Mo-attenuation redox attenuation in FeMo-MIL-88B enable stables EDLC-like supercapacitor performance

Mo-induced redox attenuation in Fe-based metal-organic frameworks (MOFs) from pseudocapacitive to capacitive represents a promising but largely unexplored strategy for achieving stable, high-power electrochemical performance. Here, we report the microwave-assisted synthesis of heterometallic FeMo-MIL-88B and show that Mo incorporation is associated with a marked shift in charge-storage behavior, characterized by attenuated Fe³⁺/Fe²⁺ faradaic contributions and favouring electric double-layer capacitance (EDLC-like) response. The material synthetized exhibits a surface area of 50.1 m² g^-1 and a mesoporous texture consistent with its nanorod morphology. EPR spectroscopy reveals a stable high-spin Fe³⁺ signal at g ≈ 2.0038 and a positive Curie-Weiss temperature (θ ≈ +36 K), consistent with a modified electronic and magnetic environment around the Fe centers in the heterometallic framework. The electrochemical characterization of the material shows that for a symmetric two-electrode configuration in 6 M KOH, FeMo-MIL-88B displays quasi-rectangular cyclic voltammograms across 5-250 mV s^-1, a b-value of 0.998, a capacitive contribution of 98.7-99.8% (Dunn's method), and a near-ideal phase angle of 86°. The symmetric device delivers 400 mF g^-1 at 5 mV s^-1, retains more than 99% capacitance after 3000 cycles, and achieves a power density of 1125 W kg^-1. These results indicate that the heterometallic FeMo environment is associated with attenuated faradaic activity and predominantly surface-controlled charge storage, providing insight into the modulation of charge-storage mechanisms in Fe-based MOFs.