• Energy Research

Local cathodic polarizations of yttria-stabilized zirconia were carried out with a PtIr probe as the working electrode in a controlled atmosphere high temperature scanning probe microscope to investigate the reduction of zirconia. Impedance spectroscopy was performed at 650 °C during increasing and decreasing polarization, in a range between 0.5 and − 2 V in 9% H2 in N2 saturated with water vapor at room temperature (25 °C). With increased polarization, the impedance spectra changed from a simple suppressed arc at low polarizations into two capacitive arcs separated by an inductive loop and followed by an inductive loop at low frequencies. Areas with high conductance as well as significantly decreased high-frequency resistances resulted from the polarizations and indicate the introduction of electronic conductivity in YSZ. Near the probe|YSZ contacts, areas with very low conductance and accumulation of Si-containing particles were observed, pointing to additional migration of silica impurities towards the probe.

This work is a study of the impact of short-term strong cathodic polarization in a Ni|YSZ model system using Ni probes as working microelectrodes in a high temperature scanning probe microscope at 650°C in humidified 9% H2 in N2. Impedance spectroscopy revealed one to three orders of magnitude decrease in the high frequency resistance and four to five orders of magnitude decrease in the low frequency impedance with polarization from −1.06 V to −3.06 V vs E°(O2), indicating introduction of electronic conductivity and expansion of the reaction zone around the Ni microelectrode. The effect on the Ni|YSZ interface included formation of electronic conductance, reaction between Ni and YSZ and accumulation of impurities around the Ni|YSZ contact as verified by conductance scans of the polarized area. Cyclic voltammetry was used to compare three systems with different impurity levels and showed that the presence of silicates reduces the current, i.e. lowers the performance of the electrode reaction.

Abstract Fluorite-type ceria-based ceramics are well established as oxygen ion conductors due to their high conductivity, superseding state-of-the-art electrolytes such as yttria-stabilized zirconia. However, at a specific temperature and oxygen partial pressure they occasionally exhibit electronic conduction attributed to polaron hopping via multivalent cations (e.g. Pr and Ce). (Ce, La, Pr, Sm, Y)O2−δ is a high-entropy oxide with a fluorite-type structure, featuring low concentrations of multivalent cations that could potentially mitigate polaron hopping. However, (Ce, La, Pr, Sm, Y)O2−δ undergoes a structural transition to the bixbyite-type structure above 1000 °C. In this study, we introduce Zr doping into (Ce, La, Pr, Sm, Y)O2−δ to hinder the structural transition at elevated temperatures. Indeed, the fluorite structure at elevated temperatures is stabilized at approximately 10 at.% Zr doping. The total conductivity initially increases with doping, peaking at 5 at.% Zr doping, and subsequently decreases with further doping. Interestingly, electronic conductivity in (Ce, La, Pr, Sm, Y)1−x Zr x O2−δ under oxidizing atmospheres is not significant and is lowest at 8 at.% Zr. These results suggest that ceria-based high-entropy oxides can serve as oxygen ion conductors with a significantly reduced electronic contribution. This work paves the way for new compositionally complex electrolytes as well as protective coatings for solid oxide fuel cells.