Bridging Battery-Level Energy and Supercapacitor Power: NiCo ₂ O ₄ /MXene Hybrid Cathodes for High-Rate and Long-Life Aqueous Zinc-Ion Hybrid Supercapacitors

The rising demand for reliable, affordable, and high-performance energy-storage solutions has fueled the development of aqueous zinc-ion hybrid supercapacitors (ZIHSs), which effectively operate between conventional supercapacitors and batteries. However, their practical advancement has been hampered by slow Zn2+ diffusion kinetics and a lack of cathode materials that facilitate fast charge transfer and long-term durability. In this study, we engineered a conductive hybrid cathode composed of spinel NiCo2O4 integrated with multilayer Ti3C2Tx MXene (NMX) to overcome the inherent conductivity limitations of NiCo2O4. The synergistic combination of redox-active NiCo2O4 with the highly conductive MXene scaffold offers abundant electrochemically accessible sites, shortened ion-transport pathways, and enhanced interfacial charge-transfer kinetics. The optimized hybrid electrode in a three-electrode setup shows a good electrochemical performance of 1250 F/g at 1 A/g. A ZIHS device, assembled with a Zn foil anode in an electrolyte containing 3 M KOH and 0.05 M ZnSO4, achieved an excellent specific capacitance of 183 F/g at 1 A/g, yielding an energy density of 73.4 Wh/kg and a power density of 851.2 W/kg. Notably, the device retained nearly 84.9% of its capacitance after 9000 cycles at 6 A/g. Moreover, postcycling analyses confirm structural stability and reveal Zn2+ insertion, sulfate adsorption, and K+ interaction, supporting a highly reversible hybrid storage mechanism. Furthermore, two NMX//Zn in series powered a red LED for 7 min, showing their practical feasibility. Collectively, these results indicate that NiCo2O4/Ti3C2Tx cathodes are promising platforms for future aqueous zinc-ion energy storage.