One-step hydrothermal synthesis of Fe3+-doped sheet-like δ-MnO2 for high-performance hybrid supercapacitors

KMnO4 was the Mn source, FeCl3·6H2O was the Fe source, and Fe3+-doped δ-MnO2 was prepared by a one-step hydrothermal method. The effects of different addition amounts of FeCl3·6H2O on the morphology, crystal structure and electrochemical properties of MnO2 were studied. The results show that pure phase MnO2 has a sheet morphology. The doping of Fe3+ decreases the size of sheet MnO2 and increases the specific surface area of MnO2, but XRD shows that MnO2 still retains the δ-type. At the same time, the doping of Fe3+ causes MnO2 to expose high-energy crystal faces, which enhances the electrochemical activity of MnO2. Further studies found that Fe3+ of FeCl3·6H2O plays an important role, while Cl− has nothing to do with it. The electrochemical test results show that when 3 mmol Fe3+ is added, the specific capacitance of MnO2–0.3 reaches the maximum value (196 F g−1), which is much higher than that of pure MnO2 (123.4 F g−1). Finally, the hybrid supercapacitor assembled from MnO2–0.3 and activated carbon has an energy density of 19.9 Wh kg−1, which can maintain 83.3 % of the initial value after 20,000 cycles. This work provides us with a new way to control the structure of electrode materials.