At low humidity (<30 RH%), water molecules begin to fill the intermolecular spaces in the membrane. At high humidity (80 RH%) the membrane swells; the free volume size increases.

A casting method becomes used for the preparation of the polyvinyl alcohol (PVA)/TiO2 nanocomposite. Characterization of the casting membranes was done using Fourier transform infrared spectrometer (FTIR), thermal gravimetric analysis, and AC impedance method. Surface morphology, element analysis, and distributions were investigated by scanning electron microscopy/energy dispersive X‐ray analysis (SEM/EDX). Positron annihilation spectroscopy was used to examine the microstructure of the membranes under study, while their crystallinity was checked by wide‐angle X‐ray diffraction (WAXD). The FTIR spectra suggest a strong interaction between TiO2 and PVA nanoparticles, whereas WAXD results indicated that the amalgamation of TiO2 nanoparticles into the PVA matrix lowers the main crystallinity of PVA. The free volume size for PVA/TiO2 with cross‐linker decreases suggest that the filling of the cavities by Ti3+ and O− ions as well as complex formation, that is, lead to a lower value of permeability. Because of their lower cost, higher thermal and lower permeability, the cross‐linked PVA/TiO2 membrane was considered to use in alkaline direct methanol fuel cells.

The positron annihilation lifetime (PAL) technique was used to investigate the yield and lifetime of positronium (Ps) in three different samples of per‐fluorinated sulfonic acid/polytetrafluoroethylene (PTFE) proton exchange membranes. The high probability of Ps formation enables the study of the hole volume size by the PAL technique and explains the mechanism of both methanol permeation and proton conductivity of the membranes in relation to the relative humidity (RH). The influence of various membrane parameters, including water uptake, proton conductivity, and mechanical properties, was studied. The concentration of water molecules and SO3−–H3O+groups is correlated with the ion exchange capacity (IEC) of the membranes where higher IEC leads to high absorbance of water. The proton conductivity is enhanced by increasing the IEC value of the membrane. The activation energies of decomposition temperature for the F‐950 membrane are the lowest (Eb = 2.856 and 1.030 eV for the 1st and 2nd decomposition peaks, respectively, implying that the membrane with high IEC is thermally less stable. With increasing the IEC of the membranes, the concentration of the water molecules/SO3 group increases. In addition, the proton conductivity of the membrane is positively influenced by increasing water content at high RH. The fact that the variations in hole volume size and both methanol permeability and proton conductivity are highly dependent on RH suggests that hole volume size plays an important role in many parameters of hydrated perfluorinated sulfonic acid/PTFE proton exchange membranes. The outcomes data were introduced and interpreted appropriately with a theoretical model and compared with the literature.

Humidity control and water management in polymer electrolyte membranes for fuel cells are still of high importance to improve the fuel cells' efficiencies. In this study, poly (vinyl alcohol) (PVA)was crosslinked using 15 wt% sulfosuccinic acid (SSA) by a solution casting method and additionally thermally crosslinked at 100• C. Positron annihilation lifetime spectroscopy was used to study the mean free volume size and the distribution at different humidity. A slight decrease in the free volume was found up to a relative humidity of 30% whereas it increases strongly for a relative humidity of more than 30%. The volume of the voids duplicates from 0.036 to 0.078 nm 3 by changing the relative humidity from 30 to 80%. Thermogravimetric analyzer was used to determine the thermal stability of the membrane. From thermogravimetric analyzer data, it was found that the PVA with 15 wt% SSA membrane are chemically stable up to 200• C.