Membranes and Membrane Technologies最新文献

Due to the increased attention to hydrogen energy and the fact that many countries adopted the programs for its development, the question on the prospects for this area becomes relevant. Initially, Russian hydrogen energy development program was focused on producing hydrogen from natural gas. However, owing to the changed international situation and the declared course to the use of “green” hydrogen, the production of which is not associated with the emission of carbon oxides, special attention should obviously be paid to the development of fuel cells (FC) and electrolyzers. In this review, the main advantages and disadvantages of fuel cells of various types are considered. Today, the most developed industry is low-temperature fuel cells based on proton-conducting membranes (proton-exchange membrane fuel cells in English literature). At the same time, fuel cells based on anion-exchange membranes with OH^–-ion conductivity are also promising. Their key advantage is the possibility of using significantly cheaper non-perfluorinated membranes and platinum-free catalysts. Considerable attention in the review is paid to fuel cells operating at elevated temperatures. The second part of this review discusses in detail the membranes currently used in these devices and promising materials that can replace them in the near future.

The coefficients of diffusion permeability of methanol through the synthesized polymer film–sulfonated polystyrene composite membranes and a Nafion-115 membrane are measured. For several composite membranes with significantly different transport properties, the values of the diffusion flux of methanol q_diff through these membranes under the conditions of a direct methanol fuel cell (DMFC) at 60°C and a concentration of the feed solution of 1–2 M are calculated. Direct measurements of the crossover current and methanol crossover q_CVA in a DMFC based on these membranes are carried out by cyclic voltammetry (CVA). It is found that the values of q_CVA are on average by 15% lower than the corresponding values of q_diff calculated for each membrane based on its individual parameters (area, thickness, permeability coefficient of methanol). It is proposed to explain the observed ratio q_CVA < q_diff by the experimentally uncontrolled and, probably, incomplete oxidation of methanol at the cathode. It can be concluded based on the obtained data that the experimental values of the crossover q_CVA can noticeably differ from calculated q_diff and real values of methanol crossover in a DMFC without monitoring the degree of oxidation of methanol at the DMFC cathode. A comparative study of the current–voltage characteristics of DMFCs based on the synthesized composite membranes with significantly different transport properties and a Nafion-115 membrane is carried out. It is found that, at 60°C and a concentration of the feed solution of 1 M, the value of methanol crossover has practically no effect on the current–voltage characteristics of the DMFCs.

The structural and transport (conductivity, diffusion permeability) properties of cation- and anion-exchange membranes with varied dispersity of ion-exchange resin particles are studied in the work. Experimental MK-40 cation-exchange and MA-41 anion-exchange membranes with variable particle size of the ion-exchange resin of <20 to <71 μm were manufactured at LLC IE Shchekinoazot (Russia). A comparative analysis of the structural characteristics of the membranes by SEM reveals the anisotropy in the properties of the surface and section. The internal phase of the membrane is characterized by high values of the fraction and sizes of the ion exchanger and macroporosity. A comparison of the concentration dependences of the specific conductivity and diffusion permeability of the experimental membranes is performed. The analysis of the values of the model transport-structural parameters shows that an increase in the conductivity of the gel phase from 0.39 up to 0.47 S/m and from 0.15 up to 0.26 S/m for cation- and anion-exchange membranes, respectively, as well as redistribution of current transfer paths in the membrane are observed in the case of a decrease in the particle size of the ion exchanger. An increase in the contribution from transfer of the internal equilibrium solution along the channel is revealed; here, the transfer numbers of counterions slightly change. Information about the changes in the structure of the transport channels in membranes with different particle sizes of the ion exchanger obtained based on the analysis of the model parameters is consistent with the data of independent studies of the morphology of their surface and section by SEM.

This review focuses on the separation properties of membranes based on poly(urethane-imides)—modern products of the chemical modification of polyimides and polyurethanes. The membrane properties of poly(urethane-imides) are reviewed in terms of their chemical design. Membranes based on multiblock (segmented) polymers, polyimides crosslinked by polyurethanes, hybrid poly(urethane-imide) materials, and poly(urethane-imides) subjected to the selective destruction of urethane blocks are considered. The main directions of synthesis of membrane poly(urethane-imides) are described, and reactants and reaction conditions are presented. The transport and separation properties of poly(urethane-imide) membranes in separation, pervaporation, and ultrafiltration processes are addressed in detail. Applications of poly(urethane-imide) membranes are discussed. The review provides insight into the importance of poly(urethane-imide) gas separation and pervaporation membranes for separation processes.

Study has been carried out on the separation of oil–water emulsion using polyamide membranes with a pore size of 0.2 μm treated with corona discharge plasma at a voltage of 5–25 kV for 1–5 min. An increase in the productivity and efficiency of separation of oil–water emulsion with corona-treated polyamide membranes has been revealed. An increase in roughness and a change in the chemical structure of the modified membranes are shown.

Modeling of ion transport in a three-layer system containing an ion-exchange membrane and two adjacent diffusion layers makes it possible to describe the permselectivity of the membrane by determining its fixed charge density. For theoretical analysis of ion transport in such systems, the Nernst–Planck and Poisson equations are widely used. The article shows that, in the galvanodynamic mode of operation of the membrane system when the density of the flowing current is specified, the Poisson equation in the ion transport model can be replaced by the equation for the displacement current. A new model is constructed in the form of a boundary value problem for the system of the Nernst–Planck and displacement current equations, based on which the concentrations of ions, electric field strength, space charge density, and chronopotentiogram of the ion-exchange membrane and adjacent diffusion layers in a direct current mode are numerically calculated. The calculation results of the proposed model are in a good agreement with the results of the modeling based on the previously described approach using the Nernst–Planck and Poisson equations as well as with the analytical assessment of the transition time. It is shown that, in the case of the three-layer geometry of the problem, the required accuracy of the numerical calculation using the proposed model is achieved with a smaller number of computational mesh elements and takes less (about 26.7-fold for the system parameters under consideration) processor time in comparison with the model based on the Nernst–Planck and Poisson equations.

Polynaphthoylenebenzimidazoles (PNBI) with keto (PNBI-СО) and sulfonic (PNBI-SO₂) bridging groups are prepared by the solid-phase polycyclization of films of corresponding polyaminoimides (PANI) synthesized by the polycondensation of 1,4,5,8-naphthalenetetracarboxylic dianhydride with 3,3',4,4'-tetraaminobenzophenone and 3,3',4,4'-tetraaminodiphenyl sulfone in N-methylpyrrolidone, respectively. The polycondensation process and the chemical structure of the resulting PANI and PNBI are controlled by ¹Н and ¹³С NMR and IR spectroscopy. It is shown that variation in the temperature of solid-state polycyclization allows for the synthesis of polymers with various cyclization degrees. The experimental values of gas permeability and diffusion coefficients for He, H₂, N₂, O₂, CO₂, and CH₄ are measured, and the solubility coefficients and ideal selectivity for various gas pairs are calculated. It is found that in terms of the permeability/selectivity ratio completely cyclized PNBI are advantageous over incompletely cyclized ones. This finding should be taken into account when choosing a polymer and a selective layer formation method to develop novel composite membranes. The gas transport characteristics achieved for completely cyclized PNBI-SO₂ and their good film-forming properties combined with a very high thermal stability of this class polymers are of great interest for further expanding the PNBI range and prospects for applying novel polymers of this class in various gas separation processes.

A procedure has been proposed for producing ceramic substrates for filtration membranes based on a narrow fraction of fine fly ash microspheres using cold uniaxial pressing followed by high-temperature firing. It has been shown that increasing the sintering temperature from 1000 to 1150°C leads to a decrease in open porosity from 40 to 24%, a decrease in the average pore size from 1.60 to 0.34 μm, and an increase in the compressive strength from 9.5 to 159 MPa. The resulting substrates are characterized by water permeability values of 1210, 310, 240, 170 L m⁻² h⁻¹ bar⁻¹ at sintering temperatures of 1000, 1050, 1100 and 1150°C, respectively. Experiments on filtration of aqueous suspensions of fine microspheres (d_av = 2.5 µm) and microsilica (d_av = 1.9 μm) through a substrate produced at a sintering temperature of 1150°C have shown the rejection close to 100%. The proposed methodology for using ash waste in the production of membrane materials promotes the development of technologies for the integrated processing of thermal energy waste.

Сrosslinked polymer memranes are obtained by the heat treatment of films prepared from a solution of a mixture of bromine-containing poly(1-trimethylsilyl-1-propyne) (PTMSP) and polyfunctional amine polyethyleneimine (PEI) as a crosslinking agent. Crosslinked products are identified from IR spectra, elemental analysis data, and stability of reaction products to a solvent (CCl₄), in which the original brominated PTMSP is soluble. According to the IR spectra, the crosslinking reaction occurs via reactive C–Br bond in bromine-containing PTMSP with the participation of PEI amino groups at a temperature above 90°С. The crosslinking of bromine-containing PTMSP makes it resistant to organic solvents. An increase in the content of PEI in the mixture correlates with the proportion of bromine atoms involved in the reaction. For brominated PTMSP films crosslinked by PEI transport parameters for individual gases and in the mixture n-butane/methane (98.4 mol % methane and 1.6 mol % n-butane) are studied. In the sequence PTMSP–brominated PTMSP-Br–PTMSP-Br/PEI (before crosslinking)–PTMSP-Br/PEI (after crosslinking) permeability for individual gases decreases. Crosslinked PTMSP in the mixture methane/n-butane demonstrates high permeability coefficients of n-butane (({{P}_{{n{text{-}}{{{text{C}}}_{{text{4}}}}{{{text{H}}}_{{10}}}}}}) = 12 000 Barrer) and selectivity for n-butane separation from a mixture with methane (({{alpha }_{{n{text{-}}{{{text{C}}}_{{text{4}}}}{{{text{H}}}_{{10}}}{text{/C}}{{{text{H}}}_{4}}}}}) = 13).

In this work, mixed matrix membranes (MMMs) were obtained based on polyimides synthesized from a mixture of diethyltoluene diamine (DETDA) isomers and BPDA and 6FDA dianhydrides by introducing metal-organic framework compounds ZIF-8 and ZIF-67 in a concentration of up to 20 wt % in chloroform solution. The starting polyimides were synthesized by one-stage high-temperature catalytic polycondensation in a benzoic acid melt. The studied MMMs were treated with sc-CO₂ followed by decompression. The gas transport and gas selective properties of the original and modified membranes were studied. Experimental values of the effective coefficients of permeability and diffusion of gases for He, H₂, O₂, N₂, CO₂, CH₄ were obtained, and the effective solubility coefficients of these gases were calculated. It was found that treatment of the studied MMMs in sc-CO₂ can significantly increase the level of gas permeability of membranes with gas selectivity at the level of initial values, while the achieved effect of changing the permeability of membranes depends on the gas, the nature of the matrix, and the concentration of the introduced particles. It was found that the treatment effect persists over time with a slight decrease in permeability of gases, which remains at a level significantly higher than the initial values. The demonstrated effect of improving gas transport properties when treating MMMs based on polyimide matrices 6FDA-DETDA and BPDA-DETDA in sc-CO₂ can be used for further application of the proposed modification method in order to increase gas transport through MMMs based on other polymers, including highly permeable ones.