Martin Tomáš

Thermal Cycloimidization Study of Polyimides and Poly(amide imide)s Synthesized from Low-Temperature Solution Polymerization

Polyimides (PI)s and Poly(amide imide)s (PAI)s containing di/tri-structural arrangements of monomer constituents were synthesized from 1,2,4,5 benzenetetracarboxylic anhydride/3,3′,4,4′ benzophenonetetracarboxylic dianhydride, diamines and trimellitic dianhydride chloride, i.e., a mixture of dianhydrides and diamines, using low-temperature solution polymerization. The majority of the resulting polymers were readily soluble in polar aprotic solvents such as dimethylformamide, dimethylsulfoxide, dimethylacetamide, etc. Thermal cycloimidization studies by thermogravimetic analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR) revealed that the rate of cycloimidization was much faster during the initial 15 min, and then occurred at much slower rates till the completion. The activation energies (Ea) for thermal cycloimidization of poly(amic acid)s was calculated from the rate of mass loss of the polymer heated at different temperatures according to the Coats and Redfern method. PI and PAIs derived from diamines containing single cyclic/benzene ring structures, such as m-phenylenediamine and 1,3-cyclohexanebis(methylamine), showed less amount of char yield than those having two benzene rings and separated by methylene, sulfone, or ether linkages. The structure of both diamine and trimellitic chloride had a profound effect on the polymer chain mobility, as indicated by the big variation in the glass transition temperatures. Thermally stable polymers were developed into membranes and tested for their mechanical strength by dynamic mechanical analysis(DMA).

Modification of gas diffusion layers properties to improve water management

In this paper we report an approach to improve water management of commercial GDLs by introducing hydrophobicity patterns. Specifically, line and grid patterns have been created in the MPL side by laser radiation. For an in-depth investigation of these modified GDLs the current density distribution was monitored during fuel cell operation. Additionally, the physical properties of these materials were investigated by a number of ex situ methods such as Fourier transform infrared microscopy, electrochemical impedance spectroscopy and water vapor sorption. Furthermore, a comparison of the physical properties of the patterned GDLs with chemically modified GDLs (treated in H2SO4 and H2O2) is provided. Our results show a clearly improved homogeneity of current density distribution of the patterned GDLs compared to untreated GDLs. This observation is likely due to a reduced local hydrophobicity which facilitates water diffusion along the flow field of the fuel cell. However, performance of the fuel cell was not affected by the MPL irradiation.Graphical Abstract

Influence of polyfurfuryl alcohol (PFA) loading on the properties of Nafion composite membranes

ABSTRACT Composite membranes based on Nafion (N115) loaded with furfuryl alcohol (FA) were prepared by in situ acid-catalyzed polymerization technique, with the aim to improve the ionic conductivity of Nafion membranes. The functionalization, thermal stability, electrical properties and mechanical strength of N115-PFA composites was analyzed by means of Fourier transform infrared (FT-IR) attenuated total reflection (ATR) spectroscopy, small- and wide-angle X-ray scattering (SAXS/WAXS), thermogravimetric analysis (TGA), electrical impedance spectroscopy, dynamic vapor sorption (DVS) and dynamic mechanical analyser (DMA). The FA loading in the resultant composites had a positive correlation with the water uptake (Wu), water vapor uptake (Wvu), ionic conductivity and thermo-mechanical stability. At low polyfurfuryl alcohol (PFA) loading, these membranes displayed higher Wu and improved ionic and electrical properties. Further, the thermo-mechanical stability also gradually increased with the PFA loading. All the composites showed a well-defined glass transition temperature in DMA, which shifted to higher temperature with repeated PFA loading. Overall, the results indicate that the developed composite membrane are promising for low temperature polymer electrolyte membrane (PEM) fuel cells.