Clarifying the origin of pseudocapacitance from a perspective of RuO2

Pseudocapacitance is dominantly considered to store charges through a unique faradic reaction (contributing to high energy density), while exhibiting electrochemical characteristics of supercapacitors (enabling high power density and excellent stability). It is still not fully understood why and how these two fundamentally distinct electrochemical processes, i.e., battery-like and capacitor-like, can be integrated into a single electrode. Conway et al. emphasized their faradic origin, whereas Costentin et al. emphasized their capacitive origin. In this Perspective, we pave a new paradigm for understanding their origin by correlating their physical structure with their electrochemical response. Specifically, we systematically review the charge storage process in hydrous RuO₂, a benchmark material for pseudocapacitance, with particular emphasis on its microstructure, the role of water molecules in proton transport, and its metallic nature, which enables unique redox reactions. Finally, we propose a conjunctive mechanism, revealing that the pseudocapacitive response likely originates from the gain and loss of electrons, without semiconductor bandgap limitations, and from ion interfacial transfer and bulk diffusion, free of kinetic limitations.