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The possibility of developing low-cost commercial grafted and sulfonated Poly(vinylidene fluoride) (PVDF- g-PSSA) membranes as proton exchange membranes for fuel cell applications have been investigated. PVDF- g-PSSA membranes were systematically prepared and examined with the focus of understanding how the polymer microstructure (degree of grafting and sulfonation, ion-exchange capacity, etc) affects their methanol permeability, water uptake, and proton conductivity. Fourier transform infrared spectroscopy was used to characterize the changes of the membrane's microstructure after grafting and sulfonation. The results showed that the PVDF- g-PSSA membranes exhibited good thermal stability and lower methanol permeability. The proton conductivity of PVDF- g-PSSA membranes was also measured by the electrochemical impedance spectroscopy method. It was found that the proton conductivity of PVDF- g-PSSA membranes depends on the degree of sulfonation. All the sulfonated membranes show high proton conductivity at 92°C, in the range of 27 to 235 mScm−1, which is much higher than that of Nafion212 (102 mScm−1 at 80°C). The results indicated that the PVDF- g-PSSA membranes are particularly promising membranes to be used as polymer electrolyte membranes due to their excellent stability, low methanol permeability, and high proton conductivity. [ABSTRACT FROM AUTHOR]

Free volume size Vf and proton conductivity σ of a novel polymer electrolyte membrane were investigated as a function of temperature and relative humidity up to 140 °C and 80%, respectively. The free volume size Vf for Nafion® HP JP reflect the α-transition temperature at about 98 °C. In addition the free volume size Vf for Nafion® HP JP is smaller than that for Nafion® NRE212 membrane which leads to lower methanol permeability of the former membrane. The proton conductivity σ for Nafion® HP JP decreases with increase of temperature up to transition temperature due to free and bound water loss, then it starts to increase due to dynamic segmental motion. It increases for Nafion® HP JP with the increase of the relative humidity. A good correlation between Vf and σ was successfully established for both membranes which indicates that σ is governed by the free volume. In conclusion, Nafion® HP JP is a suitable membrane for a proton exchange membrane fuel cell application. [ABSTRACT FROM AUTHOR]

The proton conductivity mechanism in per-fluorinated sulfonic acid/PTFE copolymer Fumapem® membranes for polymer electrolyte membranes has been investigated. Three samples of Fumapem® F-950, F-1050 and F-14100 membranes with different ion exchange capacity 1.05, 0.95, and 0.71 meq/g, respectively, were used in this study after drying. The o-Ps hole volume size (VFV,Ps) was quantified using the positron annihilation lifetime technique while the proton conductivities (σ) were measured using LCR Bridge as function of temperature. It was found that as the ion exchange capacity increases, the proton conductivity increases and the free volume expands. Temperature dependences of proton conductivity and the o-Ps hole volume size (VFV,Ps) reflect the glass transition temperature of the membrane. A good linear correlation between the reciprocal of the o-Ps hole volume size (1/VFV,Ps) and log(σ)+ΔEa/2.303kBT, (where ΔEa is the activation energy, T is the absolute temperature and kB is the Boltzmann constant) at different temperatures indicate that the ionic motion in dry Fumapem® is governed by the free volume. A linear relationship between the critical hole size γV*i and the ion exchange capacity was also achieved. [ABSTRACT FROM AUTHOR]

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 nm³ 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. [ABSTRACT FROM AUTHOR]

The photovoltaic performance of Cu(In1-x,Gax)Se2 (CIGS) materials is commonly assumed to be degraded by the presence of vacancy-related defects. However, experimental identification of specific vacancy defects remains challenging. In this work we report positron lifetime measurements on CIGS crystals with x = 0, and x = 0.05, saturation trapping to two dominant vacancy defect types, in both types of crystal, is observed and found to be independent of temperature between 15-300 K. Atomic superposition method calculations of the positron lifetimes for a range of vacancy defects in CIS and CGS are reported. The calculated lifetimes support the assignment of the first experimental lifetime component to monovacancy or divacancy defects, and the second to trivacancies, or possibly the large In-Se divacancy. Further, the calculated positron parameters obtained here provide evidence that positron annihilation spectroscopy has the capability to identify specific vacancy-related defects in the Cu(In1-x,Gax)Se2 chalcogenides. [ABSTRACT FROM AUTHOR]

Positron annihilation lifetime technique has been applied to study the microstructural changes in steel after heat treatment (tempering process). Positron lifetime spectra for the tempered samples in sandwich configuration with the positron source were accumulated at room temperature using a conventional fast-fast coincidence timing system. The measured lifetime spectra have been analyzed. A correlation between the macroscopic mechanical properties and the positron annihilation parameters has been established. Meanwhile, with the aid of image analysis method, the American Standards for Tools and Materials (ASTM) grain size in the tempered samples was determined. The two state trapping model has been applied for interpretation of the experimental results. [ABSTRACT FROM AUTHOR]