Molecular dynamics simulation of alkaline electrolyte diffusion in anion exchange membrane

Abstract

During the operation of alkaline direct liquid fuel cells, the alkaline electrolyte is usually needed in the anode electrode to accelerate the electrochemical reaction kinetics of the liquid fuel. However, the crossover of the alkaline solution in the anode through the anion exchange membrane to the cathode can increase the transfer resistance of the oxygen in the cathode. In order to reduce the crossover of the alkaline solution, the diffusion process of the alkaline solution in the anion exchange membrane needs to be fully understood. In this work, interface models of anion exchange membrane-alkaline electrolytes are established based on the cell structure of the quaternary ammonium polysulfone (QAPS) membrane to simulate the dynamic process of the alkaline solution in the membrane. The effect of the type and the concentration of the alkaline solution on the transportation of the metal ions and OH⁻ in the membrane are studied. The results show that the agglomeration of Na⁺ is formed more easily than K⁺ in the interface model. Because of the strong interaction of Na⁺ on OH⁻, OH⁻ ions appear to be concentrated, resulting in that the diffusion coefficients of the metal ion and OH⁻ in the in Na⁺ solution are lower than those in the K⁺ solution. In addition, with the raised concentration of electrolyte solution, the aggregation degrees of the metal ions and OH⁻ can be increased, which means an enlarged mass transfer resistance of the components. Furthermore, by adding a polytetrafluoroethylene (PTFE) layer on the QAPS membrane, the distribution of metal ions tends to be concentrated, and the number of hydrophilic channels in the QAPS membrane is reduced, which significantly increases the alkali resistance of the anion exchange membrane.