Abstract:

Objective In recent years, the development of compatible energy sources by combining wearable technology and textiles to make supercapacitors by replacing traditional forms of batteries with energy storage fabrics has gained wide attentions. The three-dimensional integrated fabrics possess inherent porous structures for effective attachment of active materials. The two-dimensional transition metal carbide Ti₃C₂T_x (MXene)/zinc (Zn) three-dimensional integrated knitted structure of the flexible supercapacitor (ZSC) was designed and prepared, combining flexibility of the three-dimensional fabrics with high electrical conductivity of the MXene so as to effectively improve its energy storage performance.

Method Monolayer Ti₃C₂T_x(MXene) nanosheets were prepared by selective extraction of element "A" in MAX-Ti₃AlC₂ phase using LiF and HCl. By the constant potential electrodeposition method, Zn monomers were electrodeposited on the surface of silver-plated nylon (SPN) fibers as the anode, and SPN yarns coated with MXene was used as the cathode. The functional fibers were weft knitted using STOLL computerized flat knitting machine. Cyclic voltammetry, constant current charge/discharge and electrochemical impedance methods were used to test the storage performance and durability of three-dimensional integrated knitted supercapacitors at the electrochemical workstation.

Results The morphological characteristics of prepared MXene nanosheets and the energy storage performance of Ti₃C₂T_x (MXene)/Zn three-dimensional integrated knitted flexible supercapacitors were comprehensively investigated. The results showed that the prepared MXene nanosheets were in forms of monolayer structure and hexagonal lattice, which had a 2-D layered structure with a thickness of 1.95 nm and a size of 1.4 μm. The lamellar structure with the main components of C, O and Ti was coated with MXene coated with silver-plated nylon fibers (SPN) as the cathode, and zinc monomers were electrodeposited on the SPN fibers as the anode. It was tested by cyclic voltammetry. By galvanostatic charge-discharge test, it is shown good linearity and remarkable symmetrical quasi-triangular charge-discharge curves, indicating a high coulombic efficiency and a capacitance retention of 52.18% even at higher current densities. The investigation revealed reversible Zn deposition/stripping at its cathode and anode ion adsorption/desorption.

The supercapacitor exhibited a low resistance (R_s) of 6.74 Ω determined by the internal resistance of the electrode material and the electrolyte solution, and a charge transfer resistance (R_ct) of about 8 Ω. The energy density of 47.99 μW·h/cm² (25.04 μW·h/cm²) and power density of 0.5 mW/cm² (10 mW/cm²) in this study is better than the same type of reports. After 10 000 cycles of charging and discharging, it was found to have a capacitance retention of 93.51% and a coulombic efficiency of 92.43%. There was no significant change in the energy storage performance after leaving the supercapacitor in the air for 30 days. When two 1 cm² supercapacitor fabrics were connected in series, a small electric meter could be lit up. The capocitance retention was 94.1% after 10 h of placement, with good resistance to self-discharge.

Conclusion The three-dimensional integrated knitted structure was prepared to effectively improve the energy storage performance of the supercapacitors, and its inherent porous structure effectively attracted the active material to achieve high ion diffusion speed and charge-discharge rate. The microstructure and chemical composition of MXene were discussed and analyzed. Electrochemical testing revealed that the area capacitance was 345.56 mF/cm² at a current density of 1 mA/cm², 93.51% capacitance retention and 92.43% coulombic efficiency after 10 000 charge-discharge cycles, and a power density of 10 mW/cm² at an energy density of 25.05 μW·h/cm². The three-dimensional knitted supercapacitor has good durability. It has high voltage retention of 94.1% after 10 h in air. The promising three-dimensional integrated knitted structure for flexible supercapacitors provides a reliable and efficient power supply for wearable electronic devices.

Key words: knitting, supercapacitor, two-dimensional transition metal carbide, smart wearable