Effects of Impurities on the Electrochemical Characterization of Liquid-Phase Exfoliated Niobium Diselenide Nanosheets

Commercially available bulk niobium diselenide (NbSe2) reduced into nanomaterials upon exfoliation typically contains oxide and carbide impurities. Liquid-phase exfoliated two-dimensional (2D) nanosheets of NbSe2 obtained from bulk powders provide high charge mobility and large surface area but become self-passivated and chemically inert as the presence of oxide impurities makes them behave more semimetallic. In this article, we report the effects of inherent impurities of liquid-phase exfoliated 2D NbSe2 (intended to be integrated as supercapacitor electrodes) on the electrochemical performance. The highest specific capacitances achieved using 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) and sulfuric acid (1 M H2SO4) electrolytes were 4955.5 +/- 21.5 and 13 361.6 +/- 31.8% mF/cm(2), respectively, which were affected by the impurities, oxophilicity of niobium defects, and moisture adsorption in the cell. Galvanostatic charge-discharge profiles show moisture adsorption affecting the high-energy charging procedure in the cell for BMIMPF6, resulting in leakage and decomposition of the electrolyte. Electrochemical impedance spectroscopy provides insights into the solid-electrolyte interphase and charge-transfer mechanisms at exfoliated 2D NbSe2 nanosheets, which affect the ion intercalation through heterogenous phases of the nanosheets. Overall, the NbSe2 nanosheets offer heterogenous phases because of the coexistence of Nb2O5 that influences the charge-transfer mechanism at the exfoliated surfaces.