École polytechnique de l'université de Nantes

About: École polytechnique de l'université de Nantes is a based out in . It is known for research contribution in the topics: Heat transfer & Thermal conductivity. The organization has 324 authors who have published 424 publications receiving 15658 citations.

Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor

Abstract: The charge storage mechanism in MnO2 electrode, used in aqueous electrolyte, was investigated by cyclic voltammetry and X-ray photoelectron spectroscopy. Thin MnO2 films deposited on a platinum substrate and thick MnO2 composite electrodes were used. First, the cyclic voltammetry data established that only a thin layer of MnO2 is involved in the redox process and electrochemically active. Second, the X-ray photoelectron spectroscopy data revealed that the manganese oxidation state was varying from III to IV for the reduced and oxidized forms of thin film electrodes, respectively, during the charge/discharge process. The X-ray photoelectron spectroscopy data also show that Na+ cations from the electrolyte were involved in the charge storage process of MnO2 thin film electrodes. However, the Na/Mn ratio for the reduced electrode was much lower than what was anticipated for charge compensation dominated by Na+, thus suggesting the involvement of protons in the pseudofaradaic mechanism. An important finding o...

Influence of Microstucture on the Charge Storage Properties of Chemically Synthesized Manganese Dioxide

Abstract: α-MnO2 was synthesized by a very simple coprecipitation technique and tested as active electrode material for an electrochemical supercapacitor. The powder presents a poorly crystallized cryptomelane phase with a chemical composition of K0.05MnO2H0.10·0.15H2O. Different aqueous electrolytes were tested including 0.1 M Na2SO4, 0.5 M K2HPO4/KH2PO4 buffer solution, 0.3 M H2SO4, and 1 M NaOH, but interesting pseudocapacitance behavior was only observed in the case of 0.1 M Na2SO4. Further testing using this electrolyte showed that an average capacitance of 166 F/g can be reproducibly obtained within a voltage range −0.4/+0.5 V vs Hg/Hg2SO4 using a sweep rate of 2 mV/s. This interesting value is mainly due to the chimisorption of Na+ ions and/or protons at the surface of the α-MnO2 electrode. Nearly all the Mn surface atoms are involved in the pseudocapacitive process. Therefore, the high specific capacitance seems to be related to the high surface area of the MnO2 powder rather than intercalation of Na+ ions ...

Crystalline MnO2 as Possible Alternatives to Amorphous Compounds in Electrochemical Supercapacitors

Abstract: Manganese dioxide compounds with various structures were synthesized and tested as "bulk" composite electrodes for electrochemical capacitors. The capacitance of the set of MnO 2 compounds having Brunauer-Emmett-Teller (BET) surface areas larger than 125 in 2 g -1 reached a maximum value of about 150 F g -1 . The capacitance of all amorphous compounds (except one) is due to faradaic processes localized at the surface and subsurface regions of the electrode. Further increasing the surface area does not provide additional capacitance. The capacitance of the crystallized materials is clearly dependent upon the crystalline structure, especially with the size of the tunnels able to provide limited cations intercalation. Thus, the 2D structure of birnessite materials gives an advantage to obtain relatively high capacitance values (110 F g -1 ) considering their moderate BET surface area (17 m 2 g -1 ). ID tunnel structure such as γ or β-MnO 2 is characterized by only a pseudofaradic surface capacitance and therefore relies on the BET surface area of the crystalline materials. 3D tunnel structure such as λ-MnO 2 shows some intermediate behavior between bimessite and ID tunnel structures.

Nanostructured transition metal oxides for aqueous hybrid electrochemical supercapacitors

Abstract: In this paper, we wish to present an overview of the research carried out in our laboratories with low-cost transition metal oxides (manganese dioxide, iron oxide and vanadium oxide) as active electrode materials for aqueous electrochemical supercapacitors. More specifically, the paper focuses on the approaches that have been used to increase the capacitance of the metal oxides and the cell voltage of the supercapacitor. It is shown that the cell voltage of an electrochemical supercapacitor can be increased significantly with the use of hybrid systems. The most relevant associations are Fe3O4 or activated carbon as the negative electrode and MnO2 as the positive. The cell voltage of the Fe3O4/MnO2 device is 1.8 V and this value was increased to 2.2 V by using activated carbon instead of Fe3O4. These two systems have shown superior behavior compared to a symmetric MnO2/MnO2 device which only works within a 1 V potential window in aqueous K2SO4. Furthermore, the activated carbon/MnO2 hybrid device exhibits a real power density of 605 W/kg (maximum power density =19.0 kW/kg) with an energy density of 17.3 Wh/kg. These values compete well with those of standard electrochemical double layer capacitors working in organic electrolytes.