Ionic conductivity of tetra-n-butylammonium tetrafluoroborate in the MIL-101(Cr) metal-organic framework

Nanocomposite solid electrolytes (C4H9)(4)NBF4-MIL-101(Cr) based on pure components without any other additives were prepared and their structure and electrical properties were investigated as a function of temperature and concentration of the metal-organic framework MIL-101(Cr). According to the data of thermal analysis, the heat effect due to the melting of the salt in the composites strongly decreases and tends to zero at a molar fraction of MIL-101(Cr) x >= 0.34. This effect is assumed to be caused by the amorphization of the salt in the composites which is practically complete at high content of MIL-101(Cr). The dependence of the melting enthalpy on the molar or mass fraction of MIL-101(Cr) may be explained by filling of MIL-101(Cr) pores with the salt, provided that the salt residing outside the pores is crystalline, whereas the salt located inside the pores is amorphous. In this case, at some fraction of the MIL-101(Cr), x = x(max), all the salt will be located inside the pores, and the concentration of the salt occurring in an amorphous state reaches a maximum. At x < x(max) there is a linear dependence between melting enthalpy and molar (or mass) fraction from which allows one can determine x(max) and w(max) values from experimental data. From these data, the volume of accessible pores was estimated as V-pore = 0.92 cm(3)/g corresponding to 73% of the total pore volume determined by BET adsorption method. The thermal properties fairly correlate with the X-ray diffraction data. Reflections on X-ray diffraction patterns of the composites attributed to (C4H9)(4)NBF4 strongly decrease with the concentration of MIL-101(Cr) and at the concentration x >= 0.283 practically no reflections of the salt are observed on the X-ray patterns. The electrical properties of the composites were investigated. It was shown that the concentration dependence of conductivity has a maximum at the concentration close to x(max) value determined from the thermal analysis data. At x > x(max) temperature dependences of conductivity are not linear in Arrhenius coordinates, no sudden conductivity change is observed due to the melting of the salt. Such conductivity behaviour is typical for amorphous electrolytes. Quantitative analysis of the concentration dependence of conductivity was done using the pore filling model and the mixing equations proposed earlier for two-phase composites. Theoretical curves obtained using the mixing equations satisfactorily fit the experimental data. The maximum value of ionic conductivity, 5.10(-4) S/cm at 135 degrees C, obtained for the composite 0.675(C4H9)(4)NBF4-0.325MIL-101(Cr) is rather high assuming that BF4- anions are the most probable charge carriers.