New Insights on the Surface and Stress Behavior of Manganese-Oxide As Oxygen Reduction Reaction Catalyst in Alkaline Electrolyte

Abstract

The electro-catalysts for oxygen reduction reaction (ORR) on air-cathode have a decisive impact on metal-air batteries and fuel cells. Much effort has been invested in finding a cost effective electro-catalyst, enabling a decrease in the ORR over-potential and enhancing the power source device discharge performance.[1-2] Manganese-oxides (MnOx) are particularly interesting as non-precious ORR electro-catalysts candidates due to their rich oxidation states, chemical compositions and their variety of crystal structures[1-3]. In order to broaden our understanding on the catalytic mechanism of Manganese-Oxide and its surface chemistry during ORR, we performed unique in-situ electrochemical surface stress (ESS) measurements[4]. The in-situ ESS response was measured on an Au/MnOx electrode in both Ar and O2 saturated environments. A complex stress response was measured during the electrochemical scan caused by various crystal structure transitions[5]. The presence of oxygen resulted in a less compressive stress response during the ORR; suggesting that part of the activated sites on the surface are reduced and re-oxidized (Mn4+↔Mn3+) during the ORR. These observations evidently support the proposed ORR catalytic mechanism[3,5]. In addition, an overall tensile trend was recorded during multiple cycling, pouring some light on the poor mechanical stability of the MnOx film. This insight has major implications on the applicability of MnOx as a catalyst on practical carbon based electrodes.