Mechanisms for Enhanced Lithium-Ion Selectivity in Doped Polypyrrole-Coated Cation Exchange Membranes

Here we investigate the impact of dopants chloride (Cl−) and dodecyl sulfate (DS, C12H25SO4−) on the selective transport of Li+ in polypyrrole (PPy) modified cation exchange membranes (CEMs). Experimental results show that PPy doped with DS (PPyDS) significantly enhances selectivity of Li+ over Ca2+ despite having lower relative Li+ sorption. Using molecular dynamics (MD) simulations, we find a significant improvement in the relative diffusivity of Li+ over Ca2+ in PPyDS. The simulations corroborate the experimental observation that selectivity increases with membrane hydrophobicity. Moreover, simulations elucidate that cation dehydration and cation-dopant pairing play important roles in the improved relative Li+ diffusion. These findings highlight key factors in ion selectivity and suggest pathways for optimizing IEMs for Li+ extraction from aqueous sources.