Unipolar Pulse Electrodeposition And Electrochemical Performances Of Nico Layered Double Hydroxides Electrodes

Recently unquestionably energy crisis and environmental pollution are two crucial issues for human development.In this case,it is an important topic to explore and develop the new clean energy and efficient technologies of water treatment.Hydrotalcite-like compounds?namely Layered Double Hydroxide,LDH?is a class of anionic functional materials,with unique brucite-like lamellar structure,anionic exchangeability in the interlayer.Recently,it has been widely applied in various fields,such as catalysis,supercapacitor,lithium ion battery and water treatment.LDH is a kind of subject-object material,made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and water molecules.LDH possesses unique lamellar structure and high specific surface area which could beneficially exhibit capacitance performance for electrical double-layer capacitor and pseudo-capacitor simultaneously when used as electrode materials for supercapacitor.So LDH used as electrode material can gain a relatively high capacitance.On account of the anion in the interlayer being able to exchange with other anions,it has a wide application in the field of water treatment through the interlayer-anion exchanged with functional ligand and formingcomplexes.The purpose of this paper is to prepare NiCo LDH using a new electrochemical method to study its high supercapacitor performance.In addition,calcined NiCo LDH is used to adsorb phosphate ions due to its“memory effect”and better electroactivity.Calcined NiCo LDH is fabricated as electrochemically switched ion exchange?ESIX?hybrid film and used to remove phosphate ions in the wastewater.Firstly,a unipolar pulse electrodeposition method was successfully employed to controllably synthesize nanosheet type NiCo LDH on the carbon paper.The effect of concentration rate of Ni²⁺/Co²⁺preparation solution on crystalline structure,morphology and supercapacitive performance was investigated systematically.Experimental found that the morphology and composition of NiCo LDH was highly depend on the Ni²⁺/Co²⁺ molar ratios of preparation solution;and the obtained Ni_0.76Co_0.24 LDH materials showed uniform distribution,small nanosheet size and large specific surface area on carbon fiber electrode.Ni_0.76Co_0.24 LDH electrode was evaluated through cyclic voltammetry,galvanostatic charge-discharge,stability and electrochemical impedance spectroscopy for supercapacitor application,which revealed a high specific capacitances of 2189.8 at the current density of 1 A g^-1and a good cycle stability,retaining 70.3% of the initial capacitance after20000 charge and discharge cycles at 50 A g^-1.The Ni_0.76Co_0.24 LDH electrode has relatively low Rct value of 0.53 ?.Moreover,the Ni_0.76Co_0.24 LDH electrode exhibits a high energy density of 76 Wh Kg^-1 at a power density of250 W Kg^-1 and a high power density of 7500 W Kg^-1 at energy density of 66 Wh Kg^-1.The as-prepared Ni_0.76Co_0.24 LDH as positive electrode for asymmetric supercapacitor possesses a high specific capacitance of 179.4 F g^-1.The asymmetric supercapacitor exhibits excellent energy density of 4.1 Wh Kg^-1 at a power density of 4000 W Kg^-1.The asymmetric supercapacitor stillretains 82.7% of the initial capacitance after 20000 charge and discharge cycles at 10 A g^-1.These performances indicate the Ni_0.76Co_0.24 LDH electrode is suitable for supercapacitor.In other part work,the Ni_0.76Co_0.24 LDH was calcined under 300 ? for 2h to obtain NiCo CLDH.Cyclic voltammetry measurement indicates that the NiCo CLDH has better electroactivity than NiCo LDH.NiCo LDH and NiCo CLDH were used as electrochemically switched ion exchange hybrid film.And at 0.8 V to remove phosphate in 100 mg L^-1 phosphoric acid solution,NiCo CLDH film showed a much better performance,whose adsorption capacity of PO₄^3-reached 225.7 mg g^-1 after 6 h absorption.The adsorption capacity of NiCo CLDH film for removing 100 mg L^-1 PO₄^3-,100 mg L^-1SO₄^2-and 100 mg L^-1 Br^-mixed solutions at same condition to evaluate the selectivity.Experimental found that the adsorption capacity of PO₄^3-was larger than SO₄^2-and Br^-.The adsorption and desorption recyclability of NiCo CLDH film were measured at 0.8 V and-0.4 V in 100 mg L^-1 PO₄^3-solution.Due to“memory effect”,the desorption rate of PO₄^3-was only 81.2% at initial cycle.However after five cycles of adsorption and desorption,the adsorption capacity of PO₄^3-still reached 160.2 mg g^-1,while the desorption reached98.4%,which shows good recycling performance.