Pseudocapacitance

Pseudocapacitance is an electrochemical energy storage mechanism in which a significant portion of the stored charge arises from fast, reversible faradaic (redox) reactions at or near the surface of an electrode, contributing to capacitance beyond that produced by electric double-layer effects alone.

Pseudocapacitance can be broadly categorized as surface or near-surface redox (redox pseudocapacitance) and intercalation pseudocapacitance, where ions intercalate into the electrode’s lattice with rapid charge transfer. In surface redox processes, charge is stored through fast electron transfer involving adsorbed species or surface redox centers. Intercalation pseudocapacitance involves rapid ion insertion into the material structure with redox activity.

Common pseudocapacitive materials include transition metal oxides such as RuO2 and MnO2, conducting polymers such as polyaniline and polypyrrole, and more recently MXenes and related layered compounds. These materials enable fast, reversible electron transfer and/or rapid ion exchange at high rates, contributing additional capacitance beyond the electric double layer.

Devices that utilize pseudocapacitance typically exhibit higher capacitance and energy density than purely electric double-layer capacitors while maintaining high power capability. Performance depends on factors such as voltage window, electrolyte, and material stability. The current–voltage response is governed by surface redox kinetics rather than the diffusion-limited processes more typical of batteries.

Characterization methods include cyclic voltammetry, galvanostatic charge-discharge, and impedance spectroscopy, which help distinguish pseudocapacitive behavior from electric double-layer charging and battery-like processes. Pseudocapacitors are of interest for applications requiring rapid charge–discharge with higher energy density than conventional capacitors, such as fast-charging electronics, electric vehicles, and grid storage.