Study of Alkaline Durability and Anion Conductivity Using New Expanded Pyridinium Polymer with Self-Assembly Ability

Abstract

Polymer electrolyte fuel cells (PEFCs) are promising for practical use as the next generation clean power source. Since the anion exchange membrane fuel cell (AEMFC) operates in a basic environment, cost reduction is possible because non-platinum catalysts such as iron can be used. In addition, since the activation overpotential of oxygen reduction is smaller than that of a proton exchange membrane fuel cell (PEMFC), it is attracting attention as an alternative to PEMFC [1]. However, for practical use, further improvement of the anion conductivity and alkaline durability of the anion exchange membrane (AEM), in which a hydroxide ion (OH-) acts as an anion conductor is required. General strategy to promote anion conduction has been to move cation side chain freely, however problem is that the decomposition reaction occurs due to the attack from the OH-. In this research, a new strategy of “self-organizing cation units to form channel structure” is studied to improve alkaline durability and anion conductivity at the same time. It is considered that this self-assembly strategy can achieve both the low mobility of cation to suppress from OH- attack and the anion conduction path. To realize this concept, an extended pyridinium monomer was newly designed with reference to extended pyridinium, one of the heteropolycyclic aromatic cation. Since strong π-π interactions have been reported between extended pyridiniums, introducing an extended pyridinium into the polymer will suppress the molecular motion of the anion conduction site and will induce the formation of self-assembled channels. A new extended pyridinium monomer (TVP) was synthesized according to the synthesis method of the extended pyridinium salt of Katrizky et al [2]. Synthesized TVP was obtained as a pale yellow powder. From the 1H-NMR spectrum, vinyl group and aromatic peaks were observed, and it was confirmed that the monomer possessed high purity. The homopolymer (PTVP) was synthesized by free radical solution polymerization of TVP. Synthesized PTVP was obtained as a yellow solid. In 1H-NMR measurement, vinyl peaks were disappeared and the broadening of aromatic ring peaks were observed, suggesting that the free radical polymerization was taken place. Since PTVP is brittle and difficult to use as a film probably due to its rigid molecular structure, the introduction of a copolymer monomer was examined. When styrene and isoprene reported as an anion exchange membrane were examined as a copolymerization monomer, copolymer of PTVP-styrene and PTVP-isoprene were obtained, respectively. Although a PTVP-styrene copolymer film was brittle, a PTVP-isoprene copolymer film was obtained as a free standing film. Alkaline stability and anion conductivity of the films will be discussed. [1] G. Merle; M. Wessling; K. Nijmeijer, J. Membr. Sci, 2011, 377, 1-35. [2] A. R. Katritzky; Z. Zakarial; E. Lunt; P. G. Jones; O. Kennard, J. Chem. Soc., Chem. Commun., 1979, 6, 268-269.