Boosting the volumetric capacitance of MoO3-x free-standing films with Ti3C2 MXene

Highlights: • MoO_3-x/Ti₃C₂ MXene free-standing films show high volumetric capacitances with excellent cyclability. • The asymmetric supercapacitors delivered maximum energy density of 48.6 Wh L⁻¹ with outstanding output voltage (2.1 V). • The impressive electrochemical performance, low cost and green nature bode well for practical applications. -- Abstract: The high theoretical capacitance of molybdenum trioxide (MoO₃) renders it an attractive supercapacitor electrode material. However, its low electronic conductivity restricts charge transfer and slows its reaction kinetics. Herein, we vacuum filtered porous, free-standing, flexible and highly conductive films comprised of oxygen vacancy-rich MoO_3-x nanobelts and delaminated Ti₃C₂ MXene in a mass ratio of 80:20, respectively. When tested as supercapacitor electrodes, in a 5 M LiCl electrolyte, volumetric capacitances of 631 F cm⁻³ at 1 A g⁻¹, and 474 F cm⁻³ at 10 A g⁻¹ were obtained. To increase the energy density, asymmetric supercapacitors, wherein the anodes were MoO₃-based and the cathodes were nitrogen-doped activated carbon were assembled and tested. The resulting volumetric energy density was 48.6 Wh L⁻¹. After 20,000 continuous charge/discharge cycles at 20 A g⁻¹, 96.3 % of the initial charge remained. These values are outstanding for free-standing supercapacitor electrodes, especially in aqueous electrolytes.