Charge storage mechanisms of cobalt hydroxide thin film in ionic liquid and KOH electrolytes for asymmetric supercapacitors with graphene aerogel

Abstract

Layered 2D metal hydroxides and oxides with solvated ions inside their layers are well-known materials for energy storage applications. Although thin-film layered alpha-cobalt hydroxide (α-Co(OH)2) is recognized as one of the promising candidates among many other 2D materials, it still suffers from its poor stability in alkali solution i.e., 6 M KOH. In this work, the charge storage mechanism and the electrochemical performance of α-Co(OH)2 in a room-temperature ionic liquid (IL) namely 1-butyl-1-methyl-pyrrolidinium dicyanamide ionic liquid ([BMPyr+][DCA−] IL) were investigated and compared with those in 6 M KOH. Interestingly, α-Co(OH)2 in [BMPyr+][DCA−] IL maintains their capacity retention up to 95.9% after 10,000 cycles at 0.15 mA cm−2 for the half-cell evaluation. The asymmetric cell of thin-film α-Co(OH)2//reduced graphene oxide aerogel (rGOae) assembled in [BMPyr+][DCA−] IL electrolyte provides a high areal capacitance of 18.54 F cm−2 (28.6 F g−1) at 0.15 mA cm−2 as compared with other thin-film supercapacitors. The charge storage mechanism of α-Co(OH)2 in [BMPyr+][DCA−] IL investigated by in situ X-ray absorption spectroscopy (XAS) and ex situ X-ray photoelectron spectroscopy (XPS) confirms that [DCA−] can faradaically react with α-Co(OH)2 providing CoOOH and NH(CN)2. Understanding charge storage mechanism of 2D metal hydroxides may be useful for future development of energy storage technology.