Novel nanocomposite systems based on cesium dihydrogen phosphate: Electrotransport structural, morphological and mechanical characteristics

Abstract

CsH2PO4 is a proton conductor belonging to the family of acidic salts and exhibits a monoclinic to cubic phase transition at 230 ± 2 °C with the conductivity jump of four orders of magnitude. To change the electrotransport and mechanical characteristics the non-oxide additives, such as detonation nanodiamonds, were firstly used as a dopant for proton nanocomposites based on cesium dihydrogen phosphate. The use of carbon containing materials has made it possible to create a new type of medium-temperature, highly conductive proton electrolytes. Influence of nanodiamonds (NDs) on the electrotransport, structural and thermodynamic properties of (1-x)CsH2PO4-xNDs (x-mole fraction, x = 0–0.99) electrolytes have been firstly investigated in detail by X-ray diffraction method, electrochemical impedance, FTIR-spectroscopy and differential scanning calorimetry. The CsH2PO4 (P21/m) exists in nanocomposites in the dispersed (up to 300 nm) and partially amorphous state resulting in the strong changes of the salt thermodynamic properties with increase of x mole fraction. The heterogeneous doping by non-oxide dopant resulted in the significant increase of the low temperature conductivity of 3.5 orders of magnitude due to interface interaction, while the superionic conductivity of nanocomposites remains close to the individual CsH2PO4 (∼10-2 S/cm) up to x = 0.7 (fvNDs = 11.7 %). The jump in the conductivity during the phase transition decreases significantly and almost disappears. The conductivity of nanocomposites decreases significantly only at x > 0.95 (fvNDs = 51.8 %) due to the percolation effect. The mechanical properties of CsH2PO4-NDs were shown to be much higher than of the individual CsH2PO4 even with small NDs content. Mechanism of nanocomposites formation is associated with the partial binding of CsH2PO4 protons by surface OH– groups of nanodiamonds. The results are important for the establishing and design the next generation membrane of fuel cells and nanocomposite electrodes for medium-temperature electrochemical devices.