Oxides of Manganese As Efficient Bifunctional Electrocatalysts

Abstract

The electrolysis of water, or water splitting reaction is one of the cleanest ways of producing hydrogen and oxygen gas for fuel. Electrochemical devices employing these reactions include regenerative fuel cells and rechargeable metal-air batteries, which have the highest theoretical energy density. Considerable efforts have been dedicated towards improving the energy conversion efficiency of these devices. However, the single main obstacle for achieving this has been the lack of suitable bifunctional oxygen electrocatalysts that show simultaneous high activity for both oxygen reduction (ORR) and oxygen evolution (OER) reactions during charge/discharge cycles. The overpotential and slow kinetics of oxygen reactions are limiting parameters in the advancement of electrochemical storage devices. Taking inspiration from nature’s photosynthetic process that uses a Mn4O4Ca cluster in photosystem II to catalyze water splitting, we studied the efficiency of various phases of manganese oxide (MnOx) as electrocatalysts for OER and ORR process, including the role of defects on bifunctionality. Cyclic voltammetry was used to prove that Mn2O3 is a bifunctional catalyst by showing high activity for OER and ORR. Next, in-situ UV-Vis absorbance spectroscopy was conducted to track changes in the surface oxidation state as a result of occurring reactions. Finally, we use electrochemical quartz crystal microbalance to track ion intercalation into the material. Results from these studies were used to obtain a structure-defect-function correlation that can improve the design of electrocatalysts with bifunctional properties. Acknowledgements: The authors would like to thank Rensselaer Polytechnic Institute (RPI) and National Science Foundation, CBET award (No: 1511733), for the partial financial support and Nicholas Smieszek for his assistance with the preparation of this manuscript. I.R, and Q.W also gratefully acknowledge the partial support of Howard P. Isermann fellowship provided by the Department of Chemical and Biological Engineering at RPI.