Synthesis of silica immobilized phosphotungstic acid (Si-PWA)-poly(vinyl alcohol) (PVA) composite ion-exchange membrane for direct methanol fuel cell

Abstract

A 80 μm thick composite ion-exchange membrane was synthesized by uniformly dispersing sub-micron to nano sized silica immobilized phosphotungstic acid (Si-PWA) inorganic ion exchanger into cross-linked poly(vinyl alcohol) (PVA) matrix. ATR-IR spectrum confirmed the PVA cross-linking and presence of Si-PWA in membrane. Amorphous behavior of the membrane indicated uniform blending of crystalline Si-PWA particles with cross-linked PVA. Membrane's tensile strength (93 MPa) was much higher than Nafion 117 (34 MPa). Ion exchange capacity of the membrane (0.90 meqg−1) was higher than the values reported for the other PVA based membranes. Na+ transport number was 0.92, indicating good ion-selectivity of the membrane. Membrane showed a high water uptake of 35% while its methanol uptake was low (8.4%) and thereby reduced methanol permeability (1.6 × 10−7 cm2 s−1) compared to Nafion-117 was observed, a highly desirable property for DMFC application. Proton conductivity increased from 7.04 mS cm−1 to 10.5 mS cm−1 with increase in temperature from 30 °C to 50 °C. At 35 °C, the single cell DMFC with membrane showed higher OCV (0.8 V) and comparable peak power density to Nafion-117.