Designing thermal and electrochemical oxidation processes for δ-MnO₂ nanofibers for high-performance electrochemical capacitors

To date, the phase of electrospun MnO_x nanofibers (NFs) after thermal calcination has been limited to the low oxidation state of Mn (x < 2), which has resulted in insufficient specific capacitance. The organic contents in the as-spun MnO_x NFs, which are essential for forming the NF structure, make it difficult to obtain the optimum phase (MnO₂) to achieve high electrochemical performance. Herein, δ-MnO₂ NFs, which were obtained by galvanostatic oxidation of thermally calcined MnO_x NFs, were successfully fabricated while maintaining the 1-D nanoscale structure and inhibiting loss of the active materials. The galvanostatically oxidized Mn₃O₄ exhibited an outstanding performance of 380 F g^-1 under a mass loading of 1.2 mg cm ^-2. The effect of galvanostatic oxidation was strongly dependent on the concentration and energetic stability of the Mn^2+/3+ ions in the MnO_x phases.