Self-assembled CoSe2 nanocrystals embedded into carbon nanowires as highly efficient catalyst for hydrogen evolution reaction

Self-assembled CoSe2 nanocrystals embedded into carbon nanowires as highly efficient catalyst for hydrogen evolution reaction

Accepted Manuscript Title: Self-assembled CoSe2 nanocrystals embedded into carbon nanowires as highly efficient catalyst for hydrogen evolution reacti...

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Accepted Manuscript Title: Self-assembled CoSe2 nanocrystals embedded into carbon nanowires as highly efficient catalyst for hydrogen evolution reaction Authors: Keren Zhou, Jiarui He, Xinqiang Wang, Jie Lin, Ye Jing, Wanli Zhang, Yuanfu Chen PII: DOI: Reference:

S0013-4686(17)30365-1 http://dx.doi.org/doi:10.1016/j.electacta.2017.02.089 EA 28954

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

9-12-2016 15-2-2017 15-2-2017

Please cite this article as: Keren Zhou, Jiarui He, Xinqiang Wang, Jie Lin, Ye Jing, Wanli Zhang, Yuanfu Chen, Self-assembled CoSe2 nanocrystals embedded into carbon nanowires as highly efficient catalyst for hydrogen evolution reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2017.02.089 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Title page

Self-assembled CoSe2 nanocrystals embedded into carbon nanowires as highly efficient catalyst for hydrogen evolution reaction

Keren Zhou, Jiarui He*, Xinqiang Wang, Jie Lin, Ye Jing, Wanli Zhang and Yuanfu Chen*

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China

*Corresponding authors. Jiarui He, [email protected];Yuanfu Chen, [email protected] Tel.: +86 028 83202710

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ABSTRACT Highly active, durable and inexpensive HER catalysts for electrochemical hydrogen evolution is a significant solution to cope with the energy crisis. Herein, self-assembled CoSe2 nanocrystals embedded into carbon nanowires (CoSe2@CNWs) have been synthesized by a facile hydrothermal reaction and subsequent selenylation process. Compared to bare CoSe2 NWs, the CoSe2@CNWs show outstanding performance of hydrogen evolution reaction (HER) with a small Tafel slope of 41.3 mV dec-1 and a low onset potential of ~ 130 mV vs RHE. Moreover, the CoSe2@CNWs also demonstrate good long-term stability in acidic electrolyte with a high current retention of 94.1% after 1500 cycles. The excellent HER performance of CoSe2@CNWs is attributed to the unique architecture constructed by CoSe2 nanocrystals embedded into highly conductive carbon nanowires, which guarantees rich active reaction sites and facilitates the charge transportation in HER process.

Keywords: CoSe2 nanowire; Electrocatalyst; Hydrogen evolution reaction

1. Introduction To develop clean-energy technologies, splitting of water molecules comprises of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Splitting water by HER has been widely recognized as an effective way to reduce the need for fossil fuel. The hydrogen produced from HER makes a high efficiency and carbon dioxide-free energy conversion.[1-5] However, so far, the efficient catalysts for HER 2

are usually expensive noble metals (e.g. Pt), which limit the mass production of hydrogen. Searching for highly active, durable and inexpensive HER catalysts is significant and urgent.[6] Recently, a series of works have been done on the transition-metal dichalcogenides (TMDCs) catalysts like MoSe2,[7] MoS2,[8-12] WSe2,[13,14] WS2 [15,16], but the HER performance of them still limited by the low conductivity and less active site exposure. To address such issue, a plenty of carbonous materials were used to improve the electrocatalytic properties of the catalysts in HER, for instance, reduced graphene oxide (rGO),[17-20] metal organic framework (MOF)[21] and carbon nanotubes (CNTs).[22-26] Among the TMDCs catalysts, cobalt diselenide (CoSe2) has been recognized as an alternative catalyst due to its high conductivity[27] and earthabundant resources.[1,28-30] The crystal structure and catalysis mechanism of CoSe2 have been intensively investigated.[29,31] However, the poor active area leads to a low catalytic activity of bare CoSe2.[32] In order to increase the active area, different kinds of nanostructured CoSe2 like the nanosheets,[28] nanocrystals,[33] nanowires arrays,[34,35] and nanobelt[36] have been developed. Zhou et al. presented a new idea of protecting the CoSe2 with a carbon shell for long-term HER process. [32] However, it is still a challengeable to obtain highly effective CoSe2 catalyst with rich active reaction sites by a facile, low-cost, and large-scale synthesis method.[29,37] Herein, we report a novel approach to fabricate CoSe2 nanocrystals embedded into carbon nanowires (CoSe2@CNWs) by a facile hydrothermal reaction and subsequent selenylation process. The as-prepared CoSe2@CNWs consists of a conductive network, 3

enhancing the conductivity between CoSe2 nanocrystals, which will improve the HER performance. Compared to bare CoSe2 nanowires, the CoSe2@CNWs deliver excellent electrocatalytic property with a lower onset overpotential of 130mV vs RHE and a smaller Tafel slope of 41.3 mV dec-1. The durable test also shows the exceptional stability in acidic electrolyte. Those features suggest CoSe2@CNWs as an efficient alternative catalyst for hydrogen evolution.

2. Experimental 2.1 Synthesis of NTC-Co precursor The nitrilotriacetic acid (NTC) and CoCl2·6H2O was used as starting materials to synthesize NTC-Co precursor. In brief, 0.7g CoCl2·6H2O was dissolved in the solution consisted of 30ml of deionized water and 10 ml of 2-propanol. Then 0.6g NTC was added into the solution, which was then transferred into a Teflon vessel for solventthermal reaction at 180 oC for 6 h. Finally, the product was washed by ethanol for several times before vacuum suction filtration, and the NTC-Co precursor was obtained after dried at 60 oC for 12h. 2.2 Synthesis of CoSe2@CNWs and CoSe2 NWs The above NTC-Co precursor and 0.6g glucose were dissolved into 40 mL deionized water. After hydrothermal reaction at 180 oC for 12 h, the dark red powder was obtained after similar filtration. Then the powder was put into a quartz tube under a flow of argon for excluding air for about 30 min before annealing at 650 oC for 2h with a heating rate of 2 oC min-1. After natural cooling, the product was mixed with selenium powder and then annealed at 300 oC for 2h. Finally, the black powder product was harvested. 4

The bare CoSe2 NWs were synthesized in the same process without the hydrothermal process with the glucose. 2.3 Characterizations The morphologies of the samples were examined by scanning electron microscope (SEM, JSM-7000F, JEOL) and transmission electron microscope (TEM, Tecnai F20 at 200 kV) with an energy dispersive X-ray spectrometer (EDS). The structural characterizations were recorded by an X-ray diffractometer (XRD, Rigaku D/MAX-rA diffractometer). Raman spectroscopy was performed using a Raman microscope (532 nm, Renishaw). X-ray photoelectron spectroscopy (XPS, Kratos XSAM800 Al Kα Source Gun) was performed for detecting the chemical states and composition of the sample. The conductivity of the disc sample pressed from the powder was measured using Hall effect measurement system (ECOPIA). 2.4 Electrochemical measurements Electrochemical performance of the samples was carried out using a three-electrode cell setup and acid electrolyte (0.5 M H2SO4) at an electrochemical station (CHI660C). 4 mg of the CoSe2@CNW is dispersed in 1 mL of water/ethanol (3:1) mixed with 50 μL Nafion D-520 dispersion (Alfa Aesar). The homogeneous catalyst ink was obtained by ultrasonic processing at least 30 min. Then 5μL of the catalyst ink was deposited on a glassy carbon electrode (GCE, 3 mm in diameter) to obtain the work electrode with catalyst loading of 0.283 mg cm-2. Saturated calomel electrode (SCE) employed as reference electrodes and using platinum (Pt) electrode as counter electrodes. In addition, the reference electrode has been calibrated to reversible hydrogen electrode (RHE) 5

using Pt wires as working and counter electrode with steady hydrogen flow bubbled through the whole electrolytic cell. ERHE= ESCE + 0.254 V. The stability was evaluated by continuous cyclic voltammetry tests from -0.346 to +0.254 V vs RHE at a scan rate of 100 mV s-1 for 1500 cycles. Linear sweep voltammetry (LSV) was performed with 5 mV s-1 scan rate and electrodes were cycled at least 40 cycles prior any measurements. AC impedance measurements were performed at -0.18 V vs RHE in the frequency range 0.1 to 100 KHz with an AC voltage of 5 mV. All the current density was normalized by the geometric surface area (0.07069cm2).

3. Results and discussion The fabrication process of CoSe2@CNWs is schematically illustrated in Fig. 1(a). The NTC-Co was added into the glucose solution. After hydrothermal treatment, the glucose converted to carbon on the surface of the NTC-Co. The CoSe2@CNWs was obtained by annealing the precursor with selenium powder. The product was firstly analyzed by XRD and Raman spectroscopy. Fig. 2a shows the XRD patterns of the as-prepared CoSe2@CNWs, bare CoSe2 NWs and Co@CNWs. It’s obvious that both the orthorhombic marcasite-structure of CoSe2 (ICDD PDF card 89-2003) and the cubic pyrite-type CoSe2 (ICDD PDF card 88-1712) can be corresponded, which means that polymorphic CoSe2 was produced in annealing treatment[31]. The characteristic peaks marked with triangle symbols assign to the (101), (111), (120), (211), (131), (310) crystalline surfaces of orthorhombic CoSe2, respectively. The peaks marked with diamond symbols agree with the (211), (220) and 6

(311) surfaces of the cubic CoSe2. There are no other peaks corresponding to carbon in XRD results. In addition, Raman spectrum of CoSe2@CNWs is shown in Fig. 2b. The sharp peak at 190 cm−1 corresponds to the Se−Se stretching mode of cubic CoSe2, and the orthorhombic phase is difficult to be observed in Raman-active mode.[27] The Raman peaks at 1350, and 1578 cm−1 correspond to the D, and G bands of the carbon layer, respectively. The Raman results provide confirm the existence of carbon. To investigate the chemical composition of cobalt diselenide, the sample was examined by X-ray photoelectron spectroscopy (XPS). Spectrum in Fig.2c reveals the electron-binding energies of Co 2p3/2 at 778.4 eV correspond to Co2+ cations in CoSe2. Meanwhile, a markedly shake-up satellite peak was found at the higher side of the signal, which has been reported because of the transition from metal the 3d to the coordination antibonding orbital[38]. The peaks at 54.6 eV and 55.3 eV (Fig. 2d) correspond to the Se 3d5/2 and Se 3d3/2, implying the formation of chemical bonds between Co and Se[27]. The above results indicate that the CoSe2@C have been successfully synthesized. In order to characterize the CoSe2 nanocrystals embedded into carbon nanowires, the morphologies of the CoSe2@CNWs and bare CoSe2 NWs were performed by SEM and TEM. Low-magnification SEM image in Fig. 3a shows microstructures revealing a series of randomly oriented pipe-like CoSe2@CNWs approximately dozens of micrometers in length. The high-magnification SEM image in inset of Fig. 3a further indicates that the CoSe2@CNWs uniformly distributed with diameters about 100 nm. Compared with the neat CoSe2@CNWs in Fig. 3a, much more fractures and pieces over 7

several microns can be observed for the bare CoSe2 NWs (Fig. 3b). The difference between Figs. 3a and 3b could be attributed to the protection of carbon shell during selenylation. However, the thickness of carbon shell and the architecture of CoSe2 nanoparticles is obscure due to the limited resolution of SEM. TEM is employed to more clearly demonstrate the nanostructure of CoSe2@CNWs. The typical morphology in Fig. 3c exhibits that CoSe2 nanocrystals are embedded into carbon nanowires, and the cylindrical carbon shell has a thickness of 30 nm in average. The HRTEM image in Fig. 3d is consistent with the XRD result: both the cubic and orthorhombic CoSe2 nanocrystals can be observed; the spacings of 2.39 Å, 2.59Å and 2.90Å are is assigned to the (211) planes of orthorhombic CoSe2, and the (101) and (111) plane of cubic CoSe2, respectively[31]. The electrocatalytic properties of CoSe2@CNWs and bare CoSe2 NWs samples were evaluated in a standard three-electrode cell setup with 0.5 M H2SO4 electrolyte saturated with N2. As shown in Fig. 4a, the polarization curve of CoSe2@CNWs exhibits much higher current density of 27.5 mA cm-2 (at -250 mV vs RHE) than that of the bare CoSe2 NWs (12.5 mA cm-2) at the same conditions. Meantime, the onset overpotential of CoSe2@CNWs is as low as 130 mV vs RHE, obviously lower than that of the bare CoSe2 NWs. These results indicate that the HER performances of CoSe2@CNWs have been significantly improved due to its unique core-shell architecture. The HER mechanism had been intensively studied, and two mechanisms have been widely accepted.[16] The two-step mechanisms sharing the same first step about the 8

adsorption of a hydrated proton onto the catalyst superficies through the Volmer reaction: H3O+ + e- → Hads + H2O

(1)

Then the Heyrovsky reaction include a hydrated proton bond with the adsorbed hydrogen atom with an electron transfer (Heyrovsky step): H3O+ + Hads + e- → H2 + H2O

(2)

Or the recombination of two adsorbed hydrogen atoms (Tafel reaction): Hads + Hads → H2

(3)

According to previous reports, a slope of ∼120 mV dec-1, ∼40 and ∼30 mV dec-1 correspond with the Volmer reaction, Heyrovsky and Tafel reaction respectively as the rate-determining step during the HER. In this study, the Tafel data was collected by LSV to evaluate the activity of the catalyst to reflect an intrinsic characteristic of electrocatalyst.[39,40] In Fig. 4b, the corresponding Tafel plots (log j vs E) reveal a much lower slope of 41.3 mV dec-1 for CoSe2@CNWs compared with that of the bare CoSe2 NWs (57.4. mV dec-1), indicating the Volmer-Heyrovsky combination mechanism is operative in HER process. Compared with the previously reported values for other nanowire HER catalysts (Table S1), the Tafel slope of CoSe2@CNWs is relatively low. These results show that CoSe2@CNWs have excellent HER catalysis activity. Generally, the amount of the catalyst is critical to the HER performance. The dependence of HER performance on various catalyst loading amounts of CoSe2@CNWs on GCE has been investigated. From Fig. S2b, the current density (at overpotential of 250 mV vs RHE) increases with increasing loading, and after the 9

loading is higher than 424.4μg cm-2 the current density tends to be saturated. However, the Tafel slope increases shapely after the loading rise to 282.9μg cm-2. This can be attributed to excessive loading of catalyst, which easily causes aggregation, resulting in limited HER performance. [22] To further confirm the superior HER performance of CoSe2@CNWs, the doublelayer capacitance (Cdl) was used to evaluate the electrochemical active surface area.[41] As is known to all, cyclic voltammetry can be used to calculate the Cdl of the catalyst by measuring different charging current response as a function of scan rate. The voltammograms of the CoSe2@CNWs and bare CoSe2 NWs were carried out at various scan rates (20–200 mV s-1), as shown in Fig. S1. Fig. 4c exhibits plots of double-layer charging current vs. scan rate with linear plots, which was determinated from Fig.S1. As a result, a half of linear slope equals to the Cdl.[14] It is noted that the Cdl of CoSe2@CNWs (15.5 mF cm-2) is 4.7 times higher than that of the bare CoSe2 NWs (2.7 mF cm-2). These results reflect that the excellent structure of CoSe2@CNWs which contributes to larger electrochemically active surface area. In this regard, the large current density of the CoSe2@CNWs can be attributed to much larger active surface area of in HER process. The electrical conductivity and the AC impedance of the samples can be used to analyze the HER activity. The conductivities of the CoSe2@CNWs, and bare CoSe2 NWs are 3.423, and 2.998 S cm-1, respectively. It indicates that due to the presence of carbon shell, the conductivity of CoSe2@CNWs is 14.2% higher than that of bare CoSe2 NWs. The electrochemical impedance spectroscopy (EIS) of both samples were also 10

measured. Fig. 4d shows the Nyquist plots and their equivalent circuit of the CoSe2@CNWs and the bare CoSe2 NWs. The Rct (charge transfer resistance) of the CoSe2@CNWs is only 49 Ω, much lower than that of the bare CoSe2 NWs’ (111 Ω), which leads to faster HER kinetics, as a result of the core-shell architecture. Long-term current stability is essential for practical water electrolysis with long-time scales of operation at high current densities.[41] To investigate the role of the carbon shell, long-term cycling of the CoSe2@CNWs modified GCE was carried out using continuous LSV at an accelerated scanning rate of 100 mV s-1 for 1500 cycles. As shown in Fig. 5a, the initial current density is 28.9 mA cm-2 (at -250 mV vs RHE) and it retains 27.2 mA cm-2 even after 1500 cycles, suggesting an exceptional stability in acidic electrolyte with a high current retention of 94.1%. Owing to the protective carbon shell, the active of CoSe2 still maintains the excellent catalysis activity after long-term cycling, in which the bare CoSe2 NWs without carbon shell show severe current decay, as shown in Fig. S3a. We also utilize the time vs current density plots for supplementary specification. As shown in Fig. 5b, the curves present time dependence of current density at a constant RHE potential (−0.2 V vs RHE) with a hackle-like current curve partial enlarged detail indicating the alternate procedures of bubble with accumulation and release.[42] Only a slight loss of current density observed after 15 hours certifies an excellent stability compared with the results of bare CoSe2 NWs in Fig. S3b.

4. Conclusions

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In this study, CoSe2@CNWs were synthesized through a facile hydrothermal reaction and subsequent selenylation process. Compared with the bare CoSe2 NWs, CoSe2@CNWs shows preferable HER activity in acidic media with a lower onset overpotential of 130 mV vs RHE, smaller Tafel slope of 41.3 mV dec-1, and larger current density 27.5 mA cm-2 at the -250 mV vs RHE. The small Tafel slope of 41.3 mV dec-1 is promising to drive a large current density at low overpotential. In the meantime, CoSe2@CNWs also shows excellent stability with a current retention of 94.1% after 1500 cycles. The unique architecture constructed by CoSe2 nanocrystals embedded into carbon nanowires profits not only electrocatalytic activity but durable stability. The CoSe2@CNWs can be with low cost, which is promising as a highly efficient non-noble catalyst for hydrogen evolution.

Acknowledgments The research was supported by the National Natural Science Foundation of China (Grant Nos. 51372033, 51202022, and 61378028), National High Technology Research and Development Program of China (Grant No. 2015AA034202), and the 111 Project (Grant No. B13042).

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Figure captions

Fig.1. Schematics of the synthesis route of CoSe2@CNWs. Fig.2. (a) XRD patterns of CoSe2@CNWs, bare CoSe2 and Co@CNWs. (b) Raman spectra of CoSe2@CNWs. XPS spectra of (c) Co 2p, (d) Se 3d of the CoSe2@CNWs. Fig. 3. (a) SEM image of (a) CoSe2@CNWs and (b) bare CoSe2 NWs. (c) TEM image of CoSe2@CNWs. (d) HR-TEM image of CoSe2@CNWs. Fig. 4. (a) Polarization curves at 5 mV s-1 with current density normalized by geometric surface area and (b) corresponding Tafel plots of CoSe2@CNWs, bare CoSe2 NWs, and Pt/C catalyst. (c) Estimated Cdl of the CoSe2@CNWs, bare CoSe2 NWs. (d) Nyquist plots and equivalent circuit of the CoSe2@CNWs, bare CoSe2 NWs at −0.18V vs RHE. Fig. 5. (a) Durability test for CoSe2@CNWs after 1500 CV cycles under air atmosphere. (b) Time dependence vs current density and its partial enlarged detail of CoSe2@CNWs.

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