G. V. Lavrova

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.

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.

Surface and bulk conduction and thermodynamic properties of ionic salt CsH5(PO4)2

Transport properties of ionic salt CsH5(PO4)2 are studied by the impedance method. The salt’s bulk conductivity ranges from 10−8 to 10−4 S cm−1 in the temperature interval 90 to 145°C. The apparent activation energy is high (1.6–2.0 eV). The conductivity is slightly anisotropic: it is maximum in the [001] direction and minimum in the [100] direction (∼5.6 and 1 times × 10−6 S cm−1, respectively, at 130°C). The conductivity of polycrystalline samples is higher by 1–2 orders of magnitude, and the activation energy drops to 1.05 eV due to the formation of a pseudoliquid layer with a high proton mobility at the intercrystallite boundary. The salt’s thermodynamic properties are examined by differential scanning calorimetry and thermogravimetry. No phase transitions are discovered in the salt up to the melting point (151.6°C), with the melting enthalpy equal to ∼34 kJ mol−1. The crystallization occurs at lower temperatures (107°C) and the crystallization enthalpy (−18 kJ mol−1) is lower than the melting enthalpy. The melting is accompanied by slow decomposition of the salt. Factors affecting the proton transport in the salt are analyzed.

Disordering of Pentacesium Trihydrogen Tetrasulfate in Cs5H3(SO4)4–SiO2 Composite Proton Electrolytes

Abstract(1 – x)Cs5H3(SO4)4 · yH2O–xSiO2 composite electrolytes were prepared in the composition range x = 0.3–0.9 and were characterized by impedance measurements, differential scanning calorimetry, and x-ray diffraction analysis. The results indicate that the introduction of fine SiO2 particles stabilizes the high-conductivity, disordered (nearly amorphous) state of the salt, which is similar to that in pure Cs5H3(SO4)4 above the phase transition (> 420 K). The conductivity of the composites is independent of SiO2 content up to x = 0.7 and decreases at x ≥ 0.8 owing to percolation disruption. The disordering of Cs5H3(SO4)4 · yH2O is reversible and is due to changes in the content of water of hydration.

Proton conductivity and interphase interaction in CsH2PO4-SrZrO3 composites

Microstructure, transport, and thermal properties of the composites of (1 − x)CsH2PO4/xSrZrO3 (x = 0.01–0.2) are studied. Introduction of highly dispersed strontium zirconate results in an increase in low-temperature conductivity by 1–3 orders of magnitude as depedent on the composition, leveling, and disappearance of a superionic phase transition in CsH2PO4. The methods of differential scanning calorimetry and thermogravimetry were used to show that a significant change in thermodynamic properties of CsH2PO4 in composites at x = 0.1–0.2 is observed due to salt amorphization. According to the data of X-ray phase analysis, there is no chemical interaction between the components and no new compounds are formed. An increase in conductivity is caused by salt disordering due to the interphase surface interaction.