Construction And Electrochemical Energy Storage Properties Of Transition Metal Selenide Nanoarrays

More and more attention has focused on green,low-cost and advanced energystorage devices in recent years,because both of the ever-increasing environmental destruction and the rapid depletion of exhaustible energy.Of all the energy storage devices that human used to store and convert energy,one of the most promising devices has been the electrochemical supercapacitors,thanks to their higher power density,superior charge-discharge ability and longer lifespan.However,in order to obtain high-performance hybrid supercapacitors,new-type materials with advanced electrochemical performance should be constructed.Transition mental compounds（like metal oxides,hydroxides and sulfides）have been investigated extensively for supercapacitors as battery-type electrodes.Among them,sulfides have much lower optical band gap energy and higher conductivity compared with metal oxides or hydroxides.It is well known that the elements O,S and Se belong to the same main group in the periodic table and show parallel electrochemical properties.The electric conductivity of Se(1×10^-33 S m^-1)is considerably higher than that of S(1×10^-28 S m^-1),which implies that transition metal selenides might possess higher conductivity and better electroactivity than the corresponding oxides or sulfides.Based on above considerations,Ni0.85Se nanosheet arrays supported on nickel foams have been successfully synthesized via a one-step hydrothermal approach.By using the Ni0.85Se nanosheet arrays as precursor,ternary selenides（Ni,Co）0.85Se nanosheet arrays are successfully synthesized by a facile and effective cation-exchange process.In addition,both of the two selenides are characterized and their electrochemical performances are tested.Apart from these,Ni_0.85Se//activated carbon hybrid supercapacitors are assembled with Ni_0.85Se nanosheet arrays as the positive electrode and activated carbon as negative electrode.And aqueous and all-solid-state hybrid supercapacitors are assembled with metallic（Ni,Co）0.85Se nanosheet arrays as binder-free positive electrode and nitrogen-doped biomass carbon as negative electrode.Electrochemical energy storage properties of all the as-assembled hybrid devices have been explored.The specific information is as following:（1）Synthesis and electrochemical performance of Ni0.85Se nanosheet arraysHoneycomb-like metallic nickel selenide（Ni0.85Se）nanosheet arrays supported on nickel foams are successfully synthesized via a one-step hydrothermal approach.characterization results reveal that the as-obtained Ni0.85Se nanosheets belong to hexagonal crystal phase with a relatively uniform thickness of¹5 nm.Ni_0.85Se nanosheet arrays exhibit splendid electrochemical properties,e.g.:higher specific capacitance(3105 F g^-1 at 1 A g^-1 and 1460 F g^-1 at 10 A g^-1),outstanding cyclic stability（90.1%capacitance retention after 5000 cycles）,and remarkable conductivity.Therefore,Ni_0.85Se nanosheet arrays hold the great potential in high-performance supercapacitors.（2）Synthesis and electrochemical performance of（Ni,Co）_0.85Se nanosheet arraysMetallic ternary（Ni,Co）_0.85Se nanosheet arrays have been successfully synthesized by a facile and effective cation-exchange process and directly used as electrode materials for supercapacitors.The morphology and chemical composition could be modified by altering the reaction time.By virtue of the intrinsic metallicity,synergistic effect derived from cobalt and nickel ions,super-hydrophilic surface and the intriguing nanoarray features,（Ni,Co）0.85Se nanosheet arrays with Ni/Co ratio of0.5:0.5 show better electrochemical performance,including:higher specific capacitance(3917 F g^-1 at 1 A g^-1 and 2150 F g^-1 at 10 A g^-1),outstanding cyclic stability（85.25%capacitance retention after 3000 cycles）,and remarkable conductivity.（3）Assembly and electrochemical energy storage properties of aqueous Ni_0.85Se//activated carbon hybrid supercapacitorsAqueous hybrid supercapacitors are assembled with Ni_0.85Se nanosheet arrays as the positive electrode and activated carbon as negative electrode,which show remarkable energy-storage characteristics,including:a high energy density of 65.62Wh kg^-1 at the power density of 103.33 W kg^-1,only 6.3%capacitance loss after10000 cycles,as well as a wider voltage window of 0-1.6 V.Besides,two hybrid supercapacitors in series based on Ni_0.85Se nanoarrays can light two light emitting diodes and drive a mini fan.All the results show the good application value of the Ni_0.85Se//activated carbon hybrid supercapacitors（4）Assembly and electrochemical energy storege properties of aqueous and all-solid-state（Ni,Co）_0.85Se//biomass carbon hybrid supercapacitorsBiomass carbon is prepered from chitosan.And the electrochemical test shows that the nitrogen-doped biomass carbon has outstanding electrochemical performance.Aqueous and all-solid-state hybrid supercapacitors have been assembled with metallic（Ni,Co）0.85Se nanosheet arrays as binder-free positive electrode and nitrogen-doped biomass carbon as negative electrode.The aqueous hybrid device delivers a superb energy density of 76.76 Wh kg^-1 at the power density of 206.22 W kg^-1.All-solid-state hybrid device exhibits a high stack energy density of 1.92 mWh cm^-3 at the current density of 2.5 mA cm^-2.Meanwhile,both of these two kinds of hybrid supercapacitors exhibit wider voltage windows,better cycle stabilization and higher applicable value.Such impressive results evidently prove that two kinds of hybrid supercapacitors based on（Ni,Co）_0.85Se nanosheet arrays and nitrogen-doped biomass carbon have great potential in energy-storage application.