Meta-analysis of electrically conductive membranes: A comparative review of their materials, applications, and performance

Production of Stable Electrically Conductive PVDF Membranes Based on Polydopamine-Polyethyleneimine-Assisted Deposition of Carbon Nanotubes

Electrically conductive membranes (ECMs) have emerged as a multifunctional separation technology that integrates membrane filtration with electrochemical reactions. Physical stability remains a critical challenge for ECMs synthesized by coating polymer membranes with conductive materials. In this article, polydopamine (PDA) and polyethyleneimine (PEI) were used to facilitate the synthesis of significantly more stable ECMs using poly(vinylidene fluoride) (PVDF) ultrafiltration membranes and carbon nanotubes (CNTs). Four different synthesis methods were compared in terms of the final surface stability and separation properties: (1) CNTs deposited on PEI-crosslinked PDA-coated PVDF membranes, (2) PEI-crosslinked CNTs deposited on PDA-coated PVDF, (3) PDA, PEI and CNTs sequentially deposited layer-by-layer on PVDF, and (4) PEI-crosslinked PDA deposited on CNT-coated PVDF. The results revealed that method 1 generated ECMs with the greatest physical stability, highest electrical conductivity (18,518 S/m), and sufficient permeability (395.2 L/(m2·h·bar). In comparison, method 2 resulted in membranes with the highest permeability (2128.5 L/(m2·h·bar), but with low surface conductivity (502 S/m) and poor physical stability (i.e., 53-75% lower peel-off forces compared to other methods). Overall, methods 1, 3, and 4 can be used to make highly conductive membranes with a 97-99% removal of methyl orange by electrochemical degradation at -3 V.

Facile Synthesis of Electrically Conductive Membranes

A facile and effective strategy that can be used to fabricate electrically conductive membranes (ECMs) of diverse filtration performance (i.e., water productivity and solute rejection) is not available yet. Herein, we report a facile method that enables the fabrication of ECMs of a broad performance range. The method is based on the use of polyethylenimine (PEI), glutaraldehyde, and any of a diverse set of conductive materials to cast an electrically conductive layer atop any of a diverse set of substrates (i.e., from microfiltration to reverse osmosis membranes). We developed the reported ECM fabrication method using graphite as the conductive material and PVDF membranes as substrates. We demonstrate that graphite-PVDF ECMs were stable and electrically conductive and could be successfully used for solute filtration and electrochemical degradation. We also confirmed that the PEI/glutaraldehyde-based ECM fabrication method is suitable for conductive materials other than graphite, including carbon nanotubes, reduced graphene oxide, activated charcoal, and silver nanoparticles. Compared with the substrates used for their fabrication, ECMs showed low electrical sheet resistances that varied with conductive material, increased solute rejection, and reduced water permeance. Taken together, this work presents a promising general strategy for the fabrication of ECMs for environmental applications from diverse substrates and conductive materials.

Modelling the Performance of Electrically Conductive Nanofiltration Membranes

Electrically conductive membranes are a class of stimuli-responsive materials, which allow the adjustment of selectivity for and the rejection of charged species by varying the surface potential. The electrical assistance provides a powerful tool for overcoming the selectivity-permeability trade-off due to its interaction with charged solutes, allowing the passage of neutral solvent molecules. In this work, a mathematical model for the nanofiltration of binary aqueous electrolytes by an electrically conductive membrane is proposed. The model takes into account the steric as well as Donnan exclusion of charged species due to the simultaneous presence of chemical and electronic surface charges. It is shown that the rejection reaches its minimum at the potential of zero charge (PZC), where the electronic and chemical charges compensate for each other. The rejection increases when the surface potential varies in positive and negative directions with respect to the PZC. The proposed model is successfully applied to a description of experimental data on the rejection of salts and anionic dyes by PANi-PSS/CNT and MXene/CNT nanofiltration membranes. The results provide new insights into the selectivity mechanisms of conductive membranes and can be employed to describe electrically enhanced nanofiltration processes.