Cathodic reduction of aqueous MnO 4− ions in the presence of various alkaline metals except for Na + ions led to the deposition of birnessite-type layered MnO 2 . The highest crystallinity was obtained when electrolyzed in a 2 mM KMnO 4 and 50 mM KCl at a constant potential of 0 V vs Ag/AgCl. The cathodic formation of MnO 2 was prevented by the presence of divalent cations, and Na + ions gave rise to an amorphous or low crystalline product. The birnessite film thus formed exhibited an excellent pseudocapacitive behavior in the as-deposited state, with a specific capacitance as high as 322 F g −1 at a scan rate of 2 mV s −1 , which is much larger than that (75 F g −1 ) of the birnessite film grown anodically. The resulting pseudocapacitive electrode functioned as an efficient catalyst toward the oxidation of L-cysteine, where the anodic overpotential was reduced by 0.15 to 0.3 V.
We have investigated the degradation behavior of a typical asymmetric supercapacitor cell operating in an aqueous solution of Na 2 SO 4 with a voltage window of 2 V, where α-MnO 2 and activated carbon (AC) were chosen as the active materials for positive and negative electrodes, respectively. In this study, potentials of α-MnO 2 and AC electrodes of the asymmetric cell were monitored with respect to an Ag/AgCl reference electrode during repetitive charge/discharge cycling. Since the charges passed at positive and negative electrodes are the same, a change in a certain parameter should be compensated by changes in the other parameters. An irreversible current due to reduction of water on AC electrode triggered capacitance fade in the initial charge/discharge cycling. This phenomenon made the lowest potential more positive to be deviated from the potential region where water reduction can occur, resulting in the following stable operation. Then, α-MnO 2 facilitated the oxidation of water, which deactivated α-MnO 2 itself probably due to an irreversible increase in reduced Mn sites. The deactivation of α-MnO 2 was compensated by expansion of its potential window in the cathodic scan, leading to further capacitance fade due to dissolution.Supercapacitor, also called electrochemical capacitor, can be categorized into two classes according to their charge-storage mechanisms; i.e., (i) electrochemical double-layer capacitor (EDLC) and (ii) pseudocapacitor. EDLCs store charge electrostatically via reversible ion adsorption/desorption at an electrode/electrolyte interface. Here, carbon-based materials with high conductivity and large surface area are commonly used as the electrode. The charge stored in EDLCs is limited in a range of 3-8 Wh kg -1 , which is much smaller than the energy density (>100 Wh kg -1 ) of batteries. 1 Extensive research has recently been devoted to pseudocapacitors which utilize faradaic reactions at the surface (and the bulk) of electroactive materials. The materials providing pseudocapacitance involve electrically conducting polymers (polyanilines, polythiophenes, polypyrroles, etc.) and transition metal oxides (ruthenium oxide, manganese oxide, iron oxide, nickel oxide, etc.). Among them, manganese oxides (mainly MnO 2 ) are often considered the most promising materials because of their high theoretical specific capacitance (1370 F g -1 ), low cost, and environmental benignity. 2,3 Pseudocapacitance of MnO 2 arises from the following redox reactions:where C + denotes electrolyte cations (Na + , K + , Li + , etc.). According to a comparative study performed by Devaraji et al., α-type MnO 2 has the highest specific capacitance (297 F g -1 at 0.5 mA cm -2 ) among various crystalline forms. 4 An expansion of the potential window of MnO 2 electrode is also an important subject with maintaining both electrochemical reversibility and rate capability because the energy density (E, Wh kg -1 ) is proportional to the square of the operating voltage (V, V).where C (F g -1 ) is the specific capacitance. Aque...
Cathodic reduction of an aqueous solution of potassium permanganate in the presence of various alkaline metals except for Na + ions led to a deposition of birnessite-type layered MnO 2 . The highest crystallinity was achieved in a bath containing 2 mM KMnO 4 and 50 mM KCl by applying a constant potential of 0 V vs Ag/AgCl. The cathodic formation of birnessite was prevented by the presence of divalent cations, and Na + ions yielded an amorphous or low crystalline product. The birnessite film thus formed showed excellent pseudocapacitive characteristics in its asdeposited state; i.e., a specific capacitance as high as 322 F g -1 at a potential scan rate of 2 mV s -1 , which is much higher than that (75 F g -1 ) of the birnessite film prepared anodically from Mn 2+ ions.
The inclusion complexation behavior between 10-undecyn-1-ol and cyclodextrin (CD) derivatives, namely, randomly methylated beta-CD (RM-beta-CD) and hydroxypropyl-beta-CD (HP-beta-CD), was studied in terms of solubility improvement, apparent stability constant, and the inclusion ratios of the resultant inclusion complexes. The aqueous solubility of 10-undecyn-1-ol was greatly improved through complexation with the CD derivatives. RM-beta-CD is comparatively more efficient in solubilizing 10-undecyn-1-ol with an apparent stability constant outstripping that of HP-beta-CD by about an order of magnitude. Comparative in vitro evaluations of the growth inhibition effects of inclusion complex solutions toward Rosellinia necatrix, a phytopathogenic fungus, were performed. In comparison with the positive control, appreciable improvements of the antifungal activity of 10-undecyn-1-ol through the addition of CD derivatives were observed visually. The improvement was evaluated in terms of area covered by the mycelia of Rosellinia necatrix and their growth rate. RM-beta-CD was proven to be more effective compared to HP-beta-CD with regard to the reduction of both fungal mycelium-covered area and growth rate constant, presumably owing to greater solubility enhancement by RM-beta-CD and thus the bioavailability of 10-undecyn-1-ol. Inclusion complexation of 10-undecyn-1-ol with CD derivatives suggests a potential means for production of an environmentally friendly 10-undecyn-1-ol-based fungicide to counteract R. necatrix.
We have fabricated birnessite-type layered manganese oxide bycathodic reduction of aqueous permanganate ions (MnO4-) in thepresence of various alkaline metals. The electrodeposition andcharge storage processes were monitored by means ofelectrochemical quartz crystal microbalance (EQCM). Molarmasses acquired for the electrodeposition process revealed thatMnO2 grows concurrently with the incorporation of the coexistingcations and water molecules. The molar mass decreases in theorder of Li > Rb ~ K ~ Cs > Na, which is in good agreement withthe order of crystallinity. Clearly, the degree of crystallinity in theformation of birnessite depends on the incorporation amount of theguest ions and water. The birnessite-modified electrode increasesand decreases in mass during cathodic and anodic scans in Na2SO4electrolyte. The molar masses obtained were 21.6 and 24.9 g permole of electrons, respectively, both of which coincide with theatomic weight of Na (23.0 g).
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