L. Tshwenya, F. Marken, K. Mathwig and O. A. Arotiba, ACS Applied Materials and Interfaces 12 (2020) 3214. [link, pdf]

Membrane materials with semi-permeability for anions or for cations are of interest in electrochemical and nanofluidic separation and purification technologies. In this study, partially hydrolyzed poly-acrylonitrile (phPAN) is investigated as a pH-switchable anion/cation conductor. When switching from anionic to cationic semi-permeability, also the ionic current rectification effect switches for phPAN materials deposited asymmetrically onto a 5, 10, 20, or 40 µm diameter microhole in a 6 µm thick polyethylene-terephthalate (PET) film substrate. Therefore, ionic rectifier behavior can be tuned and used to monitor and characterize semi-permeability. Effects of electrolyte type and concentration, and pH (relative to the zeta potential at approximately 3.1) are investigated by voltammetry, chronoamperometry, and impedance spectroscopy. A computational model provides good qualitative agreement with observed electrolyte concentration data. High rectification effects are observed for both cations (pH > 3.1) and anions (pH < 3.1), but only at relatively low ionic strengths.