Sustainable energy storage systems require the development of non-precious metal catalysts for water splitting. Cryptomelane-type alpha-(K)MnO2 is one of the few oxides of manganese with promising electrocatalytic activity for the oxygen evolution reaction (OER). We report a strategy to boost the performance of cryptomelane-type alpha-(K)MnO2 in OER electrocatalysis in alkaline electrolyte by thermal treatments in specific gas atmospheres. The thermal treatment of alpha-(K)MnO2 at 300 degrees C in He, H2O/He and air can lower the potential required to reach a current density of 10 mA cm(-2) for the OER by up to 60 mV. We discuss the structural changes on the atomic level, by combining X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, transmission electron microscopy and electrochemical impedance spectroscopy. The simultaneous presence of three structural features for MnOx-based water oxidation catalysts is considered essential: (i) Mn3+. sites, (ii) structural defects, and (iii) mono- and dip-oxo bridges, which can be tailored on the surface and in the bulk. Notably, extended X-ray absorption fine structure measurements suggest that more Mn(3+ )sites are detected in the corner-sharing positions, which are suggested to be the favorable sites for the electrocatalytic water oxidation. (C) 2019 Elsevier Inc. All rights reserved.