Study On The Preparation Of Lead Powder And Foam Lead By Electrolytic Reduction Of Lead Oxide By Eutectic Solvent

Because lead is of good ductility and plasticity,lead powders have been extensively used as lubricant in the fields of bearing and electrical carbon products,as well as active material in lead acid battery plates.And porous lead foam has been acknowledged as a favorable material for the grids of lead acid battery,duo to its mang desirable properties such as good porous structure,high porosity,high specific surface area and low density.However,their preparation methods have obvious disadvantages,which include complex and long reaction process,requiring high-level equipment,introducing impurities easily.Deep eutectic solvent（DES）,as a new environmental friendly solvent with low steam pressure,high conductivity,cheap,nontoxicity and other outstanding characteristics,has been developed into a promising electrolyte for the preparation of lead powders and lead foam.So it would be a potential alternative,which has importantly scientific significance and wide application foreground,to slove the problems existing in traditional techniques and improve product qualities.In this paper,the choline chloride-urea（ChCl-urea）and choline chloride-ethylene glycol（ChCl-EG）DES are synthesized,and the solubilities of lead oxide（PbO）in these solvents are also determined.The existence form of PbO in ChCl-urea DES is examined by masss spectrometry and infrared spectrometry.Then they are used as electrolytes for the electrochemical reduction of PbO to lead powders and lead foam for the first time.The electrochemical behavior of the preparation of lead powders in ChCl-urea-PbO has been investigated systematically by cyclic voltammetry,cathodic polarization and chronoamperometry.The corresponding relationship or regulation mechanism between the morphologies,crystal orientation of lead powders and the electrodeposition process conditions are determined by controlling experimental parameters.Moreover,the direct reduction mechanism of bulk solid PbO in ChCl-EG DES is studied by cyclic voltammetry and a new elucidated model account for the electrhemical process is proposed.The effects of cell voltage,temperature,reaction time and pore-forming agent content on the composition and morphologies of products are also examined.The major research results are drawn as follows:（1）The solubility of PbO in ChCl-urea and ChCl-EG are 0.116 mol·L-1 and 0.001798 mol·L-1,,respectively.Three-coordinated complexes {[PbO CO（NH2）2]·Cl-}-are formed through sp3 hybrid orbitals of Pb（Ⅱ）and three sp hybrid orbitals are all filled with lone pair electrons which are donated by Cl-,O^2-and O atom of urea respectively in ChCl-urea-PbO.Temperature and PbO concentration have significant effects on the viscosity and electrical conductivity of ChCl-urea DES.The electrical conductivity increases with the increasing of temperature,while increases firstly and then decreases with PbO concentration,and the viscosity has a contrary tendency at the same time.（2）The electroreduction of Pb（Ⅱ）to metallic lead in ChCl-urea-PbO is a diffusion controlled quasi-reversible process in the range of 323～343 K and 40～130 A·M-2.The raising of temperature leads to the increasing in particle size of lead deposits.This phenomenon is associated with an increase in the deposition efficiency and a drop in the specific energy consumption.The refinement of lead wires is observed with the increasing of current density.Temperature has a more obvious effect in comparison with the current density.Meanwhile,the current efficiency and specific energy consumption both increase slightly.Purified lead is obtained with face-centered cubic crystal structure,and the preferred orientation is（200）plane.（3）The cyclic voltammetry of Pb（Ⅱ）/Pb redox on a stainless steel electrode in PbO-ChCl-urea with different PbO concentrations indicates that the increasing of PbO concentration makes the reduction potential Ept（Ⅱ）/pb and cathodic peak current shift positively.The exchange current density increases obviously and it becomes more evident with PbO concentration.In addition,the current efficiency increases while the specific energy consumption decreases with the increasing of PbO concentration.Different shapes of lead powders including coral-like,rod-like,needle-like,fern-like and dendritic,can be electrodeposited from PbO in ChCl-urea DES.（4）The electrocrystallization of lead at a stainless steel electrode in ChCl-urea-PbO is occurred by 3D instantaneous nucleation on the active sites with nuclei growing at the diffusion-limited rate.The gas produced during the reaction process is mainly O₂ mixed with CO₂ and H2.The reactions on the electrodes as follow:Cathode:Pb（Ⅱ）+ 2^e-→ Pb0（s）;[C₅H₁₄NO]+ + e’ →（CH₃）₃N（g）+[C2H50]·Anode:O^2-→1/2 O₂（g）+ 2^e-;O^2-+ 1/2 C（s）→1/2 CO₂（g）+ 2^e-（5）The PbO powders filled in the Pt cavity microelectrode can be reduced into metallic lead in ChCl-EG and the reduction process of PbO on Pt cavity microelectrode involves the PbO（s）--Pb0（s）+O₂（g）direct electrodeoxiedation and PbO（s）→ Pb（Ⅱ）+O^2-→ PbO（s）+O₂（g）dissolving-electrodeposition.Experiments prove that during the electrolysis of solid PbO in ChCl-EG,the intimate contact between PbO bulk and current collector,as well as the penetration of electrolyte thoroughly in the bulk are important factors to accelerate the reduction rate and improve efficiency.（6）During the electrolysis of bulk PbO,the cell voltage,temperature,pore-forming agent content and reaction time have remarkable effects on the reaction process and products.With the increasing of cell voltage,the current increases and the unreduced area of the bulk is gradually reduced.The morphology of the products is changed from large granular to regular and smooth dendritic with cell voltage.Rising temperature is in favor of the diffusion of DES into PbO bulk,the dissolving of PbO,and also can accelerate the transport of three phase interfaces（3PIs）as well as the diffusion rate of Pb（Ⅱ）.High porosity is benefical to the diffusion of DES into the interior of the bulk,and the products become agglomerated rods or needles from small flakes.The current increases firstly,then decreases and finally becomes steady with the extension of time.The electrodeoxidation reactions begin at the surface of the bulk,and then penetrate into the interior of it.Meanwhile,the number and size of the formed flaky particles in products are increasing continuously with reaction time.（7）During the electrolysis of bulk PbO in ChCl-EG DES,the formed metallic lead accumulated numerous electrons is regarded as local cathodic region,and the unreducted PbO is served as a temporary anode region,thus oxygen is generated at the interface of the PbO and electrolyte.The reactions on the electrodes as follow:Cathode:①cathodic region（Pb）:PbO（s）+ 2^e-→ Pb0（s）+ O^2-;Pb（Ⅱ）+ 2^e-→ Pb0（s）②anodic region（PbO）:PbO（s）→ Pb（Ⅱ）+ O^2-;O^2-→1/2 O₂（g）+ 2^e-Anode:O^2-→ 1/2 O₂（g）+ 2^e-;O₂₊1/2 C（s）→ 1/2 CO₂（g）+ 2^e-In present paper,the preparations of lead powders and lead foam by electrochemical reduction in DES provide new methods which are different from the traditional production techniques.The systematic studies of basic theory give a theoretical basis for the production of lead powders and lead foam in DES.