CoreâShell MoS2@CoO Electrocatalyst for Water Splitting in Neural

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C: Energy Conversion and Storage; Energy and Charge Transport

Core-shell MoS2@CoO Electrocatalyst for Water Splitting in Neural and Alkaline Solutions Pengfei Cheng, Chen Yuan, Qingwei Zhou, Xianbiao Hu, Jing Li, Xiaozi Lin, Xin Wang, Mingliang Jin, Lingling Shui, Xingsen Gao, Richard Nözel, Guofu Zhou, Zhang Zhang, and Jun-Ming Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10954 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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The Journal of Physical Chemistry

Core-shell MoS2@CoO Electrocatalyst for Water Splitting in Neural and Alkaline Solutions Pengfei Cheng1, Chen Yuan1, Qingwei Zhou3, Xianbiao Hu1, Jing Li1, Xiaozi Lin1, Xin Wang2, Mingliang Jin2, Lingling Shui2, Xingsen Gao1, Richard Nötzel2, Guofu Zhou2, Zhang Zhang1*, Junming Liu3 1. Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China. 2. National Center for International Research on Green Optoelectronics, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, P. R. China. 3. Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, P. R. China Corresponding author: Zhang Zhang *E-mail: [email protected] Phone number: +8613660357067

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Abstract Core-shell nanostructured materials with synergetic effects have gained increasing attention for their widespread applications in electrochemical water splitting field. However, for most electrocatalysts, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) can only work efficiently in neutral solution or alkaline solution, which hinders their further applications using untreated natural waters with a wide pH range, such as seawater (pH～7.5-8.5). In this paper, we report the synthesis of core-shell MoS2@CoO bifunctional electrocatalysts. The MoS2@CoO coated carbon cloth electrode demonstrates excellent electocatalytic properties for both OER and HER, being with 325 and 173 mV at a current density of 10 mA/cm2 and Tafel slopes of 129.9 and 83.0 mV·dec-1 in 1 M KOH solutions, respectively. Additionally, the electrode can also work efficiently in neutral solutions (1 M PBS, pH=7), showing good OER and HER activities for potential electrocatalytic water splitting applications in untreated natural waters.

Introduction Electrochemical water splitting for clean energy productions of hydrogen (H2) and oxygen (O2) can be one of effective alternatives to fossil fuels, which has been increasingly exploited against environmental pollution, greenhouse effect and energy shortage.1–4 H2, as a clean and high energy density energy source, can promise to reduce carbon dioxide (CO2) emission and cope with the problems of energy shortage. On the other hand, O2 can apply in rechargeable metal-air batteries and regenerative fuel cells.5–7

Basically, water electrolysis devices require the cost-effective

electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with high current densities at low overpotentials.6 However, to the best of our knowledge, the most effective OER and HER electrocatalysts are ruthenium/iridium oxides and platinum,


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respectively.6,8 The limited reserves and high costs of these noble metals and noble metal oxides hinder their widespread applications. Furthermore, most electrocatalysts can be only operated in alkaline mediums for OER and HER, and a few electrocatalysts are capable of working under neutral conditions.6,9–12 Therefore, developing non-noble-metal bifunctional electrocatalysts with both high efficiency and being able to work in both alkaline and neutral solutions is highly urgent and also desirable. In recent years, transition metal dichalcogenides (TMDs), transition metal phosphides (TMP), transition metal oxides, graphene-based nanocompounds and carbon nitride-based materials have emerged as novel non-noble-metal based OER or/and HER catalysts.13–24 Especially, building bifunctional catalysts for both OER and HER with desirable electrocatalytic synergetic effect working in a wide pH range has been one of the research focuses.6,25 Zhu et al. reported core-shell Co8S9@MoS2 with excellent HER and OER performances in 1 M KOH solutions due to their synergistic effect.25 Li et al. demonstrated that in situ oxidized CoO domains on CoSe2 nanobelts could produce synergistic effect to enhance OER and HER reactions in neutral media.26 Furthermore, the stabilities in their own systems have been greatly improved duo to the synergistic effect. As far as we know, the synergistic effect of core-shell MoS2@CoO for electrocatalytic water splitting in alkaline and neutral media has not been reported. Herein, we report the synthesis of bifunctional core-shell MoS2@CoO electrocatalysts for water splitting in alkaline and neutral solutions. The MoS2@CoO consists of a CoO nanocrystal core wrapped by a shell of 2H-MoS2 nanoflakes. Such a core-shell hybrid nanostructure dramatically enhances the eletrocatalytic performances compared with pure CoO nanocrystals and 2H-MoS2 flakes. Specifically, the electrode of MoS2@CoO coated carbon cloth demonstrates excellent



electocatalytic properties for both OER and HER, being with 325 and 173 mV at a current density of 10 mA/cm2 and Tafel slopes of 129.9 and 83.0 mV·dec-1 in 1 M KOH solutions (pH=14), respectively. Additionally, the electrode can also work efficiently in neutral solutions (1 M PBS, pH=7), showing good OER and HER activities for potential electrocatalytic water splitting applications in untreated natural waters.

Experimental Section Materials Synthesis: CoO was synthesized by a hydrothermal method. 1 mmol cobaltous acetate tetrahydrate (Co(CH3COO)2·4H2O) was served as Co source and 10 mmol triphenyphosphine (P(C6H5)3) as sacrificial agent, which were dissolved in 35 ml oxidized oleylamine as O source and solvent. The mixture was stirred for 3 min with a glass rod to form a uniform system. Then, the mixture was put into a reaction still and transferred it to oven at 180 °C for 18 h. The final product was obtained by a centrifugation rate of 5000 rpm and washed with absolute ethyl alcohol 3 times, followed by drying naturally at room temperature for 10 h and we finally obtained ~50 mg CoO. Then, 20 ml absolute ethyl alcohol was added into the as-prepared CoO, then 2H-MoS2 flakes with different masses was added in it. The mixture was sonicated for 60 min and then annealed at 450°C in Ar gas to form core-shell MoS2@CoO. Co(CH3COO)2·4H2O and 2H-MoS2 flakes were purchased from Shanghai Macklin Biochemical Co., Ltd. P(C6H5)3, oleylamine were purchased from Beijing Energy Engineering Technologies Co., Ltd. All of the chemicals were used without further purification. Electrode preparation: The carbon cloth electrode was cut into 3 x 10 mm2 and orderly washed for 5 min by deionized water, acetone and absolute ethyl alcohol with the assistance of


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sonication, and blow-dried by N2 prior to use. The prepared MoS2@CoO mixture was sonicated for 10 min and was dropped onto the carbon cloth to form a ~2 mg·cm-2 catalyst work electrode (WE). The fabrication processes of MoS2 and CoO electrodes were similar. Then, the electrodes were separately annealed at 450 °C for 30 min in Ar (99.999%) to remove solvents and make the catalysts contact the substrates tightly. Characterization: Scanning electron microcopy (SEM) images were obtained with a FESEM, Model ZEISS Gemini 500. Powder X-ray diffraction (XRD) patterns were obtained on a Bruker D8-Advance using Cu Kα radiation, PANalytical X’ Pert PRO. Raman spectrum was used with a Renishaw in Via Raman microscope. Transmission electron microscope (TEM, JEM-2100HR) and X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi) were also used to study the crystalline structure and surface composition. Electrochemical Measurement: Electrochemical measurements were conducted with CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai). The electrochemical cell was set up with a 3-electrodes system: A saturated calomel electrode (SCE) as the reference electrode, a Pt wire (OER) or carbon rod (HER) as the counter electrode, the sample coated carbon cloth electrode as the working electrode. The electrolyte solutions were used fresh 1 M KOH (pH=14) or 1 M phosphate buffered saline (PBS, pH=7) or 1 M electrolyte adjusted by them both. Before measurements, the electrolyte solutions were saturated with N2. Reversible hydrogen electrode potentials were calculated by potential vs RHE (V) = E vs SCE + E θ SCE + 0.059pH (V). All electrochemical data were presented without iR compensation.

Results and discussion Electrocatalysts Preparation and Characterization:



The fabrication process of CoO nanocrystals was similar to previous report.27 Distinguishingly, we treated the hydrothermal products in the atmosphere at room temperature. A two-step preparation route was applied to synthesize the core-shell MoS2@CoO. First, the CoO nanocrystals were prepared by a hydrothermal method, and the core-shell MoS2@CoO was further obtained by ultrasonication of a mixture solution of 2H-MoS2 flakes and CoO nanocrystals. To verify the core-shell hybrid configuration, SEM and TEM were carried out. As shown in Figure 1a and 1b with different magnifications, the as-prepared CoO nanocrystals have a big size distribution ranging from several tens to several hundreds nanometers. By comparison, as shown in Figure 1c and 1d, besides the CoO nanocrystals stayed the similar morphologies after sonication and annealing, most surfaces of the CoO are covered by a rough outside layer, which consists of ultrasonically broken MoS2 nanoflakes. The core-shell hybrid nanostructures have larger specific surface areas than the pure nanocrystals, which could probably improve the corresponding electrocatalytic performances.25,28,29 Moreover, two types of distinct morphologies of CoO nanocrystals can be recognized as octahedron and spherical polyhedron. These two types of nanocrystals all have large portions of flat facets, which should facilitate the wrapping by two-dimensional (2D) MoS2 nanoflakes. A single core-shell MoS2@CoO was illustrated in the TEM image of Figure 1e, with a typical spherical polyhedron CoO core. A selected thin surface area was marked to further perform the high resolution (HR) TEM observation (as shown in Figure 1f). Clearly, a multilayered 2D MoS2 tightly covered onto the CoO surface. The lattice fringes with a measured lattice spacing of 0.62 nm correspond to the (002) planes of 2H-MoS2.30 The microscopy observations indicate that the ultrasonic mixing and annealing can successfully transform CoO nanocrystals and 2H-MoS2 flakes into the core-shell MoS2@CoO hybrid system.


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Figure 1. SEM images of (a), (b) CoO nanocrystals, and (c), (d) MoS2@CoO with different magnifications. (e) TEM image of a single core-shell MoS2@CoO, and the marked surface area is illustrated in (f) HR-TEM image of multilayered 2H-MoS2 coating.

To further verify the crystalline structure and valence state of elements, XRD, Raman and XPS measurements were conducted. Figure 2a presents XRD patterns of 2H-MoS2 flakes, as-prepared CoO nanocrystals and core-shell MoS2@CoO, respectively. 2H-MoS2 flakes gave 15 strong diffraction peaks, corresponding to the (002), (004), (100), (101), (102), (103), (006), (105), (106), (110), (008), (107), (108), (203) and (116) planes of 2H-MoS2 (PDF#37-1492), respectively. 4 strong diffraction peaks corresponding to (111), (200), (220) and (311) planes (PDF#65-2902) can be observed in CoO nanocrystals. After the ultrasonic mixing, the 2 strongest diffraction peaks corresponding to (002) and (103) planes of 2H-MoS2 flakes and all the diffraction peaks of CoO nanocrystals can be found in core-shell MoS2@CoO. Raman spectroscopy was also conducted to further identify the composition (see Figure S1). The characteristic peak positions of 2H-MoS2 were located at 384 and 410 cm-1, and the ones belonging to CoO were found at 555, 675 and 1087 cm-1, respectively31,32. Furthermore, the existence of both MoS2 and CoO on surface of the core-shell hybrid was confirmed by the XPS measurements illustrated in Figure 2b. Obviously, the peaks at 796.4 and 781.2 eV were attributed to the binding energies of Co 2p1/2 and Co 2p3/2. The satellite peaks at 785.6 and 802.9 eV suggested the Co 2p came from Co+2 in CoO.26,33 It confirmed the existence of Co+2 on the MoS2@CoO. The O1s spectrum was presented, with the



peak of 530.1 eV belonging to O2-, and the peak at 532.7 eV was attributed to O-Si binding energy on the SiO2/Si substrate.34 The peaks of 232.8 and 229.5 eV in Mo 3d spectrum correspond to Mo 3d5/2 and Mo 3d3/2, respectively, and the peak of 226.4 eV was assigned to S 2s.35 The binding energies of S at 163.4 and 161.8 eV correspond to S 2p1/2 and 2p3/2, respectively.35 The XPS results were well consistent with the XRD and Raman characterizations.

Figure 2. (a) XRD patterns of MoS2, CoO and MoS2@CoO. (b) XPS measurements of MoS2@CoO transferred on SiO2/Si substrate.

Electrocatalytic OER and HER Performances: To optimize the electrocatalytic performances of core-shell MoS2@CoO in a N2-saturated 1 M KOH solution, different mass ratios of MoS2 to CoO were analyzed (see Figure S2a). Clearly, the MoS2@CoO electrocatalyst with a mass ratio (CoO:MoS2=50:8) displayed the best electrocatalytic performance with a loading density of 2.0 mg·cm-2 on the carbon cloth. Meanwhile, the electrocatalytic performances of the working electrodes with different loading densities of the optimal MoS2@CoO were compared (see Figure S2b), and the loading density of 2.0 mg·cm-2 corresponds to the best electrocatalytic performance, indicating an optimal coverage of electrocatalyst on the carbon cloth. First, we evaluated the OER activities from different electrodes of core-shell MoS2@CoO, CoO nanocrystals and 2H-MoS2 flakes with the same loading density of approximately 2 mg·cm-2 on


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carbon cloth, in a N2-saturated 1 M KOH solution at a 5 mV·s-1 scan rate. For comparison, RuO2 and bare carbon cloth were also used as reference electrodes. Figure 3a presents their polarization curves without iR corrections. Normally, both carbon cloth and MoS2 flakes coated electrodes showed poor OER performances, and the OER performance of CoO coated electrode was much worse than RuO2 and MoS2@CoO electrode. However, carbon cloth electrode coated with the optimal MoS2@CoO displayed a significantly enhanced OER performance, being with onset overpotential of 270 mV and overpotential of 325 mV at 10 mA·cm-2. In comparison, to reach the same current density of 10 mA·cm-2, CoO electrode, RuO2, 2H-MoS2 flakes electrode and bare carbon cloth required overpotentials of 399, 269, 500+ and 500+ mV, respectively. Tafel slopes were calculated by Tafel equation η=blog(j/(mA/cm2))+a, where η, b, j and a correspond to the overpotetial , the Tafel slope, the current density and the exchange current density, respectively.16 As plotted in Figure 3b, the MoS2@CoO electrode demonstrates the smaller Tafel slope of 83.0 mV·dec-1 compared with the MoS2@CoO, CoO, MoS2 and carbon cloth electrodes, showing the comparable performance to RuO2. Besides, stability of the MoS2@CoO electrode in OER was investigated (see Figure S3). After 1000 cycles of potential sweeps in 1M KOH solution, the polarization curve exhibits a current density loss of about 30 % (vs. RHE=1.7 V) compared with the initial one. The OER current density after 10 h also declined to 80.1% of its starting value. SEM images before 1st potential sweep and after 1000th potential sweep were compared (see Figure S4). Clearly, part of catalysts detached from the surfaces of carbon cloth after 1000 potential sweeps. Meanwhile, the anodic corrosion of MoS2 may have a negative influence on the electrode stability during the OER process.36 Besides, XPS and XRD measurements after the 1000 cycles of potential sweeps (see Figure S5 and S6) have been provided to confirm that the element



valence and hybrid composition don’t change after 1000 potential sweep.

Figure 3 (a) OER polarization curves of different electrodes in N2-saturated 1 M KOH solution with a scan rate of 5 mV·s-1, and (b) corresponding Tafel plots. (c) HER polarization curves of different electrodes in N2-saturated 1 M KOH solution with a scan rate of 2 mV·s-1, and (d) corresponding Tafel plots.

Their electrocatalytic HER performances were also evaluated in 1 M KOH solution with a scan rate of 2 mV·s-1. As illustrated in Figure 3c and 3d, to reach a current density of 10 mA·cm-2, the carbon cloth, MoS2, CoO, MoS2@CoO and Pt wire electrodes required overpotentials of 500+, 357, 272, 173 and 46 mV, respectively, corresponding to Tafel slopes of 500.1, 235.4, 176.4, 129.9 and 41.3 mV·dec-1. Normally, the commercial Pt wire demonstrated the best HER performances. However, compared with the CoO and MoS2 electrodes, both electrocatalytic OER


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and HER performances of the core-shell MoS2@CoO electrode were significantly enhanced. Besides, its electrocatalytic activities for both OER and HER were compared with other representative non-noble-metal based bifunctional electrocatalysts (see Table S1).2,3,16,37–40 After 1000 cycles of potential sweeps in 1 M KOH solution (see Figure S7a), the electrocatalytic performance had declined about 10 % (vs. RHE=-0.4 V) than the initial one, indicating a relatively stable HER activity of core-shell MoS2@CoO electrocatalyst working in alkaline solutions. And the HER could be maintained with a negligible degradation for 10 h (see Figure S7b).

Figure 4 (a) OER and (b) HER performances of MoS2@CoO and bare carbon cloth electrodes in 1 M electrolytes with different pH values.

We further investigated electrocatalytic performances of the MoS2@CoO electrode working in pH changing solutions from neutral to alkaline (pH=7.0-14.0). Figure 4 illustrates polarization curves of the MoS2@CoO electrode and bare carbon cloth without iR corrections. For the OER activities (Figure 4a), to reach a current density of 10 mA/cm2, overpotentials of 325, 331, 340, 320 mV were required for pH=7, 9, 11, 14, respectively, while the carbon cloth substrate had a negligible OER activity. As the pH increased from 7.0 to 11.0, the corresponding electrocatalytic performances were gradually decreased. The electrode presented the best OER performance



working in both neutral and alkaline solutions. As plotted in Figure 4b, the MoS2@CoO electrode also exhibited the best HER activity in neutral and alkaline solutions. The bare carbon cloth presented a negligible HER activity, and an overpotential of 176 mV was required for the MoS2@CoO electrode to arrive at 10 mA/cm2 in 1 M PBS (pH=7.0). Besides, stability of the MoS2@CoO electrode in 1 M PBS was also investigated. The OER performance was gradually attenuated during the 10 h (see Figure S8), while the HER performance was slightly increased (see Figure S9). In general, its electrocatalytic activities for both OER and HER have obvious advantages compared to other non-noble-metal bifunctional electrocatalysts working in neutral solutions (pH=7.0) (See Table S2).11,12,26,41,42 To explain the electrocatalytic mechanism, electrochemical impedance spectroscopy (EIS) had been conducted to analyze the electrode kinetics at 1.6 V vs. RHE during the OER process. Nyquist impedance plots presented three semicircles of different diameters (see Figure S10), which correspond to MoS2, CoO and MoS2@CoO from large to small, respectively. And the inset stands for the equivalent circuit. According to our previous work, the semicircles correspond to charge-transfer resistances (Rct) at eletrocatalyst/electrolyte interface, which usually reflect the catalytic activity.30 Accordingly, the charge-transfer resistance for MoS2@CoO was only 6.5 Ω, which is smaller than that of pure CoO (10.2 Ω) and MoS2 (15.6 Ω). Since lower charge-transfer resistance and smaller Tafel slope indicate higher electrical conductivity and faster reaction kinetics, the MoS2@CoO electrode demonstrated the excellent eletrocatalytic performances.

Conclusions In summary, bifunctional electrocatalysts of core-shell MoS2@CoO have been firstly


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synthesized through a simple two-step method. The optimized MoS2@CoO coated carbon cloth electrode demonstrates excellent electrocatalytic performances for both OER and HER in 1 M KOH, being with 325 and 173 mV at a current density of 10 mA/cm2 and Tafel slopes of 83.0 and 129.9 mV/dec, respectively. Additionally, the electrode can also work efficiently in neutral solutions (1 M PBS, pH=7). The non-noble-metal based core-shell MoS2@CoO electrocatalyst was realized to enhance both OER and HER activities with capability of working in both alkaline and neutral solutions, making it potential for applications in untreated natural waters.

Conflict of Interest Disclosure The authors declare no competing financial interests. Supporting Information Raman spectrum analysis; engineering the optimal ratio and loading of MoS2@CoO; OER and HER j–t (current density vs time) curves of MoS2@CoO in 1 M KOH solution and 1 M PBS solution; SEM images, XPS and XRD characterizations of before and after potential sweeps (+1.8 to + 1.2 V vs. RHE) at a scan rate of 5 mV·s-1 for 1000 cycles; EIS spectra of MoS2@CoO, CoO and MoS2 for OER at 1.6 V vs. RHE, stabilities of OER and HER in 1 M KOH and 1 M PBS solutions and their performances compared with recent reported bifunctional electrocatalysts. This information is available free of charge via the Internet at http://pubs.acs.org Acknowledgement This work was supported by the National Key R&D Program of China (grant no. 2016YFB0401501), Guangdong Innovative Research Team Program (grant no. 2013C102), the Pearl River S&T Nova Program of Guangzhou (grant



no.201506010019), Cultivation project of National Engineering Technology Center (grant no. 2017B090903008), Xijiang R&D Team (X.W.),Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (Grant No. 2017B030301007), National Natural Science Foundation of China (51561135014), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No. IRT_17R40), MOE International Laboratory for Optical Information Technologies and the 111 Project.

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