Electrostatic self-assembly of MXene and carbon nanotube@MnO2 multilevel hybrids for achieving fast charge storage kinetics in aqueous asymmetric supercapacitors

Abstract

Nanostructured birnessite (δ-MnO₂) has a high specific capacitance and virtually perfect capacitive behaviors as an essential electrode material for high-power energy storage devices. However, simultaneously achieving excellent capacitive properties and adequate structural stability is particularly challenging. Herein, a conductive and freestanding pseudocapacitive electrode (Ti₃C₂T_z/CNT@MnO₂ abbreviated as TCM) is fabricated by electrostatically assembling Ti₃C₂T_z nanosheets and δ-MnO₂in situ grown on carbon nanotubes (CNT@MnO₂). Benefitting from the unique structure, strong interfacial interactions and synergistic effects between Ti₃C₂T_z nanosheets and CNT@MnO₂, the TCM electrode shows a high capacitance value (384 F g⁻¹ at 0.5 A g⁻¹), excellent rate capability, and superior stability (92.2% retention after 10 000 cycles). The outstanding capacitive charge storage of TCM originates from reversible Na⁺ intercalation/deintercalation, according to electrochemical quartz crystal microbalance (EQCM) and in situ Raman spectroscopy. Remarkably, an assembled asymmetric supercapacitor (ASC) based on the TCM film and nitrogen-doped reduced graphene oxide (NRGO) delivers landmark energy/power densities (44 W h kg⁻¹ and 43.4 kW kg⁻¹) and ultra-long cycle performance. Furthermore, the ASC exhibits a high voltage of 2.4 V with an impressive energy density of 58 W h kg⁻¹ in 10 M NaClO₄ salt-in-water electrolyte. This work offers a vital insight and clues to engineering stable birnessite materials for aqueous supercapacitors.