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.

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