Understanding Mono- and Bivalent Ion Selectivities of Nanoporous Graphene Using Ionic and Bi-ionic Potentials

Abstract

Nanoporous graphene displays salt-dependent ion permeation. In this work, we investigate the differences in Donnan potentials arising between reservoirs, separated by a perforated graphene membrane, containing different cations. We compare the case of monovalent cations interacting with nanoporous graphene with the case of bivalent cations. This is accomplished through both measurements of membrane potential arising between two salt reservoirs at different concentrations involving a single cation (ionic potential) and between two reservoirs containing different cations at the same concentration (bi-ionic potential). In our present study, Donnan dialysis experiments involve bivalent MgCl 2 , CaCl 2 , and CuCl 2 as well as monovalent KCl and NH 4 Cl salts. For all salts, except CuCl 2 , clear Donnan and diffusion potential plateaus were observed at low and high salt concentrations, respectively. Our observations show that the membrane potential scaled to the Nernst potential for bivalent cations has a lower value (≈50%) than for monovalent cations (≈72%) in the Donnan exclusion regime. This is likely due to the adsorption of these bivalent cations on monolayer graphene. For bivalent cations, the diffusion regime is reached at a lower ionic strength compared to the monovalent cations. For Mg 2+ and Ca 2+ , the membrane potential does not seem to depend upon the type of ions in the entire ionic strength range. A similar behavior is observed for the KCl and NH 4 Cl membrane potential curves. For CuCl 2 , the membrane potential curve is shifted toward lower ionic strength compared to the other two bivalent salts and the Donnan plateau is not observed at the lowest ionic strength. Bi-ionic potential measurements give further insight into the strength of specific interactions, allowing for the estimation of the relative ionic selectivities of different cations based on comparing their bi-ionic potentials. This effect of possible ion adsorption on graphene can be removed through ion exchange with monovalent salts.

Charge Regulation at a Nanoporous Two-Dimensional Interface

“…In our previous work, we have investigated the ion transport properties of perforated graphene by varying the concentration of monovalent and bivalent cations to understand how the single-layer membranes behave. 14,15 For all the salts under investigation (KCl, LiCl, K 2 SO 4 , MgCl 2 , CaCl 2 , and NH 4 Cl), we observed clear Donnan and diffusion dominated regimes due to Donnan exclusion of ions and differences in the self-diffusion coefficients of ions, respectively. These membranes further exhibited strong adsorption phenomena for bivalent cations.…”

“…Perforated monolayer graphene is a two dimensional material in which pores have been created in a controlled manner by, e.g., heavy ion beam bombardment, focused ion beams, electrical pulse method, and oxygen plasma etching. − Nanoporous graphene membranes have potential applications in the fields of separation, filtration, and biomolecular translocation. − As ions can diffuse through these pores in graphene, it can be used as electrodes for lithium ion batteries, spacers, as well as supercapacitors. , To achieve all these potential applications in practice, it is important to study the transport characteristics through these two-dimensional nanoporous materials. In our previous work, we have investigated the ion transport properties of perforated graphene by varying the concentration of monovalent and bivalent cations to understand how the single-layer membranes behave. , For all the salts under investigation (KCl, LiCl, K 2 SO 4 , MgCl 2 , CaCl 2 , and NH 4 Cl), we observed clear Donnan and diffusion dominated regimes due to Donnan exclusion of ions and differences in the self-diffusion coefficients of ions, respectively. These membranes further exhibited strong adsorption phenomena for bivalent cations.…”

Enhancing Ion Transport through Nanopores in Membranes for Salinity Gradient Power Generation

“…Besides the passive methods described above for controlling ion transport through thin membrane structures (e.g., pore size control, the use of bipolar membranes, ultrathin structures, composite 2D membranes, hydrogels, and so on), the literature also contains a growing number of proposals for enhancing the power generation by means of active factors. Generally speaking, the aim of these factors is to boost the salinity-based power generation capability of 1D (i.e., ultrathin perforate graphene), , 2D (i.e., aramid nanofiber/graphene oxide), and 3D (i.e., natural wood) , membranes from laboratory to industrial scale (see Figure ). Ion transport typically exploits three different driving forces, namely electric fields, pressure gradients, and concentration gradients (see the boron nitride membrane in Figure ).…”

Carbon Nanomaterials for Wastewater Treatment

“…This 3D graphene architecture provided an opportunity for complete removal of graphene from treated water. The selectivity of graphene for mono-and divalent cations is also assessed by controlling the pore size in graphene [79].…”