Effect of Hydration on the Intermolecular Interaction of Various Quaternary Ammonium Based Head Groups with Hydroxide Ion of Anion Exchange Membrane Studied at the Molecular Level

Abstract

Currently, the main limitation of Anion Exchange Membrane Fuel Cells is related to their low chemical stability under alkaline conditions due to the degradation of quaternary ammonium-based head groups, which lowers the transportation of hydroxide ions as well. The knowledge of the intermolecular interaction of various quaternary ammonium head groups with hydroxide ions is the key to improving hydroxide ion’s diffusivity and chemical stability of various quaternary ammonium-based head groups. Consequently, the intermolecular interaction of hydroxide ions with different quaternary ammonium head groups of anion exchange membranes is investigated at the different hydration levels via classical all-atom Molecular Dynamics and molecular well-tempered MetaDynamics simulation methods in this work. Several quaternary ammonium head groups (a) pyridinium, (b) 1,4-diazabicyclo [2.2.2] octane, (c) benzyltrimethylammonium, (d) n-methyl piperidinium, (e) guanidium, and (f) trimethylhexylammonium were investigated in detail. Classical all-atom molecular dynamic simulations illustrate that the results of radial distribution function between the nitrogen atoms of six different quaternary ammonium head groups and hydroxide ion are as follows: (a) > (c) ≥ (f) > (d) > (e) > (b). In addition, from the diffusion coefficient values it was found that the mobility of hydroxide ion by quaternary ammonium head group (f) was lower than (c) at the different hydration levels.