Enhancing Faradaic Charge Storage Contribution in Hybrid Pseudocapacitors

Abstract

This study aims to provide metrics and design rules for hybrid pseudocapacitors consisting of a transition metal oxide pseudocapacitive electrode and an inert carbon electrode with asymmetric and binary electrolyte. Hybrid pseudocapacitors are often limited by intercalation of Li+ ions in the pseudocapacitive electrode. Then, during cyclic voltammetry, only a portion of the potential window is dominated by faradaic current while the electric double layer (EDL) formation and dissolution dominate the rest. However, faradaic reactions result in a significantly larger current magnitude and capacitance than EDL charge storage. Hence, it is beneficial to extend the fraction of the potential window dominated by the faradaic current. This fraction increases with a combination of thinner pseudocapacitive electrode, faster intercalation, and slower scan rate. To study the interplay between these variables, a scaling analysis was performed to identify the relevant dimensionless similarity parameters governing Li+ transport and intercalation in the pseudocapacitive electrode. The fraction of the potential window dominated by faradaic reactions was a unique function of a dimensionless parameter Πf accounting for the respective contributions of the electrode thickness, Li+ diffusion coefficient, and scan rate. Above a critical value of Πf, the faradaic current dominated the entire potential window. Then, the device was no longer limited by intercalation in the electrode and the performance of the electrode was maximal.