V. G. Ponomareva

Composite protonic solid electrolytes in the CsHSO4-SiO2 system

Abstract Transport, thermal and structural properties of the composite solid electrolytes (1 − x )CsHSO 4 xSiO 2 (where x = 0–0.8) were investigated. The composites were prepared by mechanical mixing of components followed by heating at temperatures near CsHSO 4 melting point (483 K). The dependence of low temperature phase conductivity on x has a maximum with a value 2.5 orders of magnitude higher than that of pure CsHSO 4 and conductivity is governed by protons. Heterogeneous doping is shown to change markedly the thermodynamic parameters of the ionic component. The phase transition temperature CsHSO 4 in the composites decreases from 414 to 350 K with the increase of the content of heterogeneous additive SiO 2 from 0 to 0.7. As x raises CsHSO 4 the amorphization takes place and the relative change of ionic conductivity at phase transition diminishes, the phase transition becomes diffusive and disappears for the 0.2CsHSO 4 0.8SiO 2 composite.

The investigation of disordered phases in nanocomposite proton electrolytes based on MeHSO4 (Me=Rb, Cs, K)

Abstract Properties of alkali metal hydrogensulphate MeHSO 4 (Me=Cs, Rb, K) in (1− x )MeHSO 4 – x A (A=Al 2 O 3 , TiO 2 , SiO 2 ; x =0–0.9) nanocomposite solid electrolytes were studied by X-ray powder diffraction, differential scanning calorimetry methods and conductivity measurements. The conductivity of the composites was shown to exceed that of the individual salts by more than 1–3.5 orders of magnitude and to depend on composition. The conductivity and thermal stability of (1− x )CsHSO 4 – x A composites increase in the order Al 2 O 3 2 2 . The properties of composites (1− x )MeHSO 4 – x SiO 2 depend markedly on the grain and pore size of silica. The optimum SiO 2 pore size was in the range 35–100 A, where the highest composite conductivity was observed. For these composites, the enthalpies of CsHSO 4 and RbHSO 4 phase transitions and melting decreased considerably and the thermal stability increased. The ionic component became amorphous. Analysis of calorimetric data indicated the presence of two crystalline phases with different temperatures of phase transition and melting in the composites with silica of pore size 170 A. The MeHSO 4 state changed slightly when the SiO 2 pore size was 1000 A. In the systems with pore size of 14 A, both crystalline and amorphous salts were observed.

Influence of dispersed TiO2 on protonic conductivity of CsHSO4

Abstract The ionic conductivity of composite solid electrolytes (1− x )CsHSO 4 – x TiO 2 ( x =0–0.7) has been investigated. The low-temperature conductivity of the composites was shown to exceed by more than two orders of magnitude that of pure CsHSO 4 . The conductivity enhancement depends on the dispersoid concentration and its specific surface area. This effect is caused by the strong interface interaction between CsHSO 4 and TiO 2 in composites.

Effect of SiO2 morphology and pores size on the proton nanocomposite electrolytes properties

Abstract The composite solid electrolytes (1− x )MeHSO 4 – x SiO 2 , (where Me=Cs, Rb, x =0–0.8) have been studied by complex impedance, DSC and X-ray diffraction methods. The used SiO 2 varied in specific surface areas (13–580 m 2 g −1 ), pores size ( R =14–1000 A) and pores size distribution. The low-temperature conductivity of the composites was shown to exceed by 1–3 orders of magnitude that of the individual salts. It depended on SiO 2 content, silica pores size and their distribution. There is optimum silica pores size in a range of 35–100 A, where the most composite conductivity increase takes place; the ionic component becomes either partially or completely amorphous with x increasing (“dimensional effect”). The MeHSO 4 dispergation mainly proceeds in composites with R =170 A. The MeHSO 4 state does not change when the SiO 2 pores size is 1000 A. In systems with R =14 A both MeHSO 4 low-temperature phase and amorphous state are observed.

High Proton Conductivity and Spectroscopic Investigations of Metal–Organic Framework Materials Impregnated by Strong Acids

Strong toluenesulfonic and triflic acids were incorporated into a MIL-101 chromium(III) terephthalate coordination framework, producing hybrid proton-conducting solid electrolytes. These acid@MIL hybrid materials possess stable crystalline structures that do not deteriorate during multiple measurements or prolonged heating. Particularly, the triflic-containing compound demonstrates the highest 0.08 S cm(-1) proton conductivity at 15% relative humidity and a temperature of 60 °C, exceeding any of todays commercial materials for proton-exchange membranes. The structure of the proton-conducting media, as well as the long-range proton-transfer mechanics, was unveiled, in a certain respect, by Fourier transform infrared and (1)H NMR spectroscopy investigations. The acidic media presumably constitutes large separated droplets, coexisting in the MIL nanocages. One component of proton transfer appears to be related to the facile relay (Grotthuss) mechanism through extensive hydrogen-bonding interactions within such droplets. The second component occurs during continuous reorganization of the droplets, thus ensuring long-range proton transfer along the porous structure of the material.

Composite Solid Electrolytes

Studies of composite ionic conductors are overviewed. Mechanisms of defect formation at ionic crystal surfaces and at interphase boundaries in the composites are discussed; the Stern model that allows calculating surface potential has been involved. Methods for the calculating of the composite’s electrical conductance and other physicochemical characteristics are suggested. Thermodynamic stability of nanocomposites and the genesis of their morphology during sintering are analyzed. General regularities of changes in ionic-salt properties over wide range of the “ionic salt-oxide” systems, as well as size effects are discussed.

Composite protonic electrolytes in the system (NH4)3H(SO4)2–SiO2

Abstract Transport, thermal and structural properties of (1− x )(NH 4 ) 3 H(SO 4 ) 2 – x SiO 2 composite solid electrolytes ( x =0–0.8) were investigated, and an increase in (NH 4 ) 3 H(SO 4 ) 2 (II) low temperature phase conductivity was shown. The dependence of the low temperature conductivity on x has a smooth maximum with values five to six times higher than that of the pure polycrystalline salt and two orders of magnitude higher than that of single crystals. The (NH 4 ) 3 H(SO 4 ) 2 phase transition temperature in the composites decreases by 20 K with an increase of SiO 2 content from 0 to 0.7. The observed jump in conductivity that coincides with the phase transition becomes more diffuse and disappears at x =0.8. The thermal stability of (NH 4 ) 3 H(SO 4 ) 2 in such composites increases markedly as compared with the pure salt.

Electrical conductivity and thermal stability of (1 − x)CsH2PO4/xSiPyOz (x = 0.2–0.7) composites

The physicochemical properties of (1 − x)CsH2PO4/xSiPyOz (x = 0.2–0.7) composites containing fine-particle silicon phosphates as heterogeneous additives have been studied at different humidities. The introduction of silicon phosphates suppresses the superionic phase transition of CsH2PO4 and increases the low-temperature conductivity of the materials, which depends significantly on humidity. The CsH2PO4-SiPyOz materials offer high conductivity (∼3 × 10−3 to 10−2 S/cm at ∼110–230°C) at low water vapor pressures (3 mol % H2O). Amorphization of the CsH2PO4 in the composites markedly changes its thermodynamic properties. The effect of long-term isothermal holding (210°C, 3 mol % H2O) on the conductivity of the composites has been studied.

Effect of silica porous structure on the properties of composite electrolytes based on MeNO3 (Me=Rb, Cs)

Abstract Properties of RbNO 3 and CsNO 3 in (1− x )MeNO 3 – x SiO 2 ( x =0–0.9) nanocomposite solid electrolytes were studied by X-ray powder diffraction, differential scanning calorimetry methods and conductivity measurements. The used highly-dispersed silicas with narrow pore size distribution were different in their specific surface areas (13–580 m 2 /g) and pore size ( R =14–1000 A). The composite conductivity was shown to exceed that of individual salts by more than 1.5–4 orders of magnitude and to be maximum at x =0.5–0.7. In nanocomposites based on alkali nitrates and silica the ‘dimensional effect’ was observed. The properties of composites depended markedly on pore size of silica. The optimum pore size of heterogeneous dopant was in a range of 35–100 A, where the most composite conductivity increase took place and thermodynamic and structural properties of ionic salts changed markedly. For composites based on these silicas the enthalpies of RbNO 3 (CsNO 3 ) phase transitions and melting decreased considerably. The ionic component became either partially or completely amorphous (in particular with x increase). The MeNO 3 state changed slightly when the SiO 2 pore size was 1000 A. In systems with pore size 14 (both crystalline low temperature RbNO 3 (IV) and amorphous salt were observed.

Hydrogen sensor based on antimonium pentoxide-phosphoric acid solid electrolyte

Abstract A solid composite electrolyte with high proton conductivity based on antimonium pentoxide with additives of phosphoric acid has been obtained. A potentiometric solid-state gas sensor using this electrolyte has been developed for detecting small amounts of hydrogen (10–2000 ppm) in gas mixtures at ambient temperature. The sensor consists of the reference electrode: Ag or Ag/(Ag + Ag2SO4), the solid composite electrolyte and H2-sensitive electrode: Pt or Pd. The electromotive force (e.m.f.) of the sensor varies logarithmically with H2 concentration for hydrogen partial pressures in the range 100–2000 ppm and depends on the oxygen partial pressure. The slope of e.m.f.-log(pH2) dependence is 170 and 200 mV for Pt and Pd, respectively, which exceeds the Nernst value, presumably due to the formation of a mixed potential. The sensor can operate at a wide range (20–95%) of a relative humidity.