Development of Flexible High-Efficient Aluminum ion Supercapacitors With 2D Niobium MXene Electrode

A high-performance aluminum-ion supercapacitor is fabricated using 2D few-layered Nb₂CT_x MXene, as an active electrode material and Al₂(SO₄)₃ electrolyte for efficient energy storage. Nb₂CT_x MXene has been synthesized from Nb₂AlC MAX phase using HF. Nb₂CT_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 Al₂(SO₄)₃ electrolyte is exceptionally high compared to those (≤32.2 F g⁻¹) in H₂SO₄, Na₂SO₄, and MgSO₄ 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.