Defect-Rich Α-MnO2 – Revealing the Optimum Mn4+/Mn3+ Cation Defect Density for High Electrocatalytic Activity

Abstract

The Oxygen Reduction (ORR) and Oxygen Evolution Reaction (OER) are of great importance for a variety of electrochemical energy storage and conversion applications, i.e., fuel cells, rechargeable metal-air batteries, and electrolyzers. To efficiently enable both oxygen reactions, catalysts demonstrating high (bifunctional) electrocatalytic activity are of crucial importance.[1,2] Though precious metal catalysts, i.e., carbon supported platinum, ruthenium, iridium, or alloys thereof, are considered as the state-of-the-art in terms of catalytic activity towards the ORR and OER, respectively, their scarcity and high costs have given rise to extensive research and development in the past decade focusing on lower cost alternatives as well as strategies to improve catalytic activity.[3] Among a variety of catalyst materials, manganese oxides have gained significant interest as potentially suitable alternatives as they are inexpensive, readily available and reasonably catalytically active.[4,5] More important, tunnel-type α-MnO2, which is built of double chains of MnO6 octahedra arranged at its edges and corners, demonstrates electrocatalytic activity rivaling commercial precious-metal catalysts.[6,7] Besides various structural and physicochemical properties identified to be relevant for electrocatalysis, manganese cation defects along with oxygen vacancies created from an alternation of the Mn oxidation state, viz., from Mn4+ to Mn3+, are considered as true catalytically active sites.[8,9] Though a few studies attempted to correlate Mn4+/Mn3+ cation defects and electrocatalytic activity, the optimum defect density still remains unclear.[10–12] The aim of this work is to unravel the true nature and relationship between the electrocatalytic activity and cation defects within α-MnO2. Electrocatalytic activity with respect to the ORR and OER, i.e., half-wave potentials, overpotentials and Tafel slopes, of α-MnO2 prepared from different synthesis routes and conditions as well as commercial manganese oxides are investigated using the Thin-film Rotating Disk Electrode (TF-RDE) technique. Mn4+/Mn3+ cation defects of various manganese oxides were evaluated by means of X-ray photoelectron spectroscopy (XPS). Through a successive variation of synthesis conditions the relationship of Mn4+/Mn3+ cation defect formation on the electrocatalytic activity of α-MnO2 is evaluated and the optimum Mn4+/Mn3+ cation defect density for highest electrocatalytic activity identified.