Enhanced Pseudocapacitance of Ionic Liquid/Cobalt Hydroxide Nanohybrids

Abstract

Development of nanostructured materials with enhanced redox reaction capabilities is important for achieving high energy and power densities in energy storage systems. Here, we demonstrate that the nanohybridization of ionic liquids (ILs, 1-butyl-3-methylimidazolium tetrafluoroborate) and cobalt hydroxide (Co(OH)₂) through ionothermal synthesis leads to a rapid and reversible redox reaction. The as-synthesized IL-Co(OH)₂ has a favorable, tailored morphology with a large surface area of 400.4 m²/g and a mesopore size of 4.8 nm. In particular, the IL-Co(OH)₂-based electrode exhibits improvement in electrochemical characteristics compared with bare Co(OH)₂, showing a high specific capacitance of 859 F/g at 1 A/g, high-rate capability (∼95% retention at 30 A/g), and excellent cycling performance (∼96% retention over 1000 cycles). AC impedance analysis demonstrates that the introduction of ILs on Co(OH)₂ facilitates ion transport and charge transfer: IL-Co(OH)₂ shows a higher ion diffusion coefficient (1.06 × 10^–11 cm²/s) and lower charge transfer resistance (1.53 Ω) than those of bare Co(OH)₂ (2.55 × 10^–12 cm²/s and 2.59 Ω). Our density functional theory (DFT) calculations reveal that the IL molecules, consisting of anion and cation groups, enable easier hydrogen desorption/adsorption process, that is, a more favorable redox reaction on the Co(OH)₂ surface