Flexible Free‐Standing MoO3/Ti3C2Tz MXene Composite Films with High Gravimetric and Volumetric Capacities

Abstract Enhancing both the energy storage and power capabilities of electrochemical capacitors remains a challenge. Herein, Ti3C2Tz 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 Ti3C2Tz flakes bridge the nanobelts, facilitating electron transfer; the randomly oriented, and interconnected, MoO3 nanobelts, in turn, prevent the restacking of the Ti3C2Tz nanosheets. Benefitting from these advantages, a MoO3/Ti3C2Tz 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/Ti3C2Tz 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.


Procedures to calculate C g and C v :
The electrode's gravimetric specific capacity, C g , was calculated from the galvanostatic charge/discharge (GCD) curves according to the following equation: where I is the discharge current, Δt is the discharging time, and m is the mass of the active materials in the electrodes. Note here no binders or conducting additives were used in any of the MXene or MoO 3 containing films.
The volumetric capacity (C v ) was obtained from the following equation: where ρ is the density of the films, which was calculated assuming: where m, S and d are the mass, area and thickness of the electrode, respectively.
For the hybrid capacitor devices, the mass ratio of the negative to positive electrodes was decided based on charge balance theory (q + =q -). The charge stored (q) by each electrode was estimated assuming: The gravimetric energy, E g , and power, P g , densities were calculated assuming: Where U is the output voltage of the ASC devices.
The volumetric energy, E v , and power densities, P v , were calculated assuming:
The self-discharge rate in this work is compared with others shown in Figure S30.