Transition from `supercapacitor´ to `battery´ behavior in electrochemical energy storage

The storage of electrochemical energy in battery supercapacitor, and double-layer capacitor devices is considered. Supercapacitor systems based on 2-dimensional underpotential deposition reactions are highly reversible and their behavior arises from the pseudocapacitance associated with potential-dependence of 2-dimensional coverage of electroactive adatoms on an electrode substrate surface. Such capacitance can be 10 to 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-2-dimensional systems, such as hyper-extended hydrous RuO₂ , also exhibit large pseudocapacitance. Other examples are the conducting polymer electrodes and Li intercalate systems. The impedance behavior of an RuO₂ supercapacitor is illustrated but is far from that expected for an electrostatic capacitor