Spin State Modulation of MnO2 by Zn-Doping for Enhanced Supercapacitor and Hydrogen Evolution

Manganese dioxide (MnO2) stands out as a promising multifunctional material for supercapacitors and hydrogen evolution reaction (HER) owing to its high theoretical capacitance, rich valence states, low cost, and environmental friendliness. However, the strong Jahn-Teller effect induces electron localization in Mn3+, giving rise to poor intrinsic conductivity, sluggish charge transport, and structural instability, which severely restricts its practical applications. Herein, a series of Zn-doped MnO2 was synthesized through a facile hydrothermal method to modulate the spin state and electronic structure of MnO2. Zn doping triggered local lattice distortion, broke the symmetric octahedral field of [MnO6], and optimized the occupation of Mn 3d orbitals, thereby enhancing spin polarization and promoting d-electron delocalization. Systematic characterizations and density functional theory (DFT) calculations verified that Zn doping reduced the bandgap of MnO2 from 1.7 eV to 1.1 eV, increased the eg orbital occupancy, and generated abundant oxygen vacancies and electrochemically active sites, which significantly improved electron mobility and ion diffusion kinetics. As the supercapacitor cathode, the assembled Zn-MnO2//La-MoO3/GQDs asymmetric supercapacitor achieved a maximum energy density of 23.9 Wh kg−1 with 80.9% capacitance retention after 6000 cycles. Meanwhile, as an electrocatalyst for HER in 1 M KOH, 1% Zn-MnO2 exhibited a lower overpotential of 161 mV at 10 mA cm−2 and a small Tafel slope of 85 mV dec−1, with outstanding stability after 5000 cycles. This work demonstrated that Zn doping-induced spin state modulation effectively addressed the inherent limitations of MnO2, providing a feasible strategy for designing high-performance Mn-based materials for integrated energy storage and conversion applications.