Ph.D Thesis


Ph.D StudentOmri Mazar
SubjectDefect Chemistry and Electrical Properties of Donor-doped
Proton Conducting Pervoskite-type Oxides
DepartmentDepartment of Chemical Engineering
Supervisor Professor Tsur Yoed
Full Thesis textFull thesis text - English Version


Abstract

Protonic conductivity of acceptor-doped perovskites, specifically in materials such as BaCeO3 (BC) has fascinated the scientific world since its discovery. Protonic conduction in perovskites holds great potential for energy production-related applications, mainly solid oxide fuel cells (SOFCs). Construction of a SOFC using a perovskite proton conductor as electrolyte encompasses several significant advantages as improved catalysis (without noble metals), heat management and usability, and offer reduced cost.

Doping a perovskite with acceptors, typically trivalent rare-earth ions, results in the formation of oxygen vacancies. Subjection to water vapor forms protonic defects which enable proton migration. Hence, doping a perovskite with donors will result in the opposite outcome, metal vacancies will form, oxygen vacancies will deplete and the protonic conduction will be diminished.

This dissertation depicts the emergence of protonic conductivity within donor-doped BC. This is achieved by the formation of a novel and unique inside-out core-shell (IOCS) structure. The IOCS structure is composed of a donor-doped BC core enveloped by a nominally undoped BC. The structure is named inside-out due to the common tendency of perovskites to form the exact opposite structure. After the IOCS structure is formed, the metal vacancies are allowed to migrate outward from the core to the shell. Since metal vacancies are negatively charged, they can act as effective acceptors on the nominally undisturbed shell. Hence, oxygen vacancies followed by protonic defects are formed in the shell. Furthermore, since the proton conduction mechanism is based on proton hopping from an oxygen site to its adjacent, diminishing of the unoccupied oxygen vacancies will increase the proton mobility.

Impedance spectroscopy analysis revealed that the synthesized IOCS samples did obtain improved proton conductivity in the bulk over an undoped reference sample. Furthermore, the difference between the conductivity under wet vs. dry nitrogen atmospheres is enhanced significantly in the IOCS samples in comparison with the reference sample. The non-stoichiometric samples demonstrated improved conductivity and humidity-induced conductivity gain (HiCG) over the stoichiometric sample; this is believed to be established by the higher concentration of metal vacancies in the core in the non-stoichiometric compositions. The barium-deficient sample exhibited significantly higher HiCG over the barium-surplus sample. Hence, barium vacancies have much higher mobility than the cerium vacancies. Moreover, samples under wet atmospheres exhibit larger currents than under dry atmospheres indicating of the dominance of protonic conduction.