Melamine-derived carbon electrode for efficient H2O2

Electrochimica Acta 261 (2018) 375e383

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Melamine-derived carbon electrode for efficient H2O2 electrogeneration Yingshi Zhu a, Shan Qiu a, *, Fang Ma a, b, Guojun Li a, Fengxia Deng a, Yanshi Zheng a a b

School of Environment, Harbin Institute of Technology, Harbin 150090, China State Key Laboratory of Urban Water Resource and Environment, Harbin 150090, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 August 2017 Received in revised form 20 November 2017 Accepted 17 December 2017 Available online 20 December 2017

A facile one-step fabrication of a highly porous nitrogen-enriched graphitic carbon (NGC) cathode derived from melamine was proposed. It was the very first time for the NGC cathode to be used in the electro-Fenton (EF) process for evaluating electro-generated H2O2. The surface characteristics of melamine carbonized at different temperature (NGC-800, NGC-850 and NGC-900) were systematically investigated, including the microstructure, composition, electrochemical properties by the methods of scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS). Results showed that NGC samples carbonized at different temperature were highly porous with a micrometer size of skeletons (1.5 e2.2 mm). Considering the H2O2 ability, NGC-900 was most efficient cathode in electro-generated H2O2 with a H2O2 concentration of 87.19 mmol/L (add H2O2 concentration) among NGC-800, NGC-850 and NGC-900. Moreover, the high efficient H2O2 generation ability kept stable in a wide pH range from 3 to 9. Combined the technologies, including XPS and electrochemical technologies CV, the high efficient H2O2 capacity attributed to the pyrrolic N structure, together with the improved electroconductivity. Therefore, the simple fabrication approach for melamine-derived carbon cathode is a promising low-cost cathode for EF. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Nitrogen-carbon material Pyrrolic N Hydrogen peroxide Electro-fenton Melamine-derived cathode

1. Introduction Developing green technologies like integrating advanced oxidation processes (AOPs) to decompose refractory pollutants using various superoxide to produce reactive oxygen species, particularly hydroxyl radicals ( OH) [1,2] is urgent due to the pressure of reducing refractory pollutants in wastewaters. OH species with a high standard redox potential (Eo (OH/H2O) ¼ 2.80 V/ SHE [3]), is the second strongest oxidizer known after fluorine and can react with refractory pollutants non-selectively until their total mineralization (conversion into CO2, water, and inorganic ions) [4]. Among AOPs techniques Fenton reactions have been widely used in wastewater treatment, in which H2O2 is decomposed by ferrous ions (Fe2þ) to produce OH. However, Fenton suffers some issue, like generation a large amount of iron sludge and the risk of H2O2 transportation. In order to overcome the above-mentioned concern, the electro-Fenton was proposed, in which H2O2 can

* Corresponding author. E-mail address: [email protected] (S. Qiu). https://doi.org/10.1016/j.electacta.2017.12.122 0013-4686/© 2017 Elsevier Ltd. All rights reserved.

continuously supply from the two-electron reduction of oxygen (Eq. (1)) to induce the classical Fenton reaction via Eq. (2) [5]. Meanwhile, the cathodic regeneration of Fe2þ (Eq. (3)), which lowers the generation of waste iron sludge [6]. O2 þ 2Hþ þ 2e / H2O2

(1)

Fe2þ þ H2O2 / Fe3þ þ OH þ OH

(2)

Fe3þ þ e / Fe2þ

(3)

As is can be seen from the mechanism in the EF process (Eqs. (1)e(3)), H2O2 generation plays a crucial role in the EF system and is affected by the properties and types of cathode materials. To improve the oxygen reduction reaction (ORR) activity, various carbonaceous materials have been widely tested as cathodes, including carbon/graphite felt [7,8], carbon sponge [9,10], activated carbon fiber (ACF) [11,12], gas-diffusion electrodes [13,14] and transition metal modified carbon electrode material [15,16]. Recently, nitrogen-doped porous carbon materials have attracted

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researchers' attention due to their potential to active ORR [17,18]. According to previous studies, the catalytic activity of N-doped carbon materials for the four-electron reduction of oxygen improves from the active sites on N-doped structures such as pyridinic-N and quaternary-N [19e21]. Moreover, apart from the contribution for the 4-electron activation, it has been reported that N-doped structures contribute to a mixed reaction of two-electron and four-electron reduction ways [22e25]. Therefore, it is worth investigating the number of electrons involved in the ORR, especially the N-doped structures for two-electron reduction for electro-generated H2O2 in EF. However, according to author's knowledge, there is no related report on using melamine-derived carbon electrode as cathode in the EF process. Therefore, a facile method to fabricate macroporous NGC material by in situ carbonization of porous melamine foam (PMF), a kind of cost-effective commercially available polymer material, was put forwarded. The characteristic of the new NGC, including surface morphology, pore size, main component, state of the main element and electrochemical properties, were analyzed by SEM, XRD, XPS, CV and EIS, respectively. Meanwhile, the performance for H2O2 generation in EF by this melamine-derived cathode was investigated. Rhodamine B degradation (as pollutant probe) by NGC in EF was evaluated, along with the stability of the NGC. 2. Experimental Sodium sulfate (Na2SO4), potassium permanganate (KMnO4), hydrogen peroxide (H2O2, 30%), sodium oxalate (Na2C2O4, 99.8%) and rhodamine B (C28H31CIN2O3) were obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Potassium phthalate monobasic (C8H5KO4, 99.8%), potassium iodide (KI) and ammonium molybdate ((NH4)6Mo7O24$4H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade. Millipore-Q water was used throughout the investigation. Unless stated otherwise, all the experiments were done at room temperature (25 C) and 1.0 atm pressure. 2.1. Material preparation The commercial available melamine foam, obtained from Jie Mi in China, was directly used as the precursor for preparing macroporous NGC material via an electric furnace heated to a defined temperature between 800 C and 900 C, with a heating rate of 5 C/min under highly pure N2 atmosphere (flow rate of z 100 mL/ min). 2.2. Characterization The scanning electron microscopy (SEM) was carried by FEI Quanta 200. The X-ray diffraction (XRD) measurements were collected on a D8 ADVANCE X-ray diffractometer with Cu Ka

radiation (40 kV, 40 mA over the 2q range, 10e90 ). The X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALAB 250Xi (ThermoFisher, USA) which employed Al monochromator (hy ¼ 1486.69 eV) irradiation as the photosource. The Raman spectra were recorded on HORIBA LabRam 800 equipped with an Ar laser source (wavelength: 532 nm). The thermogravimetric-differential scanning calorimetry (TG-DSC) experiments were carried out by using a NETZSCH STA 449 C Instrument. The electrochemical properties were characterized using an electrochemical workstation with a three-electrode system (CHI 660E Shanghai Chenhua Instrument Co., Ltd., China). Electrochemical impedance spectroscopy (EIS) was used to determine the catalysts' conductivity at the open-circuit potential over the frequency range from 1 105 to 1 103 Hz at an amplitude of 5 mV. Cyclic voltammetry (CV) measurement scan rates were 5e100 mV/ s. The concentration of H2O2 was determined by spectrophotometrically using the iodide method (detection limit of z 106 mol/L) at l ¼ 351 nm [26]. 2.3. The performance of H2O2 generation and rhodamine B degradation by EF The H2O2 generation experiment was performed in an undivided cell (100 mL) in 0.05 mol/L Na2SO4 solution, stirring at 300 rpm with a magnetic bar. In this three-electrode system, Pt sheet (1 cm 2 cm 0.1 cm) and a saturated calomel electrode (SCE) acted as reference electrode and counter electrode, respectively. The amount of anhydrous sodium sulfate aqueous solution of 0.05 mol/L was 50 mL and its initial pH was adjusted in the range from 3 to 9 with sulfuric acid and sodium hydroxide prior to electrolysis. And the NGC-800 (heating at 800 C), NGC-850 and NGC-900 electrode (working area of 2 cm2, 2 mm thick) were used as working electrodes. The distance between the anode and cathode was 2 cm with current density 1 mA/cm2. At time intervals, 1 mL samples were taken for analyzing the concentration of the H2O2. The Rhodamine B treatment researches were performed at the same conditions with the H2O2 electro-generation experiments and the initial concentration of rhodamine B was 20 mg/L. 3. Results and discussion 3.1. Characterization of materials The morphologies of NGC-800, NGC-850 and NGC-900 were examined by SEM images and the typical results are shown in Fig. 1. It can be seen that all the NGC cathodes have a highly porous open structure with the pore size of 15e25 mm, and smooth clathratelike polymer skeletons with the thickness in the range of 5e8 mm. Considering the influence by carbonization temperature, the higher the carbonization temperature, the more serious thinning and fracture skeleton of the cathode NGC, shown in Fig. 1. After

Fig. 1. SEM images of carbonized PMF at 800 C, 850 C and 900 C.


carbonization treatment, shrinkage appeared on the surface of the original PMF. Aside from the change of the surface characteristic, the carbonized cathodes got elasticity, which gradually decreases as the increase of the temperature. By combining the results of the XPS and SEM, the deposit on fibers can be corresponded to Na2CO3 and due to the thermal decomposition of Na2CO3 took place from its melting point (850 C) and continued to occur as the temperature was increased, the deposit of NGC-900 was almost nonexistent [27,28]. As showed in Fig. 1, the formed micrometer size of skeletons (1.5e2.2 mm) and high void volume favors mass transfer for oxygen, which is beneficial to ORR. Fig. 2 shows the XRD patterns of NGC samples with various carbonization temperatures. All the three kinds of NGC samples have two peaks at 2q ¼ 13.96 and 2q ¼ 40.89 , which respectively assigns to the graphite (001) lattice plane and (011) plane of carbon nitride (C3N4, MDI Jade 6.5). However, a diffraction peak at 2q ¼ 27.43 only appeared in NGC-850 and NGC-900 samples, which corresponding to the graphite (002) reflection plane. It demonstrated that temperature higher than 800 C is high enough for graphitization carbonization, which is in accordance with previous report [29]. In addition, the NGC-800 sample has a different peak at 2q of 29.39 , indexing as (003) plane of C3N4. It is reported that C3N4 derived from some materials like melamine can't survive under temperature higher than 900 C due to its limited thermal stability [30]. To have a better understanding of the different nitrogen and oxygen structure of NGC samples carbonized at different temperatures, XPS studies were conducted and the binding energies analysis was corrected for specimen charging by referencing C1s to 284.8 eV. Fig. 3a shows the peaks with the binding energy of approximately 285, 400, 531 and 1071 eV, which corresponds to C1s, N1s, O1s and Na1s. Fig. 3a shows the C1s core-level spectrum and the peak at 288.2 eV is assigned to the NeC]N group in graphitic carbon nitride [31]. Fig. 3a shows that the O1s peaks center at the binding energies of 530.8e533.7 eV, which is probably mainly from oxygen-containing functional groups and physically absorbed oxygen since graphitic carbon is susceptible to oxygen adsorption [32]. It was reported that oxygen functional groups can provide hydrophilic properties to the cathode and it's convenient for dissolved oxygen to diffuse to cathode surface and enhance ORR reaction [33,34], but the percentage of O atom in NGC samples were 8.42%, 8.96% and 8.22% for NGC-800, NGC-850 and NGC-900, respectively, in which the value was almost the same. In addition,

377

(c)

Fig. 3. (a) XPS survey spectra of NGC samples resulting from different carbonize-tion temperatures; (b) XPS N1s spectra of NGC materials; (c) Schematic diagram of the nitrogen-doped carbon structure. (Blue balls: nitrogen atoms, gray balls: carbon atoms). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

the peak at 1071.4 eV could be assigned to Na2CO3 [35]. Fig. 3b displays the high-resolution XPS spectra of N1s in a narrow binding energy range from 393 eV to 407 eV. N XPS spectra were fitted into three components: 397.8e398.3, 399.2e400.0, and 401.5 eV, corresponding to pyridinic N (nitrogen in a six-atom Fig. 2. XRD patterns of the carbonized NGC samples treated at high temperature.

378


Fig. 4. (a) The Raman spectrum of NGC samples as a function of different heat treatment temperature (HTT); Curve fitting of (b) NGC-800, (c) NGC-850 and (d) NGC-d900.

heterocyclic ring), pyrrolic N (nitrogen in a five-atom heterocyclic ring), and quaternary N (or graphitic N, sp2-hybridized N neighbored with three sp2-C), respectively [33e36]. Considering the total nitrogen percentage transformation, it rose from 1.04% to 3.90% as temperature increased to 850 C. However, the relative nitrogen content dropped to 1.01% with a further temperature increasing. It is noteworthy that, the percentage of the pyrrolic-like nitrogen is elevated with the carbonization temperature augment and it changes to pyrrolic-like nitrogen and graphite-like nitrogen at 900 C. Pyridinic N and pyrrolic N are at the edge of the carbonaceous graphene structure (Fig. 3c), which is easy to contact the reactive ions in the electrolyte. Therefore, pyrrolic N and pyridinic N can effectively improve the electrochemical performance of NGCsamples [37]. Based on the above XPS analysis, we can draw the conclusion that among different melamine carbonized temperature, 850 C was the optimal temperature for pyrrolic-like nitrogen enhancement (3.34%). Raman spectra was used to distinguish different bonding types and domain sizes of NGC and results under different temperatures were shown in Fig. 4a. As it can be seen from Raman spectra acquired in the spectral range 900e2000 cm1, it exhibits two main peaks positioned at ~1360-1390 cm1 and ~1580 cm1, denoted conventionally by D and G band, respectively [38]. The G band is actually the graphite-like sp2 nanocrystals, while the D band is the linkage with nitrogen atoms [39]. Fig. 4bed is the result of the curve fitting of NGC samples by various temperatures. Table 1 lists the Raman spectral parameters of each sample in accordance with the Lorrentz curve. It is obvious that the D band shifts with the temperature increasing, indicating the structure is less orderly due to the low treatment temperature [40].

FWHM is the full width at half maximum of each band, while R in Table 1 is representing the ratio of integral intensity (area ratio). It is known that the intensity ratio of D to G band (ID/IG) is used to correlate the structural purity of graphitic materials to the graphite crystal domain size [41]. As can be seen from Table 1, the G band barely moves with temperature increasing, but the FWHM of both D band and G band decrease. Moreover, the value of R also decreases illustrating the carbon structure of the NGC samples tend to be graphitization [42]. This conclusion is in accordance with the XRD result. The thermogravimetric-differential scanning calorimetry experiments with a detected temperature from room temperature to 1000 C at a heating rate of 10 C/min, was conducted to evaluate the thermal stability and crystalline formation of NGC. The TG and DSC thermograms for PMF are shown in Fig. 5. The first weight loss (estimated 22.7%) with two endothermal peaks at 64.8 C and 228.2 C are ascribed to the water removal, which is from agent and formaldehyde left in the PMF [43]. The second weight loss in the temperature range from 371.6 C to 396.6 C, with sharp peak at 389.6 C is attributed to the removal of NH3 and HCN. The further weight loss is 55.36% (in the range of 396.6e878.6 C), with two endothermal peaks at 478.5 C and 836.0 C, which imply that the PMF begin to be carbonized and dehydrogenated, and the skeletons gradually turn into the nitrogen-enriched graphitic carbon along with further emission of NH3 and HCN [44]. The above-mentioned TG-DSC results are in consistent with the result of XRD and Raman characterization. In addition, the NGC samples can be further decomposed until release the nitrogen completely with the final products of stable graphitic carbon [45]. The TG-DSC results imply the thermal condensation of NGC from pyrolysis of PMF.


379

Table 1 Raman parameters of NGC samples with different HTT. D-band/cm1

HTT/ C

800 850 900

G-band/cm1

R (Area)

Raman shift

FWHM

Raman shift

FWHM

1386.1 1371.0 1364.9

363.8 314.0 296.76

1581.1 1581.6 1582.9

107.7 101.2 99.1

2.02 1.80 1.74

Fig. 5. TG-DSC curves of PMF under N2 atmosphere.

3.2. Electrochemical properties of the NGC samples Fig. S1 shows the effects of the different heating temperature on the NGC performance. The NGC-900 sample exhibits the highest current response, and the wide separation between the oxidative and reductive curve of NGC-900 sample is more remarkable than others. In addition, the voltammetric charge, which is one of the important indicators of the catalytic activity of electrode, can be calculated by the following formula:

q* ðnÞ ¼

1

n

ZE2

iþ i dE

(4)

E1

where n is the scan rate (V/s), in this case it is 50 mV/s, q* ðnÞ is voltammetric charge; E1 and E2 are the starting and ending points of the scan range, respectively; iþ and i are the positive and negative current density for the same potential value (mA/cm2). After calculation, the voltammetric charge of NGC-850 sample and NGC-900 sample are 8.59 mC/cm2 and 28.69 mC/cm2, respectively. The CV curves of NGC-800 sample is almost parallel to the horizontal line, so it can not compute the voltammetric charge. It is obvious that the electro-catalytic activity of NGC-900 sample is three times higher than that of the NGC-850 sample, indicating that the NGC-900 sample has the best catalytic performance for ORR reaction. It can be seen from Fig. 6a, the CV curves of NGC-850 sample has no significant reduction peak and as the scanning rate increases, the current of electrode increases correspondingly and the deviation of the balance of electrode increases. It indicates that the process of CV is involving with the transfer of mass and charge, which is not instantaneous and related to the resistance of the samples. As the scan rates become faster, the process is less complete, so the graph becomes more and more out of the ideal shape.

Fig. 6. CV curves of the (a) NGC-850 and (b) NGC-900; (c) The relationship of the reduction peak current versus scan rate for NGC-900. Operating conditions: pH ¼ 6, [Na2SO4] ¼ 0.05 mol/L, V ¼ 50 mL and current density ¼ 1 mA/cm2.

As observed in Fig. 6b, the CV curves of NGC-900 sample has a reduction peak, which corresponds to the production of H2O2. It will be proved by the following H2O2 generation experiment. The linear relationship between the reduction peak current (Ip) versus the scan rate (n) in the range of 5e100 mV/s is shown in Fig. 6c. The corresponding equations is expressed as Ip ¼ 1.88E-4þ9.14E-6v

380


(R2 ¼ 0.9952). It suggests that it is an adsorption-controlled process when NGC-900 as cathode [46]. To obtain information about the electron transfer ability of NGC samples, electrochemical impedance was measured. The pattern in impedance spectra can be illustrated by the equivalent circuit shown in the inset of Fig. 7a. The Rs represents the internal or equivalent series resistance of bulk which both NGC-850 sample and NGC-900 sample are 19.6 U. The C and Rct are the related double layer capacitance and its charge transfer between the electrolyte and working electrode. After best-fit estimating with Zview software, the charge transfer resistance for NGC-850 sample and NGC-900 sample are 13.07 U and 3.67 U, respectively. All in all, it is apparently seen that charge-transfer resistance of NGC samples increased with the temperature falling, which adequately explains why NGC-900 sample exhibits higher electrochemical activity than NGC-850 sample as the CV curves have proved. In addition, as Fig. 7b shows that the phase angle of the NGC-900 sample in the low frequency region is 72.63 , which is significantly higher than that of the NGC-850 sample (37.71 ), indicating faster ion diffusion in the electrolyte and higher ion adsorption onto the electrode surface [47].

hence H2O2 production ability with different carbonization temperature cathodes and different pH values were investigated and H2O2 accumulation results showed in Fig. 8. H2O2 accumulation rate increase with time during the first 30 min, however, the H2O2 accumulation reaches a platform, meaning its decomposition rate equals to generation rate. Here, in this EF, oxidation of H2O2 at the anode or may be the self-decomposition of H2O2 both contribute to the decomposition approaches (Eqs. (5) and (6)) [48,49]. H2O2 / O2 þ 2Hþ þ 2e

(5)

2H2O2 / 2H2O þ O2

(6)

Among these three cathodes, NGC-900 has the highest yield of H2O2 with a H2O2 generation of 87.19 mmol/L for 60 min electrolysis, suggesting that the pyrrolic-like nitrogen and electroconductivity may play important roles for promoting H2O2 production. Fig. 8b shows the H2O2 concentration after 60 min of powering with its initial pH range from 3 to 9. In terms of H2O2 generation influenced by different pH, H2O2 yields has no significant change in the pH range of 3e7, nevertheless it decreased appreciably with the increase in initial pH from 7 to 9. It can be explained that a low pH is

3.3. H2O2 productivity and stability of NGC samples Since H2O2 accumulation is a crucial indicator for EF efficiency and is highly dependent on the characteristics of the cathode,

Fig. 7. (a) Nyquist plots of NGC samples; (b) The angle of impedance phase as a function of frequency of NGC samples. Operating conditions: pH ¼ 6, [Na2SO4] ¼ 0.05 mol/L, V ¼ 50 mL and current density ¼ 1 mA/cm2.

Fig. 8. (a) The concentration of H2O2 using NGC-samples; (b) The concentration of H2O2 using NGC samples at different pH. Operating conditions: pH ¼ 6, [Na2SO4] ¼ 0.05 mol/L, V ¼ 50 mL and current density ¼ 1 mA/cm2.


Fig. 9. The stability of NGC-900 sample in yielding H2O2. Operating conditions: pH ¼ 6, [Na2SO4] ¼ 0.05 mol/L, V ¼ 50 mL and current density ¼ 1 mA/cm2.

favorable for the production of H2O2 because the conversion of dissolved oxygen to H2O2 consumes protons in acidic solution, according to Eq. (1) [50]. In alkaline solutions H2O2 predominantly exists as HO1 2 and this ion, especially at pH > 7, can catalyze H2O2 decomposition [51]. To confirm the high efficiency for NGC cathode on H2O2 generation, carbon felt, the well-known carbonaceous cathode for high H2O2 production [52] was chosen as the comparison and H2O2 accumulation experiments were conducted under same conditions. It can be seen from Fig. S2 that both NGC-900 and NGC-850 could produce H2O2 87.19 mmol/L and 60.83 mmol/L, respectively, outweighing carbon felt with an 18.49 mmol/L H2O2. The abovementioned data confirmed the superior ability for this proposed new cathode. From the applications perspective, the stability of NGC-900 is an important issue which needs to be consider, especially for the new cathode. As seen from Fig. 9, the H2O2 concentration did not decreased significantly during the reuse of 10 times, indicating that the NGC-900 is stable and reusable in the pH of 6. The stability of NGC was further confirmed by morphology structure of the used electrode after reaction via SEM. The porous structure was very stable and almost unchanged, even after reacting in acidic solution (Fig. 10). All of these observations indicate the strong feasibility utilizing this novel cathode for EF system.

381

Fig. 11. The degradation of rhodamine B using NGC samples in EF system. Operating conditions: [Na2SO4] ¼ 0.05 mol/L, [Fe2þ] ¼ 20 mmol/L, [rhodamine B] ¼ 20 mg/L, V ¼ 50 mL and current density ¼ 1 mA/cm2.

3.4. Degradation of rhodamine B by EF process Fig. 11 represents the degradation of rhodamine B (a mode pollutant) using NGC samples in EF system. As it can be seen from Fig. 9 an approximately 97.5% rhodamine B degraded with NGC-900 as cathode with 60 min electrolysis at the pH value of 3. The rhodamine B degradation efficiency was higher than use the same cathode at pH value was 6, which indicating that ferrous ions in the solution was easy to oxidized and then precipitated. Meanwhile, the NGC-900 cathode exhibited excellent performance in removing rhodamine B, which was matched greatly well with the ability of producing H2O2. 4. Conclusions In summary, NGC electrode was obtained by the carbonization of PMF in the presence of nitrogen. SEM test shows that a highly porous carbon material was successfully fabricated. XRD and Raman indicate that the NGC samples were well graphitized. TGDSC illustrates that the NGC can be further decomposed until the nitrogen is released completely with the formation of stable graphitic carbon. XPS, CV, EIS and the yield of H2O2 suggest that the pyrrolic N structure, together with the improved electroconductivity were contributing to the electro-generation of H2O2

Fig. 10. SEM images of NGC-900 before and after the experiment.

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through the ORR. The degradation efficiency of rhodamine B further verified that the NGC electrode was suitable for EF cathode. This study provides an easily obtained, high efficiency, reusable and environmentally friendly cathode for applications in EF systems. Acknowledgements This project was supported by the National Key Research and Development Plan of China (No. 2016YFC0401102). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2017.12.122. References [1] S. Qiu, D. He, J. Ma, T. Liu, T.D. Waite, Kinetic modeling of the electro-fenton process: quantification of reactive oxygen species generation, Electrochim. Acta 176 (2015) 51e58. [2] T.X.H. Le, T.V. Nguyen, Z. Amadou Yacouba, L. Zoungrana, F. Avril, D.L. Nguyen, E. Petit, J. Mendret, V. Bonniol, M. Bechelany, S. Lacour, G. Lesage, M. Cretin, Correlation between degradation pathway and toxicity of acetaminophen and its by-products by using the electro-Fenton process in aqueous media, Chemosphere 172 (2017) 1e9. [3] E. Mousset, L. Frunzo, G. Esposito, E.D.V. Hullebusch, N. Oturan, M.A. Oturan, A complete phenol oxidation pathway obtained during electro-Fenton treatment and validated by a kinetic model study, Appl. Catal. B Environ. 180 (2016) 189e198. [4] E. Isarain-Ch avez, R.M. Rodríguez, J.A. Garrido, C. Arias, F. Centellas, P.L. Cabot, E. Brillas, Degradation of the beta-blocker propranolol by electrochemical advanced oxidation processes based on Fenton's reaction chemistry using a boron-doped diamond anode, Electrochim. Acta 56 (2010) 215e221. [5] F.C. Moreira, R.A.R. Boaventura, E. Brillas, V.J.P. Vilar, Electrochemical advanced oxidation processes: a review on their application to synthetic and real wastewaters, Appl. Catal. B Environ. 202 (2017) 217e261. [6] E. Bocos, O. Iglesias, M. Pazos, M. Angeles Sanrom an, Nickel foam a suis alternative to increase the generation of Fenton's reagents, Process Saf. Environ. 101 (2016) 34e44. ~ uelos, O. García-Rodríguez, A. El-Ghenymy, F.J. Rodríguez-Valadez, [7] J.A. Ban L.A. Godínez, E. Brillas, Advanced oxidation treatment of malachite green dye using a low cost carbon-felt air-diffusion cathode, J. Environ. Chem. Eng. 4 (2016) 2066e2075. [8] J. Tian, J. Zhao, A.M. Olajuyin, M.M. Sharshar, T. Mu, M. Yang, J. Xing, Effective degradation of rhodamine B by electro-Fenton process, using ferromagnetic nanoparticles loaded on modified graphite felt electrode as reusable catalyst: in neutral pH condition and without external aeration, Environ. Sci. Pollut. Res. 23 (2016) 15471e15482. chaud, N. Oturan, E. Mousset, D. Huguenot, E.D. van Hullebusch, [9] C. Trellu, Y. Pe G. Esposito, M.A. Oturan, Comparative study on the removal of humic acids from drinking water by anodic oxidation and electro-Fenton processes: mineralization efficiency and modelling, Appl. Catal. B Environ. 194 (2016) 32e41. ~ uelos, A. Rico-Zavala, L.A. Godínez, F.J. Rodríguez[10] O. García-Rodríguez, J.A. Ban Valadez, Electrocatalytic activity of three carbon materials for the in-situ production of hydrogen peroxide and its application to the electro-fenton heterogeneous process, Int. J. Chem. React. Eng. 14 (2016) 843e850. [11] H. Lan, W. He, A. Wang, R. Liu, H. Liu, J. Qu, C.P. Huang, An activated carbon fiber cathode for the degradation of glyphosate in aqueous solutions by the Electro-Fenton mode: optimal operational conditions and the deposition of iron on cathode on electrode reusability, Water Res. 105 (2016) 575e582. [12] Y. Gong, J. Li, Y. Zhang, M. Zhang, X. Tian, A. Wang, Partial degradation of levofloxacin for biodegradability improvement by electro-Fenton process using an activated carbon fiber felt cathode, J. Hazard Mater. 304 (2016) 320. [13] L. Ma, M. Zhou, G. Ren, W. Yang, L. Liang, A highly energy-efficient flowthrough electro-Fenton process for organic pollutants degradation, Electrochim. Acta 200 (2016) 222e230. rez, J. Llanos, C. Sa ez, C. Lo pez, P. Can ~ izares, M.A. Rodrigo, Electro[14] J.F. Pe chemical jet-cell for the in-situ generation of hydrogen peroxide, Electrochem. Commun. 71 (2016) 65e68. [15] Y. Wang, G. Zhao, S. Chai, H. Zhao, Y. Wang, Three-Dimensional homogeneous ferrite-carbon aerogel: one pot fabrication and enhanced electro-fenton reactivity, ACS Appl. Mater. Inter. 5 (2013) 842e852. [16] H. Zhao, L. Qian, X. Guan, D. Wu, G. Zhao, Continuous bulk FeCuC aerogel with ultradispersed metal nanoparticles: an efficient 3D heterogeneous electrofenton cathode over a wide range of pH 3e9, Environ. Sci. Technol. 50 (2016) 5225e5233. [17] V.V. Strelko, N.T. Kartel, I.N. Dukhno, V.S. Kuts, R.B. Clarkson, B.M. Odintsov, Mechanism of reductive oxygen adsorption on active carbons with various surface chemistry, Surf. Sci. 548 (2004) 281e290.

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