Abstract |
Membrane processes are nowadays well-established technologies, which have been
extensively used in different fields from gas separation to fuel cells.
The aim of the present Thesis was the development and thorough study of new nanocomposite
dense polymeric membranes with improved properties in order to examine their suitability for
energy and environmental applications with an emphasis on polymer electrolyte membrane
fuel cells (PEMFCs) and mixed matrix membrane (MMM) systems for gas separation
applications.
In this context, as a first step, the possibility of improving the most widespread polymer
electrolyte membrane for fuel cells, “Nafion®”, was investigated. This was pursued by
combining Nafion with new or suitably modified nanoparticles. PEMFCs are considered very
promising next generation power sources for portable/vehicular applications, due to their highenergy
efficiency and power density. However, PEMFC operation also faces particular
challenges, associated e.g. with the durability of its key components (such as membranes) and
the need for complex and expensive heat and water management systems. An appealing
strategy to improve the PEMFCs performance at operating temperatures up to 120–130 °C is
the development of nanocomposite membranes by incorporating hydrophilic inorganic
particles into Nafion®, in order to impart higher proton conductivity and higher thermal
stability. In this respect, the present Thesis particularly focused on the development of spherical
colloidal silica nanoscale ionic materials (NIMs), and 2D - organosilica layered materials
bearing different functional groups. Hybrid nanocomposite membranes (1, 3, 5 wt% loadings)
were prepared by solvent casting methods. The initial nanoparticles and the final
nanocomposite membranes were characterized by a combination of experimental techniques
(XRD, IR, TGA, SEM, TEM), the mechanical properties of the polymer films as well as their
proton conductivity were tested by dynamic mechanical analysis (DMA) over a wide
temperature range, while the water dynamics which is an important factor for the efficiency of
the fuel cells, was examined by pulsed field gradient NMR spectroscopy.
On the other hand, membrane technology has gained particular interest over the last decades
also for the separation of gaseous mixtures, as it can lead to more efficient processes in
industrial, energy and environmental applications (e.g. biogas upgrade, CO2 capture, etc.). In
particular polymeric membranes have been extensively studied for gas separation processes
mainly due to their low cost and facile fabrication. However, they also have a range of
important disadvantages. In this respect, significant efforts have been devoted towards
producing membrane systems with higher thermal stability, tolerance to contaminants,
resistance to plasticization, and ability to compete with other well-established technologies.
One of the most widespread approaches is the development of mixed matrix membranes
(MMMs) that combine an organic phase (polymer) with inorganic particles and exploit the
synergistic advantages from each phase such as, the separation potential of the dispersed fillers
with the facile processability of the polymers. On this basis, one of the main objectives of the
Thesis was to develop and evaluate specific types of MMMs focusing on CO2 gas separation
processes. More specifically, two different types of polymeric materials (the rubbery Pebax®
MH1657 and glassy 6FDA-DAM), that are widely used in applications involving the removal
of CO2 from gas mixtures, were combined with metal organic framework (MOF) particles.
Pebax® MH1657 is a PA/PEO copolymer (PA/PEO = 40/60, where PA: polyamide and PE:
polyethylene; 6FDA-DAM is a high-performance glassy polyimide with high free volume and
thermal stability that satisfies most gas separations, including CO2 /CH4, even under high
pressures. The choice of MOFs, which are a special class of hybrid microporous crystalline
materials, was based on their with excellent adsorption properties that can be controlled by
tailoring their topology and porous structure. More specifically, focus was placed on the use of
materials from the UiO and ZIF families in different percentages (from 5 to 20 wt%), as they
also offer increased stability when they are incorporated in Pebax® MH1657 and 6FDA-DAM
polymers. The performance of all membranes was evaluated experimentally with adsorption
measurements at various pressure and temperature conditions to assess the corresponding CO2
solubility, as well as single gas (CO2, CH4, H2) permeability measurements to enable the
calculation of the ideal selectivity for CO2 / CH4 and CO2 /H2. In all cases, along with the
evaluation of the performance of the membranes, their physicochemical characterization
(XRD, FT-IR, SEM, TGA/DSC) was performed to evaluate their properties before and after
the incorporation of MOFs.
Overall, the above work confirmed that with an appropriate synthetic procedure and inorganic
filler selection, it is possible to obtain nanocomposite membranes with significantly improved
properties that can be used in a range of environmental and energy applications.
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