The surface properties of electrically conductive membranes (ECMs) govern their advanced abilities. During operation, these properties may differ considerably from their initially measured properties. Depending on their operating conditions, ECMs may undergo various degrees of passivation. ECM passivation can detrimentally impact their real time performance, causing large deviations from expected behaviour based on their initially measured properties. Quantifying these changes will enable consistent performance comparisons across the active and electrically conductive membrane research field. As such, consistent methods must be established to quantify ECM membrane properties. In this work, we proposed three standardized methods to assess the electrochemical, chemical, and physical stability of such membrane coatings: 1) electrochemical oxidation, 2) surface scratch testing, and 3) pressurized leaching. ECMs were synthesized by the most common approach - coating support ultrafiltration (UF) and/or microfiltration (MF) polyethersulfone (PES) membranes with carbon nanotubes (CNT) cross-linked with polyvinyl alcohol (PVA) and two types of cross-linkers (either succinic acid (SA) or glutaraldehyde (GA)). We then evaluated these ECMs based on the three standardized methods: 1) We evaluated electrochemical stability as a function of electro-oxidation induced by applying anodic potentials. 2) We measured the scratch resistance to quantify the surface mechanical stability. 3) We measured physical stability by quantifying the leaching of PVA during separation of a model foulant (polyethylene oxide (PEO)). Our methods can be extended to all types of electrically conductive membranes including MF, UF, nanofiltration (NF), and reverse osmosis (RO) ECMs. We propose that these fundamental measurements are critical to assessing the viability of ECMs for industrial MF, UF, NF, and RO applications.•Anodic-oxidation was used to check the electrochemical stability of ECMs•Depth of penetration resulted from scratch test is an indicator of the electrically conductive membrane coating's mechanical stability•The leaching of the main components forming the nanolayer was quantified to assess the membranes' physical stability.

Stability of electrically conductive membranes (ECM) is critical for expanding their application in separation-based technologies. In this work, ECMs were synthesized by coating polyethersulfone membranes with carbon nanotubes (CNT) crosslinked to polyvinyl alcohol (PVA) using two types of crosslinkers (succinic acid or glutaraldehyde). ECMs demonstrated a 21% reduction in flux over 4 h under cathodic potential (2 V) in comparison to a 69% reduction in flux for control experiments when filtering a realistic bacterial suspension. Subsequently, the electrochemical, physical, and mechanical stability of the ECMs were explored using chronoamperometry and cyclic voltammetry, an evaluation of polymer leaching from membranes, and micro mechanical scratch testing, respectively. ECMs were shown to be unstable under anodic potentials (2–4 V) with the glutaraldehyde crosslinking demonstrating the highest electrochemical stability. PVA was shown to be a physically unstable crosslinking agent for CNTs under concentration polarization conditions. Instability was moderated by extending CP layers through thicker and less dense nanolayers. ECMs showed higher mechanical stability and resistance to surface damage, in particular when coated with glutaraldehyde. We quantified the relationship between ECM surface instability and their physical and electrochemical properties. In so doing, we provide guidance for making practical and scalable electrically conductive membranes. • Applied potential impedes the development of membrane biofouling. • Electrically conductive membranes are unstable under operating conditions. • Physical, mechanical, and electrochemical stability of membranes were investigated. • PVA and GA protect CNT from anodic electro-oxidation.

Electrically conductive membranes (ECMs) self-induce antifouling mechanisms at their surface under certain applied electrical currents. Quantifying these mechanisms is critical to enhancing ECMs’ self-cleaning performance. Local pH change and H2O2 production are among the most important self-cleaning mechanisms previously hypothesized for ECMs. However, the impacts of these mechanisms have not previously been isolated and comprehensively studied. In this study, we quantified the individual impact of electrochemically induced acidic conditions, alkaline conditions, and H2O2 concentration on model bacteria, Escherichia coli. To this end, we first quantified the electrochemical potential of carbon nanotube-based ECMs to generate stressors, such as protons, hydroxyl ions, and H2O2, under a range of applied electrical currents (±0–150 mA, 0–2.7 V). Next, these chemical stressors with similar magnitude to that generated at the ECM surfaces were imposed on E. coli cells and biofilms. In the flow-through ECM systems, biofilm viability using LIVE/DEAD staining indicated biofilm viabilities of 39 ± 9.9%, 38 ± 4.7%, 45 ± 5.0%, 34 ± 3.1%, and 75 ± 4.9% after separate exposure to pH 3.5, anodic potential (2 V), pH 11, cathodic potential (2 V), and H2O2 concentration (188 μM). Electrical current-induced pH change at the membrane surface was shown to be more effective in reducing bacterial viability than H2O2 generation and more efficient than bulk pH changes. This study identified antibiofouling mechanisms of ECMs and provides guidance for determining the current patterns that maximize their antifouling effects.

Abstract Detecting the onset of membrane fouling is critical for effectively removing membrane foulants during microfiltration (MF) separation. This work investigates the use of electrical impedance spectroscopy (EIS) on the surface of electrically conductive membranes (ECMs) to measure early development of membrane surface fouling. An electrochemical cell was developed in which an ECM acted as a working electrode and a graphite electrode acted as the counter electrode. Conductive membranes were fabricated by coating single-walled/double-walled carbon nanotubes (f-SW/DWCNT) on microfiltration polyethersulfone (PES) supporting membranes. Membrane fouling was simulated by pressure depositing different amounts of latex beads onto the surface of the membrane in a dead-end filtration cell. Changes in membrane water permeability were correlated to the degree of membrane fouling. Clean membranes had water permeability of 392 ± 28 LMH/bar. Reduction of membrane water permeability of 13.8 ± 3.3%, 15.8 ± 4.7%, 17.8 ± 0.5% and 27.1 ± 4.6% were observed for membranes covered with 0.028 mg/m2, 0.28 mg/m2, 1.40 mg/m2 and 2.80 mg/m2 on the membranes, respectively. These small differences in fouling degree were statistically resolvable in measured Nyquist plots. It was observed that the diameter of the higher frequency charge transfer region (104–106 Hz) of the Nyquist plot semicircles increased with greater fouling. These observations were hypothesized to correspond to decreasing surface conductivities of the membranes by the incorporation of insulating materials (latex beads) within the porous conductive coating. This proposed hypothesis was supported by measured EIS results modeled with a theoretical equivalent circuit. Fouled membrane surface conductivity, surface hydrophilicity, and pore size were measured by SEM, four-point probe conductivity, contact angle, and MWCO experiments, respectively, to compare conventional characterization techniques with non-destructive EIS measurements.