Accelerated ceria–zirconia solubilization by cationic diffusion inversion at low oxygen activity

Fast elemental diffusion at the Gd-doped ceria/Y-stabilized zirconia interface occurs under reducing conditions at low oxygen activity (pO₂ < 10⁻¹² atm) and high temperature (1400 °C). This effect leads to formation of thick ceria–zirconia solid solution reaction layers in the micro-range vs. thin layers of few tens of nanometers under oxidative conditions (i.e. in synthetic air at pO₂ = 0.21 atm). The fast dissolution occurs by an inversion of the dominating limiting mechanism from the expected Zr⁴⁺ diffusion into the CGO lattice at high pO₂ to an unexpected Ce³⁺ diffusion into the YSZ component under reducing conditions. The diffusion coefficient of 8-fold coordinated Ce3+ in YSZ at 1400 °C and pO₂ = 10⁻¹³ atm is estimated to be around 10⁻¹¹ cm² s⁻¹. This value is around 3 orders of magnitude higher than Zr⁴⁺ interdiffusion in CGO under oxidative conditions and about 8 orders of magnitude higher than Ce⁴⁺ self-diffusion in CGO in air at the same temperature.