| Supercapacitors can be fully charged and discharged within several minutes,quite suitable for driving the equipments with fast and/or urgent requirement of electricity.However,low energy density restricts the widespread application of supercapacitors.The electrode of supercapacitor decides the level of energy storage.It is thus the key to develop the electrode with high performance to enhance the energy density.Due to high theoretical capacity,wide work potential window and low cost,manganese dioxide（Mn O2）has been viewed as the promising electrode for supercapacitors.However,there is still a large gap between practice capacity and theoretical capacity of Mn O2electrodes caused by sluggish transfers of electron and ion,making its development full of challenge.Herein,Mn O2-based electrodes have been prepared with the regulation of oxygen defects and/or morphology,which show favorable electron and ion transfers,thus obtaining high capacity,and the supercapacitors packaged by them deliver both high energy and power densities.The detailed contents are given as below:1.Realizing the regulation of nanosheet morphology and oxygen defect ofδ-Mn O2 by modifying substrate with surface oxygen.Surface-modified carbon cloth（SMCC）rich in oxygen is obtained by carbonizing the hydrothermal products of cetyltrimethylammonium bromide（CTAB）and glucose solution on carbon cloth（CC）.With the SMCC as the substrate,δ-Mn O2 is then in-situ grew under the hydrothermal reaction of KMn O4 and H2SO4 solution.Structural characterizations reveal that theδ-Mn O2anchoring on the SMCC（Mn O2/SMCC）forms the dense ultrathin nanosheets structure with abundant oxygen defects.Benefiting from the structure characteristics,superior electron and ion transfers can be obtained by the Mn O2/SMCC,which promotes the efficient redox reaction of Mn2+,Mn3+and Mn4+.At three electrode system,the Mn O2/SMCC under 1 A g-1 delivers the capacity of 792.5 C g-1.The asymmetric supercapacitor of Mn O2/SMCC shows high energy and power densites.2.Discovering that electrochemical activation enables the morphology and oxygen defect structure reconstruction of Fe-doped Mn O2（Fe-Mn O2）.Fe-Mn O2 is synthesized by hydrothermal method using KMn O4 and Fe（NO3）2·9H2O as the Mn and Fe precursors,respectively.After electrochemical activation,it is revealed by structural characterization that Fe-Mn O2 nanosheet is reconstructed to be a composite structure of Fe-doped and oxygen-deficient nanosheets and nanowires.A co-activation system reflects the reconstruction of Fe-Mn O2realized by a mechanism of dissolution and redeposition.The performance measurements indicate that the reconstructed Fe-Mn O2delivers a specific capacity of 800.2 C g-1 at 1 A g-1,193.5 C g-1 higher than that of Fe-Mn O2.Density functional theory calculation and finite element simulation are combined to research the reconstructed structure characteristics,which unveil the composite structure of nanosheet and nanowire with Fe-doping and oxygen defect in the reconstructed Fe-Mn O2 can optimize the electronic structure and ion adsorption,thus improving the energy storage.3.Revealing that the oxygen defect can be induced by regulating low cut-off work potential（LCWP）of Mn O2/Mn2O3 electrode to improve charge storage.Mn O2/Mn2O3 is prepared by electrodeposition with a constant potential.The performance measurement shows that the specific capacity of the Mn O2/Mn2O3 varies with the change of LCWP applied under the same high cut-off potential.At the potential range of0-1.2 V,the Mn O2/Mn2O3 deliveries a specific capacity of 300.6 C g-1 at1 A g-1,and it can obtain 786.7 C g-1 while the potential window turns to be-0.3-1.2 V.The Mn O2/Mn2O3 electrode discharging to-0.3 V,revealed by charge storage mechanism,possesses abundant oxygen defect,which introduces a new impurity level near the conduction band and reduces the adsorption energy of Na in discharge state to accelerate electron and ion transfers,thus obtaining increased capacity.Additionally,the asymmetric supercapacitor Mn O2/Mn2O3//Mo S2 is confirmed to deliver a high specific energy density of 41.3 Wh kg-1 at a large specific power density of 1200.4 W kg-1.4.Constructing a Mn O2/Co3O4 composite electrode with inner electron transfer by Mn O2 nanosheet and Co3O4 nanoneedle.The Mn O2/Co3O4 electrode can be prepared by step hydrothermal method and pyrolysis method,forming a structure with Co3O4 nanoneedle as core and Mn O2 nanosheet as shell,in which an obvious electron transfer among Co,Mn and O is observed.When used as the electrode for supercapacitors,the charge storage of the Mn O2/Co3O4 largely outperforms those of the Mn O2 and Co3O4,displaying an areal specific capacity of 1092.1 m C cm-2 at 1 m A cm-2.The prominent performance of Mn O2/Co3O4 can be ascribed to the favourable electron and ion transfers benefited from its unique structure.The performance evalution of asymmetric supercapacitor with Mn O2/Co3O4 cathode and active carbon anode is conducted,indicating a prominent specific energy density of0.162 m Wh cm-2 at a specific power density of 0.7 m W cm-2. |
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