Mina Nishi

Electronic Conductivity of Scandia-Stabilized Zirconia Doped with 1 mol% CeO2

Scandia-stabilized zirconia (ScSZ) has been extensively studied for the application to solid oxide fuel cells (SOFCs) because of its higher oxide ion conductivity compared to conventional yttria-stabilized zirconia (YSZ). The electronic conductivity of ScSZ is significantly lower than the ionic conductivity but is important to determine the performance of SOFC materials. In the case of nickel – stabilized zirconia cermet anode, when the scandiastabilized zirconia was used, the durability against methane fuel was enhanced [1]. The minor carriers like electron and proton in ionic conductor may affect the reaction mechanism at the cermet anode as proposed in literature [2]. ScSZ with 10 mol% Sc2O3 has the highest ionic conductivity among different content of Sc2O3 at 1273 K [3] and is generally doped with 1 mol% CeO2 to suppress the transformation of cubic to rhombohedral phase [4]. Since the valence state of cerium ion varies from 4+ to 3+ with decreasing oxygen partial pressure (p(O2)), the concentration of electron carrier in ScSZ would be affected by doping with CeO2 if the electrons on the trivalent Ce ions behave as carrier (CeZr+1/2OO Ce’Zr+1/2VO+1/4O2). In our previous study on the electronic conductivity of [(CeO2)x(ZrO2)1x]0.8(YO1.5)0.2 solid solution [5], it was observed that the electronic conductivity of the solid solution with CeO2 content of x 0.1 was 4 – 5 order-of-magnitude larger than that of x = 0, and that the electronic conductivity of solid solution with CeO2 had unusual p(O2) dependency like the concentration product of trivalent and tetravalent Ce ions. In this study, the electronic conductivity of 10 mol% Sc2O3-stabilized ZrO2 doped with 1 mol% CeO2 (1Ce10ScSZ) was measured using a modified HebbWagner ion blocking cell reported elsewhere [6]. The contribution of 1 mol% CeO2 to the electronic conduction of ScSZ was investigated by clarifying the p(O2) and temperature dependence of electronic conductivity of 1Ce10ScSZ. Figure 1 shows the electronic conductivity of 1Ce10ScSZ at 1173 – 1273 K in the p(O2) range of 10 – 10 MPa. At p(O2) > 10 MPa, the electronic conductivity at 1273 K was proportional to the 1/4 power of p(O2). The 1/4 power dependence was also observed at 1173 K in p(O2) > 10 MPa. Such 1/4 power dependence is usually explained by the p(O2) dependence of electronic hole concentration which is derived from the hole formation reaction (1/2VO +1/4O2 h + 1/2OO). At p(O2) > 10 MPa, the electronic conductivity at 1273 K was proportional to the -1/4 power of p(O2), which is usually interpreted as the p(O2) dependence of electron concentration which is derived from the electron formation reaction (1/2OO e’ + 1/2 VO +1/4O2). The solid line in fig. 1 shows the fitting curve of measured conductivity data with following equation, e = h[p(O2) / 0.1MPa] + e[p(O2) / 0.1MPa] (1) where h and e are constants. In the p(O2) range of 10 – 10 MPa, the p(O2) dependence of electronic conductivity at 1273 K deviated from the 1/4 and -1/4 power law expressed in eq. (1). The upper deviation of electronic conductivity from the 1/4 and -1/4 power law is attributed to the increase of electron concentration due to the reduction of doped CeO2. It is meaning that the electron on trivalent Ce ion behaves as electron carrier. Because of the limited amount of Ce dopant (1 mol%), the -1/4 power dependence due to the ZrO2 matrix appeared at lower p(O2) (< 10 MPa). To investigate the contribution of 1 mol% CeO2 to the electronic conduction of ScSZ in detail, the electronic conductivity of 1Ce10ScSZ is measured at 1173 K in the lower p(O2) region.