Cationic Rectifier Based on a Graphene Oxide-Covered Microhole: Theory and Experiment

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.

Asymmetric heterojunctions between size different 2D flakes intensify the ionic diode behaviour

Here we report on the facile formation of asymmetric heterojunctions between laterally size different 2D flakes, which leads to a prominent gradient in charge distribution at the nanocontact interface and triggers ionic diode-like transport behaviour with a rectification ratio of 110.

Controllable ion transport induced by pH gradient in a thermally crosslinked submicrochannel heterogeneous membrane

Solid-state nanochannels have attracted considerable attention for their similar ion transport properties to biological ion channels. The construction of porous ion channels with good stability at the submicro/micrometer scale is very beneficial to develop large-area ion channel devices. In this manuscript, based on in-situ thermal crosslinking of a small organic molecule containing triphenylamine and styrene groups, we construct a heterogeneous membrane with asymmetrical charge and wettability on cylindrical anodic aluminum oxide (AAO) channels (D ≈ 319 nm). This heterogeneous membrane has typical ion current rectification characteristics with a high rectification ratio of 36.9 and good stability. This work provides an effective strategy for the construction of submicrochannel heterogeneous membranes and also broadens the application range of bionic ion channels.

Electric Migration of Hydrogen Ion in Pore-Voltammetry Suppressed by Nafion Film

Micro-hole voltammetry exhibiting rectified current-voltage curves was performed in hydrochloric acid by varying the lengths and the diameters of the micro-holes on one end of which a Nafion film was mounted. Some voltammetric properties were compared with those in NaCl solution. The voltammograms were composed of two line-segments, the slope of one segment being larger than the other. They were controlled by electric migration partly because of the linearity of the voltammograms and partly the independence of the scan rates. Since the low conductance which appeared in the current from the hole to the Nafion film was proportional to the cross section area of the hole and the inverse of the length of the hole, it should be controlled by the geometry of the hole. The conductance of the hydrogen ion in the Nafion film was observed to be smaller than that in the bulk, because the transport rate of hydrogen ion by the Grotthuss mechanism was hindered by the destruction of hydrogen bonds in the film. In contrast, the conductance for the current from the Nafion to the hole, enhancing by up to 30 times in magnitude from the opposite current, was controlled by the cell geometry rather than the hole geometry except for very small holes. A reason for the enhancement is a supply of hydrogen ions from the Nafion to increase the concentration in the hole. The concentration of the hydrogen ion was five times smaller than that of sodium ion because of the blocking of transport of the hydrogen ion in the Nafion film. However, the rectification ratio of H+ was twice as large as that of Na+.

An ionic diode based on a spontaneously formed polypyrrole-modified graphene oxide membrane

Asymmetric membranes derived from the stacking of graphene oxide (GO) nanosheets have attracted great attention for the fabrication of ionic diodes. Herein, we described an ionic diode based on a polypyrrole-modified GO membrane with a vertical asymmetry, which was achieved by a spontaneous oxidation polymerization of pyrrole monomers on one side of the GO membrane in vapor phase. This asymmetric modification resulted in an asymmetric geometry due to the occupation of the interlayer space of one side of the GO membrane by polypyrrole. Our ionic diode demonstrated an obvious ionic rectification behavior over a wide voltage range. A calculation based on Poisson-Nernst-Planck equations was used to theoretically investigate the role of asymmetric modification of polypyrrole.

Switching Anionic and Cationic Semipermeability 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.

Engineering Smart Nanofluidic Systems for Artificial Ion Channels and Ion Pumps: From Single-Pore to Multichannel Membranes

Biological ion channels and ion pumps with intricate ion transport functions widely exist in living organisms and play irreplaceable roles in almost all physiological functions. Nanofluidics provides exciting opportunities to mimic these working processes, which not only helps understand ion transport in biological systems but also paves the way for the applications of artificial devices in many valuable areas. Recent progress in the engineering of smart nanofluidic systems for artificial ion channels and ion pumps is summarized. The artificial systems range from chemically and structurally diverse lipid-membrane-based nanopores to robust and scalable solid-state nanopores. A generic strategy of gate location design is proposed. The single-pore-based platform concept can be rationally extended into multichannel membrane systems and shows unprecedented potential in many application areas, such as single-molecule analysis, smart mass delivery, and energy conversion. Finally, some present underpinning issues that need to be addressed are discussed.