Dielectric relaxation and proton conductivity of polyantimonic acid doped with vanadium ions

Abstract

Purpose. To improve the operating characteristics of proton-conducting polymer membranes, nanoparticles of heteropoly acids and hydrated oxides of polyvalent elements are being infused. Such membranes are used in condensers of huge capacity, memory devices, selective sensors, fuel cells and ion-selective membranes. One of the perspective heteropoly acids, crystallized within the pyrochlore-type structure (sp. gr. Fd3m), is the polyantimonic acid H₂Sb₂O₆·nH₂O, 2<n<4 (PAA). In a number of researches, it is shown that the proton-crystal cell interaction degree infl uences the proton-conducting properties of PAA. Therefore, PAA transport properties can be increased by means of partial substitution of antimony ions for ions with the approximate ionic radius, but different in the electronegativity values. It is expected that this kind of substitution within the pyrochlore-type structure may lead to the degradation of the interaction energy between protons with the anion sublattice and their migration within the system of hydrogen bonds, formed by water molecules. In this regard, the aim of this work is to synthesize and study the proton transport in PAA doped with vanadium ions.

Methods and methodology. Substitutional solid solutions have been obtained with the help of the co-precipitation method. The X-ray phase analysis and the elemental analysis have been conducted in order to clarify the phase composition. Earlier in the range of works it has been shown that polyantimonic acid doping with vanadium ions within the pyrochlore-type structure has a concentration limitation: substitutional solid solution H₂Sb_2-хV_х O₆·nH₂O is formed when 0 < x < 0,48. That is why the samples, containing 0 (PAA); 10 (x = 0.2); 20 (x = 0.40); 24 (x = 0.48) % of vanadium have been chosen as subjects of the present research. By means of impedance spectroscopy method the proton-conducting properties with the relative humidity of 58 %, which was obtained by storing the measurement cell with the sample, placed inside the fl ask, above NaBr saturated solution surface, have been studied. A specially designed cell was used that represented a fl at capacitor with graphite electrodes, between which the studied powder was compacted. In order to study the conductivity under the lower temperatures the cell, previously held within 24 hours in the fl ask with RH = 58 %, was placed into a thermostat, allowing to vary the temperature from 218 to 298 K by cooling with solid carbon dioxide. According to the measuring data of the actual variable Z￠ and the imaginary variable Z≤ of the complex impedance parts the dielectric properties have been determined.

Results. Synthesized samples represented green-coloured powders with the coherent-scattering region sizes, according to the X-ray spectral analysis, of 40 nm. On the X-ray diffraction patterns there is an identical set of diffraction maximums. However, with the increase of vanadium quantity there is a shift into a wide diffraction angles’ area, which is proved by the reduction of unit cell parameters. Impedance samples hodograph for the temperature of 298 K and RH = 58 % represents a slightly stretched semicircumference and a straight line at a slight angle to the X-axis. With the help of the equivalent circuit these dependencies are described with the model ‘of grains and interlayers’.

Conclusions. Proton conductivity of polyantimonic acid doped with vanadium ions has been studied. It has been shown that the partial substitution of antimony ions within the pyrochloretype structure for ions with the lower electronegativity value increases proton conductivity and reduces its activation energy. In this case for the extreme solid substitution solution H₂Sb_1.52V_0,48 O₆ ·nH₂O the value of proton conductivity at RH = 58 % and 298 К is 66 mS/m. According to the analysis of dielectric spectroscopy data at temperatures of 218–298 K the activation

energy of conductivity is 30±2 kJ/mol, also, the proton transport mechanism is proposed. In doped PAA conductivity is carried out through the system of hydrogen bonds, formed by water molecules, located in hexagonal channels within the pyrochlore-type structure and oxygen anions of octahedron, forming the structural framework.

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