Polyoxomolybdate-derived MoS2/nitrogen-doped reduced graphene oxide hybrids for efficient hydrogen evolution

https://doi.org/10.1016/j.ijhydene.2020.02.179Get rights and content

Highlights

  • Different POM clusters at different pH values led to different microstructures.

  • More defects and larger interlayer spacing of MoS2 were led by decreasing pH value.

  • Exposed active site and accessible area are enhanced by introducing high-nuclear POM.

  • MoS2/NGO hybrids prepared at low pH exhibit superior electrocatalytic HER activity.

Abstract

Integrating MoS2 with carbon-based materials, especially graphene, is an effective strategy for preparing highly active non-noble-metal electrocatalysts in the hydrogen evolution reaction (HER). This work demonstrates a convenient hydrothermal method to fabricate molybdenum disulfide nanosheets/nitrogen-doped reduced graphene oxide (MoS2/NGO) hybrids using polyoxomolybdate as the Mo precursor. Introducing more defects and expanding interlayer spacing of MoS2 can be achieved through decreasing the pH value of the reactive system due to the existed high-nuclear polyoxometalate clusters. MoS2/NGO hybrids prepared at low pH exhibit superior HER activity to those obtained at high pH. MoS2/NGO-pH1.5 exhibits an ultralow overpotential of 81 mV at 10 mA cm−2, a low Tafel slope of 60 mV·dec−1 and good stability in alkaline electrolyte. Such excellent electrocatalytic activity is contributed by the abundant HER catalytic active sites, the increased electrochemically-accessible area and the synergetic effects between the active MoS2 catalyst and NGO support.

Introduction

Nowadays the global energy crisis and environmental deterioration have severely restricted the social development [[1], [2], [3]]. Hydrogen has the characteristics of high energy density, abundant reserves and no pollution. As an environmentally friendly renewable secondary energy source, it will be one of promising choices to replace fossil energy in future [3,4]. Hydrogen production from electrolyzing water is considered to be one of more feasible approaches [2,5,6]. It is of significance to develop highly efficient electrocatalysts for hydrogen evolution reaction (HER) to provide high current density and low overpotential [[5], [6], [7]]. Pt group metals have been figured as the most popular electrocatalysts. But their global reserve scarcity and high cost limit their practical applications [8,9]. Developing effective non-noble-metal HER electrocatalysts with high activity and low cost is still a great challenge [2,3,[7], [8], [9], [10], [11], [12]].

Layered transitional-metal dichalcogenides, such as MoS2 [5,13], WS2 [14], MoSe2 [15] and WSe2 [16] have recently been considered as promising alternatives for Pt group metals as electrocatalysts for HER. Among them, MoS2, composed of stacked S-Mo-S layers through weak van der Waals interactions, has been attracted extensive attention [8,17]. MoS2 exhibits an ideal hydrogen adsorption energy (ΔGH) near to Pt [8,18]. It was verified by the experimental results and the computational simulations that the exposed edge sites of MoS2 are responsible for its HER activity, and that the unsaturated S atoms on the layer edges take charge of the adsorption/desorption of hydrogen species during the hydrogen generation [[17], [18], [19], [20]]. Nevertheless, its instinctive stacking nature between MoS2 layers badly reduces the number of exposed active sites [18,21]. Increasing the density of exposed active sites for MoS2 can improve its electrocatalytic efficiency [20,22]. Along with the defect engineering normally used to elevate more unsaturated S atoms [20,23,24], design and preparation of MoS2 microstructures to provide abundant edge sites is the most employed methodology to enhance the number of exposed active sites [22,[25], [26], [27], [28]]. It was found that increasing the interlayer spacing of MoS2 can promote the HER activity as well [29,30]. Additionally, the poor intrinsic conductivity of MoS2 is another obvious drawback for the electrochemical HER [8,17,18]. One of efficient pathways is integrating nanostructured MoS2 with electroconductive carbonaceous materials, especially reduced graphene oxide (rGO) [18,27,28]. It is well-known that graphene possesses high chemical stability, excellent electric conductivity and a large specific surface area. The integrated graphene can accelerate the electron transfer and retard the convergence of MoS2 nanolayers during the electrochemical HER procedure [[29], [30], [31], [32], [33], [34]].

Herein, we report a simple hydrothermal route to synthesize molybdenum disulfide/nitrogen-doped reduced graphene oxide (MoS2/NGO) hybrids using phosphomolybdate acid (H3PMo12O40, PMo12), GO and thiourea as precursors. The PMo12O403− anion is the typical Keggin hetero-polyoxomolybdate cluster with a polynuclear metal-oxo structure assembled by edge- and corner-sharing MOn coordination polyhedra. It is well-known that the exact existence form of polyoxometalate (POM) clusters in the aqueous solutions is strongly pH-dependent under hydrothermal conditions [35,36]. Different POM clusters will exist under different pH conditions. Owing to the hydrogen bonding or electrostatic interactions, the POM clusters could be adsorbed onto the surface of GO films. GO could be reduced to rGO by the POM, and the POM anions will be subsequently reduced by the adsorbed thiourea to form ultrathin MoS2 nanosheets in situ on the rGO surface under the hydrothermal conditions. Excessive thiourea precursor can be decomposed to NO2, which reacts with GO to generate N-doped rGO nanosheets [30]. It is expected that different POM clusters will lead to different morphology of MoS2/NGO hybrids, which will exhibit different HER activity. It is the first time to prepare MoS2 composite hybrids under various pH conditions using PMo12, GO and thiourea as precursors. It was reveals that more defects and larger interlayer spacing of MoS2 can be introduced by decreasing the pH value of the reactive system. MoS2/NGO hybrids prepared at low pH show much better HER activity than those obtained at high pH. MoS2/NGO-pH1.5 exhibits an ultralow overpotential and long-term stability in alkaline electrolyte.

Section snippets

Preparation of GO

Graphene oxide (GO) suspension was synthesized according to a modified Hummers’ method. In brief, natural graphite powder (1 g) was added to a cold solution containing H2SO4 (100 mL) and KNO3 (0.5 g). Keeping stirring at 10 °C, KMnO4 (7.5 g) was added into the mixture. After stirring at 35 °C for 12 h, deionized water (200 mL) was slowly added until a homogeneous solution was obtained. Upon addition of 10 mL of H2O2 (30 wt%), the color of the mixture turned into bright yellow. After that, the

Morphological and structural characterization

As plotted in Fig. 1a, the XRD profiles of MoS2/NGO-x hybrids prepared under different GO additions, are very similar to each other. All reflections are in good agreement with those of the hexagonal 2H-MoS2 (JCPDS no. 37-1492). From the (002) peak at 13.9°, the interlayer spacing of MoS2 layers in MoS2/NGO-x can be determined to be 0.65 nm. From the full width at half maximum value of this reflection, the Scherrer equation gave the thickness of MoS2 nanosheet along the c axis to be 7.1 nm,

Conclusions

In summary, we fabricated MoS2/NGO hybrids through a facile one-spot hydrothermal route. The as-obtained hybrids are composed of ultrathin MoS2 nanosheets grown on the surface of NGO films. Introducing more defects and expanding interlayer spacing between MoS2 layers can be achieved by decreasing the pH value of the reactive system. The origin of the different microstructures and performances of MoS2/NGO, prepared at different pH values, was investigated in detail. The excellent HER activity of

Acknowledgements

This work was supported by Fujian Provincial Key Science and Technology Program of China (2019H0013), the Natural Science Foundation of Fujian Province of China (2017J01014, 2019J05090), Quanzhou City Science and Technology Program of China (2018C079R) and the Postgraduates Innovative Fund of Huaqiao University (17013081049). The authors also thank the faculty from Instrumental Analysis Center of Huaqiao University for their kind help in the instrument measurements.

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