Hydrothermal assisted morphology designed MoS2 material as alternative cathode catalyst for PEM electrolyser application

https://doi.org/10.1016/j.ijhydene.2016.05.285Get rights and content

Highlights

  • Simple and environmentally benign route is demonstrated to prepare MoS2 material in three different morphologies.

  • A fine control over morphology and purity was achieved.

  • Shape dependence in catalytic activity towards HER was observed.

  • High surface area of nano-sphere MoS2 morphology is responsible for superior electro-catalytic activity.

Abstract

In this work, we developed a simple and cost-effective hydrothermal route to regulate the formation of molybdenum disulfide (MoS2) in different morphologies, like, nano-sheet, nano-capsule and nano-flake structure by controlling the reaction temperature and sulphur precursor employed. Such a fine tuning of different morphologies yields a leverage to obtain novel shapes with high surface area to employ them as suitable candidates for hydrogen evolution catalysts. Moreover, we report here the first time observation of MoS2 nano-capsule formation via environmentally benign hydrothermal route and characterized them by X-ray diffraction (XRD), nitrogen adsorption and desorption by Brunaer–Emmett–Teller (BET) method, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray photo-electron spectroscopy (XPS) techniques. MoS2 nano-capsules exhibits superior activity towards hydrogen evolution reaction (HER) with a low over-potential of 120 mV (RHE), accompanied by large exchange current density and excellent stability in 0.5 M H2SO4 solution. MoS2 nano-capsule catalyst was coated on solid proton conducting membrane (Nafion) and IrO2 as anode catalyst. The performance of the catalyst was evaluated in MEA mode for 200 h at 2 V without any degradation of electrocatalytic activity.

Introduction

Search towards efficient, cost-effective catalysts for the large scale production of hydrogen gas is under immense scrutiny for the realisation of hydrogen based economy. The existing catalysts of Pt group metals for the hydrogen evolution reaction (HER) are highly efficient [1] but too expensive and their scarcity on earth make them not viable for mass production. In this contest, replacing the expensive and rare catalysts with earth-abundant materials attracts scientific curiosity toward making the hydrogen production more economic and competitive [2]. In the recent past, many potential alternate catalysts have been developed for HER which includes metal sulfides [3], [4], [5], [6], [7], selenides [8], [9], borides [10], carbides [10], [11] and nitrides [12], [13], [14]. Among all these alternatives, molybdenum disulfide (MoS2) has received significant attention due to the earth-abundant composition and high activity, leading to the development of various kinds of MoS2-based HER electrocatalysts in the form of the crystalline [15], [16], [17], [18], [19], [20], [21], [22] or amorphous nature [23], [24], [25].

MoS2 belongs to a large family of two dimensional (2D) layered metal chalcogenide materials. Similar to the graphene layers in graphite, individual sandwiched Ssingle bondMosingle bondS layers are held together by weak vander Walls interactions in hexagonally packed structures. Both experimental [18] and computational [26] studies have concluded that the catalytic activity arises from active sites located along the edges of 2D MoS2 layers, while the basal surface are catalytically inert. It is well known that the unsaturated sulphur atoms on the edges play a crucial role in HER catalysis. Hence, increasing the number of unsaturated sulphur atoms is an efficient pathway to enhance the HER activity. Besides the aspect of active sites, the electric conductivity of catalyst is another crucial factor to affect the electro-catalytic activity because a high conductivity ensures a fast electron transport during the catalytic process [27], [28], [29], [30].

The rational design and construction of materials with structure-sensitive properties at the nanoscale is extremely important in developing advanced nanomaterials. Novel properties may arise when the size of a material is decreased to nanoscale and the dimensionality lowered due to quantum confinement effect and edge effects [31], [32]. Although MoS2 has been proved as an exciting hydrogen evolution reaction (HER) catalyst but its activity is limited by constrains in exposure of active edge sites, poor electrical transport and inefficient electrical contact. As a result, tremendous efforts have been made to design and engineer the structure of MoS2 catalysts with exposed active sites. Moreover, the morphology and distribution of the catalytically important edge sites of MoS2 are found to be sensitive to preparation conditions. In this work, we highlighted a scalable pathway to accomplish the task of engineering MoS2 nano-shape which in turn manifests for a high surface area with more exposed active sites. We achieved this by designing a reaction with suitable Mo precursor along with sulphur source during hydrothermal process in order to realize controllable morphology with more active sites. We have employed both TEM and HAADF-STEM microscopic technique to uncover the facts behind methodology tuned nanoscale morphology. Moreover, the resultant MoS2 materials have been employed as electro-catalysts toward HER in 0.5 M sulphuric acid by using linear sweep voltammetry (LSV) technique to demonstrate the structure–activity relationship.

Section snippets

Experimental section

Sodium molybdate (Na2MoO4; 98% purity) from Sigma Aldrich, thioacetamide (CH3CSNH2; 98% purity) from Merck, thiourea (H2N-CS-NH2; 99% purity) from Sigma Aldrich, Nafion from Sigma Aldrich, hydrochloric acid, ethanol were of analytical grade and used without further purification. Sulfuric acid (AR), hydrogen peroxide (30% H2O2), isopropyl alcohol (CH3CHOHCH3, >99.0% purity) from Merck and Nafion®117 membrane from DuPont, USA are used.

Influence of reaction condition and precursor on MoS2 formation

The synthesized electrocatalysts have been characterized by X-ray diffraction in order to understand the formation of MoS2 material. Fig. 1 shows the XRD patterns of three different samples prepared in this work. The figure clearly indicates that MoS2 prepared from sodium molybdate and thioacetamide precursor at 200 °C (i.e., S01) displays a single peak in XRD positioned at 2θ value of 15° which corresponds to (002) plane of MoS2 sample (JCPDS card number 37-1492) may due to preferential

Conclusions

In summary, we have synthesized an efficient MoS2 catalyst with a controlled nano-capsule by controlled hydrothermal reaction between sodium molybdate and thioacetamide precursors at 240 °C. The structure of nano-capsule MoS2 has been confirmed by FE-SEM, TEM and HAADF-STEM analysis. This specific structured MoS2 presents excellent catalytic performance for HER. The significant enhancement of catalytic activity of nano-capsules when compared with other two shapes prepared in this work is

Acknowledgements

The authors appreciate the financial support from CSIR through 12th FYP projects MULTIFUN (CSC 0101) and Hydrogen Energy Initiative Programme (CSC-0122). We would like to thank our Director Dr. Vijayamohanan K. Pillai for his constant support and suggestions. The authors also thank the staff members of Central Instrument Facility of CSIR-CECRI for their tremendous help and valuable contributions.

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