Mixed ionic-electronic conductors (MIECs) which have both ionic and electronic species as charge carriers have a wide range of applications, such as electrodes in fuel cells, electrocatalytic reactors, and gas separating membranes. They may have either electronic or ionic species as the majority charge carriers. In addition to the single-phase mixed conductors, they may be fabricated by mixing two different phases of materials. Although these composites have been less studied than the single phase MIECs, the combined properties are often superior to single phase MIECs, and properties not seen in an individual phase may appear in the composite phase.
YSZ-based composite systems were chosen to test the effect of transition-metal-oxide (TMO) addition on the electronic conductivity of composite. To induce mixed conductivity, electronic-conducting TMOs such as NiO and Mn2O3 were added into YSZ above the solubility limit. While the solid solubility of NiO in YSZ is limited that of Mn2O3 is large.
In this work, mixed conducting yttria (8 mol%) stabilized zirconia (YSZ) - TMO composites were prepared in full composition range and the electrical conductivity of the composites was measured by 4-probe d.c. conductivity. Electromotive force (emf) measurements of the galvanic cell, current-voltage (I-V) measurements in ion blocking condition and the oxygen-partial-pressure dependent conductivity have been used to determine the contribution of the ionic and electronic charge carriers on the conductivity. Thus the composition-dependent electrical properties were used to explain the percolation behavior of electronic charge carriers in ionic matrix.
Although the total conductivity of dense YSZ-TMO composite was variable with TMO content, the partial-electronic conductivity increased and the ionic conductivity decreased. The composition-dependent conductivity was discussed.