Heterostructured bimetallic–sulfide@layered Ti₃C₂T_x–MXene as a synergistic electrode to realize high-energy-density aqueous hybrid-supercapacitor

Aqueous hybrid supercapacitors (AHSCs) exhibit promising electrochemical performance with long cyclic stability and high power density. However, the low-energy density restricted their development to commercialization. To improve the energy density, we proposed a heterostructured (HS) composite of nickel–cobalt–sulfide (NCS) nanoflowers embedded in exfoliated Ti₃C₂T_x−MXene layers (HS−NCS@MXene). The NCS nanoflowers were uniformly dispersed inside the MXene layers and formed a sandwich-like structure. The HS−NCS@MXene exhibited remarkable pseudocapacitive performance in a three-electrode system. The capacitance can reach 2637 F g⁻¹ (1582 C g⁻¹) at 2.5 A g⁻¹ with stable cycling life over 10,000 cycles and retained 96% capacity of the initial value. Post−mortem investigations confirmed that the charge storage mechanism in HS−NCS@MXene composite is a combination of Faradic and electrochemical double-layer storage. An AHSC was assembled by coupling the HS−NCS@MXene as a positive electrode and activated carbon as a negative electrode (HS−NCS@MXene//AC–AHSC). The HS−NCS@MXene//AC–AHSC can operate in a potential range up to 1.6 V and deliver a high capacitance of 226 F g⁻¹ at 1.5 A g⁻¹ with stable cyclic life (92%) up to 20,000 cycles. Moreover, the HS−NCS@MXene//AC−AHSC also possessed a high energy density of 80 Wh kg⁻¹ at a power density of 1196 W kg⁻¹, which exceeds most recently published works. The synergistic effect of NCS and MXene enables the HS−NCS@MXene composite to deliver outstanding electrochemical performance for AHSCs.