Membranes and Membrane Technologies最新文献

This work is aimed at studying the characteristics of a homogeneous perfluorinated membrane at various stages of introducing zirconium hydrogen phosphate (ZrHP) in situ. A sample modified with ZrHP by pore-filling method, a sample processed the same as the first one, excluding treating with ZrHP precursor solution, and a sample of initial membrane are studied. A significant increase in the diffusion permeability and specific conductivity of the membrane in the solutions of hydrochloric acid after its treatment with heated immersion solutions is observed. This effect reduces after ZrHP nanoparticles appearance. The transport-structural parameters of the microheterogeneous model have been calculated. Both the internal equilibrium solution volume and the mobility of coions and counterions in the gel phase increase after treating with immersion solutions, due to greater swelling of the gel phase in alcohols. The specific power density of the PEMFC MEA with the ZrHP-modified membrane used as a polymeric electrolyte is significantly lower compared to the initial membrane, but annealing the membrane compensates this effect.

The effect of the anion-exchange layer of the copolymer N,N-diallyl-N,N-dimethylammonium chloride and ethyl methacrylate on the electrochemical properties of a homogeneous cation-exchange membrane based on perfluorosulfonic polymer has been studied. The application of a modifying layer with a thickness of 5 µm onto a 215 µm-thick membrane leads to a reduction in electrical conductivity by no more than 35%, while the diffusion permeability decreases more than fivefold and becomes independent of concentration. During testing of both cation-exchange and bilayer membranes in the process of limiting concentration of sodium chloride solution, comparable degrees of concentration have been achieved. The effectiveness of the bilayer membrane for selective limiting electrodialysis concentration has been demonstrated. When concentrating a solution containing sodium and calcium chlorides, the specific selective permeability coefficient P(Na⁺/Ca²⁺) for the cation-exchange membrane has ranged from 0.5 to 1.2. The use of the bilayer membrane significantly increase the specific selective permeability coefficient to 1.5–2.7, depending on the current density, allowing for efficient separation of electrolytes containing singly and doubly charged cations.

Previously published experimental data and new data on cloud points were used to plot ternary phase diagram with temperature axis for the poly(vinylidene fluoride) (PVDF)–dimethyl acetamide (DMAc)–water system. The topology of the plotted diagram is different from the published previously. It shows that an increase in temperature leads to a shift of the boundary curves (liquid equilibrium binodal, polymer crystallization curve and swelling curve) to the composition range enriched by the non-solvent. At the same time, a decrease in temperature leads to degeneration of liquid equilibrium binodal. Taking into account the plotted diagram, the expected morphologies of the membranes prepared via non-solvent induced phase separation (NIPS) in both isothermal and non-isothermal conditions were derived. Morphology and properties of the samples prepared from the dope solution of defined composition via NIPS at different temperatures and via thermally assisted non-solvent induced phase separation, T-NIPS (or NIPS in non-isothermal conditions) were studied. It was shown that an increase in temperature leads to formation of cellular structure, resulting from the liquid–liquid phase separation; a decrease in through pore size and improvement of the mechanical properties. Decrease in pore size from 110 to 35 nm accompanied with decrease in permeance from 8.1 to 0.25 L/(m² h bar) and increase in blue dextran rejection from 20 to 94%. A decrease in temperature changes the structure formation process from liquid–liquid phase separation (induced by the mass transfer processes) to solid–liquid phase separation (induced by temperature decrease). In the T-NIPS process (cooling) layers with finger-like pores and sponge-like structure between them, with spherulites surrounded by sponge-like structure and with spherulites connected with each other were formed in the structure of the membrane. T-NIPS process allows to obtain membranes with permeance 6.1 L/(m² h bar) and Blue dextran rejection of 91%.

The degree of swelling of the lithium form of the perfluorosulfonic acid membrane Nafion in alcohols (ethanol, 2-propanol), water-alcohol mixtures, and highly polar aprotic solvents (N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP)) was studied, as well as the thermodynamics of membrane-solvent interaction using microcalorimetry. It was shown that the equilibrium swelling degree of the membrane correlates with the donor number of the solvent and the enthalpy of polymer swelling. The enthalpy of membrane swelling in all studied solvents is negative, indicating polymer solvation. Concentration dependences of the swelling and mixing enthalpies in DMF and NMP were studied in greater detail. The negative values of the swelling enthalpy across the entire concentration range of the solvents indicate good thermodynamic compatibility of the membrane with the solvent and highlight the advantage of using these solvents to produce Nafion dispersions due to their strong solvating properties.

A method has been developed for calculating the rate constants of rate-limiting steps of the water splitting reaction in the generating contacts of heterogeneous bipolar membranes (BPMs) containing particles of a catalytic additive. The method is based on the use of the equation of the current–voltage characteristic of the bipolar region of a heterogeneous BPM that contains generating contacts of two types. For the case when the catalytic additive is a cation exchanger (CE), one of the contacts is formed by CE particles and anion exchanger (AE) particles contained in BPM layers, and the other is formed by catalytic additive particles and AE particles contained in BPM layers. The order of the rate constants for the rate-limiting steps of the water splitting reaction in the studied membranes is consistent with the catalytic activity series, the constants of which are calculated based on the proton transfer reactions between water molecules and ionogenic groups contained in the BPM layers.

The rapidly growing field of portable energy sources requires the search for and development of efficient materials for such devices. To enhance the safety of the most common metal-ion batteries (lithium-ion and sodium-ion), it has been proposed to replace the liquid electrolyte with a unipolar conductive gel-polymer electrolyte based on a Nafion-like electrolyte (Inion) plasticized with aprotic solvents. This study presents the results of investigating the thermal stability, molecular and supramolecular structure, as well as the ionic conductivity of Inion membranes in lithium and sodium forms plasticized with propylene carbonate, using methods including synchronous thermal analysis, IR spectroscopy, small-angle X-ray scattering (SAXS), and impedance spectroscopy.

Tartrate stabilization of wine materials by electrodialysis makes it possible to speed up and automate this process and reduce the loss of valuable components. Widespread implementation of electrodialysis in industrial wine production is hampered by fouling of ion-exchange membranes with wine components and by the very limited range of membranes currently used. This study is devoted to a comparative analysis of properties of relatively inexpensive heterogeneous ion-exchange membranes MA-41, MK-40, AMH-PES, and CMH-PES before and after their use in tartrate stabilization of wine materials by electrodialysis. It is shown that the mechanisms of fouling and its effect on transport characteristics, as well as on the development of electroconvection and generation of H⁺ and OH^– ions, are largely determined by counterions that are transferred through cation-exchange (transition metal cations) and anion-exchange (carboxylic acid anions) membranes. Membranes MA-41 and MK-40 demonstrate higher resistance to fouling during operation of an electrodialysis device for less than 15 h.

The catalytic properties of samples containing Pd and Co metals on carbon supports (IR-pyrolyzed chitosan (CT) with an activated surface and detonation nanodiamonds (DNDs) have been studied in the ethanol steam reforming process. CT is a promising catalyst support due to its developed surface and the presence of nitrogen-containing groups capable of sorbing water molecules. The use of a membrane reactor with a Pd–Ru–In membrane has significantly increased the efficiency of the ethanol steam reforming process due to removing hydrogen from the reaction zone. The hydrogen yield in the membrane reactor increases twofold or more compared to a conventional reactor, while the proportion of reaction byproducts (CO and acetaldehyde) decreases. The highest hydrogen yield (15.8 mol/h per gram of catalyst) in the membrane reactor is achieved using a Pd–Co/CT_KOH catalyst.

Composite materials based on cation-exchange membranes MK-40 and ceria, including phosphate-functionalized, have been obtained and characterized by SEM, TGA, and IR spectroscopy. The conductivity of the membranes in the sodium form decreases from 6.2 to 3.5 mS/cm and in the calcium form, increases slightly from 1.3 to 1.6 mS/cm. It has been shown that selectivity to divalent ions grows. For example, during electrodialysis desalination selective permeability coefficients Р(Ca²⁺/Na⁺) and Р(Mg²⁺/Li⁺) increase to 3.6 and 6.6, respectively. Furthermore, additional phosphate functionalization of ceria improves the fouling resistance of the materials.

Three methods for modification of polyacrylonitrile (PAN) ultrafiltration membranes with polyelectrolytes are considered: (1) bulk modification by introducing polyacrylic acid (PAA) into the casting solution, (2) surface modification by using aqueous solutions of polyethyleneimine (PEI) as a coagulation bath, and (3) a combination of methods 1 and 2. In all three cases, modification of membranes with polyelectrolytes leads to effective hydrophilization of the surface of ultrafiltration membranes (the contact angle decreases from 41° to 15°–25°). It has been found that the bulk modification of PAN membranes by introducing 0.05–0.2 wt % PAA into the casting solution leads to a decrease in the pure water flux from 110 to 96 L/m² h. The maximum polyvinylpyrrolidone K30 rejection coefficient of 96% was observed at a PAA concentration of 0.05 wt %; with a subsequent increase in the PAA content, the rejection coefficient decreases to 70–73%. Surface modification of PAN membranes with polyethyleneimine leads to a more than twofold increase in their water flux (up to 233–294 L/m² h), while the rejection coefficient for polyvinylpyrrolidone K30 was 82–96% depending on the PEI concentration in the coagulation bath. It is shown that the combined modification method reduces the water flux to 44 L/m² h, which is associated with the formation of a polyelectrolyte complex and compaction of the membrane structure. It has been found that the combined modification method allows obtaining ultrafiltration PAN membranes with a high flux recovery ratio after filtration of model solutions of polyvinylpyrrolidone (73–100% compared to 65% for the unmodified membrane) and humic acids (80% compared to 73% for the unmodified membrane).