Interface Engineering by Molecular Layer Deposition on Polymer Membrane for Selective Ion Transport

Molecular layer deposition (MLD) of ethylene glycol-alucone (EG-alucone) on the Nafion cation exchange membrane is investigated to understand its impact on the morphology of the composite and consequent enhancement of ion transport selectivity. X-ray photoelectron spectroscopy, scanning electron microscopy, Density functional theory, and Doppler broadening positron annihilation spectroscopy are comprehensively employed to examine the morphology of the composite, particularly the engineered interface between EG-alucone and Nafion. These studies reveal the diffusion and subsequent reaction of the Lewis-acidic trimethyl aluminum precursor with the polymer substrate during MLD. The Al─F bond, thus formed, modifies the well-defined microstructures of Nafion and creates a distinct hybrid region with size-based exclusion properties. Post-MLD hydration partially converts the deposited EG-alucone to alumina, reshaping the surface morphology and potentially forming a conformal top layer. The positively charged nature of this top layer, along with the steric sieving effect at the EG-alucone-polymer interface, contribute to the observed improvement in monovalent ion selectivity (Cs⁺/Na⁺ = 1.99 ± 0.08 & Cs⁺/Ba²⁺ = 1.50 ± 0.05) without significantly compromising membrane conductivity. A three-layer model of the composite membrane, supported by electrochemical impedance spectroscopy and radiotracer-based transport measurements, elucidates the structure-property relationship responsible for the observed selective ion transport.