Surface modified MXene film as flexible electrode with ultrahigh volumetric capacitance

In recent years, the rapid development of portable equipment and flexible wearable devices urgently requires supercapacitor electrode material with high volumetric capacitance. Ti₃C₂T_x, a member of MXenes family, is one of the most promising supercapacitor electrode material candidates. However, how to further enhance the volumetric capacitance of the Ti₃C₂T_x is still a great challenge at present. In this paper, a simple and eco-friendly method is presented by preparing flexible Ti₃C₂T_x supercapacitor electrode using self-assembly of few-layer Ti₃C₂T_x followed by alkalinizing and annealing treatment. The alkalinized and annealed Ti₃C₂T_x film (denoted as ak-Ti₃C₂T_x film-A) exhibits an ultrahigh volumetric capacitance of 1805 F cm⁻³ at 1 A g⁻¹, and an impressive cycling stability up to 98% capacitance retention after 8000 cycles. The significant improvement in volumetric capacitance performance is mainly due to the increase of redox-active sites exposed by removal of surface terminal groups during alkalinizing and annealing, and the enhancement in conductivity of the film owing to the annealing induced increase of crystalline order. Moreover, the supercapacitor fabricated with ak-Ti₃C₂T_x film-A displays an outstanding volumetric energy density of 45.2 Wh L⁻¹. This work provides a new strategy for design and development of other MXenes-based materials with high volumetric capacitance performance for flexible and highly integrated supercapacitors.