Synthesis And Characterization Of Nafion/Na-X Zeolite Composite Proton Exchange Membranes

Nowadays, energy and environmental problems are the focus of global concern. Proton exchange membrane fuel cell has the advantages of high energy conversion efficiency, abundant and renewable fuel and environmental friendly and has become a favourite of researchers. The proton exchange membrane is a key component of a PEMFC (proton exchange membrane fuel cell). Currently the most widely used is the DuPont's Nafion membrane. In order to improve proton conductivity and decrease methanol permeability in high temperature and low humidity conditions, inorganic modification of the Nafion proton exchange membrane become a research hotspot. Zeolite with porous, high surface area, high water absorption and ion exchange capacity become one of the most promising inorganic filler.In this paper, submicro Na-X zeolites with different alumininun content were synthesized and Nafion/zeolite composite membranes were prepared. The zeolites are low Al/Si ratio (0.75:4.25)zeolite, norm Al/Si ratio (1:4)zeolite and high Al/Si ratio (1.25:3.75)zeolite, respectively.The submicro Na-X zeolite(200~300nm) with three different Al/Si ratio were synthesized by hydrothermal method, the composite membranes were prepared by recasting method. X-ray diffraction (XRD), scanning electron microscope (SEM), fourier transform infrared spectra (FT-IR), thermal gravimetric analysis (TGA), and Brunauer, Emmett and Teller (BET) analysis method was applied to investigated the characters of zeolite and composite membranes, and the water uptake and ionic exchange capacity(IEC)of the membranes were measured.The results show that zeolite with high surface area and more hydrophilic active sites in favor of PEMs'water absorption and proton exchange capacity. With the increasing of aluminum content, the pore volume, pore radius and the number of hydrophilic active sites in the zeolite increased, but the zeolite surface area reduced. In the other hand, zeolite with small pore radius can decrease methanol permeability. The hydrophilic active sites number and surface area determine the zeolite water absorption. The normal Al/Si ratio (1:4) zeolite has a reasonable balance of surface area and hydrophilic active sites, so it has the highest water absorption (26.72%). The synthesized Na-X zeolites improved the proton exchange membranes' performance, the Mafion/zeolite composite membranes have higher water uptake than the blank Nafion membrane. The Nafion/norm Al-Si ratio (1:4)zeolite composite membrane has the highest water uptake(26.45%), high water uptake is conducive to proton conductive. The addition of Na-X zeolite has great improvement on the membranes'ionic exchange capacity (IEC). On the one hand, more sulfonate groups could be exposed with higher surface area zeolite, the high surface area zeolite increased the chance of the SO3H groups combine with H+. Meanwhile the Na-X zeolites introduced extra ionic sites such as Br?nsted acid sites. The high Al/Si ratio (1.25:3.75) zeolite modified proton exchange membrane has the highest IEC (1.17). Strength test results showed that all the composite proton exchange membranes are qualified to the PEM. All the results indicate that the normal Al/Si ratio (1:4)zeolite modified composite membrane has the excellent performance characteristics.