Three-Dimensional Fibrous Network of Na_0.21 MnO₂ for Aqueous Sodium-Ion Hybrid Supercapacitors

Sodium-ion hybrid supercapacitors are potential energy-storage devices and have recently received enormous interest. However, the development of cathode materials and the use of nonaqueous electrolyte remain a great challenge. Hence, aqueous Na-ion hybrid supercapacitors based on a three-dimensional network of NaMnO₂ were developed. The cathode material was synthesized by the electro-oxidation of potassium manganese hexacyanoferrate nanocubes. The oxidized compound was confirmed to be Na_0.21 MnO₂ by various physical characterization methods. Manganese dioxide is a well-characterized material for aqueous asymmetric pseudocapacitors, but its usage at high operating voltages is limited due to the electrochemical stability of water. Nevertheless, high-potential and high-performance aqueous supercapacitors exhibiting a cell potential of 2.7 V were developed. Further, the practical applicability of an asymmetric supercapacitor based on NaMnO₂ (cathode) and reduced graphene oxide (anode) was demonstrated by powering a 2.1 V red LED.