| Electrochemical capacitors are used for applications requiring high power output and high cycle capacity. Ruthenium oxide has been used for pseudocapacitors; however, raw material costs are prohibitive to large-scale commercial production and manganese oxides are becoming popular. In general, there are certain requirements necessary in order to develop capacitance from transition metal oxide materials. The films are porous and amorphous to allow for easy movement of protons through the structure and the active sites (sites that develop capacitance) must be hydrated with a 4+ valence (e.g., MnO 2•nH2O).; A procedure combining physical vapor deposition and electrochemical oxidation was developed to produce a film that has excellent capacitance (∼500F/g). The physical vapor deposition step is used to deposit a thin layer of Mn/MnO onto a standard Si substrate with Ti/Pt metallization. In the as-deposited state, the film is crystalline, dense, 0/2+ valence, with no hydration and develops no capacitance. The film is subsequently oxidized in a solution of 1M Na2SO4 using a constant current up to a potential of 0.9V vs. Ag/AgCl. During this step in the processing, the manganese near the surface of the film is oxidized to the 4+ valence, develops significant hydration and becomes both porous and amorphous. It is after this oxidation step that the film becomes highly capacitive.; The porous surface layer responsible for the capacitive behavior can be produced on any PVD deposited manganese layer regardless of the starting film morphology (e.g. dense or porous). Since the electrochemical oxidation step is critical to the formation of the porous layer, adjusting some of the oxidation parameters can have a significant effect on the porous layer and resulting capacitance. Overall, the capacitance of the films increases as the oxidizing current density increases. Increasing the oxidation current density results in a thinner porous layer, but greater hydration content. The thickness of the porous surface layer is also a function of the thickness of the deposited film. As the starting film thickness increases, so does the resulting porous layer thickness. The hydration remains the same and therefore the thicker films produce greater areal capacitance (F/cm2) but the specific capacitance (F/g) remains constant. |
