Baseline BTMA Degradation as a Standard for Cationic Degradation in AEMFCs

Anion-exchange membrane fuel cells (AMFCs) are of increasingly popular interest as they enable the use of non-Pt fuel cell catalysts, the primary cost limitation of proton exchange membrane fuel cells.  Benzyltrimethyl ammonium (BTMA) is the standard cation that has historically been used in anion exchange membranes (AEMs)/AMFCs. BTMA degradation has been a topic of past studies; however, methodologies of degradation were not standardized or fully validated.  In fact, in our efforts on the topic we had become perplexed by the variability of results between tests.  In order to advance the state of the art technology in AMFC and the cations used therein, a standardized degradation method is necessary and would be of great value to the community. BTMA is a logical choice for developing a standardized method for cationic degradation in AMFCs. 

The method that will be presented in this presentation focuses on BTMA degradation studies in 2M KOH utilizing Teflon Parr reactors at varying temperatures and concentrations. NMR analysis was used to find the concentration of remaining BTMA at specific time points with GCMS analysis verifying product distribution.  Our earlier studies involved the use of glass or quartz vessels and the use of internal NMR standards that were not isolated from the basic solution.  High temperature, high hydroxide concentration studies have the potential of resulting in unintended reactions that can impact data interpretation regarding rates (e.g., our initial studies in glass vessels resulted in the appearance of precipitates/etched glass within the reaction vessels).  Our current system contains only BTMA, additional base, and water, which are in contact with a sealed Teflon vessel.  This experimental setup resulted in slower, more reproducible rates being observed for the degradation of BTMA, suggesting that the BTMA cation may be more robust than previously believed.  Because BTMA is a relatively compact, easily synthesized, and potentially low cost cation, any other cations being pursued for use in AEMs should offer substantial advantages over BTMA for consideration for use in AEMs.  Also appropriate baseline performance needs to be established.  Although a caveat to this is that the stability of the free cation in solution may be different than that of the tethered cation in the AEM.

The main pathway of BTMA decomposition leads to trimethyl amine and benzyl alcohol.  Our findings conclude that, under these alkaline conditions, BTMA at 80 °C has negligible degradation over 2000 hours.  Elevated temperatures (e.g., 120 and 160 °C) were utilized to accelerate the cation degradation process (Figure 1).  BTMA was found to have a half-life of 600 hours (120 °C) and 9 hours (160 °C) at concentrations of 0.01 and 0.1M BTMA.  These results show higher stability than previously reported.^1,2The degradation rate appears to be identical at concentrations of 0.01 and 0.1M BTMA; however, at a concentration of 1.0 M, degradation rates were observed to be much higher.  Additionally at 1.0 M, a second phase was observed and appeared to coincide with an increase in degradation rate.  

Employing a standardized degradation method, with BTMA stability as a baseline, will allow for a more accurate assessment of the relative stability of the next generation cations for use in AEMs.  In addition, a more relevant and straightforward comparison between different cations and experimental conditions will be possible.

References:

                (1)          Bauer, B.; Strathmann, H.; Effenberger, F. Desalination 1990, 79, 125.

                (2)          Enisla, B.; Chempath, S.; Pratt, L., Boncella, J.; Rau, J.; Macomber, C.; Pivovar, B. ECS TransactionsI 2007, 11, 1173.