Pseudokapacitance

Pseudocapacitance is a phenomenon observed in electrochemical systems that mimics the behavior of a capacitor but arises from faradaic (redox) processes rather than electrostatic charge accumulation. Unlike ideal double-layer capacitance, where charge is stored at the interface between an electrode and an electrolyte, pseudocapacitance involves rapid and reversible surface or near-surface redox reactions.

These redox reactions contribute to the charge storage mechanism. When a potential is applied, ions from the electrolyte intercalate or adsorb onto the electrode material, undergoing oxidation or reduction. This charge transfer is accompanied by a change in the electrode's oxidation state, effectively storing charge. The rate of these faradaic reactions is typically very fast, allowing for rapid charge and discharge cycles, similar to true capacitors.

Materials that exhibit pseudocapacitance often include transition metal oxides, sulfides, and conducting polymers. Examples include ruthenium dioxide (RuO2), manganese dioxide (MnO2), and polyaniline. The charge storage in these materials is a function of both the amount of active material and the kinetics of the redox reactions.

Pseudocapacitance is distinguished from battery-type charge storage by the reversibility and speed of the faradaic processes. While batteries involve bulk solid-state diffusion and phase changes, pseudocapacitive charge storage is generally limited to the surface or near-surface regions of the electrode material and occurs much faster. This makes pseudocapacitive materials attractive for applications requiring high power density, such as supercapacitors, hybrid energy storage devices, and electrochemical sensors. The energy density of pseudocapacitors is typically higher than that of electric double-layer capacitors but lower than that of batteries.