Switching Anionic and Cationic Semi-Permeability in Partially Hydrolyzed Polyacrylonitrile:A pH-Tunable Ionic Rectifier

Membrane materials with semipermeability for anions or for cations are of interest in electrochemical and nanofluidic separation and purification technologies. In this study, partially hydrolyzed polyacrylonitrile (phPAN) is investigated as a pH-switchable anion/cation conductor. When switching from anionic to cationic semipermeability, 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 semipermeability. 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 the 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

Understanding Transient Ionic Diode Currents and Impedance Responses for Aquivion® Coated Microholes

Ionic diode based devices or circuits can be applied, for example, in electroosmotic pumps or in desalination processes. Aquivion ionomer coated asymmetrically over a Teflon film (5 μm thickness) with a laser-drilled microhole (approximately 10 μm diameter) gives a cationic diode with a rectification ratio of typically 10-20 (measured in 0.01 M NaCl with ±0.3 V applied bias). Steady state voltammetry, chronoamperometry, and electrochemical impedance spectroscopy data are employed to characterize the ionic diode performance parameters. Next, a COMSOL 6.0 finite element model is employed to quantitatively assess/compare transient phenomena and to extract mechanistic information by comparison with experimental data. The experimental diode time constant and diode switching process associated with a distorted semicircle (with a typical diode switching frequency of 10 Hz) in the Nyquist plot are reproduced by computer simulation and rationalized in terms of microhole diffusion-migration times. Fundamental understanding and modeling of the ionic diode switching process can be exploited in the rational/optimized design of new improved devices.</p

Driving Electrochemical Membrane Processes with Coupled Ionic Diodes

Ionic diodes have emerged repeatedly in the literature for gel interfaces, for nanopores and channels, for nano-/micro-fluidic systems, and for asymmetrically ionomer-covered microholes. Concentration polarisation is likely to be the key to understanding the diode function and the diode time constant τdiode, i.e. the time for approaching steady state following a potential/polarity switch. For frequencies higher than ωdiode = 2πf = (τdiode)−1, the polarization mechanism is too slow for ion current rectification. Below the frequency associated with the diode time constant, irreversibility in ion flow is induced and the diode switches between two resistive states at opposite potentials (“open” and “closed”). The irreversible flow of ions allows energy conversion from electrical to electrochemical. For energy conversion, two coupled ionic diodes are necessary driven by alternating current (AC) electricity to minimise driver electrode electrolysis and energy losses. Opportunities for AC-desalination and for electroosmotic water harvesting with coupled ionic diodes are discussed.</p

Effective electroosmotic transport of water in an intrinsically microporous polyamine (PIM-EA-TB)

Tertiary-amine-based Polymers of Intrinsic Microporosity (PIMs) provide a class of highly porous molecularly rigid materials for the electrochemical transport of both ionic and neutral species. Here, the transport of water molecules together with chloride anions (i.e. the electroosmotic drag coefficient) is studied for the intrinsically microporous polyamine PIM-EA-TB immersed in aqueous 0.01 M NaCl (i) when protonated for pH  4. Preliminary data suggest that in both cases a high electroosmotic drag coefficient is observed based on direct H2O transport into a D2O-filled compartment (quantified by 1H-NMR). For PIM-EA-TB there is a strong pH dependence with a higher electroosmotic drag coefficient in less acidic solutions (going from approx. 400 H2O per anion at pH 3 to approx. 4000 H2O per anion at pH 7), although the underlying absolute rate of water transport at a fixed voltage of −1 V appears to be essentially pH independent. Water transport through the PIM-EA-TB microchannels is rationalised based on the relative populations of chloride anions and of water in the micropores (essentially a ‘piston’ mechanism)

Driving electrochemical membrane processes with coupled ionic diodes

Ionic diodes have emerged repeatedly in the literature for gel interfaces, for nanopores and channels, for nano-/micro-fluidic systems, and for asymmetrically ionomer-covered microholes. Concentration polarisation is likely to be the key to understanding the diode function and the diode time constant τdiode, i.e. the time for approaching steady state following a potential/polarity switch. For frequencies higher than ωdiode = 2πf = (τdiode)−1, the polarization mechanism is too slow for ion current rectification. Below the frequency associated with the diode time constant, irreversibility in ion flow is induced and the diode switches between two resistive states at opposite potentials (“open” and “closed”). The irreversible flow of ions allows energy conversion from electrical to electrochemical. For energy conversion, two coupled ionic diodes are necessary driven by alternating current (AC) electricity to minimise driver electrode electrolysis and energy losses. Opportunities for AC-desalination and for electroosmotic water harvesting with coupled ionic diodes are discussed.</p

Ionic Diode and Molecular Pump Phenomena Associated with Caffeic Acid Accumulated into an Intrinsically Microporous Polyamine (PIM‐EA‐TB)

The polymer of intrinsic microporosity PIM-EA-TB provides a molecularly rigid micropore structure containing tertiary amine sites and is shown here to interact with hydrogen bonding guest molecules such as caffeic acid. Voltammetric data with a PIM-EA-TB film on glassy carbon electrodes show that in both acidic solution (pH 2; PIM-EA-TB is protonated) and in neutral solution (pH 6; PIM-EA-TB is not protonated) caffeic acid is slowly accumulated into the microporous host. Binding constants are estimated and suggested to be linked to hydrogen bonding causing accumulation of caffeic acid. When employing PIM-EA-TB as an asymmetric membrane coated onto a 5 μm thick Teflon support film with 10 μm diameter microholes (using either a single microhole or a 10×10 array of microholes), binding of caffeic acid is shown to cause a modulation of the ionic current without affecting the pH-dependent ionic diode behaviour. Two complementary types of effects of caffeic acid guests are discussed based on blocking anion diffusion pathways and based on removal of positive charges. The caffeic acid transport mechanism/efficiency is investigated in view of selective molecular pumping