Membranes Based on Polyvinylidene Fluoride and Radiation-Grafted Sulfonated Polystyrene and Their Performance in Proton-Exchange Membrane Fuel Cells.

Proton-exchange membranes based on gamma-irradiated films of PVDF and radiation-grafted sulfonated polystyrene with an ion-exchange capacity of 1.8 meq/g and crosslinking degrees of 0 and 3% were synthesized. A solvent-free, environmentally friendly method of styrene grafting from its aqueous emulsion, with a styrene content of only 5 vol.% was used. Energy dispersive X-ray mapping analysis showed that the grafted sulfonated polystyrene is uniformly distributed throughout the membrane thickness. The obtained materials had a proton conductivity up to 132 mS/cm at 80 °C and a hydrogen permeability of up to 5.2 cm2/s at 30 °C, which significantly exceeded similar values for Nafion®-212 membranes. The resulting membranes exhibited a H2/O2 fuel cell peak power density of up to 0.4 W/cm2 at 65 °C. Accelerated stability tests showed that adding a crosslinking agent could significantly increase the stability of the membranes in the fuel cells. The thermal properties and crystallinity of the membranes were investigated through differential scanning calorimetry and X-ray powder diffraction methods. The conductivity, water uptake, and mechanical properties of the membranes (stress-strain curves) were also characterized.

Hydration and Diffusion of H+, Li+, Na+, Cs+ Ions in Cation-Exchange Membranes Based on Polyethylene- and Sulfonated-Grafted Polystyrene Studied by NMR Technique and Ionic Conductivity Measurements.

The main particularities of sulfonate groups hydration, water molecule, and alkaline metal cation translation mobility were revealed by nuclear magnetic resonance (NMR) and ionic conductivity measurements techniques in cation-exchange membranes MSC based on cross-linked sulfonated polystyrene (PS) grafted on polyethylene with ion-exchange capacity of 2.5 mg-eq/g. Alkaline metal cation hydration numbers (h) calculated from temperature dependences of 1H chemical shift of water molecule for membranes equilibrated with water vapor at RH = 95% are 5, 6, and 4 for Li+, Na+, and Cs+ ions, respectively. These values are close to h for equimolar aqueous salt solutions. Water molecules and counter ions Li+, Na+, and Cs+ diffusion coefficients were measured by pulsed field gradient NMR on the 1H, 7Li, 23Na, and 133Cs nuclei. For membranes as well as for aqueous chloride solutions, cation diffusion coefficients increased in the following sequence: Li+ < Na+ < Cs+. Cation and water molecule diffusion activation energies in temperature range from 20 °C to 80 °C were close to each other (about 20 kJ/mol). The cation conductivity of MSC membranes is in the same sequence, Li+ < Na+ < Cs+ << H+. The conductivity values calculated from the NMR diffusion coefficients with the use of the Nernst-Einstein equation are essentially higher than experimentally determined coefficients. The reason for this discrepancy is the heterogeneity of membrane pore and channel system. Ionic conductivity is limited by cation transfer in narrow channels, whereas the diffusion coefficient characterizes ion mobility in wide pores first of all.