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Phosphonated SBA-15/Phosphonated PSEBS Composite Membranes for High Temperature Proton Exchange Membrane Fuel Cells

Dharmalingam Sangeetha

Proton exchange membrane fuel cells (PEMFC) are increasingly becoming an attractive energy source for the future due to their portability, silent operation and high power density. Efforts have been made to improve their efficiency as well as in making the technology affordable. Several parameters come into play in the context of fuel cell efficiency, of which the operating temperature is of prime importance. Specifically, high temperature PEM fuel cell (HTPEMFC) has greater merits such as higher efficiency, improved tolerance of the electrodes against carbon monoxide poisoning, faster reaction kinetics, and effective heat transfer. Since the proton conductivities of commonly used perfluorinated membranes, such as Nafion, is highly dependent on external humidification, their operating temperature is limited to 100 °C. Hence one of the biggest challenges in PEMFC is fabricating a thermally stable membrane which can operate at temperatures above 100 °C under anhydrous conditions.

In the present work phoshonated SBA-15/phosphonated Poly(styrene-ethylene-butylene-styrene) (PSEBS) composite membranes are developed for high temperature fuel cell electrolyte. Mesoporous Santa Barbara Amorphous (SBA-15) was synthesized and it was grafted with phosphonate functionality using a simple two-step process involving chloromethylation and subsequent phosphonation. The phosphonated SBA-15 (PSBA-15) was characterized using Fourier transform infra-red (FTIR) spectroscopy, solid state 13C Nuclear magnetic resonance (NMR), 29Si NMR, 31P NMR for confirming successful modification. Morphology features were verified by small angle X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Transmission electron microscopy TEM analyses. Poly(styrene-ethylene-butylene-styrene) (PSEBS) was chosen as the base polymer and phosphonic acid functional groups were grafted onto the polymer using the aforementioned approach, where chloromethyl (-CH2Cl) groups were attached to the main chain using Friedel Craft’s alkylation, followed by the phosphonation of the chloromethylated polymer by the Michaels-Arbuzov reaction resulting in phosphonated PSEBS (PPSEBS). The functionalisation was confirmed using NMR and FTIR spectroscopy studies. Composite PPSEBS/PSBA-15 membranes were fabricated with different filler concentrations (2, 4, 6, and 8%) of PSBA-15. Various studies such as water uptake, ion exchange capacity and the proton conductivity of the composite membranes were undertaken with respect to fuel cell applications. From the studies, it was found that the PPSEBS/PSBA-15 membrane with 6% wt of filler exhibited maximum proton conductivity of 8.62 mS/cm at 140 °C. Finally, membrane electrode assembly (MEA) was fabricated using PPSEBS/6% PSBA composite membrane, Platinium (Pt) anode, Pt cathode and was tested in an in-house built fuel cell setup. A maximum power density of 226 mW/cm2 and an open circuit voltage of 0.89 V was achieved at 140 °C under un-humidified condition.