Molecular dynamics study of hydroxide ion diffusion in polymer electrolytes

Abstract

Understanding the transport of hydroxide ion through polymer films designed to function as alkaline exchange membranes in fuel cells or solid-state alkaline electrolytes in batteries is critical to optimizing these materials for high power performance in these devices. We use molecular dynamics (MD) simulations to interrogate solid polymer electrolytes comprising methylated poly(dimethylaminomethyl styrene) (“pDMAMS+”) bearing hydroxide counterions to counterbalance the positively charged quaternized amino moiety. We elucidate the effects on hydroxide ion diffusivity of ion exchange capacity (IEC, 1.5–5.2 mEq g−1), hydration level (λ = 0–3 water molecules per ion), and co-polymerization with 4-butylstyrene (0–75 mol %), a charge-neutral monomer with similar properties to DMAMS. At 300 K, MD simulations yield a hydroxide ion self-diffusion coefficient in anhydrous pDMAMS+ of ∼ 0.02 μm2 s−1 and a glass-transition temperature (Tg) of ∼ 800 K, values that remain relatively unchanged for films with up to 50% 4-butylstyrene content. Increasing the 4-butylstyrene content to 75% results in a polymer with a lower Tg, ∼500 K, and a corresponding three-fold increase in the hydroxide ion diffusion coefficient to ∼ 0.06 μm2 s−1. As expected, increasing the hydration level increases both hydroxide diffusivity and conductivity across the entire range of compositions. 4-butylstyrene content lowers the available ion content, eventually decreasing overall hydroxide conductivity. MD simulations provide a powerful method to estimate the ion and co-monomer contents that will maximize hydroxide conductivity for a given range of polymer compositions and hydration, greatly reducing the need to synthesize and test the full range of compositions experimentally.

Keywords

• pDMAMS
• Molecular dynamics
• Hydroxide ion
• Polymer electrolytes
• Diffusion
• Glass transition