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 as hydroxide conductor in anion exchange membranes. Several past studies have focused on the effect of different aromatic ammonium cations on stability in caustic conditions, but few have studied the effects of substituents on the aromatic ring as it applies to caustic stabilities. Herein we study the effects of substitutions on the aromatic ring; namely the effects of methoxy and nitro substituents, on the stability of the cation in a hydroxide solution at elevated temperatures. Our unique method to study cation stability in caustic conditions at elevated temperatures utilizes Teflon Parr reactors suitable for use under various temperatures and cation concentrations. NMR analysis was used to determine remaining cation concentrations at specific time points with GCMS analysis verifying product distribution. We then compare the experimental results with calculated modeling stabilities.<br/>The main pathway of hydroxide based BTMA decomposition is SN2 debenzylation resulting in benzyl alcohol and trimethyl amine. Methoxy and Nitro substituted BTMA degradations also proceed via SN2 reaction resulting in similar products. Our studies show that the electron donating methoxy groups slightly increase stability (compared to that of BTMA), while the electron withdrawing nitro groups greatly decrease stability in base. These results give insight into possible linking strategies to be employed when tethering a BTMA like ammonium cation to a polymeric backbone; thus synthesizing an anion exchange membrane.

Stability of substituted benzyltrimethyl ammonium cations in caustic environments

249th National Meeting (2015)

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