PbO₂ modified with TiO₂-NTs composite materials with enhanced OER electrocatalytic activity for Zn electrowinning

Abstract

The high oxygen evolution overpotential of the Pb–Ag anode is one of the main reasons for the high energy consumption in Zn electrowinning. PbO₂, owing to its high conductivity, good corrosion resistance and low cost, is widely used as an excellent coating material. In present research, a novel composite Ti/TiO₂-NTs/PbO₂ material was synthesized through a facile anodization, annealing, electrochemical reduction and galvanostatic deposition. The surface morphology, internal structure and the mechanisms of TiO₂-NTs enhancing electrochemical performance were discussed. The results show that the self-organized high aspect ratio TiO₂-NTs with diameter of ∼120 nm and length of ∼8 μm were obtained on Ti substrate. The Ti/TiO₂-NTs/PbO₂ composite material exhibits excellent oxygen evolution performance and good stability in Zn electrowinning simulation solution (50 g L⁻¹ Zn²⁺, 150 g L⁻¹ H₂SO₄) at 35 °C. Its oxygen evolution overpotential is only 630 mV under current density 50 mA cm⁻², which is 332 m lower than that of Pb-0.76 wt% Ag (η = 962 mV) and only increases 22 mV after 5000 cycles of CV scanning. Its outstanding electrochemical performance is mainly ascribed to the introduction of TiO₂-NTs in Pb(CH₃COO)₂ media since it refines the crystal grains, increases the electrochemical surface area, greatly reduces the charge transfer resistance (25.4 Ω cm² to 2.337 Ω cm²) and enhances corrosion resistance. Therefore, the Ti/TiO₂-NTs/PbO₂ material prepared in Pb(CH₃COO)₂ medium may be an ideal anode for Zn electrowinning.

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1 Introduction

Zinc extraction processes can be classified into hydrometallurgy and pyrometallurgy, and 80% of the world's zinc is extracted by hydrometallurgy.^1–4 The energy consumption of Zn electrowinning is a key point accounting for the majority of the total energy consumption in the whole process.⁵ Oxygen Evolution Reaction (OER) occurs at the anode in zinc electrowinning, one of effective ways to decrease energy consumption is reducing the oxygen evolution overpotential of the anode material.⁶ It is worth noting that present Pb–Ag alloy anode exhibits many drawbacks including high OER overpotential, low mechanical strength, low service life and consumption of noble element Ag, which limits the advancement of zinc hydrometallurgy.^7–10 Therefore, it is urgent to design and synthesise a novel energy-saving anode in zinc hydrometallurgy.

Titanium-based coating insoluble anode possesses stable dimensions, high corrosion resistance and long service life, which have attracted attention from research¹¹ and it has been widely used in hydrometallurgy, electrolytic wastewater, chlor-alkali industry and other fields.^12–15 PbO₂ is one of the best options for active coatings due to its low price, high conductivity and excellent corrosion resistance in acid solution.^16–18 However, its shortcoming, such as poor adhesion, large interface resistance and low electrocatalytic activity cannot be ignored.^19,20 To our best knowledge, Sn, Sb oxide, α-PbO₂ and other conductive layers can usually serve as intermediate layers to prolong service life and enhance OER performance.^13,20,21 Besides, the active PbO₂ coating doped with foreign elements or active particles, such as Bi,²² Ag,¹¹ RE,¹⁸ SiO₂,²³ ZrO₂,²⁴ Co₃O₄,¹⁴ MnO₂,²⁵ and carbon nanotubes(CNTs)²² can increase its electrocatalytic activity and corrosion resistance.

The discovery of CNTs by lijima was inspiring the research of other nanomaterials.²⁶ In 1990s, Zwilling and colleagues first reported that titanium could form self-organized nanotubes in electrolytes containing hydrofluoric acid.²⁷ Over the past 20 years, TiO₂ nanotubes (TiO₂-NTs) has been extensively studied, from initial preparation in aqueous solutions to ionic and organic solutions to multiple anodization processes. Up to now, TiO₂-NTs with high aspect ratio and perfect arrangement can be obtained with controllable diameter and length.^28–32 It possesses larger specific surface area, higher electron mobility, stronger hydrophilicity and stronger adsorption capacity and higher photoelectric catalytic activity than TiO₂ (quantum size effects), which is widely used in solar cells, electrochromic, sensors, photocatalytic decomposition of water, self-cleaning function and electrolytic wastewater and other fields.³³ It is a n-type semiconductor with wide band gap (E_g ≈ 3.0 eV) which is appropriate for photocatalytic reactions. However, a large amount of energy is required in the process of the transition of valence electrons to the conduction band becoming free electrons because of the wide band gap, which limit its widely application in the field of electrode materials.³⁴ In recent years, some researches on the conductivity of TiO₂-NTs have been reported in many literatures, such as crystal directional growth,³⁵ electrodeposition Cu,³⁶ foreign element doping³⁷ and self-doping reduction.³⁸ These methods can increase the carrier concentration of the nanotubes so as to enhance its conductivity, making TiO₂-NTs possible to be used in an anode materials for zinc electrowinning.

Lead nitrate medium is one of the most fashion media used for PbO₂ electrodeposition. Besides that, lead acetate,³⁹ lead methyl sulfonate,⁴⁰ lead sulfamate,⁴¹ lead fluoroborate⁴² and other media have also been reported. In present research, TiO₂-NTs may be used in Zn electrowinning anode for the first time. Titanium was selected as the substrate to fabricate TiO₂-NTs in situ as the intermediate layer via anodization. Additional anodization, annealing and electrochemical reduction methods were used to enhance the conductivity of the nanotubes and adhesion. PbO₂ was deposited on Ti/TiO₂-NTs through galvanostatic deposition in lead acetate (Pb(CH₃COO)₂) medium and lead nitrate (Pb(NO₃)₂) medium. A novel Ti/TiO₂-NTs/PbO₂ material with excellent OER performance and good corrosion resistance was successfully prepared in the Pb(CH₃COO)₂ medium. The physical structure was studied by XRD and FE-SEM, and the electrochemical performance were tested by LSV, CV, EIS and Tafel. And the mechanisms of TiO₂-NTs enhancing OER performance and corrosion resistance were explored.

2 Experimental

2.1 Preparation of Ti/TiO₂-NTs

Titanium sheet (20 mm × 10 mm × 1 mm) was used to fabricate TiO₂-NTs via anodization. First, the titanium sheet was chemically polished in a mixed solution of HF, HNO₃ and deionized water in a ratio of 1 : 1 : 2 for 10 s. Then it was ultrasonic for 5 min in anhydrous ethanol, acetone and deionized water, respectively, following dried in the air for 20 min. After that, each titanium sheet was anodized 2 times, with titanium sheet as the anode and stainless steel as the cathode. Step 1: the anodization occurred in 100 ml ethylene glycol (EG) solution containing 0.3 wt% NH₄F and 2 vol% deionized water, DC supply voltage 60 V, time 1 h. Step 2: it was transferred to 100 ml ethylene glycol solution containing 5 wt% H₃PO₄ and oxidized for 1 min so as to eliminate the influence of fluorine-rich layer (FRL) and increase the adhesion of nanotube layer and conductivity.⁴³ Finally, it was annealed for 3 h at an air atmosphere of 450 °C with a heating rate of 3 °C min⁻¹ so that amorphous TiO₂-NTs was converted to anatase.

2.2 Electrodeposition of PbO₂

Before electrodeposition of PbO₂, the electrochemical reduction of TiO₂-NTs was conducted in 0.1 M H₂SO₄ solution for 5 min with the TiO₂-NTs as the cathode and Pt sheet as the anode in order to improve the conductivity of TiO₂-NTs as much as possible, current density 5 mA cm⁻². Then, Ti sheet and Ti/TiO₂-NTs were electrodeposited with PbO₂ in lead acetate medium (I) containing 0.2 M Pb(CH₃COO)₂ and 0.2 M Na(CH₃COO), respectively, and marked as Ti/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (I). In addition, PbO₂ was deposited on Ti/TiO₂-NTs in lead nitrate medium (II) containing 0.1 M HNO₃ and 0.2 M Pb(NO₃)₂, and marked as Ti/TiO₂-NTs/PbO₂ (II). Ti sheet and Ti/TiO₂-NTs are the working electrode and stainless steel is the counter electrode, current density 20 mA cm⁻², temperature 40 °C, magnetic stirring speed 300 rpm, and electrodeposition time 1 h. The general experimental procedures are shown in Fig. 1 and the experimental installation are shown in Fig. 2.

Fig. 1 The general experimental procedures of the work.

Fig. 2 Experimental installation.

2.3 Characterization of materials

The phase structure and surface morphology of the materials were characterized by D/max2200 X-ray diffractometer (XRD, Cu Kα radiation) and a Nova Nano-SEM450 field emission scanning electron microscope (FE-SEM) respectively. Linear Sweep Voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel plots were tested by electrochemical workstation (CHI760E) under the three electrodes system. The electrolyte is Zn electrowinning simulation solution containing 50 g L⁻¹ Zn²⁺ and 150 g L⁻¹ H₂SO₄ at 35 °C.The composite materials were the working electrodes and only 1 cm² was left as the working area. Mercuric Sulfate Electrode (MSE) was the reference electrode, and stainless steel was the counter electrode.

2.4 Stability

After Ti/TiO₂-NTs/PbO₂ (I) was prepared, the i–t curve was tested by chronoamperometry at an anodic potential of 2 V (vs. MSE) and the time of 27 h. Then, its XRD pattern and SEM image were collected in order to deeply analyze the microscopic changes of the material during use. In addition, a 5000-cycle CV scan was adopted, the voltage range was 0–2 V, and the scan rate was 0.05 V s⁻¹, after that, the anodic polarization curve of the material was tested and compared with fresh Ti/TiO₂-NTs/PbO₂ (I).

3 Results and discussion

3.1 The phase structure and surface morphology

The phase composition of the active coating corresponding to Ti/PbO₂ (I), Ti/TiO₂-NTs/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (II) materials are shown in Fig. 3. As displayed in the Fig. 3a, The main diffraction peaks of Ti/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (I) appear in 36.37°, 58.12°, 60.74° and 77.23°and the corresponding crystal planes are (200), (310), (311) and (400), indicating that the phase only consists of α-PbO₂ (PDF 72-2400), and preferential growth at (200) crystal planes can be seen obviously. The crystal planes (200) and (400) of Ti/TiO₂-NTs/PbO₂ (I) are stronger compared with Ti/PbO₂ (I), suggesting that the introduction of TiO₂-NTs is beneficial to the formation of PbO₂ crystals. When the medium is changed into Pb(NO₃)₂, the phases of Ti/TiO₂-NTs/PbO₂ (II) consist of α-PbO₂ and β-PbO₂ (PDF 41-1492). The α-PbO₂ that appears in 28.57°, 32.62°, 34.31°and 49.64° isn't found in Ti/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (I). This may also be because the (200) crystal plane preferentially grows obviously in the Pb(CH₃COO)₂ medium, which leads to the inconspicuous peaks of those angles. Besides that, in Fig. 3b, it can be obtained that the FWHM of Ti/PbO₂ (I) (0.290 rad) is smaller than that of Ti/TiO₂-NTs/PbO₂ (I) (0.320 rad) in the (200) crystal plane by XRD analysis, revealing that TiO₂-NTs promotes the formation of smaller PbO₂ grains.

Fig. 3 (a) XRD pattern of different PbO₂ materials; (b) a partial enlarged view of the XRD pattern.

The SEM images of all samples are exhibited in the Fig. 4. The Ti sheet shows a rough surface after chemical polish (Fig. 4a′), which is conducive to the formation of TiO₂-NTs since the surface electric field on flat surface is so scattered that it is easy to form irregular nanopores.⁴⁴ Fig. 4a presents the surface after anodization, and some obvious change can be seen. Specifically, the detail can be observed in high magnification image (Fig. 4b, b'and b′′). A self-organized high aspect ratio TiO₂-NTs layer is formed onto the surface of Ti sheet with a diameter of ∼120 nm, length of ∼8 μm and a closed bottom.

Fig. 4 SEM of different materials: (a) TiO₂-NTs middle layer at low magnification and (a′) Ti sheet after chemical polishing; (b) TiO₂-NTs at high magnification, (b′ and b′′) bottom and side of TiO₂-NTs; (c–e) Ti/PbO₂ (I), Ti/TiO₂-NTs/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (II), deposition time 1 h; (f–h) Ti/PbO₂ (I), Ti/TiO₂-NTs/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (II), deposition time 3 min; (i) EDS of Ti/TiO₂-NTs/PbO₂ (I), deposition time 3 min.

The Fig. 4c and d represent the PbO₂ deposited onto titanium without/with TiO₂-NTs in the Pb(CH₃COO)₂ medium respectively. Comparing two images, it can be observed that the introduction of TiO₂-NTs eliminates the cracks and refines the PbO₂ crystal. In addition, the PbO₂ coating presents a circle shape which is consistent with α-PbO₂. The result agrees with the XRD pattern. The Fig. 4e is the morphology of PbO₂ deposited onto Ti/TiO₂-NTs in the Pb(NO₃)₂ medium. It can be seen that the PbO₂ shows a pyramid shape of β-PbO₂, which is totally different from that in the Pb(CH₃COO)₂ medium.

In order to better understand the behaviors of PbO₂ deposition, the SEM images were captured while fixed deposition time was 3 min and shown in Fig. 4f–h. In the Fig. 4f (Ti/PbO₂, Pb(CH₃COO)₂), there are only a small amount of PbO₂ on Ti substrate. But it can be seen from Fig. 4g′ (Ti/TiO₂-NTs/PbO₂, Pb(CH₃COO)₂) that the Ti/TiO₂-NTs surface is covered with a layer of PbO₂, which demonstrates TiO₂-NTs promotes the formation of PbO₂. Moreover, the cross section of the nanotube was collected and shown in Fig. 4g (Ti/TiO₂-NTs/PbO₂, Pb(CH₃COO)₂). A great amount of nanotube channel is filled with PbO₂, which has been confirmed in EDS results (Fig. 4i). However, in Fig. 4h (Ti/TiO₂-NTs/PbO₂, Pb(NO₃)₂) that is the deposition in the Pb(NO₃)₂ medium for 3 min, the PbO₂ isn't found at the nanotube orifice, while it exists outside the nanotube. It suggests that PbO₂ may preferentially grow outside the nanotube resulting in a low filling rate of PbO₂ in the nanotube channel. In addition, in the Fig. 4h′, PbO₂ does not completely cover Ti/TiO₂-NTs surface, comparing with Fig. 4g′, which means that the deposition rate of PbO₂ in the Pb(NO₃)₂ medium is slower than in the Pb(CH₃COO)₂ medium.

In general, the deposition behavior of PbO₂ can be summarized. First of all, TiO₂-NTs may be able to provide a larger surface area and more nucleation sites, resulting in an increase in nucleation rate and a decrease in PbO₂ grain size. Moreover, it can eliminate the surface crack and increase the adhesion of PbO₂ coating, Secondly, the deposition behavior of PbO₂ in nanotube channel is different in the two media. According to a literature reported by Velichenko,⁴⁵ the electrodeposition mechanism of PbO₂ can be described as following:

H₂O → OH_ads + H⁺ + e⁻ (1)

Pb²⁺ + OH_ads → Pb(OH)²⁺ (2)

Pb(OH)²⁺ + H₂O → Pb(OH)₂²⁺ + H⁺ + e⁻ (3)

Pb(OH)₂²⁺ → PbO₂ + 2H⁺ (4)

According to reactions (1–4), as the electrodeposition of PbO₂ goes on, more H⁺ will produce, and the formation of PbO₂ can be inhibited due to decrease of pH. When PbO₂ is electrodeposited onto TiO₂-NTs, the ion exchange is not timely due to the small diameter of the nanotube. The pH in the nanotube channel will decrease with the development of electrodeposition so that the continuous generation of PbO₂ in the nanotube channel is inhibited. However, in the Pb(CH₃COO)₂ medium, the CH₃COO⁻ can play the role of buffer to effectively hinder the pH reduction. The PbO₂ can be deposited gradually in channel, as shown in the Fig. 5. The nanotube channel has a high filling rate, which can cause the PbO₂ coating and TiO₂-NTs have a good combination.

Fig. 5 Schematic diagram of the influence of deposition medium on PbO₂ materials.

3.2 Oxygen evolution activity of the materials

3.2.1 LSV curve analysis. In oxygen evolution kinetics, oxygen evolution overpotential oxygen evolution overpotential is, the easier it is to produce oxygen. In order to study the η parameters of these materials, LSV technology was adopted in simulated Zn electrowinning solution (50 g L⁻¹ Zn²⁺, 150 g L⁻¹ H₂SO₄), scanning rate 0.05 V s⁻¹, temperature 35 °C. In addition, the LSV curve of Pb-0.76 wt% Ag was also tested in order to compare with it. The measured LSV curves and η value are shown in Fig. 6.

Fig. 6 (a) Anode polarization curve of different PbO₂ materials; (b) oxygen evolution potential and overpotential of different PbO₂ materials.

Their OER catalytic activity can be visually compared from Fig. 6a. The catalytic activity from low to high is Ti/TiO₂-NTs/PbO₂ (II), Pb-0.76wt%Ag, Ti/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (I). It can be seen from Fig. 6b that the η of Pb-0.76 wt% Ag is 962 mV. The high η is one of the main reasons for high energy consumption in Zn electrowinning. The η of Ti/PbO₂ (I) is 779 mV, although it is lower than Pb-0.76 wt% Ag, the PbO₂ coating has a lot of cracks and poor adhesion, which is not an ideal electrode. However, Ti/TiO₂-NTs/PbO₂ (I) has the best oxygen evolution performance (η = 630 mV). Its η is 149 mV lower than that of Ti/PbO₂ (I) and 332 mV lower than that of Pb-0.76 wt% Ag, indicating that the introduction of TiO₂-NTs enhances the OER catalytic activity. Moreover, it is worth noting that the η of Ti/TiO₂-NTs/PbO₂ (II) (η = 1004 mV) has a significant increase compared with Ti/TiO₂-NTs/PbO₂ (I), which shows that Pb(CH₃COO)₂ medium may be more suitable for electrodeposition of PbO₂ on TiO₂-NTs in order to improve OER catalytic activity.

3.2.2 CV curves analysis. So far, OER mechanisms have been fully studied through dynamics and density functional theory (DFT), and one of the accepted mechanisms is shown in the Fig. 7.^46–49 S represents the active sites on the oxide electrode, while OH_ads and O_ads stand for the adsorbable hydroxyl and oxygen-group species, respectively. The S plays a crucial role in the whole reaction path. In the oxide electrode, only active sites can chemically adsorb and desorb intermediate products (OH_ads and O_ads), thereby providing places for chemical reactions. Therefore, the larger the specific surface area, the more active sites, the stronger the oxygen evolution capacity. The specific surface areas can be qualitatively compared by obtaining the voltammetry charge (q*) from the integral CV curves, the larger the q*, the larger the specific surface area, at the same scanning rate (v).²¹

Fig. 7 The oxygen evolution mechanism of PbO₂ materials in acidic solution.

In this work, scan rates 0.01 V s⁻¹, 0.03 V s⁻¹, 0.05 V s⁻¹, 0.07 V s⁻¹ and 0.09 V s⁻¹ were adopted, and the measured CV curves at 0.05 V s⁻¹ are shown in the Fig. 8a. The following relations exist between q* and v:

The k and k′ are constants for the slope, is the real charge on the surface of the electrode which consists of the outercharge and the innercharge The relationships of q* versus v^−1/2 and (q*)⁻¹ versus v^1/2 is shown in Fig. 8c and d.

Fig. 8 (a) CV curves of different PbO₂ materials; (b) voltammetry charge of different materials at 0.05 V s; (c) relationship of q* versus v^−1/2; (d) relationship of (q*)⁻¹ versus v^1/2.

In the Fig. 8a, the CV curve of Ti/PbO₂ (I) is smaller than Ti/TiO₂-NTs/PbO₂ (I), and Ti/TiO₂-NTs/PbO₂ (II) has the smallest CV curve. The calculated q* at 0.05 V s⁻¹ is shown in the Fig. 8b. The q* of Ti/PbO₂ (I) is 3.586 C cm⁻², after the introduction of TiO₂-NTs, the q* of Ti/TiO₂-NTs/PbO₂ (I) increased to 4.478 C cm⁻², which means that TiO₂-NTs promote the formation of a larger specific surface area PbO₂ coating. This may because TiO₂-NTs can provide a large surface area for PbO₂ nucleation, and the increase in nucleation rate leads to an increase in electrode surface area. However, the q* of Ti/TiO₂-NTs/PbO₂ (II) is the smallest. The result shows that the specific surface area of PbO₂ electrodeposited in the Pb(CH₃COO)₂ medium are larger than those in the Pb(NO₃)₂ medium. It may due to PbO₂ with a smaller grain size can be obtained in the Pb(CH₃COO)₂ medium, resulting in a larger specific surface area. The similar laws about specific surface area at different scan rates also can be found in Fig. 8c and d. These corresponds to LSV curves characteristics in the previous section.

3.2.3 EIS analysis. According to the OER mechanism in the Fig. 7, OER begins with the transfer of the first charge and generates OH_ads, then the second charge transfers and generates O_ads.⁴⁶ It is worth mentioning that the first charge transfer step is the rate-determining step (RDS) of OER, which has been confirmed by many researchers and is called primary water discharge.⁵⁰ The conductivity of the electrode material determines the rate of charge transfer and affects the rate of OER. Therefore, in order to further study the factors affecting the catalytic activity of OER, EIS technology was adopted at an anodic potential of 1.4 V (vs. MSE). The measured EIS curves are shown in the Fig. 9.

Fig. 9 (a) Nyquist plots of different PbO₂ materials; (b) Bode plots of different PbO₂ materials; (c) equivalent circuit of OER on the PbO₂ materials in the simulated solution (50 g L⁻¹ Zn²⁺, 150 g L⁻¹ H₂SO₄) at 35 °C.

There are two semicircles in the Nyquist plots (Fig. 9a) and two peaks in the Bode plots (Fig. 9b), indicating the existence of two time-constant. The phenomenon exists in the following cases, such as the generation of solid electrolyte interface (SEI) membranes in lithium ion batteries, the occurrence of side reactions in metal corrosion, intermediates adsorption and desorption in the reaction process and so on. Electrochemical impedance spectroscopy usually consists of three parts: the high frequency semicircle is mainly the capacitive behavior due to the above situations, the middle frequency semicircle represents the impedance behavior due to charge transfer, and the straight line at low frequency is the Warburg impedance.⁵¹ The Nyquist plots are calculated using the R(Q(R(QR))) equivalent circuit,⁵² as shown in the Fig. 9c. R_f, R_ct and R_s represent adsorption impedance of OH_ads and O_ads, charge transfer resistance and solution resistance, respectively. Q_f and Q_dl are constant phase elements (CPE), because the surface of electrode materials is not an ideal plane, and the electrochemical properties of the electrode surface vary from place to place, resulting in the dispersion of resistance and capacitance. The characteristic values of capacitance can be calculated by the following formulas:⁵³

C = Q^1/n(R_S⁻¹ + R_ct⁻¹)^(n−1)/n (8)

When R_ct » R_s

C = Q^1/nR_S^(1−n)/n (9)

The simulation results of each parameter in the equivalent circuit are listed in the Table 1. The R_s maintaining around 0.33–0.39 (Ω cm²) indicates the stability of the test solution, and R_f maintains 0.269–0.415(Ω cm²). Some interesting phenomena are found about R_ct. First, by comparing Ti/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (I), it can be found that TiO₂-NTs as an intermediate layer reduces R_ct (10.01 Ω cm² to 2.337 Ω cm²). This probably due to TiO₂-NTs can provide a large contact area with PbO₂ coating, which reduces the interface resistance, as shown in the Fig. 10. Subsequently, when the electrodeposition medium is changed from Pb(CH₃COO)₂ to Pb(NO₃)₂, the R_ct shows a significant increase (2.337 Ω cm² to 25.4 Ω cm²). It may be because the PbO₂ deposited in the Pb(NO₃)₂ medium can't fill the nanotube channel well, which isn't conducive to electron transport, as shown in the Fig. 5 in the SEM discussion.

Table 1 Parameter values of different PbO₂ materials obtained by equivalent circuit simulation

Type	Rs/Ω cm^2	Rf/Ω cm^2	Qf/mΩ^−1 s^n cm^−2	Cl/mF cm^−2	Rct/Ω cm^2	Qdl/mΩ^−1 s^n cm^−2	Cdl/mF cm^−2
Ti/PbO2 (I)	0.389	0.415	0.025 (n = 1)	0.025	10.01	4.98 (n = 0.853)	5.581
Ti/TiO2-NTs/PbO2 (I)	0.363	0.269	0.046 (n = 1)	0.046	2.337	18.63 (n = 0.705)	41.454
Ti/TiO2-NTs/PbO2 (II)	0.334	0.353	0.029 (n = 1)	0.029	25.4	7.592 (n = 0.782)	9.838

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Fig. 10 Schematic diagram of the influence of TiO₂-NTs on materials.

The electrocatalytic activity of Ti/TiO₂-NTs/PbO₂ is compared with some reported materials. As shown in Table 2 and Fig. 11.

Table 2 Comparison of related materials for OER electrocatalytic activity

Electrodes	Concentration of H2SO4	η/mV (vs. NHE, 50 mA cm^−2)	Reference
Ti/Sb–SnO2/Bi–PbO2	0.5 M	1046	54
Al/Pb–PANI–WC	1.53 M	941	8
Pure Pb	1.8 M	936	55
PbO2–Co3O4	1.63 M	858	56
PANI/CeO2/WC	1.53 M	856	57
Pb–CeO2	1.63 M	826	58
Pb–Co3O4	1.53 M	792	56
3D-Pb–Ag/F–PbO2	1.53 M	772	59
Pb–MnO2	1.63 M	765	60
3D-Ti/PbO2	1.53 M	758	14
PbO2–MnO2	1.63 M	719	61
Ti/PbO2–CeO2–graphite	1.53 M	648	19
Ti/TiO2-NTs/PbO2	1.53 M	630	This work

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Fig. 11 Oxygen evolution overpotentials of different composite materials at 50 mA cm⁻².

It can be seen from Fig. 11 that Ti/TiO₂-NTs/PbO₂ (I) exhibits an excellent OER electrocatalytic activity, and its oxygen evolution overpotential is only 630 mV. This outstanding oxygen evolution performance is mainly attributed to the nanotube structure of TiO₂-NTs and the choice of lead acetate medium. The nano-channel structure provides a large surface area for PbO₂ electrodeposition, refines the PbO₂ grains, and increases the specific surface area of the deposits. The choice of lead acetate medium can make the nanotube channel better filled, which greatly reduces the internal resistance of the material. Therefore, Ti/TiO₂-NTs/PbO₂ (I) composite material presents an excellent OER electrocatalytic activity.

3.3 Corrosion resistance

Long service life of the electrode is necessary, which is one of the effective ways to save cost. Tafel technology can measure the self-corrosion current (i_corr) of different PbO₂ electrode materials, which can be used as one of the criteria to evaluate the service life. The lower the self-corrosion current, the better the corrosion resistance of the electrode. The Tafel plots measured in the simulated solution are shown in Fig. 12a, the temperature is 35 °C, and the scanning rate is 0.001 V s⁻¹. The self-corrosion current obtained by electrochemical workstation analysis is shown in Fig. 12b.

Fig. 12 (a) Tafel plots of different PbO₂ materials at scan rate 0.001 V s; (b) self-corrosion current of different PbO₂ materials in the simulated solution at 35 °C.

It can be seen from the Fig. 12b that the self-corrosion current of Ti/PbO₂ (I), Ti/TiO₂-NTs/PbO₂ (I) and Ti/TiO₂-NTs/PbO₂ (II) are 2.474 × 10⁻³, 7.432 × 10⁻⁴ and 6.636 × 10⁻⁴(A cm⁻²) respectively. The self-corrosion current of Ti/TiO₂-NTs/PbO₂ (I) is smaller than that of Ti/PbO₂ (I), which indicates that TiO₂-NTs as an intermediate layer can increase corrosion resistance. It may be because TiO₂-NTs can effectively prevent the electrolyte from contacting and corroding the Ti substrate, and it can also eliminate the surface cracks of the PbO₂ coating. In addition, the i_corr of Ti/TiO₂-NTs/PbO₂ (II) has slight decrease compared with Ti/TiO₂-NTs/PbO₂ (I), which may be associated with the different crystal structure of PbO₂, but the too high oxygen evolution overpotential indicates that it is not an ideal anode for Zn electrowinning.

3.4 Stability of the Ti/TiO₂-NTs/PbO₂ (I)

In order to test the stability of Ti/TiO₂-NTs/PbO₂ (I), a 27 h chronoamperometry was performed under a 2 V anodic potential, and the XRD pattern and SEM images were collected after the test. In addition, a 5000-cycle CV scan was adopted, the voltage was 0–2 V, and the scan rate was 0.05 V s⁻¹, and then the LSV curve of Ti/TiO₂-NTs/PbO₂ (I) was measured. The results are shown in Fig. 13.

Fig. 13 (a) Chronoamperometry curve of Ti/TiO₂-NTs/PbO₂ (I); (b) XRD pattern of Ti/TiO₂-NTs/PbO₂ (I) after 27 h chronoamperometry test; (c) SEM image of fresh Ti/TiO₂-NTs/PbO₂ (I); (d) SEM image of Ti/TiO₂-NTs/PbO₂ (I) after 27 h chronoamperometry test; (e) LSV curve of Ti/TiO₂-NTs/PbO₂ (I) after 5000 cycles of CV scan.

It can be seen from Fig. 13a that the current density increases slightly with time. After 27 h, it increases from 271.7 mA cm⁻² to 291.1 mA cm⁻², which indicates that the OER electrocatalytic activity of Ti/TiO₂-NTs/PbO₂ (I) is enhanced. After the 27 h chronoamperometry test, as shown in Fig. 13b, it can be observed that the XRD pattern of Ti/TiO₂-NTs/PbO₂ (I) only appears a new characteristic peak located at 32.62°. This corresponds to the (002) crystal plane of α-PbO₂ (PDF 72-2400), which means that no new substance is generated. Interestingly, the surface becomes rougher and presents a cone shape similar to β-PbO₂ (Fig. 13c and d), which is beneficial to the increase of the specific surface area to enhance the OER electrocatalytic activity. This conforms to the law presented in Fig. 13a. In addition, in Fig. 13e, Ti/TiO₂-NTs/PbO₂ (I) still exhibits a stable anodic polarization curve after 5000 cycles of CV scanning, and its oxygen evolution overpotential has only increased by 22 mV at 50 mA cm⁻² compared with fresh Ti/TiO₂-NTs/PbO₂ (I). All these indicate that the composite material possesses an excellent stability and long-term application potential in acidic media.

4 Conclusion

On the whole, a novel Ti/TiO₂-NTs/PbO₂ composite material for Zn electrowinning was successfully synthesized in the Pb(CH₃COO)₂ medium. TiO₂-NTs with a diameter of ∼120 nm, a length of ∼8 μm and uniform arrangement was obtained via anodization in ethylene glycol electrolyte containing 0.3wt%NH₄F and 2% vol deionized water. In the Pb(CH₃COO)₂ medium, TiO₂-NTs as an intermediate layer can make the PbO₂ coating have small grains, few cracks, and large surface area (4.478 C cm⁻² and 41.454 mF cm⁻²). Moreover, TiO₂-NTs channel can be well filled by PbO₂, which results in a low charge transfer resistance (2.337 Ω cm²). In addition, it can act as a barrier layer to increase the corrosion resistance (24.74 × 10⁻⁴ A cm⁻² to 7.432 × 10⁻⁴ A cm⁻²). Therefore, the electrochemical performance is enhanced through the modification of TiO₂-NTs. The oxygen evolution overpotential of Ti/TiO₂-NTs/PbO₂ (I) (η = 630 mV) is 332 mV lower than that of Pb-0.76 wt% Ag (η = 962 mV) under current density 50 mA cm⁻² in the Zn electrowinning simulation solution (50 g L⁻¹ Zn²⁺, 150 g L⁻¹ H₂SO₄) at 35 °C and only increases 22 mV after 5000 cycles of CV scanning. These results indicate that Ti/TiO₂-NTs/PbO₂ prepared in Pb(CH₃COO)₂ medium may be an ideal Zn electrowinning anode material.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Project No. 51874154), the Key Project of Yunnan Province Science and Technology Plan of China (Project No. 2014FA024), the Specialized Research Fund for the Doctoral Program of the Ministry of Education of China (Project No. 20125314110011), and the Analysis and Measurement Fund (2019M20182102009) of Kunming University of Science and Technology.

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