The Role of Heat Treatment in Enhanced Activity of Manganese Oxides for the Oxygen Reduction and Evolution Reactions

Abstract

In this study we use ex-situ Mn L-edge X-ray absorption spectroscopy and electrochemical characterization to understand the relationship between surface properties of five differently prepared manganese oxides (MnOx) electrocatalysts and their activity for the oxygen reduction and evolution reactions (ORR, OER). We prepare the five catalysts by electrodepositing MnOx on polished glassy carbon electrodes and heat treating them under different conditions: no heat treatment (“no ht”), 250°C, 350°C, 450°C, and 500°C. We find that “no ht” catalyst, despite having the lowest (pseudo)capacitive current, is capable of the largest changes in Mn oxidation state, as the applied potential switches from an ORR to an OER relevant value. Heat treatment at 450°C or 500°C, leading to a significant increase in (pseudo)capacitance, is on the other hand associated with only minor changes in the surface oxidation state. Investigation of (pseudo)capacitive behavior of the same MnOx thin films electrodeposited on a high surface area rather than polished carbon support reveals that the observed differences in pseudocapactive current can be explained by changes in the surface area of the electrocatalysts. Characterization of the electrocatalytic activity of the thin films demonstrates that the catalysts exposed to the two highest temperatures are associated with enhanced bifunctional activity for the ORR and the OER. The enhancement in the ORR activity is attributed to the formation of a Mn2O3 phase, while the enhancement in the OER activity is primarily explained by the increase in the electrochemical surface area of the catalyst.