Effect of Bulky Cations on the Stability and Properties of Anion Exchange Membranes

Abstract

Alkaline anion exchange membrane fuel cells are widely investigated recently as promising alternative to well-established proton exchange membrane fuel cells. Anion exchange membranes, using as solid electrolytes, are required to be thermally, chemically and mechanically stable in base at the operation temperature and humidity, as well as have an efficient OH- transport along the membrane across two electrodes. This study aims at studying AEMs functionalized with three different bulky cations. They are 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium functionalized polyphenylene oxide (Figure 1(a)) [1], tris(2,4,6-trimethoxyphenyl) phosphonium functionalized PPO (Figure 1(b)) [2], and phenylene cobaltocenium functionalized norbornene (Figure 1(c)). Degradation study on 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium functionalize PPO and unsubstituted imidazolium PPO by soaking both two membranes into 1 M sodium hydroxide at 80°C indicates attachment of 2,4,6-trimethoxyphenyl group efficiently improve OH- stability due to the enhanced volumetric steric and basicity of the central cation. Super hydrophobic and bulky phosphonium cation attached by three 2,4,6-trimethoxyphenyl groups has the chemical stability surpassing benzyl trimethyl ammonium by 30% after degrading in 1 M KOH at 80°C for 20 days. The cobaltocenium cation, which has 18 valance electrons further improve the chemical stability by 60% higher than trimethyl ammonium after treating in 1 M NaOH/CH3OH at 80°C for 20 days (Figure 2). Besides enhanced chemical properties from these bulky cations, their influence on the other polymer properties include thermal stability, water uptake, conductivity as well as transport property are also discussed. Figure 1. Structure of 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium functionalized PPO (a), tris(2,4,6-trimethoxyphenyl) phosphonium functionalized PPO (b) and phenylene cobaltocenium functionalized norbornene (c). Figure 2. Degradation studies performed at 80°C on 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium (solid diamond, 1 M KOH), tris(2,4,6-trimethoxyphenyl) phosphonium (solid square, 1 M KOH), phenylene cobaltocenium (solid triangle, 1 M NaOH/CH3OH) and benzyl trimethyl ammonium (open square, 1 M KOH; open triangle, 1 M NaOH/CH3OH). Reference [1] Y. Liu, J. Wang, Y. Yang, T.M. Brenner, S. Seifert, Y. Yan, M.W. Liberatore, A.M. Herring, J. Phys. Chem. C 118 (2014) 15136-15145. [2] Y. Liu, B. Zhang, C.L. Kinsinger, Y. Yang, S. Seifert, Y. Yan, C.M. Maupin, M.W. Liberatore, A.M. Herring, J. Membr. Sci. 506 (2016) 50-59. Figure 1