Role of the electric field in selective ion filtration in nanostructures

Abstract

Nafion has received great attention as a proton conductor that can block negative ions. Here, we report the effect of a Nafion coating on an anodic aluminium oxide (AAO) nanoporous membrane on its function of ion rejection and filtering depending on the electric field. In our experiments, Nafion, once coated, was used to repel the negative ions (anions) from the coated surface, and then selectively allowed positive ions (cations) to pass through the nanopores in the presence of an electric field. To demonstrate the proof-of-concept validation, we coated Nafion solution onto the surface of AAO membranes with 20 nm nanopores average diameter at different solution concentration levels. Vacuum filtration methods for Nafion coating were vertically applied to the plane of an AAO membrane. An electric field was then applied to the upper surface of the Nafion-coated AAO membrane to investigate if ion rejection and filtering was affected by the presence of the electric field. Both anions and cations could pass through the AAO nanopores without an electric field applied. However, only cations could well pass through the AAO nanopores under an electric field, thus effectively blocking anions from passing through the nanopores. This result shows that ion filtration of electrons has been selectively performed while the system also works as a vital catalyst in reactivating Nafion via electrolysis. A saturated viscosity ratio of Nafion solution for the coating was also determined. We believe that this approach is potentially beneficial for better understanding the fundamentals of selective ion filtration in nanostructures and for promoting the use of nanostructures in potential applications such as ion-based water purification and desalination system at the nanoscale in a massively electrically integrated format.

Electroactive polymers made by loading redox ions inside crosslinked polymeric hydrogels. Effects of hydrophobic interactions and solvent dynamics

“…They suggest that the presence of large redox cations promote the anion transport inside the anionic ionomer. On the other hand, anion rejection by thin films of an anionic polymer (Nafion 1 ) has been shown [89].…”

Nanoporous platinum electrode grown on anodic aluminum oxide membrane: Fabrication, characterization, electrocatalytic activity toward reactive oxygen and nitrogen species

“…However, Pt nanomaterials for electro-catalysis application were depended on synergetic effect of other matrix, such as Nafion and carbon, which involves complicated fabrication process. On the other hand, porous anodized aluminum oxide (AAO) substrates have become feasible, low-cost, versatile inorganic materials for wide applications in filtration [7], catalysis [8], biosensing [9] and nanofilm-fabrication [10]. AAO substrates consist of highly-ordered and uniform nanopores with large depth/width ratio (>1000) [11].…”

Hydrophobicity effects in iron polypyridyl complex electrocatalysis within Nafion thin-film electrodes

“…10 At the same time strong anion rejection effects have been observed. 11 It has recently been noted that this anion rejection can be overcome for the case of multiply-charged catalyst cations immobilised in the Nafion host in high concentration. In this case anions can gain access and react within the Nafion composite layer.…”