Preparation Of Ni₃S₂/Polypyrrole Composite Material Supported On Nickel Foam For Supercapacitor

With its short charging time,high power density,long service life,wide operating temperature range,and environmental friendliness,supercapacitors have become one of the most promising energy storage and conversion devices besides rechargeable batteries.Compared with the rechargeable batteries,supercapacitors still have a deep gap in the aspect of energy density.In order to bridge this gap,two solutions have been proposed:one is design a more perfect electrode materias,and the other is to design asymmetric supercapacitors.In the present work,using commercial nickel foam?NF?as both substrate and nickel source,nickel disulfide?Ni₃S₂?was in situ grown on nickel foam skeleton by low temperature hydrothermal method with the aid of potassium persulfate?K₂S₂O₈?.Subsequently,a thin layer of conductive polypyrrole?PPy?was deposited on the surface of nickel disulfide to obtain PPy-Ni₃S₂@NF composite electrode material,which aims to improve the main performance indexes of the composite electrode,and especially the energy density as all solid-state asymmetric supercapacitor electrode.The specific research contents are as follows:?1?Preparation of Ni₃S₂@NF electrode material.Using commercial nickel foam as both substrate and nickel source,and thiourea as sulfur source and metal precipitant,Ni₃S₂@NF electrode was fabricated by one-step hydrothermal reaction at the low temperature with the assistance of potassium persulfate oxidation.The electrochemical performance of the resultant Ni₃S₂@NF electrode was optimized by adjusting the hydrothermal temperature,the ratio of reactants,and the hydrothermal time.The results by three-electrode test show that the specific capacity of the optimal Ni₃S₂@NF electrode at a current density of 1 mA cm^-22 was2519.5 mF cm^-2.When the current density is increased by 20 times,the rate capability is 68.5%;and after 4000 charge/discharge cycles under a large current density of 20 mA cm^-2,96.0%of its initial specific capacity is retained,displaying an excellent cycle stability.The above results indicate that Ni₃S₂ is a promising electrode material for supercapacitors.?2?Preparation of PPy-Ni₃S₂@NF electrode material.Conducting polypyrrole was deposited on the surface of the Ni₃S₂@NF electrode by electrochemical oxidation polymerization.Characterization results by SEM,TEM,XRD,and XPS techniques proved that PPy-Ni₃S₂@NF composite electrode was successfully obtained,in which PPy was found as an amorphous configuration on the surface of Ni₃S₂.The optimized PPy-Ni₃S₂@NF composite electrode has a specific capacity of 3253.5 mF cm^-22 at a current density of 1 mA cm^-2,which is increased by 29.1%than before modification;When the current density increases to 20 mA cm^-2,the rate capability of the composite electrode keeps 70.7%;and after 4000charge/discharge cycles under a large current density of 20 mA cm^-2,the cycle stability still remains at 91.2%,meanwhile the discharge curves were found to be more stable than before modification,suggesting that the electro-deposition of PPy on the surface Ni₃S₂ can improve the large charge-discharge capability.The above results show that the surface modification of polypyrrole can significantly improve the specific capacity and rate capability of the Ni₃S₂@NF electrode due to the improvement in electrical conductivity,while maintaining excellent cycling stability.?3?Assembly of all solid-state asymmetric supercapacitor devices.Ni₃S₂@NF//AC@NF and PPy-Ni₃S₂@NF//AC@NF asymmetric devices were assembled by using PVA-KOH gel as a solid electrolyte,respectively.The results of the two-electrode test show that the specific capacity of the Ni₃S₂@NF//AC@NF device at 1 mA cm^-22 is 343.0 mF cm-2,and the energy density is 32.0 Wh kg-1 at a power density of 210.8W kg^-1,the rate capacity retained 83.9%after 2000 charge/discharge cycles at a current density of 8 mA cm^-2.The specific capacity of PPy-Ni₃S₂@NF//AC@NF device at 1 mA cm^-22 is 626.3 mF cm^-2,the power density at 1 mA cm^-22 is 166.7 W kg^-1,and the corresponding energy density is up to 46.4 Wh kg^-1,at the same power density of 210.8W kg^-1,the energy density of the device after optimization is 38.6 Wh kg^-1,which is an increase of 20.6%compared to before modification.Furthermore the rate capacity retained 87.8%after 2000 charge/discharge cycles at a current density of 8 mA cm-2.The results show that both the Ni₃S₂@NF and PPy-Ni₃S₂@NF can be used as good positive electrode of asymmetric supercapacitor device,however,the composite electrode modified by conducting PPy can provide higher energy density and keep excellent cycle stability under the same conditions.In summary,Ni₃S₂ can be used as an good electrode material for supercapacitors,and the surface modification by conducting PPy can further improve its supercapacitor performance,especially the key performance index such as energy density of the all-solid asymmetric supercapacitor device.