Structural in situ study of the thermal behavior of manganese dioxide materials: toward selected electrode materials for supercapacitors.

The thermal behavior of a series of MnO2 materials was investigated toward MnO2 microstructures under inert atmospheres. The byproduct formed during MnO2 heat treatments from the room temperature to 800 °C were characterized by in situ X-ray diffraction analyses. It was found that annealing spinel and ramsdellite phases caused the formation of MnO2 pyrolusite at 200 °C, Mn2O3, at 400 °C, and then Mn3O4 at higher temperatures. In the case of cryptomelane and birnessite phases, the heating process resulted in the formation of K0.51Mn0.93O2 at 600 °C, while Mn3O4 was also formed and still present up to 800 °C. Heat-treating Ni-todorokite and OMS-5 up to about 450 °C led to the formation of NiMn2O4 and NaxMnO2, respectively, and again Mn3O4 at higher temperatures. All of these structural transformations were correlated to resulting weight losses of MnO2 powders, measured by thermogravimetric analyses, during the heating process. Cyclic voltammetry measurements were performed in the presence of 0.5 M K2SO4 aqueous solution for annealed cryptomelane, K0.51Mn0.93O2, and Mn3O4-based electrodes. It was found that MnO2 cryptomelane is electrochemically stable upon heating. The long-term charge/discharge voltammetric cycling revealed that the specific capacitance of Mn3O4-based electrode is significantly improved from 14 F·g(-1) (after 20 cycles) to 123 F·g(-1) (after 500 cycles).

Microstructural effects on charge-storage properties in MnO2-based electrochemical supercapacitors.

The charge-storage mechanism in manganese dioxide (MnO2)-based electrochemical supercapacitors was investigated and discussed toward prepared MnO2 microstructures. The preparation of a series of MnO2 allotropic phases was performed by following dedicated synthetic routes. The resulting compounds are classified into three groups depending on their crystal structures based on 1D channels, 2D layers, or 3D interconnected tunnels. The 1D group includes pyrolusite, ramsdellite, cryptomelane, Ni-doped todorokite (Ni-todorokite), and OMS-5. The 2D and 3D groups are composed of birnessite and spinel, respectively. The prepared MnO2 powders were characterized using X-ray diffraction, scanning electron microscopy, the Brunauer-Emmett-Teller technique, cyclic voltammetry (CV), and electrochemical impedance spectroscopy. The influence of the MnO2 microstructure on the electrochemical performance of MnO2-based electrodes is commented on through the specific surface area and the electronic and ionic conductivities. It was demonstrated that the charge-storage mechanism in MnO2-based electrodes is mainly faradic rather than capacitive. The specific capacitance values are found to increase in the following order: pyrolusite (28 Fx g(-1)) < Ni-todorokite < ramsdellite < cryptomelane < OMS-5 < birnessite < spinel (241 Fx g(-1)). Thus, increasing the cavity size and connectivity results in the improvement of the electrochemical performance. In contrast with the usual assumption, the electrochemical performance of MnO2-based electrodes was not dependent on the specific surface area. The electronic conductivity was shown to have a limited impact as well. However, specific capacitances of MnO2 forms were strongly correlated with the corresponding ionic conductivities, which obviously rely on the microstructure. The CV experiments confirmed the good stability of all MnO2 phases during 500 charge/discharge cycles.

The bridging bidentate perchlorato group in ReO3(ClO4), ReO3(ClO4Cl2O6 and Sb2Cl6(O)(OH)(ClO4), a vibrational analysis.

Raman and infrared analysis of the new compounds: ReO3(ClO4), an ivory-white solid, and (ClO2)xReO3(ClO4)1+x (x < or = 1), an orange-red chloryl salt, showed that bridging bidentate [ClO4] and terminal ReO3 groups are present in both complexes. Vibrational data on [ClO4] in ReO3(ClO4) were compared to those obtained experimentally and by DFT calculation on a bridging bidentate [ClO4] in Sb2Cl6(O)(OH)(ClO4).

First anhydrous gold perchlorato complex: ClO(2)Au(ClO(4))(4). Synthesis and molecular and crystal structure analysis.

Chlorine trioxide, Cl(2)O(6), reacts with Au metal, AuCl(3), or HAuCl(4).nH(2)O to yield the well-defined chloryl salt, ClO(2)Au(ClO(4))(4). The crystal and molecular structure of ClO(2)Au(ClO(4))(4) was solved by a Rietveld analysis of powder X-ray diffraction data. The salt crystallizes in a monoclinic cell, space group C2/c, with cell parameters a = 15.074(5), b = 5.2944(2), and c = 22.2020(2) A and beta = 128.325(2) degrees. The structure displays discrete ClO(2)(+) ions lying in channels formed by Au(ClO(4))(4)(-) stacks. Au is located in a distorted square planar environment: Au-O = 1.87 and 2.06 A. [ClO(4)] groups are monodentate with ClO(b) = 1.53 and ClO(t) = 1.39 A (mean distances; O(b), oxygen bonded to Au; O(t), free terminal oxygen). A full vibrational study of the Au(ClO(4))(4)(-) anion is supported by DFT calculations.