β-PbO2 electrode in zinc electrowinning

Journal of Alloys and Compounds xxx (xxxx) xxx

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Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/b-PbO2 electrode in zinc electrowinning Sheng Chen a, Buming Chen a, b, **, Shichuan Wang a, Wenkai Yan a, Yapeng He a, *, Zhongcheng Guo a, b, Ruidong Xu a, c a b c

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China Kunming Hengda Technology Co. LTD, Kunming, 650106, China State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 July 2019 Received in revised form 9 September 2019 Accepted 2 October 2019 Available online xxx

Ag doped b-PbO2 anode material was prepared on Ti/SneSb-RuOx/a-PbO2 through electrodeposition technique to improve its electrochemical properties and corrosion resistance in zinc electrowinning. The chemical composition and surface morphology of the anodes were characterized by X-ray diffraction, Xray photoelectron spectroscopy and scanning electron microscopy. The electrochemical properties and corrosion resistance of the anode materials were established via linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy and accelerated life tests. Results showed that the grain size of b-PbO2 increased with AgNO3 concentration while Agþ was oxidized to AgO and adsorbed on the electrode surface to accelerate the growth of b-PbO2 grains. We also proposed the nucleation/ deposition mechanism of b-PbO2 electrode while verifying the role of Ag ion doping. The optimum electrocatalytic performance could be achieved when the concentration of AgNO3 was 6 g L1. Compared with that of the undoped pure b-PbO2 anode, the oxygen evolution potential of the Ti/SneSb-RuOx/aPbO2/AgeF-b-PbO2 was decreased by 64 mV. The service life of the doped FeAg-b-PbO2 anode could reach up as high as 249 h, which was 1.66 times of pure b-PbO2 anode. © 2019 Elsevier B.V. All rights reserved.

Keywords: Ag doping b-PbO2 composite anode Electrocatalytic activity Corrosion resistance Zinc electrowinning

1. Introduction At present, approximately 80% of the zinc output in industries is obtained via the hydrometallurgy and zinc electrowinning is considered as the most important process in zinc hydrometallurgy [1e4]. Typically, the energy consumption of zinc electrowinning accounts for as high as 80% of the total energy of zinc hydrometallurgy [5,6]. Given that PbeAg alloy is the main anode considered in zinc electrowinning industry, its high oxygen evolution overpotential results in high energy consumption during this process. Besides, the poor mechanical properties and easy bending features of PbeAg alloy also lead to susceptible short circuit and low current efficiency during the operation period. Furthermore, PbeAg alloy

* Corresponding author. ** Corresponding author. Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China. E-mail addresses: [email protected] (B. Chen), [email protected] (Y. He).

belongs to a typical hypoeutectic alloy, and the working conditions of the PbeAg anode focus on high current density (7.5 A dm¡2) and strong H2SO4 (>150 g L1) environment. As a result, boundary corrosion occurs around the contact surface in the solution and lead dioxide (PbO2) particles formed during the anodic oxidation would detach from the solution and contaminate the cathode product. At present, considerable research has been conducted on novel anode materials and their industrial preparation techniques to solve above problems [7e9]. Especially, anode materials with high conductivity, service life, electrochemical performance, mechanical strength and convenient processing have received special attention. The new titanium-based insoluble anode materials possess excellent electrochemical performance and electrocatalytic activity, high corrosion resistance and long service life. Especially, the material is highly valued by metallurgy, environmental protection, anti-corrosion and chemical workers. Through decades of research, various titanium-based metal oxide electrodes with improved performance have emerged as high efficient electrocatalytic

https://doi.org/10.1016/j.jallcom.2019.152551 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551

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materials. Commonly, titanium-based metal oxide (TBMO) electrodes can be divided into two types according to the thickness of the coating [10,11]: (1) thinly coated TBMO electrode with coating thickness of generally several micrometers to more than a dozen micrometers; (2) thickly coated TBMO electrode with coating thickness above 500 mm including PbO2 and MnO2 electrodes. As an anode active material in H2SO4 medium, PbO2 electrode possesses the advantages of good chemical stability, high oxygen evolution overpotential, low cost and long service life [12e14], which has been employed as a catalytic material in various fields, including zinc hydrometallurgy [7], sewage treatment [15], drug degradation [16] and lead-acid batteries [17,18]. However, PbO2 electrode is not ideal for actual production because of its disadvantages, such as porosity, large internal stress and easy loss. As a solution, element and composite doping in different kind electrolytes [19e24] including F, Fe3þ, Ni2þ, Sn4þ and Agþ have been employed in the bPbO2 coating and have consequently improved the stability and electrocatalytic activity of the anode to varying degrees, which has also been demonstrated to be as an effective approach. In this work, anodic electrodeposition of b-PbO2 layer was conducted using titanium sheet as the matrix and SneSb-RuOx/aPbO2 as the interlayer. The Ag ion doping was employed to enhance the electrochemical properties of b-PbO2 coating, thus generating high-performance titanium-based composite electrode materials. Meanwhile, the effect of Ag ion on the surface morphology and crystalline structure was illustrated in details. Furthermore, the deposition and doping mechanism of PbO2 electrode were also discussed to explore the role of Ag doping. The electrochemical behavior including oxygen evolution kinetics and corrosion resistance were conducted in details to elaborate the discrepancy and origin.

NaOH. The deposition time was 1 h, current density was controlled  at 3 mA cm2, and bath temperature was controlled at 40 C. Meanwhile, magnetic stirring was performed to enhance the electrolytic diffusion property.

2. Experimental sections

2.2. Characterization and electrochemical measurements

2.1. Preparation of the PbO2 electrodes

The surface morphology of PbO2 electrode was characterized by scanning electron microscopy (SEM, SU8010, HITACHI, Japan), X-ray diffraction (XRD, Rigaku D/Max 2550, Japan) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB, USA) techniques. The flame atomic absorption spectrophotometry (FAAS, Rayleigh WFX 320, China) was conducted to determine the elemental content of Ag and F in the coatings. The linear sweep voltammetry (LSV), cyclic voltammetry (CV), chronoamperometry measurements and electrochemical impedance spectroscopy (EIS) tests were performed on an electrochemical workstation (CHI760E, China) using a three-electrode system. The working electrode is a single-sided insulated 1.0 cm2 electrode connected to a reference electrode (MSE), which is also connected to a saturated potassium sulphate agar salt bridge as the reference electrode. The Pt sheet was served as the counter electrode. The electrolyte solution was composed of 50 g L1 Zn2þ and 150 g L1 H2SO4, and the tempera ture was maintained at 35 ± 0.5 C. The corresponding potential range of the CV and LSV tests was 0.5e2.0 V, EIS measurement was conducted in the frequency range from 100 kHz to 10 mHz with amplitude of 5 mV. The potential values in this work were all quoted with respect to MSE. Formula (1) was used to investigate the oxygen evolution overpotential (OEP, h) of the electrode material [8]:

2.1.1. Ti substrates pretreatment The first process involved the pretreatment of titanium substrate with four steps [25]. Firstly, titanium plates (TA1) with a dimension of 10 mm  20 mm  2 mm were immersed in 10 wt% NaOH solution at 70  C for 30 min and then washed with deionised water to remove grease or oil. The cleaned titanium plates were etched in mixed acid (VHF/VHNO3/VH2O ¼ 1:4:5) solution for 3 min to remove the oxide layer and then washed with deionised water. Next, the cleaned titanium plates were etched in 20 wt% HCl so lution for 2 h at 90 C to form a rough surface. Finally, the pretreated titanium substrate was stored in ethanol solution and 2% oxalic acid at room temperature before use. 2.1.2. Preparation of SneSb-RuOx interlayer The precursor solutions for the SneSb-RuOx preparation mixture of SnCl4$5H2O, SbCl3 and RuCl3$3H2O were dissolved in a mixed solvent at a molar ratio of 6:1:2. The chloride was completely uniformly dissolved at a certain temperature to obtain a coating liquid, which was evenly brushed on the acid-etched titanium  plate. Then, the plate was placed in an oven at 120 C, removed after 5 min and placed under room temperature. Finally, the coating was  placed under 500 C for high-temperature calcination for 10 min. The brushing, drying and calcining steps were repeated 10 times. At the last time, the coating was placed in a muffle furnace for 1 h. 2.1.3. Preparation of a-PbO2 middle-layer The a-PbO2 coating was prepared on Ti/SneSb-RuOx interlayer via anodic electrodeposition with Ti mesh as the cathode, and the corresponding solution containing 22.32 g L1 PbOþ140 g L1

2.1.4. Preparation of b-PbO2 top-layer Ag doped b-PbO2 coating on Ti/SneSb-RuOx/a-PbO2 was prepared using anodic electrodeposition method with Ti mesh as the cathode. The corresponding electrolyte contained 250 g L1 Pb(NO3)2, 10 g L1 HNO3, 0.5 g L1 NaF and 0e8 g L1 AgNO3. The deposition time was 1 h, current density was controlled  at 30 mA cm2, and bath temperature was controlled at 60 C. Magnetic stirring was performed to enhance the electrolytic diffusion property. After that, the experimental samples obtained under different conditions were denoted as PbO2ex, where x (x ¼ 0, 2, 4, 6, 8) presents the AgNO3 concentration. Besides, the current efficiency of the formation PbO2 was calculated based on the loading capacity of PbO2 and total electric charge. That is, the current efficiency was obtained from the Qox/QT ratio, where Qox is the oxidation charge from soluble Pb2þ to PbO2 during the electrodeposition, and QT the total charge passed during electrolysis. As a result, the current efficiency for the PbO2 deposition for all the conditions was around 94e95%, where the current efficiency below 100% could be ascribed to the oxygen evolution side reaction during anodic oxidation. Moreover, there was no obvious difference between different anodes while the value agreed with well previous report about the deposition of PbO2 under galvanostatic deposition [23,26].

h ¼ E þ 0:640V  1:241V  iRs

(1)

The oxygen evolution overpotential h and the current density J exhibit a semi-logarithmic relationship, as shown in formula (2):

h ¼ a þ blgJ

(2)

where a and b are Tafel parameters during anodic polarization:

Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551

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a ¼  2:3ðRT = bnFÞlgJ0

(3)

b ¼ 2:3RT=bnF

(4)

E represents the OEP with respect to the reference electrode (MSE) measured by the anodic polarization curve in the experiment, and 1.241 V is the standard potential in the measurement system (50 g L1 Zn2þþ150 g L1 H2SO4, 35  C). The oxygen evolution equilibrium potential is calculated using the Nernst equation. 0.640 V is the potential of the MSE relative to the standard hydrogen electrode. J is the measured current density at the relative potential. Rs is the electrolyte resistance between the reference electrode and the working electrode. As for the accelerated life test, the prepared electrode was used as the anode, whereas the titanium mesh served as the cathode and the test was conducted in the 150 g L1 H2SO4 electrolyte. The current density of the accelerated life test was maintained at  1 A cm2 with temperature 40 C. The variation curve of the cell voltage (U) versus service time (t) was simultaneously recorded, and the service life of the electrodes was considered terminated when the cell voltage reached up to 10 V. 3. Results and discussion 3.1. XRD analysis of top PbO2 layer The XRD spectrum of b-PbO2 electrodes prepared at different AgNO3 concentration are shown in Fig. 1. The electrode surface presents the b-PbO2 crystal form only, and there is no PbF2 phase. Given that the F content in the coating is small, F phase is difficult to form with Pb. Previous report [27] believed that F may participate in the phase transformation during the PbO2 growth. The F content in the electrode material is extremely low and may just replace coordination in the PbO2 crystal as the F radius is not considerably different from the O2 radius. As a result, no PbF2 phase is observed on the surface. Meanwhile, no phase containing Ag occurs on the surface of the electrode, and the reason could be probably be ascribed to the fact that the Ag content in the plating layer is extremely low and cannot form a phase with Pb. As displayed in Fig. 1, all samples exhibit the b-PbO2 phase (PDF 41e1492) regardless of the AgNO3 concentration in the electrolyte. The crystal intensity of the (200) and (310) planes firstly increase and then decrease with the increase in AgNO3 concentration in the

3

plating solution. The result demonstrates that Ag addition in the bPbO2 coating could promote the growth of b-PbO2 crystal along the (200) and (310) planes, but the effect is weakened when the value exceeds 6 g L1. 3.2. XPS elemental analysis XPS is employed to analyze the elemental valence and chemical composition of the b-PbO2 layer and understand the role of Ag ion. Fig. 2a reveals the XPS full spectrum of PbO2-6 and PbO2-0 electrodes, which exhibit Pb, C, O, F and Ag elemental signals. The corresponding XPS data of the binding energies of Pb4f, O1s and Ag3d of the b-PbO2 electrode are displayed in Fig. 2b, c, and d, respectively. The detailed spectrum of Pb is shown in Fig. 2b. More precisely, the Pb4f7/2 of PbO2-0 electrode is divided into two independent peaks located at 137.5 and 138.1 eV, which represent Pb(IV)4f7/2 and Pb(II)4f7/2, respectively. The peak-to-peak area ratio of Pb(IV)4f7/2 and Pb(II)4f7/2 is 1:2 based on the calculation. Similarly, the Pb4f7/2 of PbO2-6 electrode can be divided into two independent peaks representing the peak areas of Pb(IV)4f7/2 and Pb(II)4f7/2 [28], where the corresponding area ratio locates at 1:1. Moreover, the binding energy between 4f7/2 and 4f5/2 of the PbO26 and PbO2-0 electrodes is nearly 4.9 eV, which is similar to that reported in previous literature [29]. The analysis indicates that the valence states of the PbO2-6 electrode are þ2 and þ4. On the other hand, the PbO2-0 electrode is nearly twice as large as the Pb(II)4f7/2 of the PbO2-6 electrode. The above conclusion could be related to the following reasons. Firstly, the addition of AgNO3 into the electroplating solution leads to a considerably high oxidation of Pb(II) to Pb(IV). Furthermore, the higher grain growth rate than the nucleation growth rate also results into a relatively low electrode surface area. Fig. 2c depicts the O1s spectrum of the electrode surface. The peak at 531.5 eV represents the characteristic peak of free hydroxyl groups on the electrode surface and its physical adsorption capacity. The characteristic peak located at 529.6 eV represents the lattice oxygen of b-PbO2, which in turn reflects the electrochemical catalytic performance of the electrode. However, the peak area of the lattice oxygen peak of PbO2-6 is reduced by 49.3% while the peak area of the free hydroxyl group is decreased by 26.5%, and those of the lattice oxygen and free hydroxyl groups of the electrode are almost reduced by half. This occurrence is probably due to the addition of AgNO3. Generally speaking, Ag ion enters crystal lattice to replace the Pb(II) site during the electrodeposition process, and largely decreases the lattice oxygen and adsorbed oxygen. Fig. 2d shows the peak at Ag3d on the electrode surface at 367.5 and 373.6 eV, indicating that Ag is successfully doped in the oxide layer and the corresponding form is Ag(II)3d at the same time from the XPS data. This result is also consistent with those of the XPS analyses of lead and oxygen [30]. In addition, the FAAS for the doped coatings are conducted to determine the additive content in the PbO2 electrode, and the results are displayed in Fig. 3. More precisely, the F content is kept at around 0.30 ± 0.03% due to the same NaF concentration for all PbO2 coatings. Whereas, the Ag content in the doped PbO2 increases gradually with the increase of AgNO3 concentration in the electrolyte, and the value reaches basically steady-state with 169.6 ppm at 8 g L1, demonstrating the presence of F and Ag elements. Furthermore, the result also reveals that the Ag content in the doped PbO2 has a positive correlation with the AgNO3 concentration. 3.3. The surface morphology of Ag doped PbO2

Fig. 1. XRD patterns of PbO2 electrodes prepared under different AgNO3 concentration.

Fig. 4 reveals that all the SEM images show the pyramid-shaped

Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551

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Fig. 2. (a) The XPS full-spectrum, (b) Pb4f spectrogram, (c) O1s spectrogram and (d) Ag3d spectrogram of PbO2-0 and PbO2-6 anodes.

AgNO3 amount of is 8 g L1, the increased internal stress results in the formation of grain cracks. In addition, the roughness of the electrode surface and the size of the crystal grain can reflect the active surface area and electrocatalytic activity of the electrode [31], where rough electrode surface with small crystal grain generally indicates large active surface area. Though the increase of crystal grains reduces the relative specific surface area of the electrode, the adsorption of Ag ion leads to the extremely great adsorption capacity to the oxygen-containing group on the electrode surface and improvement of the electrochemical performances. Nevertheless, the excessively increase of the crystal grain would reduce the effective contact area of Ag ion, free hydroxyl and lattice oxygen with the reactant, thereby weakening the electrochemical performances. Finally, we find that the addition of AgNO3 increases the electrode grain size and improves the electrochemical performance, and 6 g L1 is considered as an optimal parameter. Fig. 3. The variation of Ag and F content in b-PbO2 coatings.

b-PbO2 grain shape while keeping the dense distribution. With the increase of AgNO3 concentration, the grain size and pyramid shape of b-PbO2 grain gradually increase, and the characterization becomes highly evident. Meanwhile, we also calculate the average crystal sizes of prepared Ag doped b-PbO2 electrodes by Scherrer’s equation from the XRD data, which are 16.13, 18.81, 19.59, 22.10 and 19.75 for PbO2-0, 2, 4, 6 and 8 electrodes, respectively, further supporting the observation on the increasing grain size of SEM images. The reason could be probably related to the increasing amount of Ag adsorbed with the increase of the Agþ content, leading to the faster grain nucleation rate. However, when the

3.4. Deposition mechanism of PbO2 and role of Ag ion Fig. 5a presents the cyclic voltammetry curves of Ti/SneSbRuOx/a-PbO2As under electrolyte with different AgNO3 concentration. The oxidative peak I corresponds to the formation of PbO2 and oxygen evolution reaction while reductive peak II could be attributed to the reduction of Pb(II) species. According to Fig. 5b and relevant literature [32], the nucleation ring has two intersections, in which the potential can be defined to be at least close to the nucleation potential (En) and reversible potential (Ec). Table 1 lists the nucleation overpotential (DEn) under different Agþ contents from the potential values at the intersections. Under the same condition, a high Agþ content stands for small DEn value and fast nucleation rate.

Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551

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Fig. 4. SEM images of different Ag doped b-PbO2 coatings and evolution of crystal grain size, (a, f) PbO2-0, (b, g) PbO2-2, (c, h) PbO2-4, (d, i) PbO2-6, (e, j) PbO2-8.

Fig. 5. (a, b) Cyclic voltammetry curves of Ti/SneSb-RuOx/a-PbO2 electrodes in 0.8 M Pb(NO3)2þ10 g L1 HNO3þ0.5 g L1 NaF þ0e8 g L1 AgNO3, (c) schematic electrodeposition mechanism of the PbO2 coating on the Ti/SneSb-RuOx/a-PbO2 electrode.

Table 1 The nucleation overpotential (DEn) from the nucleation loops of the cyclic voltammetry curves. Anode sample PbO2-0 PbO2-2 PbO2-4 PbO2-6 PbO2-8

Ec (V) 0.767 0.769 0.790 0.871 0.806

En (V) 1.476 1.367 1.369 1.323 1.157

DEn (V) 0.709 0.598 0.579 0.452 0.351

The deposition mechanism of PbO2 and the role of Ag ion are elaborated in this part. The electrodeposition of PbO2 layer may follow a stereotactic growth mechanism and is not just simple electrochemical oxidation deposition [33]. Based on previous results, the intermediate product of electrodeposition process is Pb(II), and this finding is confirmed by the XPS data. According to the above analysis, a possible nucleation mechanism involving addition of AgNO3 to b-PbO2 can be achieved. A possible scheme for

Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551

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preparing Ag-doped PbO2 on Ti/SneSb-RuOx/a-PbO2 electrode is described in Fig. 5c. In the first step, water electrolysis produces the adsorbed hydroxyl radicals, which are then chemisorbed on Ti/ SneSb-RuOx/a-PbO2 to form oxygen-containing substances. H2O/OHads þ Hþþ e

(5)

OHads þ Pb2þ/OePbþþHþ

(6)

induction period t0, which could be caused by accumulation of intermediate products involved in the crystallisation stage. After that, the current response reaches a maximum value im at tmax, and the current decreases slowly to a steady-state value owing to the electrochemical oxidative reaction [37e39]. Meanwhile, induction period t0 maximum current imax and tmax depend on the applied potential and Ag ions concentration. That is, the increase of deposition potential and Ag ions concentration lead to a decrease of the induction time and increase of the steady-state current, revealing an acceleration of PbO2 crystallisation in the presence of high potential and Ag ions. Therefore, the addition of AgNO3 facilitates the electrocrystallisation of PbO2 grains [40], where the PbO2 grain nucleation rate is higher than crystal growth rate. According to the nucleation/growth of Scharifker-Hill’s mode, the curves of (i/im)2 vs. t/tm in different systems are estimated, and the results are depicted in Fig. 6b, d. Obviously, the experimental results under different applied potentials are close to the theoretical curves for instantaneous nucleation, revealing of the electrodeposition of PbO2 primarily follows the instantaneous nucleation mode. Furthermore, the introduction of Ag ion into the system does not alter the nucleation mode [41].

In the second step, the lead oxide crystals, which may be adsorbed on the surface of the electrode, grow continuously until PbO is formed. With the continuous PbO synthesis, the Pb(II) to Pb(IV) oxidation also occurs inside the electrode coating. In this process, oxygen transfer may transpire with the combined water. However, when the PbO layer grows to a certain thickness, the PbO inside the electrode cannot easily come in contact with the Pb2þ and H2O in the solution. OePbþþH2O/OePb(OH)þHþ

(7)

OePb(OH)þPb2þ/2PbO þ Hþ

(8)

PbO þ H2O/PbO2 þ2Hþ þ2e

(9)

AgþþOHads/AgO þ Hþ

3.5. Oxygen evolution activity of PbO2 electrode

(10)

Commonly, it is desirable to obtain a low oxygen evolution potential anode to lower the cell voltage and energy consumption during the zinc electrowinning [42]. The properties of the anode materials depends mainly on the roughness and chemical properties of the oxide, which in turn leads to changes in the bond strength of the oxygen-containing material chemisorbed on the electrode surface [43]. Fig. 7 displays the anodic polarization curves of different b-PbO2 samples. It could be concluded that the oxygen evolution potential of the electrode decreases with the addition of AgNO3 concentration under the current density 500 A m2 commonly used in industrial electrowinning zinc. As the addition of Agþ ion can provide more active sites for the oxygen evolution reaction, the electrocatalytic activity is gradually improved. More details, the oxygen evolution potential of the PbO2-6 electrode is 64 mV lower than that of the PbO2-0 electrode, indicating that the addition of Agþ can effectively improve the electrocatalytic activity of the b-PbO2 electrode. However, when the concentration is greater than 6 g L1, there is not too much difference between the oxygen evolution potential, and the increase of the Agþ causes the

As for the addition of AgNO3, the Ag ions could form a highvalence AgO adsorption layer on the anode surface with strong electron-donating ability [34], which could provide an active site for active oxygen, such as OH or O, to promote the formation of O2 by the adsorbed oxygen and O [35]. Meanwhile, the AgO adsorption layer could accelerate the oxidation conversion form Pb(II) to Pb(IV) and growth of PbO2 layer. Then, the titanium-based PbO2 electrode with pyramidal pattern, well crystal and excellent electrocatalytic activity is obtained, as shown in Fig. 4. Chronoamperometry is commonly employed to elaborate the nucleation mode of electrodeposition process, and ScharifkereHill’s mathematical mode is considered as one typical theoretical mode [36], where there are instantaneous and continuous nucleation modes for the nucleation/growth process. The corresponding relationship between (i/im)2 vs. t/tm in nondimensional form can be described in following equations, where i is the current response at time t, and im is the maximum current occurring at time tm.


Instantaneous nucleation


Continuous nucleation

2

i im

i im






2

1:256

6 ¼ 1:954241  e




2 2:3367

6 ¼ 1:225441  e

 32 t= tm

7 5

2 32 t= tm

7 5




1 t=t m




To better understand some details of PbO2 nucleation under initial electrodeposition conditions, we performed chronoamperometry measurements at Ti/SneSb-RuOx/a-PbO2 electrode. Fig. 6a, c exhibits the current response of Ti/SneSb-RuOx/aPbO2 electrode at different potentials and solutions. As observed, a sharp initial increase of the current response occurs due to the double-layer charge process and reaches to a minimum i0 at the

(11a)

1 t=t m

(11b)

excessively increase of b-PbO2 crystal grain, which in turn results into the decrease of the surface roughness and active area participating in the reaction. Meanwhile, excellent electrocatalytic activity of Ag doped PbO2 electrode could be attributed to the strong adsorption capacity for oxygen-containing groups Ag(II) could also to some extent. To further explain the effect of AgNO3 on the oxygen evolution

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Fig. 6. (a) i-t curves of the Ti/SneSb-RuOx/a-PbO2 electrode in 0.8 M Pb(NO3)2þ10 g L1 HNO3þ0.5 g L1 NaF þ6 g L1 AgNO3 at different potentials, (c) i-t curves of the Ti/SneSbRuOx/a-PbO2 electrode in 0.8 M Pb(NO3)2þ10 g L1 HNO3þ0.5 g L1 NaF þ0e8 g L1 AgNO3 with potential 1.20 V, (b, d) represent the corresponding comparison between experimental data and standard instantaneous/continuous nucleation mode.

Fig. 7. Anodic polarization curves of different b-PbO2 samples under scan rate 5 mV s 1 .

Fig. 8. The EIS curves of oxygen evolution reaction on various b-PbO2 coatings.

capacity of b-PbO2, we conduct the EIS studies on the b-PbO2 coatings. Fig. 8 shows the electrochemical impedance of PbO2 electrodes with different AgNO3 content in 150 g L1 H2SO4þ50 g L1 Zn2þ solution with applied potential 1.40 V, which corresponds to the oxygen evolution zone. An equivalent circuit presented in Fig. 9 is proposed to simulate the electrochemical process of the oxygen evolution reaction [44]. The simulation data for each parameter in Fig. 9 are listed in Table 2. Rs represents the ohmic resistance, including the resistance of the electrolyte and the active material. Rt

represents the charge transfer resistance that reflects the oxygen release reaction activity. Element C represents the electric double layer capacitance of the electrode surface. Generally, the electrode catalytic activity is related to charge transfer resistance and diffusion capacitance, which are in turn related to the radius of the Nyquist curve. A small radius of the curvature indicates good catalytic activity [45]. The diameter of the five semicircles in Fig. 8 represents the Rt value, and Table 2 lists the charge transfer resistance Rt of the oxygen evolution reaction on

Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551

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S. Chen et al. / Journal of Alloys and Compounds xxx (xxxx) xxx Table 3 Overpotential and kinetic parameters for oxygen evolution on various b-PbO2 samples. Anode sample

Fig. 9. Equivalent circuit diagram of AC impedance spectrum.

Table 2 The equivalent circuit parameters of various b-PbO2 coatings. Anode sample

Rs (U cm2)

Rt (U cm2)

C (mF cm2)

PbO2-0 PbO2-2 PbO2-4 PbO2-6 PbO2-8

0.9427 1.421 1.096 1.059 0.8853

19.12 10.6 8.373 4.219 4.496

3602 6339 1265 3023 4032

different samples. The Rt value of the PbO2-6 electrode is 4.219 U cm2, which is lower than that of the other electrodes. The undoped PbO2 electrode possesses the largest Rt value of 19.12 U cm2. Moreover, the change of the charge transfer resistance Rt value gradually decreases with the addition of AgNO3. A minimum value is reached when the addition amount is 6 g L1. On the contrary, the Rt value increases slightly when the concentration reaches 8 g L1, indicating that doping with AgNO3 can greatly improve the catalytic activity of the coating and the addition of 6 g L1 AgNO3 results in an optimum catalytic activity. According to the reaction mechanism of the oxygen evolution of the electrode, such activity primarily depends on the active site of the PbO2 coating, and the large active sites mean the good reactivity. This conclusion is in accordance with the electrodes having high oxygen evolution activity and low Rt from previous research [46]. The oxygen evolution overpotential can be obtained by calculating the anodic polarization curve and AC impedance value, and the oxygen evolution activity can be determined by comparing the oxygen evolution overpotential. More precisely, the anodic polarization curve is converted into the Tafel curve (h-lgi) and linearly fitted using Origin software to obtain a and b value, and the results are displayed in Fig. 10 and Table 3. The kinetic parameters for

PbO2-0 PbO2-2 PbO2-4 PbO2-6 PbO2-8

h (V)

500 A m2 0.944 0.931 0.926 0.903 0.921

a (V)

b (V)

1.246 1.279 1.224 1.171 1.216

0.232 0.268 0.229 0.206 0.227

oxygen evolution on different electrodes at current densities of 500 and 1000 A m2 are calculated and listed in Table 3. Table 3 shows that under the conditions of 500 and 1000 A m2, PbO2-6 electrode exhibits the lowest OEP value (0.903 V and 0.965 V), indicating the high probability of oxygen evolution reaction. When the amount of AgNO3 increases from 0 to 6 g L1, the OEP gradually decreases, and the OEP begins to increase at 8 g L1. The electrode electrocatalytic performance can be expressed with the electrode kinetic parameters. Commonly, value a could denote the cell voltage [47], which is that the smaller a value, the lower the cell voltage. Table 3 shows that value a of the PbO2-6 electrode and PbO2-0 differs by 0.075, and the variation of a value and OEP follows the same tendency. From the values of b presented in Table 3, the electrode prepared with 6 g L1 AgNO3 exhibits the highest catalytic activity for oxygen evolution reaction. The mechanism of oxygen evolution on PbO2 electrode in an acid solution may be as follows [48,49]:

S þ H2 O ¼ S/OH þ Hþ þ e

(12)

S/OH ¼ S/O þ H þ þ e

(13)

S/OH þ S/OH ¼ S/O þ S þ H2 O

(14)

S/O þ S/O ¼ 2S þ O2

(15)

where S represents the number of active sites on the surface of the electrode, and S/O represents the oxygen-containing group adsorbed on the surface of the electrode. Therefore, the larger the S value of the PbO2 electrode, the more favourable the precipitation of oxygen and the greater the number of high electrochemical active sites on the surface of the electrode. The amount of voltammetric charge (q*) associated with the actual surface area and number of active sites can reflect the electrochemical activity of the electrode, which could be obtained by integrating the cyclic voltammetry curve. A large q* indicates a high electrode activity. We calculate q* value to estimate the electrode activity using the method in previous report [50e52], and the corresponding equation can be expressed as follows:

ðq* Þ1 ¼ ðqT Þ1 þ kv1=2

Fig. 10. Tafel curves of various b-PbO2 samples.

1000 A m2 1.014 1.011 0.995 0.965 0.989

(16)

where qT is the true charge of the electrode surface, and k represents the reciprocal of the voltammetric charge and the square of scan rate. The voltammetric curves of different PbO2 samples over the entire potential range 0.1 Ve0.7 V under different sweep speed are exhibited in Fig. 11. Here, q* is measured in an acid ZnSO4 solution to determine the electrochemically active surface area of PbO2 electrode [53]. Therefore, the capacitance region q* of PbO2 can be defined as the electrochemically active surface area of PbO2 in the sulphate electrolyte. The relationship between the reciprocal of q* and the square root of scan rate is shown in Table 4 and Fig. 12. For the PbO2 electrode, q* at low scan rates reflects the deep

Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551

S. Chen et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

9

release is that more active surface sites are formed on the PbO2-6 electrode. Through the measurement of the active area, the effective surface area of the PbO2-6 electrode is higher than that of the other electrodes. Therefore, the electrode having a low Rt value and a low oxygen evolution potential is confirmed to have more active sites and higher oxygen evolution activity. 3.6. Corrosion resistance evaluation

Fig. 11. The CV of b-PbO2 coatings prepared from different concentrations of AgNO3.

Table 4 The detailed voltammetric charge (q*, C cm2) of b-PbO2 coatings prepared from different AgNO3 concentration with various scan rate (mV s1). Scan rate PbO2-0 PbO2-2 PbO2-4 PbO2-6 PbO2-8

5 0.00413 0.00548 0.00560 0.00716 0.00587

10 0.00331 0.00420 0.00438 0.00547 0.00474

25 0.00249 0.00295 0.00306 0.00353 0.00338

50 0.00198 0.00231 0.00243 0.00282 0.00267

100 0.00152 0.00171 0.00192 0.00228 0.00205

150 0.00108 0.00123 0.00150 0.00182 0.00151

200 0.00095 0.00106 0.00126 0.00151 0.00128

discharge performance, whereas q* at high scan rates reflects power performance. The q* and k constants of the PbO2 electrodes with scan rates of 5 and 200 mV s1 are listed in Table 4 based on the slope of the simulation line. The k constant represents the change rate of the electrochemically active surface area with the scan rate. The smaller the k constant, the slower the change trend of the electrochemically active surface area decreases with the scan rate. Therefore, compared with the q* and k values, the results show that the electrode with 6 g L1 AgNO3 has a higher activity. The explanation for the high electrochemical activity of oxygen

Fig. 12. The dependence of reciprocal of q* on v1/2 of different PbO2 samples.

Service life is one of the important factors in the practical application of electrodes to evaluate the corrosion resistance. As shown in Fig. 13, the PbO2-6 electrode possesses the longest service life of 249 h, exhibiting well stability, which is 1.66 times of the PbO2-0 electrode life (150 h). This result is further compared with those of the PbO2 anodes prepared from other report, as shown in Table 5. The results show that the addition of AgNO3 to the b-PbO2 plating solution can improve the electrochemical stability of the bPbO2 film. The possible reason for the increase in service life is that the addition of AgNO3 improves its morphological structure [54]. The surface layer of AgO oxidizes more PbO to PbO2, and the latter is more stable than former, then enhancing the corrosion resistance. The corrosion life of the PbO2-6 electrode is longer than those of the other electrodes. This result may be attributed to the growth of the PbO2 crystal grain, which reduces the gap between the grain boundaries, making the electrode surface denser. The grain of the PbO2-8 electrode is larger than that of the PbO2-6 electrode, but the service life is decreased, probably because the nucleation rate of the PbO2-8 electrode is considerably larger than the growth rate of the grain, thereby increasing grain addition. Moreover, the internal stress of the crystal grain is increased and the surface is cracked; hence, the density of the electrode surface and corrosion resistance are reduced. This occurrence is attributed to the fact that the compact surface can block the penetration of the supporting electrolyte into the titanium substrate through the crack and delay the formation of the nonconductive layer, further increasing the electrode life. 4. Conclusions In summary, we investigate the role of Ag ion doping to boost the electrochemical performance and corrosion resistance of bPbO2 electrode in zinc electrowinning. The doping of Ag ion is

Fig. 13. The accelerated life test of b-PbO2 coatings prepared from different AgNO3concentration.

Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551

10

S. Chen et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

Table 5 Comparison of accelerated service life with previous literatures. Electrode

Solution composition

Current density (A cm2)

Lifetime (h)

Ref.

PbO2-6 Ti/SneSb-RuOx/b-PbO2 Ti/SnO2eSb2O3-CNT/b-PbO2 LaFeTi/SbeSnO2/PbO2 Ti/F-CCB-PbO2 Ti/Sn-SbOx/a-PbO2/Bi-CNT-b-PbO2 TieSn/Sb/RuO2-b-PbO2

150 g L1 H2SO4 150 g L1 H2SO4 þ1 g L1 Cl 3 M H2SO4 1 M H2SO4 1 M H2SO4 2 M H2SO4 0.5 M H2SO4

1.0 1.0 0.5 4 0.5 1.0 4.0

249 96 291 33 1.33 233 48

This work [11] [53] [55] [56] [57] [58]

confirmed by the XPS measurement, and the nucleation and deposition mechanism of Ag doped PbO2 electrode is proposed and verified. The oxygen evolution reaction kinetics, microscopic surface morphology and corrosion phase of the anode layers vary with the AgNO3 content. As a result, Ag doping could enlarge the grain size and accelerates the nucleation rate of b-PbO2. The oxygen evolution overpotential h and charge transfer resistance Rt primarily exhibit a declining trend with the increase of Agþ concentration, and this depolarization may be due to the catalytic effects of AgO. The optimum electrocatalytic performance could be achieved when the concentration of AgNO3 is 6 g L1. Furthermore, the accelerated life of PbO2-6 electrode is 249 h, exhibiting well stability and the value is 1.66 times of the undoped PbO2 electrode. These attractive results are important for the development of novel anodes materials and promoting the promising applications as enormous potential electrode material in zinc electrowinning in the future.

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Acknowledgments This research is funded by the Natural Science Foundation of China (Project No. 51564029, 51504111 and 51874154), China Postdoctoral Science Foundation (2018M633418), Technology Innovation Talents Project of Yunnan Province (2019HB111) and Applied Basic Research Program of Yunnan Province (2019FD066). References [1] M. Bestetti, U. Ducati, G. Kelsall, G. Li, E. Guerra, Use of catalytic anodes for zinc electrowinning at high current densities from purified electrolytes, Can. Metall. Q. 40 (2001) 451e458. [2] M.K. Jha, V. Kumar, R. Singh, Review of hydrometallurgical recovery of zinc from industrial wastes, Resour. Conserv. Recycl. 33 (2001) 1e22. [3] S. Gürmen, M. Emre, A laboratory-scale investigation of alkaline zinc electrowinning, Miner. Eng. 16 (2003) 559e562. [4] M. Karbasi, E.K. Alamdari, E.A. Dehkordi, Electrochemical performance of PbCo composite anode during Zinc electrowinning, Hydrometallurgy 183 (2019) 51e59. [5] C. Cachet, C. Rerolle, R. Wiart, Kinetics of Pb and Pb-Ag anodes for zinc electrowinning-II. Oxygen evolution at high polarization, Electrochim. Acta 41 (1996) 83e90. [6] I. Ivanov, Y. Stefanov, Z. Noncheva, M. Petrova, T. Dobrev, L. Mirkova, R. Vermeersch, J. Demaerel, Insoluble anodes used in hydrometallurgy: Part I. Corrosion resistance of lead and lead alloy anodes, Hydrometallurgy 57 (2000) 109e124. [7] C. Yang, S. Park, Electrochemical behavior of PbO2 nanowires array anodes in a zinc electrowinning solution, Electrochim. Acta 108 (2013) 86e94. [8] R. Xu, L. Huang, J. Zhou, P. Zhan, Y. Guan, Y. Kong, Effects of tungsten carbide on electrochemical properties and microstructural features of Al/Pb-PANI-WC composite inert anodes used in zinc electrowinning, Hydrometallurgy 125 (2012) 8e15. [9] B. Chen, W. Yan, Y. He, H. Huang, H. Leng, Z. Guo, J. Liu, Influence of F-doped bPbO2 conductive ceramic layer on the anodic behavior of 3D Al/Sn rod Pb0.75% Ag for zinc electrowinning, J. Electrochem. Soc. 166 (2019) E119eE128. [10] S. Ellis, N. Hampson, M. Ball, F. Wilkinson, The lead dioxide electrode, J. Appl. Electrochem. 16 (1986) 159e167. [11] B. Chen, S. Wang, J. Liu, H. Huang, C. Dong, Y. He, W. Yan, Z. Guo, R. Xu, H. Yang, Corrosion resistance mechanism of a novel porous Ti/Sn-Sb-RuOx/bPbO2 anode for zinc electrowinning, Corros. Sci. 144 (2018) 136e144. [12] L.S. Andrade, R.C. Rocha-Filho, N. Bocchi, S.R. Biaggio, J. Iniesta, V. GarcíaGarcia, V. Montiel, Degradation of phenol using Co-and Co, F-doped PbO2

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Please cite this article as: S. Chen et al., Ag doping to boost the electrochemical performance and corrosion resistance of Ti/SneSb-RuOx/a-PbO2/ b-PbO2 electrode in zinc electrowinning, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152551