Ultrathin Polypyrrole Layers Boosting MoO₃ as Both Cathode and Anode Materials for a 2.0 V High-Voltage Aqueous Supercapacitor

An aqueous supercapacitor is an emerging energy storage unit on account of its low cost, fast energy delivery rate, and long service life. The energy density of an aqueous supercapacitor can be enlarged via extending the voltage window of electrode materials, while the aqueous electrolyte remains thermodynamically constant at 1.23 V. Herein, an aqueous supercapacitor with a 2.0 V high-voltage window is realized by core–shell MoO3–x/polypyrrole (MP) nanocomposites as both cathode and anode materials. The ultrathin PPy layer on the MoO3 core not only improves the conductivity and cycle stability of the nanocomposites but also acts as a reductant, leading to the formation of oxygen vacancies in the MoO3 core. When used as a cathode material, the potential range of the as-obtained MP nanocomposite is up to 1.0 V. As an anode material, the stable potential range could reach −1.0 V. Due to the large potential range of the cathode and anode, the as-obtained 2.0 V aqueous supercapacitor shows a remarkably high delivery energy of 58.5 Wh kg–1. The synthesis of MP nanocomposites is simple and the electrode performance is significantly enhanced; thus, it is a suitable candidate for high-energy-density aqueous supercapacitors.