Aerosol synthesis of molybdenum diselenide–reduced graphene oxide composite with empty nanovoids and enhanced hydrogen evolution reaction performances
Introduction
Ever-increasing needs for energy consumption and depletion of conventional fossil fuels call for innovation in sustainable energy conversion technologies. Hydrogen, a promising energy carrier, has been widely utilized as an important fuel in sustainable energy conversion systems to replace conventional fossil fuels owing to its environmental friendliness and high energy density [1], [2], [3]. As for the production of hydrogen, the electrochemical water splitting has been proved to be eco-friendly and the most efficient technique, where hydrogen is generated from the electrochemical reduction of H+ [1], [2], [3], [4], [5]. Platinum (Pt) and its alloy have been widely known as the best catalysts owing to their extremely high catalytic activity and small overpotential. However, the mass production of hydrogen has been hampered by the low abundance and high cost of Pt. Considering this, many studies have centered on replacing Pt with earth-abundant catalysts having high activity.
Layered transition metal dichalcogenides (MX2, M = Mo, W; X = S, Se), a class of two-dimensional (2D), graphene-like materials, have attracted extensive interest in many fields, including electrocatalysis, energy storage, and gas sensing [1], [2], [3], [4], [5], [6], [7], [8]. Nanostructured MX2 materials with the maximum number of active edge sites and high electrocatalytic activity have recently been studied as noble metal-free electrocatalysts for the hydrogen evolution reaction (HER) [9], [10], [11]. Indeed, electrochemical water splitting has been demonstrated to be an eco-friendly and highly efficient technique in which hydrogen is generated from the electrochemical reduction of H+. Substantial efforts have been focused on maximizing the number of active edge sites and the conductivity of MX2-based electrocatalysts to enhance their HER performance [12], [13]. The nanostructured MX2 composites with carbon-related materials, such as carbon nanotubes (CNTs) and graphene, have also been studied to improve the HER performance by ameliorating the low conductivity of metal dichalcogenides [14], [15], [16]. Such carbon-related materials can also maximize the number of active edge sites of MX2 by minimizing the crystal growth during the preparation process [17], [18], [19].
Among the various transition metal dichalcogenides, MoS2 materials have been widely studied because of their high chemical stability and excellent electrocatalytic performance [20], [21], [22]. The improved HER performances of composites of MoS2 and carbon-related materials have also been investigated. MoSe2’s crystal structure is similar to that of MoS2 [23], [24]. Therefore, a few research groups have studied the synthesis and HER performances of composites of MoSe2 and carbon-related materials [25], [26], [27], [28]. Mao et al. demonstrated a high-performance HER catalyst composed of a 3D graphene network supporting perpendicularly oriented MoSe2 nanosheets [15]. Qu et al. synthesized ultrathin MoSe2 nanosheets with rich defects grown on the surface of carbon fiber cloth by a facile solvothermal method [27]. The resulting nanosheets exhibited excellent HER activity, including a small onset potential, a large exchange current density, and a small Tafel slope. Huang et al. synthesized a unique hierarchical nanostructure consisting of few-layered MoSe2 nanosheets that were perpendicularly grown on CNTs through a one-step solvothermal reaction [29]. Their nanosheets exhibited HER activity with a low onset potential of −0.07 V vs. the reversible hydrogen electrode (RHE), a small Tafel slope of 58 mV dec−1, and excellent long-term cycling stability. In previous studies, the composites of MoS2 and carbon-related materials have mainly been prepared via liquid solution processes. However, the use of an easily scalable process to produce composite powders of MoSe2 and carbon-related materials with regular morphologies has not been investigated.
The spray-drying process, which is a gas-phase reaction process, has been established as a powerful and efficient method for the pilot-scale production of composites of metal compounds with carbon-related materials, including reduced graphene oxide (rGO), CNTs, and amorphous carbon [30], [31], [32], [33], [34], [35], [36], [37]. The energy storage-related electrochemical properties of nanostructured metal oxides, sulfides, and selenides composited with carbon-related materials prepared by a simple spray-drying process have been widely studied [31], [33], [34], [35], [36]. However, to the best of our knowledge, the synthesis of MoSe2–rGO composite materials with high rGO contents and their HER performances have not been studied. Here, the MoSe2–rGO composite powders that have unique structures including empty nanovoids and exhibit excellent HER performances were prepared via a pilot-scale spray-drying process.
Section snippets
Sample preparation
MoSe2–rGO composite powders with three different rGO contents were prepared by a simple two-step process. The composite powders of ammonium molybdate, selenous acid, and GO nanosheets were prepared from a spray solution by a pilot-scale spray-drying process, as shown in Fig. S1. The temperatures at the inlet and outlet of the spray dryer were fixed at 250 °C and 120 °C, respectively. A two-fluid nozzle was used as an atomizer, and the atomization pressure was 2.0 bar. The concentration of ammonium
Results and discussion
The formation mechanism of macroporous MoSe2–rGO composite with empty nanovoids created by applying a pilot-scale spray-drying process is described in Scheme 1. A droplet (Scheme 1-1) with a size of several tens of microns containing ammonium molybdate, selenous acid, and GO nanosheets was formed with a pneumatic nozzle. Drying this droplet created the filled-structure composite powder (Scheme 1-2) of ammonium molybdate, selenous acid, and GO nanosheets. The Mo and Se components did not
Conclusions
The synthesis of MoSe2–rGO composite materials with high rGO contents via a spray-drying process and their HER performances were studied for the first time. The simple two-step procedure involving a pilot-scale spray-drying process produced MoSe2–rGO composite powders that had unique structures containing empty nanovoids and showed excellent HER performances. Eliminating the excess Se component resulted in empty nanovoids within the MoSe2–rGO composite powders. The MoSe2/rGO ratios of the MoSe2
Acknowledgements
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-015R1A2A1A15056049).
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These two authors contributed equally to this work.