Aqueous Manganese Dioxide Ink for Paper‐Based Capacitive Energy Storage Devices

We report a simple approach based on a chemical reduction method to synthesize aqueous inorganic ink comprised of hexagonal MnO 2 nanosheets. The MnO 2 ink exhibits long‐term stability and continuous thin films can be formed on various substrates without using any binder. To obtain a flexible electrode for capacitive energy storage, the MnO 2 ink was printed onto commercially available A4 paper pretreated with multiwalled carbon nanotubes. The electrode exhibited a maximum specific capacitance of 1035 F g −1 (91.7 mF cm −2 ). Paper‐based symmetric and asymmetric capacitors were assembled, which gave a maximum specific energy density of 25.3 Wh kg −1 and a power density of 81 kW kg −1 . The device could maintain a 98.9 % capacitance retention over 10 000 cycles at 4 A g −1 . The MnO 2 ink could be a versatile candidate for large‐scale production of flexible and printable electronic devices for energy storage and conversion.