High-performance binder-free hybrid supercapacitor featuring in-situ electrochemical transformation of Ni3S2 to Ni(OH)2

Abstract

Although nickel hydroxide (Ni(OH)₂) offers high theoretical capacity for supercapacitors, its practical deployment is often hindered by its intrinsic conductivity limitations and structural degradation. Herein, we introduce a robust, binder-free electrode architecture designed to circumvent these bottlenecks via the radio-frequency magnetron sputtering (RFMS) of a nickel sulfide (Ni₃S₂) film on nickel foam along with an in-situ electrochemical activation strategy. Unlike traditional methods, we utilize a sputtered nickel sulfide (Ni₃S₂) precursor that undergoes a controlled oxidative transformation during cycling, evolving into a highly active and mechanically stable Ni(OH)₂ phase. This conversion process preserves nanoscale morphology while ensuring intimate electrical contact with the current collector, effectively eliminating the contact resistance issues in binder-based systems. The optimized electrode demonstrates a superior areal capacitance of 590 mF cm⁻² at 1 mA cm⁻², corresponding to an outstanding gravimetric capacitance of 3562 F g⁻¹. Furthermore, a hybrid supercapacitor device assembled with a graphene-based negative electrode delivers an areal capacitance of 204.2 mF cm⁻², instead of degrading, the device exhibits an "electro-activation" phenomenon, yielding a 14.7% increase in capacitance after 5,000 charge–discharge cycles. These results substantiate the efficacy of the in-situ conversion route as a scalable and superior alternative to chemical routes for developing high-durability energy storage systems.