Exploring the mechanical stability of manganese oxide as an electrocatalyst via in-situ surface stress and electrochemical quartz crystal microbalance studies

We report in this study on the combination of resonance frequency shift measurements with the use of Electrochemical Quartz Crystal Microbalance (EQCM), alongside in-situ electrochemical surface stress (ESS) analysis, which provides a comprehensive description of both the microscopic and macroscopic processes occurring on the surface of the oxygen reduction reaction (ORR) electrocatalyst. The resonance frequency shift was measured on an Au/MnO_x electrode in both de-oxygenated and O₂ saturated electrolytes. The results obtained with the use of frequency shift measurements (with the use of 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 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.