Application of aprotic ionic liquids based on bis(trifluoromethylsulfonyl)imide anion as polymer gel electrolytes for cobalt oxide symmetric supercapacitors

Abstract

Mesoporous Co3O4 nanowires were uniformly deposited onto Ni foam by using a facile hydrothermal route. The as-prepared Co3O4@Ni materials were applied as electrodes for symmetric supercapacitors. The electrochemical performances of Co3O4@Ni were investigated in the presence of aprotic ionic liquids (ILs) and compared to KOH electrolyte. The selected ILs are composed by the same bis(trifluoromethylsulfonyl)imide (TFSI−) anion, paired with 1-Allyl-3-methylimidazolium (AMIM+) or 1-Ethyl-3-methylimidazolium (EMIM+) cations. The electrolytes composed by KOH and ILs were prepared in a polymer gel form. Independently on the electrolyte nature, the profiles of cyclic voltammograms and galvanostatic charge/discharge curves are indicative of an electrochemical double-layer charge storage mechanism. Combining our Co3O4@Ni electrodes with selected ILs polymer gel electrolytes showed good electrochemical stabilities with 85% and 97% retentions of the initial capacitances after 10,000 cycles in [AMIM-TFSI] and [EMIM-TFSI], respectively. At a current rate of 0.2 A g−1, the lowest values of specific energy (2.8 Wh kg−1) and specific power (201.9 W kg−1) were exhibited in the presence of KOH based gel electrolyte. This poor electrochemical performance was considerably improved by using aprotic ILs. For [AMIM-TFSI] and [EMIM-TFSI] based polymer gel electrolytes, high specific powers up to 811 and 680 W kg−1 were delivered at 0.2 A g−1 with enhanced specific energies of 29.4 and 23.3 Wh kg−1, respectively. Hence, the specific energies of the devices assembled by PVDF-HFP/AMIM-TFSI or PVDF-HFP/EMIM-TFSI polymer gel electrolytes were 8 to 10 times higher than that of the device assembled by PVA/KOH gel electrolyte.