Chaojie
Lyu
a,
Jinlong
Zheng‡
a,
Rui
Zhang
a,
Ruqiang
Zou
b,
Bin
Liu
c and
Wei
Zhou
*a
aSchool of Chemistry, Beihang University, Beijing 100191, China. E-mail: zhouwei@buaa.edu.cn
bBeijing Key Laboratory for Theory and Technology of Advanced Battery Materials, Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China
cInstitute for Food & Bioresource Engineering, College of Engineering, Peking University, Beijing 100871, China
First published on 11th December 2017
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.
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.
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 2p3/2 and Co 2p1/2 from CoP, while those at 781.1 and 796.6 eV could be indexed to Co 2p3/2 and Co 2p1/2 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 2p3/2 and P 2p1/2 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.
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.
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.
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.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7qm00533d |
‡ The author contributed equally to the first author. |
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