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 ͑BET͒ surface areas larger than 125 m 2 g −1 reached a maximum value of about 150 F g −1 . The capacitance of all amorphous compounds ͑except one͒ 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 ͒. 1D tunnel structure such as ␥or ␤-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 -MnO 2 shows some intermediate behavior between birnessite and 1D tunnel structures.

This paper provides new insight on the charge storage mechanism in crystallized Mg-doped sodium birnessite-type manganese dioxide used as active supercapacitor electrode material that involves the intercalation/deintercalation of cations between the sheets during electrochemical reduction/oxidation, respectively. An increase of the interlayer spacing from 0.710 to 0.720 nm upon electrochemical oxidation in the presence of Na+ cations in the electrolyte is associated with the deintercalation of Na+ and the intercalation of H2O between the layers. A progressive crystallinity loss of the material is also observed upon potential cycling between the oxidized and reduced states. Despite these structural changes, the birnessite electrode exhibits a stable capacitance of 145 F/g over 1100 cycles.

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International audienceNa-doped Birnessite-type manganese oxide (δ-MnO2) has been synthesized using the chemical method and characterized through X-ray diffraction and SEM, showing the lamellar structure and high crystal structure. A comparative study of the electrochemical performances of this material with those of the commercial Cryptomelane-type MnO2 has then been undertaken in ten neutral aqueous electrolytes for supercapacitor applications. Aqueous electrolytes, containing a lithium salt, LiX (where X = SO42-, NO3-, CH3CO2-, CH3SO3-, ClO4-, C7H15CO2-, TFSI-, Beti-, BOB-, or Lact-), have been first prepared under neutral pH conditions to reach the salt concentration, providing the maximum in conductivity. Their transport properties are then investigated through conductivities, viscosities, and self-diffusion coefficient measurements. Second, the thermal behaviors of these electrolytic aqueous solutions are then evaluated by using a differential scanning calorimeter from (213.15 to 473.15) K in order to access their liquid range temperatures. Cyclic voltammograms (CV) in three electrode configurations are thereafter investigated using Na Birnessite and Cryptomelane as working electrode material from (−0.05 to 1.5) V versus Ag/AgCl at various sweep rates from (2 to 100) mV*s-1. According to anion nature/structure and manganese oxide material type, different CV responses are observed, presenting a pure capacitive profile for Beti- or C6H13CO2- and an additional pseudocapacitive signal for the smallest anions, such as ClO4- and NO3-. The capacitances, energies, and efficiencies are finally calculated. These results indicate clearly that electrolytes based on a mineral lithium salt under neutral pH condition and high salt concentration (up to 5 mol*L-1) have better electrochemical performances than organic ones, up to 1.4 V with good material stability and capacity retention. The relationship between transport properties, electrostatic and steric hindrance considerations of hydrated ions, and their electrochemical performances is discussed in order to understand further the lithium intercalation-deintercalation processes in the lamellar or tunnel structure of investigated MnO2

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Anodic electrodeposition of layered manganese oxides has been carried out in a MnSO 4 solution heated at various bath temperatures from 25 to 80°C. The crystallinity of the deposits increased with an increase in the bath temperature. Although Co 2+ ions were not involved in the deposition reaction at room temperature, at the elevated temperature the addition of Co 2+ gave rise to a catalytic current suggesting the formation of Co/Mn mixed oxide. No new X-ray diffraction peaks appeared with the insertion of Co 2+ ions in the structure, but their addition caused broadening of the peaks due to the Mn dioxide phase. This indicates that Co ions are not accommodated in the interlayer space but are well dispersed within the MnO 2 layers composed of edge-shared MnO 6 octahedra. The Co-doped Mn oxide film thus obtained exhibited much better pseudocapacitive performance than the undoped counterpart.Pseudocapacitors store energy by utilizing the pseudocapacitance based on the first and reversible faradaic redox reactions of electroactive materials having multiple oxidation states, such as transitionmetal oxides and conducting polymers. They are fundamentally different from electrical double-layer capacitors arising from the nonfaradaic charge separation at the electrode/electrolyte interface, focusing on carbon materials. Of all the metal oxides investigated as pseudocapacitor materials, hydrated ruthenium oxide is considered as the most promising material offering high specific capacitance ͑760 F g −1 ͒ with excellent cyclability. 1 However, ruthenium is a very expensive and toxic material, and the capacitor requires a strongly acidic electrolyte ͑such as sulfuric acid͒, which makes it inadequate for a wide range of commercial applications. Manganese oxides, characterized by low price, facile synthesis, rich abundance of Mn elements, and environmental compatibility, can serve as a low cost replacement for RuO 2 . The capacitive behavior of amorphous Mn oxide was first recognized about 10 years ago by Lee and Goodenough. 2 Since then, intensive research effort has been devoted to develop the synthetic methods for MnO 2 -based materials with good pseudocapacitance, including oxidation of Mn 2+ , 2 reduction of MnO 4 − , 3 sol-gel method, 4 and anodic deposition. 5 Capacitive properties of Mn oxides critically depend on their morphologies, crystalline structures, valence states, and defect chemistry. Brousse et al. 6 and Devaraj and Munichandraiah 7 examined exhaustively the relationship between crystalline structures and their pseudocapacitive properties. As a conclusion, both research groups pointed out that birnessite, a two-dimensional layered structure with an interlayer distance of ϳ0.7 nm, comprised of edge-shared MnO 6 octahedra and the intercalated alkaline cations, has higher electrochemical performance than one-and three-dimensional crystals. This feature can be ascribed to the bicontinuous networks of solid and pores in birnessite, allowing both electrons and cations to move fast and reversibly. 8 In such a layere...