Synthesis and Characterization of Highly Cross-Linked Anion Exchange Pore-Filling Membrane for High Performance Solid-State Alkaline Fuel Cells
Solid-state alkaline fuel cells (SAFC) recently draw much attention compared to proton exchange membrane fuel cells (PEMFC) particularly due to the specific benefits associated with them such as incorporation of non-noble metal catalysts and liquid fuel enabling them as superior high density energy conversion devices. However, OH- ion transporting membranes are hardly used for practical applications in contrast to proton exchange membrane fuel cell (PEMFC) where nafion is used as a standard proton transporting membrane. Development of high performance anion exchange membrane is very essential for formulating such advanced systems. However, it is a challenging task because the ultimate performance not only depends on ionic conductivity but also depends on various other properties of the membrane such as mechanical strength, suppression of swelling, low fuel permeation under humidified condition and high chemical durability under alkaline condition.
In the present work, a novel type of aromatic monomer having three reaction sites that can simultaneously act as cross-linking and quaternization site was synthesized and in-situ cross-linked to fabricate the membrane. The monomer was polymerized inside the pores of porous cross-linked polyethylene substrate to obtain highly cross-linked anion exchange pore-filling membrane with high ion exchange capacity. The cross-linking density of the material inside substrate was analyzed as a measure of ion exchange capacity by elemental analysis of the substituted halide counter anion. The pore-filling membrane showed high ionic conductivity of 80mS at 80 oC owing to high ion exchange capacity even though it has a very low water uptake. Further, no significant weight change was observed for the membrane after soaking into hot water for 72h indicating the anion exchange polymer inside the substrate is highly cross-linked and stable even at high temperature conditions. In addition, the membranes showed very good stability under hot alkaline condition evidenced by the ionic conductivity measured after treating with 1M KOH solution at 60 oC for one month. More significantly, methanol permeability of the membrane was observed extremely low when compared to anion exchange membrane fabricated with a poly-vinylbenzyl trimethylammoniumchloride with 1-wt-% cross-link density. This further highlights that not only the filling electrolyte polymer but also the degree of cross-linking density of the membrane contributes significantly in suppressing the methanol permeation. These results demonstrate the high performance of pore-filled anion exchange membrane fabricated using the highly-cross-linked polyelectrolyte with high ionic exchange capacity.
 Yamaguchi et al J. Membrane Sci., 373(1-2),107-111(2011)