Syntheses And Electrochemical Performance Of Nickel/cobalt-based Sulfides And Oxalate Complexes

Consumption of fossil energy is gradually increasing,which has caused environmental pollution,greenhouse effect and other problems.In recent years,clean energy(such as solar energy,wind energy and tidal energy)has attracted great attention due to their renewable property and environmental compatibility.However,the utilization of clean energy is easily restricted by some conditions such as time,location and so on.Therefore,it is necessary to develop efficient energy storage and conversion devices to convert discontinuous energy into continuous one.Supercapacitor and hydrogen production by water splitting are two kinds of highly efficient energy storage and conversion devices.Supercapacitor is an energy storage medium,which can convert discontinuous energy into continuous one to fully utilize energy.Hydrogen production by water splitting converts electrical energy into hydrogen energy for storage,which involves oxygen evolution reaction(OER)at the cathode and hydrogen evolution reaction(HER)at the anode.For these devices,the major factor affecting their energy storage performance is the electrode material.Generally,electrode materials should have good conductivity,larger specific surface area and abundant active sites.Iron,cobalt and nickel are widely used in electrocatalytic reaction due to their abundant and environmentally compatible.Many researchers found nickel/cobalt chalcogenides and Fe,Co,Ni-based complexes possess good conductivity,electrocatalytic activity and hydrogen evolution and oxygen evolution performance,respectively.Therefore,these materials are used in the fields of supercapacitors and water splitting respectively.However,their electrochemical performance is much lower than the theoretical expectation.Therefore,it is very necessary to design electrode materials reasonably to improve their electrochemical performance.Nickel foam as a substrate for constructing three-dimensional electrode material has attracted much attention.The composite of electrode material and nickel foam can not only simplify the preparation steps of electrode but also avoid the decrease of conductivity of electrode due to the presence of binder during the preparation.At the same time,the unique structure and large specific surface area of nickel foam are beneficial for ion and electron transfer,thus improving the electrochemical performance.In addition,the doping of heteroatoms into metal compounds also promotes the electrochemical performance of materials.Based on the above considerations,we adopt the strategies,such as the composite of electrode materials and nickel foam,or the doping of heteroatoms into electrode materials,to prepare a series of electrode materials,including chromium-doped nickel/cobalt sulfides and transitional metal complexes using simple hydrothermal and electrodeposition methods.Their compositions and the morphologies have been determined by a variety of characterization methods,and their electrochemical performance has been evaluated by electrochemical workstation.The major works are as follows:(1)A series of(Co,Ni)₃S₄/Co₉S₈/Ni₃S₂ composites with different Cr contents have been prepared by controlling the ratio of raw materials,Co(NO₃)₂ and Cr(NO₃)₃.When the raw material ratio of Co(NO₃)₂ and Cr(NO₃)₃ is 4:1,the prepared material,Cr-(Co,Ni)₃S₄/Co₉S₈/Ni₃S₂/NF-4 delivers a higher specific capacitance of 310.3 m Ah g^-1 at 1 A g^-1(320.3 m Ah cm^-2 at 1 m A cm^-2).The material shows excellent electrochemical performance mainly due to the heterostructure of Co₃S₄,Ni₃S₄,Co₉S₈ and Ni₃S₂ as well as the incorporation of Cr element.It is found that when Cr is doped into Ni₃S₂ or Ni₃S₄,it can improve the conductivity of the material,promote the transfer of electrons from Ni/S to Cr,improve the electrochemical performance of the material.(2)The Fe(ox)(H₂O)₂ complex has been synthesized via a facile one-step electrodeposition method using nickel foam as substrate.It has been explored the effects of deposition potential,deposition time,the species of solvents and metal ions on the structures,morphologies and electrochemical performance of materials.It is found when the deposition potential and the deposition time are-1.4 V vs Ag/Ag Cl and 15 minutes,respectively,the prepared sample Fe(ox)(H₂O)₂/NF-(-1.4)-15 nanorods show outstanding electrocatalytic oxygen evolution reaction(OER)performance.At the current density of40 and 100 m A cm^-2,the overpotentials are 270 and 340 m V,respectively.At the same time,it exhibits excellent long-time OER activity,which can endure 80 h-electrolysis at20 m A cm^-2.At the same time,the formation mechanism of Fe(ox)(H₂O)₂ nanorods and the rate limiting step in the OER process have been investigated.