Tuning the Ion-Selectivity of Thin-Film Composite Nanofiltration Membranes by Molecular Layer Deposition of Alucone

Interface Engineering by Molecular Layer Deposition on Polymer Membrane for Selective Ion Transport

Molecular layer deposition (MLD) of ethylene glycol-alucone (EG-alucone) on the Nafion cation exchange membrane is investigated to understand its impact on the morphology of the composite and consequent enhancement of ion transport selectivity. X-ray photoelectron spectroscopy, scanning electron microscopy, Density functional theory, and Doppler broadening positron annihilation spectroscopy are comprehensively employed to examine the morphology of the composite, particularly the engineered interface between EG-alucone and Nafion. These studies reveal the diffusion and subsequent reaction of the Lewis-acidic trimethyl aluminum precursor with the polymer substrate during MLD. The Al─F bond, thus formed, modifies the well-defined microstructures of Nafion and creates a distinct hybrid region with size-based exclusion properties. Post-MLD hydration partially converts the deposited EG-alucone to alumina, reshaping the surface morphology and potentially forming a conformal top layer. The positively charged nature of this top layer, along with the steric sieving effect at the EG-alucone-polymer interface, contribute to the observed improvement in monovalent ion selectivity (Cs+/Na+ = 1.99 ± 0.08 & Cs+/Ba2+ = 1.50 ± 0.05) without significantly compromising membrane conductivity. A three-layer model of the composite membrane, supported by electrochemical impedance spectroscopy and radiotracer-based transport measurements, elucidates the structure-property relationship responsible for the observed selective ion transport.

Current Density Dependence of Transport Selectivity of Metal Ions in the Electrodriven Process across the Cation Exchange Membrane

Understanding the mechanisms leading to the selective transport of cations in an electrodriven process across a cation exchange membrane is important to design and control the potential gradient-based separation process. In this study, a comprehensive description of the current density (I, over a broad current regime) dependence of transport selectivity (Si) between cations of the same/different valence is presented. The role of conventional transport mechanisms such as diffusion, electromigration, and electroconvection in controlling the Si was identified theoretically as well as by multiple experimental approaches. These parameters were found to be dependent on the limiting current density (Ilim). In general, irrespective of the cations involved, Si (over Na+) decreased gradually with increasing I and then increased slowly (and saturated) after Ilim. This extent of variation of Si was heavily dependent on the charge and hydration state of the cations. At I < Ilim, both diffusion and electromigration processes contributed and, notably, the sorption selectivity outweighed the migration selectivity. At I → Ilim, diffusion was the solitary mechanism responsible for cation transport and migration selectivity was the major contributor in Si. At I > Ilim, as also validated by the Peclet numbers, the overall transport was dictated by electroconvection.

Atmospheric-pressure atomic layer deposition: recent applications and new emerging applications in high-porosity/3D materials

Atomic layer deposition (ALD) is a widely recognized technique for depositing ultrathin conformal films with excellent thickness control at Ångström or (sub)monolayer level. Atmospheric-pressure ALD is an upcoming ALD process with a potentially lower ownership cost of the reactor. In this review, we provide a comprehensive overview of the recent applications and development of ALD approaches emphasizing those based on operation at atmospheric pressure. Each application determines its own specific reactor design. Spatial ALD (s-ALD) has been recently introduced for the commercial production of large-area 2D displays, the surface passivation and encapsulation of solar cells and organic light-emitting diode (OLED) displays. Atmospheric temporal ALD (t-ALD) has opened up new emerging applications such as high-porosity particle coatings, functionalization of capillary columns for gas chromatography, and membrane modification in water treatment and gas purification. The challenges and opportunities for highly conformal coating on porous substrates by atmospheric ALD have been identified. We discuss in particular the pros and cons of both s-ALD and t-ALD in combination with their reactor designs in relation to the coating of 3D and high-porosity materials.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.

Thin-Film Composite Membrane Prepared by Interfacial Polymerization on the Integrated ZIF-L Nanosheets Interface for Pervaporation Dehydration

Thin-film composite (TFC) membranes are attracting wide attention because their ultrathin selective layer usually corresponds to the higher membrane flux for pervaporation. However, the direct preparation of the TFC membranes on ceramic substrates confronted with the great difficulties because the larger pores on ceramic substrate surfaces are detrimental to the formation of an intact polyamide (PA) selective layer produced by interfacial polymerization (IP) reaction. Here, the integrated ZIF-L nanosheets were proposed to be used as an assistance interlayer for the first time to eliminate the existence of the pores of the ceramic support, and provides a better basis for the formation of an intact PA selective layer by IP reaction between TMC and ethylenediamine (EDA). The experimental data obtained in pervaporation (PV) show that the increased flux from 1.1 to 2.9 kg/m²h corresponds to the decreased separation factor from 396 to 110 when the feed concentration of ethanol decreases from 95 wt % to 80 wt % at 50 °C. In addition, the membrane flux increases from 0.8 to 2.5 kg/m²h with a change of the separation factor from 683 to 111 when the operational temperature varies from 30 to 60 °C. These results demonstrate the great potential of the fabricated TFC membranes in practical application for PV dehydration of organic solutions.