NEW MEDIUM-TEMPERATURE NANOCOMPOSITE ELECTROLYTES BASED ON CESIUM DIHYDROGEN PHOSPHATE: COMPARISON OF PROTON CONDUCTIVITY AND STRUCTURAL, MORPHOLOGICAL, MECHANICAL CHARACTERISTICS

The article is concerned with the preparation of medium-temperature highly conductive composite proton electrolytes of the new type by introducing a nanodiamond (ND) additive to cesium dihydrogen phosphate, and investigation of their properties. The data on changes in the structural properties of the salt in the composite, morphology, mechanical strength and proton conductivity depending on the composition (1-x)CsH2PO4-xND (where x = 0-0.98, the molar fraction of ND) are considered. Valuable information explaining the mechanism of nanocomposite formation was obtained: the salt protons are partially bound to OH-groups of nanodiamond, which results in the arrangement of a weaker hydrogen bond system of the salt. It has been shown that there is no chemical interaction between the components in the composites, and the structure of CsH2PO4 (P21/m) is preserved during dispersion and partial amorphisation of the salt with an increase in the mole fraction of ND. The composites are characterised by the uniform particle distribution. The introduction of small ND concentrations leads to the stabilization of the size of salt particles (250±20 nm) in nanocomposites, as a result of the interfacial surface interaction of the components. Based on the change in the enthalpy of the superionic phase transition and X-ray diffraction data, the proportion of the amorphous phase in the composites was estimated to increase substantially with an increase in ND molar fraction, reaching 50 % at x = 0.8. A significant increase in the proton conductivity of the low-temperature phase of CsH2PO4 is observed, up to 3.5 orders of magnitude with a maximum at x = 0.9, and a decrease at x > 0.95 due to the conductor-insulator percolation effect. The superionic conductivity of the composites does not change up to x = 0.7 (11.7 vol% ND) and remains close to the value characteristic of initial salt CsH2PO4 (~10-2 S/cm). An assessment of the strength characteristics of nanocomposites using the Vickers method shows that, due to the high hardness of nanodiamonds, the microhardness of the composites is significantly higher than that of initial CsH2PO4 even with a small content of ND additives (x = 0.3, which corresponds to 2.64 vol%). The studied composite electrolytes have high proton conductivity, chemical stability and mechanical strength required for medium-temperature proton membranes of the new type for fuel cells.