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 of this work is that, unlike thin film electrodes, no change of the manganese oxidation state was detected for a thicker composite electrode because only a very thin layer is involved in the charge storage process.

There are an increasing number of studies regarding active electrode materials that undergo faradaic reactions but are used for electrochemical capacitor applications. Unfortunately, some of these materials are described as "pseudocapacitive" materials despite the fact that their electrochemical signature (e.g., cyclic voltammogram and charge/discharge curve) is analogous to that of a "battery" material, as commonly observed for Ni(OH) 2 and cobalt oxides in KOH electrolyte. Conversely, true pseudocapacitive electrode materials such as MnO 2 display electrochemical behavior typical of that observed for a capacitive carbon electrode. The difference between these two classes of materials will be explained, and we demonstrate why it is inappropriate to describe nickel oxide or hydroxide and cobalt oxide/hydroxide as pseudocapacitive electrode materials.

R-MnO 2 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 K 0.05 MnO 2 H 0.10 ‚0.15H 2 O. Different aqueous electrolytes were tested including 0.1 M Na 2 SO 4 , 0.5 M K 2 HPO 4 /KH 2 PO 4 buffer solution, 0.3 M H 2 SO 4 , and 1 M NaOH, but interesting pseudocapacitance behavior was only observed in the case of 0.1 M Na 2 SO 4 . 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/Hg 2 SO 4 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 R-MnO 2 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 MnO 2 powder rather than intercalation of Na + ions and/or protons in the structure of R-MnO 2 . An optimum composition of 80% of active material in the composite electrode was determined. With such a composition, the R-MnO 2 electrode can withstand 1000 cycles with 100% capacitance retention.

The push towards miniaturized electronics calls for the development of miniaturized energy-storage components that can enable sustained, autonomous operation of electronic devices for applications such as wearable gadgets and wireless sensor networks. Microsupercapacitors have been targeted as a viable route for this purpose, because, though storing less energy than microbatteries, they can be charged and discharged much more rapidly and have an almost unlimited lifetime. In this Review, we discuss the progress and the prospects of integrated miniaturized supercapacitors. In particular, we discuss their power performances and emphasize the need of a three-dimensional design to boost their energy-storage capacity. This is obtainable, for example, through self-supported nanostructured electrodes. We also critically evaluate the performance metrics currently used in the literature to characterize microsupercapacitors and offer general guidelines to benchmark performances towards prospective applications.

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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