Electrochemical supercapacitors (ECs), characteristic of high power and reasonably high energy densities, have become a versatile solution to various emerging energy applications. This critical review describes some materials science aspects on manganese oxide-based materials for these applications, primarily including the strategic design and fabrication of these electrode materials. Nanostructurization, chemical modification and incorporation with high surface area, conductive nanoarchitectures are the three major strategies in the development of high-performance manganese oxide-based electrodes for EC applications. Numerous works reviewed herein have shown enhanced electrochemical performance in the manganese oxide-based electrode materials. However, many fundamental questions remain unanswered, particularly with respect to characterization and understanding of electron transfer and atomic transport of the electrochemical interface processes within the manganese oxide-based electrodes. In order to fully exploit the potential of manganese oxide-based electrode materials, an unambiguous appreciation of these basic questions and optimization of synthesis parameters and material properties are critical for the further development of EC devices (233 references).

A simple methodology is developed to directly synthesize three-dimensional (3D) electrochemically supercapacitive arrays, consisting of multiwalled carbon nanotubes conformally covered by nanocrystalline vanadium nitride, firmly anchored to glassy carbon or Inconel electrodes. These nanostructures demonstrate a respectable specific capacitance of 289 F g–1, which is achieved in 1 M KOH electrolyte at a scan rate of 20 mV s–1. The well-connected highly electrically conductive structures exhibit a superb rate capability; at a very high scan rate of 1000 mV s–1 there is less than a 20% drop in the capacitance relative to 20 mV s–1. Such rate capability has never been reported for VN and is highly unusual for any other oxide or nitride. These 3D arrays also display nearly ideal triangular voltage profiles during constant current charge–discharge cycling. Analysis of the post-electrochemically cycled samples indicates negligible changes occurring in the VN nanocrystallite morphology, but a modification in the structure of the oxidized surface. We envision that the direct synthesis approach employed in this study may serve as a “drop-in” platform for large-scale commercial fabrication of a variety of carbon nanotube-supported functional materials that require excellent electrical conductivity to the underlying support.

No abstract

The crystal structure of anodically electrodeposited MnO2 nanocrystals can be manipulated by introducing complexing agents in the electrodeposition solutions. MnO2 nanocrystals with three types of crystal structures were observed: hexagonal ε-MnO2 (complex-free), defective rock salt MnO2 (ethylenediaminetetraacetic acid), and defective antifluorite MnO2 (citrate). The capacitive performance of the MnO2 nanocrystals depends strongly on their crystal structures. MnO2 with defective rock salt and antifluorite structures exhibit better capacitive properties than ε-MnO2. The electrochemical capacitance differences can be explained in terms of the crystal chemistry. In both the defective rock salt and antifluorite MnO2, an anomalous trend was observed. The specific capacitance does not decrease with increasing scanning rate. A possible reason is that certain physicochemical changes, such as phase transformations or morphology changes, occur preferentially at high cycling rates.

The carbon nanotube-silver composite (Ag-CNT) is a new class of multifunctional materials with potential applications such as sensors, catalysts, biodisinfection, and sorbents. A simple method combining wet-chemistry and thermal reduction was adopted to synthesize silver on the surface of the CNT. The synthesized Ag-CNT was tested as a sorbent for the removal of elemental mercury from flue gases of coal-fired power plants and as a mercury trap for elemental mercury analysis. A complete capture of mercury by the Ag-CNT was achieved up to a capture temperature of 150 °C, similar to the temperature of flue gases in coal-fired power plants. The captured mercury could be quickly and completely released by simple heating at 330 °C, to restore its mercury adsorption capacity. Silver on the Ag-CNT was shown to be the main active component for the mercury capture via an amalgamation mechanism in contrast to simple physical adsorption on the undoped CNT . Compared to silver-coated quartz beads (Ag-beads) and gold-coated quartz beads (Au-beads), which is conventionally used as a mercury trap for mercury measurements, the Ag-CNT showed a much higher mercury capture capacity and a minimal memory effect. With the Ag-CNT as a mercury preconcentration trap, calibration results showed a satisfactory linear coefficient of ≥0.9998 between known amounts of standard mercury and their corresponding fluorescence signals of cold vapor atomic fluorescence spectrophotometry (CVAFS). The presence of SO2, NO x , CO2, or O2 showed a negligible impact on the mercury capture performance of the Ag-CNT.