Computational Modeling of Degradation of Substituted Benzyltrimethyl Ammonium

Interest in alkaline fuel cells has been renewed in recent years with the development of the alkaline membrane fuel cells (AMFCs). Inside AMFCs, an anion exchange membrane (AEM) allows hydroxide (OH^-) to transport across the membrane between the electrodes and prevents carbonates from forming precipitation. However, AEMs can degrade over time due to the reactions between OH^- and the cations attached to the polymer backbone of the membrane, which remains as a challenge for many potential applications of AMFCs.

Benzyltrimethyl ammonium (BTMA⁺) is the most commonly used cation in AEMs and has been investigated extensively for its application in AMFC. Our experimental measurements had shown that unsubstituted BTMA⁺ cation degrades ~10% within 29 days in 5M NaOH at 80°C.¹ Two major OH^- degradation pathways for BTMA⁺ are benzyl S_N2 pathway and methyl S_N2 pathway^2,3 and their transition state (TS) barriers (ΔG^≠) are 23.3 kcal/mol and 25.1 kcal/mol respectively at 160°C and 1 atm.⁴ Thus, the benzyl S_N2 pathway is the dominant degradation pathway for BTMA⁺. In search for BTMA derivative cations with higher degradation barriers, in this research, we modeled the BTMA⁺ with different substituent groups at different positions (Fig 1) and calculated their ΔΔG^≠ values compared with unsubstituted BTMA⁺ using the density functional theory (DFT) method as reported in our previous paper.⁴

Based on our calculations results, for every cation, its ΔG^≠ of methyl S_N2 reaction is higher than its benzyl ΔG^≠, and thus, the benzyl S_N2 reaction is always the dominant degradation pathway. For the benzyl S_N2 reaction (Fig 2), the largest ΔΔG^≠ is 1.6 kcal/mol from the double-meta substituted −NH₂ and −N(CH₃)₂, and the ΔΔG^≠ values for all −NO₂ substitutions are less than 0. All of the benzyl S_N2 ΔG^≠ values for the double-meta substitutions with electron-releasing substituents are larger than the unsubstituted BTMA⁺, indicating that this type of substitutions is a promising candidate for cations with higher stability. Encouraged by the large benzyl S_N2 ΔΔG^≠ values of double-meta −NH₂ and −N(CH₃)₂, we further investigated substituents in –NRR’ form at double-meta positions. We studied four substituents: −NH(CH₂CH₂CH₂CH₃),  −NH(CH₃), −N(CH₂CH₃)₂, and −N(CH₂CH₂CH₃)₂. However, none of them results in a higher benzyl S_N2 ΔΔG^≠.

Besides the benzyl and methyl S_N2 pathways, BTMA⁺ may also take the benzyl S_N1 pathway. For the double-meta −N(CH₃)₂ substitution, the benzyl S_N1 ΔG^≠ is 27.7 kcal/mol, still larger than its benzyl S_N2 ΔG^≠ (24.9 kcal/mol). The para- −N(CH₃)₂ has a benzyl S_N1 ΔG^≠ as small as 11.3 kcal/mol, much smaller than its benzyl S_N2 ΔG^≠, 22.0 kcal/mol and the benzyl S_N1 pathway becomes the dominant degradation pathway. Therefore, in search for the BTMA⁺ derivative cations with high stability, meta- substitutions with electron releasing substituents are preferred, not only because meta- substitutions have larger benzyl S_N2 ΔG^≠, but also because ortho- and para- substitutions may have very small benzyl S_N1 ΔG^≠.

Based on our calculation results, the cations with the largest improvement found in this research only resulted in a ΔΔG^≠ of 1.6 kcal/mol at 160°C and 1.5 kcal/mol at 80°C. A ΔΔG^≠ of 1.5 kcal/mol will result in a decrease in degradation rate of 8.5 times. While potentially meaningful for some applications, this increase is not as large as would be desired and not experimentally validated.  Overall, our calculation results suggest that the improvement of the BTMA⁺ stability by adding substituent groups on the benzyl ring is relatively limited.

References:

            (1)        Einsla, ECS Transactions 2007, 11, 1173.

            (2)        Chempath, J Phys Chem C 2010, 114, 11977.

            (3)        Chempath, J Phys Chem C 2008, 112, 3179.

            (4)        Long,  J Phys Chem C 2012, 116, 9419.