Effect Of The Trivalent Metal Ions (Al³⁺/Fe³⁺) On Structural And Electrochemical Properties Of Nickel-Cobalt-Based Layered Double Hydroxides

Layered double hydroxides?LDHs?as layered material with unique lamellar structure and the redox of transition metal ions in metal hydroxide layer are promising for next-generation supercapacitors in that electrical double layered capacitance and faradaic pseudocapacitance can be simultaneously acquired.NiCoFe-LDH,NiCoAl_0.5Fe_0.5-LDH and NiCoAl-LDH was synthesized by hydrothermal method with nickel nitrate,cobalt nitrate,aluminum nitrate,iron nitrate,urea as raw materials.The molar ratio of divalent metal ions(Ni²⁺,Co²⁺)effect on the microstructure and electrochemical properties of NiCoFe-LDH and the trivalent metal ions(Al³⁺/Fe³⁺)effect on structural and electrochemical properties of nickel-cobalt-based LDH were discussed.The XRD?SEM?TEM?FT-IR?BET?XPS and electrochemical workstation were employed to characterize the materials.The results are as follows:1)To understand the molar ratio of divalent metal ions(Ni²⁺,Co²⁺)effect on the microstructure and electrochemical properties of NiCoFe-LDH.Three molar ratio of metal ions was synthesized by hydrothermal method with nickel nitrate,cobalt nitrate,iron nitrate,urea as raw materials.The XRD results shows that the molar ratio of metal ions has little effect on the phase composition of the meteriors.It is found that the microstructure of NiCoFe-LDH shows a three-dimensional pore structure when the molar ratio of Ni:Co:Fe is2:1:1.Ni_2.5Co_0.5Fe₁-LDH?Ni₂Co₁Fe₁-LDH?Ni₁Co₂Fe₁-LDH were used as electrode materials.The electrochemical properties of Ni_2.5Co_0.5Fe₁-LDH?Ni₂Co₁Fe₁-LDH?Ni₁Co₂Fe₁-LDH were stutied by AC impedance,cyclic voltammetry,galvanostatic charge-discharge.Comparative studies show that Ni₂Co₁Fe₁-LDH electrode exhibits a superior pseudocapacitance of 1393F·g^-1,815 F·g^-1,and 663 F·g^-1 at high current densities of 1 A·g^-1,2 A·g^-1and 5 A·g^-1,all the largest of the capacitance values of the three LDH electrode material.2)To understand trivalent metal ions(Al³⁺/Fe³⁺)effect on the structural and electrochemical properties of nickel-cobalt-based layered double hydroxides?LDH?.NiCoAl-LDH,NiCoAl_0.5Fe_0.5-LDH and NiCoFe-LDH was synthesized by hydrothermal method with nickel nitrate,cobalt nitrate,aluminum nitrate,iron nitrate,urea as raw materials.The XRD?SEM?TEM?FT-IR?BET?XPS results shows that:Al³⁺is conducive to improve the stability of lamellar structure of nickel-cobalt-based layered double hydroxides and provide theoretical support for the excellent stability of electrode material.Additionally,the increased the crystallinity and hydrophilicity of ternary-component LDH via the incorporation of Al³⁺into the mixture metal hydroxide layers of LDH are favorable for electrolyte accessibility and the charge transfer.Al³⁺can improve the electrochemical activity of the reversible reaction between Ni²⁺,Co²⁺and OH-in metal hydroxide layer which is beneficial to improve the specific capacitance of electrode material of LDH.NiCoAl-LDH,NiCoAl_0.5Fe_0.5-LDH and NiCoFe-LDH were used as electrode materials.The electrochemical properties of NiCoAl-LDH,NiCoAl_0.5Fe_0.5-LDH and NiCoFe-LDH were stutied by AC impedance,cyclic voltammetry,galvanostatic charge-discharge.Comparative studies show that NiCoAl-LDH exhibited superior specific capacitance of 1153 F·g^-1 at 6 A·g^-1,and excellent cycling stability?ca.92.97%capacitance retention after 3000 cycles?.We attribute such remarkable enhancement in cyclic performance to the framework stability of Al³⁺in the multi-metal LDH matrix layer.