Understanding the mechanisms in lithium-ion selectivity enhancement by coating cation exchange membranes with doped polypyrrole

Abstract

Here we study the effects of three types of dopants—chloride (Cl⁻), p-toluene sulfonate (TS, $C_7{H}_7{O}_3{S}^{-}$), and dodecyl sulfate (DS, $C_{12}{H}_{25}{\mathrm{SO}}_4^{-}$)—on the hydrophobicity of the polypyrrole (PPy) modified cation exchange membranes (CEMs) and their selective transport of Li⁺. The modified CEMs are referred to as PPyCl-CEM, PPyTS-CEM and PPyDS-CEM, corresponding to the dopants used. Using membrane capacitive deionization (MCDI), we find that the CEMs modifications by PPyDS or PPyTS exhibit enhanced selective transport of Li⁺ over Ca²⁺. In addition, the selective transport of Li⁺ increases with increasing surface hydrophobicity, with PPyDS-CEM exhibiting the highest hydrophobicity and the greatest enhancement in Li⁺ selectivity —over 60 % improvement in MCDI and a 38-fold increase in the diffusion cell—relative to Ca²⁺. Combining experimental sorption tests and computational molecular dynamics (MD) simulations, we find that while PPyDS modification results in lower relative Li⁺ sorption compared with Ca²⁺, it promotes relative Li⁺ diffusion, leading to enhanced selective transport of Li⁺ over Ca²⁺. The MD simulations further elucidate that both cation dehydration and cation-dopant ion pairing play important roles in the improved relative Li⁺ diffusion.