Thin films of manganese dioxide were formed on nickel foils by electrodeposition and by both dip-coating and drop-coating with manganese dioxide suspensions (sols) and their subsequent gelation and calcination. The performance of these films as ultracapacitors was studied by cyclic voltammetry in the range 0.0-0.9 V (SCE) and by chronopotentiometry in unbuffered Na 2 SO 4 solution. The cyclic voltammograms of ultrathin, dip-coated sol-gel-derived films indicated better capacitive behavior and gave differential specific capacitance values as high as 698 F/g compared to values half to two-thirds as great for the electrodeposited films. Multilayer drop-coated sol-gel films were prepared to attain film thicknesses comparable to the electrodeposited films, and these were found to provide charge-storage capacity as high as 15 mC/cm 2 , more than three times greater than that of the electrodeposited films. All films, except electrodeposited films that were not thermally cured, exhibited good cycling stability, losing not much more than 10% of capacity after 1500 cycles.
Material characterization of sol-gel-derived and electrodeposited MnO2 thin films showed that their microstructures are highly porous in nature. While sol-gel-derived films are nanoparticulate, electrodeposited films showed macropores of random and irregular platelike structures, comprising much denser surface layers and highly porous underlying layers. On the basis of calculated and theoretical density values of 1 and 4.99 g/cm3, respectively, the porosity of sol-gel-derived MnO2 films was determined to be as high as 80%, which is substantially higher than electrodeposited films at 67%. Apart from their higher specific capacitance, sol-gel-derived MnO2 films appeared to exhibit higher cycling stability and reversibility than electrodeposited MnO2 films. In the case of sol-gel films, thinner films appeared to exhibit higher cycling stability than thicker films. There was less alteration in surface morphology and microstructure, and the rate of loss in charge-storage capacity upon voltammetric cycling was not as significant for sol-gel MnO2 thin films
Reversible conductance transitions are demonstrated on the molecular scale in a complex of 3-nitrobenzal malononitrile and 1, 4-phenylenediamine, by application of local electric field pulses. Both macroscopic and local current-voltage (I/V) measurements show similar electrical bistability behavior. The mechanism of the electrical bistability is discussed.
Stable colloidal tetrapropylammonium manganese oxide ͑TPA-MO͒ was formed by the reduction of tetrapropylammonium permanganate with 2-butanol at room temperature. Thin films of TPA-MO were prepared using the sol-gel process by dip-coating directly onto clean nickel foils followed by heat-treatment under controlled conditions. The microstructure evolution of TPA-MO films at various calcination temperatures was characterized using Brunauer-Emmett-Teller method, X-ray diffraction, and scanning electron microscopy. The performance of these films as supercapacitors was evaluated using cyclic voltammetry in various aqueous electrolytes. These thin films exhibited excellent capacitive behavior with a specific capacitance of 720 F/g. These films also showed good reversibility and cycling stability, losing little more than 20% of their charge capacity after 1500 cycles. 4,5 As compared to other transition metal oxides such as NiO 2 , CoO, RuO 2 , and V 2 O 5 , these materials have received much attention and interest due to the low cost of raw materials, low toxicity, and environmentally friendly character.Studies of Pang et al. 4,5 have shown that sol-gel-derived MnO 2 thin films are a promising electrode material for supercapacitors due to their high reversibility, good cycling stability, and their high specific capacitance of 700 F/g. However, in spite of their favorable characteristics, the potential use of such sol-gel-derived MnO 2 films for fabricating practical devices is limited by the very dilute concentration (10 Ϫ3 M) of the MnO 2 colloidal suspension employed in coatings. Stable nanoparticles of manganese oxides having higher concentrations are difficult to prepare because of the strong tendency of manganese oxides to precipitate or coagulate during synthesis. Any increase in the MnO 2 concentration invariably leads to the destabilization of the colloidal suspension. Considerable efforts have therefore been directed toward exploring alternative precursors and approaches in order to prepare stable colloidal suspensions with higher concentrations of manganese dioxide particles.Recently, stable colloidal manganese oxides with concentrations as high as 0.57 M have been successfully prepared by incorporating tetralkylammonium cations ͑alkyl ϭ ethyl, propyl, and butyl͒ to prevent agglomerization of the negatively charged manganese oxide particles.6 Materials derived from such colloidal manganese oxides are reported to have lamellar structure with tetraalkylammonium cations intercalated between the manganese oxide layers. The manganese oxides have an average oxidation state of 3.70-3.79. 6 The pore size distribution of materials derived from such organic cation template-based manganese oxides can be well controlled by the size of the particles that constitute the templates, which can be subsequently removed by heat-treatment.In this paper, we report on the microstructural characterization of sol-gel-derived tetraproplyammonium manganese oxide ͑TPA-MO͒ thin films on nickel substrates and the electrochemical charact...
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