Effect of Carbonate Anions on Quaternary Ammonium-Hydroxide Interaction

Currently, there are two main challenges in state-of-the-art anion-exchange membrane fuel cells (AEMFCs)—first, cation degradation in the presence of hydroxide anions; second, carbonation process during AEMFC operation. Both degradation and carbonation processes lead to a significant decrease in the ionic conductivity of the anion exchange membranes (AEMs), and, in turn, in the AEMFC performance. In this work, we use molecular dynamics simulations to bring first insights into the contributing factors that lead to changes in the degradation of quaternary ammonium cations due to the presence of carbonate anions. Focusing on low hydration levels, we explore the behavior of benzyltrimethylammonium cation (BTMA+) in the presence of a mixture of hydroxide and carbonate anions at different water:cation ratios. Water is shown to have a stronger affinity toward carbonate than hydroxide. Thus, the introduction of carbonate anions effectively lowers the concentration of free hydroxide anions and thereby decreases the conductivity of the AEM. Lower hydration of the hydroxide anion, in turn, leads to higher coordination of hydroxide compared with carbonate around BTMA+, hence increasing the probability of degradation of the cation. Nonetheless, carbonate competes with hydroxide in its interaction with cation, leading to approximately 20% reduction in hydroxide coordination around the BTMA+ when carbonate is present. We examine in detail these two competing factors—steric shielding of BTMA+ by carbonate and effectively lower hydration of the hydroxide—which are critical for understanding the effect of carbonate on the stability of quaternary ammonium cations.