Importance of Polymer Backbone Stability of Anion Exchange Polymer Electrolytes

Anion exchange membranes have received attention for solid-state alkaline membrane water electrolysis. However, the harsh operating condicitions of alkaline membrane water electrolysis require good chemical stability of the membrane. Chemical degradation of anion exchange membranes can occur in the cationic functional group, cationic functional group-polymer linkage, and polymer backbone. The alkaline stability of cation-functionalized polyaromatic membranes has mainly been discussed in the context of cation stability. Thus far, lifetime studies of cation-fucntinalized polyaromatics indicate that cation stability is of paramount importnace for stable operation of alkaline membrane water electrolysis. However, equally important is the stability of the polymer backbone which may impact the key membrane properties such as mechanical toughness, hydroxide conductivity and water uptake.[1]

In this study, comparative studies on cationic group vs. polymer backbone stabilities of cationic group functionalized polyaromatic membranes were made by computational modeling and experimenatal method. The computational modeling study suggests that the facile backbone degradation of anion exchange membranes is not only due to the electron deficincy of the polymer backbone but also due to the low bond dissociation energy of the molecular stucture. The relatively lower bond dissociation energy of heteroatom substitutuents in the polymer backbone of anion exchange membranes caused facile backbone degradation before cationic functional group degradation. For example, benzyl trimethyl ammonium has very high electonegativity but the degradation occurs after aryl-ether degradation due to the high bond dissociation energy of carbon-carbon or carbon-nitrogen bonds.

Experimental study on degradation of benzyl trimethyl ammonium functionalized fluoirnated poly(arylene ether) membranes suggests further insight regarding polymer backbone degradation. The C-N stretching peaks of the poly(arylene ether) membranes is shifted to the higher wave number (blue shift) due to the cationic group degradation of the cationic group attached to the phenolic structure occurs before that of the cationic group attached to the un-degraded polymer backbone. This inidcates accessibility of water molecules also play a critical role in polymer backbone degradation. We will provide some other evidences on facile backbone degradation of polyaromatic polymers and their impact on membrane properties at the meeting. This presentation emphasizes on the importance of polymer backbone stability.

[1] Fujimoto, C.; Kim, D. S.; Hibbs, M.; Wrobleski, D.; Kim, Y. S. J. Membr. Sci. 2012, 423, 438−449.