Water and hydrogen evolution properties and protonic conducting behaviors of Ca 2+-doped La 2Zr 2O 7 with a pyrochlore structure

Abstract

Undoped and Ca 2+-doped La 2Zr 2O 7 with a pyrochlore structure, i.e. (La 2 − x Ca x )Zr 2O 7 − δ , La 2(Zr 2 − x Ca x )O 7 − δ and (La 2 − x / 2Ca x/2 )(Zr 2− x/2 Ca x/2 )O 7 − δ , were prepared by a conventional ceramic method. Composition dependencies of a lattice parameter showed a slight decrease, a sharp increase and a slight increase with increasing x in systems of (La 2− x Ca x )Zr 2O 7− δ , La 2(Zr 2− x Ca x )O 7 − δ and (La 2− x/2 Ca x / 2)(Zr 2− x / 2Ca x / 2)O 7− δ , respectively. Proton concentrations in these oxides were evaluated by detection of evolved H 2O gas, which was generated by thermal desorption, using a quadrupole mass spectrometer. Although no evolutions of H 2O, H 2 or O 2 gases were detected for undoped La 2Zr 2O 7− δ , clear evolutions of H 2O gas were observed for all Ca 2+-doped samples. The amount of evolved H 2O per mole of oxide, [H 2O], was found to be 3.0×10 −2 for (La 1 . 9 2Ca 0 . 0 8)Zr 2O 7− δ , 8.4×10 −3 for La 2(Zr 1 . 9 6Ca 0 . 0 4)O 7− δ and 8.2×10 −3 for (La 1 . 9 8Ca 0 . 0 2)(Zr 1 . 9 8Ca 0 . 0 2)O 7 − δ . Further, small but clear evolutions of both hydrogen and oxygen were detected only for (La 1 . 9 2Ca 0 . 0 8)Zr 2O 7− δ . Differences in the amount of evolved water, and the starting and maximum temperatures of the water evolution were discussed with reference to characteristics of the pyrochlore structure. E.m.f. measurements for various gas concentration cells with the sintered (La 1.98Ca 0.02)(Zr 1.98Ca 0.02)O 7− δ pellet as a solid electrolyte showed that the mean transference number of proton was 1 at ≤873 K and 0.82 at 1073 K. Thus, Ca 2+-doped La 2Zr 2O 7− δ with a pyrochlore structure is a promising protonic conductor at high temperature.