Boosting Hybrid Supercapacitor Performance via Synergy between Ti-Doped Ni_0.85Se Nanospheres and Ferrocyanide-Enhanced Electrolyte

The trade-off between energy and power density remains a major hurdle for supercapacitors (SCs). This work demonstrates a substantial performance enhancement in aqueous hybrid supercapacitors through synergistic electrode and electrolyte engineering. First, titanium-doped nickel selenide nanospheres (Ti-Ni_0.85Se) are successfully synthesized via a hydrothermal method using trace Ti₃C₂T_x MXene as a controllable titanium source. The incorporated Ti atoms induce a slight lattice expansion and optimize the electronic structure, significantly improving the intrinsic conductivity and charge-transfer kinetics of the material. In a 3 mol L^-1 KOH electrolyte, the Ti-Ni_0.85Se electrode delivers a high specific capacity of 586 C g^-1 at 1 A g^-1. Furthermore, introducing K₄[Fe(CN)₆] as a redox-active additive provides additional Faradaic capacity via the reversible [Fe(CN)₆]^3-/4- couple, boosting the specific capacity of Ti-Ni_0.85Se to 861 C g^-1. The assembled Ti-Ni_0.85Se//activated carbon hybrid supercapacitor achieves a high energy density of 35.7 Wh kg^-1 at a power density of 1156 W kg^-1, with 71.6% capacitance retention after 3000 cycles. This study validates the synergistic strategy of electrode doping and electrolyte modulation as an effective pathway to break the performance bottlenecks of supercapacitors.