I. A. Stenina

Complex phosphates with the NASICON structure (MxA2(PO4)3)

Data on the synthesis and properties of NASICON complex phosphates MxA2(PO4)3 are summarized. The crystal chemistry and homology of these compounds are surveyed. The existence of large channels and numerous voids in the structure and various occupations of these voids by mobile cations generate many valuable physical and chemical properties of the MxA2(PO4)3 complex phosphates. The details of phase transitions, ionic conductivity, thermal expansion, ion-exchange properties, and sensor properties of the NASICON materials are considered, as well as the possibility of purposefully changing these properties through partial heterovalent substitution for the metal or phosphorus atoms.

Synthesis and characterization of MF-4SK + SiO2 hybrid membranes modified with tungstophosphoric heteropolyacid

Composites based on MF-4K perfluorinated cation-exchange membranes doped with hydrous silica nanoparticles, which were precipitated under various conditions, and with tungstophosphoric heteropolyacid nanoparticles were synthesized. The proton conductivity of the composites was studied as a function of temperature and relative ambient humidity. As a result of modification, the water content and ion conductivity of the membranes in low humidities increase by 2.5 orders of magnitude compared to unmodified MF-4SK membranes and the material is rendered less water-dependent.

Cation mobility and ion exchange in acid tin phosphate

Abstract Thermodynamics and kinetics investigation of ion exchange H + /M + (M=Li, Na) in tin acid phosphate has been carried out. The equilibrium constants and the cation diffusion coefficients in the exchange products have been determined. It was shown that the diffusion coefficient value in M x H 2− x Sn(PO 4 ) 2 · n H 2 O depends strongly on proton concentration in the solution. The cation mobility in tin acid phosphate and its ion exchange products has been investigated by impedance spectroscopy too. It was shown that the ion conductivity of these compounds is determined by the surface contribution and increases with increase in the hydration degree.

Ion Transport in MF-4SK Membranes Modified with Hydrous Zirconia

Composites based on perfluorinated cation-exchange membranes MF-4SK with embedded hydrous zirconia were synthesized. The composites have higher proton conductivity, lower diffusion permeability, and higher ion-transport selectivity than unmodified MF-4SK membranes. Diffusion permeability anisotropy was found in samples with heterogeneous dopant distribution across the membrane.

Kinetics of hydrogen-sodium ion exchange in acid zirconium phosphate

Abstract Proton diffusion in NaHZr(PO4)2–5H2O and Na2Zr(PO4)2–3H2O phases was examined by studying the kinetics of hydrogen–sodium ion exchange. Three regions characterized by an almost linear dependence of the apparent diffusion coefficients of the hydrogen ion on the pH values were obtained at pH

Microstructure and ion transport in Li1 + xTi2 − xMx(PO4)3 (M = Cr, Fe, Al) NASICON-type materials

Li1 + xTi2 − xMx(PO4)3 (M = Cr, Fe, Al) NASICON-type materials have been prepared by the Pechini process and solid-state reactions and characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy. We have identified the factors that determine the rate of ion transport in nanocrystalline and bulk samples at low and high temperatures. The effects of the preparation procedure and heterovalent doping on the ionic conductivity of the materials have been assessed. Heterovalent doping is shown to have a considerably stronger effect on the ionic conductivity in comparison with the microstructure of the materials.

Effect of Precipitation pH and Heat Treatment on the Properties of Hydrous Zirconium Dioxide

The effect of the precipitation pH and subsequent heat treatment is studied on the properties of hydrous zirconium dioxide precipitated by ammonia from nitrate solutions. Precipitation at pH ≤ 6 generates hydrous zirconium dioxide, which contains excess sorbate nitrate ions; the product precipitated at pH ≥ 7 contains excess ammonium ions. This distinction considerably affects the course of thermolysis and the morphology of products. The exotherm associated with the formation of the crystal structure of zirconia becomes more pronounced with rising precipitation pH. In addition, the samples prepared at pH ≥ 7 have a more developed surface. The morphologic and microstructural evolution of hydrous zirconium dioxide samples during thermolysis is described.

Water self-diffusion and ionic conductivity in perfluorinated sulfocationic membranes MF-4SK

Water self-diffusion and ion mobilities in various ionic forms (H+, Li+, Na+, Rb+, Cs+, and Ba2+) of perfluorinated sulfocationic membranes MF-4SK were studied by NMR and impedance spectroscopy. When degrees of hydration are low, the self-diffusion coefficients of water and ionic conductivities are considerably affected by the water content of the membrane. The self-diffusion coefficients decrease in the order H+ > Ba2+ > Cs+ > Rb+ > Na+ > Li+, whereas the ion mobility decreases in the order H+ > Li+ > Na+ > Cs+ > Ba2+.

Diffusion mobility of alkali metals in perfluorinated sulfocationic and carboxylic membranes as probed by 1H, 7Li, 23Na, and 133Cs NMR spectroscopy

The hydration, state, and ion mobility of protons and alkali metal cations (Li+, Na+, and Cs+) in perfluorinated sulfocationic membranes (MF-4SK) and carboxylic membranes (F-4KF) were studied using NMR and impedance spectroscopy. The hydration numbers and translational mobilities of ions and the formation conditions for cation/ionogenic group tight ion pairs were ascertained proceeding from analysis of chemical shifts and 1H, 7Li, 23Na, and 133Cs NMR line widths. NMR data correlate with the results of ion conductivity measurements. The translational mobility of cations and ionic conductivity in the sulfocationic membranes are higher than in the carboxylic membranes and increase in the order Cs+-Na+-Li+.

Transport properties of MF-4SK membranes modified with inorganic dopants

Proton transport in MF-4SK perfluorinated sulfo cationic membranes modified by silica and zirconium hydrogen phosphate were studied using pulsed-field gradient NMR and impedance spectroscopy. At high water contents, water molecules are involved in proton transfer. At low water contents, the proton conductivity of modified membranes considerably exceeds the conductivity of an unmodified membrane. The most likely reason for this effect is the generation of an extra H-bond network involving the dopant and sulfo groups of the membrane.