Confinement Effects on the Rate Performance of Redox Active Molecules for Pseudocapacitive Flowable Electrodes

The pseudocapacitive flowable electrodes typically show high energy density because of the contribution of the faradaic charge of redox-active organic materials and the electric double layer charge of carbon materials. However, the redox reaction kinetics of organic molecules are slow due to poor diffusion kinetics. We recently reported that a pseudocapacitive flowable electrode exhibited bell-shaped cyclic voltammograms (peak separation (ΔEp) = 0 mV); specifically, the molecules were confined within slit-shaped graphitic micropores of activated carbon (AC). Herein, we studied the relationship between charge storage and the reaction mechanism to tailor the electrochemical performance of a pseudocapacitive flowable electrode by half-cell study. The results show that the redox reaction of the confined molecules entailed a charge-transfer-controlled mechanism, while the unconfined molecules exhibited a mass-transfer-controlled system. This difference inhibited the fast charging and discharging of the pseudocapacitive flowable electrode. This study demonstrates that half-cell studies are crucial for clarifying the relationship between the charge storage and rate performances of pseudocapacitive flowable electrodes.