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Uniform Deposition of Co3O4 Nanosheets on Exfoliated MoS2 Nanosheets as Advanced Catalysts for Water Splitting

Uniform Deposition of Co3O4 Nanosheets on Exfoliated MoS2 Nanosheets as Advanced Catalysts for Water Splitting

Electrochimica Acta 212 (2016) 890–897 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

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Electrochimica Acta 212 (2016) 890–897

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Uniform Deposition of Co3O4 Nanosheets on Exfoliated MoS2 Nanosheets as Advanced Catalysts for Water Splitting Xudan Wanga , Yuanyuan Zhenga , Junhua Yuana,b,* , Jianfeng Shenc,** , Ai-jun Wanga , Li Niud , Shengtang Huangb a

College of Life Sciences and Chemistry, College of Geography and Environmental Science, Zhejiang Normal University, Jinhua, Zhejiang, 321004, China School of Pharmacy, Hubei University of Science and Technology, Xianning, Hubei, 437100, China [1_TD$IF]Institute of Special Materials and Technology, Fudan University, Shanghai 200433, China d State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin, 130022, China b c

A R T I C L E I N F O

Article history: Received 20 April 2016 Received in revised form 18 June 2016 Accepted 14 July 2016 Available online 17 July 2016 Keywords: water splitting electrocatalysis oxygen evolution reaction transition metal hydroxides molybdenum disulfide

A B S T R A C T

Sluggish oxygen evolution reaction (OER) is the bottleneck for water splitting. Herein, an ultrathin Co3O4 nanosheet is uniformly deposited on the chemical exfoliated MoS2 nanosheet (ex-MoS2) via an in-situ hydrolysis and epitaxial growth. These novel noble-metal-free OER catalysts were proposed as an excellent platform for water splitting to promote the oxygen evolution activity and stability. MoS2 introduction can improve the conductivity of Co3O4 nanosheets, generate more Co3+ species in Co3O4 nanosheets during electrochemical activation and render high durability against dissolution of Co3O4 nanosheets in high potential polarization. As a result, these novel Co3O4/ex-MoS2 hybrids exhibit an excellent OER activity with a remarkable low Tafel slope (ca. 36 mV dec1), and a substantially small overpotential (ca. 290 mV required for 10 mA cm2). Their OER stability is also outstanding, with 95.2% OER activity retention for 10000 potential cyclings. The superior OER performance is compatible to, and even better than state of art OER catalysts reported recently. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Hydrogen economy is a promising solution to fossil fuel depletion and natural environment deterioration [1–3]. Water splitting can generate a clean, renewable and inexhaustible hydrogen fuel [4–6]. However, any processes involved with the H2 conversion from water undergo at a high kinetic overpotential associated with oxygen evolution reaction (OER), leading to a loss in the overall efficiency of water splitting [7–10]. Noble metals or its oxide, Pt, Ir and Ru, are usually used as ideal catalysts for this sluggish four-electron electrolysis [7,11]. But their practical use is prohibited by their scarcity and high cost. Recently, numerous efforts were dedicated to exploit alternatives for these OER catalysts so as to improve catalytic efficiency and reduce product cost. Transition metal oxide, such as Ni, Co, Mn oxide derivatives,

* Corresponding author at: College of Life Sciences and Chemistry, College of Geography and Environmental Science, Zhejiang Normal University, Jinhua, Zhejiang, 321004, China. Tel.: +86 579 82282269; fax: +86 579 82282269. ** Corresponding [12_TD$IF]authorat: Institute of Special Materials and Technology, Fudan University, Shanghai 200433, China.[13_TD$IF] Tel.: +86 021 55664095; fax: +86 021 55664095. E-mail addresses: [email protected] (J. Yuan), [email protected] (J. Shen). http://dx.doi.org/10.1016/j.electacta.2016.07.078 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

stand out from the candidates of OER catalysts due to their abundance and remarkable environment compatibility [12,13]. Among transition metal oxide, cobalt-based nanomaterials have emerged as a particularly efficient and affordable OER catalyst for water splitting [13]. However, these Co-based nanomaterials show low OER activity and stability due to their poor conductivity. To solve this problem, carbon supports, such as graphene, carbon nanotube and carbon fiber, were introduced into Co-based OER catalysts to accelerate electron transport and maximize catalytic efficiency [14–16]. However, for most carbon-support Pt catalysts in polymer electrolyte fuel cell, carbon supports are susceptible to oxidation at high overpotential, and the presence of O2 will speed up carbon corrosion in basic solution [17]. This support failure also accounts for the loss of activity and stability for OER catalysts [18]. Therefore, carbon-free supports should be recommended for design of OER catalysts [19,20]. Over the past decades, two-dimension molybdenum disulfide (MoS2) has attracted intensive attention as a versatile electrocatalyst owing to its prominent performance in application of catalysis, biosensor, battery and supercapacitor [21]. Herein, Co3O4 nanosheets were uniformly grown on the surface of exfoliated MoS2 by in-situ hydrolysis and epitaxial growth. The as-prepared

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Co3O4/ex-MoS2 hybrids show excellent OER activity and stability. Our works will offer a novel strategy to solve the bottleneck of water splitting. 2. Experimental 2.1. Chemicals and materials MoS2, Cobalt (II) acetate and isopropanol were purchased from Acros. Water was purified by a Milli-Q system. IrO2/C catalysts was prepared according to [22]. 2.2. Chemical exfoliation of MoS2 Bulk MoS2 is exfoliated according to the reference [23]. In brief, bulk MoS2 150 mg was sonicated in BRANSON 5510-MTH with the frequency of 40 kHz in the solution mixed with 60 mL isopropanol and 40 mL water. The MoS2 dispersion was centrifuged to collect the supernatants using Thermo Scientific Sorvall Legend X1 centrifuge at 1000 rpm for 10 min. This process was repeated thrice to further remove non-exfoliated materials.

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2.3. Preparation of Co3O4/ex-MoS2 hybrids A certain amount of Cobalt (II) acetate was dispersed and dissolved in the as-prepared MoS2 dispersion. The Co2+ ion will adsorbed on the surface of ex-MoS2. Its pH value was adjusted to be 9-10 with 1 M NH3H2O, and the mixture solution was stirred for 3 h. Co(OH)2 will be formed and grow on ex-MoS2. This precipitate was recovered by centrifugation and rinsed with alcohol and water. Finally, the precipitate was dried under vacuum at 60  C and annealed to transform Co(OH)2 into Co3O4 for 3 h in air at 300  C. For convenience, Co3O4/ex-MoS2 hybrids was recorded as Co3O4/ ex-MoS2 hybrids (x:y), x:y was defined as the molar ratio of Co3O4 and MoS2. For comparison, Co3O4 nanosheets were prepared in a strategy similar to that for Co3O4/ex-MoS2 hybrids without ex-MoS2. 2.4. Composition and structure analysis The morphology of samples was imaged by transmission electron microscopy (TEM, JEOL 2010 microscope). The composition was analyzed by Raman spectra at Renishaw Invia Raman microscope with an excitation wavelength of 633 nm, X-ray

[(Fig_1)TD$IG]

Fig 1. TEM images of Co3O4/ex-MoS2 hybrids with different Co3O4/MoS2 molar ratio: (A, B) 1:2, (C, D) 1:1 and (E, F) 2:1. Scale: (A, C, E) 200 nm, (B, D, F) 5 nm.

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photoelectron spectroscopy (XPS, ESCALAB-MKII spectrometer) and inductively coupled plasma-atom emission spectroscopy (ICPAES, TJA Atomscan Advantage instrument). Their crystalline data were obtained from X-ray powder diffraction (XRD, Philips PW3040/60 diffractometer). 2.5. Electrochemical investigations Electrochemical measurements were performed in a threeelectrode setup interfaced to an electrochemical station (CHI 760C). Typically, 2 mg of OER catalysts and 20 mL of Nafi[email protected] alcohol solution (0.5 wt%) were mixed by ultrasonic dispersion in 200 mL aqueous solution to form a homogeneous ink. Then, 8 mL of the OER catalysts dispersion was applied onto a glassy carbon disk electrode (5 mm diameter) controlled by Pine rotating electrode system. Pt plate was served as the auxiliary electrodes, and the saturated calomel electrode (SCE) was acted as reference electrodes, respectively. The potentials were recalculated against the reversible hydrogen electrode (RHE) in line with ERHE = ESCE + EuSCE + 0.059 pH [24]. The iR drop is automatically corrected by CHI software. For the electrochemical measurements, 0.1 M KOH was used as electrolyte, which was bubbled with oxygen for 30 min in advance. Linear sweep voltammetry (LSV) was operated at a scan rate of 10 mV s1. Chronoamperometry was conducted at 2.0 V with 1000 rpm. The electrical impedance spectroscopy was recorded at 1.45 V with ac voltage amplitude of 5 mV. The frequency ranged from 106 to 0.01 Hz. 3. Results and discussion The morphology of Co3O4/MoS2 hybrids can be observed by TEM images. Fig. 1 shows ultrathin Co3O4 nanosheets grown on exMoS2, and form a three-dimensional structure, which can be

observed from SEM images (Fig. S1). High-resolution TEM images reveal that the dominant plane of Co3O4 nanosheets is exposed with ca. 0.24 nm lattice fringe, indicating the (311) facet of cubic crystal of Co3O4 [14]. The Co3O4 nanosheets are uniformly distributed on Ex-MoS2.The ex-MoS2 edge exhibited a 2 H hexagonal structure with a lattice parameter of 0.27 nm corresponding to its (100) plane [25], demonstrating intact maintenance of outstanding crystallinity during the exfoliation process and subsequent Co3O4 deposition [26]. The amount of Co3O4 nanosheets on ex-MoS2 can be tuned by controlling its loading. For comparison, a few-layer ex-MoS2 and ultrathin Co3O4 nanosheets were also prepared and shown in Fig. S2. The lateral size of these ex-MoS2 sheets was typically hundreds of nanometers (Fig. S1), and folded edges with more than three layers were rarely observed. Co3O4 nanosheet is well-crystallized, and its thickness is 5 nm at most. The elemental map of Co3O4/ex-MoS2 hybrids is imaged by energy-filtered TEM equipped with an electron energy-loss spectroscope. Fig. 2 exhibits uniform distribution of Co and Mo elements throughout the entire of Co3O4/ex-MoS2 hybrids. The structure of Co3O4/ex-MoS2 hybrids is further characterized by Raman spectroscopy and XRD pattern (Fig. 3). For MoS2, as shown in Panel A, Raman frequencies of in-plane E2g1 mode around 384 cm1 and out-of plane A1g mode around 407 cm1 are often used as reliable and convenient features to identify the number of layers. Red shift of the E1g2 mode and blue shift of the A1g mode were observed for bulk MoS2 by comparison with ex-MoS2, indicating success in solution exfoliation of bulk MoS2 into fewlayer MoS2. There is no obvious difference in Raman spectra between ex-MoS2 and Co3O4/ex-MoS2 hybrids. The results suggest that the few-layer structure of ex-MoS2 remains the same despite Co3O4 on ex-MoS2 [23,27,28]. The XRD patterns show a variety of peaks in bulk MoS2 due to its multilayer structure [29] (see Panel B). A series of strong diffraction peaks at 14.4 , 39.6 and 49.8 are

[(Fig._2)TD$IG]

Fig. 2. Elemental mapping of Co3O4/ex-MoS2 hybrids (1:1): (A) TEM image, (B) Mo L, (C) Co K and (D) MoCo. Scale 200 nm.

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[(Fig._3)TD$IG]

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Fig. 3. Raman spectra (A) and XRD pattern (B) of the Bulk MoS2, ex-MoS2, Co3O4 and Co3O4/ex-MoS2 hybrids (1:1).

assigned to (002), (103) and (105) facet of hexagonal lattice of MoS2, respectively (JCPDF (37-1492) and (84-1398)) [30]. After solution exfoliation, these diffractions obviously weaken, and most of them are inconspicuous. In addition, the signal of the (002) reflection is broadened significantly, indicative of few-layer planar crystal structure of ex-MoS2 [26], For Co3O4 nanosheets, the peaks at 19.06 , 31.27, 36.9 , 59.5 , and 65.4 can be ascribed to (111), (200), (311), (5111) and (440) facet of a face-centered cubic structure (JCPDS no. 42-1467), respectively [16,31], For Co3O4/exMoS2 hybrids, a combination of two sets of diffraction peaks corresponding to ex-MoS2 and Co3O4 can be observed, further demonstrating the success of Co3O4 deposition on ex-MoS2 and the maintenance of few-layer structure of MoS2 in hybrids.

The composition of Co3O4/ex-MoS2 hybrids is investigated by XPS, as shown in Fig. 4, the survey spectra clearly indicts the presence of Mo, S, Co and O (see Panel A). It gives the Co3O4/MoS2 molar ratio of 1:1, in good agreement with ICP-AES data (Table 1). The S element exhibits two major peaks at 163.8 and 162.7 eV, as shown in Panel B, which are attributed to the S 2p 1/2 and S 2p 3/2 orbitals of Mo-S bonding, respectively. For Co3O4/ex-MoS2 hybrids, another couple of S 2p peaks were presented at 161.5 and 163.3 eV, and overlapped with Mo-S signals, which can be assigned to 2p 3/2 and S 2p 1/2 of Co-S bonding, indicative of the formation of CoS species during Co3O4 deposition [32]. Panel C depicts the highresolution scan of the Mo 3d electrons with two major peaks at 229.6 eV and 232.8 eV, consistent with Mo 3d doublet of Mo 3d 3/2

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Fig. 4. XPS survey spectra (A) of ex-MoS2, Co3O4 and Co3O4/ex-MoS2 hybrids (1:1), (B) S 1s core level spectra of ex-MoS2 and Co3O4/ex-MoS2 hybrids, (C) Mo 3d core level spectra of ex-MoS2 and Co3O4/ex-MoS2 hybrids and (D) Co 2p core level spectra of Co3O4 and Co3O4/ex-MoS2 hybrids.

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Table 1 Co, Mo content, Co/Mo atomic ratio in OER catalysts and Co3+ content in [10_TD$IF]samples. Content Samples Ex-MoS2 Co3O4/Ex-MoS2 (1:1) Co3O4/Ex-MoS2 (1:2) Co3O4/Ex-MoS2 (2:1) Co3O4 a b c d

Co%a

44.5 31.2 56.3 49.4

Mo%a

Co/Mo Atomic ratioa

Co3+ content in Co3O4 matrixcb

59.6 23.8 34.0 15.0

3 1.5 6

50.5c 50.2c 49.8c 49.5c

84.6d 75.2d 58.2d 54.8d

These data recorded from ICP-AES. These data recorded from XPS. These data recorded before electrochemical activation. These data recorded after electrochemical activation.

and 3d 5/2 for ex-MoS2, respectively. For ex-MoS2, S 2 s peak also appears in the low binding energy region. S signal of Mo-S binding is at 225.8 eV, while S signal of Co-S binding is at 224.6 eV [33]. In the high binding energy region, an insignificant peak takes place at 236.4 eV, indicating the presence of Mo6+ state as a result of MoS2 oxidation during solution exfoliation. This Mo6+ peaks become more obvious after Co3O4 deposition [34,35]. Panel D depicts highresolution scans of Co 2p electrons, which can be fitted into two spin-orbit doublets and one couple of shakeup satellites, the fitting Co 2p doublet at 780.7 and 795.8 eV are attributed to Co2+ species, while another at 779.4 and 794.3 eV belong to Co3+ species [36,37]. The Co2+/Co3+ atomic ratio can be calculated to be 1:1 both for Co3O4 and Co3O4/ex-MoS2 hybrids by integrating the area of their XPS curves (Table 1). In addition, for Co3O4/ex-MoS2 hybrids, an unapparent Co 2p doublet was also presented with Co 2p 1/2 at 778.5 eV and Co 2p 3/2 793.6 eV, which can be derived from the formation of Co-S binding [32]. The OER activity of Co3O4/ex-MoS2 hybrids was evaluated in a three-electrode system by linear scanning voltammetry (LSV) and their Tafel plots (Fig. 5). Individual components (Co3O4 nanosheets and ex-MoS2) and reference (IrO2/C catalysts) were also tested for comparison. Fig. 5 records the iR-corrected LSV curves at a scan rate of 10 mV s1. For Co3O4, as shown in Fig5 inset (upper), a small peak occurs at about 1.10 V, which can be assigned to Co2+ oxidation [38]. Differently, for Co3O4/ex-MoS2 hybrids, the Co2+ oxidation current appears an upward platform, and overlaps with OER current around 1.35 V. The peak area of Co2+ oxidation was integrated as an aim to evaluate the Co3+ content in Co3O4 matrix. For Co3O4/ex-MoS2 hybrids (1:1), the electrochemically generated

[(Fig._5)TD$IG]

Fig. 5. LSV curves (A) of ex-MoS2, Co3O4, IrO2/C and Co3O4/ex-MoS2 hybrids with different Co3O4/MoS2 molar ratio in 0.1 M KOH solution at the scan rate of 10 mV s1. (B) Their enlarged part of LSV under low current density. (C) Co3+ content in Co3O4 matrix for Co3O4 and Co3O4/ex-MoS2 hybrids.

Co3+ content (e-Co3+) was significantly increased by fourfold that in the case of Co3O4, suggesting a favorable Co2+/Co3+ transformation in Co3O4 matrix after its deposition on ex-MoS2 (Fig5 inset (down)). The Co3+ species is believed to be crucial to the active sites of Co3O4 and its hybrids for OER [10,38]. The total Co3+ content (TCo3+) after electrochemical activation was also estimated based on XPS analysis (Table 1). The Co3O4/ex-MoS2 hybrids also exhibit a higher Co3+ content than that of Co3O4, accounting for their higher OER activity. Oxygen generation will start at about 1.47 V for Co3O4, 1.45 V for ex-MoS2 and 1.40 V for IrO2/C catalysts, respectively. Co3O4/ex-MoS2 hybrids display a smaller onset OER potential ranged from 1.30 to 1.38 V dependent of Co3O4 loading, much higher than that of its component and reference. The optimal atom ratio of Co/Mo is 3:1, this sample exhibits a largest OER current up to 226 mA cm1 with a smallest OER onset potential of 1.30 V. Fig. 5 inset (upper) also shows OER potential at 10 mA cm2, this potential is of merit for OER catalysts due to its amount to 10% efficient solar water-splitting [7,39]. Obviously, these Co3O4/exMoS2 hybrids can decrease substantially the overpotential by 0.25 V by comparison with the reference IrO2/C catalyst, and 0.29 V in contrast with its component Co3O4, demonstrating the superior OER activity for Co3O4/ex-MoS2 hybrids. The excellent OER activity was also manifested from Tafel slopes in accordance with the Tafel equation (Fig. 6) [7]:

h¼b

logj j0

where h is the overpotential, b the Tafel slope, j the current density and j0 the exchange current density. The current density j was specified slightly over the potential region of Co2+ oxidation to deduct its current contribution. Among Co3O4/ex-MoS2 hybrids, as shown in Panel A, the sample with 3:1 Co/Mo atom ratio shows a smallest Tafel slope of about 36 mV dec1. This b value is much less than those of Co3O4 (74 mV dec1), ex-MoS2 (210 mV dec1) and IrO2/C catalysts (45 mV dec1). Therefore, the lower Tafel slope of Co3O4/ex-MoS2 hybrids renders a greatly enhanced current response at high potential, a case in point is 200 mA cm2 current density generated at 2.00 V for Co3O4/ex-MoS2 hybrids (1:1), almost 1.5 times that of IrO2/C catalysts at the same potential. In order to demonstrate the superior activity of Co3O4/ex-MoS2 hybrids as advanced OER catalysts for water splitting, two OER parameters, Tafel slope b and overpotential to generate 10 mA cm2, were selected for comparison with state of art OER catalysts reported recently [9,40–47]. As shown in Panel B, the as-prepared Co3O4/ex-MoS2 hybrids (1:1) can be listed among the most kinetically active candidates for OER catalysts used in water-splitting application due to its lower overpotential and smaller Tafel slope as compared to most reported transition metal oxide, including Co3O4-based materials in 0.10 M KOH.

[(Fig._6)TD$IG]

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Fig. 6. Tafel plots of ex-MoS2, Co3O4, IrO2/C and Co3O4/ex-MoS2 hybrids with different Co3O4/MoS2 molar ratio in 0.1 M KOH solution) Comparison on kinetics (Tafel slope) and activity (the overpotential required to achieve 10 mA cm2) for exMoS2, Co3O4, IrO2/C and Co3O4/ex-MoS2 hybrids with references all measured in 0.1 M KOH solution.

Stability is another important criterion for OER catalysts regarding their wide practical applications. The chronopotentiometric curves and LSV data of Co3O4/ex-MoS2 hybrids (1:1) were examined to check their durability during water oxidation (Fig. 7). Panel A shows the chronoamperometric response at a potential of 2.0 V using RDE system with 1000 rpm. The fluctuations in i-t curve resulted from the disturbance of O2 bubbles formed on the surface [48]. The fast current drop is ascribed to the formation of doublelayer-capacitance in initial stage (<500 s) [49]. Subsequently, water oxidation is trigged and proceeds for 4500 s. The OER current of Co3O4/ex-MoS2 hybrids remains almost stable, while there is continuous loss of OER current for Co3O4 probably as a result from its dissolution and loss. Interestingly, a substantial increase of OER current can be observed from i-t curves of ex-MoS2, even so, its OER current over 5000 s potentiodynamic polarization is still low (5.6 mA cm2), 1/2 that of Co3O4 (28.4 mA cm2) and 1/17 that of Co3O4/ex-MoS2 hybrids (95.2 mA cm2). The results also demonstrate the remarkable OER activity of Co3O4/ex-MoS2 hybrids, in good agreement with data of CV curves. Panel B shows the variation of OER activity as a function of CV cycle numbers. The OER current at 2.0 V is chose as an index of OER activity for comparison between Co3O4 and Co3O4/ex-MoS2 hybrids. The Co3O4/ex-MoS2 hybrids maintain 95.2% OER activity in contrast of 49.8% that for

[(Fig._7)TD$IG]

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Fig. 7. Chronoamperometry (i-t) curves (A) for ex-MoS2, Co3O4 and Co3O4/ex-MoS2 hybrids (1:1) measured at 2.0 V in 0.1 M KOH solution. The OER activity retention (B) of Co3O4 and Co3O4/ex-MoS2 hybrids for 10000 potential cyclings. Inset: LSV curves of Co3O4 and Co3O4/ex-MoS2 hybrids (1:1) in 0.1 M KOH solution for 1st and 10000th cycle at the scan rate of 10 mV s1.

Co3O4 for 10000 potential cyclings, further confirming the robust durability of Co3O4/ex-MoS2 hybrids. The higher stability of Co3O4/ ex-MoS2 hybrids may be relevant with the formation of Co-S bonding, which possesses a higher resistance to decomposition, leading to a high stability of Co3O4/ex-MoS2 hybrids against dissolution during water splitting process under high potential polarization. The OER kinetics was elucidated by AC impedance spectroscopy. Fig. 8 shows the representative Nyquist plots of ex-MoS2, Co3O4 and Co3O4/ex-MoS2 hybrids modified with GC electrode in quiescent condition. All the impedance spectra consist of two frequency regions, i.e., a semicircle at high frequencies and a linear part at low frequencies. The semicircle is corresponding to faradic resistances (Rc) caused by the interfacial charge transport in electrode interface. The straight line is associated with Warburg resistance (Rw) and double-layer capacitance (Cdl), which is subjected to the nature of OER catalysts [14]. For Co3O4/ex-MoS2 hybrids, their smaller diameter indicates more kinetically favorable for OER by comparison with that of Co3O4. The large of Rc arch means poor OER activity for ex-MoS2. The straight slope is subjected to the nature of OER catalysts. For ex-MoS2, the slope of this line at low frequencies is below 45 degree, indicative of a higher Warburg resistance (Rw) due to a slower ion diffusion process at the electrode interface [50]. For Co3O4 and Co3O4/ex-

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[(Fig._8)TD$IG]

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Fig. 8. Nyquist plots of ex-MoS2, Co3O4 and Co3O4/ex-MoS2 hybrids (1:1) recorded at 1.45 V in 0.1 M KOH solution.

MoS2 hybrids, the line at low frequencies is much steeper than that of ex-MoS2 hybrids, suggesting a higher double layer charging capacity of the solid-liquid junction as a result of their high active specific surface area. The X-intercept of the Nyquist plots represents the equivalent series resistance (Rs), which is estimated to be 12.6, 47.5 and 21.1 V for the ex-MoS2, Co3O4 and Co3O4/exMoS2 hybrids, respectively. For ex-MoS2, its Rs value is smaller relative to Co3O4, demonstrating its higher conductivity compared to Co3O4. Therefore, the combination of ex-MoS2 with Co3O4 can greatly speed up the OER activity of water splitting. 4. Conclusion An ultrathin Co3O4 nanosheet was deposited on ex-MoS2 via insitu hydrolysis and epitaxial growth. These novel Co3O4/ex-MoS2 hybrids shows excellent OER performance in terms of activity and stability, which outperform IrO2/C catalysts and compete favorably against state of art OER catalysts reported recently, with an extremely low Tafel slope (ca. 36 mV dec1), a considerably small overpotential (ca. 350 mV required for 10 mA cm2), and robust durability (ca. 95.2% OER activity retention for 10000 potential cyclings). Besides as a promising alternative to advanced OER catalysts for water splitting, the as-prepared Co3O4/ex-MoS2 hybrids is expected to be inspiring and applicable in various applications, such as heterogeneous catalysis, sensors, energy conversion and storage. Acknowledgements This work was financially supported by the Zhejiang Provincial Public Welfare Project (No. 2016C33011) and the National Natural Foundation of China (No. 21275130) Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.07.078. References [1] J. Chow, R.J. Kopp, P.R. Portney, Energy Resources and Global Development, Science 302 (2003) 1528–1531. [2] N.S. Lewis, D.G. Nocera, Powering the planet: Chemical challenges in solar energy utilization, PNAS 103 (2006) 15729–15735.

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