A Review on Pseudocapacitors: Advances in Nb Oxide - Sodium-Ion Systems

Abstract

Pseudocapacitive supercapacitors have emerged as an important alternative to storage electrochemical energy. Among the several possible configurations of material for electrodes and electrolyte composition, the combination of oxides containing niobium and electrolytes based on sodium-ion has been presented as a very promising set. This review summarizes the main advances in the development of supercapacitors that use the Nb oxide - sodium-ion system. The electrochemical energy storage mechanisms are described and the influence of the type of electrolyte (aqueous or non-aqueous) is discussed. It was possible to verify that non-aqueous electrolytes are widely more used to assemble the Nb oxide - sodium-ion arrangements. For these systems the energy storage is controlled by the mechanism of intercalation/deintercalation of sodium-ions in the oxide structure. Despite non-aqueous electrolytes exhibit the advantage of operating in a wider window potential, they have disadvantages such as low electrical conductivity and sluggish Na+ kinetics. To overcome these aspects, works in the field have generally focused on improving the properties of the oxides, especially concerning its conductivity through core@shell systems, composites or doping. On the other hand, few studies were found in the literature concerning the Nb oxide - sodium-ion systems that use aqueous electrolytes. Nevertheless, these works showed promising results such as an expansion of the potential window usually used in aqueous electrolytes or the possibility to apply the Nb oxide as cathode or anode.