One-step hydrothermal synthesis of Fe3+-doped sheet-like δ-MnO2for high-performance hybrid supercapacitors

KMnO4was the Mn source, FeCl3·6H2O was the Fe source, and Fe3+-doped δ-MnO2was 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 MnO2were studied. The results show that pure phase MnO2has a sheet morphology. The doping of Fe3+decreases the size of sheet MnO2and increases the specific surface area of MnO2, but XRD shows that MnO2still retains the δ-type. At the same time, the doping of Fe3+causes MnO2to 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.