Nanostructured transition metal oxides for aqueous hybrid electrochemical supercapacitors

In this paper, we wish to present an overview of the research carried out in our laboratories with low-cost transition metal oxides (manganese dioxide, iron oxide and vanadium oxide) as active electrode materials for aqueous electrochemical supercapacitors. More specifically, the paper focuses on the approaches that have been used to increase the capacitance of the metal oxides and the cell voltage of the supercapacitor. It is shown that the cell voltage of an electrochemical supercapacitor can be increased significantly with the use of hybrid systems. The most relevant associations are Fe₃O₄ or activated carbon as the negative electrode and MnO₂ as the positive. The cell voltage of the Fe₃O₄/MnO₂ device is 1.8 V and this value was increased to 2.2 V by using activated carbon instead of Fe₃O₄. These two systems have shown superior behavior compared to a symmetric MnO₂/MnO₂ device which only works within a 1 V potential window in aqueous K₂SO₄. Furthermore, the activated carbon/MnO₂ hybrid device exhibits a real power density of 605 W/kg (maximum power density =19.0 kW/kg) with an energy density of 17.3 Wh/kg. These values compete well with those of standard electrochemical double layer capacitors working in organic electrolytes. PACS 82.47.Uv; 82.45.Fk; 82.45.Yz