Preparation Of New Energy-efficient Anode Material And Electrochemical Performance Study

At present, the lead alloys containing small amounts of silver, tin, calcium or antimony have been widely used as insoluble anodes in zinc electrowinning industry. The anodes can meet the needs of zinc electrowinning, but oxygen overpotential of the anodes is still high. Lead ions dissolved in the electrolyte can be reduced on the cathode and contaminate the cathode zinc. A new type of PbO2-coated metal anode, which has been widely used in electrolysis, is made up of four layers. The base is made of titanium plate and is covered with a conductive undercoating(such an undercoating is necessary for protecting the substrate from passivation) as bottom. The intermediate coating is composed ofα-PbO2, andβ-PbO2 is electrodeposited as the surface layer. Titanium is not, however, a viable substrate for practical electrodes in electrodepositing nonferrous metals for its cost. Aluminum is relatively cheap and has a good conductivity. The electrode material by electrodepositing lead dioxide on Al substrate has a huge market prospects.Firstly, thermodynamic analysis for the feasibility of electrodepositing PbO2 and MnO2 from aqueous solution was discussed. Secondly, lead dioxide on Al substrate was prepared through anodic deposition from aqueous solutions. Foreign ions or fine particles were added to the electrolyte aims at incorporate with the lead dioxide. Therefore, the electrocatalytic activity and the stability of the PbO2 electrodes can be enhanced. For example, the intermediate coating, consisting ofα-PbO2-CeO2-TiO2, was prepared by electrodepositing lead dioxide with doping nano-titanium dioxide and rare-earth oxide. The surface coating, consisting ofβ-PbO2-WC-ZrO2(orβ-PbO2-MnO2-WC-ZrO2), was prepared by doping WC and nano-ZrO2 particles. The morphology, crystal phase, surface composition, microstructure, particles size and microhardness of the electrode were characterized by means of SEM, XRD, EDS and so on. The electrocatalytic properties of electrodes have also been studied by linear sweep voltammetry, A.C.Impedance, Tafel plot and Cyclic voltammetry. The anticipated service lives of electrodes were measured by accelerated life test using in 150g/L H2SO4 solution at a current density of 2A/cm2. The electrocatalytic activity and service life of the composite layers in the oxygen evolution reaction have been investigated with aim of assessing the effect of fine particles. The E-pH diagrams of Pb-H2O and Mn-H2O system were calculated and drawn by a number of thermodynamic data. The results showed that the potential of electrodepositedα-PbO2 andγ-MnO2 were changed more obviously than that ofβ-PbO2 by increasing the temperature. On the basis of standard redox potentials, anodic deposition of MnO2 was easier than deposition of PbO2.α-PbO2,α-PbO2/β-PbO2 (NO3-) andα-PbO2/β-PbO2 (CH3COO-) were obtained from alkaline,nitrate and acetate bath, respectively. The surface roughness ofα-PbO2/β-PbO2 (NO3-) on A1 substrate was the largest, resulting in a considerable electrocatalytic activity. The solid solubility ofα-PbO2 inβ-PbO2 (NO3-) presented the maximal value, providing a way to improve the conductivity and corrosion resistance. The structural models of A1/α-PbO2 and A1/α-PbO2/β-PbO2 (NO3-) electrodes were proposed. In the reaction mechanisms for oxygen evolution, the relatively high values of Tafel slope were interpreted by the decreasing of the number of activity particles covered on electrode surface.The electrodeposition process ofα-PbO2 coating on the A1 substrate was studied. Applied current density during the deposition of PbO2 had a strong influence on the morphology of the prepared film. A compact and uniform layer of lead dioxide was obtained at the current density≤3mA/cm2. A further increase in current density resulted in a smaller particle with high porosity. The morphology and particle size distribution could be improved when the HPbO2- concentration was 0.12M or higher and the bath temperature was 40℃. However, theα-PbO2 coating with a few porosities was obtained at a very long plating time. The PbO2 deposited in alkaline conditions was highly non stoichiometric. The PbO impurities were formed on the surface of the electrode besides theα-PbO2 phase.Theα-PbO2-CeO2-TiO2 composite electrodes prepared by electrochemical anodic codeposition technique by doping CeO2 and TiO2 in alkaline bath were studied. The best formula and process conditions were got as follows:4M NaOH with litharge PbO, current density 0.5A/dm2, TiO2 15g/L, temperature 40℃, electrodeposition time 3h, CeO2 10g/L. On this condition, theα-PbO2-(3.77 wt.%)TiO2-(2.13 wt.%)CeO2 codeposited electrode material could be prepared. The thickness, hardness and film density of composite coating was better than that of the pureα-PbO2 coating. The apparent energies of activation of theα-PbO2-(3.77wt.%)TiO2-(2.13wt.%)CeO2 electrode was the lowest in alkaline bath. In the potential of 0.35V, the lead dioxide deposition was influenced by the kinetic control and diffusion control. In H2SO4 solution, the Al substrateα-PbO2-(3.77wt.%)TiO2-(2.13wt.%) CeO2 electrode showed the best catalytic activity and corrosion resistance. The Guglielmi model for CeO2 and TiO2 codeposition withα-PbO2 was proposed.WC-ZrO2/β-PbO2 composite coatings were prepared on the Al/α-PbO2-CeO2-TiO2 surface by anodic codeposition ofβ-PbO2, nano-ZrO2 and WC particles. The best formula and process conditions were got as follows:Pb(NO3)2 250g/L, HNO3 15g/L, WC 40g/L, ZrO2 50g/L, temperature 50℃, current density 3.0A/dm2, electrodeposition time 4h. On this condition, theβ-PbO2-(6.56wt.%)WC-(3.74wt.%) ZrO2 codeposited electrode material with a thickness of 408μm and a microhardness of Hv 723 could be prepared. Theβ-PbO2-(6.56wt.%)WC-(3.74wt.%) ZrO2 showed the best electrocatalytic activity and good corrosion resistance.A typical rutile structure was observed in undoped-β-PbO2 coating. The doped-WC-β-PbO2 coating was composed of a lot of bigger crystal cell, and there was a larger crack between the crystal cells. The crystal cell was made up of the clusters of nano-grains. The WC particles had the random structure on the composite coating. In addition, the composite coating possessed an unclear outline of the rutile crystallite ofβ-PbO2. The doped nano-ZrO2-β-PbO2 coating had a fine grain and uniform distribution besides a clear outline ofβ-PbO2 structure. The composite coating possessed a uniform and dispersive distribution of nano-ZrO2 particles. The doped nano-ZrO2-WC-β-PbO2 electrode possessed more fine grain, and a uniform, dispersive distribution of nano-ZrO2 and WC particles within theβ-PbO2 matrix oxide. The codeposition of WC and nano-ZrO2 particles induced a markedly increase in the roughness of PbO2 deposits. The new crystallites were easily nucleated on the WC particle. The new crystallites could not be nucleated on nano-ZrO2 particles. However, they could absorbe the nano-ZrO2 particles. There was a synergic effect of the nano-ZrO2 and WC particles.Compared with undoped-β-PbO2, the intensities of the undoped-WC-β-PbO2 peaks became less intense, and the halfwidth increased as the degree of crystallinity decrease. The peak intensity of WC phase was high in the doped-WC-β-PbO2 composite coating, especially the WC(110) and WC(101) planes, which were higher than theβ(200) andβ(211). Compared with undoped-β-PbO2, the doped-nano-ZrO2 particle could decrease the content of a-phase. The phase obtained in the doped-ZrO2-β-PbO2 composite coating had a strong preferential orientation of the crystallites. The presence of peak(2θ=27.43,32.75,44.738°) was regarded as the attributive indicator of the tetragonal PbWO4, and the PbWO4(112)'s peak was the highest intensity. The service life of the composite coating was longer than the undoped-β-PbO2 in H2SO4 solution, especially the anticipated service life of Al/α-PbO2-CeO2-TiO2/β-PbO2-(6.58wt.%)WC-(3.78wt.%) ZrO2 which could reach 20.1y in an industrial current density(1000A/m2).WC-ZrO2/β-PbO2-MnO2 composite coatings were prepared on the Al/α-PbO2-CeO2-TiO2 surface by anodic codeposition of PbO2,MnO2,nano-ZrO2 and WC particles. When the current density and Mn(NO3)2 concentration in electrolyte were controlled at 1.0A/dm2 and 80g/L, the contents of the nano-ZrO2 and WC particles of the composite coatings could reach 6.63% and 3.49%, respectively, and the service life of the composite electrode could reach 368h in a current density 2A/cm2. The increase of the current density leaded to the upgrowth of the grain structure and decreased the content of MnO2 in the composite coating. However, the increase of the current density had a little effect on the phase. The increase of the Mn(NO3)2's concentration made the grain structure refine, and the content of MnO2 of the composite coating increased exponentially. When Mn(NO3)2 concentration >80g/L, the crack of the composite coating appeared and the structure of deposit changed from crystalline to amorphous step by step.