Design and Formulation of Engineered 2D Nb₂CT_xMXene for Ultra-Stable NH₄⁺Aqueous Hybrid Supercapacitors

Two-dimensional (2D) transition metal carbides/nitrides (MXenes), which possess layered structures with excellent electrical conductivity, are auspicious materials for energy storage applications. The presence of van der Waals interactions between Nb₂CT_xMXene nanosheets tends to induce self-stacking, which reduces the accessible surface area and the number of electrochemically active sites. This structural limitation adversely affects cycling stability and specific capacitance when employed in supercapacitor applications. Herein, nitrogen (N)-functionalized Nb₂CT_xMXene (N-Nb₂CT_x) materials were optimized by annealing Nb₂CT_xin an ammonia (NH₃) atmosphere at different temperatures (denoted as N-Nb₂CT_x-300 °C, N-Nb₂CT_x-400 °C, and N-Nb₂CT_x-500 °C). Based on the experimental results in the three-electrode system, it was found that N-Nb₂CT_x-400 °C exhibited the most favorable electrochemical performance, attributed to effective nitrogen doping and enhanced structural stability. Annealing at 400 °C helps nitrogen atoms to be more evenly doped into the lattice of Nb₂CT_x, thereby increasing the material’s surface area and porosity while minimizing significant structural agglomeration and partial decomposition or oxidation. N-Nb₂CT_x-400 °C shows a specific capacitance of 382 F/g at 1 A/g, retaining 140 F/g at a current density of 20 A/g in ammonium sulfate ((NH₄)₂SO₄) electrolyte. Assembled NH₄⁺ion hybrid supercapacitors (AHSCs) exhibit a specific energy density of 23.36 Wh/kg at a power density of 900 W/kg. Through density functional theory (DFT) calculations, it is revealed that N-Nb₂CT_xMXene exhibits superior thermodynamic and diffusion properties toward SO₄^2–and NH₄⁺ions, providing theoretical guidance for the design and development of high-performance electrode materials for energy storage devices.