Alkaline Stability of Benzyl Trimethyl Ammonium Functionalized Polyaromatics: A Computational and Experimental Study

Alkaline stability of benzyl trimethylammonium (BTMA)-functionalized polyaromatic membranes was investigated by computational modeling and experimental methods. The barrier height of hydroxide initiated aryl-ether cleavage in the polymer backbone was computed to be 85.8 kJ/mol, a value lower than the nucleophilic substitution of the α-carbons on the benzylic position of BTMA cationic functional group, computed to be 90.8 kJ/mol. The barrier heights of aryl–aryl cleavage (polymer backbone) are 223.8–246.0 kJ/mol. The computational modeling study suggests that the facile aryl–ether cleavage is not only due to the electron deficiency of the aryl group but also due to the low bond dissociation energy arising from the ether substituent. Ex situ degradation studies using Fourier transform infrared (FTIR) and 1H nuclear magnetic resonance (NMR) spectroscopy indicated that 61% of the aryl–ether groups degraded after 2 h of treatment in 0.5 M NaOH at 80 °C. BTMA cationic groups degraded slowly over 48 h under the same conditions. In situ degradation studies validate the calculated results: anion exchange membrane fuel cells and water electrolyzer using poly(arylene ether) membranes exhibit a catastrophic, premature failure during lifetime tests, while no sudden performance loss is observed with an ether-free poly(phenylene) membrane. Despite the gradual performance loss due to the degradation of BTMA cation functional group, the membrane electrode assembly using the poly(phenylene) membrane exhibited a lifetime of >2000 h in the alkaline water electrolyzer mode at 50 °C.