Anion Exchange Membranes (AEMs), based on Polyamine Obtained by Modifying Polyketone, for Electrochemical Applications

Polymeric anion exchange materials can be key components for forming membranes for use in several electrochemical applications. Polyketones seem particularly promising as materials for making anion exchange membranes (AEMs), not only because the starting monomers, carbon monoxide and ethylene, are relatively inexpensive (pointing to the feasibility of producing polyketone at a more competitive cost than other membranes), but also because the presence of 1,4-dicarbonyl units along the backbone is an important chemical feature for the purposes of chemically modifying these polymers. It allows for post-manufacturing functionalization through the so-called Paal-Knorr reaction, which introduces N-substituted pyrrole units along the polymer backbone. An anion exchange membrane (AEM) was made with a modified polyketone using a solvent casting method, followed by iodomethylation and ion exchange with KOH (PK-PDAPm). Every step in the synthetic process was confirmed by Fourier Transform InfraRed spectroscopy (FTIR). Nuclear Magnetic Resonance (NMR) spectroscopy was also used to characterize the structure of the modified polyketone in detail. The results obtained revealed the formation of a pyrrole ring along the polyketone backbone. Polyamines modified in this way are amenable to structural rearrangements to form N-substituted pyrrole crosslinked with dihydropyridine units. Scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction techniques were also used to study the morphological, thermal, and structural characteristics of the modified polyketone, as well as the membranes derived therefrom. Thermogravimetric analyses demonstrated the thermal stability of the material up to 200oC, with no significant mass loss or degradation. The conductivity of the AEM was studied at temperatures up to 120oC, and the highest value of 9x10-4 S.cm-1 was reached at 120oC for the ionic conductivity of the membrane in iodide form, with values of the same order of magnitude (10-4 S.cm-1) for the membrane in OH form. Polyamine (PA-SiNH2)m, membranes containing silica formed by sol-gel reactions of 3-aminopropyltriethoxysilane (APTES) in hydrolytic conditions were prepared by solution casting, followed by methylation and an ion exchange process, in an effort to improve the properties of the AEM. FTIR and NMR were used to investigate the chemical features of the silica and its interaction with the polyamine polymer. The influence of amino-functionalized silica (Si-NH2) on the properties of the membrane obtained was investigated. The results demonstrated: a significant improvement in thermal stability up to 300oC, and an increase in water uptake and ion exchange capacity by comparison with the AEM (PK-PDAPm) containing no silica. The maximum conductivity obtained for (PA-SiNH2)m-I and (PA-SiNH2)m-OH was 2.4 ×10-4 S cm-1 at 130oC, and 4.8 ×10-4 S cm-1 at 120oC. These details may serve as an initial guide to the use of the above-described AEM in electrochemical applications.