Effect of the Current Density on Electrodepositing Alpha-Lead
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Acta Metall. Sin.(Engl. Lett.)Vol.22 No.5 pp373-382 October 2009 E®ect of the current density on electrodepositing alpha-lead dioxide coating on aluminum substrate Buming CHEN, Zhongcheng GUO ¤, Hui HUANG, Xianwan YANG and Yuandong CAO Faculty of Materials and Metallurgical Engineering, Kunming University of Science and Technology, Kunming 650093, China Manuscript received 22 September 2008; in revised form 13 March 2009 The ®-PbO2 electrodes are prepared by anodic electrodeposition on A1/conductive coating electrode from alkaline plumbite solutions in order to investigate the e®ect of the di®erent current densities on the properties of ®-PbO2 electrodes. The physic- ochemical properties of the ®-PbO2 electrodes are analyzed by using SEM, EDS, XRD, Tafel plot, linear sweep voltammetry (LSV) and A.C. impedance. A compact and uniform layer of lead dioxide was obtained at the current density of 3 mA¢cm¡2. A further increase in current density results in smaller particles with high porosity. EDS and XRD analyses have shown that the PbO2 deposited in alkaline conditions is highly non stoichiometric, and the PbO impurities are formed on the surface layer besides the ®-PbO2. The corrosion resistance of ®-PbO2 at the low current density is superior to that of the high current density. It can be attributed to a porous layer of deposited ¯lms at high current densities. When used as anodes for oxygen evolution 2+ ¡1 ¡1 in aqueous Zn 50 g¢L ,H2SO4 150 g¢L , the Al/conductive coating/®-PbO2 exhibits lower potential compared to Pb electrode. Al/conductive coating/®-PbO2 electrode with the best electrocatalytic activity was obtained at current density of 1 mA¢cm¡2. The lowest roughness factor was obtained at 1 mA¢cm¡2. KEY WORDS Alpha lead dioxide; Electrodeposition; Current density; Anodes; Oxygen evolution 1 Introduction At present, the lead alloys[1;2] containing small amounts of silver, tin, calcium or an- timony have been widely used as insoluble anodes in zinc electrowinning industry. The anodes can meet the need of zinc electrowinning, but oxygen overpotential of the anodes is still high. Lead ions dissolved in the electrolyte can be produced at the cathode and con- taminate the zinc metal[3]. Various kinds of the metal oxides anodes for oxygen evolution [4] [5;6] [7] reaction (OER) were developed, such as RuO2+TiO2 , IrO2+MnO2 , IrO2+Ta2O5 . All of these anodes, so called dimensionally stable anodes (DSAs), have better electrocatal- ¤Corresponding author: Professor, PhD; Tel.: +86 871 8352598. E-mail address: [email protected] (Zhongcheng GUO) DOI: 10.1016/S1006-7191(08)60111-8 ¢ 374 ¢ ysis. However, DSAs are of limited service life at high anode potential[8] and have not been used extensively in electrowinning operations mainly due to cost. Lead dioxides have been used frequently in electrocatalysis and industrial application because of its excellent properties such as good conductivity, low cost, high stability and relatively high service life. The lead dioxide of electrodepositing is known to exist in two polymorphs: orthorhombic ®-lead dioxide and tetragonal ¯-lead dioxide[9]. It is well known that the crystal structure of PbO2 deposited depends on the pH of the electroplating [9] solution: ®-PbO2 is obtained from bases, ¯-PbO2 from acids . The ®-PbO2 has a more compact structure than ¯-PbO2, which results in better contact between the particles. The more compact structure makes the ®-PbO2 more di±cult to discharge compared to the ¯- [10] [11] [12] PbO2 . RÄuetschi and Feng et al: show that the ®{PbO2 has a higher catalytic activity than ¯-PbO2 in dilute H2SO4 solution. [13;14] A new type of PbO2-coated metal anode has been widely used in electrolysis . This electrode consists of four layers: the base is a titanium plate, which is covered with a conductive undercoating (such an undercoating is necessary for protecting the substrate from passivation) as bottom, on which an ®-PbO2 coating as the intermediate layer and ¯nally ¯-PbO2 as the surface layer. Titanium is not, however, a viable substrate for practical electrodes in electrodepositing nonferrous metals. Aluminum is relatively cheap and has good conductivity. The electrode material by electrodepositing lead dioxide on Al substrate has a huge market prospects. A stress-free intermediate ®-PbO2 coating is produced by electrodeposition from an alkaline lead bath[15]. It plays a role of binder on the top ¯-PbO2 coating and can improve the serve life of electrode. However, the alpha-lead dioxide ¯lms deposited from basic solution are of highly porosity[16;17] and the research work is con¯ned to the internal stress measurement[15]. Methods of preparation of alpha lead dioxide as well as their characteristics have been reviewed[9]. The alpha-form, deposited from neutral and alkaline solution, has the cobumbite form[12;18]. It is claimed that the current density has signi¯cant e®ect on the phase composition and properties of alpha lead dioxide[17]. In this study the method of electrochemical anodic deposition was adopted to obtain alpha lead dioxide coating from alkaline plumbite solutions by di®erent values of current density on Al/conductive coating substrate. The e®ect of the current density on the structure and physicochemical properties of the deposited lead dioxide coating were discussed. 2 Experimental 2.1 Preparation of the PbO2/Al anode The PbO2/Al anode was produced by applying to an Al substrate with a conductive undercoating, then covering the undercoating with a coating consisting of the ®-PbO2 deposit. The substrates were aluminum rods with 60 mm in length and 9 mm in diameter, which were roughened by sand-blasting, degreased and chemically etched, then coated with a conductive coating. The procedure was described as follows: ¯rstly, the conductive coating solution was applied to the substrate by brushing; secondly, the substrate was dried under ultraviolet lamp for surface drying, and ¯nally dried in electricity box at 423 K for 2 h. The undercoating produced in this study was about 20 ¹m thick. The electrodeposition of ¢ 375 ¢ the ®-PbO2 was carried out under the following condition; 4 M NaOH with PbO(s) (the ¡ ¡2 soluble Pb(II) species were HPbO2 anions), pH>14, anode current density 1{5 mA¢cm , mild stirring using a magnetic stirrer, bath temperature 40 ±C, and electroplating time 2 h. 2.2 Characterization of PbO2/Al electrode The surface morphology of the coatings was examined by SEM (XL30 ESEM, Philip, Holand). The crystalline compositions of the ¯lms were determined by using EDS (PHOENIX, EDAI, USA). The crystalline structure of the ¯lms was studied by X-ray di®ractometer (using CoK® radiation, D8ADVANCE, Bruker, Germany). 2.3 Electrochemical measurement A single three-electrode compartment cell system was employed. Al/conductive coat- ing/®-PbO2 anode as the working electrode, Hg/Hg2Cl2 (KCl, saturated) as the refer- ence electrode, and a graphite as the auxiliary electrode. Typical anodic potentiody- namic polarization curves and anodic polarization curves were measured at 25 ±C in Zn2+ ¡1 ¡1 50 g¢L +H2SO4 150 g¢L solutions with a CHI660C electrochemical workstation. Electrochemical impedance measurement was carried out in the 4 M NaOH solution at 25 ±C with frequency range from 0.1 Hz to 100 kHz. The amplitude of the ac signal was 5 mV. The operating potential of 0.1 V was selected. The impedance data were converted into Nyquist data format, and then ¯tted to appropriate simulative circuits. 3 Results and Discussion 3.1 Surface morphological studies Fig.1 shows the SEM of deposited alpha lead dioxide on Al/conductive coating electrode from solution containing 4 M NaOH saturated with PbO(s) at di®erent current densities. Figs.1a{e are the microstructures at 2000 ampli¯cation with the current density of 1, 2, 3, 4 and 5 mA¢cm¡2 respectively, Figs.1a0{e0 are that at 10000 ampli¯cation with the ¡2 current density of 1, 2, 3, 4 and 5 mA¢cm respectively. The surface of PbO2 deposited at 1 mA cm¡2 consisted of crystals of large size with surface uniformity (Fig.1a0). At ¡2 0 current density of 2 mA¢cm (Fig.1b ), the PbO2 surface becomes more uniform consisted of rod-like grains. Further increasing the current density, there are a large number of small crystals without clear crystal edges (Fig.1c0). The ¯lm exhibits a ¯ber texture with good orientation and is recrystallized into the oriented ¯ber texture on standing in surface layer ¡2 of the formed ¯lm. At current density of 4 mA¢cm or higher, however, the PbO2 ¯lms of the ¯ber texture are randomly oriented and highly porous. A comparison between Fig.1d0 and e0 reveals that the clear crystal edges are appeared at 4 mA¢cm¡2. It is interesting to note that increasing of current density from 1 to 5 mA¢cm¡2 causes to decrease the diameter of the ¯ber. The ¯ber structure of the alpha lead dioxide deposit may be the result of complex- ity of lead cation in the electrodeposition solution. It has been reported[17;19] that PbO ¡ dissolve in alkaline solutions with formation of biplumbite ion HPbO2 and, to a lesser extent, polynuclear complexes. It seems probably that oxidation of such complexes can 2¡ lead to polynuclear complexes containing PbO3 . The hydrolysis of such species follows the oxidation on the electrode, thus the deposition rate of ®-PbO2 is slow. At high current densities the solid oxide may precipitate at some distance from the electrode surface due to the increased concentration of the complexed Pb(IV) cations. The agglomeration of ¢ 376 ¢ ± Fig.1 SEM photographs of ®-PbO2 prepared from 4 M NaOH saturated with PbO(s) (40 C) on Al/conductive coating electrode for 2 h at a current density of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5 mA/cm2, a0, b0, c0, d0 and e0 are at higher magni¯cation of Fig.1(a), (b), (c), (d) and (e), respectively ¢ 377 ¢ such precipitates on the surface coatings can lead to a randomly oriented surface, also, at ¡ the high current, the oxidation of HPbO2 to PbO2 (reaction 1) in an aqueous electrolyte, Oxidation of water to oxygen (reaction 2) can also take place simultaneously.