Effective Concentration of Mobile Oxygen-Vacancies in Heavily Doped Cubic Zirconia: Results from Combined Electrochemical Impedance and NMR Spectroscopies

The effective concentration of mobile oxygen vacancies in heavily doped solid electrolytes (SE) used in electrochemical applications such as oxide fuel cells, electrolyzers, and gas sensors has long been of great interest. It is well-known that not all the oxygen vacancies in such heavily doped SEs are sufficiently mobile due to their association with the dopant cations, and thus, the effective number of mobile oxygen vacancies is smaller than the total number created by acceptor doping. However, the effective concentration of oxygen vacancies in a heavily doped SE cannot be directly obtained from electrical conductivity measurements, unless their mobility is known a priori. Here, we report the combined application of dopant-cation magic angle spinning nuclear magnetic resonance (MAS NMR) and electrochemical impedance (El) spectroscopic techniques to determine the effective concentration and the mobility of the oxygen vacancies in a heavily doped (59 cat % Y) yttria stabilized zirconia (YSZ). The results clearly demonstrate that the effective concentration of oxygen vacancies in this SE is lower than their nominal concentration by more than a factor of 2, even 700 degrees C. Furthermore, their mobility in this heavily doped YSZ is lower than that in 17 cat % Y-doped zirconia, one of most widely used SE, by orders of magnitude and is characterized by an activation energy (1.38 eV) that is significantly higher in the former than (1.02 eV) in the latter. These results provide a unique and direct mechanistic understanding of the respective roles of concentration and mobility in controlling ionic transport in SEs.