Development of Flexible High‐Efficient Aluminum ion Supercapacitors With <scp>2D</scp> Niobium <scp>MXene</scp> Electrode

ABSTRACT A high‐performance aluminum‐ion supercapacitor is fabricated using 2D few‐layered Nb 2 CT x MXene, as an active electrode material and Al 2 (SO 4 ) 3 electrolyte for efficient energy storage. Nb 2 CT x MXene has been synthesized from Nb 2 AlC MAX phase using HF. Nb 2 CT x MXene coated on carbon cloth (Nb@CC) displays a capacitance of 307 F g −1 with 90% coulombic efficiency. The specific capacitance of Nb@CC in Al 2 (SO 4 ) 3 electrolyte is exceptionally high compared to those (≤32.2 F g −1 ) in H 2 SO 4 , Na 2 SO 4 , and MgSO 4 electrolytes. Both symmetric and asymmetric aluminum ion supercapacitors are fabricated with Nb@CC electrode. The Nb@CC//Nb@CC symmetric device exhibits a capacitance of 122 F g −1 with a high energy density (E d ) of 33.2 Wh kg −1 at 1.41 kW kg −1 power density (P d ). An asymmetric supercapacitor device (ASC), Nb@CC//CNT@CC, with carbon nanotube (CNT@CC) cathode delivers a maximum E d of 24.7 Wh kg −1 at 3.41 kW kg −1 P d and excellent stability with 90% capacitance retention after 4000 cycles. A remarkably high P d of 34 kW kg −1 is maintained with 13.2 Wh kg −1 E d , and the rate capability is 53% for a 10‐fold increase in current density. These results offer the feasibility of efficient aqueous supercapacitors with Al‐ion as guest species, presenting new possibilities for supercapacitors.