Asymmetric Electrokinetic Proton Transport through 2D Nanofluidic Heterojunctions

Abstract

Nanofluidic ion transport in nacre-like 2D layered materials attracts broad research interest due to subnanometer confined space and versatile surface chemistry for precisely ionic sieving and ultrafast water permeation. Currently, most of the 2D-material-based nanofluidic systems are homogeneous, and the investigations of proton conduction therein are restricted to symmetric transport behaviors. It remains a great challenge to endow the 2D nanofluidic systems with asymmetric proton transport characteristics and adaptive responsibilities. Herein, we report the asymmetric proton transport phenomena through a 2D nanofluidic heterojunction membrane under three different types of electrokinetic driving force, that is, the external electric field, the transmembrane concentration gradient, and the hydraulic pressure difference. The heterogeneous 2D nanofluidic membrane comprises of sequentially stacked negatively and positively charged graphene oxide (n-GO and p-GO) multilayers. We find that the preferential direction for proton transport is opposite under the three types of electrokinetic driving force. The preferential direction for electric-field-driven proton transport is from the n-GO multilayers to the p-GO multilayers, showing rectified behaviors. Intriguingly, when the transmembrane concentration difference and the hydraulic flow are used as the driving force, a preferred diffusive and streaming proton current is found in the reverse direction, from the p-GO to the n-GO multilayers. The asymmetric proton transport phenomena are explained in terms of asymmetric proton concentration polarization and difference in proton selectivity. The membrane-scale heterogeneous 2D nanofluidic devices with electrokinetically controlled asymmetric proton flow provide a facile and general strategy for potential applications in biomimetic energy conversion and chemical sensing.

Electric‐Field‐Induced Ionic Sieving at Planar Graphene Oxide Heterojunctions for Miniaturized Water Desalination

“…[ 14 ] Besides size‐based exclusion, they do not sufficiently take advantage of the novel transport phenomena in nano‐ or sub‐nanofluidic systems, [ 25–27 ] such as asymmetric ion transport, [ 28,29 ] ion concentration polarization, [ 30,31 ] and heterogeneous membrane structure. [ 32–35 ]…”

Solvation‐Involved Nanoionics: New Opportunities from 2D Nanomaterial Laminar Membranes

“…interactions between ions (or solvents) and materials are directly affected. Examples of this approach include chemical functionalization and thermal annealing . Additionally, the electrons inside the materials can be modulated by the means of voltage bias or light illumination so that the electrostatic interactions (e.g., Debye screening in EDL) between ions and materials could be indirectly regulated through the coupling of electrons and ions .…”

“…This strategy allows the cost‐effective production of 2D‐NLMs with tunable nanochannels at large scale . Given the ease of synthesis, high‐level structural tunability and versatility, 2D‐NLMs have been extensively explored for confined molecular and ion transport by many research groups including Gao, Geim, Gogotsi, Guo, Huang, Jin, Li, Mi, Nair, Ren, Zhu, and more. The following section will lay out some of the typical attributes of 2D‐NLMs obtained by cascading to correlate with the advances in solvation‐involved nanoionics research.…”

“…and organic solvent recovery . Third, the solution processability of 2D nanomaterials enables various assembling strategies for membranes, where asymmetric or even more complex structures can be obtained through alternative filtration, simple cutting and pasting procedures . This enables novel ionic applications, such as fabrication of ionic diodes and transistors 122a.…”

Rectified Ion Transport Through 2D Nanofluidic Heterojunctions

“…However, so far, most 2D nanofluidic architectures are homogeneous. It is challenging to endow the 2D nanofluidic devices with asymmetric transport properties and tunable selectivity . For example, Wang et al fabricate a spiropyran decorated graphene oxide membrane with both rectified ion transport properties and light responsiveness …”