Preparation and characterization of PbO₂ electrodes modified with polyvinyl alcohol (PVA)

Abstract

Novel PbO₂ electrodes were successfully synthesized with polyvinyl alcohol (PVA) modification through electro-deposition technology. The morphology and crystalline structure of the electrodes were characterized by SEM and XRD, respectively. In addition, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and accelerated life stability testing were also carried out to analyze the chemical state, electrochemical performances and stability of the electrodes. The results showed that PVA could refine the grain size and increase the oxygen evolution potential (OEP). After PVA modification, the predominant phase of the PbO₂ electrodes was unchanged and all were pure β-PbO₂. Besides, for modified electrodes, the electrode film impedance reduced and the proportion of adsorbed hydroxyl oxygen (O_ad) on the electrode increased, implying the fast charge transfer and excellent degradation efficiency for organics. In the process of dye oxidation, the PbO₂-2.0 vt% electrode showed the highest electrocatalytic activity for ARG degradation due to its highest OEP and massive O_ad. Moreover, the accelerated service life tests revealed that the PbO₂-2.0 vt% electrode exhibited the highest stability and the accelerated service life was 329.5 h, which was more than 3 times longer than that of the PbO₂-0 vt% electrode (96 h).

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1. Introduction

Electro-catalytic oxidation technology (EOT) is becoming an attractive approach for wastewater treatment on account of its high efficiency, easy control, versatility, and environmental friendliness.^1,2 In the process of electro-catalytic oxidation, the electrode is the core which strongly affects the efficiency of organic pollutant oxidation.^3,4 Therefore, it is very important to develop a novel electrode material with high catalytic activity and stability. So far, many kinds of electrodes have been studied, including graphite, Pt, boron doped diamond (BDD), RuO₂, IrO₂, SnO₂ and PbO₂, etc.^5–11 Among these electrodes, Ti/PbO₂ was considered to be a more promising and attractive electrode in wastewater treatment processes due to its low cost, ease of preparation, high OEP and long service lifetime.^12–14 However, the fragile PbO₂ coating is easily to peel off and dissolved in the process of electrolysis, which could lead to the reduction of the electrocatalytic activity and stability of electrode as well as cause the secondary water pollution.^15–17 Hence, it is necessary to modify the PbO₂ electrode to meet the standards of industrial electrodes.

Doping is a frequently-used and effective way to modify PbO₂ electrode for improving its performance. Lots of foreign materials have been adopted, such as metallic element (Bi,¹⁸ Co,¹⁹ Fe,²⁰ Ce,²¹ Cu²²), non-metallic element (F^19,23,24), redox ion ([Fe(CN)₆]³⁻ (ref. 13)) and compounds (TiO₂,²⁵ clay,²⁶ carbon nanotubes,²⁷ fluorine resin (FR),¹⁵ Polypyrrole (PPy)²⁸ and polytetrafluoroethylene (PTFE)²⁹), etc. The results show that the modified PbO₂ electrode exhibits excellent electrocatalytic ability and stable performance. Alcohols can also be used as modifiers for electrodes. Alcohols addition can affect the rate of metal ions migration and crystallization. Recently, to the best of my knowledge, there are two reports about the alcohols modified PbO₂ electrode. Xu et al.³⁰ studied the effect of ethylene glycol (EG) modification on the electrochemical properties of PbO₂ electrode. Yang et al.³¹ reported that the PbO₂ electrode was modified with polyethylene glycol (PEG). Based on the two reports, it could be concluded that after the modification by alcohols, the electrodes both have excellent electro-catalytic activity and stability.

Polyvinyl alcohol (PVA) is an important chemical raw material and has been widely used for many applications including textile, paper, adhesives, adhesives, and so on.³² In addition, PVA is also a kind of nonionic surfactant and used in the production of emulsifier and dispersant. Adding proper amount of surfactant into the electroplating solution can effectively improve the morphology of the coating and the performance of the electrode.³³ Meanwhile, PVA is nontoxic to organisms and is the only known xenobiotic carbon chain polymer to biodegrade at high molecular masses.³⁴ Therefore, using PVP to modify the PbO₂ electrode would be very interesting and promising. But at present, to the best of our knowledge, studies of polyvinyl alcohol (PVA)-doped PbO₂ electrode have not been previously reported.

Therefore, in the present work, polyvinyl alcohol (PVA) was adopted to modify the PbO₂ electrode by electrochemical deposition. The morphology, crystalline structure and electrochemical performances were characterized. Acid red G (ARG, C₁₈H₁₃N₃Na₂O₈S₂, CAS number: 3734-67-6) was chosen as the model pollutant for electro-catalytic oxidation to evaluate electrochemical activity of the electrodes. Besides, Pb element leaching of PbO₂ electrodes was studied during electrolysis process to evaluate the safety. Furthermore, the accelerated life test was also carried out to assess its stability.

2. Experimental

2.1. Materials and reagents

All chemicals (analytical grade) were obtained from Sinopharm Chemical Reagent Xi'an Co., Ltd and used without further purification. Pure (>99.6%) titanium plates with 0.5 mm thickness were purchased from BaoTi Co., Ltd. and used as the substrate. Deionized water with conductance of 18 MΩ cm was prepared by an EPED-40TF water purification laboratory system (Yipuyida Technology development Ltd., Nanjing, China).

2.2. Electrode preparation

Ti plates (3 cm × 5 cm × 0.5 mm) were used as the electrode substrate. The pre-treatment of Ti plates and the fabrication of the Sb–SnO₂ inner layer (brush coating-thermal deposition method) were carried out according to our previous work.^13,22 The surface β-PbO₂ layer was deposited on the Ti/Sb–SnO₂ surface through the electrochemical deposition method. The electro-deposition solution contained 0.5 mol L⁻¹ Pb(NO₃)₂, 0.1 mol L⁻¹ Cu(NO₃)₂, 0.01 mol L⁻¹ NaF, 0.1 mol L⁻¹ HNO₃ and the solution dissolved different volume fractions of PVA (0 vt%, 1.0 vt%, 2.0 vt% and 5.0 vt%, marked as PbO₂-0 vt%, PbO₂-1.0 vt%, PbO₂-2.0 vt% and PbO₂-5.0 vt%, respectively). The pretreated Ti/Sb–SnO₂ was used as anode in electro-deposition solution (65 °C) for 120 min under the current density of 10 mA cm⁻¹. The copper sheet with the same size was adopted as the counter cathode. The average amount of PbO₂ coating on the electrode surface is 91.6 ± 0.3 mg cm⁻² exclusive of PbO₂-5.0 vt% (PbO₂-5.0 vt%: 83.5 mg cm⁻²).

PVA (Type 1799) was dissolved as follows: firstly, adding 2 g PVA slowly to a beaker filled with 100 mL deionized water (25 °C). The beaker would be sealed which could reduce the water evaporation. Secondly, the solution was fully swollen at room temperature (25 °C) for 12 h and performed at a heating rate of 5 °C min⁻¹ from 25 °C to 90 °C. Thirdly, the solution was kept at 90 °C for 3 h, until the tiny particles in the solution disappeared. Finally, the solution was filtered through 80 mesh of stainless steel to remove impurities and prepared for PbO₂ electrodes modification.

2.3. Characterization analysis

The morphology and crystal structure of PbO₂ electrodes are analyzed by scanning electron microscopy (SEM, JEOL, JSM-6390A) and X'pert PRO MRD diffractometer (XRD, PAN alytical, Holland) using Cu K_α source (λ = 0.15416 nm). The chemical states of O on the electrodes were analyzed by the X-ray photoelectron spectroscopy (XPS) performed on Axis Ultra spectrometer (Al K_α radiation; 1486.6 eV). Binding energy of the C 1s peak (284.8 eV) was used as the reference for calibration.

Electrochemical properties were tested on the CHI660D electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) in a standard three-electrode cell (25 °C). The fresh PbO₂ electrodes were served as the working electrode. Copper sheets were acted as the counter electrode and Ag/AgCl (sat KCl) was acted as the reference electrode. The electrolyte was 0.5 mol L⁻¹ Na₂SO₄ solution. Cyclic voltammetry (CV) measurements were performed between 0 and 2.5 V with a sweep rate of 50 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements were carried out in a range of 10⁵ Hz to 0.1 Hz at a potential of 0 V (vs. Ag/AgCl) with a sine wave of 5 mV amplitude.

The accelerated lifetime test was carried out at an anode current density of 500 mA cm⁻² using an electrochemical workstation (LK3000A, Tianjin Lanlike, China). The electrolyte was 3 mol L⁻¹ H₂SO₄ (35 °C ± 2 °C). The working electrode was considered as deactivation when the cell voltage reached to 10 V.

2.4. Electro-catalytic oxidation of ARG

Electro-catalytic oxidation of ARG was conducted in an undivided electrolytic cell under constant current density of 15 mA cm⁻² equipped with a magnetic stirrer. The volume of dye solution was 200 mL and the initial ARG concentration was 100 mg L⁻¹. 0.1 mol L⁻¹ Na₂SO₄ was used as the supporting electrolyte. The PbO₂ anodes have an area of 18 cm² and copper sheet with the same area was used as the cathode. The gap between the electrodes was 1 cm. Electrolysis were carried out at room temperature for 120 min and the liquid samples were withdrawn from the electrolytic cell every 15 min. The dye concentrations were measured by UV-vis absorption (Agilent 8453, Agilent) at the characteristic wavelengths of 505 nm and COD values (mg L⁻¹) of the samples were determined by CSB/COD Reactor (ET 125 SC). The decolorization rate (η_ARG) and COD removal rate (η_COD) in electrochemical oxidation could be calculated as follows:where A₀ and A_t are the absorbance value in 505 nm of initial sample and electrolysis at the given times t, respectively.where COD₀ and COD_t are the initial COD concentration and COD concentration value at given time t, respectively.

The instantaneous current efficiency (ICE) and energy consumption (E_p) were calculated as follows:

where COD_t1 and COD_t2 are the chemical oxygen demand at times t₁ and t₂, respectively, I is the current (A), F is the Faraday constant (96 487 C mol⁻¹), U is the cell voltage (V) and V is the volume of the electrolyte (L).

In order to evaluate the safety performance of the PbO₂ electrode in the electrolysis process. The fresh prepared electrode was electrolyzed at 15 mA cm⁻² for 120 min in 0.1 mol L⁻¹ Na₂SO₄ solution. The concentration of Pb element dissolved in electrolyte (no coating detachment was found) was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Shimadzu, Japan).

3. Results and discussion

3.1. Surface morphological analysis of PbO₂ electrodes

Fig. 1 shows the SEM images of the PbO₂ electrodes modified with different PVA volume fractions. From Fig. 1a, it could be observed that the PbO₂-0 vt% electrode showed typical pyramid structure (octahedral crystals) with different sizes on the surface. The electrode surface was relatively rough, and there were a number of micro holes distributed on the surface of the coating. The PbO₂ grains were randomly arranged and there were not obvious crystal orientation. This structure could easily lead to the infiltration of the electrolyte into the coating, reducing the stability of the electrode.³⁵ In addition, the O₂ generated by electrolysis of water would be released from the micro holes and furtherly damaged the coating. As shown in Fig. 1b and d, it could be seen that after PVA adding into the deposition solution, the modified PbO₂ electrode still had the pyramid structure, but it was ordered and uniform compared with unmodified electrode. With the increase of the PVA doping amount, the particles of the electrode surface became smaller and smoother. The reasons could be explained as follows: firstly, the surface tension of the plating solution could be reduced by the addition of surfactants (PVA), making the coating smooth and compact. Secondly, PbO₂ crystal nucleus would be coated by PVA molecule and inhibited the growth and aggregation of PbO₂ grains. Therefore, the particle size of PbO₂ crystals decreased. Thirdly, the addition of PVA increased the viscosity of electroplating solution, reducing the migration rate of Pb ions and making the current density distribute on the electrode uniformly. Therefore, the electrode coating became compacted and smooth. The special structure could suppress the infiltration of the electrolyte into the coating layer and prolong the service life of the electrode.

Fig. 1 SEM images of the different PbO₂ electrodes ((a) PbO₂-0 vt%, (b) PbO₂-1.0 vt%, (c) PbO₂-2.0 vt%, (d) PbO₂-5.0 vt%).

3.2. Structure of PbO₂ electrodes

The XRD patterns of the PbO₂ electrodes are shown in Fig. 2. For PbO₂-0 vt% electrode, the crystal structure is pure β-PbO₂ and the main diffraction peaks at 25.4°, 32.0°, 36.2° and 49.0° were assigned to the (110), (101), (200) and (211) crystal faces, respectively. After PVA addition, the crystalline orientation of the electrodes did not change greatly and the predominant phases were still β-PbO₂. However, the diffraction intensities of (101) and (211) planes for PbO₂-1.0 vt% and PbO₂-2.0 vt% increased significantly, indicating that the preferred orientation along the (101) and (211) directions. In addition, the diffraction intensities of (110) and (200) planes decreased. Moreover, when the PVA volume fraction increased to 2.0 vt%, the peak of (220) plane disappeared. This phenomenon could be account for the different adsorption and encapsulation of PVA on the different crystal surfaces. The planes of (110), (200) and (220) were suppressed and the planes of (101) and (211) were preferential growth. However, when the PVA volume fraction increased to 5.0 vt%, the intensities of diffraction peaks were all weak compared with other electrodes. After calculated by Debye–Scherrer formula, it could be seen that the average grain sizes of PbO₂ crystals decreased with the addition of PVA (Table 1). The average grain sizes of PbO₂ crystals were 21.20 nm, 20.91 nm, 19.37 nm and 19.13 nm, respectively, which was consistent with SEM results.

Fig. 2 XRD patterns of the different PbO₂ electrodes.

Table 1 The different parameters of selected PbO₂ electrodes^a

	PbO2-0 vt%	PbO2-1.0 vt%	PbO2-2.0 vt%	PbO2-5.0 vt%
a OEP represents oxygen evolution potential.
Average loading capacity/mg cm^−2	91.9	91.6	91.3	83.5
Grain size/nm	21.2	20.9	19.3	19.1
OEP/V	1.845	1.849	1.862	1.837
Accelerated lifetime/h	96	256.5	329.5	92.5

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3.3. Cyclic voltammetry (CV) test

Fig. 3 shows the typical cyclic voltammograms of different PbO₂ electrodes in 0.5 mol L⁻¹ Na₂SO₄ solution at the scan rate of 50 mV s⁻¹. As shown in Table 1, it could be seen that the oxygen evolution potential (OEP) of the unmodified PbO₂ electrode was 1.845 V. After the modification of PVA, the OEP increased for PbO₂-1.0 vt% (1.849 V) and PbO₂-2.0 vt% (1.862 V), respectively. However, when the volume fraction of PVA reached to 5.0 vt%, the OEP decreased to 1.837 V. It was well accepted that electrodes with a high OEP were praised, because the oxygen evolution reaction and energy loss could be suppressed.^10,36,37 Therefore, this result implied that the PbO₂-2.0 vt% with the highest OEP could exhibit a better performances for organics removal than that of others.

Fig. 3 Cyclic voltammograms curves of PbO₂ electrodes in 0.5 mol L⁻¹ Na₂SO₄ solution, scan rate: 50 mV s⁻¹.

In addition, the current response (at the section of oxygen evolution) of PbO₂-2.0 vt% was the most obvious. Xu et al.³⁸ considered that the high current response in the section of oxygen evolution could promote the water splitting and accelerate the generation rate of HO˙ free radical, which was also advantageous to the degradation of organic matter for PbO₂-2.0 vt%.

3.4. Electrochemical impedance spectroscopy (EIS)

Fig. 4 shows the EIS Nyquist plots of freshly prepared PbO₂ electrodes in 0.5 mol L⁻¹ Na₂SO₄ solutions and the simulated data is shown in Table 2. The Nyquist plots for all electrodes fit the equivalent circuit R_s(R_fQ) very well. And R_s, R_f and Q represent the solution resistance, electrode film resistance and the constant phase element (CPE), respectively. From this figure, it could be seen that arc diameter of electrodes decreased with the increase of the volume fraction of PVA, revealing that the PVA addition could reduce the electrode film resistance and accelerate the electron transfer. As shown in Table 2, the PbO₂-2.0 vt% showed the smallest R_f (42.47 Ω cm⁻²), which was lower than that of PbO₂-0 vt% (102.1 Ω cm⁻²). In addition, the PbO₂-2.0 vt% also showed the largest CPE value (0.000523 Ω⁻¹ sⁿ cm⁻²) compared with other electrodes, reflecting a high specific area and more active sites on the electrode.³⁹

Fig. 4 The fitted curves of PbO₂ electrodes in EIS and equivalent circuit model (the inset).

Table 2 Simulated data of each parameter (EIS)

	Rs/Ω cm^−2	Rf/Ω cm^−2	Q/Ω^−1 s^n cm^−2	n
PbO2-0 vt%	3.272	102.1	0.000245	0.695
PbO2-1.0 vt%	3.410	82.21	0.000262	0.601
PbO2-2.0 vt%	2.909	42.47	0.000523	0.684
PbO2-5.0 vt%	3.821	87.39	0.000191	0.701

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3.5. Electrochemical oxidation for ARG

Fig. 5 shows the performance of the PbO₂ electrodes for ARG degradation. From Fig. 5a, it could be clearly seen that the decolorization rates all reached more than 90% after 120 min electrolysis. The highest decolorization rate was obtained on PbO₂-2.0 vt% and decolorization rate could reach 89.3% within 60 min, which was higher than that of PbO₂-0 vt% (72.4%). After the kinetic fitting, we found that the removal of ARG was according to the first order kinetics (the inset). The rate constants (k) for PbO₂-0 vt%, PbO₂-1.0 vt%, PbO₂-2.0 vt% and PbO₂-5.0 vt% were 2.52 × 10⁻² min⁻¹, 3.63 × 10⁻² min⁻¹, 4.69 × 10⁻² min⁻¹ and 2.87 × 10⁻² min⁻¹, respectively. And the corresponding t_1/2 values were 33 min, 25 min, 23 min and 29 min, respectively (Table 3). As shown in Fig. 5b, the PbO₂-2.0 vt% possessed the highest COD removal rate value of 66.8% within 60 min, which was higher than that of others. Meanwhile, the PbO₂-2.0 vt% also showed the highest ICE (34.5%) and lowest E_p (0.041 kW h g_COD⁻¹) than those of the other PbO₂ electrodes. The best performance for ARG degradation at PbO₂-2.0 vt% could be ascribed to the high oxygen evolution potential, good electric conductivity and high specific area. These properties were helpful to generate more HO˙ free radicals and remove ARG at low energy consumption.⁴⁰

Fig. 5 Performance of the PbO₂ electrodes for ARG degradation. (a) ARG decolorization rate; (b) COD removal rate.

Table 3 The kinetics for the electrochemical degradation of ARG (electrolysis time: 120 min)

Electrode	Rate constants^a (k, 10^−2 min^−1)	Half-lives (t1/2, min)	R^2	ICE^b (%)	Ep^c (kW h gCOD^−1)
a Pseudo-first-order rate constant of electrochemical degradation.b The values of ICE was obtained at the time of 50% COD removal.c The values of Ep was obtained at the time of 50% COD removal.
PbO2-0 vt%	2.52	33	0.991	24.8	0.057
PbO2-1.0 vt%	3.63	25	0.993	32.0	0.044
PbO2-2.0 vt%	4.69	23	0.990	34.5	0.041
PbO2-5.0 vt%	2.87	29	0.988	28.1	0.050

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After ICP-AES detection, we found that the concentration of dissolved Pb element in the electrolyte decreased after the PVA modification (Table 4). This result could be explained as that after PVA modification, the morphology became smooth, compact and the grain sizes became smaller, which could slow down the erosion rate of electrolyte on the PbO₂ coating. As shown in Table 4, it could be seen that the concentration of Pb element for PbO₂-0 vt%, PbO₂-1.0 vt%, PbO₂-2.0 vt% and PbO₂-5.0 vt% were 0.023 mg L⁻¹, 0.011 mg L⁻¹, 0.006 mg L⁻¹ and 0.016 mg L⁻¹, respectively. It could be observed that the PbO₂-2.0 vt% had the least amount of Pb dissolution, implying the best safety performance in application.

Table 4 The concentration of Pb element in the electrolyte (electrolysis time: 120 min)

Electrode	PbO2-0 vt%	PbO2-1.0 vt%	PbO2-2.0 vt%	PbO2-5.0 vt%
Concentration (mg L^−1)	0.023	0.011	0.006	0.016

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3.6. X-ray photoelectron spectroscopy (XPS) analysis

In order to further explore the reasons for the good catalytic performance of PbO₂-2.0 vt% electrode, the XPS test was carried out for PbO₂-0 vt% and PbO₂-2.0 vt% (Fig. 6) and XPS data of chemical states of O on the electrode surface is shown in Table 5. Fig. 6a shows the XPS whole spectra for PbO₂-0 vt% and PbO₂-2.0 vt%. From Fig. 6a, it could be seen that the peaks of Pb, C and O existed in PbO₂-0 vt% electrode. As for PbO₂-2.0 vt%, the peaks showed no obvious changes, indicating that the structure of PbO₂ remained intact after PVA modification. From Fig. 6b and c, there existed two types oxygen on the electrode surface. The peak at around 529 eV was attributed to the lattice oxygen (O_L) and the peak at around 531 eV was attributed to adsorbed hydroxyl oxygen (O_ad).⁴¹ As shown in Table 5, it could be observed that the percentage of the O_ad to the total oxygen (O_L + O_ad) for PbO₂-2.0 vt% (76.81%) was higher than that of PbO₂-0 vt% (68.95%). O_ad was the most active oxygen and could generate more HO˙ free radicals, favoring the organics oxidation.^42,43 Therefore, PbO₂-2.0 vt% with a higher percentage of the O_ad implied a higher performance for ARG removal.

Fig. 6 XPS spectra of PbO₂ electrode PbO₂-0 vt% and PbO₂-2.0 vt%. (a) XPS fully scanned spectra; (b) O 1s core level spectrum for PbO₂-0 vt%; (c) O 1s core level spectrum for PbO₂-2.0 vt%.

Table 5 XPS data of chemical states of O on the electrode surface^a

Electrode	Binding energy (eV)		ε%	η
	O 1s (OL)	O 1s (Oad)	Oad/(Oad + OL)100%	(Oad/OL)
a Oad: adsorbed hydroxyl oxygen; OL: lattice oxygen.
PbO2-0 vt%	529.06	531.82	68.95	2.22
PbO2-2 vt%	529.01	531.02	76.81	3.31

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3.7. Electrochemical stability test

Stability is an important index to evaluate the quality of electrodes. Therefore, the stabilities of the PbO₂ electrodes were investigated through accelerated service life tests (Fig. 7). It was clearly seen that the cell potential of the selected electrodes maintained relatively stable below the cell potential of around 4.0 V. Then the cell potential began to rise sharply in a short period of time, leading to deactivation. For PbO₂-1.0 vt% and PbO₂-2.0 vt%, the service life were 329.5 h and 256.5 h, respectively, which were higher than that of PbO₂-0 vt% (96 h). The enhancement of service life could be account for the smooth and compact morphology which could inhibit the infiltration of electrolyte into the coating and the formation of TiO₂ passivation layer. In addition, because of the improvement of oxygen evolution potential (OEP) for PbO₂-1.0 vt% and PbO₂-2.0 vt%, the diffusion of reactive oxygen species into the Ti substrate was also inhibited. Furthermore, the PVA addition could effectively reduce the internal stress of PbO₂ coating and improve the bonding strength between coating and substrate. For these reasons, the electrode stability had been greatly improved.

Fig. 7 Accelerated life test of selected PbO₂ electrodes (H₂SO₄: 3 mol L⁻¹; current density: 500 mA cm⁻²).

However, when the PVA volume fraction increased to 5.0 vt%, the service life decreased to 92.5 h, which was lower than that (96 h) of PbO₂-0 vt%. This result could be interpreted as that PVA addition could increase the viscosity of the electroplating solution and reduce the mass transfer rate of ions. Thus, when adding excess of PVA into the electroplating solution, the migration rate of Pb²⁺ ion to Ti/Sb–SnO₂ substrate would slow down and the average loading capacity of PbO₂ coating would decrease (only 83.5 mg cm⁻²) within a constant time. Therefore, the lower average loading capacity implied that the coating could be consumed in a relatively short period of time, leading to the deactivation of electrode rapidly.

Fig. 8 shows the SEM images and EDX spectrum of the deactivated PbO₂ electrodes from Fig. 8a and d, it could be seen that the PbO₂ coating still existed. However, compared with morphology of the fresh PbO₂-0 vt% and PbO₂-5.0 vt%, the typical rectangular pyramid structure of β-PbO₂ disappeared and the coating became very loose. After EDX analysis, it was found that there were still a large number of Pb element in the coating (Fig. 8e and h). This result indicated that the deactivation pattern of the two electrodes may be the substrate passivation. Although most of the PbO₂ surface coating still existed, the electrode was inactive. For PbO₂-1.0 vt% and PbO₂-2.0 vt%, the coating have been largely depleted and Ti substrate has been exposed. In addition, more Ti content was detected on the PbO₂-1.0 vt% and PbO₂-2.0 vt% after EDX analysis, which indicated that the depletion of the coating lead to the inactivation of PbO₂-1.0 vt% and PbO₂-2.0 vt%. It was known to all, the coating consumption was a much slower process than direct passivation of substrate.⁴⁴ Therefore, it could also explain the reason for the longer service life of PbO₂-1.0 vt% and PbO₂-2.0 vt% compared with PbO₂-0 vt% and PbO₂-5.0 vt%.

Fig. 8 SEM images and EDX spectrum of the deactivated PbO₂-0 vt% electrode (a and e), PbO₂-1.0 vt% (b and f), PbO₂-2.0 vt% (c and g) and PbO₂-5.0 vt% (d and h).

4. Conclusions

This study reported the preparation of PbO₂ electrode modified with polyvinyl alcohol (PVA) through electro-deposition technology. The results of SEM and XRD showed that the grain size of the modified electrode decreased and the morphology became compacted and smooth. The PVA addition could effectively improve the oxygen evolution potential (OEP) and reduce the electrode film resistance (R_f). This result showed that the PbO₂-2.0 vt% possessed the highest OEP (1.862 V) and lowest R_f (42.47 Ω cm⁻²). From the electrochemical oxidation performances for ARG degradation, it could be seen that the PbO₂-2.0 vt% electrode showed the highest electrocatalytic activity for ARG degradation with the lowest energy consumption (E_p) due to its highest oxygen evolution potential (OEP), good electric conductivity and massive O_ad. After the kinetic fitting, the removal of ARG was according to the first order kinetics and the rate constants (k) for PbO₂-2.0 vt% was 4.69 × 10⁻² min⁻¹, which was 1.86 times higher than that of PbO₂-0 vt% (2.52 × 10⁻² min⁻¹). In addition, the dissolution reaction of Pb ion was suppressed after the PVA modification, ensuring the safety performance in application. Furthermore, the result of accelerated service life tests showed that the PbO₂-2.0 vt% electrode exhibited the highest stability and the accelerated service life was 329.5 h, which was more than 3 times longer than that of PbO₂-0 vt% electrode (96 h). Thus, the PbO₂-2.0 vt% electrode was considered to be the most optimal electrode in this study.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21507104).

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