Ti/SnO2-Sb/Rare Earth Electrodes Containing Different Contents of Ni Intermediate Layer for Efficient Electrochemical Decolorization of Rhodamine B

Water contamination by dyes discharged from many industries is an environmental issue of great matter. Electrochemical oxidation is an advanced approach for wastewater treatment. In this study, the composite electrodes of Ti/SnO2-Sb-Ni/rare earth have beenmodified using rare earth elements (Re) Gd, Ce, Eu, and Er and various molar ratios of tin and nickel intermediate layer, and their electrochemical oxidation effects were scrutinized. To analyze the decolorization performance of the electrodes, Rhodamine B (RhB) dye was utilized as a target pollutant. Accelerated life testing indicated that the longer service life could be observed in Ni (3.5%)/Re and Ni (5%)/ Re electrodes compared with other modified Ni (0%, 1%, and 2%)/Re electrodes. Compared with the color removal efficiencies of the Ni (2%)/Re electrodes, the decolorization rate of 90% after treatment for 60min and the low energy consumption of 3.621 kWh·m can be achieved at the Ni (2%)/Gd electrode under the experimental condition of 100mg·L RhB. 'e best decolorization rate was observed at the Ni (2%)/Re electrodes among other Ni and no adding Ni-doped Re electrodes. 'e characterization of the electrodes was described, consisting of surface morphology, oxygen evolution potential, and a crystallographic and elemental combination of the coatings.


Introduction
Pollution of water sources by wastewater from synthetic dye and textile industries is the main cause of water pollution since their released wastewater could contain continual organic dyes and harmful by-products. Leftover coloring matters are distinguished by intense color, high organic content, and steady chemical composition due to the existence of azo functional groups. erefore, they have influenced serious hazards for the environment and human beings [1,2]. ese environmental impact dyes are degraded by various treatment techniques such as biological degradation system [3], adsorption [4], coagulation-flocculation [5], Fenton technique [6], membrane separation [7], photocatalysis [8], son-catalysis, and ozonation [9,10]. e pollutants oxidized by the electrochemical processes have also been accepted as an impressive method for decolorizing wastewaters and to degrade dyes because of their advantages: safety and ease to operate, wide scope of treatment conditions, and ecological similarity [11][12][13][14][15][16]. e hydroxyl radicals (֯OH) are generated when the electrochemical oxidation transforms several pollutants into carbon dioxide, water, and some inorganic anions [17][18][19]. Four requirements have been needed to possess an excellent modified electrode: remarkable electrocatalytic activity, high oxygen evolution potential, excellent service life, and good conductivity [20]. Titanium-based SnO 2 -Sb is considered a suitable electrode material due to its higher OEP and superior electrochemical performance. However, the poor adhesion coating layer, easy detachment, short stability, and not high degradation achievement of the SnO2-Sb electrodes obstruct their widespread applications [21][22][23][24]. e insertion intermediate layer between an active layer and the Ti base, Ti/TiH x /Sb-Sn [25], Ti/Cu-NRs/Sb-Sn [26], Ti/TiO x H y / Sb−Sn [27] Ti/IrO 2 /Sb-Sn [28], Ti/Pt/Sb-Sn [29], Ti/TiO 2 -NT/Sn-Sb [30], significantly improves the performance of SnO 2 -Sb electrodes. In addition, Mn is also applied as interlayers of Ti-based SnO 2 electrode because of not only cost-effective preparation but also uncomplicated preparation processes. Introducing the rare earth Gd [31], Eu, Dy [32,33], Er [34] into the outer layer of the electrodes as dopants can also greatly affect the degradation efficiency.
In the present work, the SnO 2 -Sb active layer is doped with rare earth elements (Gd, Ce, Er, and Eu), and Ni intermediate layers and no intermediate layer were prepared by brush-coating thermal pyrolysis procedure. e purpose of this work is to focus on the effects of the different intermediate layer contents on electrode electrocatalytic performance and compare them using a different initial dye concentration of RhB and stability to choose the most excellent modified electrode materials. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and linear sweep voltammetry (LSV) were performed to characterize the modified electrodes.

Preparation of Electrodes.
Titanium substrates (20 mm × 50 mm × 0.5 mm, high purity, Bao-Ti Co. Ltd, China) were refined with abrasive papers and then ultrasonically degreased with 1 : 1 volume by volume mixture solution of 1.0 mol·L −1 NaOH and acetone in an ultrasonic bath for 30 min, followed by boiling in a 10% (wt.%) oxalic acid at 98°C for 120 min. e intermediate coating solution was prepared by adding a proper quantity of hydrochloric acid including 1.0 mol·L −1 SnCl 4 ·5H 2 O (Sigma Aldrich), SbCl 3 0.3 mol·L −1 (99.5%, Merck), and x mol·L −1 NiCl 2 ·6H 2 O (98%, Merck) (x � 0.01, 0.02, 0.035 and 0.05) into n-butanol. e prepared solution for the active layer was consisting of SnCl 4 .5H 2 O and SbCl 3 and suitable rare earth (Sn/Sb/Re of 100 : 3:2, molar ratio) which were added insolvent that was used the same as above. e coating layer solution for the intermediate layer was uniformly covered on the titanium plates by using a soft brush. After brushing the coating, the solution on the Ti sheet was evaporated at the oven at 120°C for 10 min. e procedure of brush and dry was done again three times, and then, thermal oxidation of the coating layer was formed at about 400°C for 10 min by a muffle furnace. is process was replicated 8 times. e last thermal decomposition process was operated at about 500°C for 60 min. Lastly, identical procedures were employed to overlay and thermally oxidized to develop the outer layer.

Characterization of Electrodes.
e microstructure of the electrodes was characterized by a Bruker D8 ADVANCE X-ray diffractometer (Germany) with Cu-Kα radiation with an operating voltage of 40 kV and a current of 40 mA. e micromorphology and the microanalysis of the content of the element were observed using S-4700 field emission SEM (Hitachi Industrial Equipment Systems Co., Ltd, Japan). e oxygen evolution potential measurements of electrodes were investigated in a conventional three-electrode cell system at the electrochemical workstation (Chenhua Instrument Shanghai Co. Ltd. China). e OEP of the electrodes was tested at 10 mV·s −1 in 0.1 mol·L −1 Na 2 SO 4 solution at room temperature. e as-prepared electrode (10 × 10 mm 2 ) was served as a working electrode. A platinum plate and Ag/ AgCl electrode were served as a counter electrode and a reference electrode, respectively. To decrease the observational duration, the accelerated service lifetime test was conducted at a stable current density of 1000 mA·cm −2 in a 1.0 mol·L −1 sulfuric acid solution at 40°C. e modified electrodes served as the anode (10 × 10 mm 2 ) and a Ti sheet was employed as the cathode by spacing the electrodes adjusted to roundabout 10 mm. e cell potential is simultaneously noted constant time interval for evaluating electrode inactivation when the value reached 10.0 V.

Electrochemical Decolorization of Rhodamine B.
In each electrochemical decolorization experiment, 200 ml dye solution with 0.1 mol·L −1 Na 2 SO 4 as an electrolytic solution was used in a two-electrode system using 250 ml capacity of a cylinder cell. e initial dye concentration (20,40, and 100 mg L −1 ) was investigated to gauge the efficiency of the electrodes because actual wastewater involves various concentrations of RhB. e modified electrodes with an effective area (20 × 20 mm 2 ) and Ti plate were employed as an anode and cathode which were separated at approximately 20 mm of the interelectrode distance space, and a magnetic bar was used to mix thoroughly the dye solution with a steady rotation rate during the electrocatalytic process. e applied current was fitted at a consistent current intensity of 10 mA·cm −2 using an SS-605 switching direct current power supply. e tested samples were eliminated from the experimental system at fixed meantime examined during the dye decolorization processes. e absorbed radiation of the tested sample was measured spectrophotometrically on the model 2200 UV-visible spectrophotometer (Ocean Optics, Dunedin, FL) at the maximal absorption wavelength (554 nm). e decolorization efficiency (η) was evaluated by the following equation: where C 0 and C t were before degradation and after degradation time t of the concentration of RhB, respectively.  [35,36]. e coating morphology of the Ti/SnO 2 -Sb-Ni (2%)/Gd electrode is composed of smaller cracks, and a smoother coating surface is obtained (Figure 1(c)). e crack width of the Ti/SnO 2 -Sb-Ni (0%)/Gd electrode (Figure 1           e peak intensities of the SnO 2 were increased when the contents of Ni increased from 1 to 5. In any case, the decolorization performance was decreased with including Ni amount of 3.5 to 5%; as a result, these intensities increased.

Results and Discussion
us, the inclusion of Ni 2% for all prepared electrodes is ideal for improving the decolorization activities of all as-prepared electrodes. No characteristic peaks of TiO 2 were observed, so it strongly proposed that the Ti support was not oxidized during the electrode preparation. e characteristic peaks of Sb, Ni, and Re elements were not observed in Figures 5(a)-5(d), indicating that the Sb, Ni, and Re phases were mutually entered into SnO 2 unit cells at high temperatures [37,38]. e presence of Ni, Sn, Sb, and rare earth (Gd, Ce, Eu, and Er) could be acquired by EDS (Table 1).
All of the abovementioned results reported that the modified Ni intermediate layer-based rare earth electrodes significantly improve the performance of electrochemical decolorization of RhB. In the cases of organic substances oxidation, hydroxyl radicals (֯OH) which were initiated from H 2 O oxidation (equation (2)) act as a mediator. e ֯OH can either accelerate the oxidation of the organic pollutants (equation (3)).
R + M(°OH) n ⟶ M + Oxidation products such as CO 2 + nH + + ne − In other reports, the color removing kinetics effort on each electrode was observed to be related properly to the pseudo-first-order [40][41][42]. e dye concentration decreased exponentially with time and the kinetic rate can be assessed by the following mathematical formula [43]: where C 0 and C t are the dye concentration (mg·L −1 ) at the initial condition and at experimental time t, respectively. e k app (min −1 ) is the detected rate constant. e reduction in energy demands is one of the curial potions in ascertaining the achievability of the electrocatalytic destruction of organic contaminants. e magnitude of electrical energy (kWh) needed to decrease the concentration of contaminant in a unit volume (1000 dm 3 ) by one order of magnitude was defined as electric energy per order (E EO ) due to the decolorization kinetics of RhB obeyed pseudo-first-order. e values of E EO (kW h m −3 order −1 ) can be evaluated from the following equation [43]: where P is the electrical power (kW), V is the volume of dye solution (m 3 ), and k 1 ′ is the pseudo-first-order rate constant. Table 11 presents the variation of the energy per order for the electrochemical decolorization on Ti/SnO 2 -Sb-Ni/Re electrodes for various Ni contents and different rare earth elements at a dye concentration of 20 mg·L −1 . According to these results, the value of E EO is 6.8 times lower for the Ni (2%)/Gd electrode than for the Ni (0%)/Gd electrode, reduced to 2.3 times lower for the Ni (2%)/Ce than for the Ni (0%)/Ce, decreased 3.3 times lower for the Ni (2%)/Eu than for the Ni (0%)/Eu, and down to 2.4 times lower for the Ni (2%)/Er than for the Ni (0%)/Er. e resultant values of E EO were observed in Table 12 which is the decolorization of 40 mg·L −1 initial concentration. It can be observed that the value of E EO decreased at the Ni (2%)/Gd electrode. is value is 4.3 times lower than that of the Ni (0%)/Gd electrode applied. e value of E EO for the Ni (2%)/Ce electrode is 3.8 times lower than that of the Ni (0%)/Ce electrode, reduced to 4 times lower for the Ni (2%)/Eu than for the Ni (0%)/Eu, and declined 3.1 times lower for the Ni (2%)/Er than for the Ni (0%)/Er. Table 13 shows the variation of the energy per order on Ti/SnO 2 -Sb-Ni/Re electrodes with Ni (2%) at 100 mg L −1 dye concentration. e value of E EO for the Gd electrode is 1.7 times, 1.3 times, and 1.4 times lower than that of the Ce, Eu, and Er electrodes, respectively. e E EO values presented here demonstrate that the use of Ni (2%) intermediate layer was energy savings against other Ni content modified electrodes.

Electrode Stability Tests.
Electrode strength is a prominent aspect that concludes whether Ti-and Sn-based electrodes are to be exploited in the industrial range. e service life is acutely dominated by the experimental circumstances: temperature, current density, and electrolyte concentration.
e real service lifetime of electrodes, because of utilization at moderate current density operation, is comparatively lengthy, which is inconvenient to be evaluated. Since Correa-Lozano et al. have introduced the experimental correlation between the actual lifetime (t 1 ) and the accelerated lifetime (t 2 ), this correlation was employed to estimate the actual service time of Ti-and Sn-based anodes at various current densities [44], as displayed in where i 1 is the current density (10 mA·cm −2 ) for the actual condition. And i 2 is the current density for the accelerated test [26].
In this work, the test conditions for accelerated duration are comparatively high, with a current density (1000 mA·cm −2 ), concentrated sulfuric acid solution (1.0 mol·L −1 ), and temperature (40°C). us, the electrode accelerated service time acquired during this condition is comparatively short. Figures 16(a)  of the electrode prohibits the outer layer from fall down, in that way extending the lifetime of the electrode and the passivation will be delayed [49][50][51].

Conclusions
e Ti/SnO 2 -Sb-Ni/Re electrodes obtained are very helpful for the electrochemical degradation of RhB. Compared with Ti/SnO 2 -Sb-Ni/Re, experimental results exhibited that these modified electrodes had a different electrochemical decolorization performance. Compared with modified Ni (0%, 1%, and 2%)/Re electrodes, the longer accelerated life could be observed in Ni (3.5%)/Re and Ni (5%)/ Re electrodes. Compared to those without intermediate layer electrodes, the electrodes with Ni (2%) intermediate layers exhibited increased electrochemical performance and efficiency for decolorization of RhB. Additionally, their service lifetimes increased more than 1.9 times, 1.69 times, 1.76 times, and 1.47 times for Gd, Ce, Eu, and Er, respectively. Compared with the color removal efficiencies of the Ni (2%)/Re electrodes of decolorization efficiencies, the color removal rate of RhB reaches 90% after treatment for 60 min. Relatively low energy consumption of 3.621 kWh m −3 of wastewater can be acquired under the experimental condition of 100 mg·L −1 RhB. It is concluded that the moderate content Ni could enhance the decolorization performance, but higher Ni amounts did not result in better performance.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.