Hybrid MnO₂–disordered mesoporous carbon nanocomposites: synthesis and characterization as electrochemical pseudocapacitor electrodes

MnO₂–mesoporous carbon hybrid nanocomposites were synthesized to achieve high values of redox pseudocapacitance at scan rates of 100 mV s⁻¹. High-resolution transmission electron microscopy (HRTEM) along with energy dispersive X-ray spectroscopy (EDX) demonstrated that 1 nm thick MnO₂ nanodomains, resembling a conformal coating, were uniformly distributed throughout the mesoporous carbon structure. HRTEM and X-ray diffraction (XRD) showed formation of MnO₂ nanocrystals with lattice planes corresponding to birnessite. The electrochemical redox pseudocapacitance of these composite materials in aqueous 1 M Na₂SO₄ electrolyte containing as little as 2 wt% MnO₂ exhibited a high gravimetric MnO₂ pseudocapacitance (C_MnO2) of 560 F g_MnO2⁻¹. Even for 30 wt% MnO₂, a high C_MnO2 of 137 F g_MnO2⁻¹ was observed at 100 mV s⁻¹. Sodium ion diffusion coefficients on the order of 10⁻⁹ to 10⁻¹⁰ cm² s⁻¹ were measured using chronoamperometry. The controlled growth and conformal coating of redox-active MnO₂–mesoporous carbon composites offer the potential for achieving high power energy storage with low cost materials.