PbO2 and SnO2 are two promising anode materials for electrochemical oxidation. In order to highlight the difference between two kinds of electrodes in an electrochemical oxidation process, their morphology, structural, oxygen evolution overpotential (OEP), electrochemical activity and service life-time were compared in detail in this paper. Surface characterization by scanning electron microscope shows that the film of the PbO2 electrode is even, compact, non-porous, and non-cracked, while many cracks are present on the film of the SnO2 electrode. Electrochemical studies based on linear sweep voltammetry (LSV) and cyclic voltammetry (CV) prove that the OEP for the SnO2 electrode was much higher than that of the PbO2 electrode, and the electron-transfer kinetics and the reversibility of electrode reaction of the SnO2 electrode were superior to those of the PbO2 electrode. In electrochemical decomposition of p-nitrophenol, the degradation ratios at PbO2 and SnO2 anodes achieved 86.9% and 96.5%, respectively, after 120 min electrolysis, which verified the results of LSV and CV. The accelerated lifetime tests show that the service life time of the SnO2 electrode is far shorter than that of the PbO2 electrode, even though it was shown to be superior to the PbO2 in electrocatalytic activity.
Fresh water scarcity has been a severe problem in recent years. Moreover, an increase in water pollutant levels is directly threatening the safety of drinking water for humans and animals. Thus, extensive wastewater treatment technologies have been developed, among which electrochemical oxidation is a promising approach for the degradation of refractory organic pollutants in water due to its many distinctive advantages including strong oxidation performance, environmental compatibility, versatility, cost effectiveness and amenability to automation (Zhuo et al. 2011; Liu et al. 2012).
The degradation efficiency of organic pollutants in an electrochemical oxidation process depends on the activity of the anode materials (Zhao et al. 2014). Besides high activity, high chemical stability is one of the basic features for the optimal electrode. Typical anode materials include graphite (Chen et al. 2010), Pt (Torres et al. 2003), activated carbon fiber (Yi et al. 2008), boron-doped diamond (Nasr & Abdelatif 2009), PbO2 (Wang et al. 2010), SnO2 (Río et al. 2010), IrO2 (Tolba et al. 2010), RuO2 (Tran et al. 2009) and MnO2 (Lin et al. 2012). Among all these anode materials, PbO2 and SnO2 are two promising candidates in view of their effective degradation for organic pollutants, simple preparation process and low cost. In previous studies, the degradation applications of a variety of organic wastewaters were reported for these two electrodes, such as dye wastewater (Río et al. 2010), landfill leachate (Panizza & Martinez-Huitle 2013), carwash wastewater (Panizza & Cerisola 2010), tannery wastewater (Costa et al. 2008), and various toxic organics (Samet et al. 2010; Zhu et al. 2010; Hamza et al. 2011). Thus, in this paper, PbO2 and SnO2 electrodes were prepared and their electro-catalytic characterizations were analyzed and compared by scanning electron microscopy (SEM), X-ray diffraction (XRD), linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical degradation of p-nitrophenol (p-NP). Also, the service life of PbO2 and SnO2 electrodes was determined by accelerated life tests.
The PbO2 electrode was prepared by electro-deposition. The specific details of the preparation method can be found in Duan et al. (2013).
The SnO2 electrode was prepared by thermal deposition. Firstly, Ti substrates underwent a preparation of sandblasting, 10 min of ultrasonic cleaning in acetone, 10 min of ultrasonic cleaning in deionized water, 2 h of etching in boiling aqueous 15% oxalic acid, and rinsing with deionized water. Then, the pretreated Ti substrates were brushed with a solution containing 20 g SnCl4·5H2O and 2 g SbCl3 in 100 mL of isopropanol-HCl mixture, dried at 120 °C for 10 min, and baked at 500 °C for 10 min. This procedure was repeated 10 times. Finally, the Ti substrate was annealed at 500 °C for 1 h.
SEM was carried out on a Hitachi S-570 model instrument. XRD patterns of samples were obtained with an XRD (Rigaku D-max/3C) using Cu Kα radiation (45 kV, 30 mA). LSV and CV were executed in the PGSTAT302 electrochemical workstation. LSV and CV measurements were both carried out with a conventional three-electrode system. The fabricated PbO2 and SnO2 electrodes were used as the working electrode, a platinum sheet as the auxiliary electrode, and a saturated calomel electrode as the reference electrode.
The electrochemical degradation experiments of p-NP were carried out by batch processes and the apparatus mainly consisted of a direct-current power supply, a heat-gathering style magnetic stirrer and a glass reactor. The anode (PbO2 or SnO2 electrode) and cathode (stainless steel sheet) were positioned vertically and parallel to each other at a distance of 1 cm. The initial concentration of p-NP during all the experimental runs was 50 mg L–1 of a volume of 200 mL; 0.05 M Na2SO4 was used as the supporting electrolyte. The reaction temperature was kept at 30 °C during all the experimental runs. Electrochemical degradation was performed at a current density of 30 mA cm–2. During the experiments, samples were drawn from the reactor at certain intervals and then analyzed. The disappearance of p-NP during electrolysis was analyzed using a spectrophotometric technique (λ = 273 nm).
Stability tests for the electrodes were carried out in a three electrode cell in 2 M H2SO4 solution at 60 °C with a constant anodic current density of 1 A cm–2. The fabricated PbO2 or SnO2 electrodes were used as the anode, a stainless steel sheet as the cathode, and a saturated Ag/AgCl as the reference electrode. The anode potential was periodically monitored with time. The service lifetime of an electrode is defined as the operation time at which the anodic potential increased rapidly by 5 V (vs. Ag/AgCl).
RESULTS AND DISCUSSION
Surface morphology and phase structure
The SEM images of PbO2 and SnO2 electrodes are shown in Figure 1. The PbO2 electrode (Figure 1(a)) consists of a large number of typical pyramid-structure crystal particles. The film of the PbO2 electrode is even, compact, non-porous, and non-cracked. However, the SnO2 electrode presents a ‘mud crack’ morphology (Figure 1(b)), which was a specific feature of thermal deposition. In addition, there are many cracks on the film of the SnO2 electrode, which was a result of mechanism tension caused by the plasticity of the coating and the difference of the thermal expansion coefficient between the base metal and the coating (Comninellis & Vercesi 1991; Wang et al. 2010).
Figure 2 presents the XRD patterns of PbO2 and SnO2 electrodes. The XRD pattern of the PbO2 electrode presents diffraction peaks of pure β-PbO2. However, from the XRD pattern of the SnO2 electrode, two kinds of crystalline structures including SnO2 and Ti are observed. The presence of diffraction peaks of Ti demonstrates that the formed SnO2 film was still so thin that the X-ray went to the Ti substrate. In addition, it can be found that no diffraction peak of Sb2O3 was observed in the pattern of the SnO2 electrode because there was too little Sb dopant in the film to be detected by XRD analysis.
The oxygen evolution overpotential (OEP) of the PbO2 and SnO2 electrodes was measured by means of LSV in 0.5 M Na2SO4 solution at a scan rate of 50 mV s–1, and the potential corresponding to the inflection point of the polarization curve was defined as the OEP of the electrode. Figure 3 represents typical polarization curves for two kinds of electrodes. According to the polarization curves, the OEP for the PbO2 electrode is approximately 1.55 V (vs. SCE), while the OEP value on the SnO2 electrode is 1.85 V (vs. SCE), much higher than that of the PbO2 electrode. The higher OEP of the SnO2 electrode indicates that the SnO2 electrode should have better electrocatalytic activity and degradation performance for organic pollutants in the electro-catalytic oxidation process (Amadelli et al. 2002; Yao et al. 2014). However, this result is not in agreement with the previous reports for the OEP on Pb/PbO2 and Ti/SnO2 anodes, in which Pb/PbO2 material presented higher OEP than Ti/SnO2 material (Vazquez-Gomez et al. 2012). This conflict may be attributed to the difference in the preparation processes of the electrodes.
The CV measurements in 1.0 M KCl solution containing 5.0 mM K3Fe(CN)6 and 5.0 mM K4Fe(CN)6 were conducted to estimate the electrochemical activity of PbO2 and SnO2 electrodes. Figure 4(a) and 4(b) shows the CV curves recorded for the PbO2 and SnO2 electrodes, respectively, at scan rates of 20 mV s–1, 50 mV s–1, 80 mV s–1, 100 mV s–1, 130 mV s–1, and 150 mV s–1. There are a pair of distinct anodic and cathodic peaks observed for two kinds of electrodes corresponding to the redox reactions through a direct electron-transfer process between Fe(CN)64– and Fe(CN)63– at all scans. Comparing two sets of CV curves, it also can be found that sharper, larger and narrower redox peaks appeared on the CV curves of the SnO2 electrode. Besides, the peak potential separation (ΔEp) for the SnO2 electrode was significantly smaller than that of the PbO2 electrode. For example, at a scan rate of 100 mV s–1, the ΔEp values were 356 mV and 823 mV for the SnO2 and PbO2 electrodes, respectively. All these results suggest that the electron-transfer kinetics and the reversibility of the electrode reaction of the SnO2 electrode were superior to those of the PbO2 electrode.
CV measurements were also performed in 0.5 M Na2SO4 solution containing 200 mg L–1p-NP to investigate the direct electrochemical oxidation performance of the PbO2 and SnO2 electrodes. The cyclic voltammograms are shown in Figure 5(a) and 5(b). For comparison, the cyclic voltammograms in blank Na2SO4 solution are also shown in Figure 5(c) and 5(d).
From the curves of the PbO2 electrode (Figure 5(a) and 5(c)), it can be observed that there were three oxidation peaks in the anodic branches and one reduction peak in the cathodic branch with and without the presence of p-NP. Interestingly, the current response of the third oxidation peak (at about 1.2 V vs. SCE) with p-NP is much higher than that without p-NP in the first cycle, but in the subsequent cycles, if p-NP existed in the solution, this peak decreased to total disappearance, if not, it increased slowly and moved negatively. These differences indicate that the third oxidation peak in Figure 5(a) should be attributed to the oxidation reaction of p-NP on the surface of the PbO2 electrode, which may overlap the oxidation peak of formation of PbO2 in Figure 5(c). The gradual disappearance of peaks in Figure 5(a) with the increase in scan times means that a passivation film was formed on the surface of the PbO2 electrode during the oxidation of p-NP in Na2SO4 solution, which prevented all reactions further occurring on the surface of the electrode.
In the voltammogram of the SnO2 electrode in Na2SO4 solution containing p-NP (Figure 5(b)), an irreversible oxidation peak presented at 1.25 V vs. SCE in the first cycle. Compared with that in blank Na2SO4 solution (Figure 5(d)), it can be determined that this oxidation peak was ascribed to the oxidation of p-NP on the surface of the SnO2 electrode. It can also be observed that, in subsequent cycles, the oxidation peak decreased gradually during the successive cyclic voltammetric sweeps, but it still exists until the tenth cycle, which shows that passivation also occurred on the SnO2 electrode, but the rate of passivation was slower than that of the PbO2 electrode.
Degradation of p-NP on different electrodes
Electrochemical degradation experiments of p-NP at PbO2 and SnO2 anodes were performed by applying 30 mA cm–2 of current density. Figure 6(a) demonstrates the p-NP removal percentage variation by utilizing PbO2 and SnO2 as anodes during the electrochemical oxidation process. It can be observed that p-NP could be degraded on these two anodes and the p-NP removal percentages for both electrodes increased with the increase in reaction time. However, it is also worth noting that the degradation rates of p-NP varied according to the type of anode. At the SnO2 electrode, the removal percentage was 96.5% after 2 h. However, at the PbO2 anode, the corresponding removal percentage was 86.9%, a lower value than that obtained at the SnO2 electrode. The plots of ln(C0/C) versus time are shown in Figure 6(b). The linear relationships manifest the electrochemical oxidation of p-NP via a pseudo-first-order mechanism, with rate constants of 0.0174 min–1 at the PbO2 electrode and 0.0285 min–1 at the SnO2 electrode. Thus, we can conclude that the SnO2 electrode had a stronger oxidation performance for p-NP, which was in agreement with the above deduction in polarization curves.
Figure 7 compares the stability of the PbO2 and SnO2 electrodes by accelerated life tests. As seen in Figure 7, the PbO2 electrode had a service life of 56 h, which was 37 times longer than that of the SnO2 electrode. This is attributed to the different morphologies of PbO2 and SnO2 electrodes. The PbO2 electrode was prepared by electrodeposition, as shown in Figure 1, its film was compact, non-porous, and non-cracked, which not only reduced the penetration of electrolyte through cracks or pores (Yao et al. 2014), but also suppressed the formation of TiO2 on the Ti substrate and internal O2 evolution under the film. On the contrary, the electrolyte easily penetrated the cracks on the SnO2 electrode, leading to the breaking off of the coating.
In this work, the electro-catalytic activities and service lifetime of PbO2 and SnO2 electrodes were compared in detail for the electrochemical oxidation process. The main conclusions are as follows: the film of the PbO2 electrode is even, compact, non-porous, and non-cracked, while there were many cracks present on the film of the SnO2 electrode. LSV proves that the OEP for the SnO2 electrode is much higher than that of the PbO2 electrode. The electron-transfer kinetics and the reversibility of the electrode reaction of the SnO2 electrode were superior to those of the PbO2 electrode in CV measurements. The results of bulk electrolysis show that p-NP can be readily degraded at PbO2 and SnO2 anodes, and the decay kinetics of p-NP on both anodes follow a pseudo-first-order reaction. Although high over voltage anode material SnO2 was shown to be superior to the PbO2, a comparison of service lifetime of the two electrode materials showed that the PbO2 electrode had a service life of 56 h, which is 37 times longer than that of the SnO2 electrode.
This project was supported financially by the Young People Fund (20150520079JH) and Natural Science Fund (20140101215JC) of the Jilin Science and Technology Department, China. This work was also supported by the Development Program (2014055) of the Siping Technology Bureau, China.