Construction Of Micro-nanostructured Nickel-based Compounds For Energy Storage

Hybrid supercapacitors（HSCs）assembled from battery-type electrode and capacitive electrode can exhibit higher energy densities than the traditional electrical double-layer supercapacitors（SCs）and higher power densities than the electrochemical batteries.Thereinto,the battery-type electrode material plays an important role in improving the energy storage performance of HSCs.In particular,transition metal nickel-based hydroxides and oxides（such as Ni（OH）₂ and NiO,etc.）are considered as excellent electrode materials for HSCs due to their excellent electrochemical energy storage properties.However,the low conductivity of Ni（OH）₂/NiO inhibits the release of its specific capacity,which is not conducive to obtaining HSCs with higher energy density.On the other hand,transition metal nickel-based compounds(such as Ni₃N,Ni₃Se₂,Ni₅P₁₂,etc.)exhibit higher power density,energy density and excellent rate performance due to their high electron conduction rate,excellent mechanical stability and large specific capacitance,etc.,and are developed as potential battery-type materials for high-performance HSCs.Meanwhile,the energy storage performance of HSCs is influenced not only on the inherent properties of electrode materials,but also on the nanos-microstructure of electrode materials.Up to now,a large number of studies have shown that the preparation of battery-type electrode materials with larger specific surface area and more active sites is the key approach to improve the energy density and electrochemical performance of HSCs.Therefore,it is very important to the design and preparation of battery-type electrode material with micro-nanostructures and high specific surface area process important significance for the electrochemical energy storage performance improvement of electrode materials and devices.In addition,the composite material is also an effective method to improve the conductivity of nickel-based oxides and hydroxides.Based on the above discussion,the main contents of this research are as follows:（1）Nickel nitrides are considered to be promising for hybrid supercapacitors（HSCs）as electrochemical energy storage materials,owing to their good electrical conductivity.However,nickel nitride is prone to agglomeration in high-temperature preparation process,resulting in lower specific surface area and porosity,further inhibiting its energy storage performance in HSCs.Consequently,a three-dimensional Ni₃N mesh with a mesoporous structure（3D Ni₃N mesh）was constructed as the electrode material for HSCs via a facile hydrothermal reaction and nitrogenization treatment in this work.The electrochemical tests demonstrated that the specific capacity of the as-prepared 3D Ni₃N mesh could reach 254.2 m Ah g^-1 at 1 A g^-1 in the three-electrode cell.The specific capacity of the 3D Ni₃N mesh is still up to 108.3 m Ah g^-1,even at a high current density of 30 A g^-1.The changes of micro-nano structure,chemical composition and energy storage mechanism of the 3D Ni₃N mesh electrode materials in the charge-discharge process were also analyzed and discussed.To further demonstrate its potential applications,a HSCs was also assembled with the 3D Ni₃N mesh and the activated carbon（AC）as positive and negative electrode respectively.It is worth mentioning that that the specific capacity of the 3D Ni₃N mesh//AC HSCs is 59.3 m Ah g^-1 at 0.5 A g^-1,while the energy density also reaches 56.2 Wh kg^-1 at a power density of 468 W kg^-1.In addition,the 3D Ni₃N Mesh//AC HSCs still maintains 77.4%capacity along with the average 98.7%in coulombic efficiency after 5000continuous cycles,demonstrating good cyclic reversibility.（2）Nickel hydroxide has attracted particular attention due to its high theoretical specific capacity,excellent electrochemical reversibility and environmental friendliness.However,the poor electron conductivity of the hydroxide/oxide substantially inhibits the electrochemical redox processes during charging and discharging,resulting in reduced specific capacity and multiplicative performance.Nickel-synthesised semi-metallic nickel-based compounds are attracting increasing interest due to their better electron transfer capability.Numerous studies have shown that the electrical conductivity of nickel selenides is superior to that of NiO and Ni_xS_x.As a result,composites of nickel-based selenides and hydroxides can improve the efficiency of electrodes’energy storage.In this work,hydrothermal and continuous potential deposition were used to create Ni（OH）₂@Ni₃Se₂ NWs@NF with nanowire shape.This active material’s micro-nano-graded structure helps with electron transport and ion diffusion during electrochemical processes.The findings demonstrate that the electrodes in the three-electrode setup have a specific capacitance of up to1.79 m Ah cm^-2(10 m A cm^-2).Even with a high current of 50 m A cm^-2,the specific capacity maintained at0.9 m Ah cm^-2.Ni（OH）₂@Ni₃Se₂ NWs@NF was used as the positive electrode during HSC assembly,and AC served as the negative electrode.Electrochemical testing of the assembled hybrid device showed a large energy density of 51.6 Wh kg^-1 at a power density of 322.5 W kg^-1.Even after 5000 charge/discharge cycles at a current density of 30 m A cm^-2,the HSCs still exhibited good cycling stability.