M.Sc Thesis


M.Sc StudentKeisar Or
SubjectSurface Stress Study of Oxides Electro-Catalysis during
ORR in Metal-Air Battery Cathodes
DepartmentDepartment of Energy
Supervisors PROF. Yair Ein-Eli
DR. Yair Cohen


Abstract

The electro-catalysts for oxygen reduction reaction (ORR) on air-cathodes have a decisive impact on metal-air batteries and fuel cells performance. Manganese-oxides (MnOx) are particularly interesting as non-precious electro-catalysts candidates. In this study I report on the combination of in-situ electrochemical surface stress (ESS) analysis, alongside resonance frequency shift measurements preformed with the use of electrochemical quartz crystal microbalance (EQCM). The surface stress analysis is used to probe changes in the surface stress on the electrocatalyst-electrolyte interface, while the frequency shift measurement is used to track mass changes on the electrocatalyst film. The conjugation of the two techniques provides a comprehensive description of both the microscopic and macroscopic processes occurring on the surface of the electrocatalyst.

The electrochemical surface stress as well as the resonance frequency shift were measured on an Au/MnOx electrode in both de-oxygenated and O2 saturated alkaline electrolytes. A complex stress response was measured during the electrochemical scan, caused by various crystal structure transitions and surface reconstruction processes. It was found that the presence of oxygen resulted in a less compressive stress response during the ORR. In addition, an overall tensile trend was recorded during multiple cycling, explaining the poor mechanical stability of the MnOx film.

The results obtained with the use of frequency shift measurements (EQCM) are in good agreement with the interpretations of the data originated from the surface stress response. It was found that the structural changes identified by the surface stress led to the insertion and release of water molecules into and out of the manganese-oxide film. These findings are used to further explain the mechanical failure of the electrocatalyst film subsequent to multiple charge/discharge cycles, hence revealing an additional valuable information on the poor mechanical stability of the manganese-oxide film. Moreover, the presence of oxygen in the electrolyte resulted in mass changes, being qualitatively similar to those recorded in de-oxygenated electrolyte, albeit with different intensities during the ORR, further validating the proposed catalytic mechanism. These findings have major implications on the applicability of MnOx as catalyst.