Ti₃C₂T_x//AC dual-ions hybrid aqueous supercapacitors with high volumetric energy density

Supercapacitors with an advanced high volumetric energy density require a match of both electrodes materials and suitable electrolyte. Here, aqueous asymmetric supercapacitors are designed by employing layered Ti₃C₂T_x as negative electrodes and highly compact activated carbon (AC) as positive electrodes. Ti₃C₂T_x//AC asymmetric supercapacitors (Ti₃C₂T_x//AC ASCs) with 1.7 V voltage window are designed by regulating the mass ratio of both electrodes, which results from the shift of the open circuit potential with different applied voltages. Then, dual-ion hybrid supercapacitors (Ti₃C₂T_x//AC-KBr HSCs) are also constructed by introducing redox active (KBr) into traditional electrolyte (H₂SO₄) for Ti₃C₂T_x//AC ASCs to achieve a high volumetric energy density, in which fast hydronium ion intercalated pseudocapacitance happens at the Ti₃C₂T_x negative electrode, while redox reaction of Br⁻/Br₃⁻ within the highly compact AC happens at the positive electrode. As a result, the Ti₃C₂T_x//AC-KBr HSC achieves a high volumetric capacitance of 250 F cm⁻³ at 2 mV s⁻¹, a volumetric energy density of as high as 102.8 Wh L⁻¹ at 424 W L⁻¹ and an outstanding cycling stability with 122% capacitance retention after 6000 cycles. Furthermore, the self-discharge performance for the Ti₃C₂T_x//AC-KBr HSC is obviously improved, which is mainly ascribed to the adsorption of oxidized species at the AC surface.