Abstract:

A proton exchange membrane of poly(vinylidene fluoride) (PVDF) grafted onto polystyrene sulfonated acid (PVDF-g-PSSA)was prepared as follows. Styrene was first added to N-methyl pyrrolidone (NMP) solution containing PVDF that was modified with plain sodiumsilicate. Benzoyl peroxide (BPO) was then added as an evocating agent and polystyrene was directly grafted onto the PVDF that was modified with plain sodium silicate. Lastly, the formed membrane was sulfonated. The microstructures, morphologies, and mechanical properties of the membranes and the distributions of sulfur and silicon were investigated using Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and a multifunctional material experiment machine. The influence of styrene content on the proton conductivity and methanol permeability of these membranes was studied using an impedance analyzer and a gas chromatograph. Results showed that, depending on the content of styrene, styrene was easily grafted onto the PVDF that was modified by plain sodium silicate. A sulfonation reaction was also found to occur in the membranes and the mechanical properties had improved. As the styrene content increased, the proton conductivity of the PVDF-g-PSSA membranes also increased. The degree of PVDF-g-PSSA swelling was 20.4% at 20% (w, mass fraction) styrene content and 8% Na4SiO4 and at 25 ℃. The membranes possess a methanol permeability of around 10-7 cm2·s-1, which is about ten times as few as that of Nafion115. These composite membranes have high selectivity and are promising for use in direct methanol fuel cells.

Key words: Proton exchange membrane, Na4SiO4, Poly(vinylidene fluoride), Styrene