▎ 摘　　要

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 (alpha-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 alpha-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, alpha-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 alpha-Co(OH)(2)//reduced graphene oxide aerogel (rGO(ae)) 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 alpha-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 alpha-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. (C) 2019 Elsevier Ltd. All rights reserved.