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The storage of electrochemical energy in battery, "supercapacitor," and double-layer capacitor devices is considered. A comparison of the mechanisms and performance of such systems enables their essential features to be recognized and distinguished, and the conditions for transition between supercapacitor and "battery" behavior to be characterized. Supercapacitor systems based on two-dimensional underpotential deposition reactions are highly reversible and their behavior arises from the pseudocapacitance associated with potential-dependence of two-dimensional coverage ofe]ectroactive adatoms on an electrode substrate surface. Such capacitance can be 10-100 times the double-layer capacitance of the same electrode area. An essential fundamental difference from battery behavior arises because, in such systems, the chemical and associated electrode potentials are a continuous function of degree of charge, unlike the thermodynamic behavior of single-phase battery reactants. Quasi-two-dimensional systems, such as hyperextended hydrous RuO2, also exhibit large pseudocapacitance which, in this case, is associated with a sequence of redox processes that are highly reversible. Such oxide redox systems give rise to the best supercapacitor behavior and capacitances of farads per gram can be achieved. Other examples are the conducting polymer electrodes and Li intercalate systems. These systems provide examples of the transition between battery and supercapacitor behavior arising from a range of degrees of oxidation/ reduction that arise over an appreciable range of potentials. The impedance behavior of an RuO2 supercapacitor is illustrated but is far from that expected for an electrostatic capacitor.

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