Flexible Free-Standing MoO₃/Ti₃C₂T _z MXene Composite Films with High Gravimetric and Volumetric Capacities

Enhancing both the energy storage and power capabilities of electrochemical capacitors remains a challenge. Herein, Ti₃C₂T _z MXene is mixed with MoO₃ nanobelts in various mass ratios and the mixture is used to vacuum filter binder free, open, flexible, and free-standing films. The conductive Ti₃C₂T _z flakes bridge the nanobelts, facilitating electron transfer; the randomly oriented, and interconnected, MoO₃ nanobelts, in turn, prevent the restacking of the Ti₃C₂T _z nanosheets. Benefitting from these advantages, a MoO₃/Ti₃C₂T _z film with a 8:2 mass ratio exhibits high gravimetric/volumetric capacities with good cyclability, namely, 837 C g^-1 and 1836 C cm^-3 at 1 A g^-1 for an ≈ 10 µm thick film; and 767 C g^-1 and 1664 C cm^-3 at 1 A g^-1 for ≈ 50 µm thick film. To further increase the energy density, hybrid capacitors are fabricated with MoO₃/Ti₃C₂T _z films as the negative electrodes and nitrogen-doped activated carbon as the positive electrodes. This device delivers maximum gravimetric/volumetric energy densities of 31.2 Wh kg^-1 and 39.2 Wh L^-1, respectively. The cycling stability of 94.2% retention ratio after 10 000 continuous charge/discharge cycles is also noteworthy. The high energy density achieved in this work can pave the way for practical applications of MXene-containing materials in energy storage devices.