SnO2/Na–SnO2@MXene hybrid electrode materials for supercapacitor applications

Abstract

Correlations among the restacking tendency of MXenes’ 2D layers, encapsulation of 2D MXenes with metal–oxide nanostructures, the effect of alkali metal loading, and electrochemical activities of MXenes are matters of debate and involve a deep understanding of their functionality and pseudocapacitive properties. Herein, MXene sheets were encapsulated with SnO₂ and Na–SnO₂ nanoparticles (NPs) and investigated for their structural, electronic, surface morphological, and electrochemical properties. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results revealed the formation of MXenes and SnO₂-based nanocomposite architectures. X-ray absorption spectroscopy (XAS) measurements, measured at the Sn M-edge and Ti L-edge, confirmed the presence of Sn⁴⁺ and Ti⁴⁺ ions in SnO₂@MXene and/or Na–SnO₂@MXene nanocomposites. A low concentration (∼1%) of Na loading in SnO₂ NPs or SnO₂@MXene nanocomposites facilitated supplementary redox features and, thus, offered nearly two times higher specific capacitance values than their bare counterparts. The log scan rate vs log peak current graphs unveiled a dominating surface-related charge storage mechanism in bare SnO₂ NPs. Na loading enabled an appreciable diffusion-controlled charge storage mechanism, surface-related charge storage in the Na–SnO₂@MXene nanocomposites, and a specific capacitance of 91.2 F g⁻¹ at a scan rate of 5 mV s⁻¹. The three-electrode cell of Na–SnO₂@MXene nanocomposites exhibited ∼89% retention for 3000 cycles. A two-electrode-based symmetric supercapacitor device, a Swagelok cell, was tested for the Na–SnO₂@MXene sample with 1 M KOH electrolyte and 2 V LED. The symmetric supercapacitor offered a high energy density of ∼75 W h kg⁻¹ (at a power density of 7500 W kg⁻¹) and a high-power density of 27 000 W kg⁻¹ (at an energy density of 30 W h kg⁻¹).