Identifying pseudocapacitance of Fe2O3 in an ionic liquid and its application in asymmetric supercapacitors

Abstract

We identify the pseudocapacitive behaviour of Fe ₂O ₃ in an ionic liquid electrolyte, and build high-performance asymmetric supercapacitors by utilizing such pseudocapacitance.

Abstract

Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions, as observed with transition metal oxides in alkaline aqueous electrolytes. Here, we demonstrate that pseudocapacitive behavior of Fe ₂O ₃ can occur in a 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF ₄) ionic liquid (IL), and it is closely related to the chemical state variation between Fe ³⁺ and Fe ²⁺ on the surface of a Fe ₂O ₃ electrode during the charging/discharging process. By taking advantage of such pseudocapacitance, we prepared a promising electrode material, i.e., graphene nanosheet-supported Fe ₂O ₃ nanoparticles (denoted as Fe ₂O ₃@GNS), and then built high-performance asymmetric supercapacitors (ASs) using Fe ₂O ₃@GNS as the battery-type electrode material, commercial activated carbon (AC)/or activated polyaniline-derived carbon nanorods (denoted as APDC) as the capacitor-type electrode material, and EMIMBF ₄ IL as the electrolyte. The as-made ASs are able to work reversibly in a full operation voltage region of 0–4 V and exhibit very high energy density. Especially, the AS of Fe ₂O ₃@GNS//APDC achieves an extremely high energy density of 177 W h kg ⁻¹ and shows a superior combination of high energy and power density (the energy density still remains 62.4 W h kg ⁻¹ even at a high power density of 8 kW kg ⁻¹).