Anti-corrosion porous RuO₂/NbC anodes for the electrochemical oxidation of phenol

Abstract

Efficient anode materials with porous structures have drawn increasing attention due to their high specific surface area, which can compensate for the slow reaction rate of electrochemical oxidation. However, the use of these materials is often limited due to their poor corrosion resistance. Herein, we report a facile scale-up method, by carbothermal reduction, for the preparation of porous niobium carbide to be used as an anode for the electrochemical oxidation of phenol in water. No niobium ions were detected when the anodes were under aggressive attack by sulfuric acid and under electrochemical corrosion tests with a current density less than 20.98 mA cm⁻². The porous niobium carbide was further modified by applying a ruthenium oxide coating to improve its catalytic activity. The removal rates of phenol and chemical oxygen demand by the RuO₂/NbC anode reached 1.87 × 10⁻² mg min⁻¹ cm⁻² and 6.33 × 10⁻² mg min⁻¹ cm⁻², respectively. The average current efficiency was 85.2%. Thus, an anti-corrosion, highly catalytically active and energy-efficient porous RuO₂/NbC anode for the degradation of aqueous phenol in wastewater was successfully prepared.

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Introduction

Electrochemical methods are frequently used to treat wastewater that contains refractory organic compounds and offer many distinct advantages, such as environmental compatibility and high efficiency.^1–6 It has been determined that the electrochemical treatment of wastewater depends on a wide variety of factors, including anode materials, reaction conditions and waste characteristics.

Electrochemical oxidation has been reported with different anodes, including dense and porous materials. Dense materials, such as metal electrodes,^7–9 metal oxide electrodes (PbO₂, Bi₂O₅–PbO₅, IrO₂ and SnO₂),^10,11 boron-doped diamond (BDD) electrodes^12–15 and dimensionally stable anodes (DSA, i.e., Ti/IrO₂, Ti/RuO₂ and Ti/SnO₂),¹⁶ have attracted interest from many researchers because of their improved conductivity and catalytic activity. Porous electrodes, such as active carbon fibres,¹⁷ carbon nanotubes¹⁸ and other porous materials,^19–25 have been proposed as a result of their high specific surface areas, electrical conductivity and chemical resistance. The porous structures of the electrode also promote the mass transport and reaction rate in a flow-through electrolytic reactor.²¹ Therefore, porous anodes hold great promise in wastewater treatment. However, the main problem associated with the corrosion susceptibility of anode materials, especially the leaching of metal ions, is the risk of secondary pollution. Many researchers have coated noble metals on the surface of anodes, such as DSA materials, to increase their anti-corrosion performance. This method is not suitable for porous anodes because an excessively thick anti-corrosive layer can block the pores, and it is difficult for a thin layer to display anti-corrosion performance. Therefore, producing a self-anti-corrosion porous anode is an essential strategy to solve this problem. A homemade porous carbon membrane as the anode to remove organic pollutants by electrochemical oxidation was attempted early in our group.²⁶ The final effluent became yellow after 12 h, and the turbidity increased slightly due to the corrosion of the carbon membrane anode. We also studied the corrosion resistance of other anode materials, such as porous titanium, titanium modified by RuO₂ or Ta₂O₅ and substoichiometric TiO₂ anodes.^26–30 Although the modified anode materials exhibited superior corrosion resistance compared with that of the pristine anodes, none of these porous anodes possessed ideal corrosion resistance properties. Therefore, the development of an anti-corrosion porous anode is desired to overcome this challenge.

Transition metal carbides are well known for their extremely high hardness and high melting points. These materials are primarily used as cutting tools and thin-film coatings of microcircuit devices. Transition metal carbides also show high catalytic activities that are similar to those of noble metal catalysts, such as Pt, Pd and Ir.¹⁹ Niobium carbide (NbC) is a compound with a high melting point (>3500 °C) and high resistance to chemical attack or thermal shock, which makes it useful for high-temperature and high-stress engineering applications.³¹ However, there are few studies on the electrocatalytic properties of NbC.

Recently, NbC was synthesized via a few methods, including the direct reaction of Nb with carbon,³² gas–solid reactions between Nb and carbon with iodine as catalysts,³³ the carbothermal reduction of metal oxides at high temperatures³¹ and magnetron sputtering.³⁴ However, all of these methods are mainly used for the preparation of bulk NbC.

The aim of the present work was to develop a simple scale-up method to synthesize anti-corrosion, conductive, porous NbC anodes by carbothermal reduction of metal oxides at medium temperatures. The obtained porous NbC anodes showed excellent corrosion resistance and high electrocatalytic activity for phenol degradation. Moreover, the porous RuO₂/NbC anode showed high catalytic activity and high current efficiency.

Experimental

Electrode preparation

Niobium pentaoxide nanoparticle (nano-Nb₂O₅) powders were synthesized using the combined citrate acid and ethylenediaminetetraacetic acid (EDTA) complexing method.³⁵ Pre-weighted niobium oxalate was first dissolved in an EDTA–NH₃·H₂O solution under heating and stirring. After stirring for a certain time, citric acid was introduced, and the mole ratio of EDTA : citric acid : total metal ions was controlled to be approximately 1 : 1.5 : 1. Since precipitation might occur after the addition of citric acid, NH₃·H₂O was added to adjust the pH value to ∼6, and the solution became transparent immediately. With the evaporation of water, a dark brown gel was obtained. The gel was then heated at 140 °C for 48 h to obtain a loose and porous black powder, which was further sintered at 650–1000 °C for 1 h at a heating rate of 1 °C min⁻¹ to obtain the white Nb₂O₅ powder. The prepared powders mixed with a small amount of carboxymethyl cellulose were pressed into a stainless steel mould (20 mm in diameter) under a pressure of 20 MPa for 30 s. These white discs were sintered at 850 °C for 2 h in air atmosphere and then carbothermally reduced at a certain temperature for 8 h (at a heating rate of 2 °C min⁻¹ and in a 10% methane/argon atmosphere (CH₄/Ar)) to obtain the porous NbC anode.

To improve the catalytic activity, the porous NbC anode was modified with RuO₂. The NbC anode was immersed in a ruthenium chloride solution (5%) in a hot reflux system for 4 h, and then the air-dried samples were heat treated at 400 °C for 2 h (under an Ar atmosphere) in a box furnace to prepare a ruthenium oxide-modified porous NbC anode. By repeating the modification process, different amounts of RuO₂ were coated.

Electrochemical removal of phenol

In the undivided cell, the porous NbC anode and platinum (Pt) were used as the anode and cathode, respectively. The active dimension of the anode (4.5 cm²) was equal to that of the cathode. A saturated calomel electrode (SCE) was chosen as the reference electrode. A Luggin capillary connected the reservoir, thus connecting the reference electrode with the main electrolyte compartment. The synthetic phenol wastewater contained 200 mL phenol solution at an initial concentration of 100 mg L⁻¹. Additionally, 0.1 M sodium sulfate (Na₂SO₄) was used as the electrolyte. An electrochemical workstation (CHI660E; Shanghai Chenhua Apparatus Co., China) supplied a constant current.

Corrosion resistance experiments

Electrochemical corrosion resistance was evaluated in the same way as performing the electrochemical removal of phenol. For the chemical corrosion resistance test, we put the porous NbC anode in 200 mL of 50 wt% sulfuric acid (70 °C) for 9 h. The corrosion resistance of the porous materials was evaluated by the content of Nb ions in solution, which was analysed by UV-Vis spectroscopy (UV2400, Shanghai Shunyuhengping Co., China). As reported in the literature, the UV-Vis spectroscopy method is a stable, facile and reliable method for detecting Nb ions in water.^36–38

Analysis

The crystal structures of the nano-Nb₂O₅ powder and porous NbC anode were characterized by X-ray diffraction (XRD; LabX XRD-6000). The surface and cross-section morphologies of the porous NbC anode were investigated by scanning electron microscopy (SEM; Quanta FEG 250). The atomic ratio of Ru was detected by an energy spectrum analyser (EDX, INCA Energy 250). The elemental contents of the electrode surface were analysed by X-ray photoelectron spectra (XPS, Thermo Scientific Escalab 250Xi).

Cyclic voltammetry (CV) experiments, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements were carried out at room temperature using an electrochemical workstation with three electrodes in a 0.1 M Na₂SO₄ solution. The porous NbC anode (4.5 cm²) and flat Pt were used as the working electrode and counter electrode, respectively. An SCE was chosen as the reference electrode.

After each electrocatalysis process, the phenol in solution was tested by UV-Vis spectroscopy on a spectrophotometer. The chemical oxygen demand (COD) was measured using a COD reactor (HACH) and a spectrophotometer. The average current efficiency (ACE) for the anodic oxidation of phenol was calculated from the COD:

where COD₀ and COD_t are the concentrations of the chemical oxygen demand (mg L⁻¹) at the initial time and time t (h), I is the current (A), F is the Faraday constant (96 485 C mol⁻¹), V is the volume of electrolyte (L), and 8 is the equivalent mass of oxygen (g eq.⁻¹).

Results

Characterization of the porous NbC electrode

The structural and phase formation analyses of Nb₂O₅ and NbC at different calcination temperatures were performed by XRD. The XRD pattern resembled that of orthorhombic Nb₂O₅ below 800 °C according to the Joint Committee on Powder Diffraction Standards (JCPDS) number 28-0317 (Fig. 1a). Interestingly, the phase changed to monoclinic Nb₂O₅ after a medium temperature treatment (1000 °C). Fig. 1b shows the XRD patterns of the porous NbC anode prepared under a 10%

Fig. 1 XRD patterns of Nb₂O₅ nanoparticles prepared under different temperatures (a) and porous flat NbC prepared under different temperatures and an atmosphere of 10% CH₄/Ar (b), SEM images of the surface of porous NbC (c) and a cross-section of porous NbC (d).

CH₄/Ar atmosphere at different temperatures. These well-defined diffraction peaks at 35°, 40°, 58°, 69°, 73° and 75° correspond to the (111), (200), (220), (311), (222) and (400) lattice planes of NbC (JCPDS no. 89-3690), respectively, when the temperatures were higher than 1100 °C. The obtained NbC showed a well-crystallized structure. The SEM images of porous NbC are shown in Fig. 1c and d. The particles were sintered and connected to form a porous and smooth surface with a pore size of 1–2 μm, which correspond with the pore distribution results (Fig. S1†) tested by bubble-point method. The cross-section of NbC (Fig. 1d) displayed an abundance of pores resulting in a high surface area. Fig. 2a displays the C1s XPS peak of the porous NbC anode. Two deconvoluted peaks were observed in the spectrum. The strong peak centred at 284.8 eV was assigned to the carbon component in NbC (C–Nb bonds).³⁹ The peak at 286.3 eV was identified as carbon in the amorphous matrix (C–C bonds). The XPS results indicate that there is some amorphous carbon in the obtained NbC.³⁴ For the NbC, two apparent peaks at 207.2 eV and 204.3 eV were assigned to Nb 3d3/2 and Nb 3d5/2 of Nb, respectively (Fig. 2b).⁴⁰ Therefore, we obtained porous NbC through an easy scale-up method.

Fig. 2 XPS spectra of NbC. C 1s (a) and Nb 3d (b).

Corrosion resistance of the porous NbC electrode

We detected the concentration of Nb ions in the solution at different time intervals when we used a porous NbC anode at different current densities. For comparison, we also examined the corrosion resistances of commercial substoichiometric TiO₂. The Nb ions were not detected in the solution after 9 h when the current density was lower than 20.98 mA cm⁻² (Table 1). The concentration of Nb ions increased from 3.96 to 58.25 μg L⁻¹ when the corrosion time increased from 1 h to 9 h at the applied current density of 100 mA cm⁻². However, the concentration of Ti ions increased from 7.05 μg L⁻¹ to 216.66 μg L⁻¹ when the corrosion time increased from 1 h to 9 h at an applied current density of 20.98 mA cm⁻². These results imply that the porous NbC shows better anti-corrosion capacity. To evaluate the chemical corrosion resistance, we put the porous NbC anode in a solution of 50 wt% sulfuric acid (70 °C) for 9 h, and no metal ions were detected in the solution. Therefore, the porous NbC showed promising corrosion resistance.

Table 1 Concentration of metal ion in the solution after the electrochemical corrosion with the porous NbC and commercial electrode materials (substoichiometric TiO₂) as the anodes

Electrode	Current density (mA cm^−2)	Time (h)	Concentration of metal ion (μg L^−1)
Porous NbC	0	9	0
	2.01	9	0
	8.77	9	0
	20.98	9	0
	100.00	1	3.96
	100.00	3	27.97
	100.00	5	31.79
	100.00	7	47.74
	100.00	9	58.25
Substoichiometric TiO2	20.98	1	7.05
		3	40.51
		5	75.74
		7	147.97
		9	216.66

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Electrochemical removal of phenol

In Fig. 3, the cyclic voltammogram curve for the 100 mg L⁻¹ phenol solution showed an obvious current increase at 0.4 V versus SCE. This result indicated that phenol could be electrochemically oxidized at a very low potential. No obvious peak could be seen from the CV curves because of the characteristics of carbon materials themselves.⁴¹ The porous NbC anode had an oxygen evolution potential higher than 1.5 V versus SCE when the anode current at 1.5 V was 13.93 mA cm⁻² (Fig. 3), which suggested that the electrochemical catalytic performance of porous NbC was slightly higher than that of other DSAs.⁴² The higher oxygen evolution potential is desirable because the low probability of hydroxyl radical reactions to form molecular oxygen favours organic oxidation by hydroxyl radicals,⁴³ and anodic oxygen evolution also represents an unwanted power loss and reduces the overall current efficiency.⁴⁴

Fig. 3 The CV curves of porous flat NbC using a 0.1 M Na₂SO₄ electrolyte with and without 100 mg L⁻¹ phenol.

When we used NbC as the anode, the maximum phenol, COD removal and ACE values were 82.62%, 31.08% and 51.62%, respectively, when the current density and electrode distance were 20 mA cm⁻² and 4 mm with 100 mg L⁻¹ phenols (Fig. S2†). This result indicated that the mineralization rate (31.08%) was still relatively low even if the porous NbC showed better phenol removal (82.62%).To improve the mineralization rate of NbC, RuO₂ was used to modify the porous NbC. As shown in Fig. 4a and b, the phenol removal, COD removal and ACE reached maximum values of 100.00%, 71.30% and 85.22%, respectively, when the anode was modified with RuO₂ three times. Compared with the values of porous NbC (Fig. S2†), the electrocatalytic activity of the modified anode (RuO₂/NbC electrode) was dramatically improved (Fig. 4).

Fig. 4 The effects of the number of RuO₂ coating times on phenol removal (a), COD removal and ACE at 8 h (b) with a current density of 20 mA cm⁻² and an initial phenol concentration of 100 mg L⁻¹.

Overall, we may find it difficult to evaluate the actual electrocatalytic performance of porous RuO₂/NbC electrodes in this work compared with different electrodes reported in other literature because of the different reactors used. The pollutant removal rate (per unit time and electrode area) was reasonably used to evaluate the electrode electrocatalytic activity and reactor efficiency. Our main results (shown by the removal rate) compared with other literature values can be seen in Table 2. The phenol and COD removal rates by the RuO₂/NbC anode were 1.87 × 10⁻² mg min⁻¹ cm⁻² and 6.33 × 10⁻² mg min⁻¹ cm⁻², respectively; these results along with the high ACE value of 85.22% mean that the porous RuO₂/NbC electrode showed an excellent mineralization rate and an excellent current efficiency compared with other electrodes. These results clearly confirm that the RuO₂/NbC electrode is a promising anti-corrosion anode for the electrochemical oxidation of organic pollutants in water.

Table 2 Comparisons phenol and COD/TOC removal rate and ACE in this work with those in the literates

Compound	Anode materials	Concentration /electrolyte	Current density (mA cm^−2)	Initial concentration (mg L^−1)	Compound removal rate (mg min^−1 cm^−2)	COD removal rate (mg min^−1 cm^−2)	TOC removal rate (mg min^−1 cm^−2)	ICE (%)	ACE (%)	MCE (%)	Reference
a The values are calculated by the given values in the literature, and the calculation equation can be seen in the ESI.
Phenol	RuO2/NbC	0.10 M/Na2SO4	19.7	100.00	1.87 × 10^−2	6.33 × 10^−2	—	—	85.2	—	This work
Phenol	TiO2/RuO2/Pt	0.05 M/NaCl	20.0	8.00	1.60 × 10^−3^a	—	4.08 × 10^−3^a	∼40	—	25.52^a	62
Phenol	Ti/SnO2–Sb2O3–Nb2O5/PbO2	7.50 g L^−1/NaSO4	20.0	500.00	6.06 × 10^−2^a	2.66 × 10^−2^a	—	∼24.00	26.76^a	—	63
Phenol	BDD nanowire	0.50 M/Na2SO4	30.0	94.00	9.50 × 10^−3^a	2.00 × 10^−2^a	6.56 × 10^−3^a	∼60.00	13.42^a	13.68^a	64
Phenol	β-PbO2	1.00 g L^−1/K2SO4	27.5	100.00	3.46 × 10^−2^a	—	—	<5.00	—	—	65
Phenol	Ti/SnO2/Pb	0.25 M/Na2SO4	20.0	490.00	2.18 × 10^−2^a	—	5.21 × 10^−3^a	—	—	52.09^a	43
Phenol	Ti/MnOx	0.10 M/Na2SO4	2.5	100.00	2.14 × 10^−3^a	—	9.94 × 10^−4^a	—	—	24.86^a	66
Phenol	MnO2/Nano-G|foam-Ni/Pd	0.10 M/Na2SO4	39.0	100.00	5.14 × 10^−3^a	—	3.40 × 10^−3^a	—	—	5.45^a	67
Phenol	Nb/BDD	0.20 M/KH2PO4 + 100 mM/NaCl	6.46	50.00	2.69 × 10^−2^a	—	1.23 × 10^−3^a	—	—	11.95^a	68
Tetracycline	Ti/Ti4O7	0.03 M/Na2SO4	15.0	5.00	4.69 × 10^−4^a	—	2.20 × 10^−4^a	—	—	9.90	1
Tetracycline	BDD	5.00 g L^−1/Na2SO4	300.0	150.00	6.25 × 10^−4^a	1.00 × 10^−3^a	3.23 × 10^−4^a	—	2.50^a	0.06^a	69
Tetracycline	BDD	0.05 M/Na2SO4	20.8	100.00	2.89 × 10^−3^a	—	1.68 × 10^−3^a	—	—	4.44^a	70
Tetracycline	Ti/SnO2–Sb	0.01 M/Na2SO4	25.0	5.00	1.13 × 10^−3^a	—	5.69 × 10^−4^a	—	—	1.25^a	71
Tetracycline	porous Ti4O7	0.10 M/Na2SO4	0.5	10.00	1.57 × 10^−4^a	—	3.90 × 10^−5^a	—	—	4.28^a	72
Tetracycline	Ti/RuO2–IrO2	0.10 M/Na2SO4	47.6	200.00	9.21 × 10^−3^a	—	—	—	—	—	73
Tetracycline	Ti/RuO2–IrO2	0.10 M/Na2SO4	565.0^a	200.00	2.21^a	0.60^a	—	—	21.23^a	—	74
Benzoic acid	BDD	0.05 M/Na2SO4	352.9^a	100.00	5.71 × 10^−2^a	0.14^a	—	22.00	4.42^a		75
Benzoic acid	TiO2-NTs/SnO2	0.10 M/Na2SO4	14.7	140.00	9.18 × 10^−3^a	1.44 × 10^−2^a	—	26.40	19.67^a	—	76
4-Chloro-3-methyl phenol	Ti/SnO2–Sb/PbO2	0.25 M/Na2SO4	10.0	99.81	2.77 × 10^−3^a	3.08 × 10^−3^a	8.33 × 10^−4^a	6.50	6.19^a	—	77
4-Chlorophenol	Er–PbO2 electrode	0.20 M/Na2SO4	20.0	128.56	9.52 × 10^−3^a	1.29 × 10^−2^a	—	16.00	12.98^a	—	78
2,4-Dichlorophenol	Ti/SnO2–Sb	0.01 M/Na2SO4	30.0	100.00	1.39 × 10^−3^a	—	5.69 × 10^−4^a	—	—	—	79
Procion blue dye	Ti/RuOx–TiOx	—	40.0	—	—	7.19 × 10^−2^a	—	—	36.13^a	—	80
Methyl orange	SnO2/TiO2-NTs/Ti	0.10 M/Na2SO4	30.0	500.00	1.85 × 10^−2^a	—	5.18 × 10^−3^a	6.00	—	11.73^a	81
Methyl orange	Ti/Ir–Pb	0.05 M/Na2SO4	70.0	50.00	2.92 × 10^−3^a		1.10 × 10^−3^a	—	—	1.67	82
Bisphenol A	Nb/BDD	0.10 M/Na2SO4	6.5	150.00	—	1.89 × 10^−2^a	—	58.00	58.53^a	—	83
Bisphenol A	Ti/SnO2–Sb2O5/PbO2	0.02 M/Na2SO4	40.0	22.83	—	8.78 × 10^−3^a	—	—	4.41^a	—	84
Bisphenol A	BDD	0.04 M/NaCl	15.0	13.70	5.71 × 10^−2^a	—	3.51 × 10^−3^a	—	—	33.59	85
o-Nitrophenol	Ti/Bi–PbO2	0.10 M/Na2SO4	30.0	50.00	2.50 × 10^−2^a	2.88 × 10^−2^a	—	—	31.49	—	86
o-Nitrophenol	Ti/Bi–PbO2	0.10 M/Na2SO4	30.0	50.00	2.50 × 10^−2^a	—	3.92 × 10^−2^a	75.60^a	—	58.29^a	87
Atrazine	Ti/Ru0.3Ti0.7O2	0.10 M/NaCl	50.0	20.00	5.34 × 10^−3^a	—	1.33 × 10^−3^a	—	—	3.12^a	88
Atrazine	BDD	0.03 M/Na2SO4	18.2	0.10	1.06 × 10^−4^a	—	1.44 × 10^−6^a	—	0.10	0.26^a	89

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Discussion

Analysis of phenol removal on a porous RuO₂/NbC electrode

The high electrocatalytic activity of the RuO₂/NbC anode may be attributed to the interface coordination effect between the porous NbC support and the RuO₂ coating. The abundant Lewis acid sites on RuO₂ particles are more likely to combine hydroxyl radicals generated by electrochemical oxidation processes to form Lewis acid–base pairs,⁴⁵ which can oxidize organics by direct oxidation mechanisms.⁴⁶ Oxidized ruthenium ions on the surface may display a rather strong Lewis acidity.⁴⁷ NbC is a well-known interstitial compound with a high density of states (DOS) at the Fermi level. This is correlated with d-band contraction caused by the expansion of the metal crystal lattice and the increase in the intermetal distance.⁴⁸ The adsorbed substance (hydroxyl radicals) on the surface of NbC can influence the charge transfer dramatically.^49,50 The electrodes coated by metal oxide, such as the RuO₂ modified electrode, possessed various redox pairs or Lewis acid–base pairs in the potential range between the hydrogen and oxygen gas evolution regions.^51,52 The NbC support may transfer electron charge to RuO₂ and result in redox pair formation, which is the main factor producing hydroxyl radicals.^53,54 Strong adsorption between aromatic species and transition metal oxides due to weak π bonding is a dominant factor determining electrocatalytic activity. Therefore, more phenol can be adsorbed on the RuO₂.⁵⁵ Additionally, the porous structure of NbC provided more surface area for the RuO₂ coating, which can increase the electrocatalytic activity.

Kinetic analysis of phenol removal on the porous RuO₂/NbC electrode

The removal of phenol on the NbC anode followed apparent pseudo-first-order kinetics according to the following rate equation:where C is the concentration of phenol at time t (h), while the kinetic parameters (k: the pseudo-first-order rate constant, t_1/2: half-life (h)) can be calculated by plotting ln(C₀/C_t) versus t (Fig. 5).⁵⁶ In general, the phenol removal had positive linear correlations with k under all the experimental conditions (R² = 0.87–0.98). The values of k and t_1/2 are shown in Table 3. Compared with NbC, the electrode that was modified three times showed a decrease in the t_1/2 value from 5.67 to 1.49 h, while the k values increased from 0.14 to 0.80 h⁻¹, indicating faster degradation efficiencies at the RuO₂/NbC anode.

Fig. 5 First-order kinetics for the phenol removal by RuO₂/NbC and NbC anodes.

Table 3 Pseudo-first-order rate constants for removal phenol

Modified times by RuO2	Phenol removal (%)	COD removal (%)	ACE (%)	Reaction rate constant (h^−1)	Half-life (t1/2, h)	R^2
0	68.22	20.64	34.29	0.14	5.67	0.91
1	91.96	59.50	49.00	0.34	2.60	0.96
2	99.25	66.74	69.14	0.60	1.76	0.93
3	100.00	71.30	85.22	0.80	1.49	0.87
4	98.69	64.90	57.87	0.54	1.65	0.98

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These results were consistent with the changes in phenol removal (from 68.22% to 100%), COD removal (from 20.64% to 71.30%) and ACE (from 34.29% to 85.22%), which implies that the RuO₂/NbC anode shows a higher mineralization rate and current efficiency. It is worth mentioning that the k value showed little change when the coating was modified four times, implying that a thicker RuO₂ layer may block the holes in porous NbC and decrease the surface area. This result was possibly attributed to the limited number of catalytically active sites in the porous RuO₂/NbC anode.

The corrosion resistance of the porous NbC electrode

The corrosion resistance of porous anodes is always low, which limits the application of anodes for industrial wastewater treatment. According to some studies,^57–59 the most frequently used method to solve the above-mentioned problem is slurry coating sintering, which is suitable for dense materials except for porous materials. However, this method may block the pores and reduce the surface area, which would limit the reactor efficiency. Therefore, the development of an anti-corrosion support is a promising strategy. We present an anti-corrosion porous NbC anode with higher electrocatalytic properties than those of other anodes. To understand the anti-corrosion properties of the NbC electrode, an EIS experiment was conducted. The impedance spectra for the NbC anode and RuO₂/NbC anode in 0.1 M Na₂SO₄ solution are presented as Nyquist plots in Fig. 6. The Nyquist plots (Fig. 6a) showed a single depressed semicircle, and the diameter of the semicircle increased after modifying NbC with RuO₂. The results indicated that the RuO₂ coating can slightly improve the corrosion protective properties, in which the RuO₂/NbC electrode showed the best performance.⁶⁰ The Bode plots shown in Fig. 6b and c also help explain these results. Potentiodynamic polarization measurements were carried out immediately after the EIS experiments. Fig. 6d shows the polarization curves of the RuO₂/NbC and NbC anodes in 0.1 M Na₂SO₄ solution. The electrochemical parameters can be summarized in Table 4, including the corrosion current density (I_corr), corrosion potential (E_corr), anodic Tafel slopes (β_a) and cathodic Tafel slopes (β_c). Overall, a lower I_corr and a higher E_corr mean better corrosion protective properties.⁶¹ The I_corr of the RuO₂/NbC anode was 4.07 μA cm⁻², which was lower than that of the bare NbC anode (25.40 μA cm⁻²). Moreover, the E_corr of the RuO₂/NbC anode (−0.74 V) was more positive than that of the bare NbC (−0.81 V). These results mean that the RuO₂/NbC anode displayed superior anti-corrosion properties compared with the bare NbC anode. According to the values shown in Table 1, the porous NbC electrode presented better anti-corrosion properties than those of substoichiometric TiO₂. Therefore, the porous RuO₂/NbC anode combines superior corrosion resistance and high electrocatalytic activity, making it a preferred anode material for the electrochemical oxidation of pollutants.

Fig. 6 Nyquist plots (a), Bode plots (b, c), and Tafel plots (d) of the NbC substrate and RuO₂/NbC in 0.1 M Na₂SO₄.

Table 4 Dynamic parameters of electrodes immersed in 0.1 M Na₂SO₄ solution

Sample	Ecorr (V)	Icorr (μA cm^−2)	βc (mV dec^−1)	βa (mV dec^−1)
NbC	−0.81	25.40	17.00	−10.00
RuO2/NbC	−0.74	4.07	2.74	−6.13

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Conclusions

We developed a simple scale-up method for the preparation of anti-corrosion porous NbC anodes by the carbothermal reduction of porous Nb₂O₅. The resultant porous anodes showed good performance in various corrosion tests. By modifying RuO₂, the porous RuO₂/NbC anode exhibited high electrocatalytic activity and high current efficiency in the electrochemical oxidation of phenol. The achieved phenol and COD removal rates were 1.87 × 10⁻² mg min⁻¹ cm⁻² and 6.33 × 10⁻² mg min⁻¹ cm⁻², respectively, with the ACE value reaching 85.22%. These promising results pave a way to develop porous anodes for the electrochemical oxidation of organic waste.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51678019).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra03353j

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