ReaxFF Reactive Molecular Dynamics Simulation of Functionalized Poly(phenylene oxide) Anion Exchange Membrane

Three functionalized poly(phenylene oxide) (PPO) anion exchange membranes (AEMs), PPO-trimethylamine (PPO-TMA), PPO-dimethylbutylamine (PPO-DMBA), and PPO-dimethyloctylamine (PPO-DMOA), at two hydration levels (λ = 8.3 and 20.8) have been studied by ReaxFF reactive molecular dynamics simulations. Our simulations reveal that with increasing hydration the microstructures of membrane swell and water molecules are more likely to form a channel, which improves the diffusion of hydroxide ion (OH^-). Our study of OH^- diffusion demonstrates that PPO-TMA hydrated membrane provides the biggest diffusion constant at the high hydration level. However, from comparison of the structural and dynamical properties of the three membranes at the same water content, it is found that when one methyl group of quaternary ammonium center is replaced by a long alkyl chain group, the hydrophobic effects block the OH^- approaching nitrogen, resulting in a lower rate of degradation and an improved alkaline stability of PPO-DMOA hydrated membrane. On the basis of these simulation results, we expect that a high performance AEM fuel cell should balance the conductivity with stability of the membrane.