Manganese oxide/carbon aerogel composite electrodes are prepared by a self-limiting anodic-electrochemical deposition of manganese oxide into a binder-enriched carbon aerogel layer, drop-cast on a graphite substrate, using 0.1 M Mn(CH3COO)2·4H2O as the electrolyte. Manganese oxide grows in the form of thin nanofibers along the backbone of the carbon aerogel, leaving adequate working space for the electrolyte and enabling a fuller extent of the utilization of the manganese oxide to make the composite an outstanding supercapacitor electrode material. The manganese oxide is determined to be Mn3O4 with the Raman spectroscopy and high-resolution transmission electron microscopy. The rectangularity of the cyclic-voltammogram loops of the composite electrode is excellent and remains that way for scan rates up to a very-high value of 500 mV s−1, indicating extremely good redox reversibility and cycle efficiency. At a scan rate of 25 mV s−1, the specific capacitance, as measured in 0.5 M Na2SO4 for a potential window of 0.1–0.9 V vs. Ag/AgCl, reaches a maximum value of 503 F g−1 and experiences only a negligible decay of less than 1% at the 6000th cycle, implying an extraordinary cycling stability. The cycling efficiency is as high as 98% at a current density of 8 A g−1 cm−2, showing an almost-ideal capacitive behavior. The power density reaches 48.5 kW kg−1 and the energy density 21.6 W h kg−1 at a scan rate of 500 mV s−1, well above the specifications of current state-of-the-art supercapacitors.
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