Electrochemical activation enabling structure reconstruction of Fe-doped MnO2 for enhancing pseudocapacitive storage

Monitoring and understanding of materials reconstructed under working conditions are vital to accurately identify active sites, clarify reaction mechanism and reasonably design advanced electrodes. Here, a Fe doped MnO2 (Fe-MnO2) nanosheet with unique features is demonstrated, unfolding dynamic reconstruction on both morphology and defect structures toward self-optimized pseudocapacitive storage, which can be easily controlled by galvanostatic charge/discharge activation. The results show that the Fe-MnO2 after activation is reconstructed from nanosheets to a composite structure of Fe-doped and oxygen-deficient nanosheets and nanowires. This composite structure endows the reconstructed Fe-MnO2 with accelerated electron and ion transfers and high electrochemical active surface area. Density functional theory (DFT) calculation and finite element simulation reveal that the co-existence of Fe doping and oxygen defects arouses more delocalized charges, and the nanowires on nanosheets display tip-enhanced electric field effects for attracting more ions, which effectively improves electron and ion transfer kinetics. The reconstructed Fe-MnO2 delivers a specific capacitance of 500.1 F g−1 at 1 A g−1, a significant self-optimized energy storage compared with Fe-MnO2 with 379.2 F g−1. These observations demonstrate active morphology and crystallinity for outstanding pseudocapacitive storage of the Fe doped MnO2 electrode, offering a guidance to design the electrode materials with superior performance.