Research paperSynthesis and evaluation of MoWCoS/G and MoWCuS/G as new transition metal dichalcogenide nanocatalysts for electrochemical hydrogen evolution reaction
Introduction
Recently, the global energy crisis in the supply of energy resources to the world’s demands, the increase in the planet population, the finitude of the fossil fuels, as well as environmental hazards of these types of fuels are the factors that actuate the human to use the new sources of clean and renewable energies [1], [2]. Hydrogen as a clean and renewable fuel guaranteeing much less environmental contaminations is potentially a good candidate for macro scale applications in the coming decades. One of the most important concerns in the field of energy is the conversion of the latent energy in hydrogen to electrical energy that could be carried out by developing hydrogen fuel cells as the cheap and stable electrocatalysts for hydrogen evolution reaction (HER) [3], [4]. Although a group of 2-dimensional composites, namely dichalcogenides, such as WS2, MoS2, show a suitable electrocatalytic activity, but they suffer from some drawbacks like weak electrical conductivity as well as a shortage of active electrocatalytic sites which are mainly concentrated between the layers’ edges.
Platinum is assumed as an important catalyst for energy generation from hydrogen but it would be too expensive and scarce [5]. The foregoing 2-dimensional composites could be the best substitutions for platinum [6], [7]. It is worth mentioning that in order to achieve better results in the electrocatalytic activity, a series of defects must be imposed to these composites. Numerous works have been performed for promoting the number of the composites’ active sites, such as growing vertical and aligned layers [8], [9], synthesis of nanoflakes [10], synthesis of clusters [11], and the most recently, the creation of quantum dots via laser [12].
Any method considered for increasing the electrical conductivity of 2-dimensional electrocatalysts must ease the electron transportation to the active electrocatalytic sites. One solution can be combining these materials as composites or hybrids, as well as inserting metals while the synthesis process is taking place so that, both higher amounts of active sites and electrical conductivity lead to the promotion of electrocatalytic activity [7], [13], [14], [15], [16], [17].
MoS2 is supposed as one of the most applicable and promising materials if used in a layered form instead of a bulk material and it is well known as a semiconductor having a direct band gap of 1.8 eV [18]. In any case, modification and promotion of MoS2’s electrocatalytic activity are considered serious issues.
In this work, we synthesized two composites comprising tungsten, molybdenum, cobalt, and copper in the forms of Mo2S3, MoS2, Co4S3, CoS2, WS2, CuS and Cu2S in different phase structures. The XRD patterns confirm the success of this one-step synthesis. In order to study the morphology and crystal structure of the samples, SEM and TEM micrographs were obtained. Also, EDX analysis confirmed the presence of the mentioned elements in both composites. Surveying the change in the diffusion coefficient of hydrogen in the two synthesized composites with or without graphene in hydrogen contained materials was performed by using electrochemical linear sweep voltammetry technique, and the Tafel slopes and stability test were also carried out for the prepared nanocomposites. To the best of our knowledge, that was the first investigation of these two new transition metal dichalcogenides as three component nanocomposites for HER prepose.
Section snippets
Materials and instrumental
Nafion 117 solution (5%) in a mixture of lower aliphatic alcohols and water was purchased from Sigma-Aldrich. All other materials such as sodium molybdate, sodium tungstate, thioacetamide, cobalt chloride and copper chloride were of the highest quality and analytical grade procured from Merck. A Potentiostat/galvanostat model Origaflex 500 (France) along with a glassy carbon (GC) electrode (diameter: 3 mm) were used in all the experiments.
Synthesis of porous MoWCoS and MoWCuS nanocomposites
“Solution A” containing 4 mmol sodium molybdate (Na2MoO4) and 10 mmol thioacetamide was dissolved in 15 mL deionized water and then magnetically stirred at ambient temperature for 15 min. “Solution B” consisting of 4 mmol sodium tungstate (Na2O4W) and also 10 mmol thioacetamide was dissolved in 15 mL deionized water followed by magnetic stirring at ambient temperature for 15 min. “Solution C” including 4 mmol cobalt chloride and 10 mmol thioacetamide was likewise diluted and stirred in the same
Synthesis of graphene oxide, MoWCoS/G, and MoWCuS/G
Graphene oxide was synthesized following modified Hummers’ method [19]. In order to create reduced graphene (G), 0.3 mg/mL of the suspension was transferred to 5 mL of the mixture of ammonia 0.25% and hydrazine 0.35% with a 4:1 M ratio, and then it was refluxed at 80 °C for 1 h. After the pre-treatment step, 0.5 mL of the yielding suspension along with 0.5 mL Nafion solution 0.5% was mixed with 0.5 g MoWCoS or MoWCuS. Subsequently, 10 μL of the obtained mixture was loaded on GC electrode.
Material characterization
Fig. 1a shows the XRD pattern of synthesized MoWCoS nanocomposite. According to the JCPD card, the observed diffraction peaks at angles 32.38, 36.34 and 41.35° are related to the scattering from the (−1 1 1), (−1 1 2) and (2 1 0) planes of Mo2S3 in monoclinic phase, respectively. These peak positions are found to be in agreement with JCPD data card (01-078-1332). The peaks at 44.38, 52.00 and 54.08 correspond to the reflections from the hexagonal phase of MoS2 (JCPD 00-002-0132). Moreover, the
Conclusion
In this work, a couple of new catalysts, MoWCuS and MoWCoS, were successfully prepared for HER applications through hybridizing with reduced graphene. A combination of these compounds with reduced graphene was loaded on GC electrode to study their electrocatalytic properties in the hydrogen evolution reaction. The obtained results showed that the amount of overpotential in MoWCoS and MoWCuS are respectively equal to 44.1 mV/dec and 49 mV/dec showing a significant decrease in comparison with
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2019, Journal of Molecular StructureCitation Excerpt :The average size of particles was determined using Scherer equation: D = 0.9λ/βcosθ, where D is crystallite size, λ is the wavelength of X-ray radiation, β shows FWHM (full width at half maximum), and θ is the Bragg angle. The approximated size of the synthesized nanocomposite corresponded to 17.6 nm [15]. In favor of confirming the successful synthesis of synthetic nanocomposites, EDX analysis was done.