Designing thermal and electrochemical oxidation processes for δ-MnO2 nanofibers for high-performance electrochemical capacitors†
To date, the phase of electrospun MnOx 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 MnOx 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 MnOx 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 Mn2+/3+ ions in the MnOx phases.