Fabrication of Mo1.33CTz (MXene)–cellulose freestanding electrodes for supercapacitor applications

Abstract

MXenes are two-dimensional (2D) transition metal carbides/nitrides with high potential for energy storage devices owing to their high flexibility, conductivity and specific capacitance. However, MXene films tend to suffer from diffusion limitation of ions within the film, and thus their thickness is commonly reduced to a few micrometers (mass loadings <1 mg cm⁻²). Herein, a straightforward one-step protocol for synthesizing freestanding Mo_1.33CT_z–cellulose composite electrodes with high MXene loading is reported. By varying the amount of the cellulose content, a high gravimetric capacitance (up to 440 F g⁻¹ for 45 wt% cellulose content, ∼5.9 μm thick film) and volumetric capacitance (up to 1178 F cm⁻³ for 5 wt% cellulose content, ∼4.8 μm thick film) is achieved. These capacitance values are superior to those for the pristine MXene film, of a similar MXene loading (1.56 mg cm⁻², ∼4.2 μm thick film), delivering values of about 272 F g⁻¹ and 1032 F cm⁻³. Interestingly, the Mo_1.33CT_z–cellulose composite electrodes display an outstanding capacitance retention (∼95%) after 30 000 cycles, which is better than those reported for other Mo_1.33CT_z-based electrodes. Furthermore, the presence of cellulose inside a thick composite electrode (∼26 μm, MXene loading 5.2 mg cm⁻²) offers a novel approach for opening the structure during electrochemical cycling, with resulting high areal capacitance of about 1.4 F cm⁻². A symmetric device of Mo_1.33CT_z–cellulose electrodes featured a long lifespan of about 35 000 cycles and delivered a device capacitance up to 95 F g⁻¹. The superior performance of the Mo_1.33CT_z–cellulose electrodes in terms of high gravimetric, volumetric, and areal capacitances, long lifespan, and promising rate capability, paves the way for their use in energy storage devices.