Homologous Co₃O₄‖CoP nanowires grown on carbon cloth as a high-performance electrode pair for triclosan degradation and hydrogen evolution

Abstract

As a widely used antimicrobial agent in toothpastes and body washes for decades, triclosan (TCS) is causing great harm to the environment through wastewater. Herein, we used a two-electrode system to remove TCS comprising Co₃O₄ nanowires/carbon cloth (Co₃O₄ NWs/CC) as anode and CoP nanowires/carbon cloth (CoP NWs/CC) as cathode. At a constant current density of 10 mA cm⁻², the degradation efficiency for TCS is 95% in 60 min and it is transformed into important chemical raw materials, including phenol, 1,2-dihydroxybenzene, and 2-phenoxyphenol. Subsequently, hydrogen is generated on the CoP NWs/CC cathode with Faradaic efficiency of nearly 100%. The synergetic effect of the superhydrophilic surfaces of the Co₃O₄ NWs/CC anode and the CoP NWs/CC cathode as well as the superaerophobicity of the cathode assures full contact of the electrolyte solution and the active materials and high-efficiency generation of H₂ bubbles.

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Wei Zhou

Dr Wei Zhou received her PhD degree in Materials Science and Engineering from Beihang University under the supervision of Prof. Lin Guo in 2009. She once worked as a visiting scholar in Prof. Hong Yang's group in University of Illinois at Urbana-Champaign (UIUC) in 2012–2013. Currently, she is a professor at School of Chemistry in Beihang University. Her research interests focus on iron triad (Fe, Co, Ni) and their compounds’ nanomaterials for applications in energy, adsorption and catalysis.

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Introduction

As an antimicrobial agent, triclosan (TCS, 5-chloro-2-(2,4-dichlorophenoxy)-phenol) has been added to many consumer products, such as toothpastes, soaps, body washes, detergents, and surgical cleaning treatments, for decades.¹ It has been reported that over 100 tons of TCS enters municipal wastewater treatment plants in the U.S. every year, about half of which remains in the streams with little removal under anaerobic digestion.² Unfortunately, mass residue has been detected in effluents, biosolids, and even in human breast milk.³ Recently, the U.S. Food and Drug Administration (FDA) decided that TCS cannot be treated as GRAS for use in consumer antiseptic wash products due to lack of sufficient information.⁴ Moreover, the potential risk for the environment worries many people as TCS can transform into the more toxic and persistent pollutant, viz., chlorodioxin.⁵ Thus, it is urgent to develop an efficient TCS degradation method for wastewater treatment.

Various methods have been used to degrade TCS, including ozonation, biological treatment, photolysis, oxidizer oxidation, and electrochemical processes. Among them, the ozonation technique, biological treatment, and photolysis technique are hindered by high cost, prolonged degradation time, and secondary pollution.^6–10 The chemical oxidants, such as KMnO₄, K₂FeO₄, and S₂O₈²⁻, can degrade TCS effectively, but they make the treated water coloured and generate sludge.^11–14 The simple and flexible electrochemical process has attracted much attention as it is low cost and highly efficient.^15,16 For instance, Niu et al. fabricated the Ti/SnO₂–Sb/Ce–PbO₂ anode and obtained a high degradation rate of 99.9% during 5 min at 2–10 mA cm⁻² with pH = 3–11.¹⁵ However, some chlorinated intermediates including 2,4-dichlorophenol, 5-chloro-3-(chlorohydroquinone)phenol, and 2-chloro-5-(2,4-dichlorophenoxy)benzene-1,4-diol were produced, which might cause reproductive toxicity, neurotoxicity, and endocrine system disturbance. Alternatively, Wang et al. used a boron doped diamond film electrode and TCS was completely oxidized to CO₂ at current density above 2 mA cm⁻² with increase in electrolysis time.¹⁶ Therefore, it is very important to develop suitable electrodes to achieve high-efficiency degradation with intermediates of nonchlorinated organic compounds.^16,17

Moreover, electrocatalytic water splitting has been recognized as an ideal approach to produce clean H₂ fuel to replace fossil fuels.¹⁸ However, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) of water splitting require efficient electrocatalysts to reduce the electrical energy consumption.¹⁹ At present, some transition metal phosphides, sulphides, and selenides have been reported as having promising HER catalytic performances,^20–22 while some transition metal oxides, nitrides, and hydroxides exhibit attractive OER activities.^23–25 Herein, we choose homologous cobalt compounds as electrocatalysts to realize energy-saving hydrogen production at cathode and TCS degradation at anode. Moreover, the electro-oxidation of TCS might reduce the bottleneck of the OER process, similar to the reported addition of methanol, ethanol, glycerol, hydrazine, and urea.^26–30 Thus far, the bi-functional system has not been reported using homologous cobalt compounds as electrode materials. Notably, the degradation products of TCS are valuable chemical raw materials, including phenol, 1,2-dihydroxybenzene, and 2-phenoxyphenol, instead of chlorinated products.

Results and discussion

The precursor of Co(CO₃)_0.5OH·0.11H₂O nanowires (NWs) in diameters of about 100–200 nm with an orthorhombic structure was prepared on carbon cloth (precursor/CC) by a hydrothermal method (Fig. S1, ESI†).³¹ The corresponding CoP and Co₃O₄ NWs on carbon cloth (CC) with similar morphologies were obtained through low-temperature phosphidation and heat treatment processes, respectively. Their components were confirmed by XRD patterns and EDX spectrum shown in Fig. S2 and S3 (ESI†). The morphological and structural characterization of CoP NWs/CC and Co₃O₄ NWs/CC are shown in Fig. 1. As shown in Fig. 1a and c, the CoP and Co₃O₄ microflowers assembled by NWs ∼100 nm in diameter were grown uniformly on CC. Single CoP and Co₃O₄ NWs composed of numerous tiny particles were confirmed by TEM as shown in Fig. 1b and d, respectively. Their corresponding HRTEM images showed lattice spacings of 0.23 and 0.29 nm, corresponding to the (201) planes of CoP and the (220) planes of Co₃O₄. In addition, the sample of Co₃O₄ octahedron/CC (Co₃O₄ Oct/CC) was prepared for comparison by a hydrothermal method (see Experimental) with related characterizations in Fig. S4 (ESI†).

Fig. 1 (a) SEM image of CoP NWs/CC with magnified CoP NWs inseted. (b) TEM image of a single CoP NW with corresponding HRTEM inseted. (c) SEM image of Co₃O₄ NWs/CC with magnified Co₃O₄ NW insetted. (d) TEM image of a single Co₃O₄ NW with corresponding HRTEM image insetted.

Fig. 2 shows the X-ray photoelectron spectroscopy (XPS) results of the CoP (Fig. 2a and b) and the Co₃O₄ (Fig. 2c and d). As shown in Fig. 2a, the peaks at 778.4 and 793.7 eV could be assigned to Co 2p_3/2 and Co 2p_1/2 from CoP, while those at 781.1 and 796.6 eV could be indexed to Co 2p_3/2 and Co 2p_1/2 from oxidized Co species.^32,33 The oxidized Co might be arising from superficial oxidation due to contact with air.³³ In Fig. 2b, the P 2p signal showed two main peaks at 129.7 and 130.8 eV, corresponding to P 2p_3/2 and P 2p_1/2 from CoP, while the broad peak at 134.2 eV could be assigned to oxidized phosphate species on the surface of the sample.³⁴ The XPS results of CoP/CC were in agreement with the XRD pattern (Fig. S2, ESI†), despite slight oxidation of the surface. Fig. 2c shows the Co 2p XPS spectra of the Co₃O₄ sample. The peaks at 779.8 and 795.0 eV could be assigned to Co(II), while the peaks at 781.5 and 796.8 eV to Co(III), which implies the formation of Co₃O₄.³⁵Fig. 2d shows the XPS spectra of the O 1s, which could be deconvoluted into three peaks. The smaller binding energy peak at 529.5 eV could be ascribed to Co–O species,³⁵ while the other two peaks located at 530.9 and 532.0 eV are consistent with the adsorbed oxygen species.³⁶ The XPS results of Co₃O₄/CC were in accordance with the XRD characterization (Fig. S3, ESI†).

Fig. 2 High-resolution XPS spectra of (a) Co 2p and (b) P 2p for CoP. High-resolution XPS spectra of (c) Co 2p and (d) O 1s for Co₃O₄.

We then examined the HER activity of the CoP NWs/CC with a scan rate of 2 mV s⁻¹ using a three-electrode electrochemical station in 0.5 M H₂SO₄ solution (pH = 0), 1 M PBS buffer solution (pH = 7), and 1 M KOH solution (pH = 14), separately. Fig. 3a shows the linear sweep voltammetry (LSV) curves vs. the reversible hydrogen electrode (RHE). The overpotential for a current density of 10 mA cm⁻² is an important parameter for OER because it represents the current density from a realistic device with 12% solar to hydrogen efficiency.³⁷ As shown in Fig. 3a, the overpotentials at the current density of −10 mA cm⁻² were 118 mV (pH = 0), 130 mV (pH = 7) and 69 mV (pH = 14). According to the results, the CoP NWs/CC showed best HER performance in alkaline condition. The HER kinetics of the CoP NWs/CC were examined by Tafel plots (η vs. log|j|) derived from LSV. The linear regions of the Tafel plots were fitted via the Tafel equation (η = b × log|j| + a, where η is the overpotential, b is the Tafel slope, and j is the current density). Fig. 3b shows the Tafel slopes of 73, 144 and 70 mV dec⁻¹ for CoP NWs/CC at pH values of 0, 7 and 14, respectively. Recently reported Co-based HER catalysts in solution with pH = 0, 7, and 14 are summarized in Table S1 (ESI†). The HER performance of CoP NWs/CC in H₂SO₄ solution (pH = 0) and PBS buffer solution (pH = 7) was comparable to some reported Co-based catalysts. However, the HER performance of the CoP NWs/CC in KOH solution (pH = 14) with overpotential of 69 mV was superior to most reported Co-based catalysts (overpotentials higher than 100 mV). It is well known that high stability and durability are necessary for the application of electrocatalysts. Thus, chronopotentiometry tests at a current density of −10 mA cm⁻² were performed to confirm the stability of the CoP NWs/CC (Fig. 3c). The CoP NWs/CC showed initial potentials of −0.32, −0.74, and −1.09 V with retention of 98.6%, 95.5%, and 98.4% after 24 h at pH = 0, 7, and 14, respectively. Considering the overpotential, Tafel slope and stability, the CoP NWs/CC exhibited better HER performance in KOH solution than in H₂SO₄ solution and PBS buffer solution. Fig. 3d shows the overpotential and potential retention of our study in comparison with other reported Co-based catalysts in KOH solution. The details are also summarized in Table S1 (ESI†). From data shown in Fig. 3d and Table S1 (ESI†), it could be inferred that the CoP NWs/CC exhibited excellent HER performance with lower overpotential of 69 mV and higher potential retention of 98.4% than other catalysts,^38–44 which may be attributed to the superhydrophilic and superaerophobic property of CoP NWs/CC (Fig. S5, ESI†).^45,46 The superhydrophilic surface ensured full contact of the CoP NWs/CC electrode with the electrolyte solution during water splitting, while the superaerophobic property reduced gas bubble adhesion and made H₂ bubbles leave quickly from surfaces of the CoP NWs/CC.^47,48 The superficial characters of the CoP NWs/CC significantly improved the efficiency of H₂ generation, showing excellent HER performance.

Fig. 3 (a) LSV curves of CoP NWs/CC for HER at pH values of 0, 7, and 14 with a scan rate of 2 mV s⁻¹. (b) Corresponding Tafel plots of CoP NWs/CC for HER at pH values of 0, 7, and 14. (c) Chronopotentiometry technology at a constant current density of −10 mA cm⁻² of CoP NWs/CC cathode at pH values of 0, 7, and 14. (d) Comparison of the overpotential and potential retention of our work with recent reported works.

Both TCS degradation and OER test were carried out in 1 M KOH solution under a three-electrode system, using the Co₃O₄ NWs/CC as the working electrode, Pt wire as counter electrode, and saturated Ag/AgCl electrode as reference electrode. Fig. S5a (ESI†) shows the LSV curves with and without TCS under the three-electrode system. Clearly, OER process would compete with the oxidation of TCS through the electrocatalytic degradation.⁴⁹ In the absence of TCS, the Co₃O₄ NWs/CC required an overpotential of 360 mV to achieve a current density of 10 mA cm⁻² in 1 M KOH solution, which was comparable to some previously reported Co₃O₄ electrocatalysts, such as Co₃O₄ nanoflakes,⁵⁰ Co₃O₄ film⁵¹ and Co₃O₄/rGO composite.⁵² Upon the addition 40 mg L⁻¹ TCS, the corresponding overpotential decreased to 310 mV, indicating that the TCS degradation was easier than OER process using the Co₃O₄ NWs/CC anode. The corresponding Tafel slopes are shown in Fig. S5b (ESI†). The Tafel slope was 123 mV dec⁻¹ for Co₃O₄ NWs/CC in 1 M KOH solution with TCS, which was superior to that without TCS (140 mV dec⁻¹), suggesting more favorable kinetics to drive large current density at low overpotential with TCS addition.⁵³ To determine the impact of the additional TCS on HER performance of the CoP NWs/CC, we evaluated the HER performance with and without TCS in 1 M KOH solution under three-electrode system. As demonstrated in Fig. S6 (ESI†), the LSV curves of the CoP NWs/CC for HER with and without TCS almost overlapped and the Tafel slopes were nearly equal, implying that the added TCS showed no evident negative impact on HER in the CoP NWs/CC cathode.

Since the excellent HER and TCS degradation performance in 1 M KOH solution with TCS, we assembled a two-electrode electrolyzer with Co₃O₄ NWs/CC as anode and CoP NWs/CC as cathode (Co₃O₄ NWs/CC‖CoP NWs/CC) in a KOH solution with and without TCS (Fig. 4a). We chose Co₃O₄ as anode active material due to its excellent OER performance as reported before.^54–58 Based on the excellent HER performance of CoP NWs/CC in alkaline condition, it was selected as cathode. We further confirmed the competitive result of TCS degradation and OER process at the anode as the LSV data show in Fig. 4b.⁴⁹ Without TCS, the cell requires a voltage of 1.68 and 1.76 V to achieve current density of 10 and 20 mA cm⁻² in 1 M KOH solution, respectively. Adding TCS with concentration of 40 mg L⁻¹, it only needed potentials of 1.63 and 1.70 V, respectively, implying a much higher energy conversion efficiency of TCS degradation than oxygen generation. According to this LSV result, we degraded TCS by chronopotentiometry at a current density of 10 mA cm⁻² using the Co₃O₄ NWs/CC‖CoP NWs/CC electrode pair (Fig. 4c). The Co₃O₄ Oct/CC and the hydrophilic bare CC (treated by HNO₃) were also tested as anodes under the same conditions for comparison. As shown in Fig. 4c, the degradation rates were 95%, 82% and 52% in 60 min using the Co₃O₄ NWs/CC‖CoP NWs/CC, the Co₃O₄ Oct/CC‖CoP NWs/CC and the CC‖CoP NWs/CC electrode pairs, respectively. The concentration change of TCS by HPLC in the degradation process is shown in Fig. S7 (ESI†). The electrochemical degradation of TCS has attracted much attention in recent years.^59–63 For example, a degradation rate of 62% was obtained in 180 min using a boron-doped diamond electrode. However, it worked at a high current density of 28.5 mA cm⁻² and a low TCS concentration (1.19 mg L⁻¹).⁵⁹ Using the Ti/SnO₂–Sb/Ce–PbO₂ anode, TCS degradation rate of 99.9% was achieved at 10 mA cm⁻² in 5 min, but the TCS concentration was only 4 mg L⁻¹.¹⁵ As exhibited in Table S2 (ESI†), most studies could realize decent degradation results in a low TCS concentration, but were incapable in a relatively high concentration. Compared with these reported results, the high degradation concentration (40 mg L⁻¹) and degradation rate (95%), short degradation time (60 min) and easily assembled Co₃O₄ NWs/CC‖CoP NWs/CC pair provide the possibility for practical application of TCS degradation. In addition, the Co₃O₄ NWs/CC‖CoP NWs/CC pair could also produce H₂ in the degradation process. For the Co₃O₄ NWs/CC‖CoP NWs/CC, the Faradaic efficiency of H₂ evolution on cathode was calculated by comparing the gas amount experimentally quantified with that theoretically calculated. As shown in Fig. S8 (ESI†), the measured amount effectively matched the calculated amount based on the passed charge, suggesting a Faradaic efficiency close to 100%.⁴⁹ Considering the practical application, we further explored the stability and cyclability of TCS degradation and H₂ generation by the Co₃O₄ NWs/CC‖CoP NWs/CC pair. We repeated a two-hour chronopotentiometry electrolysis for 6 cycles. As shown in Fig. 4d, the TCS degradation rates were 100%, 98.2%, 97.0%, 96.3%, 95.4%, and 94.3%, in sequence, using Co₃O₄ NWs/CC anode. Moreover, there was no apparent decrease in H₂ Faradaic efficiency in the CoP NWs/CC cathode simultaneously. Before and after 6 successive electrolysis cycles, optical photos of the CoP NWs/CC and the Co₃O₄ NWs/CC (Fig. S9, ESI†) indicated the stability of the active materials grown on CC. The SEM images of the CoP NWs/CC and the Co₃O₄ NWs/CC in Fig. S10a and b (ESI†) show that there was no visible change in morphology and structure after 6 successive electrolysis cycles. The XRD patterns in Fig. S10c and d (ESI†) show that the CoP NWs/CC and the Co₃O₄ NWs/CC, after 6 cycles, had the same components as their separate initial electrodes. In addition, the XPS spectra of the CoP and the Co₃O₄ samples after 6 cycles indicated there was no remarkable component change for the two electrodes compared with their separate initial electrodes (Fig. S11, ESI†). The above results illustrate that the Co₃O₄ NWs/CC‖CoP NWs/CC pair show excellent stability and cyclability for TCS degradation and H₂ generation.

Fig. 4 (a) Schematic illustration for TCS degradation and H₂ generation by a Co₃O₄ NWs/CC‖CoP NWs/CC pair. (b) LSV curves of water electrolysis for the Co₃O₄ NWs/CC‖CoP NWs/CC pair with a scan rate of 2 mV s⁻¹ with and without TCS. (c) Degradation rates of TCS with different anodes at the current density of 10 mA cm⁻². (d) TCS degradation rates and H₂ Faradaic efficiencies of Co₃O₄ NWs/CC‖CoP NWs/CC pair in 1 M KOH solution with 40 mg L⁻¹ TCS for 6 successive electrolysis cycles.

To test for a better performance of the Co₃O₄ NWs/CC than the Co₃O₄ Oct/CC and the bare CC anodes, the effect of wettability on degradation efficiency was studied in Fig. 5. The different wettability was checked by measuring the contact angles of TCS solution towards different electrode surfaces. Fig. 5a represents the optical photo of Co₃O₄ NWs/CC with a smooth and dry surface before the test. Fig. 5b shows the Co₃O₄ NWs/CC surface adsorbing a liquid drop with a contact angle of 0°, indicating the superhydrophilic property of the surface.^44,45 However, the liquid contact angle of Co₃O₄ Oct/CC surface is about 15.5° in Fig. 5c, proving its surface was highly hydrophilic. The hydrophilicity varies along with the surficial morphologies of the Co₃O₄ micro/nanostructures, that is, the flower-like structure composed of nanowires from Co₃O₄ NWs/CC showed better hydrophilic property than octahedrons from Co₃O₄ Oct/CC. Although the hydrophilic property of the CC improved from contact angle of 140.2° (Fig. S12, ESI†) to 95.7° (Fig. 5d) by HNO₃ treatment, it was still poorer than the electrodes covered by Co₃O₄ NWs or octahedrons.

Fig. 5 (a) Co₃O₄ NWs/CC surface before wettability test. Co₃O₄ NWs/CC (b), Co₃O₄ Oct/CC (c), and HNO₃-treated bare CC (d) with liquid contact angle of 0°, 15.5°, and 95.7°, respectively. The insets show their corresponding surficial morphologies.

Comparing the wettability results, the Co₃O₄ NWs/CC exhibited the best hydrophilicity among the three anodes, owing to its typical 3D architecture with rough surface. It could be inferred that when the Co₃O₄ NWs/CC was immersed into TCS solution, the solution could easily diffuse into the electrode. In this case, sufficient contact of TCS molecules with the active components on the electrode was guaranteed, leading to an excellent degradation efficiency.⁴⁶ Owing to its weaker hydrophilicity, the Co₃O₄ Oct/CC showed poorer degradation than the Co₃O₄ NWs/CC (Fig. 4c). For the bare CC, the electrolyte solution could not wet the surface completely and only limited TCS molecules could contact the CC surface. In addition, the CC also showed negligible catalytic ability for TCS degradation. Simultaneously, the superhydrophilicity of the CoP NWs/CC cathode also assured that the electrolyte solution wet the surficial active materials thoroughly and the superaerophobicity enabled H₂ bubbles to generate efficiently and release rapidly (Fig. S13, ESI†). Therefore, the combination of the Co₃O₄ NWs/CC anode and the CoP NWs/CC cathode achieved high-efficiency TCS degradation and hydrogen generation with suitable morphologies providing optimal wettability.

The electrochemical degradation mechanism was proposed based on the intermediates identified by the technique of liquid chromatograph-mass spectrometer (LC-MS). Three main peaks at m/z 186, m/z 110, and m/z 94 were identified from the MS derived from TCS degradation. The details are shown in Table S2 (ESI†). According to the three detected intermediates and their changes with respect to retention time, two main pathways of TCS degradation were proposed as illustrated in Fig. 6. The dechlorination process in path 1 led to the first intermediate of 2-phenoxyphenol (m/z 186). Then, the 2-phenoxyphenol would be further degraded in two different pathways, including C–O–C bond-breaking following path 2a with product of phenols (m/z 94), and following path 2b with products of benzene and 1,2-dihydroxybenzene (m/z 110). Both of these intermediates from the two pathways would be further oxidized through ring opening reactions and then degraded into water and carbon dioxide, followed by mineralization. Clearly, the intermediates obtained in our proposed system show lower toxicity and less harm to the human body and plants compared to chlorinated intermediate products. Notably, these phenols are important chemical raw materials with a wide range of organic synthetic applications in fibres, rubber, plastics, pharmaceuticals, pesticides, spices, dyes, and coatings.

Fig. 6 Probable degradation pathways of TCS on the Co₃O₄ NWs/CC anode.

Conclusions

In summary, we have demonstrated a facile strategy for triclosan degradation and H₂ production effectively using the Co₃O₄ NWs/CC as anode and the CoP NWs/CC as cathode under a two-electrode system. The degradation efficiency of TCS at the anode is 95% in 60 min and the Faradaic efficiency of H₂ production at the cathode is nearly 100%. Compared to chlorinated organic compounds, the degradation products are important chemical raw materials, including phenol, 1,2-dihydroxybenzene, and 2-phenoxyphenol. As the anode, the liquid contact angle of Co₃O₄ NWs/CC is about 0°, much smaller than those of Co₃O₄ Oct/CC (∼15.5°) and bare CC (∼95.7°). The superhydrophilicity of Co₃O₄ NWs/CC makes the TCS solution fully contact the electrode surfaces easily, leading to an excellent degradation efficiency. In addition, the superhydrophilicity and superaerophobicity of the CoP NWs/CC cathode assures full contact of electrolyte solution with superficial active materials and more efficient H₂ generation. Given the low cost and excellent cycling stability of the catalysts and the high efficiency of TCS degradation and H₂ production, this study would open a new opportunity towards the development of an electrochemical water splitting system for energy-saving hydrogen generation and high-efficiency TCS degradation. It might provide a feasible route to treat the large amounts of TCS now existing in the aquatic ecosystem in the future.

Experimental

Materials

CoCl₂·6H₂O was purchased from Guangdong Guanghua Sci-tech Co., Ltd. Urea, Na₂SO₄, NaH₂PO₂·H₂O, and Co(NO₃)₂·6H₂O were purchased from Beijing Chemical Works. NH₄F and cetyltrimethyl ammonium bromide (CTAB) were purchased from Aladdin Ltd, Shanghai, China. NaH₂PO₄ and Na₂HPO₄ were used for preparation of PBS buffer solution. TCS was purchased from Sigma-Aldrich (USA). H₂SO₄ solution and NaOH were used to change the pH of the electrolyte. All chemicals were used as received. Carbon cloth was provided by Shanghai Hesen Electric Co., Ltd. The deionized (DI) water used throughout all experiments was purified through a Millipore system. HPLC-grade acetonitrile was used for sample analyses.

Preparation of precursor/CC, CoP NWs/CC cathode and Co₃O₄ NWs/CC anode

A piece of carbon cloth (CC, 2 × 3 cm²) was immersed into concentrated HNO₃ for 24 h to remove surficial impurities, followed by washing with deionized water and ethanol. In a typical synthesis, CoCl₂·6H₂O (0.30 g), NH₄F (0.095 g), and urea (0.30 g) were mixed in 20 mL DI water under stirring for 20 min until complete dissolution. The solution was then transferred into a Teflon-lined stainless autoclave (25 mL), and the CC was immersed into the solution. The autoclave was sealed and maintained at 120 °C for 6 h in an oven. After the autoclave cooled down naturally, the precursor/CC was taken out and washed with water and ethanol several times, then dried at 60 °C for 5 h. To prepare the CoP NWs/CC cathode, the precursor/CC and NaH₂PO₂·H₂O (300 mg) were placed at two separate positions in a porcelain boat with NaH₂PO₂·H₂O at the upstream side of the furnace. Subsequently, the samples were heated at 300 °C for 2 h at a heating rate of 2 °C min⁻¹ in a N₂ atmosphere and then, naturally cooled to ambient temperature under N₂. The CoP NWs/CC was finally obtained. The Co₃O₄ NWs/CC was obtained by annealing the precursor/CC at 300 °C for 2 h in a N₂ atmosphere and then, cooled down to ambient temperature under N₂.

Preparation of Co₃O₄ Oct/CC anode

Co(NO₃)₂·6H₂O (0.30 g) and CTAB (0.15 g) were dissolved in a mixture of 4 mL of DI water and 32 mL of methanol. After sonication for 30 min, the resultant solution was then transferred to 50 mL Teflon autoclaves and the CC was immersed in the solution for 30 min. Then, the Teflon autoclaves were heated in an oven at 180 °C for 12 h. After the autoclave cooled down slowly to room temperature, the sample was taken out and washed with DI water and ethanol several times, then dried at 60 °C.

Characterizations

The morphologies of the samples were studied using a field-emission gun scanning electron microscope (SEM, Hitachi 7500). Transmission electron microscopy (TEM) investigations were carried out using a JEOL JEM-2100F microscope. X-ray diffraction (XRD) spectra of the samples were recorded by a Rigaku Dmax 2200 X-ray diffractometer (XRD) with Cu-K_α radiation (λ = 1.5416 Å). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 photoelectron spectrometer. The concentrations of the TCS were quantified by HPLC (Agilent 1260 series) equipped with a C18 column (Agilent Technology) using a gradient composition of acetonitrile and water (volume ratio of 70 : 30) at a constant flow rate of 1 mL min⁻¹ and UV detector of 220 nm. Under these conditions, the retention time of TCS was 5.2 min. The intermediate products were analyzed by liquid chromatograph-mass spectrometer (LC-MS, Waters Xevo TQ-S Micro). High performance liquid chromatography (HPLC) was performed on a reverse phase liquid chromatographic column (Waters Xterra 5 μm MS C18 column) using a Waters 2695 XE LC instrument (Waters Corp., Milford, MA) and flowing phase of acetonitrile, water (70 : 30) and acetic acid (0.1%) at a constant flow rate of 0.5 mL min⁻¹. Atmospheric pressure ionization tandem mass spectrometry analysis was performed on a benchtop triple quadrupole mass spectrometer operated using a negative electrospray ionization source. The surface wettability of the electrodes was characterized using a KSV CAM200 Goniometer instrument at ambient temperature in air. The liquid contact angles were determined by averaging measurements taken from at least five different positions on the measured surfaces.

Electrocatalytic measurements

HER measurements were performed on Gamry Interface 1000 electrochemical workstation with a three-electrode system. The CoP NWs/CC (1 × 1 cm²), a graphite electrode, and a saturated Ag/AgCl electrode were used as the working electrode, the counter electrode, and the reference electrode, respectively. The Ag/AgCl electrode could be converted to the reversible hydrogen electrode (RHE) scale by adding a value of (0.197 + 0.0591 × pH) V. The linear sweep voltammetry (LSV) measurements of HER were conducted in 0.5 M H₂SO₄ solution (pH = 0), 1 M PBS buffer solution (pH = 7), and 1 M aqueous KOH solution (pH = 14) separately at a scan rate of 2 mV s⁻¹. The OER measurements were conducted at a scan rate of 2 mV s⁻¹ in 1 M KOH solution with and without TCS under a three-electrode system with saturated Ag/AgCl as the reference electrode and Pt wire as the counter electrode. The overall water splitting was performed using the Co₃O₄ NWs/CC‖CoP NWs/CC pair in 1 M KOH solution with and without TCS at a scan rate of 2 mV s⁻¹. The degradation of TCS was also performed on Gamry Interface 1000 electrochemical workstation in a two-electrode system with the Co₃O₄ NWs/CC (2 × 3 cm²) as the anode and the CoP NWs/CC (2 × 3 cm²) as the cathode. Hydrogen generated from the cathode was collected by gas drainage set with inverted cylinder filled with TCS degradation solution and the CoP NWs/CC electrode at the bottom of the cylinder during the degradation process. The Faradaic efficiency was calculated by comparing the amount of measured with calculated hydrogen.

Degradation of TCS

The degradation of TCS was performed with a two-electrode system. The CoP NWs/CC (2 × 3 cm²) was used as the cathode and the Co₃O₄ NWs/CC (2 × 3 cm²) as the anode. The degradation solution was 1 M KOH with the TCS concentration of 40 mg L⁻¹. The solution of TCS in the reactor was stirred continuously using a magnetic stirrer. The degradation process was carried out by chronopotentiometry with a current density 10 mA cm⁻².

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (51622204, 51472014 & 51438011), Beijing Nova Program (Z171100001117071), the Foundation for the Author of National Excellent Doctoral Dissertation of China (201331), the Fundamental Research Funds for the Central Universities, the National Postdoctoral Program for Innovative Talents (BX201600007), and the Project funded by China Postdoctoral Science Foundation (2016M600894).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00533d

‡ The author contributed equally to the first author.

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