Water and hydrogen evolution properties and protonic conducting behaviors of Ca2+-doped La2Zr2O7 with a pyrochlore structure

Undoped and Ca2+-doped La2Zr2O7 with a pyrochlore structure, i.e. (La2-xCax)Zr2O7-delta, La-2(Zr2-xCax)O7-delta and (La2-x/2Cax/2)(Zr2-x/2Cax/2)O7-delta, 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 (La2-xCax)Zr2O7-delta, La-2(Zr2-xCax/2)O7-delta and (La2-x/2Cax/2)(Zr2-x/2Cax/2)O7-delta, respectively. Proton concentrations in these oxides were evaluated by detection of evolved H2O gas, which was generated by thermal desorption, using a quadrupole mass spectrometer. Although no evolutions of H2O, H-2 or O-2 gases were detected for undoped La2Zr2O7-delta, clear evolutions of H2O gas were observed for all Ca2+-doped samples. The amount of evolved H2O per mole of oxide, [H2O], was found to be 3.0 x 10(-2) for (La1.92Ca0.08)Zr2O7-delta, 8.4 x 10(-3) for La-2(Zr1.96Ca0.04)O7-delta and 8.2 x 10(-3) for (La1.98Ca0.02)(Zr1.98Ca0.02)O7-delta. Further, small but clear evolutions of both hydrogen and oxygen were detected only for (La1.98Ca0.02)Zr1.98O7-delta. 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 (La1.98Ca0.02)(Zr1.98Ca0.02)O7-delta pellet as a solid electrolyte showed that the mean transference number of proton was 1 at less than or equal to 873 K and 0.82 at 1073 K. Thus, Ca2+-doped La2Zr2O7-delta with a pyrochlore structure is a promising protonic conductor at high temperature.