In contrast to EDLCs, pseudocapacit

In contrast to EDLCs, pseudocapacitive electrode materialsstore charge via Faradaic processes that involve fast, reversibleredox reactions at the surface or near surface of the activematerials. This mechanism is associated with a valence statechange of the electrode material as a result of electrontransfer.127−130 Ruthenium dioxide (RuO2) was the firstelectrode material reported to exhibit pseudocapacitivebehavior.29,68 Even though the charge storage from a chargetransfer reaction on an RuO2 thin film electrode is a type ofFaradaic reaction, the CV curve exhibits close to a rectangularshape, which is a typical capacitive feature.128 The termpseudocapacitance is formally used to identify electrodematerials whose electrochemical signatures are capacitive butcharge storage occurs by charge-transfer Faradaic reactionsacross the double layer.131 This process is Faradaic in originfrom the fast and reversible surface redox thermodynamicprocesses, but capacitance arises from the linear relationshipbetween the extent of charge (ΔQ) and the potential change(ΔU). The active centers that contribute to the pseudocapacitance are located near the surface of the metal oxides, at adistance, ≪(2Dt)1/2, where D is the diffusion coefficient forcharge-compensating ions (cm2/s) and t is the diffusion timerange (s).132 Energy storage involving pseudocapacitanceexhibits the intermediate electrochemical behavior betweenpure electrostatic EDLCs and solid-state diffusion dominated byFaradaic reactions in bulk battery-type materials.