Ultrafine Amorphous MnO_<i>x</i> Nanoparticles for a High Mass Loading Electrode

High electrochemically active manganese dioxide (MnO2) has been deemed to be a promising active material for a high mass loading electrode. Here, an amorphous MnOx nanostructure constituted by ultrafine MnOx nanoparticles was fabricated by a simple, controllable, and cost-effective strategy. The electrochemical properties of the as-prepared MnOx nanostructures were significantly improved due to their amorphous crystalline structure. For a high mass loading electrode, an optimized content of amorphous MnOx, carbon black, and carbon nanotubes was measured to be 79.1%, 13.5%, and 7.4%, respectively. A high mass loading electrode of 13.5 mg·cm–2 was fabricated with a capacitance of 371.3 F·g–1 calculated based on the weight of pure amorphous MnOx. Hence, a high mass loading asymmetric supercapacitor assembled by amorphous MnOx (12.6 mg·cm–2) and activated porous carbon (APC, 22.4 mg·cm–2) delivers an ultrahigh areal energy density of 780.3 μW h·cm–2. Furthermore, a good cycling stability with 98% of the initial capacitance retention was achieved after 8000 cycles, meeting the requirements of a commercial energy storage device.