C@MoS 2 CoreâShell Nanofiber Electrocatalyst for Water Splitting

Subscriber access provided by Karolinska Institutet, University Library

Letter

Bifunctional and efficient CoS2-C@MoS2 coreshell nanofiber electrocatalyst for water splitting Yun Zhu, Lifei Song, Na Song, Meixuan Li, Ce Wang, and Xiaofeng Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05462 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Bifunctional and Efficient CoS2-C@MoS2 Core-shell Nanofiber Electrocatalyst for Water Splitting

Yun Zhu, Lifei Song, Na Song, Meixuan Li, Ce Wang, Xiaofeng Lu*

Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, 2699 Qianjin Street, Gaoxin District, Changchun 130012, P. R. China *Corresponding Author. Tel.: (+86)-431-8516-8292; fax: (+86)-431-8516-8292. E-mail address: [email protected] (Xiaofeng Lu).

KEYWORDS: CoS2-C@MoS2 Core-shell nanofiber, HER, OER, Synergistic effect, Electrocatalysis.

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT It is highly desirable to develop alternative non-noble metal hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts with the advantages of low cost and high efficiency. In this study, we report the synthesis of CoS2-C@MoS2 core-shell nanofibers by using electrospun Co-carbon nanofibers as templates, followed by a one-step hydrothermal reaction accompanied with a sulfurization process. Benefited from the unique structure and the synergistic effect between the components, the resultant CoS2-C@MoS2 core-shell nanofibers exhibit excellent electrocatalytic activity and stability toward HER, showing the overpotential of approximately 173 mV at 10 mA cm-2 and virtually immobile current density after 1000 cycles. Furthermore, the CoS2-C@MoS2 core-shell nanofibers also show a good OER catalytic activity, providing them as efficient candidates for bifunctional water splitting electrocatalysts.

2


Page 2 of 22

Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION The energy crisis and environment pollution are imminent worldwide problems, which induce dramatically increased demand for producing a sustainable, renewable and clean energy source.1-7 Over the past decades, many efforts has been made for developing solar and wind source as promising energy sources, however, their intrinsic property of unpredictability and intermittency greatly inhibits their actual applications. Recently, electrochemical water splitting as a promising solution to produce clean-fuel hydrogen and oxygen has attracted more and more attention because it could equalize the intermittent generation of electrical energy from solar and wind sources. Generally, electrochemical water splitting mainly contains two half-cell reactions: hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode, in which noble metals and their oxides exhibit effective catalytic ability leading to a significant decrement of uphill-reaction overpotentials, such as Pt and Ir oxides.8-10 Nevertheless, it is difficult to put these noble metals into large-scale applications on account of their low abundance and high costs. In recent years, there are much research work focusing on the development of low-cost earth-abundant metal catalyst with a high activity, long-term stability and wide availability.11-14 Transition metal chalcogenides, such as CoxS, NiS2, and MoxS, have been regarded as one of the most high-profile candidates to catalyze water splitting reaction.15-20 Among them, molybdenum disulfide (MoS2) is in fore due to its unique layer structure with short-range ordering and excellent electrochemical performance. In the earlier report from Chorkendorff’s group, MoS2 was proposed as an effective catalyst for HER.21 By comparing the HER activity of MoS2 nanoparticles with different sizes, the electrocatalytic activity in a linear correlation with the amount of edge sites on MoS2 nanoparticles rather than basal planes was investigated. On account of this tissue, considerable efforts have been devoted for the design of varied types of MoS2 nanostructure in case of aggregation, including monolayer MoS2,22 amorphous MoS2 porous films,23 and 2D MoS2 nanosheets.24 Nevertheless, there remains a nonnegligible obstacle for MoS2 to be a perfect substitute for noble metals because of 3



its poor conductivity.25, 26 To promote the catalytic efficiency of MoS2 nanomaterials, the hybridization with other materials is the most eurytopic route because it is beneficial for the electron transport. As we know, CoS2 is a kind of half-metallic material which has already been accepted as an alternative to ruthenium and iridium oxide. Due to the intrinsic appreciable metal-like conductivity (6.7 × 103 S cm-1 at 300 K) and the proper integration of electrochemical active sites, the catalytic performance of CoS2 in OER is remarkably superior to many other materials.27 As mentioned in the previous report, the electrochemical catalytic performance of transition metal sulfide also relies heavily on the surface composition which contains hydride acceptor sites and proton acceptor sites, inducing a weak bonding between intermediates in splitting reaction and sulfur on the surface.28 Recently, Yang and co-workers demonstrated that the introduction of CNTs into CoS2 and the design of porous nanostructure could achieve an enhanced OER catalytic activity, which should be attributed to the improved electrical conductivity for rapid electron transfer, and large exposure of active surface sites. Based on the considerations mentioned above, we proposed an effective approach to fabricate CoS2-C@MoS2 core-shell nanofibers as catalysts with enhanced HER and OER performance. CoS2-C@MoS2 core-shell nanofibers show superior HER kinetics with an over potential of approximately 173 mV at 10 mA cm-2 and Tafel slope of 61 mV dec-1, which should be ascribed to their unique structure and synergistic effect between components. CoS2-C@MoS2 core-shell nanofibers also made a good performance on stability during the electrochemical test, showing virtually immobile current density after 1000 cycles. Furthermore, CoS2-C@MoS2 core-shell nanofibers displayed a good performance on OER with a Tafel slope as small as 46 mV dec-1, and favorable performance maintained for 1000 cycles CV test, providing them as efficient candidates for bifunctional water splitting electrocatalysts.

RESULTS AND DISCUSSION 4


Page 4 of 22

Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hierarchical CoS2-C@MoS2 core-shell nanofibers were synthesized by using Co-C nanofibers as hard templates, followed by a one-step hydrothermal reaction for the decoration of MoS2 nanosheets and the sulfurization process of cobalt. The electrospinning technique was reported as an efficient approach to produce nanofibers in quantity, inducing it a first-best solution to prepare polyacrylonitrile (PAN)/Co(Ac)2 nanofibers.29 Next, the PAN/Co(Ac)2 nanofibers were calcined in Ar atmosphere at 650°C, leading to the formation of Co-C composite nanofibers. As shown in Figure 1a, the as-synthesized Co-C nanofibers possess diameters ranging from 108 to 153 nm, with a rough surface and well distribution of cobalt nanoparticles. To grow MoS2 nanosheets on the surface of Co-C nanofibers, a typical hydrothermal route was carried out as described in many previous researches, using (NH4)2MoS4 as both Mo and sulfur source in the reaction system. During the formation of MoS2 nanosheets, the cobalt nanoparticles inside could also be etched

Figure 1 SEM and corresponding inset TEM image of (a) Co-C nanofibers, (b) CoS2-C@MoS2-5, (c) CoS2-C@MoS2-25 and (d) CoS2-C@MoS2-50 core-shell nanofibers. 5



Page 6 of 22

and sulfurized into CoS2. Finally, the as-synthesized products possess MoS2 nanosheets as shell coating on the surface of CoS2-C nanofibers. The shell thickness of core-shell nanofibers could be controlled simply by varying the amount of (NH4)2MoS4. As can been seen in Figure 1b, c and d, the shell thickness increases with the increment of the quantity of (NH4)2MoS4 in the reaction system, respectively. When 5 mg of (NH4)2MoS4 was introduced, a thin layer of MoS2 nanosheets with several to ten nanometers were formed on the CoS2-C nanofibers. After increasing the content of (NH4)2MoS4 to 25 mg, CoS2-C@MoS2 nanofibers with a core-shell structure with shell thickness from 30 to 70 nm were observed. The TEM image also exhibits an intense interface between MoS2 nanosheets and CoS2-C nanofibers, which is beneficial for charge transfer and thus enhances the electrocatalytic activity. Furthermore, when 50 mg of (NH4)2MoS4 was added into the reaction system, CoS2-C nanofibers were coated with a dense and thick MoS2 nanosheets which exhibited darker contrast than others, making it difficult to determine the CoS2-C core in the TEM image. For comparison, C@MoS2 and CoS2-C nanofibers have also been prepared, which showed a core-shell structure decorated with MoS2 nanosheets and solid nanofibers encapsulated within CoS2 nanoparticles, respectively (Figure S1 and S2). Furthermore,

high-resolution

transmission

electron

microscopy

(HRTEM)

characterization exhibited the exact crystalline fingerprint of CoS2 and MoS2 in CoS2-C@MoS2 core-shell nanofibers. The typical lattice plane distance of 0.28 nm was indexed to CoS2 (200) crystal plane as given in Figure 2b, revealing the formation of CoS2 phase.30 It is worth noticed that, the special 2D transition metal sulfide MoS2 was proved to have lamellar crystal structure, making its layer-to-layer spacing very distinct.31,

32

Figure 2c displayed the layer-to-layer fingerprint of 0.62

nm fitted well with the layer distance of MoS2 and the numbers of layers also suggesting the ultrathin morphology of these nanosheets. The energy dispersive X-ray (EDX) spectrum in Figure 2d showed obvious peaks corresponding to Co, Mo, S, C, N, O, Cu and Si elements, respectively, but the signals of Si and Cu were resulting from the instrument and carbon-coated copper grid. Furthermore, elemental mapping 6


Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. HRTEM analysis of CoS2-C@MoS2-25: (a) TEM image, (b) and (c) HRTEM images focus on different positions, (c) EDX spectrum, (e) HAADF-STEM image and EDX mapping of (f) Co, (g) Mo, (h) S, (i) C and (j) O. was applied to analyze the elemental distribution of CoS2-C@MoS2 core-shell nanofibers as given in Figure 2e-g. We could notice that the distribution of Mo and S elements was wider than Co, C, and N elements, suggesting the formation of hierarchical core-shell structure. The crystalline phase was further investigated with X-ray diffraction (XRD) analysis to ensure the existence of CoS2 and MoS2. In Figure S3, the XRD pattern of CoS2-C NFs showed five distinct peaks at 32.3°, 36.3°, 39.9°, 46.4°, and 55.1° corresponded to (200), (210), (211), (220) and (311) planes of the cubic structured CoS2 in cattierite phase (JCPDS no. 41-1471).31 On the other hand, the characteristic peaks at 14.2°, 33.2° and 58.7° were observed for the C@MoS2 core-shell nanofibers, which are fitted well with (002), (100) and (110) planes of MoS2 (JCPDS no. 37-1492).33 Compared with these two materials containing single sulfide component, the prepared CoS2-C@MoS2-25 nanofibers displayed both diffraction peaks from MoS2 and CoS2, demonstrating the formation of MoS2 nanosheets on the surface of 7



CoS2-C nanofibers. Especially, the diffraction signal located at 14.2° of the CoS2-C@MoS2-25 nanofibers was in accordance with (002) reflection of MoS2 (JCPDS Card no. 37-1492), corresponding to the interlamellar distance of 0.62 nm as measured in HRTEM image.33 As illustrated in previous report, the appearance of this diffraction peak reveals the existence of periodic sequence of MoS2 which was connected with weak van der Waals force, demonstrating that it is easy to be intercalated and beneficial for the exposure of more MoS2 edges. The surface component and chemical state were also characterized with X-Ray photoelectron spectroscopy (XPS) ulteriorly as shown in Figure S4. The spectrum confirmed the presence of Co, Mo, S, C, N, and O elements in the obtained CoS2-C@MoS2 core-shell nanofibers (Figure S4a). In the fine spectra of Co 2p, the electron signals at 794.2 and 779.2 eV displayed in Figure S4b were ascribed to Co 2p1/2 and Co 2p3/2, respectively, which were fitted well with the previous reported results of Co 2p in CoS2.34 The electron binding signals in Figure S4c confirmed the chemical state of Mo specie, in which the doublet binding energy peaks located at 232.2 and 229.0 eV were in agreement with 3d3/2 and 3d5/2 of Mo4+, respectively.35 Additionally, the 2s peak of S at 226.4 eV was also observed in Figure S4c. And in the fine S 2p level spectrum, there were two well-known doublet peaks at around 163.1 and 161.9 eV, which are in accordance with S 2p1/2 and 2p3/2. And the spin-orbit energy separation is 1.2 eV, suggesting the existence of S2- in CoS2-C@MoS2 core-shell nanofibers.36 Additionally, in the high resolution spectrum of C 1s (Figure S4e), the energy binding peak could be split up to four peaks corresponding to C=C-C (284.6 eV), C-S (284.9 eV), C-O/C-N (285.8 eV) and C=O-OH (288 eV) signals.37 The existence of C-S and C-N species was ascribed to the replacement of C atom with S and N atom from the sulfuration and carbonization process, respectively. The N 1s fine spectrum displayed that the peak was composed of four types of N: pyridinic N (395 eV), pyrrolic N (396.4 eV), graphitic N (399.4 eV), and oxidized N (402 eV).38 From all of the XPS results, it could be confirmed that the Co-C nanofibers have been converted into CoS2-C nanofibers as well as the formation of MoS2 on their surface. 8


Page 8 of 22

Page 9 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Electrochemical analysis of hydrogen evolution reaction. (a) LSV curves and (b) corresponding Tafel plots of CoS2-C@MoS2-5, CoS2-C@MoS2-25, CoS2-C@MoS2-50,

C@MoS2,

and

CoS2-C

nanofibers.

(c)

CV

curves

of

CoS2-C@MoS2-25. (d) The current density difference of sweeps between anodic and cathodic versus the scan rate at the middle voltage. The double-layer capacitance values of CoS2-C@MoS2-5, CoS2-C@MoS2-25, CoS2-C@MoS2-50 is obtained with the fitting lines. (e) Nyquist plots of CoS2-C@MoS2-5, CoS2-C@MoS2-25, CoS2-C@MoS2-50. (f) LSV curves of CoS2-C@MoS2-25 as working electrode initially and after 1000 CV cycles. The as-prepared CoS2-C@MoS2 core-shell nanofibers were investigated to be an efficient electrocatalyst for HER in acidic conditions. The HER activity test was carried out under Ar-saturated 0.5 M H2SO4 electrolyte with a three-electrode configuration system. We evaluated the HER activities of CoS2-C@MoS2 core-shell nanofibers with different density of MoS2 nanosheets to analyze the synergistic effect of the components. GCEs modified with CoS2-C@MoS2-5, CoS2-C@MoS2-25, 9



CoS2-C@MoS2-50, C@MoS2, and CoS2-C nanofibers were applied as working electrodes for comparison. Figure 3a exhibits the corresponding linear sweep voltammetry (LSV) curves of those materials with reversible hydrogen electrode (RHE) as abscissa axis. The material labeled as CoS2-C@MoS2-25 possessed a dramatically improvement on the HER activity compared with the other materials. The overpotential of CoS2-C@MoS2-25 was approximately 173 mV at 10 mA cm-2, which was much lower than that of the CoS2-C@MoS2-5 (210 mV), CoS2-C@MoS2-50 (199 mV), C@MoS2 (225 mV) shown in table S1, and also lower than many reported materials listed in table S2. It is worth noting that, CoS2-C showed negligible catalytic ability toward the hydrogen production process, similar with C@MoS2 as working electrode, suggesting that the excellent HER performance was owing to the cooperation of compounds. As illustrated by some researchers in previous reports, 39-41 MoS2 was known as an active HER material; however, the poor conductivity inhibited its practical application. In this work, CoS2-C as core material is beneficial for the rapid transfer of electrons, which could recover the defect of MoS2, thus promoting the HER catalytic activity. The Tafel slopes of these materials were obtained by fitting the corresponding polarization LSV curves with the typical Tafel equation (η= blogj + a), where b represented as Tafel slope and j is current density. The Tafel slope of CoS2-C@MoS2-25 was calculated to be 61 mV dec-1 as displayed in Figure 3b, which is lower than the other materials, including CoS2-C@MoS2-5 (67 mV dec-1), CoS2-C@MoS2-50 (74 mV dec-1), C@MoS2(96 mV dec-1), and CoS2-C (96 mV dec-1), representing the superior HER kinetics (Table S1).

To make a deeper investigation on the mechanism for the outstanding HER activity of CoS2-C@MoS2-25, we analyzed the electrochemically active surface area (ECSA) and electrochemical impedance (EI) of CoS2-C@MoS2 nanofibers with different density MoS2 nanosheets. The electrochemical double-layer capacitance (EDLC) measurement was applied to evaluate the ECSA by testing the function of capacitive current and scan rate (Figure 3c).42-44 The EDLC values of CoS2-C@MoS2-5, CoS2-C@MoS2-25 and CoS2-C@MoS2-50 were calculated to be 2.99, 6.32 and 3.44 10


Page 10 of 22

Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mF cm-2, respectively, as displayed in Figure 3d. As expected, CoS2-C@MoS2-25 possesses the largest electrochemical active area, revealing the existence of more effective sites. As mentioned before, the active sites of MoS2 mainly located at the edges or defects on sheet surface. The thin shell of MoS2 in CoS2-C@MoS2-5 sample couldn’t provide effective active edges of MoS2 sheets. Although the sample of CoS2-C@MoS2-50 possesses thicker shell structure, it didn’t exhibit enhanced electrochemical performance, which might be due to the formation of dense shell inhibiting the effective contact between active sites and electrolyte. The dense shell consisted of MoS2 nanosheets also lead to the delay of electrons transfer from surface to inside. The Nyquist plots in Figure 3e provided potential evidence for the poor HER activity of CoS2-C@MoS2-50. CoS2-C@MoS2-50 showed the largest EI value compared with CoS2-C@MoS2-5 and CoS2-C@MoS2-25 due to the dense MoS2 shell structure, hindering the electrons transfer from active sites. As a brief summary, the glorious HER activity of CoS2-C@MoS2 nanofibers relied heavily on the synergistic effect of different components: using MoS2 nanosheets as shell structure would increase the electrochemical active sites dramatically; meanwhile the CoS2-C acted as an effective channel to accelerate the charge transfer generated from MoS2. The stability of CoS2-C@MoS2 core-shell nanofibers is also an essential factor for the feasibility of their application. Successive cyclic voltammetry (CV) test was applied to evaluate the stability of CoS2-C@MoS2-25 in acidic electrolyte at scanning rate of 50 mV s-1. The current density could remain virtually immobile for 1000 cycles (Figure 3f). As we know, MoS2 is always regarded as a promising catalyst for HER ascribed to its excellent chemisorption capability for hydrogen, but an inert material for OER activity which is related with the sluggish proton-couple charge transfer process acted as a bottle-neck step for hydrolysis reaction. Interestingly, we found that, the integration of CoS2 and the robust construction of hierarchical core-shell structure contributed an enhanced OER activity of MoS2. As displayed in Figure 4, the OER catalytic activity of CoS2-C@MoS2-25 core-shell nanofibers was estimated in an alkaline electrolyte, and CoS2-C@MoS2-5 and CoS2-C@MoS2-50 were also tested for 11



analysis.

The

LSV

CoS2-C@MoS2-25

at

curves

in

Figure

4a

revealed

391

mV,

much

better

than

Page 12 of 22

the the

overpotential

of

overpotentials

of

CoS2-C@MoS2-5, CoS2-C@MoS2-50, and CoS2-C at 405 mV, 431 mV, and 400 mV with the current density achieved at 10 mA cm-2 (Table S1). The current density of C@MoS2 couldn’t reach 10 mA cm-2 until the overpotential increased to 520 mV, revealing the extremely poor OER catalytic ability. Accordingly, the Tafel plots were figured out from OER polarization curves. As shown in Figure 4b, CoS2-C@MoS2-25 possess lower Tafel slope of 46 mV dec-1 than CoS2-C@MoS2-5 (64mV dec-1), CoS2-C@MoS2-50 (80 mV dec-1), CoS2-C (72 mV dec-1) and C@MoS2 (103 mV dec-1) (Table S1). By comparing, CoS2-C@MoS2-25 exhibited the best OER activity which was even much better than many electrocatalysts in Table S3 produced by other groups, and the C@MoS2 possessed the worst OER performance. It is worth mentioned that the CoS2-C exhibited a lower overpotential at 10 mA cm-2 and Tafel slope than CoS2-C@MoS2-50 and C@MoS2, revealing the existence of CoS2 contributed a lot to the OER activity of as-synthesized material. However, the catalytic ability of CoS2-C nanofibers couldn’t catch CoS2-C@MoS2-25, suggesting the improvement of OER activity relies on not only the intrinsic ability of CoS2, but also the synergistic effect between CoS2 and MoS2. As illustrated in many previous reported articles, 19,42,45 the construction of core-shell structure enlarged the specific area and the porous nanosheets stacking morphology provided effective channels for electrolyte infiltrating, which would be beneficial for transport of ions and electrons during the charge-discharge process. The CoS2-C substrate contributed not only for the building of hierarchical nanosheets structure, but also reduce the internal resistance and provide a continuous pathway for electrons transfer. From the comparison among CoS2-C@MoS2-5, CoS2-C@MoS2-25 and CoS2-C@MoS2-50, it is observed that, the samples of over-covered-like CoS2-C@MoS2-50 or less-coverd-like CoS2-C@MoS2-5 couldn’t give better electrochemical catalytic performance, implying that the integration of different materials with proper proportion is important for engineering the electrocatalytic ability of materials. 12


Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CoS2-C@MoS2-25 also exhibited a favorable OER stability. As shown in Figure S5, the LSV curve after 1000 cycles test did not change much compared with the initial one. To make deep investigation on the OER performance of electrocatalyst, we characterized the final product after stability test with XPS, SEM and EDX measurements. The XPS spectra were displayed in Figure S6, which could be clearly seen that there were obvious changes occurred after stability test. In the survey spectrum, the signal from Mo and S decreased a lot comparing with the signal before stability test (Figure S6a). In contrast, the peak resulting from O rose up significantly. It is interesting that, the fine spectrum of Co in Figure S6b just showed tiny changes which could even be ignored, verifying the high stability of Co2S in OER cycle test. However, the XPS change of Mo revealed the oxidation of Mo from sulfide to oxide (Figure S6c). The two peaks located at 232.3 and 234.0 eV were coincident with 3d5/2 and 3d3/2 of Mo6+ in MoO3, respectively, which was soluble in alkaline solution.46,47 Similar with Mo species, the high-resolution XPS spectrum of S showed significant variations after the stability test (Figure S6d). The emerging peak at 168.9 eV was fitted well with the signal from S in SO42- or –SO3H form, revealing the oxidation of S after the stability test.48 The formation of soluble form of Mo and S leaded to the dramatically loss as shown in survey spectrum (Figure S6a). The O 1s peak after stability test could be split into three peaks, the additional peak labeled as O1 at 529.6

Figure 4. Electrochemical analysis of oxygen evolution reaction: (a) LSV curves and (b)

corresponding

Tafel

plots

of

CoS2-C@MoS2-5,

CoS2-C@MoS2-50, CoS2-C and C@MoS2 nanofibers. 13


CoS2-C@MoS2-25,


eV was related to metal-O band, indicating the surface oxidation of metal species (Figure S6e). The other two peaks labeled as O2 and O3 should be assigned to lattice oxygen and –OH, respectively.48 The results from XPS spectra demonstrated that Co, Mo and S should be the active species while MoS2 was not stable during the OER test. The obtained sample after stability test was further investigated with SEM and EDX measurements, and the results were shown in Figure S7. The morphology of CoS2-C@MoS2 core-shell nanofiber changed a lot by comparing the SEM image before (Figure S7a) and after (Figure S7b) the stability test. The material maintained its nanofiber structure, but the nanosheet-like shell was damaged, revealing that the OER stability test destroyed the surface component rather than core component. According to the corresponding EDX spectrum, the signal of Mo and S reduced significantly, and the content of oxygen increased a lot, consistent with the results of XPS analysis. The Co species didn’t change much, which should be due to the protection of shell and the encapsulation within carbon matrix.

CONCLUSIONS In summary, we have fabricated hierarchical CoS2-C@MoS2 core-shell nanofibers, as a non-noble metal catalyst with enhanced HER and OER performance. The electrochemical statistics demonstrate that the exposure of active sites and conductivity as two crucial factors contribute to the electrocatalytic activity. CoS2-C surmounts the drawbacks of MoS2 on conductivity, and arouses the HER and OER activity of MoS2 on account of the synergistic effect. This study demonstrates a simple and versatile solution for the fabrication of unique core-shell structured fibrous nanomaterials as efficient bifunctional electrocatalyst for HER and OER, which provides a new idea to construct many other multi-dimensional nanostructures in designing different types of high-performance electrocatalysts.

ASSOCIATED CONTENT Supporting information Experimental section, SEM and TEM images of C@MoS2 core-shell nanofibers 14


Page 14 of 22

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and CoS2-C nanofibers, XRD patterns of CoS2-C nanofibers, CoS2-C@MoS2-25 core-shell nanofibers and C@MoS2 core-shell nanofibers, XPS spectrums, LSV curves, SEM images and EDX spectra before and after OER stability test, calibration of reference electrode, tables of electrochemical catalytic performance comparison among as-prepared materials with others.

AUTHOR INFORMATION Corresponding Author Fax & Tel: +86-431-85168292; E-mail: [email protected] ORCID Xiaofeng Lu: 0000-0001-8900-9594 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51773075, 51473065, 21474043).

REFERENCES 1. Xu, J.; Lawson, T.; Fan, H. B.; Su, D. W. and Wang, G. X. Updated metal compounds (MOFs, -S, -OH, -N, -C) used as cathode materials for lithium– sulfur batteries. Adv. Energy Mater. 2018, 8 (10), 1702607, DOI: 10.1002/aenm.201702607. 2. Pumera, M. Graphene-based nanomaterials for energy storage. Energy Environ. Sci. 2011, 4 (3), 668-674, DOI: 10.1039/C0EE00295J. 3. Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.; Van, S. W. Nanostructured materials for advanced energy conversion and storage devices. Nat. Mater. 2005, 4 (5), 366-377, DOI:10.1038/nmat1368. 15



Page 16 of 22

4. Lewis, N. S. Toward cost-effective solar energy use. Science 2007, 315 (5813), 798-801, DOI: 10.1126/science.1137014. 5. Yang, N. L.; Zhai, J.; Wang, D.; Chen, Y. S.; Jiang, L. Two-dimensional graphene bridges enhanced photoinduced charge transport in dye-sensitized solar cells. ACS Nano 2010, 4 (2), 887-894, DOI: 10.1021/nn901660v. 6. Barber, J., Photosynthetic energy conversion: natural and artificial. Chem. Soc. Rev. 2009, 38 (1), 185-196, DOI: 10.1039/B802262N. 7. Wang, H. T.; Lee, H. W.; Deng, Y.; Lu, Z. Y.; Hsu, P. C.; Liu, Y. Y.; Lin, D. C.; Cui, Y. Bifunctional non-noble metal oxide nanoparticle electrocatalysts through lithium-induced conversion for overall water splitting. Nat. Commun. 2015, 6, 7261, DOI: 10.1038/ncomms8261. 8. Yu, X. W.; Zhang, M.; Chen, J.; Li, Y. G.; Shi, G. Q. Nitrogen and sulfur codoped graphite foam as a self-supported metal-free electrocatalytic electrode for water oxidation. Adv. Energy Mater. 2016, 6 (2), 1501492, DOI: 10.1002/aenm.201501492. 9. Sun,W.; Wang, Z. Q.; Zaman, W. Q.; Zhou, Z. H.; Cao, L. M.; Gong, X. Q.; Yang, J. Effect of lattice strain on the electro-catalytic activity of IrO2 for water

splitting.

Chem.

Commun.

2018,

54

(8),

996-999,

DOI:

10.1039/c7cc09580e. 10. Li, M. X.; Zhu, Y.; Song, N.; Wang, C.; Lu, X. F. Fabrication of Pt nanoparticles on nitrogen-doped carbon/Ni nanofibers for improved hydrogen evolution

activity.

J.

Colloid

Interface

Sci.

2018,

514,

199-207,

DOI:10.1016/j.jcis.2017.12.028. 11. Long, X.; Li, J. K.; Xiao, S.; Yan, K. Y.; Wang, Z. L.; Chen, H. N.; Yang, S. H. A strongly coupled graphene and FeNi double hydroxide hybrid as an excellent electrocatalyst for the oxygen evolution reaction. Angew. Chem. Int. Ed. 2014, 53 (29), 7584-7588, DOI: 10.1002/anie.201402822. 12. Song, F.; Hu, X. L. Exfoliation of layered double hydroxides for enhanced oxygen

evolution

catalysis.

Nat.

Commun.

10.1038/ncomms5477. 16


2014,

5,

4477,

DOI:

Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13. Yu, X. W.; Zhang, M.; Yuan, W. J.; Shi, G. Q. High-performance three-dimensional Ni-Fe layered double hydroxide/graphene electrode for water oxidation. J. Mater. Chem. A 2015, 3 (13), 6921-6928, DOI: 10.1039/c5ta01034a. 14. Hai, X.; Zhou, W.; Wang, S. Y.; Pang, H.; Chang, K.; Ichihara, F.; Ye, J. H. Rational design of freestanding MoS2 monolayers for hydrogen evolution reaction.

Nano

Energy

2017,

39,

409-417,

DOI:

10.1016/j.nanoen.2017.07.021. 15. Zou, X. X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 2015, 44 (15), 5148-5180, DOI: 10.1039/c4cs00448e. 16. Feng, L. L.; Yu, G. T.; Wu, Y. Y.; Li, G. D.; Li, H.; Sun, Y. H.; Asefa, T.; Chen, W.; Zou, X. X. High-index faceted Ni3S2 nanosheet arrays as highly active and ultrastable electrocatalysts for water splitting. J. Am. Chem. Soc. 2015, 137 (44), 14023-14026, DOI: 10.1021/jacs.5b08186. 17. Yang, J.; Wang, K.; Zhu, J. X.; Zhang, C.; Liu, T. X. Self-templated growth of vertically aligned 2H-1T MoS2 for efficient electrocatalytic hydrogen evolution. ACS Appl. Mater. Interfaces 2016, 8 (46), 31702-31708, DOI: 10.1021/acsami.6b11298. 18. Gu, H. H.; Fan, W.; Liu, T. X. Phosphorus-doped NiCo2S4 nanocrystals grown on electrospun carbon nanofibers as ultra-efficient electrocatalysts for the hydrogen evolution reaction. Nanoscale Horiz. 2017, 2 (5), 277-283, DOI: 10.1039/c7nh00066a. 19. Zhu, H.; Zhang, J. F.; Yanzhang, R.; Du, M. L.; Wang, Q. F.; Gao, G. H.; Wu, J. D.; Wu, G. M.; Zhang, M.; Liu, B.; Yao, J. M.; Zhang, X. W. When cubic cobalt sulfide meets layered molybdenum disulfide: a core–shell system toward synergetic electrocatalytic water splitting. Adv. Mater. 2015, 27 (32), 4752-4759, DOI: 10.1002/adma.201501969. 20. Zhu, H.; Gao, G. H.; Du, M. L.; Zhou, J. H.; Wang, K.; Wu, W. B.; Chen, X.; Li, Y.; Ma, P. M.; Dong, W. F.; Duan, F.; Chen, M. Q.; Wu, G. M.; Wu, J. D.; 17



Yang, H. T.; Guo, S. J. Atomic-scale core/shell structure engineering induces precise tensile strain to boost hydrogen evolution catalysis. Adv. Mater. 2018, 30 (26), 1707301, DOI: 10.1002/adma.201707301. 21. Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch S.; Chorkendroff, I. Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts. Science 2007, 317 (5834), 100-102, DOI: 10.1126/science.1141483. 22. Zhang, J.; Wu, J. J.; Guo, H.; Chen, W. B.; Yuan, J. T.; Martinez, U.; Gupta, G.; Mohite, A.; Ajayan P. M.; Lou, L. Unveiling active sites for the hydrogen evolution reaction on monolayer MoS2. Adv. Mater. 2017, 29 (42), 1701955, DOI: 10.1002/adma.201701955. 23. Lu, Z. Y.; Zhang, H. C.; Zhu, W.; Yu, X. Y.; Kuang, Y.; Chang, Z.; Lei, X. D.; Sun, X. M. In situ fabrication of porous MoS2 thin-films as high-performance catalysts for electrochemical hydrogen evolution. Chem. Common. 2013, 49 (68), 7516-7518, DOI: 10.1039/c3cc44143a. 24. Muralikrishna, S.; Manjunath, K.; Samrat, D.; Reddy, V.; Ramakrishnappa, T.; Nagaraju, D. H. Hydrothermal synthesis of 2D MoS2 nanosheets for electrocatalytic hydrogen evolution reaction. RSC Adv. 2015, 5 (109), 89389-89396, DOI: 10.1039/c5ra18855e. 25. Gong, F.; Ding, Z. W.; Fang, Y.; Tong, C. J.; Xia, D. W.; Lv, Y. Y.; Wang, B.; Papavassiliou, D. V.; Liao, J. X.; Wu, M. Q. Enhanced electrochemical and thermal transport properties of graphene/MoS2 heterostructures for energy storage: Insights from multi-scale modeling. ACS Appl. Mater. Interfaces 2018, 10 (17), 14614-14621, DOI: 10.1021/acsami.7b19582. 26. Chen, Z. B.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara, M. K.; Jaramillo, T. F. Core-shell MoO3-MoS2 nanowires for hydrogen evolution: a functional design for electrocatalytic materials. Nano. Lett. 2011, 11 (10), 4168-4175, DOI: 10.1021/nl2020476. 27. Faber, M. S.; Dziedzic, R.; Lukowski, M. A.; Kaiser, N. S.; Ding, Q.; Jin, S. High-performance electrocatalysis using metallic cobalt pyrite (CoS2) micro18


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and nanostructures J. Am. Chem. Soc. 2014, 136 (28), 10053-10061, DOI: 10.1021/ja504099w. 28. Chen, G. F.; Ma, T, Y.; Liu, Z. Q.; Li, N.; Su, Y. Z.; Davey, K.; Qiao, S. Z. Efficient and stable bifunctional electrocatalysts Ni/NixMy (M = P, S) for overall water splitting. Adv. Funct. Mater. 2016, 26 (19), 3314-3323, DOI: 10.1002/adfm.201505626. 29. Lu, X. F.; Wang, C.; Wei, Y. One-dimensional composite nanomaterials: synthesis by electrospinning and their applications. Small 2009, 5 (21), 2349-2370, DOI: 10.1002/smll.200900445. 30. Chen, L. L.; Yang, W. X.; Liu, X. J.; Jia, J. B. Flower-like CoS2/MoS2 nanocomposite with enhanced electrocatalytic activity for hydrogen evolution reaction. Int. J. Hydrogen Energy, 2017, 42 (17), 12246-12253, DOI: 10.1016/j.ijhydene.2017.03.054. 31. Guo, Y. X.; Gan, L. F.; Shang, C. S.; Wang, E. K.; Wang, J. A Cake‐style CoS2@MoS2/RGO hybrid catalyst for efficient hydrogen evolution. Adv. Funct. Mater. 2017, 27 (5), 1602699, DOI: 10.1002/adfm.201602699. 32. Zheng, X. L.; Xu, J. B.; Yan, K. Y.; Wang, H.; Wang, Z. L.; Yang, S. H. Space-confined growth of MoS2 nanosheets within graphite: the layered hybrid of MoS2 and graphene as an active catalyst for hydrogen evolution reaction. Chem. Mater. 2014, 26 (7). 2344-2353, DOI: 10.1021/cm500347r. 33. Li, M.; Wang, D.; Li, J. H.; Pan, Z. D.; Ma, H. J.; Jiang Y. X.; Tian, Z. J. Facile hydrothermal synthesis of MoS2 nano-sheets with controllable structures and enhanced catalytic performance for anthracene hydrogenation. RSC. Adv. 2016, 6 (75), 71534-71542, DOI: 10.1039/c6ra16084k. 34. Luan, F.; Zhang, S.; Chen, D. D.; Zheng, K.; Zhuang, X. M. CoS2-decorated ionic liquid-functionalized graphene as a novel hydrazine electrochemical sensor. Talanta 2018, 182, 529-535, DOI: 10.1016/j.talanta.2018.02.031. 35. Nan, H. Y.; Wang, Z. L.; Wang, W. H.; Liang, Z.; Lu, Y.; Chen, Q.; He, D. W.; Tan, P. H.; Miao, F.; Wang, X. R.; Wang, J. L.; Ni, Z. H. Strong 19



photoluminescence enhancement of MoS2 through defect engineering and oxygen bonding. ACS nano 2014, 8 (6), 5738-5745, DOI: 10.1021/nn500532f. 36. Deokar, G.; Vignaud, D.; Arenal, R.; Louette P.; Colomer, J. F. Synthesis and characterization of MoS2 nanosheets. Nanotechnology 2016, 28 (7), 075604. 37. Wan, S.; Liu, Y. P.; Li, G. D.; Li, X. T.; Wang, D. J.; Zou, X. X. Well-dispersed CoS2 nano-octahedra grown on a carbon fibre network as efficient electrocatalysts for hydrogen evolution reaction. Catal. Sci. Technol. 2016, 6 (12), 4545-4553, DOI: 10.1039/c5cy02292d. 38. Yang, F.; Wan, Q.; Duan, X. C.; Guo, W.; Mao, Y. H.; Ma, J. M. N-doped carbon/MoS2 composites as an excellent battery anode. RSC Adv. 2016, 6 (22), 18583-18586, DOI: 10.1039/c5ra24674a. 39. Laursen, A. B.; Kegnæs, S.; Dahl, S.; Chorkendorff, I. Molybdenum sulphides-efficient and viable materials for electro- and photoelectrocatalytic hydrogen evolution. Energy Environ. Sci. 2012, 5 (2), 5577-5591, DOI: 10.1039/c2ee02618j. 40. Kim, Y.; Tiwari, A. P.; Prakash, O.; Lee, H. Activation of ternary transition metal chalcogenide basal planes through chemical strain for the hydrogen evolution reaction. ChemPLusChem, 2017, 82 (5), 785-791, DOI : 10.1002/cplu.201700164. 41. Gao, M. R.; Liang, J. X.; Zheng, Y. R.; Xu, Y. F.; Jiang, J.; Gao, Q.; Li J.; Yu, S. H. An efficient molybdenum disulfide/cobalt diselenide hybrid catalyst for electrochemical hydrogen generation. Nat. Commun. 2015, 6, 5982. DOI: 10.1038/ncomms6982. 42. Zhang, H. C., Li, Y. J., Xu, T. H., Wang, J. B., Huo, Z. Y., Wang, P. B.; Sun, X. M. Amorphous Co-doped MoS2 nanosheet coated metallic CoS2 nanocubes as an excellent electrocatalyst for hydrogen evolution. J. Mater. Chem. A 2015, 3 (29), 15020-15023, DOI: 10.1039/c5ta03410h. 43. McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. J. Am. Chem. Soc. 2013, 135 (45), 16977-16987, DOI: 10.1021/ja407115p. 20


Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


44. Zhou, W. J.; Zhou, J.; Zhou, Y. C.; Lu, J.; Zhou, K.; Yang, L. J.; Tang, Z. H.; Li L. G.; Chen, S. W. N-doped carbon-wrapped cobalt nanoparticles on n-doped graphene nanosheets for high-efficiency hydrogen production. Chem. Mater., 2015, 27 (6), 2026-2032, DOI: 10.1021/acs.chemmater.5b00331. 45. Wang, L. N.;

Zhang, X.; Ma, Y.; Yang, M.; Qi, Y. X. Supercapacitor

performances of the MoS2/CoS2 nanotube arrays in situ grown on Ti plate. J. Phys. Chem. C 2017, 121 (17), 9089-9095, DOI: 10.1021/acs.jpcc.6b13026. 46. Sadighi, Z.; Liu, J. P.; Zhao, L.; Ciucci F.; Kim, J. K. Metallic MoS2 nanosheets: multifunctional electrocatalyst for the ORR, OER and Li–O2 batteries. Nanoscale, 2018, DOI: 10.1039/c8nr07106c. 47. Yu, Z. Y.; Duan, Y.; Gao, M. R.; Lang, C. C.; Zheng, Y. R.; Yu, S. H. A one-dimensional porous carbon-supported Ni/Mo2C dual catalyst for efficient water splitting. Chem. Sci. 2017, 8, 968-973, DOI: 10.1039/C6SC03356C. 48. Li, Y. X.; Yin, J.; An, L.; Lu, M.; Sun, K.; Zhao, Y. Q.; Gao, D. Q.; Cheng, F. Y.; Xi, P. X. FeS2/CoS2 Interface nanosheets as efficient bifunctional electrocatalyst for overall water splitting. Small 2018, 14 (26), 1801070, DOI: 10.1002/smll.201801070.

21



Page 22 of 22

Graphic for manuscript Synopsis The

as-synthesized

CoS2-C@MoS2

core-shell

nanofibers

electrocatalytic activities towards both HER and OER.

22


exhibit

excellent

C@MoS 2 CoreâShell Nanofiber Electrocatalyst for Water Splitting

Recommend Documents

C@MoS 2 CoreâShell Nanofiber Electrocatalyst for Water Splitting