Proton conductivity of functionalized zirconia colloids

Colloidal 60 nm ZrO₂ particles have been treated with aqueous solutions of phosphoric acid and sulphophenyl-phosphonic acid (SPPA). This leads to the covalent bonding of P-OH and C₆H₄-SO₃H acid groups onto the surface of the particles. The resulting acidity yields a significant proton conductivity at the surface of the particles. The proton conductivity of the ZrO₂-H₃PO₄ system is quite stable against a change of relative humidity and temperature. However, it remains limited by the low acidity of the P-OH group. On the contrary, the proton conductivity of the ZrO₂-SPPA system is almost two orders of magnitude higher at high relative humidities due to the high acidity of the C₆H₄-SO₃H group (1.10^-3 S,cm^-1 in the same conditions). But, it is much more sensitive to changes in relative humidity. A mixed grafting of both H₃PO₄ and SPPA onto the zirconia particles allows to obtain a conductivity as high as in the SPPA case whereas it remains stable with humidity and temperature as for the phosphoric acid. This probably indicates that the conductivity arises from ionization of sulphonic acid but that the weaker phosphoric acid groups contribute to the conduction mechanism.