Redox cascade engineering in urea-crystallized manganese-integrated nickel-iron metal-organic frameworks for high-performance hybrid supercapacitors

Abstract

The growing need for hybrid energy storage systems that merge the advantages of batteries and supercapacitors has sparked considerable interest. As a result, researchers have begun to explore advanced electrode materials that offer both strong redox activity and long-term structural stability. Our study proposes a dual-modification approach to engineer a highly porous and redox-enriched NiFe-based metal organic framework (NiFe-MOF) through urea-assisted synthesis and in situ integration of MnO₂. The final Mn/NiFeMOF-U hybrid shows a hierarchically porous structure with abundant faradaic sites that facilitate fast ion transport and enhanced multivalent redox coupling across Ni, Fe, and Mn centers. The results indicate surface-controlled charge storage dominated by reversible Ni²⁺/Ni³⁺, Fe²⁺/Fe³⁺, and Mn²⁺/Mn⁴⁺ transitions. Furthermore, electron hopping and a robust electron cascade are identified as the main electron transfer pathways in this system. By pairing Mn/NiFeMOF-U with reduced graphene oxide (rGO), the assembled battery-supercapacitor hybrid (BSH) demonstrates an outstanding energy density of 1.7 mWh/cm² at 6.4 mW/cm², while maintaining 91% capacitance retention and 99% Coulombic efficiency after 10,000 cycles. Overall, we believe this study not only presents a new strategy for designing redox-modulated MOF hybrids but also confirms their practical potential for next-generation electrochemical energy storage.