Electrochemical Energy Storage and Sensing of Heavy Metal Ions by High-Energy Surface-Exposed Bifunctional Mn-Doped NiSe ₂ Nano-Octahedrons

Advanced energy storage and smart environmental monitoring have driven the development of multifunctional materials exhibiting superior performance in both electrochemical energy storage (EES) and selective sensing applications. Herein, we report the advanced solvothermal synthesis of octahedron-shaped Mn-doped NiSe 2 nanostructures as bifunctional electrode materials. Present NiSe 2 exhibits superior electrical conductivity and electrochemical stability, while Mn doping enhances the charge carrier concentration, modulates electronic properties, and introduces synergistic effects through accessible high-spin Mn oxidation states. Electrochemical studies using a three-electrode configuration demonstrated that optimized Ni 0.90 Mn 0.10 Se 2 delivers a high specific capacitance of 1238 F g –1 (137.5 mAh g –1 ) at 2 A g –1, an areal capacitance of 743 mF cm –2 at 12 mA cm –2, and retains 84% of its capacitance at higher current densities. An asymmetric solid-state device incorporating Ni 0.90 Mn 0.10 Se 2 and activated carbon achieved a capacitance of 338 F g –1 (140.8 mA h g –1 ), an energy density of 52 W h kg –1, a power density of 3414 W kg –1, and a Coulombic efficiency of 94.8% over 20000 charging–discharging cycles. Simultaneously, this electrode in the presence of other interfering ions exhibited excellent electrochemical sensing capabilities for toxic heavy-metal ions (Pb 2+, Cd 2+, and Hg 2+ ), with detection limits as low as 0.0088 μg L –1 for Hg 2+ . Thus, Mn-doped NiSe 2 emerges as a cost-effective, high-performance bifunctional material that enables both rapid charge storage and highly sensitive heavy-metal-ion detection, paving the way for integrated energy storage and environmental monitoring within a single electrode.