Pyrochlore Pr2Zr2-xMxO7+δ (M = Al, Ga, In) solid-state electrolytes: Defect-mediated oxygen hopping pathways and enhanced NO2 sensing properties

The work showed the enhancement of NO2 sensing properties by molecular engineering of oxygen-defect sites in a pyrochlore-phase Pr2Zr2O7 oxygen conductor and molecular insights into the oxygen migration pathways by theoretical calculation. A B-site substitution strategy was developed to prepare a series of defective Pr2Zr2-xMxO7+δ (PZM, M = Al, Ga, In) oxygen conductors to establish the structure-performance relationship of the component-optimized conductors. The structural analysis by the combination of X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and density functional theory calculations clarified the defect-mediated oxygen transport mechanism. The electrochemical results clearly indicate that the thermal-activation energy of oxygen ions increases with the electronegativity of dopants (Al3+ > Ga3+ > In3+), independent of the lattice distortion. The In3+ doping can not only create more amounts of oxygen-defect sites, but also reduce at utmost the bond energy of 48f-oxygen ions in octahedral [ZrO6] units. As a result, the hopping of 48f-oxygen ions can proceed fast across the adjacent oxygen-defect sites as oxygen ion transfer stations. The NO2 sensing results of the (Pt)NiO/PZM/Pt sensors, which were evaluated at the applied potential of −300 mV in the mild temperature region of 500-700 °C, give a significantly-enhanced ΔI value for the optimized Pr2Zr2-xInxO7+δ (x = 0.05) conductor, as compared to the pure Pr2Zr2O7 counterpart (4.4-fold) and the commercial 8% Y2O3-ZrO2 (YSZ) conductor (2.5-fold). The PZM sensor shows the promising application in motor vehicles.