Synthesis of Ca2Mn3O8 films and their electrochemical studies for the oxygen evolution reaction (OER) of water

Based on the oxygen evolving mu-oxido-Mn4Ca cluster unit in photosynthesis II, Ca2Mn3O8 films prepared by dip-coating and screen-printing were tested as oxygen evolving catalysts. Structural analysis confirmed that smooth 100 nm films can be grown from a Ca2MnO4 sol-gel solution showing a high orientation in (1 0 0) direction when deposited on F:SnO2/quartz and quartz substrates. The distance found by X-ray diffraction between the layers exactly fitted the monoclinic layer compound of composition Ca2Mn3O8. To compare the electrochemical behavior of this layer with the bulk Ca2Mn3O8 powder, a 6 mu m thick film was prepared by screen-printing. Differential electrochemical mass spectroscopy performed in 0.1 M KH2PO4 at pH 7 and 1 M Tris(hydroxymethyl)aminomethane at pH 7.8 revealed that both films were able to oxidize water as well as organic matter with a good correlation to the slope of the current density curves; however, the dip-coated films displayed lower current densities than the screen-printed Ca2Mn3O8 layers. A closer look into the cyclic voltammetry revealed that capacity values were similar indicating that not only the different surface areas played a role, but variations in the specific capacity values possibly due to the orientation of the films or the manganese mixed valence in the surface as determined by X-ray photoelectron spectroscopy (XPS) could be responsible for these effects. (C) 2012 Elsevier Ltd. All rights reserved.