Preparation and characterization of Fe-Ce co ...

Journal of Electroanalytical Chemistry 823 (2018) 193–202

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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Preparation and characterization of Fe-Ce co-doped Ti/TiO2 NTs/PbO2 nanocomposite electrodes for efficient electrocatalytic degradation of organic pollutants

T

Mai Xua, Yulu Maoa,b, Wenliang Songc, XueMei OuYanga,b, Yunhu Hua, Yijun Weia, ⁎ ChuanGao Zhua, Wenyan Fanga, Bingchao Shaoa, Rong Lua, Fengwu Wanga, a b c

School of Chemistry and Material Engineering, Huainan Normal University, Huainan, 232038, People's Republic of China School of Chemical Engineering, Anhui University of Science and Technology, Huainan, Anhui 232001, People's Republic of China BK21 PLUS Centre for Advanced Chemical Technology, Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Pulse electrodeposition Ti/TNTs/Fe-Ce-PbO2 electrodes Methylene blue Electrochemical oxidation

To further improve the electrocatalytic activity and stability of PbO2 electrode in refractory organic pollutants treatment, Fe and Ce co-doped Ti/TiO2 nanotube (TNTs)/PbO2 electrode was successfully synthesized by pulse electrodeposition. The morphology, crystallinity and elemental composition Ti/TNTs/Fe-Ce-PbO2 electrodes were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electrochemical impendence spectroscopy (EIS) were utilized to investigate the electrochemical performance of electrodes. Results showed that Ti/TNTs/Fe-Ce-PbO2 electrodes (P) possessed finer grain size and higher specific surface than other three PbO2-based electrodes. The electrochemical measurements showed that Ti/TNTs/Fe-Ce-PbO2 electrodes (P) exhibited higher oxidation peak current, oxygen evolution potential and strong capability of ·OH generation than those of other three kinds of electrodes. The experiments on degradation of methylene blue (MB) also indicated that Ti/TNTs/Fe-Ce-PbO2 electrodes (P) have higher chemical oxygen demand (COD) removal efficiency and instantaneous current efficiency (ICE) than the Ti/TNTs/Fe-Ce-PbO2 electrodes (D). Meanwhile, high reusability and safety were achieved by Ti/TNTs/Fe-Ce-PbO2 electrodes (P) when treating wastewater containing organic dyes.

1. Introduction Contamination of freshwater caused by the organic pollutants and hazardous pollutants from the industrial production is known as a global issue. Especially, with the rapid development of synthetic dye technology, organic azo dyes have been widely used in several industries, such as textile, color paper and printing, and so on [1, 2]. The vast majority of colored wastewater has been discharged into the environment without sufficient treatment, which caused serious ecosystem problems in the past decade. Hence, the treatment of wastewater containing organic dyes is essential before discharge. Traditional various treatment strategies for the toxic of effluents containing azo dyes are inconvenient, as for example biological oxidation, chemical coagulation and activated carbon adsorption [3]. As a result, the development of the more efficient and feasibility method to completely remove the persistent organic dyes from wastewaters is essential.

During the last two decades, electrochemical oxidation technology, which is regarded as an emerging method for the decontamination of various wastewater containing toxic or refractory organic pollutants, has attracted a great deal of attention because of its many distinctive advantages, such as high oxidation efficiency, easy control, environmental compatibility, and so on [4–10]. During the anodic oxidation process, the anode material as the core component is a crucial factor which strongly affects the efficiency of organic pollutant oxidation [11]. Therefore, it is very important to develop a novel electrode material with high catalytic activity and stability. So far, various types of electrode materials [12] have been investigated to decompose the organics of wastewater, such as graphite [13], platinum [14], IrO2 [15], RuO2 [16], SnO2 [17], PbO2 [18], and boron-doped diamond (BDD) [19] electrodes. Among numerous types of electrodes, PbO2 electrode has attracted a great deal of attention and investigations as one of the optimum metal oxide electrode materials

⁎ Corresponding author at: School of Chemistry and Material Engineering, Huainan Normal University, Dongshan Road (West), 232038, Huainan, Anhui Province, People's Republic of China. E-mail addresses: [email protected], [email protected] (F. Wang).

https://doi.org/10.1016/j.jelechem.2018.06.007 Received 19 March 2018; Received in revised form 1 May 2018; Accepted 2 June 2018 1572-6657/ © 2018 Elsevier B.V. All rights reserved.


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a reference electrode). The composition of electrodeposition solution was: 0.5 mol L−1 Pb (NO3)2, 0.1 mol L−1 HNO3, 0.02 mol L−1 Fe (NO3)3·9H2O and 0.01 mol L−1 Ce (NO3)3·9H2O [24, 27, 45]. The parameters of the pulse electrodeposition were negative pulse (−50 mA cm−2, 1 ms), positive pulse (+50 mA cm−2, 10 ms), current off-time (1000 ms) and the deposition processes were carried out at 60 °C for 45 min. The obtained electrode was marked as Ti/TNTs/Fe-CePbO2 electrode (P). For comparison purposes, the direct electrodeposition was also carried out at 60 °C for 45 min, and the current density was controlled at 50 mA cm−2, which was marked as Ti/TNTs/ Fe-Ce-PbO2 electrode (D). When the Ti/TNTs/Fe-PbO2 electrodes (P) was prepared by the same pulse electrodeposition procedures of the preparation of the Ti/TNTs/Fe-Ce-PbO2 electrode (P), 0.02 mol L−1 Fe (NO3)3·9H2O was added into the electrodeposition solution (0.5 mol L−1 Pb (NO3)2, 0.1 mol L−1 HNO3). A similar methodology was followed for the preparation of the Ti/TNTs/PbO2 electrodes (P), but no Fe (NO3)3·9H2O was added into the electrodeposition solution. The approximate thicknesses of all PbO2 deposit are 16 μm.

because of its low electrical resistivity, low price, high corrosion-resistance, as well as high overpotential for the oxygen evolution reaction [20, 21]. However, it has some shortcomings which limit its further application, such as relatively large interface resistance, low catalyst activity and easily damaged [22]. Thus, the key point is to make a further improvement in the electrocatalytic performance and accelerate the practical applications of PbO2 electrodes in electrochemical technologies for water treatment. More recently, the microstructure design of electrode substrate has attracted widely interests, and it has been confirmed that the nature of substrate material can further improve the electrochemical performances and stability of electrodes, especially for PbO2 grew on the TiO2 nanotube array template [23]. In addition, many studies also proved that the electrocatalytic ability and stability (chemical or mechanical) of PbO2 electrode could be greatly improved by doping of some foreign materials into the oxide layer [24, 25]. The incorporation of foreign materials such as Co2+ [26, 27], Fe3+ [28, 29], F− [30], rare earths ions [31] and Bi3+ [32, 33] into PbO2 has been investigated, which has been proved to be beneficial to the improvement of the electrochemical performance. Direction current electrodeposition is the main preparation method of PbO2 composite electrodes by adding some foreign elements into the deposition solution [34]. Recently, pulse electrodeposition has attracted more and more interest as a convenient method to control the density of materials and the size of the crystal particles, and to achieve a uniform morphology on the surface of the electrode. Compared with direct electrodeposition, pulse electrodeposition is helpful to control the structure and the properties of the produced deposits [35]. Therefore, the doped PbO2 electrodes prepared by pulse electrodeposition possess more unique compositions and microstructures than that obtained by other electrodeposition methods [36–38]. In the present work, Fe and Ce co-doped PbO2 electrode based on TNTs arrays was prepared by pulse electrodeposition. The subsequently morphology, crystalline structure, surface composition, electrochemical performances and stability were characterized. The electrocatalytic activities of Ti/TNTs/Fe-Ce-PbO2 electrodes were further evaluated by degrading the methylene blue (MB) as the bio-refractory model organic pollutant. The COD and absorbance were recorded to investigate the removal process of MB.

2.4. Characterization of Ti/TNTs/Fe-Ce-PbO2 electrodes A SEM (Netherlands FEI Sirion-200) was employed to investigate and analyze the structure of the prepared samples, including Ti/TNTs, Ti/TNTs/Fe-Ce-PbO2 electrodes. The XRD analysis were recorded with a Japan MXPAHF X-ray diffractometer equipped with graphite monochromatized Cu Kα irradiation (λ = 0.154056 nm), employing a scanning rate of 0.02°/s1 in the 2θ range of 10–70°. XPS were taken on a Thermo ESCALAB 250 X-ray photoelectron spectrometer with Al Kα radiation (1486.60 eV, 150 W) to analyze the element state of the electrodes. 2.5. Electrochemical experiments Electrochemical behaviors of Ti/TNTs, Ti/TNTs/Fe-PbO2 (P), Ti/ TNTs/Fe-Ce-PbO2 (P) and Ti/TNTs/Fe-Ce-PbO2 (D) electrodes (1 cm × 1 cm) were measured with a standard three-electrode cell using Autolab PGSTAT302N electrochemical analysis system. The platinum was used as the counter electrode and saturated calomel electrode (SCE) was the reference electrode. The measurement of CV was conducted in 0.1 mol L−1 Na2SO4 solution at a scan rate of 50 mV s−1. The steady-state polarization curves were conducted in 0.2 mol L−1 Na2SO4 solution at a scan rate of 10 mV s−1. EIS measurements were made at a measurement potential of open-circuit potential in 0.2 mol L−1 Na2SO4 solution, the frequencies swept from 1 × 105 Hz to 0.1 Hz with an applied sine wave of 20 mV amplitude [41]. The stability tests were performed by anodic polarization at 1 A cm−2 in 2 mol L−1 H2SO4 solution at 60 °C. During the experiments, the absorbance of MB with different electrolytic times was analyzed by the UV/visible analysis Spectrophotometer (UV-3600 plus, SHIMADZU). The chemical oxygen demand (COD) was determined by a 5B-1F COD reactor and a spectrophotometer (Lianhua Tech Co., China). The COD removal efficiency (η) was calculated as follow,

2. Experimental 2.1. Materials All chemical reagents were supplied by Alfa Aesar and used as received without further purification. All solutions were prepared using deionized water. 2.2. Preparation of Ti/TNTs matrices A Ti foil (1 cm × 3 cm × 0.5 mm, 99.7%) was used as a substrate for the formation of vertically-aligned TNTs. The TNTs were prepared by anodic oxidation as described in our previous work [39]. After anodization, the obtained Ti/TNTs were cleaned in the deionized water by ultrasonic. The cleaned TNTs were air-dried and then annealed to induce crystallization in a muffle furnace, where the annealing was conducted in air at 500 °C for 120 min. In order to increase their conductivity, the prepared TNTs were electrochemically reduced in 1 M (NH4)2SO4 as described in a ref. [40].

η=

COD0 − CODt × 100% COD0

(1)

where COD0 is the COD of initial concentration and CODt is the COD at given time t. The instantaneous current efficiency (ICE) of Ti/TNTs/Fe-Ce-PbO2 electrodes (1 cm × 3 cm) for different conditions was another evaluation of the electrocatalytic ability and was calculated as [42].

2.3. Preparation of Ti/TNTs/Fe-Ce-PbO2 electrodes The surface layer of Fe, Ce and PbO2 on the Ti/TNTs were achieved in the prepared nanotubes substrates by electrodeposition method in a three-electrode system (the Ti/TiO2NTs as a working electrode, platinum foil as a counter electrode and the saturated calomel electrode as

ICE =

[(COD)t − (COD)t + Δt]FV 8IΔI

(2) −1

where CODt and CODt+Δt are the COD at time t and t + Δ t (g·O2 L ), respectively; F is the Faraday constant (96,487 C mol−1); V is the 194


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Fig. 1. SEM image of electrodes: (a) Ti/TNTs, (b) Ti/TNTs/PbO2 electrodes (P), (c) Ti/TNTs/Fe-PbO2 electrodes (P), (d) Ti/TNTs/Fe-Ce-PbO2 electrodes (P), (e) Ti/ TNTs/Fe-Ce-PbO2 electrodes (D).

(D) by the direct electrodeposition still displays typical pyramidal shape and the particle size is larger. On the contrary, the crystal grain of the only Fe-doped PbO2 electrode by the pulse electrodeposition (Ti/TNTs/ Fe-PbO2 (P)) is obviously smaller than the Ti/TNTs/Fe-Ce-PbO2 electrode (D). Hence, the pulse electrodeposition could also significantly affect the film morphology and improve the coating structure effectively. Based on the results from the SEM analyses, we can conclude that the synergistic effect of pulse electrodeposition and Fe, Ce codoping can significantly decrease the particle size of PbO2 electrode. Therefore, the Ti/TNTs/Fe-Ce-PbO2 electrodes (P) can obtain the smallest particles and has a larger specific area than other three PbO2 electrodes. It is well known that smaller grain size is favorable for the formation of large specific area and more active sites for the electrocatalytic reaction, which might contribute to the enhanced electrochemical performance in degrading the pollutants on the electrode [43].

volume of the electrolyte (L); and I is the current (A). The concentration of Pb2+ ions dissolved in solution after electrolysis was determined by inductively coupled plasma atomic emission spectroscopy (ICPAES, Shimadzu, Japan).

3. Results and discussion 3.1. Surface morphological analysis of Ti/TNTs/Fe-Ce-PbO2 electrodes As shown in Fig. 1a, the anodization of Ti foil produced the vertically grown nanotube array. The Ti/TNTs have been successfully fabricated on the surface of Ti by the anodization method. The hollow Ti/ TNTs with highly ordered structure were found to have an average pore diameter of 125 nm. Fig.1b–e shows the SEM surface micrographs of the pure PbO2 electrode and the other three doped PbO2 electrodes deposited on the Ti/TNTs by different electrodeposition methods. Compared with Fig. 1b and 1c, the surface particle sizes of the Ti/TNTs/FeCe-PbO2 electrode (P) (Fig. 1d) become smaller and the structure becomes looser. So, it can be deduced that the Ti/TNTs/Fe-Ce-PbO2 electrode (P) has a larger specific surface than other two electrodes. According to the morphological changes of Fig. 1b–d, we preliminarily inferred that the introduction of Fe and Ce into PbO2 film significantly affected the film morphology of PbO2 electrode. It's worth mentioning that, from Fig. 1e, the morphology of Ti/TNTs/Fe-Ce-PbO2 electrode

3.2. XRD and XPS analysis XRD was used to identify and determine the phase structure, crystallite size and relative crystallinity of the samples. Fig. 2a shows XRD patterns of Ti/TNTs, Ti/TNTs/PbO2 (P), Ti/TNTs/Fe/PbO2 (P), Ti/ TNTs/Fe-Ce-PbO2 (D) and Ti/TNTs/Fe-Ce-PbO2 (P) electrodes. The diffraction peaks of all samples are ascribed to the diffraction peaks of 195


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Fig. 2. (a) XRD spectra of electrodes (a:Ti/TNTs, b:Ti/TNTs/PbO2(P), c: Ti/TNTs/Fe/PbO2 (P), d: Ti/TNTs/Fe-Ce-PbO2 (D), e: Ti/TNTs/Fe-Ce-PbO2 (P)), (b)XPS survey spectrum of Ti/TNTs/Fe-Ce-PbO2 electrodes (P), (c)Fe 2p, (d)Ce 3d, (e)Pb 4f.

Ti, TiO2 and β-PbO2. Compared to the diffractogram of Ti/TNTs electrodes, XRD peak intensities of anatase steadily become weaker and the width of XRD diffraction peaks of anatase becomes slightly wider after embed with Pb, Fe and Ce. The diffraction peaks observed at 2θ = 25.4°, 32.0°, 36.2°, 49.1°, 52.2°, 58.9° and 62.5° were assigned to (110), (101), (200), (211), (220), (310) and (301) planes of β-PbO2. After Fe and Ce modification by the pulse electrodeposition, the intensities of the peaks were attenuated. The result indicated that the Ti/ TNTs/Fe-Ce-PbO2 (P) electrode had smaller crystal size than those of three PbO2 electrodes [47]. In addition, the average crystal sizes of βPbO2 crystals of electrodes were calculated by Scherrer's formula via the relative intensity of the (101) diffraction peak. The smallest grain size of 27.3 nm was achieved by the Ti/TNTs/Fe-Ce-PbO2 (P) electrode while at Ti/TNTs/Fe-Ce-PbO2 (D), Ti/TNTs/Fe-PbO2 (P) and Ti/TNTs/ PbO2 (P) electrodes those crystal sizes were 36.7, 33.4 and 35.1 nm, which was consistent with SEM results. The smaller crystal size facilitated to the formation of large specific area which was one of the most important factors influencing the activities of catalysts [44]. This also meant that PbO2 oxides were dispersed more effectively on the Ti/ TNTs/Fe-Ce-PbO2 (P) electrode, resulting in the great increase of active

sites, which were helpful to increase its service lifetime and electrocatalytic properties [45]. In addition, when Fe and Ce were doped into the PbO2 electrode, it can be observed from Fig. 2a that the XRD peak intensities of TiO2 became weaker and the width of XRD diffraction peaks of TiO2 became slightly wider, indicating the PbO2 coating of Fe and Ce modified electrodes had the better coverage of the TNTs substrate and the PbO2 coating became more dense [4]. A more dense surface could effectively prevent electrochemical corrosion, and thus help to prolong the service lifetime of the electrode [44]. In order to further analyze the surface composition of the prepared Ti/TNTs/Fe-Ce-PbO2 electrode (P) and identify the chemical status of Fe, Ce elements in the samples, XPS were carried out. Fig. 2b presents the general XPS survey spectrum of the Ti/TiO2NTs/Fe-Ce-PbO2 electrode (P). The surface composition of Ti/TiO2NTs/Fe-Ce-PbO2 electrode (P) includes Ti, O, C, Pb, Ce and Fe elements, with peaks appearing at binding energies of 458.55 eV (Ti 2p), 529.16 eV (O1s), 284.79 eV (C1s), 137.57 eV (Pb 4f), 897.51 eV (Ce 3d) and 710.42 eV (Fe 2p), respectively. The carbon peak is attributed to the adventitious hydrocarbon from XPS instrument. The binding energies of 144.2 eV and 139.5 eV are assigned to the Pb 4 f 5/2 and Pb 4 f 7/2, respectively 196


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on the Ti/TNTs/Fe-Ce-PbO2 electrode (P) and on some other types of PbO2 electrodes reported in literatures [50–52]. The Ti/TNTs/Fe-CePbO2 electrode (P) had the higher OEP than Ti/TNTs/SnO2-Sb/PbO2, Ti/TNTs/SnO2-Sb/α-PbO2/β-PbO2 electrodes, and slightly lower than Large disk Ti/TNTs/PbO2.

Table 1 XPS analysis of Ti/TNTs/Fe-Ce-PbO2 electrodes (P). Element

C 1s

O 1s

Pb 4 f

Fe 2p

Ce 3d

Binding energy (eV) Content (atm. %)

284.79 33.38

529.16 41.3

137.57 19.69

710.42 2.52

897.51 1.95

3.3.3. Electrode stability The stability of Ti/TNTs/Fe-Ce-PbO2 electrodes (P) was tested by the accelerated service life experiments at a large current density of 1 A cm−2 in a 2 mol L−1 H2SO4 solution [22]. For comparison, Ti/ TNTs/PbO2 (P), Ti/TNTs/Fe-PbO2 (P) and Ti/TNTs/Fe-Ce-PbO2 (D) electrodes were also tested under the same conditions. As shown in Fig.3d, the service lifetime of Ti/TNTs/Fe-Ce-PbO2 electrode (P) was 222 h, which was longer than 155 h, 125 h and 107 h of Ti/TNTs/Fe-CePbO2 (D), Ti/TNTs/Fe-PbO2 (P) and Ti/TNTs/PbO2 electrodes, respectively. These results indicate that Ti/TNTs/Fe-Ce-PbO2 electrode (P) exhibit excellent stability. The enhancement of service life can be attributed to the pulse electrodeposition and the introduction of Fe and Ce into PbO2 film.

(Fig. 2e). The Fe 2 p spectrum in Fig. 2c displays two well-defined and symmetric peaks centered at 711.5 eV and 724.6 eV, corresponding to Fe 2 p 3/2 and Fe 2 p 1/2, which can be ascribed to the Fe3+ ions [46, 47]. Thus, the Fe element in the samples exists mainly in the +3 oxidation state (Fe3+). As shown in Fig. 2d, five peaks at 882.6, 888.0, 897.5, 906.8 and 916.1 eV were found for the Ti/TiO2NTs/Fe-Ce-PbO2 samples and were assigned to Ce 3 d 5/2 and Ce 3 d 3/2. All these verified that cerium existed as Ce+4 [31]. Table 1 presents the results of the average coating composition in terms of contents and binding energy of each species. All of the above results demonstrate that Fe and Ce were successfully introduced into the PbO2 electrode.

3.3.4. EIS of electrodes Fig. 3e shows the EIS Nyquist plots and their equivalent circuit for different electrodes in 0.2 mol L−1 Na2SO4 solution at the frequencies swept from 1 × 105 Hz to 0.1 Hz. It can be clearly observed that the arc diameter of Ti/TNTs/Fe-Ce-PbO2 (P) electrode is the smallest, revealing that the resistance of electrode film was decreased [22, 41]. The equivalent circuit shown in the inset of Fig. 3e was used to fit the EIS data and the calculated values were listed in Table 3. In the equivalent circuit, Rs, Rc, and Q2 represent the solution resistance, the faradaic charge transfer and the oxide/electrolyte interface, respectively. Q1 and R1 takes into account the properties of porous layer [53]. The Ti/TNTs/Fe-Ce-PbO2 electrodes (P) showed the smallest Rct, indicating that Ti/TNTs/Fe-Ce-PbO2 electrodes (P) possessed the larger specific surface area and the more electrochemically active sites on the electrode than the Ti/TNTs/Fe-Ce-PbO2 electrodes (D) [54]. The low charge transfer resistance was helpful for Ti/TNTs/Fe-Ce-PbO2 electrodes (P) electrode to degrade organics in lower energy.

3.3. Electrochemical measurements 3.3.1. CV of electrodes Fig. 3a shows the CVs of Ti/TNTs and different PbO2 electrodes in 0.2 mol L−1 Na2SO4 solution at a scan rate of 50 mV s−1. When Comparing the CVs of the five electrodes, the oxidation peak current of Ti/ TNTs/Fe-Ce-PbO2 electrode (P) was the highest. It also can be seen from Fig. 3a that the oxidation peak current of Ti/TNTs/Fe-Ce-PbO2 electrode (D) is a little higher than Ti/TNTs/Fe-PbO2 electrode (P), although Ti/TNTs/Fe-PbO2 electrode (P) has a smaller grain size (Fig. 1c).The results meant the co-doping of Fe and Ce into PbO2 played an important role in improving the electrocatalytic activity of electrode. In addition, it is worth noting that the oxidation peak current of Ti/ TNTs/Fe-Ce-PbO2 electrode (P) was larger than the Ti/TNTs/Fe-CePbO2 electrode (D). These results suggested that the electrochemical activity improvement of Ti/TNTs/Fe-Ce-PbO2 electrodes (P) was attributed to synergistic effect of the increase of electro-active surface area by the pulse electrodeposition and co-doping of Fe, Ce. The result can also be further confirmed in our following study. To evaluate the direct or indirect oxidation of the organic pollutant of Ti/TNTs/Fe-Ce-PbO2 electrode (P), the CV tests in a 0.1 mol L−1 Na2SO4 solution in the absence and in the presence of 30 mg L−1 MB were investigated. As shown in Fig. 3b, when 30 mg L−1 MB was added to the 0.1 mol L−1 Na2SO4 solution, no new oxidation or reduction peak appeared, which indicated that direct electron transfer did not occur on the surface of Ti/TNTs/Fe-Ce-PbO2 electrode (P) during electrochemical oxidation of the organic pollutant. Thus, it could be concluded that the degradation of MB may be achieved via indirect electrochemical oxidation mediated by ·OH radicals [48, 49].

3.3.5. Voltammetric charge and electrochemical activity analysis The electrocatalytic efficiency and activity of electrode were also related to its real surface area and the number of active sites, which can be reflect the by Voltammetric charge (q*). For the same electrode material, larger q∗ means higher electrochemical activity. The q* value could be calculated by the method reported in literature as follows [47]:

(q∗) = (q o∗)−1 + k υ−1/2

(3)

where qo* represents the values of outer charge, which is related the outer surface of the oxide coating exposed directly to the electrolyte; υ is the scan rate and k is a constant. Fig. 3f showed the relationship of q* versus the square root of scan rate (v) in 0.2 mol L−1 Na2SO4 solution. The q* was obtained by integration of the cycle voltammetric curves over the whole potential range from 0 to 2 V. The results indicated that the q* value of Ti/TNTs/Fe-Ce-PbO2 electrode (P) was the largest among all the prepared electrodes. The results indicated that Ti/TNTs/ Fe-Ce-PbO2 electrode (P) had the highest active surface area and best electrochemical porosity, which were in good agreement with the CVs results.

3.3.2. Steady-state polarization curves The steady-state polarization curves of different PbO2 electrodes were examined in 0.2 mol L−1 Na2SO4. The anodic oxygen evolution causes power loss in organic pollutant degradation process, which reduces the current efficiency of organic oxidation. Therefore, the oxygen evolution overpotential (OEP) is an important feature for the application of anodes, and a high OEP denotes high degradation efficiency due to a low opportunity of side reaction of oxygen formation. As shown in Fig. 3c, the Ti/TNTs/Fe-Ce-PbO2 electrode (P) had the highest OEP of 2.61 V, while the OEP of Ti/TNTs/PbO2 (P), Ti/TNTs/Fe-PbO2 (P) and Ti/TNTs/Fe-Ce-PbO2 electrodes (D) were about 2.25 V, 2.48 V and 2.55 V. Therefore, it is assumed that this embedded Ti/TNTs/Fe-CePbO2 electrode (P) would be beneficial for improving the efficiency of organic pollutant degradation during the electrocatalytic oxidation process. For comparison purpose, Table 2 gives the OEP values obtained

3.4. Electrochemical degradation of MB 3.4.1. Effect of electrode The effect of the removal efficiency on the different PbO2 electrodes during electrochemical oxidation of MB was investigated and the results were demonstrated in Fig. 4a–b. It can be seen that 99% MB 197


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Fig. 3. (a) Cyclic voltammograms of different electrodes in 0.2 mol L−1 Na2SO4 solution at a scan rate of 50 mV s−1, (b) Cyclic voltammograms of Ti/TNTs/Fe-CePbO2 (P) electrode in the presence and in the absence of 30 mg L−1 MB at 50 mV s−1, (c) Steady-state polarization curves of different electrodes in 0.2 mol L−1 Na2SO4 solution at a scan rate of 10 mV s−1, (d) Reusability of Ti/TNTs/PbO2(P), Ti/TNTs/Fe-PbO2(P), Ti/TNTs/Fe-Ce-PbO2 (D) and Ti/TNTs/Fe-Ce-PbO2 (P) electrodes for the accelerated service life experiments at a large current density of 1 A cm−2 in a 2 mol L−1 H2SO4 solution, (e) Electrochemical impedance spectroscopy of different electrodes, (f) Relationship of voltammetric charge quantity (q*) versus the square root of scan rate in 0.2 mol L−1 Na2SO4 solution. Table 2 OEP comparison for pollutants degradation on different PbO2 electrodes. Electrode

OPE

Ref.

Ti/TNTs/SnO2-Sb/PbO2 Ti/TNTs/SnO2-Sb/α-PbO2/β-PbO2 Large disk Ti/TNTs/PbO2 Ti/TNTs/Fe-Ce-PbO2(P)

2.0 V 1.7 V 2.7 V 2.61 V

[46] [23] [47, 48] This work

Table 3 The parameters obtained by simulation with the equivalent circuit model Rs(Q1(Rc(R1Q2))) of Ti/TNTs/Fe-Ce-PbO2 electrodes (P) and (D).

removal efficiency and 81% COD removal efficiency was achieved for Ti/TNTs/Fe-Ce-PbO2 electrodes (P) after 120 min, and only 90% and 60% for the Ti/TNTs/Fe-Ce-PbO2 electrodes (D). Meanwhile, according to the COD values, the variation of ICE with reaction time was also shown in Fig. 4c. It can be observed that the ICE decreases dramatically with time, and the ICE of Ti/TNTs/Fe-Ce-PbO2 electrodes (P) are always the higher than that of Ti/TNTs/Fe-Ce-PbO2 electrodes (D). The

Electrode

Rs/Ω

Rc/kΩ

R1/Ω

106 × Q1/ (n·Ω−1)

n1

106 × Q2/ (n·Ω−1)

n2

(D) (P)

11.93 12.16

10.78 5.60

192.8 586

8.30 14.40

0.934 0.863

8.93 2.52

0.135 0.071

higher COD removal efficiency and ICE of Ti/TNTs/Fe-Ce-PbO2 electrodes (P) can be ascribed to the larger surface areas and higher oxygen evolution overpotential than that of Ti/TNTs/Fe-Ce-PbO2 electrodes (D). We can conclude that the Ti/TNTs/Fe-Ce-PbO2 electrodes (P) are more suitable for decomposition of MB than Ti/TNTs/Fe-Ce-PbO2 electrodes (D). In Fig. 4d, it can be seen that the MB degradation of Ti/ TNTs/Fe-Ce-PbO2 electrodes is in good agreement with the pseudo198


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Fig. 4. Comparison of (a) COD and (b) MB removal efficiency of Ti/TNTs/Fe-Ce-PbO2 electrodes (D) and Ti/TNTs/Fe-Ce-PbO2 electrodes (P), (c) Comparison of ICE of Ti/TNTs/Fe-Ce-PbO2 electrodes (D) and Ti/TNTs/Fe-Ce-PbO2 electrodes (P), (d) Comparison of kinetics of Ti/TNTs/Fe-Ce-PbO2 electrodes (D) and Ti/TNTs/FeCe-PbO2 electrodes (P) Conditions: [temperature] = 25 °C; [pH] = 5; [current density] = 50 mA cm−2; [MB concentration] = 30 mg L−1; [Na2SO4] = 0.2 mol L−1.

Ti/TNTs/Fe-Ce-PbO2 electrodes (P) prepared in this work could degrade pollutants more effectively than other PbO2 electrodes under similar conditions.

Table 4 Parameters of pseudo-first order kinetics during electrochemical degradation of MB over Ti/TNTs/Fe-Ce-PbO2 electrodes (P) and (D). Condition: temperature = 25 °C; initial concentration of MB = 30 mg L−1; current density = 50 mA cm−2; initial pH = 5; supporting electrolyte (Na2SO4) concentration: 0.2 mol L−1. Electrode

K(min−1)

R2

(P) (D)

0.0135 0.00711

0.9678 0.9374

3.4.2. The UV–vis absorption spectra analysis As shown in Fig.5, the UV–vis spectras of MB solution in different degradation conditions by Ti/TNTs/Fe-Ce-PbO2 electrodes (P) were recorded, and the UV–vis absorption peaks of initial MB solution mainly consist of four peaks at 246 nm, 291 nm, 602 nm and 664 nm. The bands at 664 nm and 602 nm can be ascribed to the presence of chromophore and other two bands are attributed to the aromatic rings in the MB molecule. During the degradation process, the intensities of absorption peaks decrease with the increasing electrolysis time. When electrolysistime reached 120 min, these four peaks almost completely disappeared, implying that the effective mineralization oxidation of MB can be obtained on the Ti/TNTs/Fe-Ce-PbO2 electrode (P) [43].

first-order model. The parameters of the kinetics are showed in Table 4. The highest k value was obtained on the Ti/TNTs/Fe-Ce-PbO2 electrodes (P). It is also evident that COD decreases more rapidly on Ti/ TNTs/Fe-Ce-PbO2 electrodes (P). Table 5 compared COD removal efficiency of different PbO2 electrodes by MB degradation experiments reported in literatures [54, 55]. From Table 5, it could be seen that the

Table 5 COD removal efficiency comparison for pollutants degradation on different PbO2 electrodes. Electrode

COD removal (%)

Degrading conditions

Ref

Pt/MnO2

70

[50]

Ti/PbO2-ZrO2

72.7

Ti/TNTs/Co-PbO2

74

Ti/SnO2-Sb2O3/CeO2-PbO2

60

Ti/TNTs/Fe-Ce-PbO2(P)

81

120 min, pH = 6–8, 7 mA/cm, 0.05 mol/LNa2SO4,40 mg/L MB 120 min, pH = 3, 50 mA/cm, 0.2 mol/LNa2SO4,30 mg/L MB 20 min, pH = 3, 50 mA/cm, 0.2 mol/LNa2SO4,30 mg/L MB 90 min, 50 mA/cm, 0.2 mol/LNa2SO4,30 mg/L MB 120 min, pH = 5, 50 mA/cm, 0.2 mol/LNa2SO4,30 mg/L MB

199

[51] [53] [18] This work


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Fig. 7. The effect of current density on COD removal efficiency with time on Ti/ TNTs/Fe-Ce-PbO2 electrodes (P). Conditions: [temperature] = 25 °C; [pH] = 5; [MB concentration] = 30 mg L−1; [Na2SO4] = 0.2 mol L−1.

Fig. 5. The variation of MB absorption spectra with the reaction time.

3.4.3. Effect of initial MB concentration From a view of practical application, it was an important step to investigate the effect of initial concentration because the concentrations of actual industrial pollutants usually changed. The effects of initial MB concentration on the removal efficiency are described in this section (Fig. 6). It can be observed that with the initial concentrations of the dye increased from 10 mg L−1 to 50 mg L−1, the COD removal efficiencies decreased from 86 % to 71 % after 120 min. The above phenomena can be explained by the adsorption of MB and produced intermediates on the electrode surface, which prevented the contact between pollutants and the active sites [42]. 3.4.4. Effect of current density Current density plays an important role in the electrochemical reaction of pollutants degradation because of its influence on the capability of hydroxyl radical formation on the anode surface and the electron transfer rate from pollutants to the anode [56]. As shown in Fig. 7, we found that higher current density can lead to better COD removal efficiency. The reason was that a higher current density resulted in a higher generation rate of hydroxyl [57]. The COD removal efficiencies reached 84 % at 70 mA cm−2 after 120 min electrolysis. However, then only a slight increase occurred when the current density increased from 50 to 70 mA cm−2. Also considering the more obvious side reaction at the higher current density. Thus, the current density of 50 mA cm−2 is the optimum parameter.

Fig. 8. The effect of initial pH values on COD removal efficiency with time on Ti/TNTs/Fe-Ce-PbO2 electrodes (P). Conditions: [temperature] = 25 °C; [current density] = 50 mA cm−2; [MB concentration] = 30 mg L−1; [Na2SO4] =0.2 mol L−1.

3.4.5. Effect of initial pH values The initial pH is of crucial importance in the electro-oxidation process. The effect of initial pH values ranging from 3 to 11 on MB degradation is presented in Fig. 8. The results show that COD removal efficiencies decrease with increasing pH. When the pH value was 5, the best degradation rate of 81 % was achieved, indicating that the degradation of MB was more favorable in acidic solutions. According to the literature, electrolyte would be consumed excessively in alkaline environment, which it can lead to the reduced of conductivity of solution for the lack of electrolyte [58, 59].

3.5. Generation capacity of ·OH The ·OH radicals played a key role in the electrocatalytic degradation of organic pollutants in aqueous solution, and the amount of ·OH radical are closely related to the electrocatalytic activity [22]. Thus, the ·OH generation ability can also be used to evaluate the electrocatalytic activity of PbO2 electrode. To evaluate the ability for ·OH generated on the different modified electrodes, the concentration of ·OH generated on the different electrodes in the electrochemical degradation process are determined quantitatively (Fig. 9) [60]. The concentration of ·OH at the different PbO2 electrodes was measured by the dimethyl sulfoxide (DMSO) trapping and high-performance liquid chromatography (HPLC,

Fig. 6. The effect of MB concentration on COD removal efficiency with time on Ti/TNTs/Fe-Ce-PbO2 electrodes (P). Conditions: [temperature] = 25 °C; [pH] = 5; [current density] = 50 mg L−1; [Na2SO4] = 0.2 mol L−1. 200


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solution was analyzed after electrolysis using Ti/TNTs/Fe-Ce-PbO2 electrode. The dissolved concentration of Pb2+ was 0.0059 mg L−1, less than the drinking ordinance limits (0.01 mg L−1) [57]. It is well known that the main reasons to the leakage of Pb element of electrodes include detachment and dissolution of PbO2 films. However, the Ti/TNTs/FeCe-PbO2 composite electrode (P) electrode has an ordered multilayer structure with TNTs as substrate and Ce and Fe Co-doped PbO2 as catalyst layer. These special microstructures significantly enhance the service lifetime and improve its stability. Therefore, we could conclude that TNTs as substrate and Ce and Fe Co- doping could significantly inhibit detachment and anodic dissolution of the PbO2 coating and improve the safety of the PbO2 electrode. This study will be helpful for the application of PbO2 electrode in a reactor for treatment of dye real effluent. 4. Conclusion

Fig. 9. Concentration evolution of ·OH radicals on the different PbO2 electrodes after electrolysis for 0.5 h and 1 h in 0.2 mol L−1Na2SO4 solution.(a: Ti/TNTs/ PbO2 electrodes (P), b: Ti/TNTs/Fe-PbO2 electrodes (P), c: Ti/TNTs/Fe-CePbO2 electrodes (D), d: Ti/TNTs/Fe-Ce-PbO2 electrodes (P)).

This study reported the preparation of Ti/TNTs/PbO2 electrodes modified which were Fe and Ce by the pulse electrodeposition. The morphology, structure and stability tests show that Ti/TNTs/Fe-CePbO2 electrodes (P) possess larger surface area, smaller grain size and stronger stability than other three PbO2 electrodes. Electrochemical degradation of MB as a model organic pollutant on the Ti/TNTs/Fe-CePbO2 electrodes (P) was investigated to achieve desirable COD removal efficiency. The results show that 81 % COD removal efficiency could be achieved when employing 0.2 mol L−1 Na2SO4 solution containing 30 mg L−1 MB. The Ti/TNTs/Fe-Ce-PbO2 electrodes (P) show higher COD removal efficiency and ICE when degrading the MB. The experimental results demonstrate that the Ti/TNTs/Fe-Ce-PbO2 electrodes (P) possess the excellent electrocatalytic properties in refractory pollutants. Furthermore, the result of accelerated service life experiments indicates that Ti/TNTs/Fe-Ce-PbO2 electrodes (P) exhibit excellent stability, recommending this electrode as an alternative material for the decontamination of refractory pollutants. Acknowledgments This work was supported by the Natural Science Foundation of the Anhui Higher Education Institutions of China (No. KJ2017ZD37, No. KJ2016A670), the Natural Science Foundation of the Anhui of China (No. 1808085ME109) and the National Natural Science Foundation of China (No. 21176099).

Fig. 10. Electrochemical degradation of MB for 120 min using Ti/TNTs/Fe-CePbO2 electrodes (P) (10 cycles).

1200, Agilent Technologies, USA) according to reference. From Fig. 9, it was clearly seen that the concentration of total ·OH radicals formed at the Ti/TNTs/Fe-Ce-PbO2 electrode (P) was higher than those of other three electrodes. Hence, the electrochemical oxidation ability of Ti/ TNTs/Fe-Ce-PbO2 electrode (P) was superior to other three PbO2 electrodes. These phenomenons demonstrate that the Co-doping of Fe and Ce by pulse electrodeposition promoted the generation capacity of ·OH radicals of PbO2 electrode.

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3.6. Reusability and safety evaluation In order to further certify the excellent stability of Ti/TNTs/Fe-CePbO2 electrodes (P) and achieve industrial application in the future, the reusability of Ti/TNTs/Fe-Ce-PbO2 electrode (P) for MB degradation was investigated (Fig. 10) under the same reaction conditions. It was observed that the COD removal rate of MB exhibited a slight decrease after ten cycles, but the MB degradation efficiencies and COD removal rate of Ti/TNTs/Fe-Ce-PbO2 electrode (P) was still up to 98.6% and 79.8 %. It indicated that the Ti/TNTs/Fe-Ce-PbO2 electrode (P) has the relative superiority in stability and reusability. In addition, the electrode was intact and no any evident drop was found on the electrode surface. Meanwhile, the leakage of Pb element in the electrolysis process was an important drawback of PbO2 electrode, which would restrict its utilization in practical application [4]. So, the safety of 201


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