Transition from 'supercapacitor' to 'battery' behavior in electrochemical energy storage

Abstract

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.