Ternary hybrid (SPEEK/SPVdF-HFP/GO) based membrane electrolyte for the applications of fuel cells: profile of improved mechanical strength, thermal stability and proton conductivity

Abstract

Ternary hybrid membranes composed of sulfonated (poly ether ether ketone) (SPEEK), sulfonated polyvinylidene fluoride-co-hexafluoropropylene (SPVdF-HFP) and 1, 3, 5 or 7 wt% graphene oxide (GO) were fabricated using a facile solution casting method. The reinforcement due to the existence of SPVdF-HFP and GO afforded good mechanical and thermal stabilities to the hybrid membranes, which was confirmed by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). The surface morphological properties and roughness of the hybrid membranes were scrutinized using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM), whereas the clenched structure with even intercalation of GO sheets in the polymer matrix was observed. The temperature dependent changes in proton conductivity, as well as the mass, length, and thickness of membranes were measured; the ternary hybrid exhibited more significant changes compared to other membranes. The chemical structure, intermolecular bond stretching, structural reorganization, and crystallinity of the membranes were analyzed using proton nuclear magnetic resonance spectroscopy (¹H-NMR), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) instrumentation techniques. In the ternary hybrid membranes, SPVdF-HFP increased the per cluster volume of SO₃H groups and GO increased the number of directional hydrogen bonds (H-bonds), which collectively provided good impact in proton conductivity. At 90 °C, the peak proton conductivity attained by the SPEEK was 68 mS cm⁻¹, while that of the ternary hybrid was 122 mS cm⁻¹, 1.7 times better conductivity. Furthermore, the ternary hybrids exhibited much lower H₂ permeability compared to that of SPEEK and Nafion-117 membranes.