Flexible Iron-Ion Hybrid Capacitor Based on a MnO₂ Electrode

Aqueous Fe-ion hybrid capacitors, with high safety, low cost, and environmental friendliness, have attracted considerable attention as an emerging energy storage device. However, Fe-ion-based energy storage systems still face challenges, such as narrow voltage windows, limited energy density, and harsh fabrication conditions. To address these issues, this work fabricates an aqueous Fe-ion hybrid capacitor using low-cost activated carbon as the anode, manganese dioxide (MnO₂) as the cathode, with an FeSO₄ + NH₄Cl aqueous electrolyte, successfully expanding the voltage window to 0-1.2 V. The atomic molecular dynamics simulations confirm the potential of MnO₂ as a cathode material, and the spiny nanostructured MnO₂ shows a large specific surface area and a stable tunnel structure, which facilitates the intercalation/deintercalation of Fe²⁺ ions. Consequently, the assembled device achieved a specific capacitance of 835 mF cm^-2 at 1 mA cm^-2 and a surface energy density of 167 μWh cm^-2 with a capacitance retention of 97.2% after 3000 cycles. Furthermore, to meet wearable electronics requirements, a flexible device was assembled by integrating a carboxymethyl cellulose-poly(vinyl alcohol) hydrogel soft-packaging material. The results show that the flexible device exhibits excellent bending resistance. The further assembled flexible Fe-ion supercapacitors demonstrate a high energy storage potential. Under a current density test of 1 mA cm^-2, the calculated specific capacitance is 682.4 mF cm^-2, and the areal energy density is 136.48 μWh cm^-2. The further assembled flexible Fe-ion supercapacitors demonstrate high energy storage potential. Under a current density test of 1 mA cm^-2, the calculated specific capacitance is 682.4 mF cm^-2, and the areal energy density is 136.48 μWh cm^-2. Similarly, in the cycling performance test, the device retains 92.6% of its capacity after 3000 cycles. This study provides technical references for the development and practical application of flexible Fe-ion-based energy storage devices.