(Invited) Interfacial and Transport Phenomena in Hybrid Pseudocapacitors

Electrochemical capacitors have attracted significant attention in recent years due to their promise as electrical energy storage devices for high energy and power applications. They are typically classified as either electric double layer capacitors (EDLCs) or pseudocapacitors depending on the energy storage mechanism. EDLCs store energy physically in the electric double layers (EDLs) forming near the electrode/electrolyte interfaces. Thus, EDLCs feature highly reversible charge storage and long cycle life. However, their energy density remains low compared with that of batteries. On the other hand, pseudocapacitors store energy via the electric double layers as well as via reversible oxidation-reduction (redox) reactions with or without insertion or intercalation. Pseudocapacitors tend to feature larger capacitances and energy densities than EDLCs because the amount of charge they can store is not limited by the surface area of the electrode/electrolyte interface. Finally, hybrid pseudocapacitors are devices combining a pseudocapacitive electrode made of a porous transition metal oxide (e.g., RuO2, MnO2, Nb2O5, MoO3) and an EDLC-type electrode typically made of porous carbon. This invited talk will present recent advances in physical modeling of interfacial, transport, and electrochemical phenomena in pseudocapacitors. It aims to provide physical interpretation of cyclic voltammograms obtained experimentally either from hybrid pseudocapacitors or from pseudocapacitive electrodes using three-electrode systems. First, it will present a continuum model simultaneously accounting for (i) charge transport in electrodes and electrolyte, (ii) the dynamics of the electric double layer, (iii) steric repulsion due to finite ion sizes, (iv) redox reactions, and (v) intercalation. Second, it will report selected numerical simulations of CV curves for different electrode thickness and electron and ion transport properties in the electrodes and electrolyte, representative of actual situations. These results will be used to elucidate the respective contributions of faradaic reactions and electric double layer formation to charge storage in pseudocapacitors. Particular attention will be paid to the so-called b-value characterizing the power law evolution of the total current with respect to scan rate for a given potential. Special effort will be made to systematically compare model predictions with experimental data.