Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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CoP Nanoparticles Combined with WSe2 Nanosheets: An Efficient Hybrid Catalyst for Electrocatalytic Hydrogen Evolution Reaction Jiahui Qian, Zhen Li, Xiaomeng Guo, Yang Li, Wenchao Peng, Guoliang Zhang, Fengbao Zhang, and Xiaobin Fan* School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300350, China S Supporting Information *
ABSTRACT: Hydrogen (H2) is a clean and efficient energy carrier, and great efforts have been focused on developing new non-Pt electrocatalysts for the hydrogen evolution reaction (HER). In this study, CoP/WSe2 nanosheets hybrids were successfully prepared, and their potential as a new non-noble HER catalyst was evaluated. We found that all the hybrids showed higher catalytic activity than pure CoP and WSe2 nanosheets, suggesting their synergistic catalytic effect in the HER. Particularly, in 0.5 M H2SO4, the hybrid with an optimized CoP-to-WSe2 mass ratio of 1:1 has the highest current density of 102.9 mA/cm2 at 300 mV vs RHE and excellent long-term stability.
1. INTRODUCTION Hydrogen (H2), as a clean and efficient energy carrier, is a potential candidate to replace traditional nonrenewable fuels, and water electrolysis is one of the most effective methods to produce H2.1−4 Many nanomaterials have been explored and used as electrocatalysts for the hydrogen evolution reaction (HER).5−10 Among them, transition-metal phosphides have attracted special attention, owing to their low price and high electrocatalytic activity in the HER.11−15 In particular, various morphologies of CoP have been successfully fabricated,1617 and enhanced electocatalytic activity was observed in their hybrids with other components, such as Al-doped and Fedoped CoP nanoarry,18−20 Zn0.08Co0.92P/TM,21 FexCo1−xP nanowire,22 and CoP nanoparticles (NPs) combined with carbon meterials.23,24 For example, CoP NPs anchored on carbon nanotubes have shown lower overpotential and better stability with activity unchanged after 400 cycles of cyclic voltammetry sweeps.23 Improved electrocatalytic activity was also demonstrated in the CoP NPs after being composited with reduced graphene oxide.24 Recently, CoP NP hybrid with transition-metal dichalcogenides (TMDs), such as MoS2 and WS2, showed obvious synergetic catalytic effect in the HER.24,25 Layered TMD materials, such as CoSe2,26 NiSe2,27 and ReSe2,28 have been demonstrated to be promising nonprecious electrocatalysts for the HER.29 As an important member of TMDs, WSe2 has attracted increasing attention as a HER catalyst,30−32 especially the metallic 1T phase WSe2. Depending on the metal coordination, WSe2 presents two different polymorph structures, the semiconductive 2H phase with trigonal prismatic metal coordination and the metallic 1T phase in trigonal antiprismatic (or octahedral).33 The chemical © XXXX American Chemical Society
exfoliation process induces a phase transition from 2H phase to the 1T phase, which has been demonstrated to have higher activities in HER.34 However, the hybrid catalysts of WSe2 and transition-metal phosphides, especially the CoP NPs/WSe2 nanosheets hybrid, have been never reported. Here we report the new CoP NPs/WSe2 nanosheets hybrid catalyst and evaluate its potential as an electocatalyst in the HER. We combined CoP NPs and WSe2 nanosheets by simple mixing with sonication (Scheme 1) and systematically investigated the electrocatalytic activities of the samples.
2. EXPERIMENTAL SECTION 2.1. Materials. WSe2 powders and n-butyl lithium (C4H9Li) were purchased from Aladdin Reagent Co, Ltd. Scheme 1. Illustration of the Formation of CoP/WSe2 Hybrid and Its Application in the HER
Received: August 26, 2017 Revised: December 7, 2017 Accepted: December 12, 2017
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DOI: 10.1021/acs.iecr.7b03537 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Cobalt nitrate [Co(NO3)2·H2O], sodium hydroxide (NaOH), sodium hypophosphite (NaH2PO2) were purchased from Yuan Li Chemical Reagent Co, Ltd. Sodium citrate (Na3C6H5O7· 2H2O) was purchased from Guang Fu Chemical Reagent Co, Ltd. 2.2. Synthesis of CoP NPs. CoP NPs were prepared by a two-step method.35 First, 0.8 g of Co(NO3)2·6H2O and 0.2 g og sodium citrate were dissolved into 200 mL of deionized water. The mixture was stirred for 20 min to form a homogeneous solution. Then, 20 mL of NaOH solution (0.5 M) was added into the mixture dropwise. After that, the dispersion turned to green because of the formation of Co(OH)2. The generated green precipitates were collected and washed with deionized water by centrifugation and then dried by freeze−drying. Second, the obtained green solid was mixed with NaH2PO2 by a mass ratio of 1:5 and ground in an agate mortar. The mixture was added into a quartz boat and heated in a tube furnace at 300 °C for 1 h under Ar atmosphere. After cooling to room temperature naturally, the obtained CoP NP sample was purified by centrifugation with deionized water four times and dried in a freeze−drying plant. The samples were stored under Ar atmosphere at around 5 °C. 2.3. Synthesis of WSe2 Nanosheets. WSe2 nanosheets were prepared by a chemical exfoliation method with lithium intercalation.36,37 For intercalation, 0.5 g of WSe2 crystals were added into a Schlenk flask under Ar atmosphere, and 5 mL of n-butyl lithium (2.5 M) was injected into the system by a syringe. Then the Schlenk flask was put in a water bath at 66 °C for 72 h. The intercalation compound was washed by centrifuged four times with hexane to remove excess n-butyl lithium. The exfoliation was achieved immediately when the fresh intercalated samples dispersed into 200 mL of water under sonication. After 5 min, the mixture was centrifuged with deionized water several times to get exfoliated WSe 2 nanosheets. The sample was dried in a freeze−drying plant and preserved under Ar atmosphere at around 5 °C to prevent surface oxidation and phase conversion of WSe2. 2.4. Synthesis of CoP/WSe2 Hybrids. The mass ratio of CoP and WSe2 were set as 4:1, 3:1, 2:1; 1:1, and 1:2, and they were marked as CoP/WSe2-1, CoP/WSe2-2, CoP/WSe2-3, CoP/WSe2-4, CoP/WSe2-5, respectively. The designed amount of WSe2 dispersion solution and corresponding mass of CoP solution underwent sonication for 15 min separately to form their own homogeneous solution. The solutions were then mixed together and underwent sonication for another 15 min, forming the CoP/WSe2 hybrids. 2.5. Characterization. The samples were characterized by X-ray diffraction (XRD) with a Bruker-Nonius D8 FOCUS diffractometer, transmission electron microscopy (TEM) with JEM-2100F transmission electron microscope operating at 200 kV, high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy with Renishaw inVia confocal microscope-based Raman spectrometer with 532 nm laser excitation, and X-ray photoelectron spectroscopy (XPS) with PerkineElmer PHI1600 spectrometer. 2.6. Electrochemical Measurements. A CHI660E electrochemical workstation was used in all electrochemical measurements, equipped with a three-electrode system in 0.5 M H2SO4 (pH 0), 1.0 M PBS (pH 7), and 1.0 M KOH (pH 14). A Pt wire was used as the counter electrode, while an Ag/ AgCl electrode (in saturated KCl solution) as the reference electrode. The working electrodes were prepared by dropping 5 μL of the prepared CoP, WSe2, and CoP/WSe2 hybrid inks
(dissolved catalysts with a concentration of 5 mg/mL) onto glassy carbon electrodes with diameter of 3 mm. Measured potentials were converted into potentials versus RHE according to the equation ERHE = EAg/AgCl + 0.2 + 0.059 pH.38 The current density calculated in this study was based on the geometric area of the glassy carbon, which is 7.065 × 10−2 cm2. The electrocatalytic activities of the hybrids were evaluated by linear sweep voltammetry (LSV) from −0.3 to 0.2 V with a scan rate of 5 mV/s. For each sample, the LSV test was performed multiple times, and we selected the best polarization curve to show the sample’s electrocatalytic activity. The stability test of the CoP/WSe2-4 hybrid was operated by conducting cyclic voltammetry between −0.3 and 0.2 V. The LSV was performed after every 500 cycles.
3. RESULTS AND DISCUSSION The WSe2 nanosheets were prepared via a chemical exfoliation method by lithium intercalation,37 and the CoP NPs were synthesized by using a two-step method.35 Then, the CoP/ WSe2 hybrids with different mass ratios were fabricated by simple mixing with sonication. The X-ray powder diffraction (XRD) pattern of the exfoliated WSe2 nanosheets (Figure 1a)
Figure 1. XRD patterns of bulk WSe2, exfoliated WSe2 nanosheets, CoP NPs, and the representative CoP/WSe2 hybrids (CoP/WSe2-4).
confirms the absence of the characteristic (002) diffraction of layered WSe 2 crystals (No. 38-1388), suggesting the completely exfoliated nature of the obtained WSe2 nanosheets. The CoP NPs before and after hybrid with the WSe2 nanosheets show nearly identical XRD patterns (Figure 1b), B
DOI: 10.1021/acs.iecr.7b03537 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
(Figure S1), the CoP NPs hybrid with the WSe2 nanosheets show less agglomeration. Lattice fringes of these anchored CoP NPs have a d spacing of 0.19 nm that corresponds to the (211) plan of orthorhombic CoP (Figure 3d), supporting the successful preparation of the CoP/WSe2 hybrid. The morphology of the CoP/WSe2-4 was also investigated by SEM test, and the images are shown in the Supporting Information (Figure S6a,c). As we can see in the X-ray photoemission spectroscopy (XPS) spectrum, the dominant peak in the Co 2p (778.45 eV, Figure 4a) is positively shifted from that of Co metal (778.1−
in which the (011), (211), and (301) planes of orthorhombic CoP (JCPDS 29-0497) are obvious.23,35 In the Raman spectra of bulk WSe2 (Figure 2), two peaks at 242.75 and 250.75 cm−1 were observed and assigned as E12g and
Figure 2. Raman spectra of bulk WSe2 and WSe2 nanosheets.
A1g, respectively.39 By exfoliation, these two peaks overlapped each other, as the A1g peak shifted to lower frequency while the E12g peak shifted to higher frequency.40 It should be noted that the characteristic peaks of bulk 2H-WSe2 in 350−400 cm−1 completely disappeared in the obtained WSe2 nanosheets, attributed to the 2H to 1T phase change.33,41 Transmission electron microscopy (TEM) measurements reveal that the WSe2 nanosheets appear transmissive to electrons, owing to their atomic scale thickness (Figure 3a). Their crumpling and folding on the TEM grids are vivid, and mixed 1T and 2H phase are confirmed in the HRTEM images (Figure 3b). As can be seen from Figure 3c, different from the dried CoP NPs that show a heavily agglomerated morphology
Figure 4. XPS spectra of (a) Co 2p, (b) P 2p, (c) W 4f,and (d) Se 2p regions of CoP/WSe2-4.
Figure 3. (a) STEM and (b) HRTEM images of WSe2 nanosheets; (c) STEM and (d) HRTEM images of CoP/WSe2-4.
778.2 eV), while the peak of the P 2p (129.35 eV, Figure 4b) is negatively shifted from that of elemental P (130.2 eV).17,35 Each of the W 3d peaks can be deconvoluted into two independent components. The doublet peaks at 33.60 and 31.60 eV should be assigned to the 1T-WSe2, while the doublet peaks at 34.25 and 32.30 eV correspond to 2H-WSe2.33 It should be noted that the surface of the WSe2 sample was partially oxidized. However, considering the penetration depth of XPS and the fact that we do not find any peaks about tungsten oxide in the XRD results, we think that only the WSe2 nanosheets on the surface of dried sample were oxidized. The XPS spectrum of the WSe2 nanosheets and the CoP NPs are addressed in the Supporting Information (Figures S3 and S4). The electrochemical behaviors of the CoP/WSe2 hybrids were investigated in 0.5 M H2SO4 (pH 0), 1.0 M PBS (pH 7), and 1.0 M KOH (pH 14) solution by using a standard threeelectrode electrochemical cell setup. The same amount of pure WSe2 nanosheets, CoP NPs, and hybrids with different mass ratios were subjected to HER without any binders, and Pt/C was also tested for comparison (Figure 5a−c). In 0.5 M H2SO4, all the hybrids show better electrocatalytic activities than bare CoP NPs and WSe2 nanosheets (Figure 5a). Remarkably, with the decrease of CoP to WSe2 mass ratios, the current density progressively increases. The current density reaches the largest value at the mass ratio of 1:1, followed by a C
DOI: 10.1021/acs.iecr.7b03537 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 5. LSV curves for different CoP/WSe2 ratio hybrids and Pt/C electrodes in (a) 0.5 M H2SO4 (pH 0), (b) 1.0 M PBS (pH 7), and (c) 1.0 M KOH (pH 14) at the scan speed of 5 mV/s at 298 K; (d−f) the corresponding Tafel plots.
operating in the HER process.45 The Tafel slopes of CoP NPs, WSe2 nanosheets, CoP/WSe2-4, and commercial 20 wt % Pt/C catalysts in 1.0 M PBS (pH 7) and 1.0 M KOH (pH 14) are shown in Table S1. Electrochemical impedance spectroscopy (EIS) revealed that the electrochemistry reaction resistance (Rct) of the WSe2 nanosheets and CoP are 418 and 265 Ω, respectively (Figure 6a). Whereas the Rct of the CoP/WSe2-4 is only 35 Ω, indicating that the hybrid has a much more efficient electron transfer process.46 These results may explain, at least in part, the enhanced catalytic activity of the hybrid. The stability test of the CoP/WSe2-4 hybrid was operated by conducting cyclic voltammetry between −0.3 and 0.2 V at an accelerated scanning rate of 100 mV s−1. It can be seen that after 1000 cycles, the current density of the CoP/WSe2-4 at 300 mV vs RHE decreased only from 85.6 to 81.0 mA/cm2, demonstrating good long-term stability of the hybrid catalyst (Figure 6 b). For further demonstrating the long-term durability of the sample, the time dependence test of the CoP/WSe2-4 hybrid was performed (shown in Figure S5), and the morphology of the sample that underwent the test was investigated by SEM (shown in Figure S6b,d). The synergistic effect between CoP and WSe2 has a great contribution to the enhanced catalytic activity of the catalyst. On one hand, the WSe2 nanosheets, acting not only as an active phase but also as supports for the dispersion of CoP NPs. As can be seen in TEM results, different from the dried CoP NPs that show a heavily agglomerated morphology, the CoP NPs anchored on WSe2 show high density and good dispersion, exposing many more catalytic active sites. On the other hand, owing to the atomic scale thickness, the WSe2 nanosheets appear transmissive to electrons, permitting highly efficient electron transfer to the catalytic sites on CoP NPs.
decrease. That is, CoP/WSe2-4 with the mass ratio of 1:1 has the best HER catalytic activity. In the study, CoP/WSe2-4 shows an onset overpotential as low as 46 mV, and further negative potential causes a rapid rise of cathodic current. CoP/ WSe2-4 needs an overpotential of 163 mV to attain the current density of 10 mA/cm2, which is much better than that of pure CoP NPs (400 mV) and WSe2 nanosheets (268 mV). At the overpotential of 300 mV, CoP/WSe2-4 exhibits the current density of 102.9 mA/cm2, which is 5.6 times and 3.8 times of those of WSe2 nanosheets (18.4 mA/cm2) and CoP NPs (27.4 mA/cm2), respectively. It should be noted that the CoP/WSe24 here shows better performance than many similar catalysts in previous reports, like the WSe2 nanoflower (18.2 mA/cm2)30 and RGO/WSe2 hybrid (38.43 mA/cm2),42 WSe2/CNTs hybrid (35.5 mA/cm2),43 and the composite CoP/MoS2 (78 mA/cm2). In 1.0 M PBS (pH 7) and 1.0 M KOH (pH 14) (Figure 5b,c), the CoP/WSe2-4 with the mass ratio of 1:1 shows the best HER catalytic activity, just like the trend in 0.5 M H2SO4. However, at the overpotential of 300 mV, the CoP/WSe2-4 exhibits the current density of 4.80 and 55.95 mA/cm2 in 1.0 M PBS (pH 7) and 1.0 M KOH (pH 14), respectively. These values are much less than that in 0.5 M H2SO4 (102.9 mA/ cm2). The efficiency of the catalytic reaction in HER progress was evaluated via the analysis of Tafel plots. The linear regions are fitted into the Tafel equation (η = b log(j) + a, where ob is the Tafel slope), and the results are shown in Figure 5d−f.44 In 0.5 M H2SO4, the Tafel slopes of CoP NPs, WSe2 nanosheets, CoP/WSe2-4, and commercial 20 wt % Pt/C catalysts are 86.4, 89.5, 76.5, and 29.5 mV dec−1, respectively (Figure 5d). The Tafel slope of CoP/WSe2-4 hybrids (76.5 mV dec−1) indicates that it was the Volmer Heyrovsky combination mechanism D
DOI: 10.1021/acs.iecr.7b03537 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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STEM and HRTEM images; EDX spectrum; XPS spectra; time-dependent current density curve for CoP/WSe2-4 hybrid under static overpotential of 225 mV for over 20 h; SEM images of the CoP/WSe2-4 before and after the time dependence test; LSV curves and corresponding Tafel plots for the as-prepared and annealed CoP/WSe2-4 hybrid; Tafel slopes (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yang Li: 0000-0003-3003-9857 Wenchao Peng: 0000-0002-1515-8287 Xiaobin Fan: 0000-0002-9615-3866 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study is supported by the National Natural Science Funds (No. 21676198) and the Program of Introducing Talents of Discipline to Universities (No. B06006).
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Figure 6. (a) Nyquist plots of pure CoP, WSe2, and CoP/WSe2-4 at −0.20 V versus RHE; (b) LSV curves of the CoP/WSe2-4 at initial condition and after 1000 cycles for durability test in 0.5 M H2SO4 solution.
The XPS results indicated that the Co in CoP has a partial positive charge (δ+), while the P has a partial negative charge (δ−).47 When the catalysts are supplied to the HER, the Co centers act as the hydride-acceptor, and the P centers act as the proton-acceptor.17 Hydrogen ions at the active sites of the catalysts are reduced by the transferred electrons, followed by releasing hydrogen gas. The WSe2 nanosheets with atomic scale thickness permit highly efficient electron transfer and promote the HER progress.
4. CONCLUSIONS In summary, to enhance the electrocatalytic activity of CoP in the HER, exfoliated WSe2 nanosheets were introduced, and systematic studies on the electrocatalytic activity of the hybrid with different mass ratios were carried out. All the hybrids showed higher catalystic activity than pure CoP and WSe2 nanosheets in HER. Particularly, in 0.5 M H2SO4, the hybrid with an optimized CoP-to-WSe2 mass ratio of 1:1 has the highest current density of 102.9 mA/cm2 at 300 mV vs RHE and excellent long-term stability. All these results suggest that the CoP/WSe2 is of great potential as a non-noble electrocatalyst for the HER.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03537. E
DOI: 10.1021/acs.iecr.7b03537 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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