Site‐Specific Selenium Substitution Enhances Charge Storage Performance in Solid‐State Flexible MnFe ₂ O ₄ ‐Based Supercapacitor Devices via Modulated d‐States

Abstract This study reports the first investigation into selenium‐substituted MnFe 2 O 4 (Se‐MFO) nanoparticles, where Se is precisely doped at Fe sites via a solvothermal method, for high‐performance solid‐state flexible supercapacitor devices. Spectroscopic analysis confirms site‐specific Se incorporation, and first‐principles calculations show that Se modifies the d‐states at the valence band edge, optimizes the electronic structure, and increases redox‐active site density by tuning oxygen vacancies. This facilitates reversible Mn 2+ /Fe 2+ transitions, improves ionic mobility, and accelerates charge transfer kinetics. As a result, Se‐MFO delivers a high energy density of 140 W h kg −1 , a power density of 4000 W kg −1 , and a Coulombic efficiency of 99.8% at 3 A g −1 . The device also exhibits excellent cycling stability, retaining ≈85% capacitance over 25 000 cycles. The electronic structure engineering through site‐specific Se substitution positions Se‐MFO as a scalable and durable electrode for next‐generation flexible energy storage technologies.