Facile gravity-steered fabrication of free-standing bilayer composite films for flexible micro-supercapacitor enhancement

Optimizing the film fabrication process of active materials for micro-supercapacitors (MSCs) is crucial for enhancing their performance. However, improper control of ink viscosity and flow during the screen-printing process can result in poor print quality and film uniformity. Vacuum filtration methods also have limitations in film transfer and large-scale production. Here, a uniform composite film is constructed on a polymer flexible substrate via room-temperature evaporation. Exploiting differential sedimentation rates in a gravitational field, this process yields a bilayer structure where the active material forming the bottom layer (in contact with the substrate) and MXene forming the top layer. FESEM and XRD analyses verify the formation of a well-defined bilayer heterostructure. Moreover, FESEM images reveal that perforation by the underlying nanowires imparts a porous architecture to the top MXene layer, thereby accelerating ion transport. Benefiting from the unique bilayer heterogeneous structure, the optimized composite film electrodes (MXene-Mo₂C@N/C) exhibit outstanding capacitive performance, achieving an areal capacitance of 70 mF cm⁻² and a volumetric capacitance of 86 F cm⁻³, both at 0.1 mA cm⁻³. These electrodes deliver a high energy density of 2.94 μWh cm⁻² at a power density of 27.5 μW cm⁻². This gravity-guided approach widens material options and identifies the key parameters for fabricating scalable, free-standing heterogeneous bilayer films.