Fuel cell membranes prepared from multi-walled carbon nanotubes and silica nanotubes-filled sulfonated polyamide/sulfonated polystyrene porous blend films

Abstract

A sulfonated polyamide (PA-S) was first synthesized through polycondensation of sulfonated 4,4′-oxydianiline and terephthaloyl chloride. The sulfonation of polystyrene (PS-S) was conducted using a mild sulfonating reagent (98% H₂SO₄). Two categories of mechanically robust and thermally stable nanocomposites, based on multi-walled carbon nanotube (MWCNT) and silica nanotube (SiNT), i.e. PS-S/PA-S/MWCNT and PS-S/PA-S/SiNT, were prepared by solution blending. Scanning electron micrographs showed good dispersion of filler and adhesion of matrix on the surface of nanotube. Accordingly, a symmetric membrane structure with dense porous top layer, porous sublayer, and fully developed micropores at the bottom were observed. The porous membrane structure was accountable for the excellent water retention capability, and so, higher proton conductivity of new hybrids. Increasing the amount of nanotube from 0.5 to 2 wt.% increased the ultimate tensile stress of functional PS-S/PA-S/MWCNT nanocomposites 51.8–62.2 MPa compared with non-functional filler (33.4 MPa) and PS-S/PA-S/SiNT. A rapport between nanotube loading and thermal stability of the materials was also observed as the glass transition in PS-S/PA-S/MWCNT nanocomposites increased from 202 to 206℃. MWCNT-based blend membranes had higher ion exchange capacity (IEC), around 2.99–3.42 mmol/g. Novel membranes with high IEC value achieved high proton conductivity of 1.28–2.23 S/cm in a wide range of humidity values at 80℃ which was higher than that of perfluorinated Nafion®117 membrane (1.1 × 10⁻¹S/cm) at 80℃ (94% relative humidity; RH) which was used as benchmark. Moreover, a H₂/O₂ fuel cell using the PS-S/PA-S/MWCNT (IEC 3.42 mmol/g) also showed better performance than that of Nafion®117 at 40℃ and 30% RH.