Proton conductivity of functionalized zirconia colloids
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Proton conductivity of functionalized zirconia colloids
D. Carrière, M. Moreau, K. Lahlil, P. Barboux and J.-P. Boilot. Laboratoire de Physique de la Matière Condensée, CNRS UMR 7643C, 1 route de Saclay, Ecole Polytechnique, 91128 Palaiseau Cedex ABSTRACT 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 POH 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 C6H4SO3H 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. INTRODUCTION Room temperature protonic conductors may find well known applications in the domain of electrochromic devices, fuel cells or supercapacitors [1]. But they suffer from a weak stability of the conductivity against changes in temperature or relative humidity. Mineralorganic hybrid systems may offer a promising compromise between the strong acidity of some acid polymers and the better stability towards dehydration of the mineral species. Polymer-phosphoric acid blends such as H3PO4 / PBI complexes or H3PO4 / N,N dimethylacetamide / PVDF composites have been proposed [2,3,4]. However, these systems contain free phosphoric acid that is lost upon water treatment. An alternative is to use acid and/or basic functions of insoluble polycondensed mineral species. This has been achieved with some crystallized lamellar zirconium phosphates and phosphonates [5,6,7] or by in-situ condensation of inorganic phosphates in a proton conducting polymer matrix [8]. Similarly, our work focuses on the role of the surface conduction on mineral colloidal particles. This requires an optimization of the synthesis and characterisation of nanometric and organically modified inorganic species. Apart from higher loadings in the membranes, the nanometric size of the particles is expected to allow an enhancement of the proton conduction properties due to a better grain-to-grain transfer and a higher surface groups content. In a previous work, we have shown that protonic conductors can be prepared by grafting phosphoric acid at the surface of colloidal zirconia particles [9]. A higher conductivity could be obtained by
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