Electrochemical oxidation of Mn/MnO films: formation of an electrochemical capacitor

An in-depth study of the oxidation step required to produce electrochemical capacitors from porous manganese-oxide materials was carried out. The oxidation process takes place under the application of a small anodic current in a solution of 1 M Na2SO4, resulting in the formation of a three-layered structure. During the oxidation process, a base layer of undisturbed zigzag material oxidizes from Mn/MnO to Mn3O4. At the same time, a second layer forms directly on the surface of the zigzag material. The layer is partly crystalline Mn3O4 and partly amorphous. The final and most important layer is the amorphous, hydrated MnO2 surface film. It is believed that this layer is solely responsible for the capacitive behavior of these films. The porosity of the electrode prior to oxidation is shown to be immaterial as oxidation of a fully dense film results in similarly high capacitive values attributed to the formation of a porous surface layer. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.