Improvement of stability and reduction of energy consumption for Ti-based MnO_x electrode by Ce and carbon black co-incorporation in electrochemical degradation of ammonia nitrogen

Well-developed methods, classified as physical technology, chemical technology and biological technology, have been applied to wastewater treatment. The physical technologies include coagulation/flocculation (Mamelkina et al. 2020), separation membrane (Tavangar et al. 2020) and adsorption (Bacelo et al. 2020; Choudhary et al. 2020; Imran et al. 2021). The biological treatments have been studied, including anaerobic process, activated sludge process and so on (Gopalakrishnan et al. 2019; Wu et al. 2021). The chemical technology, such as ozonation and oxidation with catalysts (Silva et al. 2019; Liu et al. 2021), as well as advanced oxidation processes (AOPs), has been widely used in wastewater treatment due to its effectiveness. Electrochemical oxidation (EO) technology, as one of the AOPs, has attracted considerable attention for wastewater remediation, because it could completely mineralize refractory organic pollutants and present several characteristics of environmental significance without generating secondary pollutants, such as mild operation conditions, strong adaptability, simple and reliable equipment, and so on (Brillas & Martínez-Huitle 2015; Dominguez et al. 2018; Garcia-Segura et al. 2018). In EO remediation of wastewater, anode materials play particularly important roles, which could determine the degradation efficiency and service lifetime of the electrodes. Dimensionally stable anode, an electrode prepared by active metal oxides coated on the titanium substrate, has been successfully employed to degrade various refractory pollutants, and shows potential to overcome the poor stability of traditional graphite electrodes (Zhu et al. 2019) and high cost of noble metal electrodes (Tavares et al. 2012). A variety of electrodes, such as Ti/PbO₂ (Polcaro et al. 1999; Bian et al. 2019), Ti/SnO₂ (Martínez-Huitle et al. 2008), Ti/RuO₂ (Yue et al. 2017), and Ti/IrO₂ (Baddouh et al. 2020) have been investigated as anode for the EO remediation of wastewater because they are effective in oxidizing pollutants with high oxygen overpotentials. However, the high cost and inefficient performance for high-chloride content wastewater limits their use (Kaur et al. 2017). And the large-scale application is also limited due to leakage of metal cations during preparation process and by electrochemical corrosion, which may cause secondary pollution (Li et al. 2016). Therefore, it is essential to develop electrodes with superior catalytic activity, high stability, low cost and mild toxicity.

Ti/MnO_x anodes are regarded as promising candidates in EO remediation of wastewater because of the low cost and toxicity, ease of preparation, and high electro-catalytic activity. Yang et al. reported porous Ti/MnO_x anode for degradation of phenol in electro-catalytic membrane reactor, and the phenol removal efficiency was 93%, with chemical oxygen demand (COD) and total organic carbon (TOC) removal efficiency of 79 and 68%, respectively (Yang et al. 2018). Massa's reports suggested that MnO_x could promote the direct and indirect oxidation of phenol in EO remediation of wastewater (Massa et al. 2018). Hui et al. prepared porous Ti/MnO_x anode for degradation of highly concentrated phenol in wastewater, and the phenol removal efficiency was 73% in fixed bed electro-catalytic reactor (Hui et al. 2019). Our previous results also demonstrated the superior activity of Ti/MnO_x anodes in electrochemical degradation of Acid Red B, a typical azo dye in textile wastewater (Xu et al. 2019). Even though MnO_x-based electrodes present superior electro-catalytic activity for removal of organic pollution, and have been widely studied in water oxidation, lithium-ion batteries and supercapacitors, the commercial application of Ti/MnO_x electrode is still hindered by the short lifetime (Jiang & Kucernak 2002; Martínez-Huitle et al. 2008; Wiechen et al. 2012; Xiang et al. 2015; Wang et al. 2018).

Ammonia nitrogen (NH₄⁺-N) in wastewater is an increasing problem, which can promote eutrophication and is toxic to aquatic organisms. Even though most of the NH₄⁺-N can be removed by biological method in practical application (Del Moro et al. 2016), low concentration of NH₄⁺-N is very difficult to remove, and the concentration of NH₄⁺-N needs to be in accordance with a specified detection limit. Previous studies have demonstrated that NH₄⁺-N could be decomposed to N₂ mainly by electrochemical degradation process (Zöllig et al. 2015). To the best of our knowledge and according to the literature review, there is no report on the study of Ti/MnO_x anodes in electrochemical degradation of NH₄⁺-N from an aqueous solution. In the present work, we introduced conductive carbon black (C) and Ce into the MnO_x catalytic layer to fabricate a Ti/Ce:MnO_x-C electrode. The surface characteristics and electrochemical properties of Ti/Ce:MnO_x-C electrode were studied, and the degradation efficiency, cell potential, energy consumption, and stability of Ti/Ce:MnO_x-C electrode were compared with MnO_x/Ti in electrochemical degradation of NH₄⁺-N. Studies showed that Ti/Ce:MnO_x-C electrode presented significantly improved lifetime and reduced energy consumption with the co-incorporation of C and Ce.

Titanium plate was purchased from Suzhou Shuertai Industrial Technology Co., Ltd (Suzhou, China). Isopropanol and oxalic acid were purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). Conductive carbon black was purchased from Tianjin Yiborui Chemical Co. Ltd (Tianjin, China). Ammonia sulfate, manganese nitrate (50%), cerium nitrate hexahydrate, sulfuric acid, sodium hydroxide, sodium chloride and sodium sulfate were purchased from Sinopharm Group Chemical Reagent Co., Ltd (Shanghai, China). All chemical reagents employed were of analytical purity grade. Ultra-purified water with resistivity of 18.2 MΩ·cm was obtained from the Millipore-Q system for all of the solutions.

First, titanium plate was pretreated by polishing, caustic washing and acid etching. Then 5 mL of Mn(NO₃)₂ isopropanol solution (1 mmol) was sprayed onto the pretreated Ti substrate at pressure of 4 kPa by the spray gun (caliber: 0.2 mm). After that, the Ti plate was dried at 100 °C for 10 min and then thermally treated at 200 °C for 5 min. The above processes were repeated four times. Finally, the product was calcinated at 350 °C for 20 min to obtain Ti/MnO_x electrode. Ti/MnO_x-C electrode was prepared with a certain amount of conductive carbon black (C) added into the Mn(NO₃)₂ solution during the spraying process. The molar ratio of C/Mn was 14%. As for the Ti/Ce:MnO_x-C electrode, the spraying solution was a mixture of conductive carbon black (C), Ce(NO₃)₂·6H₂O and Mn(NO₃)₂ with C/Mn molar ratio of 14% and Ce/Mn molar ratio of 1%. The active area of the prepared electrodes was 4 cm².

The powder X-ray diffraction (XRD) data were collected on X'PERT PRO type X-ray diffraction (Netherlands Spectris) with CuKα radiation (λ = 1.5418 Å) over the 2θ range of 5° to 80° with scan speed of 5°/min at room temperature. The scanning electron microscopy (SEM) was undertaken by JSM-7500F cold field emission scanning electron microscope (Japan Electronics Co., Ltd) at accelerating voltage of 20 kV. The X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB250 (American Thermo VG Co., Ltd) equipped with an Al Kα source.

Electrochemical experiments were performed in a conventional three-electrode cell system at CHI660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd, China). The prepared electrodes were employed as working electrode with platinum electrode (20 mm × 30 mm × 0.1 mm) as the counter electrode, and Ag/AgCl/0.1 M KCl (Shanghai Ciyue Electronic Technology Co., Ltd, China) as the reference electrode. The cyclic voltammetry (CV) measurement was conducted in 0.1 mol/L sodium sulfate solution at scan rate of 50 mV/s with scan region from 0 to 2.5 V. The polarization curve (linear sweep voltammetry; LSV) was tested at scan rate of 10 mV/s in 0.1 mol/L sodium sulfate and 400 mg/L sodium chloride solution. The conductivity of the electrodes was tested by electrochemical impedance spectroscopy (EIS) with the frequency range from 1 × 10⁵ to 1 × 10⁻² Hz and the amplitude of 10 mV, which was conducted at open circuit potential. All of the electrodes were activated by cyclic voltammetry at 50 mV/s before the tests.

The surface morphology of the prepared electrodes was analyzed by SEM. As shown in Figure 1(a), the surface of Ti/MnO_x electrode was rough with large particles, suggesting the aggregation of MnO_x particles during preparation process. Ti/MnO_x-C electrode possessed smooth surface without obvious cracks (Figure 1(b)), indicating that the introduction of conductive carbon black could significantly affect the morphology and improve the surface integrity of the electrode. However, pinholes with size of ca. 20 nm were observed on the surface of electrode. Ti/Ce:MnO_x-C electrode displayed more dense and compact surface with smaller pinhole size (ca. 5 nm in Figure 1(c)). The surface evenness and compaction of electrode was important for electrochemical process, and might affect the lifetime of electrode (Santos et al. 2014). The SEM images suggested the significant improvement of surface evenness and compaction originated from carbon black and Ce addition.

Typical XRD patterns of Ti/MnO_x, Ti/MnO_x-C and Ti/Ce:MnO_x-C electrodes are shown in Figure S1 (Supporting Information). All of the diffraction peaks could be assigned to the hexagonal Ti (JCPDS No. 44-1294 and No. 89-3725) originating from Ti substrate without MnO_x phase observed, which might be caused by the amorphous MnO_x formed in the present conditions. Additionally, there was no diffraction peak of TiO₂ in the XRD patterns, which suggested that the Ti substrate was not oxidized during the preparation process. Therefore, the conductivity of Ti substrate was maintained to facilitate the electrochemical reaction.

XPS measurement was performed to investigate the chemical state of the electrode surface elements. The XPS survey spectra are presented in Figure S2, and the Mn and O photoelectron peaks are clearly observed. The fine structure of Mn 2p and O 1 s was investigated because the Mn oxidation state and oxygen species play important roles in the electrochemical processes. As shown in Figure 2, the Mn 2p peaks were fitted by considering two resolved doublets, Mn 2p_1/2 and Mn 2p_3/2, located at 653.9 eV and 642.2 eV, respectively. The coexistence of Mn(II), Mn(III) and Mn(IV) was evidenced by fitting the peaks, which could be assigned to MnO, Mn₂O₃ and MnO₂ as reported in the literature (Lee et al. 2011; Li et al. 2011; Wang et al. 2017a), even though there was no diffraction peak of manganese oxide found in the XRD results. The discrepancy might be caused by the poor crystallinity of manganese oxide formed in this study. The relative content of MnO_x phase calculated by Mn 2p_3/2 is displayed in Table 1. It has been reported that MnO₂ was more beneficial for the electrochemical degradation of pollutants (Massa et al. 2018), and the results suggested that the introduction of Ce could enhance the Mn(IV) content while the addition of carbon black had little influence on the Mn oxidation state of Ti-based MnO_x electrode. The O 1s peak could be deconvoluted into the signal of lattice oxygen (O_L) and adsorbed oxygen (O_A). According to the peak fitting results shown in Table 1, the relative content of O_A, which has been reported as the most active oxygen and could play an important part in EO process, was similar for the three electrodes (Ramirez et al. 2014; Bian et al. 2019), suggesting the incorporation of carbon black and Ce had little effect on the relative O_A content of Ti-based MnO_x electrode.

The influence of additive on the electrochemical behavior of the Ti-based MnO_x electrode was investigated by voltammetry measurement. Typical CV curves of the Ti/MnO_x, Ti/MnO_x-C and Ti/Ce:MnO_x-C electrodes in 0.1 mol/L sodium sulfate solution are shown in Figure 3(a). There was no obvious redox signal observed except for the oxygen evolution, indicating that Ti-based MnO_x electrodes were electrochemically inactive in Na₂SO₄ solution (Duan et al. 2014). The electrochemical active surface area is related to the geometric area of the region enclosed by the CV curve, so the Ti/Ce:MnO_x-C electrode with larger electrochemical active surface area will present higher electrochemical activity in EO degradation of pollution. Additionally, the oxygen evolution current of Ti/Ce:MnO_x-C electrode was much higher than that of Ti/MnO_x and Ti/MnO_x-C at the same potential, which verified the superior electrochemical activity of Ti/Ce:MnO_x-C electrode (Duan et al. 2012). It has been reported that the NH₄⁺-N degradation mechanism could be direct oxidation and indirect oxidation (Siddharth et al. 2018). In this work, NH₄⁺-N was completely removed in the presence of Cl⁻ and the degradation efficiency was only 6.1% in the absence of Cl⁻ in the electrolyte (Figure S3). The electrochemical degradation results verified that NH₄⁺-N was mainly degraded by indirect oxidation in this work. Ti-based MnO_x electrode showed enhanced performance for the electrochemical degradation of NH₄⁺-N in the presence of Cl⁻ due to the generation of several chlorine-based oxidants, such as Cl₂, HOCl and ClO⁻ (Siddharth et al. 2018). As shown by the LSV profiles of electrodes in the presence of Cl⁻ (Figure 3(b)), the onset potential of Ti/Ce:MnO_x-C electrode was lower with higher current at the same potential, which indicated that Ce and C co-incorporation facilitated the electrochemical reaction of Ti-based MnO_x electrode.

In further investigation on the influence of additive on the electrochemical activity of Ti-based MnO_x electrode, EIS was used to determine the charge transfer resistance of electrodes. The Nyquist and equivalent circuit model for Ti/MnO_x, Ti/MnO_x-C and Ti/Ce:MnO_x-C electrodes are presented in Figure 4. In the equivalent circuit, the ohmic resistance and charge transfer resistance are presented as R_s and R_ct, respectively. According to the fitting results, the value of R_ct for Ti/MnO_x-C electrode was 80 Ω, which was significantly reduced compared with that for the Ti/MnO_x electrode (654 Ω), and the R_ct value was further reduced to 9 Ω for Ti/Ce:MnO_x-C electrode. The EIS measurement demonstrated that the Ce:MnO_x-C coating layer exhibited a much lower charge transfer resistance, which indicated the great improvement of electron transfer rate and conductivity of electrodes. The improvement of electron transfer rate was beneficial for the reduction of cell potential and energy consumption during the electrochemical process, which provided economic feasibility for electrochemical degradation of pollutants in wastewater (Hernández et al. 2016).

The catalytic performance of the Ti-based MnO_x electrodes for electrochemical degradation of NH₄⁺-N (23 mg/L) was investigated in Na₂SO₄ (0.1 mol/L) solution with NaCl concentration of 400 mg/L, current density of 20 mA/cm² and pH of 9.0. Figure 5(a) illustrates the concentration of NH₄⁺-N during the electrochemical degradation process with the prepared electrodes. The concentration of NH₄⁺-N decreased gradually with the reaction proceed, and reached 0.2 mg/L at 90 min for Ti/MnO_x electrode. The degradation efficiency of Ti/MnO_x-C was comparable to that of Ti/Ce:MnO_x-C electrode. For Ti/Ce:MnO_x-C electrode, the NH₄⁺-N could be completely removed within 90 min, and the residual total nitrogen was 2.0 mg/L with nitrate of 5.1% (1.2 mg/L) and dichloramine of 3.4% (0.8 mg/L), which suggested that most of the ammonia nitrogen (91.5%) was mineralized to N₂. These results suggested that Ti-based MnO_x electrode was efficient for NH₄⁺-N degradation. Figure 5(b) illustrates the corresponding cell potential during the electrochemical degradation process, which kept stable for each electrode. The cell potential of Ti/MnO_x-C electrode was lower than that of Ti/MnO_x. It is noteworthy that the cell potential of Ti/Ce:MnO_x-C electrode reduced by 35% compared to Ti/MnO_x, which was 3.4 V versus 5.2 V. The significant decrease of cell potential might be caused by the improved conductivity of electrode by co-incorporation of carbon black and Ce verified by the EIS results (Song et al. 2007; Wang et al. 2017b). The corresponding energy consumption during electrochemical process was calculated by Equation (2) and shown in Figure 5(c). The energy consumption increased as electrochemical degradation proceeded and reached 0.18 kW·h/g for Ti/Ce:MnO_x-C electrode when NH₄⁺-N was totally degraded, which was reduced by 35% compared to the Ti/MnO_x electrode (0.28 kW·h/g). The cost of energy consumption is reduced to ca. US$ 20/kg (NH₄⁺-N) for Ti/Ce:MnO_x-C electrode. These results indicated that the co-incorporation of carbon black and Ce could improve the conductivity of electrodes, leading to reduced cell potential and less energy consumption during electrochemical process (Jin et al. 2015).

The stability of electrode determines the electrode performance and commercial application, so in order to evaluate the stability of electrode, accelerated service life experiments were carried out in 0.5 mol/L sulfuric acid solution at 100 mA/cm². As displayed in Figure 6, the cell potential of Ti/MnO_x electrode increased significantly, suggesting the poor stability of Ti/MnO_x electrode, while the cell potential of Ti/Ce:MnO_x-C electrode was lower and much more stable than that of Ti/MnO_x. According to the empirical formula proposed by B. Correa-Lozano et al. (1997), the actual lifetime of Ti/Ce:MnO_x-C electrode was 54.6 h calculated by Equation (3), which was ca. 25 times that of Ti/MnO_x electrode (2.2 h). The improved stability of Ti/Ce:MnO_x-C electrode mainly originated from the co-incorporation of carbon black and Ce, which not only enhanced the conductivity but also improved the surface evenness of the electrode (Gargouri et al. 2014; Jin et al. 2015).

In summary, the Ti-based MnO_x electrode co-incorporated with carbon black and Ce was successfully prepared by spraying-calcination method. The surface properties, electrochemical performance, degradation activity and stability of Ti/Ce:MnO_x-C electrode were compared with those of the Ti/MnO_x electrode. The electrochemical results demonstrated that Ti/Ce:MnO_x-C electrode served as an efficient electrocatalyst to degrade NH₄⁺-N completely with N₂ formation of 91.5%. The cell potential and energy consumption of Ti/Ce:MnO_x-C electrode were reduced by 35% and stability was improved by ca. 25 times. The superior performance of Ti/Ce:MnO_x-C electrode might benefit from its flat and integrated surface and low charge transfer resistance by co-incorporation of carbon black and Ce. This research provided a novel method for the modification of Ti-based MnO_x electrode, and would be helpful for the application of Ti-based MnO_x electrode in electrochemical degradation of NH₄⁺-N and other organic pollutants.

This research was financially supported by National Natural Science Foundation of China (No. 21905035), LiaoNing Revitalization Talents Program (XLYC1907093), Guide Plan of Key Research and Development Program of Liaoning Province (2019JH8/10100096) and Fundamental Research Funds for the Central Universities (No. 3132019334), and the Initial Scientific Research Fund of Young Teachers in Shenyang University of Technology under Grant No. 200005782.

All relevant data are included in the paper or its Supplementary Information.