Fabrication and Characterization of a PbO2-TiN Composite Electrode by Co-Deposition Method

An excellent PbO2 electrode with high electrocatalytic activity and stability was successfully fabricated by doping TiN particles through co-deposition method (marked as PbO2-TiN). The morphology (SEM), crystalline structure (XRD), chemical state (XPS), electrochemical performances (CV and EIS) and stability (accelerated life test) were characterized. The results showed that TiN doping could obviously improve the surface morphology of the electrode, increase the electrode current response and reduce the electrode impedance. During the electrochemical oxidation process, the PbO2-0.5TiN electrode showed the best performance on degradation of Acid Red G and Methylene blue (the highest removal efficiency, the lowest energy consumption and minimum Pb dissolution). The proportion of adsorbed hydroxyl oxygen for PbO2-0.5TiN electrode was 78.75%, which was higher than that for PbO2 electrode (68.95%). The accelerated service lifetime of PbO2-0.5TiN electrode was 302 h, which was more than 3 times longer than that of PbO2 electrode (96 h). Furthermore, a likely deactivation mechanism for lifetime enhancement of PbO2-0.5TiN electrode was proposed. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0261610jes] All rights reserved.

6][7] Anode plays an important role in the electrochemical oxidation process. 8Therefore, anodes with a high catalytic activity and long service life are desired. 9,10ome of the traditional DSA electrodes, such as Ti/RuO 2 and Ti/IrO 2 with good stability exhibit little weak electrochemical oxidation ability for organics due to the low oxygen evolution potential (OEP) and the cost are high due to the noble metals 5,11 .In addition, Ti/Sb-SnO 2 has been the research focus for its high oxygen evolution potential (OEP) and strong electrochemical oxidation ability.However, the short service life of the Ti/Sb-SnO 2 affects the degradation rate of pollutants. 12,13In contrast, Ti/PbO 2 is a more promising metal oxide electrode widely used in practice due to its low cost, ease of synthesis, high electrocatalytic activity and chemical stability 14,15 .However, the leakage of Pb element in the electrolysis process may lead to the secondary water pollution. 16Besides, the catalytic performance of the Ti/PbO 2 electrode still has room for improvement.Thus, several efforts have been devoted to further improve its performance in the electrochemical oxidation capability and stability, including inserting the middle layer, 17 doping with elements (Bi, 18 Co, 19 Fe, 20 F, 21 Ce, 22 Cu, 23 etc.), compounds (TiO 2 , 24 fluorine resin (FR), 25 etc.) or nanocomposites (ZrO 2 nano-particles, [26][27][28] Co 3 O 4 nano-particles, 29 WO 3 nano-particles, 30 CeO 2 nano-particles, 31 etc.).Based on the reports above mentioned, it can be concluded that an appropriate modification can effectively improve the performances of the electrode.
Titanium nitride (TiN) is a kind of novel functional materials possessing considerable excellent characteristics such as high chemical and thermal stability, good electric conductivity, corrosion resistance, wear resistance and low cost [32][33][34] .Therefore, the application of TiN to DSA electrodes modification may be promising and interesting.As far as we know, the research on the TiN modified SnO 2 electrode has been previously reported.Wu et al. 35 have reported that the Ti/Sb-SnO 2 electrode with a TiN interlayer could improve the oxygen evolution potential and service life of the electrodes.Duan et al. 36,37 have reported that TiN was applied as a dopant to modify the Ti/SnO 2 -Sb electrode by the pulse electro-codeposition method and dip coating method.The results confirmed that both the catalytic activity and stability of the electrode were significantly improved by TiN doping.However, the studies of TiN-doping on the PbO 2 electrode have not been reported at present.z E-mail: yanwei@xjtu.edu.cn;xuhao@xjtu.edu.cnHence, in this study, the PbO 2 layer of Ti/Sb-SnO 2 /PbO 2 electrode was doped with titanium nitride (TiN) particles by co-deposition method (marked as PbO 2 -TiN).The morphology, crystalline structure, chemical state, electrochemical performances and stability were characterized.Due to the many kinds of dyes in practice, Acid red G (ARG: anionic dye) and Methylene blue (MB: cationic dye) were both chosen as the model pollutant for electrochemical oxidation.Furthermore, a possible deactivation mechanism of PbO 2 -TiN was also analyzed to explain the reason for lifetime enhancement.The purpose of this work is to make the Ti/Sb-SnO 2 /PbO 2 electrode possess the excellent characteristics of titanium nitride (good electric conductivity and corrosion resistance), and the improved catalytic performance and stability.

Experimental
Materials and reagents.-Thechemicals used were of analytical grade and were all purchased from Sinopharm Chemical Reagent Xi'an Co., Ltd.All aqueous solutions were deionized water obtained from an EPED-40TF water purification laboratory system (Nanjing, China).High-purity (>99.6%)titanium plates (BaoTi Ltd., China) with a thickness of 0.5 mm were used as the substrate in this study.Titanium nitride particles (size 700 nm) were purchased from Beijing Dk Nano technology Co., LTD.
Electrode preparation.-Thepretreatment of titanium substrate was carried out as follows: Firstly, Ti plates with a dimension of 3 cm × 5 cm × 0.5 mm were mechanically polished, and then immersed in the solution composed of 1 mol L −1 NaOH and acetone (1:1 volume ratio) at 98 • C for 10 min to remove the organic residues on the surface.Secondly, the Ti plates were etched in oxalic acid (10wt%) at 98 • C for 120 min.Thirdly, the plates were rinsed with deionized water and dried for electroplating.
The inner Sb-SnO 2 layer was prepared by brush coating-thermal deposition method.Brush coating solution was a mixed liquid consisting of ethanol, isopropanol and n-butanol containing 1 mol • L −1 SnCl 4 , 0.1 mol • L −1 SbCl 3 and 0.001 mol • L −1 NaF (pH was adjusted to 2.0 by 37% hydrochloric acid).The Ti plates were brushed by precursor solution and heated in an oven at 120 • C for 15 min.Then taking the Ti plates into the muffle furnace (KLS-1100X, Hefei Kejing Co., Ltd., China) at 500 • C for 15 min.The procedure above mentioned was repeated 10 times.The last calcination process lasted 1 h at 500 • C.
Ti/Sb−SnO 2 /PbO 2 -TiN was prepared through anodic electrochemical co-deposition of PbO 2 -TiN on Ti/Sb−SnO 2 .The copper plate with the same size was used as the counter electrode.The surface deposition solution consisted of 0.5 mol • L −1 Pb(NO 3 ) 2 , 0.01 mol • L −1 NaF, 0.1 mol • L −1 Cu(NO 3 ) 2 and certain amounts of TiN particles(0 g • L −1 , 0.25 g • L −1 , 0.5 g • L −1 , 1 g • L −1 and 2 g • L −1 ; marked as PbO 2 , PbO 2 -0.25TiN,PbO 2 -0.5TiN,PbO 2 -1.0TiN and PbO 2 -2.0TiN, respectively).The deposition solution was adjusted to acidic (pH = 2.0) by concentrated HNO 3 .Ultimately, the mixed solution stirred (1500 rmp) for 1 h in the ultrasonic environment (59 KHz) to form a suspension of TiN.In order to maintain the stability of the suspension, the whole process of electrodeposition has been kept stirring at 1500 rmp.The deposition processes were carried out at 10 mA • cm −2 with a magnetic stirrer for 120 min (65 • C).Finally, the electrodes after deposition processes were rinsed with deionized water and dried in the oven.
Characterizations.-The morphology of the electrode was analyzed by scanning electron microscopy (SEM, JEOL, JSM-6390A) and energy-dispersive X-ray analyzer (EDX) was used for elemental analysis.The crystallization was characterized by X-ray diffraction (XRD, PAN alytical, Holland) using Cu K α radiation (λ = 0.15416 nm) and the scanning angle (2θ) ranges from 10 • to 80 • .The chemical states of the coatings were studied using the X-ray photoelectron spectroscopy (XPS) analysis performed on Axis Ultra, Kratos (UK; Al K α radiation, 150 W, 15 kV and 1486.6 eV).The binding energies were calibrated reference to the C 1s peak (284.8 eV).
Electrochemical experiments.-Theelectrochemical measurements were conducted in a standard three-electrode cell by an electrochemical workstation (CHI 660D, Chenhua, Shanghai, China).The copper sheet served as the counter electrode and Ag/AgCl (sat KCl) served as reference electrode.The oxygen evolution potential (OEP) and current response of the electrode were measured by cyclic voltammetry (CV, scan rate: 0.05 V • s −1 ) technique in 0.5 mol • L −1 Na 2 SO 4 solution.EIS measurements were carried out in 0.5 mol • L −1 Na 2 SO 4 solution at a measurement potential of 0 V.The frequencies swept from 10 5 Hz to 0.1 Hz and the sine wave amplitude was 5 mV.The measured data were analyzed using the ZSimpWin software.
The accelerated lifetime test was carried out to assess the stability of the electrode (35 • C ± 2 • C).A 3 mol • L −1 H 2 SO 4 solution was adopted as the electrolyte.An electrochemical workstation (LK3000A, Tianjin Lanlike, China) were used to provide a constant current density of 500 mA • cm −2 .The prepared electrode was served as a working electrode and the copper sheets with same area were served as a counter electrode.The working electrode was considered to be deactivated when the cell voltage of the test system reached 10 V.
Electrochemical oxidation process.-Theelectrochemical oxidation for dyes (ARG and MB) were performed in 200 mL simulated wastewater containing 100 mg • L −1 dye with the supporting electrolyte of 0.1 mol • L −1 Na 2 SO 4 .The electrode (Ti/Sb−SnO 2 /PbO 2 -TiN) was used as an anode and copper sheet as a cathode.The distance between the anode and cathode was set at 1.0 cm.The electrolysis process was performed in the galvanostatic condition of 15 mA • cm −2 (the working electrode area of 18 cm 2 ) with a magnetic stirrer for 120 min.The dye solution concentrations were measured by UV-Vis absorption (Agilent 8453, Agilent) at the characteristic wavelengths of 505 nm for ARG and 664 nm for MB.The UV 505/664 removal efficiency (RE) of dyes in electrochemical oxidation could be calculated as follows: Where A 0 and A t are the absorbance value in 505 nm or 664 nm of initial wastewater sample and electrolysis at the given times t, respectively In addition, COD (ET 125 SC, CSB/COD Reactor), instantaneous current efficiency (ICE) and energy consumption (E p , kW • h/g • COD) were also characterized to analyze the performance of dye degrada-tion.The ICE and E p could be calculated as follows: Where COD t 1 and COD t 2 are the chemical oxygen demands at times t 1 and t 2 (h), respectively, F is the Faraday constant (96487 C • mol −1 ), I is the current (A), U is the cell voltage (V) and V is the volume of the electrolyte (L).
The concentration of leached metal ion in the electrolyte during the process of accelerated lifetime test and electrochemical oxidation were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICPE-9000, Shimadzu, Japan).

Results and Discussion
Surface morphological analysis PbO 2 electrodes.-Fig. 1 shows the SEM images of the pure TiN particles and PbO 2 electrodes prepared with different TiN doping amounts.From Fig. 1a, it could be observed that the TiN particles were uniform, which could facilitate the uniform distribution of TiN particles in the PbO 2 coating.Fig. 1b displays the morphology of the surface layer of the PbO 2 electrode.It could be seen that the electrode surface was rough, porous and inhomogenous.Some typical pyramidal shapes with obvious boundaries were observed on the surface.In addition, there were some cracks on the PbO 2 electrode surface layer, which could cause electrolyte to penetrate into the coating and decrease the stability of the electrodes in the electrolysis.Figs.1c-1f show the microstructures of PbO 2 electrode doped with different TiN amounts from less to more.After TiN doping, the cracks on PbO 2 electrodes surface disappeared and we could see that the surface of PbO 2 -TiN electrodes were more compact and smoother.This result indicated that introducing TiN into the coating was favorable to decrease cracks on the coating surface as well as to enhance the stability.Besides, it could been seen that with the increase of doping amount of TiN, some particles appeared on the surface of the electrodes (arrow position of Figs.1e and 1f).After EDX test, these particles were proved to be TiN particles (Figs.1g  and 1h).This phenomenon indicated that the TiN doping amount may be excessive over 1.0 g • L −1 , which would damage the order of the electrode coating and degrade the electrochemical performance of the electrode.Further validation was carried as follows.
EDX spectrums and elemental mappings.-Fig. 2 shows the EDX spectrums and elemental mappings of PbO 2 electrodes with different TiN doping amounts from 0.25 g • L −1 to 2.0 g • L −1 (EDX spectrum of pure PbO 2 electrode is not shown in Fig. 2 and no Ti element is detected).From EDX spectrums, it could be seen that the peaks of O and Pb elements were observed obviously and no Sn and Sb elements could be found, indicating that the electrodes surface were covered completely by PbO 2 layer.Besides, with the increase of TiN doping amount, the content of Ti increased gradually from 0.9% to 2.96% (atom%).Moreover, from elemental mappings, it could be observed that the Ti element evenly distributed on the surface of the PbO 2 -TiN electrodes, suggesting that TiN particles were successfully and evenly doped into PbO 2 electrodes.Furthermore, it could be clearly seen that the density of the Ti element increased with the increase of TiN doping amount.  of main diffraction peaks did not change and all samples showed a series of diffraction peaks of β-PbO 2 .However, some individual peaks intensity of (110), ( 101), ( 200) and (211) were influenced by TiN doping.From Fig. 3b, it could be seen that with the increase of doping amount of TiN, the peaks intensity of (110), ( 101) and (211) were enhanced and the peak intensity of (200) was weakened.These results indicated that the growth trend of crystal face of PbO 2 could be affected by TiN doping.In addition, the diffraction peak of TiN could not been detected in the XRD patterns, which may result from the fact that the content of TiN was too low in the surface layer, or TiN was doped into the PbO 2 crystal lattice.Moreover, the peaks of SnO 2, Sb 2 O 3 and Ti were not detected, indicating the PbO 2 coating had a better coverage of the interlayer and substrate.

Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS)
.-Fig. 4 shows the cyclic voltammograms curves and electrochemical impedance spectroscopy (EIS) of PbO 2 -TiN electrodes.9][40][41][42][43][44] In addition, after TiN doping, the current response of PbO 2 -TiN electrodes was enhanced, indicating that the electrical conductivity of the electrode increased.The maximum current respond was obtained at PbO 2 -0.5TiN electrode.Xu et al. 45 have reported that the increase of current response in the section of oxygen evolution implied that the reaction of water splitting was enhanced and the oxygen evolution activity was promoted.Therefore, the introduction of TiN could reduce the charge transfer resistance, which was in favor of removing organics with lower energy consumption.Moreover, from the figure, it could be seen that the oxygen evolution potential (OEP) of the modified PbO 2 electrodes did not decrease after TiN doping and all reached at high OEP of 1.85 V (vs sat Ag/AgCl) for oxygen evolution.It was well known that an anode with a high OEP was desirable because of the inhibition of unwanted energy loss on O 2 generation, indicating that the PbO 2 -TiN electrodes were suitable for the degradation of organics 9,46 .
The Nyquist plots of freshly prepared PbO 2 -TiN electrodes in 0.5 mol • L −1 Na 2 SO 4 solutions are shown in Fig. 4b and the simulated data is shown in Table I.The EIS data of all tested electrodes fit the equivalent circuit R s (R f Q) very well (solution resistance (R s ), electrode film resistance (R f ) and the constant phase element (Q)). 47t could be evidently found that the arc diameter for PbO 2 -TiN was smaller than that for PbO 2 , which indicated that the TiN doping reduced electrode impedance and favored electron transfer.However, when the doping amount of TiN was more than 0.5 g • L −1 , the arc diameter increased again, indicating that the excessive TiN doping was not conducive to reduce the electrode impedance.As shown in Table I, the R f value of PbO 2 -0.5TiN was 46.72 • cm −2 , which was less than that of PbO 2 (102.1 • cm −2 ).These results suggested that the electrode impedance could be reduced effectively after the introduction of TiN.The PbO 2 -0.5TiN exhibited the minimum resistance, implying that a little low energy would be consumed for organics degradation.This result was consistent with the result of CV test.Moreover, from the results, it could be concluded that 0.5 g • L −1 is the most suitable TiN doping amount for increasing current response and reducing the charge transfer resistance.In addition, excessive TiN doping would be counterproductive due to the destruction of PbO 2 lattice order.
Electrochemical oxidation of dye.-In order to compare the electrochemical degradation activity of the PbO 2 -TiN electrodes, Acid red G (ARG: anionic dye) and Methylene blue (MB: cationic dye) were both chosen as the model pollutant.
Fig. 5 shows the performance of the PbO 2 -TiN electrodes for ARG and MB degradation.As shown in Figs.5a and 5b, we could see that the UV 505 and COD removal efficiency increased after introducing the TiN particles.The PbO 2 -0.5TiN electrode exhibited the best electrocatalytic ability for ARG degradation.The UV 505 and COD removal efficiency could reach to 97.7% and 68.8%, which was higher than that of PbO 2 electrode after 60 min electrochemical oxidation (72.4% and 50.6%).After the dynamic fitting, the degradation reaction followed pseudo-first-order kinetics at both UV 505 and COD removal, and the kinetic constants (k) were listed in the inset of Figs.5a and  5b.The maximum values of k were obtained at PbO 2 -0.5TiN electrode.The k values of UV 505 and COD removal reaction were 6.06 × 10 −2 • min −1 and 2.09 × 10 −2 • min −1 at PbO 2 -0.5TiN electrode, which were also higher than those of PbO 2 electrode (2.52 × 10 −2 • min −1 and 1.10 × 10 −2 • min −1 ).This result could be explained as that the PbO 2 -0.5TiN electrode possessed the maximum current response and minimum impedance, which was more conducive to the charge transfer and organics degradation.As shown in Figs.5c and 5d, the value of ICE decreased gradually and the value of E p increased gradually for all electrodes as a functional of time.It was because that the mass transfer process of the target pollutants from the bulk solution to the electrode surface was suppressed and lots of energy was consumed for the side reactions with the electrolysis.In addition, compared with other PbO 2 electrodes in the experiment, PbO 2 -0.5TiN electrode presented the highest ICE and lowest E p for ARG degradation.After 60 min electrolysis, the value of ICE and E p were 34.2% and 0.041 kW • h/g • COD, respectively.However, for PbO 2 electrode, the value of ICE (24.8%) and E p (0.057 kW • h/g • COD) were poorer.Therefore, it could be concluded that PbO 2 -0.5TiN electrode was considered as the best anode because of the high-efficiency, higher ICE and lower E p for ARG removal in this study.
From Figs. 5e and 5f, it could be clearly found that the TiN doping of PbO 2 electrodes also had a positive influence on the decolorization and COD removal for MB.The PbO 2 -0.5TiN electrode exhibited the best performance for MB degradation.The UV 664 and COD removal   As is known to all, ARG is a common anionic dye, while MB is a cationic dye.Due to the positive charge of cationic dye, it is difficult to diffuse to the anode region, leading to a poor performance for MB probably.However, from Fig. 5, it could be concluded that the PbO 2 -0.5TiN electrode had a good catalytic oxidation ability for both ARG degradation and MB degradation.Besides, as for MB degradation, the UV 664 and COD removal efficiency could still reach to 90.0% and 79.8% after 120 min electrochemical oxidation.

Table I. Simulated data of each parameter (EIS).
Therefore, to further analyzed the reason for catalytic oxidation ability enhancement of PbO 2 -0.5TiN electrode, the XPS measurement was also conducted for PbO 2 electrode and PbO 2 -0.5TiN electrode (Fig. 6) and XPS data of chemical states of O on the electrode surface is shown in Table II.Fig. 6a exhibits the survey spectra of PbO 2 electrode and PbO 2 -0.5TiN electrode, from which it could be clearly observed that the peaks of Pb and O existed in sample PbO 2 electrode.In addition, the Ti and N were also detected in the PbO 2 -0.5TiN electrode surface, indicating the TiN was successfully doped into the PbO 2 electrode.Figs.6b and 6c show the core level Ti2p and N1s spectrum for PbO 2 -0.5TiN electrode.The binding energies of Ti2p and N1s peaks at 457.6 eV and 397.2 eV was in consistent with TiN, which also proved the existence of TiN particles.Figs.6d and 6e show the core level O1s spectra of PbO 2 electrode and PbO 2 -0.5TiN electrode.As two O1s peaks shown in Fig. 6d, the peak at the lower binding energy (around 529 eV) represented lattice oxygen (O L ), while the peak at higher binding energy (around 531 eV) represented adsorbed hydroxyl oxygen (O ad ). 37Besides, from Fig. 6e, the adsorbed water also presented simultaneously in the PbO 2 -0.5TiN electrode. 48Furthermore, from Table II, we could see that the proportion (ε) of O ad for PbO 2 -0.5TiN electrode was 78.75%, which was higher than that for PbO 2 electrode (68.95%).After TiN doping, the atom ratio η (O ad /O L ) increased from 2.22 to 3.69.As is well known, the O ad is related to active oxygen species (HO • ) which could improve the electrocatalytic efficiency for organics. 49Therefore, the PbO 2 -0.5TiN electrode with higher proportion of O ad implied a stronger electrochemical oxidation capacity for organics, which could also account for the best performance for dye in this study.
Safety evaluation of PbO 2 -TiN electrodes.-ForPbO 2 electrodes, the leakage of Pb element etc. may exist in the electrolysis process which could cause the secondary pollution.Therefore, the leaching of metals elements were detected by inductively coupled plasma atomic emission spectroscopy (ICP-AES).Table III shows the concentration of Pb, Sn, Sb and Ti elements released from the electrodes after 120 min electrolysis in 0.1 mol • L −1 Na 2 SO 4 solution without dye (the other conditions were the same as that of dye degradation).It was found that the Sn (0.021 mg • L −1 ), Sb (0.007 mg • L −1 ) and Pb (0.023 mg • L −1 ) were detected at PbO 2 electrode and the concentration of toxic Pb element had exceeded the drinking water standard (GB5749-2006, Pb<0.01 mg • L −1 ).After TiN doping, the amount of leakage of Pb decreased significantly and was below the drinking water ordinance limits.The lowest concentration of Pb (0.002 mg • L −1 ) was obtained at PbO 2 -0.5TiN electrode.In addition, Ti element was detected at PbO 2 -TiN electrodes, implying that some TiN particles may release from the PbO 2 -TiN coating.However, the Ti element was not toxic and the concentration was low, which did not affect water quality deeply.

Electrode stability and deactivation mechanism.-Accelerate life
tests.-The electrochemical stability is an important factor for the electrode quality.In order to evaluate the stability of the PbO 2 -0.5TiN electrode with the best electrocatalytic activity, the accelerate life tests of PbO 2 electrode and PbO 2 -0.5TiN electrode were carried out (Fig. 7).The average amount of PbO 2 and PbO 2 -0.5TiN coating on the electrodes surface was around 84.7 mg • cm −2 and 93.7 mg • cm −2 .As shown in Fig. 7, it could be evidently observed that the TiN doping could obviously extend the service lifetime of PbO 2 electrode.The accelerated service lifetime for PbO 2 -0.5TiN electrode was 302 h, which was more than 3 times longer than PbO 2 electrode (96 h).
SEM images and EDX spectrum of the deactivated PbO 2 -TiN electrodes.-Fig.8 shows the SEM images and EDX spectrum of the deactivated PbO 2 electrode and PbO 2 -0.5TiN electrode.From Fig. 8a, it could be seen that the PbO 2 coating still existed, however, the morphology was completely different compared with the fresh PbO 2 electrode.The PbO 2 coating became loose and the crystals of typical pyramidal shape were disappeared.In addition, lots of cracks appeared on the surface, which could cause the rapid infiltration of electrolyte.When the electrolyte reached to the Ti substrate, the passivation layer (TiO x ) would be formed, which eventually lead to the electrode deactivation. 50As shown in Fig. 8c, a large number of Pb and O elements were detected.The atom ratios of O and Pb on the deactivated PbO 2 electrode surface were 33.35% and 49.10%, respectively.This result indicated that most of the PbO 2 surface coating still existed, although the electrode was inactive.From Fig. 8b, it could be seen that most of the Ti substrate had been exposed and only a small amount of scattered and lamellar coating still existed.According to Fig. 8d, it was observed that the O, Pb, S, Sn, Sb and Ti elements were detected.The atom ratios of O, Pb, S, Sn, Sb and Ti on the deactivated PbO 2 -0.5TiN electrode surface were 47.68%, 3.51%, 6.68%, 2.90%, 0.35% and 38.88%, respectively.This result indicated that most of the surface PbO 2 coating and Sb-SnO 2 interlayer had been consumed when the electrode was deactivated.It was known that the time of coating consumption was much longer than that of Ti substrate deactivation due to the direct infiltration of the electrolyte.Therefore, this result may explain the reason for the enhancement of the longer service life of the PbO 2 -0.5TiN electrode.
Monitoring of leached Ti and Pb during the accelerated lifetime test process.-Fig.9 shows the concentration of leached Ti and Pb during the accelerated lifetime test process by ICP-AES.For PbO 2 electrode, it could be found that the concentration of Ti experienced a relatively stable stage after 63 h of electrolysis and then increased sharply.And for Pb element, the concentration was relatively stable during the first 16 h and then increased at roughly the same speed.When the electrolysis process continued to about 60 h, the concentration of Pb increased sharply.This phenomenon could be explained by  that when electrolyte had penetrated into the coating along the cracks, Pb element could be dissolved rapidly through both surface corrosion and internal corrosion of coating.When the electrolyte furtherly reach to the Ti substrate, it would cause the rapid dissolution of Ti and Pb element on the PbO 2 electrode.As for PbO 2 -0.5TiN electrode, the concentration of Ti had experienced a slow increase before 274 h and then increased sharply.While for Pb element, the trend was similar to that of Ti element.This result implied that in the process of Pb element dissolution, the dissolution or fall off of TiN particles also occurred.In addition, for PbO 2 -0.5TiN electrode, the concentration of Pb (0.61 mg • L −1 ) was far lower than that of PbO 2 electrode (3.31 mg • L −1 ) when the electrolysis process reached to about 60 h, which was also certifying the stability of PbO 2 -0.5TiN electrode was higher than that of the undoped one (PbO 2 electrode).Meanwhile, we also found that the concentration of Ti and Pb were both higher than that of PbO 2 electrode when the electrode was deactivated, which was consistent with the results of the 3.6.2chapter.

Concentration (mg
Proposed deactivation mechanism.-Afterthe analysis of the above results, the proposed deactivation mechanisms of PbO 2 electrode (left) and PbO 2 -0.5TiN electrode (right) are shown in Fig. 10.For the PbO 2 electrode, electrolyte or other active substances would infiltrate into the crack of the electrode coating easily, causing more cracks appear on the coating and accelerating the dissolution of Pb element.When the electrolyte touched the Ti substrate through a penetrating way, the passivation layer (TiO x ) would be formed between Ti substrate and Sb-SnO 2 interlayer.After the further passivation, the cell voltage increased sharply and caused the electrode deactivation. 51wever, in terms of the novel PbO 2 -0.5TiN electrode, electrolyte was difficult to invade into the interior of the coating by slowly dissolving coating way because of the less cracks on electrode surface and the good corrosion resistance and wear resistance of TiN.The PbO 2 coating mixed with 0.5 g • L −1 TiN particles was similar to the concrete, which showed a strong ability to resist corrosion of electrolyte.Therefore, the coating dissolution was a much slower process than the direct electrolyte penetration.In addition, according to the above results and related references, 36,37 the introduction of TiN could improve the conductivity of the electrode and decrease cell voltage, which could also slow down the corrosion rate of coating.Furthermore, the average surface loading amount of the PbO 2 -0.5TiN electrode was 93.7 mg • cm −2 , which was higher than that of the PbO 2 electrode (84.7 mg • cm −2 ).In general, detachment, dissolution and passivation of substrate could cause the electrode deactivation. 52Because the phenomenon of detachment could not be observed in this study, dissolution and passivation were the main reasons for the electrode deactivation.Therefore, the higher surface loading amount would lead to a longer accelerated lifetime.Taken together, the results are clearly of the longer accelerated service lifetime for PbO 2 -0.5TiN than that for PbO 2 electrode.

Conclusions
A novel PbO 2 electrode doped with TiN particles was successfully fabricated through co-deposition method.TiN doping could obviously reduce the cracks on the surface coating of electrode, increase the current response of the electrode and reduce the electrode impedance.The electrochemical oxidation of dyes (Acid Red G and Methylene blue) both followed pseudo-first-order kinetics and the PbO 2 -0.5TiN electrode showed the best degradation performance (the highest removal efficiency, the lowest energy consumption and the minimum Pb dissolution).In addition, the proportion of adsorbed hydroxyl oxygen (O ad ) for PbO 2 -0.5TiN electrode was 78.75%, which was higher than that for PbO 2 electrode (68.95%).This result indicated that the PbO 2 -0.5TiN electrode with higher proportion of O ad would exhibit a stronger electrochemical oxidation capacity for organics.The accelerated service lifetime of PbO 2 -0.5TiN electrode was 302 h, which  was more than 3 times longer than that of PbO 2 electrode (96 h).The mechanism of stability enhancement could be ascribed to that the TiN with good corrosion resistance and electrical conductivity could effectively inhibit the coating erosion rate by electrolyte and decreased cell voltage.When the PbO 2 -0.5TiN coating with concrete structure was exhausted, the electrode would be deactivated.

Figure 5 .
Figure 5. Performances of the PbO 2 -TiN electrodes of ARG and MB degradation.ARG: (a) UV 505 removal efficiency and (b) COD removal efficiency, (c) instantaneous current efficiency and (d) energy consumption; MB: (e) UV 664 removal efficiency (f) COD removal efficiency, (g) instantaneous current efficiency and (h) energy consumption.

Table II . XPS data of chemical states of O element on the electrode surface.
In addition, the decolorization and COD removal at different electrodes were also in good agreement with the pseudo-first-order reaction model.The kinetic constants (k) were shown in the inset of Figs.5e and 5f.The maximum values of k were obtained at PbO 2 -0.5TiN electrode.The k values of UV 664 and COD removal reaction were 1.65 × 10 −2 • min −1 and 1.32 × 10 −2 • min −1 at PbO 2 -0.5TiN electrode, which were also higher than that of PbO 2 electrode (1.00 × 10 −2 • min −1 and 0.75 × 10 −2 • min −1 ).As shown in Figs.5g and 5h, it could be seen that the trends of ICE and E p were the same as that of ARG degradation process with time.PbO 2 -0.5TiN electrode also presented the highest ICE and lowest E p for MB degradation.After 60 min electrolysis, the values of ICE were 24.7% and 16.3%, corresponding to PbO 2 -0.5TiN and PbO 2 electrode.And the E p values were 0.064 kW • h/g • COD and 0.085 kW • h/g • COD.Therefore, it could be concluded that PbO 2 -0.5TiN electrode also exhibited the best performance for MB degradation.