Imidazolium-based ionic liquids as supercapacitor electrolytes and the effect of copper (I) and (II) oxides nanoparticles along with MWCNT addition to electrode materials

This work investigates the electrochemical performance of five imidazolium-based ionic liquids as electrolytes for supercapacitors across a wide temperature range and examines the effect of incorporating nanomaterials into the electrode matrix. Copper(I) oxide (Cu₂O) and copper(II) oxide (CuO) nanoparticles were synthesized via thermal pyrolysis and characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Initially, supercapacitors with activated carbon electrodes were tested using 1-Hexyl-3-methylimidazolium bromide ([HMIM][Br]), 1-Butyl-3 Methylimidazolium Chloride ([BMIM][Cl]), 1-Hexyl-3-methylimidazolium Thiocyanate ([HMIM][SCN]), 1-Hexyl-3-methylimidazolium Chloride ([HMIM][Cl]), and 1-Octyl-3 Methylimidazolium Chloride ([OMIM][Cl]) as electrolytes. Direct current (DC) methods were employed to determine equivalent series resistance, parallel resistance, capacitance, and potential window. The results revealed that increasing temperature generally enhanced specific capacitance while reducing both series and parallel resistance. Among the tested electrolytes, [OMIM][Cl] demonstrated the best performance, exhibiting the highest energy (22.91 W.h. kg⁻¹ at 0°C) and power density (64.02 kW. Kg⁻¹ at 75°C). In the second phase of the study, [OMIM][Cl] was selected as the electrolyte, and multi-walled carbon nanotubes (MWCNTs), CuO, and Cu₂O nanoparticles were incorporated into the electrode material. The CuO/MWCNT-based device exhibited the highest specific capacitance, lowest series resistance, and highest parallel resistance. Meanwhile, the supercapacitor with Cu₂O nanoparticles achieved the widest voltage range, resulting in the highest energy (32.00 W.h. kg⁻¹ at 28°C) and power (33.44 kW. Kg⁻¹ at 75°C) density among nanoparticle-containing supercapacitors. These findings demonstrate the synergistic potential of imidazolium-based ionic liquids and copper oxide nanomaterials in enhancing the energy storage capabilities of next-generation supercapacitors.