Preparation Of Nickel-cobalt Bimetallic Sulfides And Their Electrochemical Properties

Supercapacitors（SCs）are a new type of energy storage device with a wide application prospect by virtue of high power density,fast charging and discharging speed,and long cycle life,but with low energy density.The electrode material,as an important component of supercapacitor,plays a key role in its energy storage performance.Meanwhile,to solve the problems of energy scarcity and environmental pollution,it is crucial to promote electrolytic water hydrogen production technology.However,the current electrolytic water hydrogen production is constrained by the large electrode polarization and slow reaction kinetics,which makes the preparation of hydrogen with high energy consumption and low conversion efficiency.Nickel-cobalt bimetallic sulfide（NiCo₂S₄）has become a very promising candidate electrode material for supercapacitor and catalyst for electrically resolved hydrogen reaction（HER）due to its abundant ionic valence and high electrical conductivity.However,there are problems of low specific capacity,poor cycling stability and high hydrogen precipitation overpotential in applications.The research work focuses on the design and synthesis of nickel-cobalt bimetallic sulfide electrode materials in order to obtain electrode materials with excellent electrochemical energy storage and electrocatalytic performance,and to optimize the electrochemical energy storage and electrocatalytic hydrogen precipitation performance of sulfide materials in terms of structural morphology control,doping modification and composite nanoarray structure design,respectively:（1）NiCo₂S₄ electrode materials with different structural morphologies were synthesized by a two-step solvothermal method by regulating the solvent ratio,and when V_ethanol:V_water=3:1,porous microspheres of NiCo₂S₄（NCS-3）with large specific surface area and high pore size distribution were prepared.The structure is conducive to the sufficient contact between the electrolyte and the active material,which can promote the rapid electron transfer and ion transport,and thus improve the electrochemical performance of the NiCo₂S₄ electrode material.The specific capacity of the NiCo₂S₄ electrode was 857 C g^-1 at a current density of 1 A g^-1 in 6 M KOH electrolyte,and the multiplicative performance was 80%when the current density was increased from 1 A g^-1 to 20 A g^-1.The asymmetric capacitor NCS-3//AC assembled using NCS-3 electrode and activated carbon（AC）electrode possesses a maximum energy density of 48 Wh kg^-1 at a power density of 801 W kg^-1.（2）The effects of transition metal ions doping on the microstructure and electrochemical properties of the prepared porous microspherical NiCo₂S₄ electrode materials were investigated using transition metal ions such as Mn²⁺,Fe³⁺,Cu²⁺and Zn²⁺for doping modification.Experimental tests and first-principles calculations showed that the crystal structure of NiCo₂S₄ was not changed by moderate Mn²⁺doping compared with other transition metal ion doping.The prepared Mn_0.5-NCS electrode material has larger specific surface area and electron density,faster ion diffusion coefficient and lower charge transfer impedance compared with NiCo₂S₄.The specific capacity was 1175 C g^-1 at 1 A g^-1 current density and the capacitance retention was81%at 20 A g^-1 current density.its capacity retention was 80%after 5000 cycles of stability test.The assembled Mn-NCS//AC button-type asymmetric device has a maximum energy density of 55.4 Wh kg^-1 at a power density of 797 W kg^-1.For HER catalysts,the hydrogen precipitation overpotential of the Mn_0.5-NCS electrode material was 95 mV at a current density of 10 m A cm^-2 with a Tafel slope of 71 mV dec^-1.（3）NiCo₂S₄@Ni（OH）₂ composite array electrode materials were prepared on nickel foam substrates,and the microstructure and electrochemical and electrocatalytic hydrogen precipitation performance of the composite electrode materials were tested.The results show that the composite electrode materials have superior electrochemical energy storage performance and electrocatalytic activity compared with NiCo₂S₄ and Ni（OH）₂ electrode materials.The specific capacity of the composite electrode is 1400C g^-1 at 1 A g^-1 current density,and it has 78%capacitance retention at 20 A g^-1 current density.The assembled NiCo₂S₄@Ni（OH）₂//AC asymmetric solid-state supercapacitor possesses a maximum energy density of 59.3 Wh kg^-1 at a power density of 652 W kg^-1after 5000 cycles at a current density of 10 A g^-1.When used for HER reaction catalysis,the NiCo₂S₄@Ni（OH）₂ composite electrode material has an overpotential of only 93mV and a Tafel slope of 78 mV dec^-1.