Ultrasonication during the synthesis of manganese oxides for electrochemical capacitors

An investigation has been conducted to examine the effect that ultrasonication has on the synthesis of manganese dioxide used as a pseudocapacitive electrode material in supercapacitors. Using the reaction between KMnO4 and MnSO4, the layered birnessite polymorph of manganese dioxide was produced with and without ultrasonication (control). X-ray diffraction analysis of the materials indicates that ultrasonication has a minimal effect on the crystallite size, although the crystallite dimensions are small anyway (8.6 vs 14.1 nm within the layers, and 9.4 vs 8.9 nm between the layers for the control and ultrasonicated samples, respectively). However, ultrasonication has had a profound effect on morphology, essentially fusing these small crystallites together to form much larger needles 100–200 nm in length. This fusion process has also dramatically decreased the Brunauer, Emmett, and Teller surface area (from 170 to 10 m2/g with ultrasonication), and furthermore electrochemical performance has also been diminished, although by not as much as the drop in surface area; e.g., 106 vs 64 F/g. These phenomena are discussed in terms of the pore volume available for charge storage.