Dual-regulation tailoring of tunnel-structured hexagonal tungsten oxide for high-performance ammonium-ion hybrid supercapacitors

Abstract

Ammonium-ion hybrid supercapacitors (A-HSCs) have emerged as promising candidates for next-generation energy storage owing to their inherent safety and environmental sustainability. Hexagonal tungsten oxide (h-WO₃), with its well-defined tunnel structure, holds great promise as a negative electrode material for NH₄⁺ storage. However, its practical application is hindered by structural instability and poor intrinsic electrical conductivity. To address these challenges, a dual-regulation strategy is proposed, integrating molybdenum (Mo) doping and NH₄⁺ pre-intercalation to concurrently optimize the tunnel structure and electronic environment of h-WO₃ (Mo-NWO). Comprehensive experimental and theoretical analyses reveal that Mo doping narrows the bandgap of WO₃ and reduces the diffusion energy barrier, thereby accelerating NH₄⁺ adsorption and diffusion. Simultaneously, NH₄⁺ pre-intercalation stabilizes the tunnel framework via hydrogen bonding, ensuring structural reversibility. As expected, the Mo-NWO/AC electrode achieves a high areal capacitance of 13.6 F cm⁻² at 5 mA cm⁻² and retains 80.14 % of its capacitance after 5000 cycles, demonstrating exceptional rate capability and cycling stability. Moreover, the assembled Mn₃O₄//Mo-NWO/AC device delivers a high energy density of 3.41 mWh cm⁻² and outstanding long-term stability (85.75 % retention after 12,000 cycles). This work provides a viable strategy for designing high-performance NH₄⁺ storage materials and advances the development of sustainable energy storage systems.