Effect of Carbonate Anions on Quaternary Ammonium-Hydroxide Interaction

Abstract

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.

Water Content and Ionic Conductivity of Thin Films of Different Anionic Forms of Anion Conducting Ionomers

“…44 The hydration of the ionomers is in part governed by the affinity of water for the cation/anion ion-pair at low RH and energy to dissociate the ions, i.e., cationic quaternary ammonium (QA) and the mobile anion, at high RH. 60 The degree of affinity of water toward the anion and cation of an ion-pair influences the hydration characteristics under drier conditions. The Hofmeister series suggests that large anions such as Br − and HCO 3 − and large cations such as QA + exhibit weak interaction with water whereas small anions such as F − and OH − and cations such H + and Li + strongly interact with water.…”

“…However, a MD simulation study has shown that at a hydration level of up to λ equal to 4, QA + HCO 3 − do not dissociate. 60 In the contact pair regime (λ < anion solvation number), the enthalpic term of hydration Gibbs energy would arise from the exothermic effect of mobile water attaining a less energetic state of sorbed water. For our experiments, the comparison between three forms can be done for λ of 2 or less and between F − and HCO 3 − forms for λ up to 4.…”

“…The hydration of the ionomers is in part governed by the affinity of water for the cation/anion ion-pair at low RH and energy to dissociate the ions, i.e., cationic quaternary ammonium (QA) and the mobile anion, at high RH . The degree of affinity of water toward the anion and cation of an ion-pair influences the hydration characteristics under drier conditions.…”

Synthesis of CeOx‐Decorated Pd/C Catalysts by Controlled Surface Reactions for Hydrogen Oxidation in Anion Exchange Membrane Fuel Cells

“…Significant progress has been achieved recently in the field of anion exchange membrane fuel cells (AEMFCs). [1][2][3] In spite of the remarkable development of this technology, the deployment of AEMFCs is still hindered by the chemical degradation of the anion exchange membrane (AEM) and ionomers, [4][5][6][7][8][9][10][11] carbonation issues, [12][13][14][15] and by the sluggish kinetics of the hydrogen oxidation reaction (HOR). [2,16,17] It has been shown that the HOR kinetics in alkaline media is two to three orders of magnitude lower than that in acidic media, [18,19] even for the most active catalysts such as Pt, [17,[19][20][21][22] Rh, [23] and Ir.…”

The Reaction Mechanism Between Tetraarylammonium Salts and Hydroxide

“…Our group has been interested in the synthesis, and decomposition, of QAs in order to try to open new directions in the development of stable cations for AEMFCs. We have recently developed an improved procedure for the synthesis of N,N ‐diaryl carbazolium salts, which allows for the synthesis of a variety of derivatives with different substituents in very few synthetic steps and significantly better yields .…”