Preparation Of Ni?OH?₂-based Binder-free Electrode Materials And Their Supercapacitor Performance

Supercapacitors,as one of a new category of energy storage devices,have attracted considerable attention because of their attractive merits such as high power density,ultrahigh cycling stability,fast charge/discharge capability,safe operation and so on.They are widely used in portable electronic devices,backup power systems and hybrid electric vehicles.However,the low energy density greatly limits their wide range of practical application.Therefore,the design and synthesis of high quality electrode materials with high specific capacity,high energy density and good cycling stability have become a research hotspot in recent years.Transition metal oxide/hydroxide-based pseudocapacitor electrode materials such as Ni?OH?₂,Co?OH?₂,MnO₂ have been favored by researchers for their ultrahigh specific capacitance.In this paper,by using Ni foam as a substrate and supporting material,Ni?OH?₂ nanoarrays and their hierarchical heterostructures were in situ synthesized,and active carbon was used as negative electrode to assemble an asymmetrical supercapacitor,which improves the energy density by maximizing the specific capacitance and working potential window of the device.The specific research results are as follows:First,three dimensional hexagon-like phosphrous doped Ni?OH?₂?P-Ni?OH?₂?rod arrays are firstly grown on Ni foam as a binder-free electrode for high performance supercapacitors,which is fabricated via a one-step hydrothermal treatment of Ni foam in H₂O₂ aqueous solution containing 5 mM NaH₂PO₄.Benefiting from 3D Ni foam which provides more efficient electrical contact and large active surface area,and P doped could help improve the intrinsic electronic conductivity of Ni?OH?₂.The hexagon-like P-Ni?OH?₂ rod arrays based binder-free electrode exhibits high specific capacitance of 3.51 F cm^-2 at 2 mAcm^-2 and good cycling stability.The assembled all-solid-state asymmetric supercapacitor?AAS?of P-Ni?OH?₂/NF//AC delivers an energy density of 81.31 Wh kg-1 at the power density of 0.63kWkg-1.Secondly,P-Ni?OH?₂ micro/nanorods in situ grown on Ni foam were used as secondary substrate,then the Ni-Co LDH nanosheets were grown on P-Ni?OH?₂ micro/nanorods to fabricate the three dimensional hierarchical porous core-shell structures of P-Ni?OH?₂@Ni-Co LDH via hydrothermal method.As a binder-free eletrode material,the excellent electrochemical properties of the P-Ni?OH?₂@Ni-Co LDH can be due to the unique heterostructure and synergistic effect,which results from the abundant active surface area provided by the Ni-Co LDH nanosheets and fast charge transportation of the P-Ni?OH?₂ rods.At a current density of 1 mA cm^-2,the areal capacitance of P-Ni?OH?₂@Ni-Co LDH/NF electrode was 13.44 F cm^-2,which is about 3.8 times higher than P-Ni?OH?₂/NF electrode(3.51 F cm^-2)and shows great cycling stability.In addition,the AAS device of P-Ni?OH?₂@Ni-Co LDH//AC also delivers a high energy density of 0.323 mWh cm^-2 at the power density of 1.6 mW cm^-2.Finally,the hierarchical core-shell heterostructure of P-Ni?OH?₂ rods@MnO₂ nanosheets are fabricated directly grown on Ni foam via a two-step hydrothermal process.As a binder-free electrode material,this unique core-shell heterostructure exhibits excellent electrochemical properties,including an ultrahigh areal capacitance of 5.75 F cm^-2 at a current density of 2 mA cm^-2 and long cycling life.Moreover,an AAS of P-Ni?OH?₂@MnO₂//AC is successfully assembled to enhance the electrochemical properties.The AAS device also shows excellent electrochemical properties including a large potential window of 0?1.6 V,high areal capacitance of 911.3 mF cm^-2 and a high energy density of 0.324 mW h cm^-2 at a power density of 0.8 mW cm^-2;it also shows outstanding cycling stability,with capacitance retention of 80%after 5000 cycles at 20 mA cm^-2.