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MoCoFeS hybridized with reduced graphene oxide as a new electrocatalyst for hydrogen evolution reaction
Chemical Physics Letters 711 (2018) 32–36
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
MoCoFeS hybridized with reduced graphene oxide as a new electrocatalyst for hydrogen evolution reaction ⁎
T
⁎
Mohammad Bagher Askaria, , Parisa Salarizadehb, , Majid seifia, Seyed Mohammad Rozatia, Amirkhosro Beheshti-Marnanic, Homa Saeidfirozehd,e a
Department of Physics, Faculty of Science, University of Guilan, Rasht 41335-1914, Iran High-Temperature Fuel Cell Department, Vali-e-Asr University of Rafsanjan, Rafsanjan 1599637111, Iran c Department of Chemistry, Payame Noor University, Tehran 19395-4697, Iran d Physics Dept., Alzahra University, Vanak, Tehran 19938-93973, Iran e School of Physics, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5531, Tehran, Iran b
H I GH L IG H T S
supported reduced graphene oxide (MCFS/rGO) catalyst were prepared by hydrothermal method. • MoCoFeS catalyst showed good electrochemical properties for HER. • MCFS/rGO content of rGO on electrochemical properties of catalyst was investigated. • The • The Tafel slope and onset potential of MCFS/rGO0.4 were lower than MCFS.
A R T I C LE I N FO
A B S T R A C T
Keywords: Reduced graphene oxide Hydrogen evolution reaction Hybrid catalyst MoCoFeS
The produce of high-efficiency materials in hydrogen evolution technology is a priority. Recent development in graphene-based material shows that it can be a good candidate for this target. Accordingly, the MoCoFeS/ reduced graphene oxide (rGO) catalyst was prepared by the hydrothermal method for hydrogen evolution reaction. The catalysts were characterized and the effect of rGO on the electrochemical properties of the catalyst was examined. Our results are encouraging that, the optimized catalyst (MoCoFeS/rGO0.4) at −110 mV overpotential with a Tafel slope of 50.26 mV dec−1 can be a novel candidate for an active non-precious metal hydrogen evolution reaction catalyst.
1. Introduction During the last two decades, the application of hydrogen energy is undergoing a revolution in terms of renewable energy [1–3]. There is a considerable studying have been focused on storage [4,5] and transforming [6] the hydrogen energy, but an industrial application of hydrogen is still an open question [7]. Up until now, major of studies claim that electrochemical hydrogen evolution reaction (HER) can be a more efficient process to produce the hydrogen [8]. Platinum group metals can be the best candidate for effective HER electrocatalysts [9,10]. The main limitation of platinum group metals is the cost of this materials. Hence, a growing body of literature investigated low cost material for HER same as Molybdenum-Sulfide (MoS2) [11–16], Tungsten disulfide (WS2) [17–21] and Nonsulfides (e.g. Phosphides [22,23], Carbides [24,25] and Borides [26]). Also, sulfides and oxides
⁎
of transition metals such as Mo, W, Co, Ni, and Cu in the form of composite and hybrids with polymers and carbon compounds have been applied for hydrogen generation and oxygen reduction reaction [27,28]. More recently, most of the literature focused on the using an extraordinary feature of a non-precious metal composition of carbon the same as nanotube, nanosheet, and graphene [29–31]. Carbonaceous materials such as reduced graphene oxide (rGO) have been used as support for many catalysts. The morphology of rGO can activate these catalysts. The large surface area and high conductivity, are responsible for catalytic performances of supported catalysts. Moreover, rGO has attracted considerable attention for HER due to its great chemical stability and fast electron transfer properties compared to the other carbon-based materials [32–35]. Nevertheless, the characteristics of a novel materials with high
Corresponding authors. E-mail addresses: [email protected] (M.B. Askari), [email protected] (P. Salarizadeh).
https://doi.org/10.1016/j.cplett.2018.09.025 Received 27 July 2018; Accepted 10 September 2018 Available online 11 September 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.
Chemical Physics Letters 711 (2018) 32–36
M.B. Askari et al.
efficiency of HER have not been dealt with in depth. This paper is, a preliminary attempt to characterize a novel composite MoCoFeS (MCFS) as a 2-dimensional nanocomposite as electrochemical hydrogen evolution. In this context, we tried to examine a various hybridizing form with different mass ratios of rGO. The marked observation was reported to emerge from the data comparison with previous HER parameters. 2. Experimental 2.1. Materials and methods Sodium molybdate (Na2MoO4) powder, cobalt chloride (CoCl2), iron nitrate Fe(NO3), thioacetamide (C2H5NS) and sulfuric acid were obtained from Merck company. Nafion 117 solution (5% w/v in water) was purchased from Sigma-Aldrich Company. Here we should mention that all chemicals were of analytical-reagent grade. In order to synthesize Graphene Oxide (GO), Modified Hummers Approach [36] was used. In summary, to achieved GO, K2S2O8 (8 g) and P2O5 (8 g) powder were dissolved in 24 mL of concentrated sulfuric acid solution. Then the obtained solution was stirred at 80 °C for 6 h. After that, graphite powder (4 g) was gently added in above the solution. As a product of this process, a dark blue material was obtained. Then it was cooled down to ambient temperature. To continue the process, the achieved product was rinsed with distilled water and followed by filtering and vacuum drying at 60 °C for 12 h. Later, pre-oxidized graphite powder (2 g) was dissolved into 92 mL of H2SO4. This process takes place in ice-bath. Then KMnO4 (12 g) powder was added and the mixture was kept under continuous stirring for 20 min. After this step, NaNO3 (2 g) was added to the product and stirred the mixture at 30 °C for 2 h again. Subsequently, 200 mL of distilled water was added while the solution was stirred. After 20 min, 560 mL of distilled water and 10 mL of H2O2 (30% w/v in water) was added to the slurry, respectively. Finally, an HCl solution (1 M) was used to remove the residual metal and rinsed it by distilled water three times. In this step, to reach the reduced form from GO, distilled water was adding to GO in an ultrasonication bath for 45 min. to enable separation of agglomerated particles, the products were kept in the dialyzed bag for 20 h. As the last stage, 10 mL of ammonia solution (25% w/v in water) and hydrazine hydrate (%50 w/v in water) was added to 1 mg/mL of graphene oxide suspension under the stirring condition at 85 °C for 1 h. The synthesis method of MCFS can be reviewed in a one-step hydrothermal method. First of all, Fe(NO3)3, Na2MoO4 and CoCl2 powder were dissolved in 40 mL of C2H5NS/distilled water solution under the magnetically stirred condition for 1 h. Here we should mention that the molar ratio of the salts and salts/thioacetamide was assumed 1:1 and 1:4, respectively. After that, the hydrothermal autoclave reactor (100 mL Teflon autoclave) was used to reach 240 °C and kept the mixture at this temperature for 24 h. The obtained black powder was rinsed three times by water/ethanol (3:1) solution and dried at 60 °C for 24 h. As the most important stage of our work, to achieve the proper hybrid of MCFS and rGO, 1 g of MCFS powder was mixed mechanically with 5 different amount of rGO powder (e.g. 0.2, 0.4, 0.6, 0.8 and 1 g). Then the achieved mixture was dissolved in 60 mL of distilled water under the stirring condition for 1 h. After that, the products were transferred to a 100 mL autoclave and kept it at 200 °C for 24 ho. Finally, the achieved powder was rinsed with water/ethanol solution, and then it was dried in a vacuum oven for 24 h, at 40 °C. The hybrid catalyst was named MCFS/rGOx, where x present the weight ratio of rGO nanosheets in the hybrid catalyst.
Fig. 1. The XRD pattern (a) and the EDS analysis (b) of the MCFS nanocomposite.
electron microscopy (FESEM, MIRAll TESCAN) and high-resolution transmission electron microscopy (HRTEM, Philips CM30) operated at 300 kV were used for investigation morphology and size of nanoparticles. Furthermore, energy dispersive X-ray spectroscopy (EDS) was accomplished to study the constituents of nanoparticles. The electrochemical measurements were conducted in a threeelectrode system on the AUTOLAB potentiostat - galvanostat at room temperature. The MCFS/rGO/GC was the working electrode, whereas the platinum wire and Ag/AgCl electrode were applied as the counter and reference electrodes, respectively. The polarization curves of catalysts were obtained in an acidic media (0.5 M H2SO4) with a scanning rate of 2 mV s−1. Electrochemical impedance spectroscopy (EIS) experiments were performed at the frequency range of 1 MHz to 0.1 Hz with a voltage amplitude of 5 mV.
3. Results and discussion 3.1. Structure and morphology The XRD patterns of MCFS nanocomposite was indicated in Fig. 1. The diffraction peaks at 2 θ of 32.76°, 36.52°, 47.47° and 55.44° corresponded to the (2 0 0), (2 1 0), (2 2 0), and (3 1 1) planes, respectively which were relating to the cubic phase of FeS2. Also, the found signals at 2 θ of 32.76°, 36.52°, and 69.64° show the reflections from the (0 0 2), (1 0 1), and (0 0 4) planes which correspond to the presence of hexagonal phase of CoFeS2. The Miller indexes, (0 1 3), (3 0 1), and (3 1 0), corresponded to CoMo2S4 was observed at 2 theta angles of 36.52°, 46.98°, and 55.44. Cubic phase of Fe3S4 was confirmed by observed peaks at 2 theta angles of 26.58°, 31.97°, 38.10°, 54.10°, and 56.26° related to (2 2 0), (3 1 1), (4 4 0), (4 0 0) and (5 3 1). The rhombohedral phase of Co4S3 was observed at the 2θ angles of 31.97°, 35.67°, and 54.10°, which are in correspondence with the (1 0 0), (1 0 1), and (1 1 0) reflections, respectively. The peaks observed at 2θ = 35.97°, 58.22°,
2.2. Characterization and electrochemical measurements X-ray diffraction (XRD) (Philips PW1800) analysis was used to characterize the MCFS composite. Additionally, field emission scanning 33
Chemical Physics Letters 711 (2018) 32–36
M.B. Askari et al.
Fig. 2. The FESEM images of MCFS (a, b) and FESEM and HRTEM micrographs of rGO (c, d) and MCFS/rGO0.4 (e, f).
3.2. Electrocatalytic properties of MCFS/rGOx
and 62.68° related to (1 0 2), (1 1 0) and (1 0 7) planes confirmed the hexagonal phase of MoS2. The XRD results represented that MCFS nanocomposite has been successfully synthesized. Mean particles size of the composite was calculated by Debye Scherrer equation:
D=
0.9λ β cos θ
Linear sweep voltammetry (LSV) was performed to examine the HER properties of the MCFS/rGOx catalysts in an acidic media (0.5 M H2SO4). The Tafel slope and overpotential were the performance indicators to evaluate HER. The polarization curves of MCFS/rGOx/GC electrodes were shown in Fig. 3a. The electrocatalytic properties related to HER were shifted to the smaller potentials and lower Tafel slopes which indicate the better coverage of the catalyst surface with Hads. Therefore, the HER activity of MCFS/rGOx increased with an increased content of rGO up to 0.4. It can be seen that the lowest onset potential and Tafel slope belonged to MCFS/rGO0.4 (Fig. 3b). The Tafel slope was obtained 50.26 mV dec−1 for MCFS/rGO0.4/GC electrode with onset potential about −110 mV which is comparable with Pt wire electrode reported by other researchers (30 mV dec−1) [19]. It is be noted that the Tafel slope and onset potential was very close to the Volmer-Hirovsky mechanism with relatively high coverage of Hads on the surface of the catalyst. But, with increasing content of rGO upper than 0.4, the onset potentials and Tafel slopes were increased as a result the HER activity declined likely due to coverage of MCFS surface with rGO and decreasing of active catalyst sites. However, MCFS/rGO0.6/GC and MCFS/rGO0.8/GC indicate still higher HER activity than that of the MCFS. The MCFS/rGO1/GC electrode exhibited −550 mV onset overpotential and 202.6 mV dec−1 Tafel slope that shows very poor HER properties. Regard to the existence of iron and molybdenum in the
(1)
where, D, β, and θ are the crystallite size, the full width in half maximum (FWHM) and the Bragg’s angle. λ is the wavelength of Cu Kα radiation [37]. The estimated size of the manufactured nanocomposite was obtained 19.9 nm. Also, constituents of the MCFS nanocomposite was approved by EDS. Fig. 1b depicts the EDS of the MCFS nanocomposite and confirms the presence of Mo, Co, Fe, and S in the composite which is consistent with XRD pattern. FESEM and HRTEM micrographs of the MCFS nanocomposite, rGO, and MCFS/rGO were shown in Fig. 2. The porosity of the MCFS nanocomposite was clearly revealed in Fig. 2a and b. Also, the nanosheet morphology of rGO with nano-size of the graphene sheet is seen in Fig. 2c and d. The FESEM and HRTEM images of MCFS/rGO catalyst was shown in Fig. 2e and f, respectively. It can be seen from Fig. 2e and f the uniform dispersion of the MCFS nanocomposite into the rGO and the size of MCFS composite about 23 nm. 34
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Fig. 5. The LSV curves of MCFS/rGO0.4/GC electrode before and after continuous 500 cycles (a) and chronopotentiometry results of MCFS/rGOx catalysts (b).
Fig. 3. Polarization curves related to MCFS/rGOx/GC electrodes (a) and corresponded Tafel slopes (b).
catalysts with rGO contents higher than 0.4, the Rct was increased as it can be seen in Fig. 4, because the rGO likely covered the MCFS active sites and thus the number of the active sites is hardly available. Also, in hydrothermal synthesis likely the rGO layers in hybrid stick together and the amount of conductivity is reduced and charge transfer resistance increased. Similar result was observed in other research [38]. Long term stability of the electrocatalyst is very important for practical application. Usually, the stability of catalyst is investigated by continues cyclic voltamogrames and chronopotentiometery. We cycled our MCFS/rGO hybrid catalyst continuously for 500 times to evaluate the stability of the MCFS/rGOx catalysts. The HER polarization curves and their corresponding Tafel slopes before and after 500 cycles was shown in Fig. 5a. HER measurements were carried out at the potential range of −550 mV and 100 mV. It was observed that I-V curves remained constant after 500 cycles the end of the cycles without significant change in cathodic current (Fig. 5a). Also, after 500 cycles, the overpotential and Tafel slope remained constant for MCFS/rGO0.4 catalyst. Therefore, the good stability of the catalysts and reducing overpotentials was observed in acidic media. Although, the active sites of the MCFS/rGO catalyst may change chemically after 500 cycles and lead to higher catalytic activity [20]. However, there is no decrease in the catalytic performance of the catalyst after 500 cycles. Therefore high stability and durability were considered for MCFS/rGOx toward HER. Also, the chronopotentiometrograms related to HER are shown in Fig. 5b. As it can be seen the cathodic potential for MCFS/rGO0.4 duration 2500 s, exhibited lower value compared to other modified electrodes which are compatible with polarization curves (Fig. 3a). A literature survey on the electrochemical properties (overpotential and Tafel slope) of the as-prepared electrocatalyst and other researches was shown in Table 1. Results indicate that the obtained overpotential and Tafel slope for HER at MCFS/rGO/GC electrode is better than or comparable with other researches.
structure of the composite, it is assumed the overpotential of hydrogen evolution on the surface of the composite will be decreased significantly. EIS was used to the investigation of electrocatalytic activity of MCFS/rGOx catalysts toward the HER reaction. The Nyquist plots of MCFS/rGOx/GC electrodes were shown in Fig. 4. All catalysts showed semicircle and their difference is significant. The Rct is an important factor correlated to the electrocatalytic kinetics at the catalyst/electrolyte interface. Rct was decreased by increasing the content of rGO up to weight ratio of 0.4. The Rct value was obtained 34.773 Ω cm−2 for MCFS/rGO0.4. The smaller Rct shows faster HER kinetics and higher reaction rate. The decrease at Rct with increasing in rGO content up to 0.4, is consistent with the values of Tafel slopes. In the MCFS/rGOx
Fig. 4. Nyquist plots of MCFS/rGOx catalysts. 35
Chemical Physics Letters 711 (2018) 32–36
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Table 1 Comparison of the electrochemical properties of the as-prepared catalyst with other researches. Composite
MoCo FeS/ rGO
CoMoS
Fe1–xCoxS2
MoS2CoMo2S4
(Co-doped FeS2) Fe0.50Co0.50S2
NiFeMoZn
(FeS2 doped MoS2) FeMoS2NF
Polybenzimiazole/ MoS2
Tafel slope (mV dec−1) Over potential (mV) References
50.26
56
46
42
52
48.2
82
50.6
−110 This work
−80 [39]
−120 [40]
−110 [41]
−150 [42]
−83.1 [43]
−136 [44]
−160 [27]
4. Conclusion
[21] J. Yi, et al., Solvothermal synthesis of metallic 1T-WS2: A supporting co-catalyst on carbon nitride nanosheets toward photocatalytic hydrogen evolution, Chem. Eng. J. 335 (2018) 282–289. [22] King, L.A., et al., Stable and Active Polymer Electrolyte Membrane Electrolyzers Utilizing Transition Metal Phosphide Hydrogen Evolution Catalysts. in Meeting Abstracts, The Electrochemical Society, 2018. [23] Z. Xing, et al., Closely interconnected network of molybdenum phosphide nanoparticles: a highly efficient electrocatalyst for generating hydrogen from water, Adv. Mater. 26 (32) (2014) 5702–5707. [24] W.-F. Chen, J.T. Muckerman, E. Fujita, Recent developments in transition metal carbides and nitrides as hydrogen evolution electrocatalysts, Chem. Commun. 49 (79) (2013) 8896–8909. [25] T. Niu, Old materials with new properties II: The metal carbides, Nano Today 18 (2018) 12–14. [26] H. Park, et al., Boron-dependency of molybdenum boride electrocatalysts for the hydrogen evolution reaction, Angew. Chem. Int. Ed. 56 (20) (2017) 5575–5578. [27] M. Dinari, et al., Polybenzimidazole and polybenzimidazole/MoS 2 hybrids as an active nitrogen sites: hydrogen generation application, RSC Adv. 5 (122) (2015) 100996–101005. [28] A.A. Ensafi, E. Heydari-Soureshjani, B. Rezaei, [PW11MO39] 5− decorated on Rureduced graphene oxide nanosheets, characterizations and application as a high performance storage energy and oxygen reduction reaction, Chem. Eng. J. 330 (2017) 1109–1118. [29] J. Deng, et al., Enhanced electron penetration through an ultrathin graphene layer for highly efficient catalysis of the hydrogen evolution reaction, Angew. Chem. Int. Ed. 54 (7) (2015) 2100–2104. [30] P. Niu, et al., Graphene-like carbon nitride nanosheets for improved photocatalytic activities, Adv. Funct. Mater. 22 (22) (2012) 4763–4770. [31] X. Zou, et al., Cobalt-embedded nitrogen-rich carbon nanotubes efficiently catalyze hydrogen evolution reaction at all pH values, Angew. Chem. Int. Ed. 53 (17) (2014) 4372–4376. [32] J. Duan, et al., 3D WS2 nanolayers@ heteroatom-doped graphene films as hydrogen evolution catalyst electrodes, Adv. Mater. 27 (28) (2015) 4234–4241. [33] J. Duan, et al., Porous C3N4 nanolayers@ N-graphene films as catalyst electrodes for highly efficient hydrogen evolution, ACS Nano 9 (1) (2015) 931–940. [34] C. Hu, L. Dai, Carbon-based metal-free catalysts for electrocatalysis beyond the ORR, Angew. Chem. Int. Ed. 55 (39) (2016) 11736–11758. [35] A. Xie, et al., Pristine graphene electrode in hydrogen evolution reaction, ACS Appl. Mater. Interf. 9 (5) (2017) 4643–4648. [36] J. Chen, et al., High-yield preparation of graphene oxide from small graphite flakes via an improved Hummers method with a simple purification process, Carbon 81 (2015) 826–834. [37] Kriz, D.A., et al., Response to Comments on the application of the Scherrer equation in “Copper aluminum mixed oxide (CuAl MO) catalyst: A green approach for the one-pot synthesis of imines under solvent-free conditions”, by Suib et al.[Appl. Catal. B: Environ, 188 [2016] 227–234, doi: 10.1016/j. apcatb. 2016.02. 007], Article Type: Correspondence. Applied Catalysis B: Environmental, 2017. 202: p. 704-705. [38] H. Li, et al., Charge-transfer induced high efficient hydrogen evolution of MoS 2/ graphene cocatalyst, Sci. Rep. 5 (2015) 18730. [39] Y.-R. Liu, et al., Ternary CoS2/MoS2/RGO electrocatalyst with CoMoS phase for efficient hydrogen evolution, Appl. Surf. Sci. 412 (2017) 138–145. [40] D.-Y. Wang, et al., Highly active and stable hybrid catalyst of cobalt-doped FeS2 nanosheets–carbon nanotubes for hydrogen evolution reaction, J. Am. Chem. Soc. 137 (4) (2015) 1587–1592. [41] X. Zhang, et al., Hybrid catalyst of MoS2-CoMo2S4 on graphene for robust electrochemical hydrogen evolution, Fuel 184 (2016) 559–564. [42] S.-Y. Huang, et al., Cobalt-doped iron sulfide as an electrocatalyst for hydrogen evolution, J Electrochem. Soc. 164 (4) (2017) F276–F282. [43] F. Crnkovic, S. Machado, L. Avaca, Electrochemical and morphological studies of electrodeposited Ni–Fe–Mo–Zn alloys tailored for water electrolysis, Int. J. Hydrogen Energy 29 (3) (2004) 249–254. [44] X. Zhao, et al., FeS2-doped MoS2 nanoflower with the dominant 1T-MoS2 phase as an excellent electrocatalyst for high-performance hydrogen evolution, Electrochim. Acta 249 (2017) 72–78.
In summary, a one-step hydrothermal method was applied to prepare MCFS/rGOx catalysts with different content of rGO for HER. Electrochemical properties were investigated by LSV, EIS and chronopotentiometry and the optimized catalyst, MCFS/rGO0.4, exhibited low overpotential equal to -110 mV and Tafel slope of 50.26 mV dec−1. Also, the stability of MCFS/rGO0.4 catalyst was investigated after 500 cycles. The result showed the constant overpotential and Tafel slope after 500 cycles for MCFS/rGO0.4 catalyst. Interestingly, this catalyst has a potential candidate for hydrogen evolution. References [1] B. Hinnemann, et al., Biomimetic hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution, J. Am. Chem. Soc. 127 (15) (2005) 5308–5309. [2] R. Ramachandran, R.K. Menon, An overview of industrial uses of hydrogen, Int. J. Hydrogen Energy 23 (7) (1998) 593–598. [3] H. Saeidfirozeh, et al., Synthesis of Galaxite, Mn0. 9Co0. 1Al2O4, and its application as a novel nanocatalyst for electrochemical hydrogen evolution reaction, Physica B: Condensed Matter 538 (2018) 172–178. [4] A.C. Dillon, et al., Storage of hydrogen in single-walled carbon nanotubes, Nature 386 (6623) (1997) 377. [5] L. Schlapbach, A. Züttel, Hydrogen-storage materials for mobile applications, in Materials for sustainable energy: a collection of peer-reviewed research and review articles from nature publishing group, World Scientific, 2011, pp. 265–270. [6] G. Marbán, T. Valdés-Solís, Towards the hydrogen economy? Int. J. Hydrogen Energy 32 (12) (2007) 1625–1637. [7] M. Felderhoff, et al., Hydrogen storage: the remaining scientific and technological challenges, PCCP 9 (21) (2007) 2643–2653. [8] M.G. Walter, et al., Solar water splitting cells, Chem. Rev. 110 (11) (2010) 6446–6473. [9] W. Sheng, H.A. Gasteiger, Y. Shao-Horn, Hydrogen oxidation and evolution reaction kinetics on platinum: acid vs alkaline electrolytes, J. Electrochem. Soc. 157 (11) (2010) B1529–B1536. [10] P.C. Vesborg, B. Seger, I. Chorkendorff, Recent development in hydrogen evolution reaction catalysts and their practical implementation, J. Phys. Chem. Lett. 6 (6) (2015) 951–957. [11] M.A.R. Anjum, et al., Efficient hydrogen evolution reaction catalysis in alkaline media by all-in-one MoS2 with multifunctional active sites, Adv. Mater. 30 (20) (2018) 1707105. [12] J.D. Benck, et al., Catalyzing the hydrogen evolution reaction (HER) with molybdenum sulfide nanomaterials, Acs Catalysis 4 (11) (2014) 3957–3971. [13] T.F. Jaramillo, et al., Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts, Science 317 (5834) (2007) 100–102. [14] Y. Sun, et al., Enabling colloidal synthesis of edge-oriented MoS2 with expanded interlayer spacing for enhanced HER catalysis, Nano Lett. 17 (3) (2017) 1963–1969. [15] H. Tributsch, Layer-type transition metal dichalcogenides—a new class of electrodes for electrochemical solar cells, Ber. Bunsenges. Phys. Chem. 81 (4) (1977) 361–369. [16] H. Tributsch, J. Bennett, Electrochemistry and photochemistry of MoS2 layer crystals. I, J. Electroanal. Chem. Interf. Electrochem. 81 (1) (1977) 97–111. [17] A. Jiang, et al., Vanadium-Doped WS2 nanosheets grown on carbon cloth as a highly efficient electrocatalyst for the hydrogen Evolution Reaction, Chem.–An Asian J. (2018). [18] D. Kong, et al., First-row transition metal dichalcogenide catalysts for hydrogen evolution reaction, Energy Environ. Sci. 6 (12) (2013) 3553–3558. [19] D. Voiry, et al., Enhanced catalytic activity in strained chemically exfoliated WS 2 nanosheets for hydrogen evolution, Nat. Mater. 12 (9) (2013) 850. [20] J. Yang, H.S. Shin, Recent advances in layered transition metal dichalcogenides for hydrogen evolution reaction, J. Mater. Chem. A 2 (17) (2014) 5979–5985.
36
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
MoCoFeS hybridized with reduced graphene oxide as a new electrocatalyst for hydrogen evolution reaction ⁎
T
⁎
Mohammad Bagher Askaria, , Parisa Salarizadehb, , Majid seifia, Seyed Mohammad Rozatia, Amirkhosro Beheshti-Marnanic, Homa Saeidfirozehd,e a
Department of Physics, Faculty of Science, University of Guilan, Rasht 41335-1914, Iran High-Temperature Fuel Cell Department, Vali-e-Asr University of Rafsanjan, Rafsanjan 1599637111, Iran c Department of Chemistry, Payame Noor University, Tehran 19395-4697, Iran d Physics Dept., Alzahra University, Vanak, Tehran 19938-93973, Iran e School of Physics, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5531, Tehran, Iran b
H I GH L IG H T S
supported reduced graphene oxide (MCFS/rGO) catalyst were prepared by hydrothermal method. • MoCoFeS catalyst showed good electrochemical properties for HER. • MCFS/rGO content of rGO on electrochemical properties of catalyst was investigated. • The • The Tafel slope and onset potential of MCFS/rGO0.4 were lower than MCFS.
A R T I C LE I N FO
A B S T R A C T
Keywords: Reduced graphene oxide Hydrogen evolution reaction Hybrid catalyst MoCoFeS
The produce of high-efficiency materials in hydrogen evolution technology is a priority. Recent development in graphene-based material shows that it can be a good candidate for this target. Accordingly, the MoCoFeS/ reduced graphene oxide (rGO) catalyst was prepared by the hydrothermal method for hydrogen evolution reaction. The catalysts were characterized and the effect of rGO on the electrochemical properties of the catalyst was examined. Our results are encouraging that, the optimized catalyst (MoCoFeS/rGO0.4) at −110 mV overpotential with a Tafel slope of 50.26 mV dec−1 can be a novel candidate for an active non-precious metal hydrogen evolution reaction catalyst.
1. Introduction During the last two decades, the application of hydrogen energy is undergoing a revolution in terms of renewable energy [1–3]. There is a considerable studying have been focused on storage [4,5] and transforming [6] the hydrogen energy, but an industrial application of hydrogen is still an open question [7]. Up until now, major of studies claim that electrochemical hydrogen evolution reaction (HER) can be a more efficient process to produce the hydrogen [8]. Platinum group metals can be the best candidate for effective HER electrocatalysts [9,10]. The main limitation of platinum group metals is the cost of this materials. Hence, a growing body of literature investigated low cost material for HER same as Molybdenum-Sulfide (MoS2) [11–16], Tungsten disulfide (WS2) [17–21] and Nonsulfides (e.g. Phosphides [22,23], Carbides [24,25] and Borides [26]). Also, sulfides and oxides
⁎
of transition metals such as Mo, W, Co, Ni, and Cu in the form of composite and hybrids with polymers and carbon compounds have been applied for hydrogen generation and oxygen reduction reaction [27,28]. More recently, most of the literature focused on the using an extraordinary feature of a non-precious metal composition of carbon the same as nanotube, nanosheet, and graphene [29–31]. Carbonaceous materials such as reduced graphene oxide (rGO) have been used as support for many catalysts. The morphology of rGO can activate these catalysts. The large surface area and high conductivity, are responsible for catalytic performances of supported catalysts. Moreover, rGO has attracted considerable attention for HER due to its great chemical stability and fast electron transfer properties compared to the other carbon-based materials [32–35]. Nevertheless, the characteristics of a novel materials with high
Corresponding authors. E-mail addresses: [email protected] (M.B. Askari), [email protected] (P. Salarizadeh).
https://doi.org/10.1016/j.cplett.2018.09.025 Received 27 July 2018; Accepted 10 September 2018 Available online 11 September 2018 0009-2614/ © 2018 Elsevier B.V. All rights reserved.
Chemical Physics Letters 711 (2018) 32–36
M.B. Askari et al.
efficiency of HER have not been dealt with in depth. This paper is, a preliminary attempt to characterize a novel composite MoCoFeS (MCFS) as a 2-dimensional nanocomposite as electrochemical hydrogen evolution. In this context, we tried to examine a various hybridizing form with different mass ratios of rGO. The marked observation was reported to emerge from the data comparison with previous HER parameters. 2. Experimental 2.1. Materials and methods Sodium molybdate (Na2MoO4) powder, cobalt chloride (CoCl2), iron nitrate Fe(NO3), thioacetamide (C2H5NS) and sulfuric acid were obtained from Merck company. Nafion 117 solution (5% w/v in water) was purchased from Sigma-Aldrich Company. Here we should mention that all chemicals were of analytical-reagent grade. In order to synthesize Graphene Oxide (GO), Modified Hummers Approach [36] was used. In summary, to achieved GO, K2S2O8 (8 g) and P2O5 (8 g) powder were dissolved in 24 mL of concentrated sulfuric acid solution. Then the obtained solution was stirred at 80 °C for 6 h. After that, graphite powder (4 g) was gently added in above the solution. As a product of this process, a dark blue material was obtained. Then it was cooled down to ambient temperature. To continue the process, the achieved product was rinsed with distilled water and followed by filtering and vacuum drying at 60 °C for 12 h. Later, pre-oxidized graphite powder (2 g) was dissolved into 92 mL of H2SO4. This process takes place in ice-bath. Then KMnO4 (12 g) powder was added and the mixture was kept under continuous stirring for 20 min. After this step, NaNO3 (2 g) was added to the product and stirred the mixture at 30 °C for 2 h again. Subsequently, 200 mL of distilled water was added while the solution was stirred. After 20 min, 560 mL of distilled water and 10 mL of H2O2 (30% w/v in water) was added to the slurry, respectively. Finally, an HCl solution (1 M) was used to remove the residual metal and rinsed it by distilled water three times. In this step, to reach the reduced form from GO, distilled water was adding to GO in an ultrasonication bath for 45 min. to enable separation of agglomerated particles, the products were kept in the dialyzed bag for 20 h. As the last stage, 10 mL of ammonia solution (25% w/v in water) and hydrazine hydrate (%50 w/v in water) was added to 1 mg/mL of graphene oxide suspension under the stirring condition at 85 °C for 1 h. The synthesis method of MCFS can be reviewed in a one-step hydrothermal method. First of all, Fe(NO3)3, Na2MoO4 and CoCl2 powder were dissolved in 40 mL of C2H5NS/distilled water solution under the magnetically stirred condition for 1 h. Here we should mention that the molar ratio of the salts and salts/thioacetamide was assumed 1:1 and 1:4, respectively. After that, the hydrothermal autoclave reactor (100 mL Teflon autoclave) was used to reach 240 °C and kept the mixture at this temperature for 24 h. The obtained black powder was rinsed three times by water/ethanol (3:1) solution and dried at 60 °C for 24 h. As the most important stage of our work, to achieve the proper hybrid of MCFS and rGO, 1 g of MCFS powder was mixed mechanically with 5 different amount of rGO powder (e.g. 0.2, 0.4, 0.6, 0.8 and 1 g). Then the achieved mixture was dissolved in 60 mL of distilled water under the stirring condition for 1 h. After that, the products were transferred to a 100 mL autoclave and kept it at 200 °C for 24 ho. Finally, the achieved powder was rinsed with water/ethanol solution, and then it was dried in a vacuum oven for 24 h, at 40 °C. The hybrid catalyst was named MCFS/rGOx, where x present the weight ratio of rGO nanosheets in the hybrid catalyst.
Fig. 1. The XRD pattern (a) and the EDS analysis (b) of the MCFS nanocomposite.
electron microscopy (FESEM, MIRAll TESCAN) and high-resolution transmission electron microscopy (HRTEM, Philips CM30) operated at 300 kV were used for investigation morphology and size of nanoparticles. Furthermore, energy dispersive X-ray spectroscopy (EDS) was accomplished to study the constituents of nanoparticles. The electrochemical measurements were conducted in a threeelectrode system on the AUTOLAB potentiostat - galvanostat at room temperature. The MCFS/rGO/GC was the working electrode, whereas the platinum wire and Ag/AgCl electrode were applied as the counter and reference electrodes, respectively. The polarization curves of catalysts were obtained in an acidic media (0.5 M H2SO4) with a scanning rate of 2 mV s−1. Electrochemical impedance spectroscopy (EIS) experiments were performed at the frequency range of 1 MHz to 0.1 Hz with a voltage amplitude of 5 mV.
3. Results and discussion 3.1. Structure and morphology The XRD patterns of MCFS nanocomposite was indicated in Fig. 1. The diffraction peaks at 2 θ of 32.76°, 36.52°, 47.47° and 55.44° corresponded to the (2 0 0), (2 1 0), (2 2 0), and (3 1 1) planes, respectively which were relating to the cubic phase of FeS2. Also, the found signals at 2 θ of 32.76°, 36.52°, and 69.64° show the reflections from the (0 0 2), (1 0 1), and (0 0 4) planes which correspond to the presence of hexagonal phase of CoFeS2. The Miller indexes, (0 1 3), (3 0 1), and (3 1 0), corresponded to CoMo2S4 was observed at 2 theta angles of 36.52°, 46.98°, and 55.44. Cubic phase of Fe3S4 was confirmed by observed peaks at 2 theta angles of 26.58°, 31.97°, 38.10°, 54.10°, and 56.26° related to (2 2 0), (3 1 1), (4 4 0), (4 0 0) and (5 3 1). The rhombohedral phase of Co4S3 was observed at the 2θ angles of 31.97°, 35.67°, and 54.10°, which are in correspondence with the (1 0 0), (1 0 1), and (1 1 0) reflections, respectively. The peaks observed at 2θ = 35.97°, 58.22°,
2.2. Characterization and electrochemical measurements X-ray diffraction (XRD) (Philips PW1800) analysis was used to characterize the MCFS composite. Additionally, field emission scanning 33
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Fig. 2. The FESEM images of MCFS (a, b) and FESEM and HRTEM micrographs of rGO (c, d) and MCFS/rGO0.4 (e, f).
3.2. Electrocatalytic properties of MCFS/rGOx
and 62.68° related to (1 0 2), (1 1 0) and (1 0 7) planes confirmed the hexagonal phase of MoS2. The XRD results represented that MCFS nanocomposite has been successfully synthesized. Mean particles size of the composite was calculated by Debye Scherrer equation:
D=
0.9λ β cos θ
Linear sweep voltammetry (LSV) was performed to examine the HER properties of the MCFS/rGOx catalysts in an acidic media (0.5 M H2SO4). The Tafel slope and overpotential were the performance indicators to evaluate HER. The polarization curves of MCFS/rGOx/GC electrodes were shown in Fig. 3a. The electrocatalytic properties related to HER were shifted to the smaller potentials and lower Tafel slopes which indicate the better coverage of the catalyst surface with Hads. Therefore, the HER activity of MCFS/rGOx increased with an increased content of rGO up to 0.4. It can be seen that the lowest onset potential and Tafel slope belonged to MCFS/rGO0.4 (Fig. 3b). The Tafel slope was obtained 50.26 mV dec−1 for MCFS/rGO0.4/GC electrode with onset potential about −110 mV which is comparable with Pt wire electrode reported by other researchers (30 mV dec−1) [19]. It is be noted that the Tafel slope and onset potential was very close to the Volmer-Hirovsky mechanism with relatively high coverage of Hads on the surface of the catalyst. But, with increasing content of rGO upper than 0.4, the onset potentials and Tafel slopes were increased as a result the HER activity declined likely due to coverage of MCFS surface with rGO and decreasing of active catalyst sites. However, MCFS/rGO0.6/GC and MCFS/rGO0.8/GC indicate still higher HER activity than that of the MCFS. The MCFS/rGO1/GC electrode exhibited −550 mV onset overpotential and 202.6 mV dec−1 Tafel slope that shows very poor HER properties. Regard to the existence of iron and molybdenum in the
(1)
where, D, β, and θ are the crystallite size, the full width in half maximum (FWHM) and the Bragg’s angle. λ is the wavelength of Cu Kα radiation [37]. The estimated size of the manufactured nanocomposite was obtained 19.9 nm. Also, constituents of the MCFS nanocomposite was approved by EDS. Fig. 1b depicts the EDS of the MCFS nanocomposite and confirms the presence of Mo, Co, Fe, and S in the composite which is consistent with XRD pattern. FESEM and HRTEM micrographs of the MCFS nanocomposite, rGO, and MCFS/rGO were shown in Fig. 2. The porosity of the MCFS nanocomposite was clearly revealed in Fig. 2a and b. Also, the nanosheet morphology of rGO with nano-size of the graphene sheet is seen in Fig. 2c and d. The FESEM and HRTEM images of MCFS/rGO catalyst was shown in Fig. 2e and f, respectively. It can be seen from Fig. 2e and f the uniform dispersion of the MCFS nanocomposite into the rGO and the size of MCFS composite about 23 nm. 34
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Fig. 5. The LSV curves of MCFS/rGO0.4/GC electrode before and after continuous 500 cycles (a) and chronopotentiometry results of MCFS/rGOx catalysts (b).
Fig. 3. Polarization curves related to MCFS/rGOx/GC electrodes (a) and corresponded Tafel slopes (b).
catalysts with rGO contents higher than 0.4, the Rct was increased as it can be seen in Fig. 4, because the rGO likely covered the MCFS active sites and thus the number of the active sites is hardly available. Also, in hydrothermal synthesis likely the rGO layers in hybrid stick together and the amount of conductivity is reduced and charge transfer resistance increased. Similar result was observed in other research [38]. Long term stability of the electrocatalyst is very important for practical application. Usually, the stability of catalyst is investigated by continues cyclic voltamogrames and chronopotentiometery. We cycled our MCFS/rGO hybrid catalyst continuously for 500 times to evaluate the stability of the MCFS/rGOx catalysts. The HER polarization curves and their corresponding Tafel slopes before and after 500 cycles was shown in Fig. 5a. HER measurements were carried out at the potential range of −550 mV and 100 mV. It was observed that I-V curves remained constant after 500 cycles the end of the cycles without significant change in cathodic current (Fig. 5a). Also, after 500 cycles, the overpotential and Tafel slope remained constant for MCFS/rGO0.4 catalyst. Therefore, the good stability of the catalysts and reducing overpotentials was observed in acidic media. Although, the active sites of the MCFS/rGO catalyst may change chemically after 500 cycles and lead to higher catalytic activity [20]. However, there is no decrease in the catalytic performance of the catalyst after 500 cycles. Therefore high stability and durability were considered for MCFS/rGOx toward HER. Also, the chronopotentiometrograms related to HER are shown in Fig. 5b. As it can be seen the cathodic potential for MCFS/rGO0.4 duration 2500 s, exhibited lower value compared to other modified electrodes which are compatible with polarization curves (Fig. 3a). A literature survey on the electrochemical properties (overpotential and Tafel slope) of the as-prepared electrocatalyst and other researches was shown in Table 1. Results indicate that the obtained overpotential and Tafel slope for HER at MCFS/rGO/GC electrode is better than or comparable with other researches.
structure of the composite, it is assumed the overpotential of hydrogen evolution on the surface of the composite will be decreased significantly. EIS was used to the investigation of electrocatalytic activity of MCFS/rGOx catalysts toward the HER reaction. The Nyquist plots of MCFS/rGOx/GC electrodes were shown in Fig. 4. All catalysts showed semicircle and their difference is significant. The Rct is an important factor correlated to the electrocatalytic kinetics at the catalyst/electrolyte interface. Rct was decreased by increasing the content of rGO up to weight ratio of 0.4. The Rct value was obtained 34.773 Ω cm−2 for MCFS/rGO0.4. The smaller Rct shows faster HER kinetics and higher reaction rate. The decrease at Rct with increasing in rGO content up to 0.4, is consistent with the values of Tafel slopes. In the MCFS/rGOx
Fig. 4. Nyquist plots of MCFS/rGOx catalysts. 35
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Table 1 Comparison of the electrochemical properties of the as-prepared catalyst with other researches. Composite
MoCo FeS/ rGO
CoMoS
Fe1–xCoxS2
MoS2CoMo2S4
(Co-doped FeS2) Fe0.50Co0.50S2
NiFeMoZn
(FeS2 doped MoS2) FeMoS2NF
Polybenzimiazole/ MoS2
Tafel slope (mV dec−1) Over potential (mV) References
50.26
56
46
42
52
48.2
82
50.6
−110 This work
−80 [39]
−120 [40]
−110 [41]
−150 [42]
−83.1 [43]
−136 [44]
−160 [27]
4. Conclusion
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