Electrostatic Self‐Assembly of Ti ₃ C ₂ T _x MXene /Cellulose Nanofiber Composite Films for Wearable Supercapacitor and Joule Heater

The titanium carbide nanosheets (MXene) hold great potential for fabricating high‐performance electronics due to their two‐dimensional layered structure, high electrical conductivity, and versatile surface chemistry. However, assembling the small MXene nanosheets into flexible macroscopic films for wearable electronics still remains a challenge. Herein, we report the hierarchical assembling of MXene nanosheets and cellulose nanofibers into high‐performance composite films via an electrostatic self‐assembly strategy induced by polyethyleneimine. Benefited from the nacre‐like microstructure of MXene “bricks” and cellulose nanofibers “mortars” interlocked by polyethyleneimine via hydrogen bonding and electrostatic interaction, composite films possess integrated superior flexibility, high tensile strength, and stable electrical conductivity, which are advantageous for wearable electronic applications. To provide a proof‐of‐concept design, a symmetric quasi‐solid‐state supercapacitor with the as‐prepared composite film as electrode is fabricated, which exhibits a specific capacitance of 93.9 mF cm −2 at a current density of 0.1 mA cm −2 and almost constant capacitive behavior under different bending states. In addition, the composite film possesses capacities of electrothermal conversion and complete degradation in a hydrogen peroxide solution. These results demonstrate that the electrostatically self‐assembled composite films hold great promise in the development of highly flexible, mechanically robust, and environmentally friendly energy storage and conversion devices.