characterized for their physico-chemical properties in terms of mechanical, thermal and dimensional stability, water sorption, proton and electrical conductivity along with methanol permeability. Improved mechanical stability for the nanocomposite membranes is observed due to interfacial interactions and good compatibility between SGNF and SPPEK. The nanocomposite membranes exhibit improved electrochemical selectivity, which in turn enhances the overall DMFC power output. The DMFC performance of the nanocomposite membrane is higher than pristine SPPEK and is on par with the Nafion-117 membrane. The stability of the nanocomposite membranes is examined by operating membrane electrode assemblies (MEAs) in the open circuit voltage (OCV) condition for 50 h. In addition, the stability of the composite membranes is also confirmed by measuring the methanol crossover current density using linear sweep voltammetry (LSV) for MEAs comprising these membranes before and after an OCV test.