Study on the performance of an improved Ti/SnO₂–Sb₂O₃/PbO₂ based on porous titanium substrate compared with planar titanium substrate

Abstract

This present work focused on systematically studying the effect of a porous Ti substrate on the surface structure and electrochemical properties of lead dioxide electrodes prepared by anodic deposition under galvanostatic conditions. Characterization experiments, including scanning electron microscopy (SEM), X-ray diffraction (XRD), linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), accelerated life time testing and a degradation experiment of methylene blue trihydrate were performed to determine the effect of different Ti substrates. Compared with the planar Ti substrate, the results showed that the porous Ti substrate decreased the grain size of lead dioxide and formed a compact and fine surface coating. The electrode had smaller crystalline particles and a more compact structure. The porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode had a higher oxygen evolution overpotential, a larger active surface area and higher electrochemical activity. The lifetime of the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode (214 h) was 3.69 times higher than that of the planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode (58 h). Moreover, the degradation rate constant of methylene blue trihydrate on the porous Ti substrate lead dioxide electrode (0.03868 min⁻¹) was 1.52 times than that of the planar Ti substrate lead dioxide electrode (0.02542 min⁻¹).

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1. Introduction

In recent years, electrochemical oxidation technology as an advanced oxidation treatment for organic pollutant wastewater has attracted increasing attention due to its high energy efficiency, environmental compatibility, and easy applicability to automation.^1–5 The research hotspot of the literature mainly focuses on the study of excellent electrode materials,⁶ the electrochemical degradation mechanism,⁷ the synergistic effect of electrochemical oxidation technology and other degradation technology.⁸ However, the development of electrodes with high electrochemical activity and stability is one of the key factors affecting the efficiency of electrochemical oxidation.⁹ The lack of suitable anodes is still a major problem.

To date, various types of electrodes, including carbon materials (such as activated carbon fiber, graphite and boron doped diamond (BDD)), metal materials (such as platinum and titanium metal and stainless steel) and dimensionally stable electrode (DSA) (such as, RuO₂, SnO₂ and PbO₂), have been investigated for electro-catalytic oxidation of organic pollutants.^4,5,10,11 Comparison of various electrodes has shown that the metal oxide electrode, based on the titanium substrate and the BDD electrode, is one of the most suitable electrodes for electrochemical oxidation. Despite BDD electrodes having the advantages of a high oxygen evolution potential and superior electrochemical stability,¹² its high cost, complex preparation process and especially the difficulties to seek an appropriate substrate for deposition has limited its application to experimental study.^10,13 Hence, the main problems hindering practical use are relatively high operating cost and short service life.

Due to its low cost, good conductivity, high oxygen over-potential and better electrochemical stability,¹⁴ the PbO₂ electrode has been widely used in electro-synthesis, electrolysis and more recently wastewater treatment processing. In order to further improve the performance of the PbO₂ electrode, a great deal of effort has been spent on some new methods, such as adding a new intermediate layer (such as Sb–SnO₂,¹⁵ MnO₂ (ref. 16) and RuO₂ (ref. 5)) between the titanium substrate and the oxidation layer, doping metal or non-metallic ions (such as Bi³⁺, F⁻,Fe³⁺,Ce³⁺) into the oxide layer,¹⁴ and using new preparation technologies.¹⁷ However, the abovementioned methods still have inevitable limitations, and some effort has been spent on researching improvements for the titanium substrate.

Due to its excellent physical and chemical properties (high tensile strength, low density and good chemical stability⁹), planar Ti has been widely accepted for coating electrode substrates, but the problem of poor adhesion of the oxide active layer deposited on planar titanium negatively affects service life. As a novel titanium matrix, porous Ti has the advantages of good conductivity, corrosion resistance, high porosity, a large surface area and good biocompatibility. It is widely used in the aerospace, medical and chemical fields.¹⁸ In addition, porous Ti substrates used as an electrode substrate material has also attracted considerably more attention. Recent studies revealed the significance of porous Ti on improving the performance of electrodes. Braga et al. investigated diamond grown on a 3D porous titanium matrix with high roughness, large porosity and large surface area with outstanding electrochemical performance.¹⁹ Sun et al. studied high quality BDD thin film electrodes deposited on porous Ti substrate that were prepared successfully using the hot filament chemical vapour deposition (HFCVD) method.²⁰ Zhang et al. found that a porous titanium substrate could effectively lower the charge transfer resistance in the electro-deposition process and that 3D-Ti/PbO₂ had abundant crystal orientations and a large electrochemically active surface area.^21,22

This work focused on investigating the structure and electrochemical performance of porous Ti as a PbO₂ electrode substrate. A systematic study was carried out to investigate the preparation and characterization of porous Ti/SnO₂–Sb₂O₃/PbO₂ and planar Ti/SnO₂–Sb₂O₃/PbO₂. In addition, research showed that Ti-substrate PbO₂ anodes coated with a Sb–SnO₂ interlayer can increase the service life and improve the performance of electro-catalytic oxidation.⁹ Hence, SnO₂–Sb₂O₃ was chosen as an intermediate layer by thermal decomposition and PbO₂ as oxides active layer by electrochemical deposition, respectively. The morphology, crystalline structure, electrochemical performance, and stability of the prepared electrodes were characterized. In order to further evaluate the electro-catalytic activity, methylene blue trihydrate was used as the model organic pollutant for electrochemical degradation. We hope that the experimental results can contribute to the development of porous titanium substrate as electrode materials.

2. Experimental

2.1 Materials and Reagents

Porous titanium (purity 99.9%, 20 mm × 10 mm × 1 mm) and pure titanium sheets (TA2, 20 mm × 10 mm × 1 mm) were purchased from Baoji Jinkai Industrial Technology Co. Ltd. SnCl₄·5H₂O, SbCl₃, HCl, Pb(NO₃)₂, NaF and other chemicals used in this study were of analytical grade and obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All the chemicals used in the experiments were received without further purification. All the solutions were prepared with deionized water.

2.2 Electrode preparation

2.2.1 Titanium surface treatment. In order to prepare for a good adhesive metal oxide film material, both the planar titanium and porous titanium substrate were pre-treated in the same manner according to the following procedure. First, a porous titanium substrate (20 mm × 10 mm × 1 mm) was mechanically polished with 600-mesh abrasive papers. Then, the porous titanium was cleansed with deionized water and acetone to remove solid particles and grease. Second, it was subsequently immersed in sodium hydroxide (15% m/m) at a temperature of 60 °C for 30 min, and then etched in boiling hydrochloric acid (30% v/v) for about 60 min to produce a gray surface with uniform roughness. Finally, it was washed by ultrasonication in ultrapure water and stored in deionized water.

2.2.2 Coating SnO₂–Sb₂O₃. In this step, the SnO₂–Sb₂O₃ intermediate layer was prepared on titanium substrates by thermal decomposition described in a ref. 23 The coating solution consisted of 6.65 g SnCl₄·5H₂O, 0.475 g SbCl₃ and 1 mL concentrated HCl dissolved in 25 mL isopropanol. The treated titanium substrate was dipped in the solution for 5 min, and then dried at about 130 °C for 10 min with excess solvent being evaporated by hot air, and then the treated titanium substrate was calcined at 500 °C for 15 min in a muffle furnace. All the above processes were repeated twelve times and the electrodes were annealed at 500 °C for 60 min in the last process. The purpose of the preparation of SnO₂–Sb₂O₃ intermediate layer is to increase the conductivity and prevent the formation of TiO₂ as described in a related ref. 15 and 23.

2.2.3 Electrochemical deposition PbO₂. PbO₂ was deposited onto Ti/SnO₂–Sb₂O₃ (geometric area: 1.0 × 1.0 cm²) under a current density of 20 mA cm⁻² at 65 °C for 1 h with magnetic stirring. The coating solution consists of 0.1 M HNO₃ acidic media containing 0.5 mol L⁻¹ Pb(NO₃)₂ and 0.04 mol L⁻¹ NaF. After electro-deposition, the modified electrode was rinsed with deionized water. A Ti/PbO₂ electrode without the interlayer made with the same method was used as a reference. In the electro-deposition system a copper foil electrode (20 mm × 20 mm) was used as the cathode.

2.3 Performance analysis

2.3.1 Physicochemical characterization. The surface morphologies of electrodes were characterized by a HITACHI S-3400N scanning electron microscope. A PANalytical X'Pert PRO X-ray diffractometer with Cu Kα (λ = 0.15418 nm) incident radiation was employed to analyse the crystal structure of electrodes. The X-ray diffraction (XRD) patterns were obtained for 2θ angles from 20° to 90° at a scan rate of 0.02° min⁻¹.

2.3.2 Electrochemical measurement. All the electrochemical measurements of the PbO₂ electrodes were tested on a CHI760D electrochemical workstation (Chenhua Instrument Shanghai Co. Ltd. China) with a conventional three-electrode cell. The three-electrode system used a PbO₂ electrode as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum sheet as the counter electrode. Linear sweep voltammetry curves were obtained to test the oxygen evolution potential in a 0.5 mol L⁻¹ H₂SO₄ solution. Cyclic voltammetry curves were recorded to calculate the voltammetric charge quantity for the different electrodes. Electrochemical impedance spectroscopy (EIS) was used to determine the charge transfer resistance of the modified electrodes. Anti-corrosion performance for electrodes was investigated using an accelerated lifetime test with a current density of 500 mA cm⁻² in 3.0 mol L⁻¹ H₂SO₄ solution. The service life of the electrodes was considered to be terminated when the cell voltage reached 10 V.

All the electrochemical experiments were carried out at room temperature (25 ± 2 °C). All the solutions were prepared with deionized water and all the reagents used in the experiments were of analytical grade.

2.4 Electrocatalytic test

Methylene blue trihydrate was used as the target pollutant for the electrochemical degradation test. The electrochemical degradation was carried out in a cylindrical single compartment cell equipped with a magnetic stirrer and a jacketed cooler to maintain a constant temperature. The porous-PbO₂ electrode or planar-PbO₂ electrodes (1 cm × 1 cm) worked as the anodes. The cathode was stainless copper foil (2 cm × 2 cm) and a distance of 1.5 cm was set between the two electrodes. The initial methylene blue trihydrate concentration was 10 mg L⁻¹ and 0.1 mol L⁻¹ Na₂SO₄ was added to the aqueous solution as the supporting electrolyte. The current density was controlled at a constant 60 mA cm⁻² by a direct current power supply (RXN-605D, China). The stirring rate was about 800 rpm. The experiments were carried out at room temperature for 120 min. During the experiments, liquid samples were withdrawn from the electrolytic cell every 20 min for the TU-1810 UV/visible analysis Spectrophotometer (Beijing Puxi Instrument Co. Ltd.). The maximum adsorption wavelength of methylene blue trihydrate is 664 nm. The colour removal efficiency of methylene blue trihydrate in the electrochemical oxidation was calculated as follows:

A₀ is the absorbance value of the initial wastewater sample at 664 nm and A_t is the absorbance value of the wastewater samples at 664 nm at the given time t.

In addition, we used the fluorescence method based on terephthalic acid, which is a well-known ˙OH scavenger that can be used for estimating the amount of ˙OH radicals generated under various conditions by a fluorescence spectrophotometer (Perkin-Elmer LS-50, American). An aqueous solution of a 200 mL volume containing 0.5 mol L⁻¹ terephthalic acid, 0.5 g L⁻¹ NaOH and 0.25 mol L⁻¹ Na₂SO₄ was used as the electrolyte solution. The anode was a prepared PbO₂ electrode and the cathode was a stainless steel sheet. Hydroxyl radical production was performed at a current density of 30 mA cm⁻² at 30 °C. During the experiments, samples were drawn from the reactor every 5 min and diluted 10 times with deionized water, then analyzed with a fluorescence spectrophotometer. The fluorescence spectra were recorded in the range of 380 to 520 nm, using a 315 nm excitation wavelength.

3. Results and discussion

3.1 Surface morphology and electrode crystal structure

3.1.1 Morphological analysis by SEM. The SEM cross-section morphology of the two electrodes is shown in Fig. 1. It can be clearly seen that the planar Ti/SnO₂–Sb₂O₃ electrode has a significant boundary between the SnO₂–Sb₂O₃ intermediate layer and the PbO₂ coating. The thickness of each layer was 293 μm and 99 μm, respectively. For the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode, SnO₂–Sb₂O₃ intermediate layer could not be easily distinguished, which was distributed upon a porous titanium substrate, including the surface and the pores, with only a PbO₂ coating layer with a thickness of 173 μm covering the surface of the electrode.

Fig. 1 SEM morphology of cross-sections for (a) porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode (×500), (b) planar Ti/SnO₂–Sb₂O₃ electrode.

Fig. 2 shows the SEM surface micrographs of the prepared electrodes with different titanium substrates. All the insets correspond to higher magnification images. As can be seen from Fig. 2(a) and (b), the porous titanium substrate had a different surface morphology and structure compared with the planar titanium substrate. It can be clearly seen that the surface of the porous titanium was very rough and had irregular pores with average sizes of 30 μm. This substrate can provide a larger specific surface area than the planar titanium substrate.

Fig. 2 SEM images of (a) porous titanium substrate (×100), (b) traditional planar titanium substrate (×100), insets in (a) and (b) are SEM images with high magnification (×500) corresponding to the electrode. (c) porous Ti/SnO₂–Sb₂O₃ electrode (×500), (d) planar Ti/SnO₂–Sb₂O₃ electrode (×500), (e) porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode (×500), (f) planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode (×500); inset in (c–f) are SEM images with high magnification (×4000) corresponding to the electrode.

Fig. 2(c) and (d) show the morphological characteristics of the SnO₂–Sb₂O₃ intermediate layer deposited on the porous Ti and planar Ti substrates by thermal decomposition, respectively. In comparison with Fig. 2(c), the electrode surface in Fig. 2(d) exhibited a specific “crack-mud” micro-morphology structure, which is typical for oxide electrodes as described by other literature.^9,24 On the contrary, the surface of the porous Ti/SnO₂–Sb₂O₃ electrode in Fig. 2(c) appeared to be more compact, crack-free and uniformly distributed, which is beneficial for electrochemical properties. This conclusion was further proven by the following experiments.

To further check the impact of the porous Ti substrate on the formation of PbO₂, Fig. 2(e) and (f) displays the electrode crystal structure and appearance of porous Ti/SnO₂–Sb₂O₃/PbO₂ and planar Ti/SnO₂–Sb₂O₃/PbO₂ with a magnification of 500×, 4000× in the insets, respectively. It is noticeable that the particle sizes of porous Ti/SnO₂–Sb₂O₃/PbO₂ became smaller than that of planar Ti/SnO₂–Sb₂O₃/PbO₂ at the same magnification, the coated particles grown on the SnO₂–Sb₂O₃ intermediate layer did not present fissures or delamination and appeared to be more compact and uniformly distributed. The grain sizes of PbO₂ deposited on the planar Ti/SnO₂–Sb₂O₃ is almost ten times more than that of PbO₂ deposited on porous Ti/SnO₂–Sb₂O₃, which ranged from 200 to 450 nm. Similar results can be found in the literature.²¹ The porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode with nanoscale particles had a larger specific surface area, which can provide more active sites for electrochemical oxidation. Hence porous Ti substrate-modified electrode was expected to have better electrochemical properties. This expectation has been proven by other authors.²⁰

In conclusion, despite using the same electrode preparation technology, the porous titanium substrates resulted in an improved electrode with a different surface microstructure compared with the structure of an electrode based on planar titanium substrates. The underlying reason for the difference in the morphology of the two types of electrodes may be complicated. However, porous titanium substrates with a larger specific surface area and three-dimensional porous structure, is conducive to the uniform dispersion of the active coating with nano-structure. In addition, owing to the large surface area and there being more active sites, the porous Ti/SnO₂–Sb₂O₃ substrate could make the current distribution uniform and reduce the actual current density of the electrode. Hence, the potential of porous Ti/SnO₂–Sb₂O₃ is lower than that of the planar Ti/SnO₂–Sb₂O₃ substrate. According to the crystal growth and nucleation theory, crystal growth is preferential at low potentials, and crystal nucleation is preferential at high potentials, the nucleation is preferential at high potentials on planar titanium substrates, large amounts of nuclei crash together to produce large particle, as shown in Fig. 2(f). While both the nucleation and growth are not preferential on the porous titanium substrates, many small crystallite particles have more chances to produce them, leading to a uniform PbO₂ electrode.

3.1.2 Structural analysis by XRD. In order to further verify the results of the SEM observation and examine the crystalline structure and lattice parameters of the electrode coating, wide-angle XRD analysis of the different prepared electrodes was performed on the planar and porous titanium substrates and is shown in Fig. 3(A). A series of diffraction peaks from SnO₂, Sb₂O₃ and titanium metal were detectable for two samples in Fig. 3(A), and it is observed that the intensity of diffraction peaks for SnO₂ (110, 101, 211, 301) or Sb₂O₃ (022, 113, 361) of the porous Ti/SnO₂–Sb₂O₃ electrode was considerably stronger than that of the planar Ti/SnO₂–Sb₂O₃ electrode. No crystallized TiO₂ peaks were recorded, which strongly suggests that the Ti substrate was not oxidized in the process of thermal decomposition, and both the porous and planar structures were not easy to be completely covered by the limited amount of sintering. In order to fully cover the porous titanium matrix material, increasing the amount of sintering could be beneficial.

Fig. 3 (A) XRD patterns of (a) porous Ti/SnO₂–Sb₂O₃ electrode, (b) planar Ti/SnO₂–Sb₂O₃ electrode. (B) XRD patterns of (a) porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode, (b) planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode, (c) porous Ti/PbO₂ electrode, (d) planar Ti/PbO₂ electrode.

It is well known that PbO₂ is a polymorphic material with two allotropic forms, α-PbO₂ and β-PbO₂. The conductivity of β-PbO₂ is higher than that of α-PbO₂, indicating that the conductivity of PbO₂ can be enhanced by increasing the β-PbO₂ content.²⁵ The corresponding patterns of the PbO₂ coatings with different substrates are shown in Fig. 3(B). It can be seen that the XRD pattern of PbO₂ coatings based on different substrates are quite similar and the peak intensities of PbO₂ deposited on the porous Ti/SnO₂–Sb₂O₃ electrodes are stronger than the peak intensities of deposition of PbO₂ on other substrate. Among them, Fig. 3(a) displays the main diffraction peaks at 2θ = 25.4°, 32.0°, 36.2°, 49.2°, 52.1°, 59.2°, 62.3°, 74.4°, and 85.9°, which are assigned to the (110), (101), (200), (211), (220), (310), (301), (321) and (411) planes of β-PbO₂, respectively, and the strong main crystal plane of β-PbO₂ is (101), (211) and (301) plane. At the same time, some weak peaks (2θ = 67.8°, 76.9°) infer the presence of α-PbO₂. However, no diffraction peaks of tin and antimony oxides are observed, indicating that the active layer of PbO₂ was uniform, and thus tin and antimony oxides were undetected by XRD. This was also confirmed by the above SEM results.

According to the electrochemical deposition mechanism of PbO₂ coatings:²⁶

H₂O → ˙OH + H⁺+ e (1)

Pb²⁺ + ˙OH → Pb(OH)²⁺ (2)

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

Pb(OH)₂²⁺ → PbO₂ + 2H⁺ (4)

In the process of electro-deposition, the ˙OH group generated from eqn (1) would adsorb on the PbO₂ crystal face and make the crystal grow via eqn (2) to (4). In addition, under the same conditions, the morphological difference of the crystal produced should be attributed to the surface morphology and structure of the substrates. Hence, the surface properties of porous Ti/SnO₂–Sb₂O₃ electrodes determine the nucleation free energy, and subsequently the nucleation and growth process also determines the crystal size, population size and morphology of the PbO₂ coating. The surface energy of the porous electrode with porous structure and roughness was greater than that of other planar electrode, which is helpful to overcome the interfacial energy and reduces the net interface energy of nucleation. Therefore, it is advantageous to adsorb Pb²⁺ and ˙OH onto the electrode surface to form a large number of crystal nuclei, but because of the limited total amount of ions in solution under the given current density, the growth of PbO₂ crystal grains is limited. Thus, the limitation of the growth of PbO₂ crystal grains increases the chance of crystal nucleus growth. This led to a uniform and smooth PbO₂ electrode surface.²⁷

3.2 Electrochemical characterization of electrode

3.2.1 Linear polarization curves. The oxygen evolution reaction (OER) takes place when current densities abruptly increase in the linear polarization curve. The overpotential for OER is obtained by an extrapolated technique from the polarization curve.^10,28–31 Fig. 4 displays the linear polarization curves of different electrodes in 0.5 mol L⁻¹ H₂SO₄ solution at a scan rate of 20 mV s⁻¹. It shows that porous Ti/SnO₂–Sb₂O₃/PbO₂ has a higher oxygen evolution potential of 1.8 V, which is higher than that of the planar Ti/SnO₂–Sb₂O₃/PbO₂, porous Ti/PbO₂ and planar Ti/PbO₂ electrodes used in this comparison experiment and other PbO₂ or Ti-based modified electrodes, such as 1.75 V for Ti/PbO₂–Sn,²⁹ 1.61 V for CNT-Bi-PbO₂ (ref. 32) and 1.6 V for β-PbO₂.³³ Tafel plot analysis is another method to evaluate the oxygen evolution reaction.^34,35 We adopted this method to further support the above results. From the inset of Fig. 4, it can be seen that the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode was characterized by a high oxygen evolution over-potential (OEP) that follows the Tafel relation, with a slope at 170 mV per decade. For the planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode, a lower Tafel slope, 140 mV per decade was found. These results indicate that the OEP of the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode is higher than that of the planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode, which is in accordance with the results of an extrapolated technique from the polarization curve. Obviously, adopting porous titanium can significantly enhance the oxygen evolution over-potential of the PbO₂ electrode. In general, a high oxygen evolution over-potential would restrain the evolution of oxygen molecules, which would be beneficial for the efficiency of organic pollutant degradation during the electro-catalytic oxidation process.

Fig. 4 Linear polarization curves of different PbO₂ electrodes in 0.5 mol L⁻¹ H₂SO₄ solution, scan rate: 20 mV S⁻¹, inset is Tafel plots for the oxygen evolution reaction.

3.2.2 Electrochemical active surface area. Electrochemically active surface area indicates that the active sites are accessible to electrolyte when electrochemical reaction occurs.³⁶ It is known that the real surface area of an electrode, especially for porous electrodes, is related to the voltammetric charge (q*).³⁷⁻³⁹ Hence, we used the method reported in other ref. 36 and 40 to quantify the electrode areas. The total electrochemical surface area (q^*_T) was calibrated through plotting the reciprocal of q* against the square root of the potential scan rate using the following equation:

(q*)⁻¹ = (q^*_T)⁻¹ + kv^−1/2 (5)

The q^*_T is composed of two fractions, q^*₀ and q^*_i, which represent electric quantity in the outer geometric and inner unattainable electrode areas, respectively. The values of the outer charge q^*₀ and q^*_i can be obtained according to the eqn (6) and (7):

q* = q^*₀ + kv^−1/2 (6)

q^*_T = q^*₀ + q^*_i (7)

The values of the inner charge (q^*_i) can be acquired by the subtraction of q^*_T and q^*₀. The electrochemical porosity is defined as the ratio between the inner and total charge (q^*_i/q^*_T). v stands for the voltage scan rate and, k is a constant.

The relationship of the reciprocal of q* versus square root of scan rate is shown in Fig. 5(A) and a satisfactory linear fitting was obtained. Through extrapolating the linear plots to v = 0, the total electrochemical surface area q^*_T was obtained. In addition, the values of the outer charge q^*₀ could be obtained from the extrapolating of v ≈ ∞ according to eqn (6) in Fig. 5(B). The values of the charges and electrochemical porosity for the different electrodes are listed in Table 1.

Fig. 5 (A) Extrapolation of q^*_T for electrodes from the representation of (q*)⁻¹ versus v^1/2, (B) extrapolation of q^*₀ for electrodes from the representation of q* versus v^−1/2: data obtained from the cyclic voltammograms obtained between 0.3 and 0.8 V versus SCE in a 0.5 mol L⁻¹ H₂SO₄ solution at scan rates from 10 to 50 mV s⁻¹.

Table 1 The data of total, outer and inner charge totals and electrochemical porosity obtained via voltammetric charge analyses of the prepared electrodes

Electrode	q^*T(C cm^−2)	q^*0 (C cm^−2)	q^*i (C cm^−2)	q^*i/qT (%)
Planar Ti/PbO2	0.000927	0.0005	0.000427	46.06
Planar Ti/SnO2–Sb2O3/PbO2	0.001979	0.0004	0.001579	79.79
Porous Ti/PbO2	0.044395	0.0014	0.042995	96.84
Porous Ti/SnO2–SbO3/PbO2	0.188360	0.0050	0.183360	97.34

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The results indicate that adopting a porous titanium substrate could cause an increase of the effective surface area and voltammetric charge. The inner quantity q^*_i of the porous Ti/PbO₂ was approximately 100 times that of the planar Ti/PbO₂. At the same time, PbO₂ deposited on the porous titanium substrate had a higher electrochemical porosity, which was in accordance with the SEM analysis of Fig. 1 and 2. Hence, porous titanium could provide more actual surface area and active sites for electro-catalytic oxidation.

3.2.3 Cyclic voltammetry. Fig. 6 shows the cyclic voltammograms of a porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode and a planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode in 1.0 mol L⁻¹ KCl solution containing 50 mmol L⁻¹ K₃Fe(CN)₆ at different scan rates from 20 to 120 mV s⁻¹. These can be used to judge the mass transfer rate and reaction activity of the electrodes.⁴¹ With increasing scan rate, both anodic peak current and cathodic peak current increase, indicating an easily reversible electrochemical reaction of the redox couple. At the same time, a good linear relationship exists between the peak current (I_p) and the square root of the scan rate (v^1/2), according to the following semi-diffusion equation:⁴²

I_p = 269n^3/2ADCv^1/2 (8)

where I_p is the peak current (A), n is the stoichiometric number of electrons involved in the electrode reaction, A is the electrode area (cm²), D is the diffusion coefficient of species (cm² s⁻¹), C is the concentration of species in solution, and v is the scan rate (mV s⁻¹). The above equation can be simplified into eqn (9):

I_p = kv^1/2 (9)

where k is a coefficient only relevant to D, because A and C are constants in this study. Thus, k (the gradient of I_p vs. v^1/2) can be used to represent the mass transfer rate of species. The higher the value of k, the faster the mass transfer rate. The porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode has a higher k value (62.29 × 10⁻⁵) than that of the planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode (1.91 × 10⁻⁵), further demonstrating that the porous Ti substrate enhanced the mass transfer rate on the PbO₂ electrode.

Fig. 6 Cyclic voltammograms of different PbO₂ electrodes in 50 mmol L⁻¹ K₃Fe(CN)₆ + 1 mol L⁻¹ KCl solution at different scan rates. (A) Porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode, (B) planar Ti/SnO₂–Sb₂O₃/PbO₂. Insets show the plots of the peak current vs. the square root of scan rate.

This linear behaviour indicates that the electrochemical reaction indicated by the quasi-reversible electron transfer kinetics is mainly controlled by diffusion. In addition, comparing the reduction peak current (I_p) at the same scan rate, the peak current of the porous titanium substrate electrode was obviously higher than that of the planar titanium electrode planar. According to eqn (8), this was mainly caused by the much larger surface area of the porous titanium substrate.

3.2.4 Electrochemical impedance. Fig. 7 displays the electrochemical impedance spectrum of the planar Ti/SnO₂–Sb₂O₃/PbO₂ (a) and porous Ti/SnO₂–Sb₂O₃/PbO₂ electrodes (b) in 0.5 mol L⁻¹ H₂SO₄ solution in the oxygen evolution potential region (1.85 V vs. SCE). The equivalent circuit, shown in Fig. 8, was used to fit the EIS data. The simulated data for each parameter in Fig. 8 is listed in Table 2. In this R_s(R_ctQ_dl) circuit, R_s represents the ohmic resistance, including the resistance of the electrolyte and active material. R_ct stands for charge-transfer resistance, reflecting the oxygen evolution reaction activity. Q_dl is introduced to replace the electric double layer capacitance.

Fig. 7 EIS plots in the 0.5 mol L⁻¹ acidic solution: (a) planar Ti/SnO₂–Sb₂O₃/PbO₂, (b) porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode. Electrode potential: 1.85 V vs. SCE.

Fig. 8 Equivalent circuit used in the analysis of the experimental EIS data.

Table 2 Simulated values determined for each electrical element

Electrode	Rs/Ω cm^2	Qdl/Ω cm^−2 s^n	Rcl/Ω cm^2	n
Planar Ti/SnO2–Sb2O3/PbO2	0.5889	0.0018	1.742	0.9653
Porous Ti/SnO2–Sb2O3/PbO2	0.3412	0.0056	0.715	0.9591

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It can be seen clearly in Fig. 7 that two obvious semicircles appeared in the electrochemical impedance spectra. The diameter of the semicircle size reflects R_ct and the resistance values were 1.742 Ω cm² and 0.715 Ω cm² (Table 2) for the planar and porous based electrodes, respectively, indicating that the oxygen evolution reaction activity of the porous Ti/SnO₂–Sb₂O₅/PbO₂ electrode was higher. According to the reaction mechanism of the electrode oxygen evolution,^16,43 the oxygen evolution activity depends on the active sites of the electrode coating and more active sites results in greater reaction activity. Hence, the porous Ti/SnO₂–Sb₂O₅/PbO₂ electrode with larger specific surface area has considerably more activity, resulting in greater oxygen evolution activity. The result was further proven by the R_s values in Table 2, which shows that the prepared porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode had the smallest R_s, indicating the largest electrochemical active surface area. From the above results, it can conclude that porous titanium substrates can reduce the R_s and R_ct, meaning the conductivity and electrochemical activity of oxygen evolution are higher than those for the planar titanium substrate based electrode. In addition, a highly electrochemical active surface is important to prepare a high performance electrode.

3.2.5 Electrode stability. As shown in Fig. 9, the porous Ti/SnO₂–Sb₂O₃/PbO₂ exhibited the best electrochemical stability and its service lifetime was the longest (214 h), as much as 3.69 times that of planar Ti/SnO₂–Sb₂O₃/PbO₂ (58 h) and longer than the porous Ti/PbO₂ (0.5 h) and the planar Ti/PbO₂ electrodes (2.3 h). The results reveal that porous titanium as the substrate material for preparing electrodes can improve the electrochemical stability of the PbO₂ electrode. It can be explained by the following reasons.

Fig. 9 Variation of cell potential with the testing time in the accelerated life for different PbO₂ electrodes.

First, from the decreased PbO₂ particle size, a compact and fine surface layer was made which is observed in the Fig. 2 SEM images. The compact surface of the porous Ti/SnO₂–Sb₂O₃/PbO₂ can baffle the penetration of the supporting electrolyte toward the titanium substrate through the cracks and pores and delay the formation of a non-conductive TiO₂ layer. Additionally, The surface properties of porous titanium itself can make the SnO₂–Sb₂O₃ interlayer or PbO₂ film and substrate combine more tightly and reduced the film detachment.²² Second, the oxygen evolution over-potential of the electrode has an important impact on the electrochemical stability. The evolution of oxygen on the anode will result in the stripping and dissolution of the PbO₂ film. Hence, when the oxygen evolution over-potential increased, the process of oxygen evolution was reduced, which could prolong the lifetime of electrode. Third, compared with Ti/PbO₂ electrode without the SnO₂–Sb₂O₃ interlayer, porous Ti/SnO₂–Sb₂O₃/PbO₂ or planar Ti/SnO₂–Sb₂O₃/PbO₂ showed better electrochemical stability. The reason is that the Sb-doped SnO₂ interlayer can further reduce the internal stress with the titanium substrate and improve the stability.⁴⁴

3.3 Electrocatalytic test

To investigate the influence of the porous titanium substrate on the electro-catalytic degradation activity of the prepared electrode, degradation experiments were carried on a different electrode. The variation of colour removal efficiency for methylene blue trihydrate with electrolysis time is shown in Fig. 10(A). It can be clearly seen that the colour removal efficiency rates were up to 90% within 70 min for the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode and only 76% for the planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode. Porous titanium electrodes showed the highest activity for colour removal. In addition, the degradation processes were fitted with a pseudo-first-order model for all the electrodes. The fitting results are shown in Fig. 10(B). According to the good linear correlation between the logarithm values of the normalized concentration and de-colorization time, the degradation of methylene blue trihydrate on these electrodes was fitted to the pseudo-first-order kinetic expression. The rate equation for the de-colorization can be expressed as follows:

C_t = C₀ e^−k_appt (10)

k_app is the apparent kinetics coefficient. The k_app values for the two electrodes were 0.03868 min⁻¹ and 0.02542 min⁻¹, respectively.

Fig. 10 Colour removal efficiency as a function of degradation time for different electrodes during the electrolysis of methylene blue trihydrate: (A) operating conditions: Na₂SO₄ concentration, 0.1 mol L⁻¹; initial concentration, 10 mg L⁻¹; current density: 60 mA cm⁻²; stirring rate: 800 rpm.(B) Kinetic analysis of the curves.

It is clear from Fig. 10 and the kinetic coefficient k_app values that the porous titanium electrodes displayed better decolorization performance for the degradation of methylene blue trihydrate. The reaction rate constant of the porous titanium electrodes was 1.52 times greater than that of planar titanium electrodes. The highest de-colorization rate can be ascribed to the highest active surface area for the porous electrodes, which can provide more active sites to generate more ˙OH radicals. At the same time, a large surface area increases the adsorption ability of reagent and ˙OH radicals, which resulted in an improvement of the decolorization ability.

It is well known that pollutants are mainly degraded by the indirect electrochemical oxidation mediated by ˙OH radicals in the electro-catalytic oxidation process. Hence, the ˙OH radicals generation ability can give more accurate information about electro-catalytic ability of electrode materials. During electrochemical treatment, terephthalic acid as a type of ˙OH radical capture agent, can readily react with ˙OH radicals to produce highly fluorescent product 2-hydroxyterephthalic acid. The amount of ˙OH radicals formed was approximately equal to the amount of 2-hydroxyterephthalic acid, which was represented as a fluorescence intensity.⁴⁵ As can be seen in Fig. 11, the fluorescence intensity of 2-hydroxyterephthalic acid around 425 nm for two electrodes increased with increasing the reaction time, indicating that ˙OH radicals were indeed formed on the anodes and played an important role in electrochemical degradation test. Comparing the fluorescence intensity for the different electrodes at different reaction times, it was found that the fluorescence intensity for the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode was higher than that for the planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode, which reveals the excellent electro-catalytic activity of the porous PbO₂ electrode. Therefore, the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode could oxidize pollutants more effectively compared with the planar electrode.

Fig. 11 Fluorescence spectrum changes observed during the electro-catalytic oxidation process in a 0.5 mol L⁻¹ aqueous solution of terephthalic acid using the prepared PbO₂: (A) planar Ti/SnO₂–Sb₂O₃/PbO₂ electrode, (B) porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode.

4. Conclusions

The porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode was successfully prepared on porous titanium substrates by thermal decomposition and electro-deposition methods. The surface morphology and structure of the porous Ti/SnO₂–Sb₂O₃ or the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrodes showed that the porous structure could be beneficial for physicochemical characterization of the electrodes. The surface structure of the porous Ti substrate and the porous Ti/SnO₂–Sb₂O₃ effectively improved the interlayer coating structure and favored the formation of PbO₂ during electrodeposition. The linear polarization curves show that the overpotential for the OER at the PbO₂ electrode based on the porous titanium substrates is higher when compared to the planar titanium substrate electrode. The voltammetric charge quantity indicated that the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode had the highest active surface area. The cyclic voltammetry analysis demonstrated that the porous Ti substrate enhanced the mass transfer rate on PbO₂ electrode. The results of accelerated life tests showed that the service life of the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode was longer than those of three other types of electrodes, which was 3.69 times that of planar Ti/SnO₂–Sb₂O₃/PbO_2. Due to its highest over-potential and a large active surface area, the porous Ti/SnO₂–Sb₂O₃/PbO₂ electrode showed the best degradation performance in the simulated wastewater treatment. Its pseudo first-order kinetics coefficient was 0.03868 min⁻¹. In summary, porous titanium substrates are important for the improvement in the performance of electrodes.

Acknowledgements

The authors are grateful for the financial support provided by the Innovative Program of Activities for University in Shanghai (no. PE2015029). Wenli Zhang (Jilin University) and Quansheng Zhang (Shanghai Institute of Technology) are also gratefully acknowledged for supplying experimental guidance and the electrochemical workstation, respectively.

References

1. M. Zhou, Q. Dai, L. Lei, C. A. Ma and D. Wang, Environ. Sci. Technol., 2004, 39, 363–370.
2. C. Comninellis, Electrochim. Acta, 1994, 39, 1857–1862.
3. M. Panizza and G. Cerisola, Chem. Rev., 2009, 109, 6541–6569.
4. C. A. Martinez-Huitle and S. Ferro, Chem. Soc. Rev., 2006, 35, 1324–1340.
5. N. Fan, Z. Li, L. Zhao, N. Wu and T. Zhou, Chem. Eng. J., 2013, 214, 83–90.
6. W. Wu, Z.-H. Huang and T.-T. Lim, Appl. Catal., A, 2014, 480, 58–78.
7. X.-y. Li, Y.-h. Cui, Y.-j. Feng, Z.-m. Xie and J.-D. Gu, Water Res., 2005, 39, 1972–1981.
8. M. M. Dong, R. Trenholm and F. L. Rosario-Ortiz, J. Hazard. Mater., 2015, 282, 216–223.
9. D. Shao, X. Li, H. Xu and W. Yan, RSC Adv., 2014, 4, 21230–21237.
10. G. Zhao, Y. Zhang, Y. Lei, B. Lv, J. Gao, Y. Zhang and D. Li, Environ. Sci. Technol., 2010, 44, 1754–1759.
11. S. Chen, Y. Zheng, S. Wang and X. Chen, Chem. Eng. J., 2011, 172, 47–51.
12. M. Panizza and G. Cerisola, Electrochim. Acta, 2005, 51, 191–199.
13. G. Zhao, P. Li, F. Nong, M. Li, J. Gao and D. Li, J. Phys. Chem. C, 2010, 114, 5906–5913.
14. W. Wu, Z.-H. Huang and T.-T. Lim, Appl. Catal., A, 2014, 480, 58–78.
15. H.-y. Ding, Y.-j. Feng and J.-f. Liu, Mater. Lett., 2007, 61, 4920–4923.
16. Z.-G. Ye, H.-M. Meng and D.-B. Sun, J. Electroanal. Chem., 2008, 621, 49–54.
17. Q. Wang, T. Jin, Z. Hu, L. Zhou and M. Zhou, Sep. Purif. Technol., 2013, 102, 180–186.
18. M. Manjaiah, S. Narendranath and S. Basavarajappa, Trans. Nonferrous Met. Soc. China, 2014, 24, 12–21.
19. N. A. Braga, C. A. A. Cairo, E. C. Almeida, M. R. Baldan and N. G. Ferreiraa, Diamond Relat. Mater., 2008, 17, 1891–1896.
20. J. Sun, H. Lu, H. Lin, W. Huang, H. Li, J. Lu and T. Cui, Mater. Lett., 2012, 83, 112–114.
21. W. Zhang, H. Lin, H. Kong, H. Lu, Z. Yang and T. Liu, Electrochim. Acta, 2014, 139, 209–216.
22. W. Zhang, H. Lin, H. Kong, H. Lu, Z. Yang and T. Liu, Int. J. Hydrogen Energy, 2014, 39, 17153–17161.
23. H. An, Q. Li, D. Tao, H. Cui, X. Xu, L. Ding, L. Sun and J. Zhai, Appl. Surf. Sci., 2011, 258, 218–224.
24. F. Montilla, E. Morallón, A. De Battisti and J. Vazquez, J. Phys. Chem. B, 2004, 108, 5036–5043.
25. X. Yang, R. Zou, F. Huo, D. Cai and D. Xiao, J. Hazard. Mater., 2009, 164, 367–373.
26. X. Hao, S. Dan, Z. Qian, Y. Honghui and W. Yan, RSC Adv., 2014, 4, 25011–25017.
27. R. Xiubin, L. Haiyan, L. Yanan, L. Wei and L. Haibo, 3-Dimensional Growth Mechanism of Lead Dioxide Electrode on the Ti Substrate in the Process of Electrochemical Deposition, Kexue Chubanshe, Beijing, China, 2009.
28. Y. Liu, H. Liu, J. Ma and J. Li, Electrochim. Acta, 2011, 56, 1352–1360.
29. H. Li, Y. Chen, Y. Zhang, W. Han, X. Sun, J. Li and L. Wang, J. Electroanal. Chem., 2013, 689, 193–200.
30. L. Zhang, L. Xu, J. He and J. Zhang, Electrochim. Acta, 2014, 117, 192–201.
31. M. Panizza, P. A. Michaud, G. Cerisola and C. Comninellis, Electrochem. Commun., 2001, 3, 336–339.
32. L. Chang, Y. Zhou, X. Duan, W. Liu and D. Xu, J. Taiwan Inst. Chem. Eng., 2014, 45, 1338–1346.
33. Y. J. Feng and X. Y. Li, Water Res., 2003, 37, 2399–2407.
34. B. Correa-Lozano, C. Comninellis and A. D. Battisti, J. Appl. Electrochem., 1997, 27, 970–974.
35. Y. Dan, H. Lu, X. Liu, H. Lin and J. Zhao, Int. J. Hydrogen Energy, 2011, 36, 1949–1954.
36. H. Vogt, Electrochim. Acta, 1994, 39, 1981–1983.
37. Y. Takasu and Y. Murakami, Electrochim. Acta, 2000, 45, 4135–4141.
38. S. Ardizzone, G. Fregonara and S. Trasatti, Electrochim. Acta, 1990, 35, 263–267.
39. F. Montilla, E. Morallón, A. De Battisti and J. L. Vázquez, J. Phys. Chem. B, 2004, 108, 5036–5043.
40. Y. Chen, L. Hong, H. Xue, W. Han, L. Wang, X. Sun and J. Li, J. Electroanal. Chem., 2010, 648, 119–127.
41. N. Vinokur, B. Miller, Y. Avyigal and R. Kalish, J. Electrochem. Soc., 1999, 146, 125–130.
42. A. J. Bard and L. R. Faulkner, Electrochemical methods: fundamentals and applications, Wiley, New York, 1980.
43. Z.-G. Ye, H.-M. Meng, D. Chen, H.-Y. Yu, Z.-S. Huan, X.-D. Wang and D.-B. Sun, Solid State Sci., 2008, 10, 346–354.
44. S. Chai, G. Zhao, Y. Wang, Y.-n. Zhang, Y. Wang, Y. Jin and X. Huang, Appl. Catal., B, 2014, 147, 275–286.
45. K.-i. Ishibashi, A. Fujishima, T. Watanabe and K. Hashimoto, Electrochem. Commun., 2000, 2, 207–210.

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